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INTRODUCTION |
We report here on the neurophysiological response to simulated auditory motion. Our fundamental questions were whether motion might affect the spatial processing of a sound and whether specialization for acoustical features specific to motion exists in the auditory domain. On a behavioral level, auditory motion processing is important for determining spatial information in the presence of sound source and/or listener movement. Accurate auditory spatial processing is particularly important for the survival of auditory predators, such as insectivorous bats. Microchiropteran bats emit high-frequency echolocation pulses and rely on information in the returning echoes for obstacle avoidance as well as for prey detection, tracking, and capture (Griffin 1958
). The acoustical image of the environment changes as bats approach stationary objects reflecting their biosonar emissions, producing an "acoustical flow field" (Lee et al. 1992). In addition, both the bat and its prey are typically in flight, and intricate paths of motion from prey echoes also may be produced. Auditory spatial processing therefore must be carried out in a continuously changing acoustic environment, and echolocating bats should rely very heavily on auditory motion processing to track and capture insects on the wing.
In comparison with visual motion, the encoding of auditory motion is problematic. Visual spatial processing is inherent in the optics and retinal organization of the eye, which preserve the spatial characteristics of a stimulus. This spatial information is maintained in place-coded activity maps throughout the retinotopically organized central visual pathway. Such maps are thought to convey the state of that mapped feature (e.g., visual location) and to facilitate higher-order feature extraction based on the mapped stimulus parameter (Knudsen et al. 1987). Higher-order specialization for motion processing is well documented in the visual system. In primate visual cortex, for example, cells that respond well to motion in one preferred direction and poorly in other "null" directions are thought to encode motion direction (e.g., Albright et al. 1984; Baker et al. 1981; Felleman and Kaas 1984; Maunsell and Van Essen 1983; Mikami et al. 1986) and target motion features are carried by neurons in a specialized "motion pathway" (Van Essen and Maunsell 1983).
By contrast to retinotopic maps of visual space, central auditory nuclei preserve cochleotopic maps of stimulus frequency. The location of an auditory stimulus is not readily available from such maps and therefore must be calculated from differences in stimulus timing or intensity between the ears. Because information about location is produced by different mechanisms in the visual and auditory systems, the basis for motion processing also would be expected to differ in the two modalities. We sought to compare motion processing in the auditory modality to its visual counterpart and to explore how the calculation of auditory space is influenced by changing the acoustic variables in the midst of those calculations.
Earlier studies have demonstrated an influence of motion direction on the responses to moving sounds using both real and apparent motion stimuli (e.g., Ahissar et al. 1992; Kleiser and Schuller 1995; Rauschecker and Harris 1989; Spitzer and Semple 1991, 1993; Stumpf et al. 1992; Takahashi and Keller 1992; Toronchuk et al. 1992; Wagner and Takahashi 1990, 1992; Yin and Kuwada 1983). While motion, direction and/or velocity were shown to influence the response to sound in these studies, the exact nature of motion sensitivity and the mechanisms producing this sensitivity are not yet fully characterized. The present study is a large-scale examination of free-field apparent motion responses in an unanesthetized mammalian preparation, explores such factors as motion velocity and orientation, and includes vertical and oblique motion orientations.
The subject of this study, Parnell's mustached bat (Pteronotus parnelli), has been particularly well studied with regard to the cellular mechanisms of stationary sound localization. Pteronotus acoustically scans its environment with "long CF/FM" sonar pulses, consisting of a long constant-frequency portion, followed by a short, downward-sweeping, frequency modulation (Novick 1963a). As in other echolocating bats (Griffin et al. 1960), the acoustic behavior of Pteronotus follows a stereotypical pattern during approach to a target that can be divided into search, approach, and terminal phases (Novick 1963b). Sonar pulse duration progressively decreases from a maximum of ~30 ms during the initial search phase to a minimum of ~6 ms immediately before contact (Novick and Vaisnys 1964). Similarly, the interval between successive pulses decreases from ~200 ms during the search phase to ~10 ms in the terminal phase (Novick 1963b). Thus information about the environment is gathered as a series of "acoustic snapshots," and the rate at which these snapshots are updated varies with distance to target.
The Pteronotus biosonar signal contains five harmonics, but the second, at ~60 kHz, is most prominent (Gooler and O'Neill 1987; Novick 1963a). The primary auditory pathway demonstrates sharp tuning and an overrepresentation of the second harmonic (Henson 1973; Kössl and Vater 1985; O'Neill 1985; Pollak et al. 1972; Ross et al. 1988; Suga and Jen 1976; Suga and Manabe 1982; Suga et al. 1975; Zook et al. 1985). About one-third of the central nucleus (ICC) of the inferior colliculus (IC) is devoted to representation of the 60-kHz harmonic, and cells tuned to that frequency range have low thresholds and are tuned extremely sharply (Grinnell 1970; O'Neill 1985; Pollak and Bodenhamer 1981). These units are clustered into the cytoarchitectonically distinct dorsoposterior division (DPD) (O'Neill et al. 1989; Pollak and Bodenhamer 1981; Zook et al. 1985), which can be considered an enormously hypertrophied isofrequency lamina (Zook et al. 1985). The DPD therefore provides a unique view into the functional organization for auditory spatial processing within a single isofrequency lamina. For example, Wenstrup et al. (1986) have demonstrated an intralaminar organization of binaural response types within the DPD and have described a map of sensitivity to interaural intensity difference (IID) with depth in the "EI area" of the DPD. However, echolocating bats are capable of both active and passive sound localization (e.g., Faure and Barclay 1992; Fuzessery et al. 1993; Kanwal et al. 1994), and there is no evidence to suggest that active and passive localization in azimuth and elevation are subserved by different neural mechanisms or structures (Fuzessery and Pollak 1985; Hutson and Kieber 1997; Pollak et al. 1995; Zook and Casseday 1982a,b). The neural processing of azimuth and elevation by echolocating bats therefore can be considered to follow the general mammalian plan.
In this study, sensitivity to free-field apparent motion was tested in the DPD, a neural substrate functionally organized for the processing of auditory space (Wenstrup et al. 1986). Apparent motion was produced by jumping a tone burst across an array of speakers in lieu of actual motion of a single speaker. In psychoacoustical experiments, this form of apparent motion gives rise to perceptions akin to real motion (Burtt 1917; Strybel et al. 1989, 1992) and has the advantage of producing none of the extraneous noise associated with a mechanically moving sound source (e.g., motors, bearings, wind noise, etc.). It also has the advantage that it approximates the acoustic stimulation normally experienced by echolocating bats, which emit temporally discrete sonar pulses and thereby experience the world "stroboscopically." This research was designed to characterize further the neural response to auditory motion, to determine relevant stimulus parameters producing a motion response, and to elucidate possible mechanisms giving rise to motion selectivity.
This work represents a portion of the dissertation by W. W. Wilson that was performed in partial fulfillment of the requirements for the PhD degree in the neuroscience program at the University of Rochester.
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METHODS |
Preparation
Six Jamaican Parnell's mustached bats (Pteronotus parnelli parnelli; Chiroptera: Mormoopidae) served as experimental subjects. The animals were maintained on a diet of fortified mealworms in a temperature- (28°C) and humidity- (85-95%) controlled flight room approximating the colony's home cave. All surgical and recording procedures were approved under the animal care and usage guidelines of the University Committee on Animal Resources and conducted in facilities with programs accredited by the American Association of Laboratory Animal Care.
Surgical procedure
Individual bats were prepared under methoxyflurane (Metofane, Pittman-Moore) anesthesia in sterile conditions. The dorsal surface of the skull was exposed by reflecting the overlying skin and musculature laterally, and a small threaded holding tube was attached to the skull with cyanoacrylate glue and dental acrylic. A sharpened tungsten indifferent electrode (125-µm diam.) then was inserted through a small hole bored in the skull and glued in place contacting the dura. The preceding procedure did not appreciably affect the normal position or motility of the pinnae.
Bats were allowed to recover from anesthesia overnight before the first recording session. Topical anesthetic [lidocaine hydrochloride (Xylocaine), 2%] was applied to the wound margins throughout sessions as needed. Before recording began, a small hole (~500-µm square) was cut in the skull to permit insertion of the recording electrode. The IC is readily visible through the thin skull of the bat and placement of the hole over the IC was accomplished visually. The specific target was the DPD of the ICC. Recordings also were carried out in the superior colliculus (SC) of one animal using the standard recording setup described in the following section. In addition, a glass micropipette (8-µm-tip diam) filled with 10% horseradish peroxidase in 0.05 M tris(hydroxymethyl)aminomethane buffer, 0.5 M KCl, pH 7.6 was used to record activity in the SC and to mark an iontophoretic injection site. Subsequent histological processing (after Mesulam 1982) was used to verify recording sites.
Recording setup
Experiments were carried out in a shielded, double-walled, temperature-controlled, soundproofed booth (IAC) lined with convoluted foam (Sonex) to attenuate echoes. The awake bat was placed into a form-fitting foam restraint in a custom-built stereotaxic frame (Schuller et al. 1986). The holding tube on the head of the bat then was attached to an arm on the stereotaxic frame such that the head was in a fixed position relative to the loudspeaker array (see further text). Synthetic lamb's wool was draped over the frame to reduce echoes. Only the bat's head, the restraining arm, and a small part of the stereotaxic frame were uncovered and directly in the sound field, and the tube and arm that held the bat's head were never directly between the sound source and the bat's ears. Recording sessions generally lasted 6-8 h per day, and water was provided to the bat at regular intervals during this time.
Parylene-coated tungsten microelectrodes (Micro-Probe) with tip exposures of 10 µm (2.0-2.5 M
impedance) or glass micropipettes filled with 3 M NaCl or 3 M KCl (impedance >10 M) were introduced into the brain with a three-dimensional micromanipulator system and a piezoelectric microdrive (Burleigh Inchworm PZ-555). Neural activity was amplified and recorded with conventional extracellular techniques. Recorded spikes were discriminated from background using a time/amplitude window discriminator (BAK Electronics model DIS-1), and the time of occurrence was recorded by a real-time clock on the laboratory computer (Digital Equipment Corporation Micro PDP 11/23+) in concert with our data acquisition/stimulus presentation package (HAL) written by H. D. Lesser. Additional motion-specific software was written in the C programming language by W. W. Wilson and integrated into the existing package.
Stimulus generation and delivery
Pure tone bursts were the acoustic stimuli used in all experiments. Continuous pure tones were generated by a calibrated function generator (Wavetek Model 111), and tone frequency was monitored by a frequency counter (Optoelectronics FC-50). The signals then were gated into tone bursts with a 1.0-ms linear rise/fall time by an electronic switch (Wilsonics BSIT), sent through a programmable attenuator (Wilsonics PATT), band-pass filtered from 5 to 150 kHz (Krohn-Hite 3202R), and broadcast from a custom-built speaker array. The speaker array's controller board was governed by the computer's parallel I/O port under software control.
The array controller consisted of an extended-bit addressing system and associated multiplexers (used as digital switches). Multiplexers were used rather than relay switches to avoid audible switching transients. The extended-bit addressing hardware used input from the computer's parallel I/O board to activate a selected multiplexer line. Each multiplexer line fed one of 130 possible speakers in the array, and only one speaker was activated at any given time. Controller switching was coordinated with stimulus presentation and data acquisition by the software, and a selected line became active 
25 ms before a tone burst was passed through it. The tone burst was routed through the active line, biased at 200 V DC and broadcast from that line's corresponding speaker in the array.
The speaker array consisted of 130 electrostatic transducers (Polaroid model T2004-C; 3.8-cm diam, 4.2° subtended angle) mounted on a frame of 10 horizontal semicircular perimeters (51.5 cm radii; Fig. 1). The speakers were mounted symmetrically about midline at 10° intervals on each perimeter, and the perimeters were separated by 10° elevation, forming a 10 × 10° grid that spanned a 90° range in elevation and a 120° range in azimuth. At the focus of the array, the speaker-to-speaker variation was ±2.05 dB at 60 kHz measured by a calibrated 1/4-in. microphone (Brüel and Kjaer model 4135) connected to a measuring amplifier (Brüel and Kjaer model 2610). With this setup, tone bursts could be presented either in succession from a single speaker (stationary stimulus) or jumping sequentially from speaker to speaker (free-field apparent motion or FFM).
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| FIG. 1.
Free-field stimulus array viewed from the front and slightly to the left. A total of 130 speakers formed a 10 × 10° grid in front of the animal at a radius of 51.5 cm. During free-field experiments, the point where the bat's interaural axis intersects midline was placed at the focal point of this array. Marked speaker shows the best location of a typical cell in the right inferior colliculus (IC) at 30° contralateral (CL) azimuth,  10° elevation. Free-field apparent motion (FFM) was produced by jumping tone bursts back and forth between the ends of the speaker array in any of the 4 orientations allowing straight line motion through the best location (smaller reproductions).
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Pinna movements in response to auditory motion might have confounded the interpretation of our results. However, we observed no pinna movement correlated with stimulus location either by direct visual examination or by examination of video recordings. Furthermore, we have obtained results similar to those reported here for apparent motion stimuli (dynamic IID and dynamic intensity stimuli) presented either through earphones or from a single loudspeaker in the free-field, i.e., conditions unaffected by pinna movements (Wilson and O'Neill 1995).
Experimental procedure
During recording sessions, the intersection of the bat's interaural axis and midline was placed at the focal point of the speaker array. The bat then was positioned such that the lower jaw and interaural axis were aligned with the horizontal axis of the speaker array. This is roughly equivalent to the plane used in other studies of this species (Fuzessery and Pollak 1985; Fuzessery et al. 1992; Makous and O'Neill 1986). The azimuth of a given speaker is expressed as degrees lateral from the midsagittal plane in the hemifield contralateral (CL) or ipsilateral (IL) to the recording site. Speakers above the plane of the jawline have a positive elevation (+e), those below, a negative elevation (e).
To isolate single units, 30-ms stationary tone bursts were presented at a rate of 5/s (200 ms onset interpulse interval, IPI) from the speaker at 30°CL, 10°e. This is roughly the point of greatest pinna amplification for 60 kHz in this species and the center of DPD spatial preference (Fuzessery and Pollak 1985; Makous and O'Neill 1986). Search stimuli were presented as the electrode was advanced until a single unit was isolated. Most units were driven by stimuli from the contralateral hemifield; however, spontaneously active but unresponsive units were tested with stimuli from midline, the ipsilateral hemifield, and with apparent motion stimuli. In initial experiments, we varied the stimulus location, intensity, and frequency to determine the speaker at which the lowest intensity stimulus at any frequency elicited a stimulus-driven response. These parameters were defined respectively as the best location, minimum threshold (MT), and best frequency (BF) of the cell. To collect more motion data before losing a cell, we later defined the best location as 30°CL, 10°e and determined the BF and MT at this location. Because BF and MT measurements depend on the directional properties of the ear (Gooler et al. 1993), a decrease in frequency tuning accuracy may have resulted from using this fixed, but not necessarily optimal position for each unit.
Apparent motion stimuli
A free-field motion stimulus was presented by sequentially changing the source of BF tone bursts from speaker to speaker through one of four possible orientations (horizontal, vertical, right oblique, and left oblique) through the best location (Fig. 1), usually at 10 dB above minimum threshold. A single "sweep" of motion typically consisted of one round-trip sequence of stimuli moving between the ends of the array, and a single "trial" of motion consisted of 20 sweeps repeated seamlessly. For any motion orientation, the direction that the stimulus moved on the first half-sweep presented was arbitrarily defined as the "forward" direction of motion. At the ends of the array, stimuli were presented twice from the same speaker so that an equal number of stimuli occurred in both directions of motion. We then could determine the effect of motion direction at any speaker by comparing the response elicited by 20 stimuli in the forward motion sequence to 20 otherwise identical stimuli but in the reverse motion sequence. The continuous round-trip nature of the stimulus was used to preclude any effect of the starting direction in the motion sequence, and our pairwise statistical analyses tested for a consistent directional effect across all sweeps. In addition, the effect of FFM starting direction was tested empirically within single units and did not influence the response to this motion paradigm.
Initial experiments indicated that a greater number of sweeps and a subsequent increase in statistical power were desirable. Due to the inherent possibility of losing the cell before a trial was complete, multiple trials with an equal number of sweeps were run, rather than a single trial with a large number of sweeps. This also allowed for a degree of post hoc analysis of the response variability between trials. We attempted to gather at least three identical trials for each stimulus condition. These trials were pooled into a single motion "set" if this proved statistically valid (see Statistical techniques).
The data acquisition software time-stamped each spike arrival with a precision of 10 µs and generated on-line peristimulus time histograms (PSTs) of the response. For each stimulus presentation, data acquisition usually began 5 ms before the stimulus began and lasted until after the cell had stopped responding. For each trial, stimulus onset times, spike arrival times, the location of the active speaker at the time each spike occurred, and motion direction were stored in individual computer files for off-line analysis. The effect of IPI, stimulus duration, motion orientation, and/or range of motion was determined in some cells.
The shortest temporal gap between stimuli that we could present was constrained by the data acquisition system to 25 ms. Standard duration/IPI combinations for apparent motion stimuli were 30s/200 ms, 30s/100 ms, and 30s/66.6 ms, although other combinations were used. This stimulus duration is typical of the initial search phase of the echolocation sequence (Novick and Vaisnys 1964), and the set of standard IPIs normally are experienced by Pteronotus during approach to a sonar target (Novick 1963b). In addition, human subjects can perceive motion from a 50-ms tone burst jumping between speakers over this range of IPIs (Strybel et al. 1989). Thus the temporal features of our apparent motion stimuli were both behaviorally relevant to the mustached bat and good approximations of conditions giving rise to motion percepts in humans.
Apparent angular velocity (
) was calculated as a function of the angular separation between speakers (
) and the IPI between stimulus onsets ( = /IPI) (Rauschecker and Harris 1989; Wagner and Takahashi 1990, 1992). Motion in the horizontal and vertical orientations ( = 10°) for our standard IPIs therefore had angular velocities of 50°/s (200 ms IPIs), 100°/s (100 ms IPIs), and 150°/s (66.6 ms IPIs). Due to the geometry of the speaker array, oblique orientations of motion ( = 14.14°) had higher apparent velocities of 71, 141, and 212°/s for the same set of IPIs.
Data analysis
On-line displays of the response as a function of stimulus location and motion direction were used to monitor to the effect of motion direction during a recording session. Only stimulus-driven spikes were included in subsequent off-line analyses. Stimulus-driven spikes were defined as those occurring in a temporal "window" for all data gathered under a given constellation of stimulus characteristics (i.e., IPI, duration, intensity, orientation, etc.). Using the window improved the signal/noise ratio and lowered statistical variability by eliminating the influence of spontaneous activity outside the window. The temporal window was obtained by subjectively determining the response onset and offset for all data files with a given stimulus configuration, displayed as PST histograms with 500-µs binwidths. These multiple estimates of response onset and offset then were used to set an overall best window that was applied as a temporal spike arrival time filter for all files with that stimulus configuration. To ensure that all stimulus-driven spikes were included in the analysis, the best window typically started at the earliest estimate of response onset and ended at the latest estimate of response offset. Output files containing the number of spikes in the analysis window for each stimulus presentation were produced and transferred to a personal computer for further analysis. Most analyses were written in the RPL language using theRS/1 data analysis package (BBN Software Products).
Statistical techniques
A brief description of our statistical analyses appears here. Detailed statistical procedures are included in the APPENDIX.
DATA POOLING.
Multiple trials with identical stimulus parameters were run for most units. Although it was advantageous to increase statistical power by pooling identical trials into a single set, blindly pooling dissimilar results would increase statistical variability. Pooling validity was tested using Kruskal-Wallis tests (Sokal and Rohlf 1981) on the spike count distribution at each speaker location for forward sweeps, reverse sweeps, and the difference between the forward and reverse sweeps.
We found that the response across trials was consistent; all replicates could be pooled for 90% of the sets with multiple trials. Boxplots were used to determine the source of any significant difference, and if a trial could not be pooled, it was removed from the set and the remaining trials were tested for pooling validity. The largest statistically valid set was used for all subsequent analyses.
DIFFERENCE IN DISCHARGE MAGNITUDE OR "DIRECTIONAL BIAS."
One way that the neural response in the two apparent motion directions might differ was for a greater overall response to occur in one direction of apparent motion over the other (i.e., a directional bias). The Wilcoxon signed-rank test was used to test whether the total number of spikes elicited by the forward sweeps of motion in a set differed from that to the reverse sweeps at a significance level of P 
0.05 without regard to the particular locations at which this effect may have occurred.
DIRECTIONALLY DEPENDENT SHIFT IN RECEPTIVE FIELD (LINEAR COMBINATION).
Another possible effect of apparent motion was a shift in a unit's receptive field location for the forward and reverse directions of motion with or without a coincident directional bias in spike count. We used a custom "linear combination" statistical procedure to determine whether such a shift was significant. The linear combination was designed to detect a shift in the location of the receptive fields in opposite motion directions by testing for a consistent pattern in the difference between them, allowing, but not requiring, the curves to cross at a single point.
The response at a given location, sweep, and direction was normalized to eliminate any response magnitude effect in the linear combination analysis. This was necessary because a directional bias summed over all locations would also produce a consistent pattern in the area between the forward and reverse curves. Because the difference in response magnitude was examined previously with the Wilcoxon test, response magnitude information was blocked out of the linear combination but was not lost. The normalization allowed us to use the linear combination to test for differences in curve shape, eliminating the influence of differences in curve size.
LATENCY ANALYSIS.
For spike latency analysis, the latency of the first spike for each stimulus presentation in a latency analysis window was analyzed. This window was similar to the spike count window except that the PST bin containing the last spike in the phasic portion of the response marked the end of the analysis window. This temporal filter prevented a skewed first spike latency distribution because of first spikes in the tonic portion of the response and reduced variability in the distribution by blocking out background activity before the evoked response. Statistical analyses were not performed on the spike latency data because many stimuli (e.g., at locations outside the receptive field) had zero spikes in the phasic response window, severely decreasing the n and invalidating statistical assumptions used in the spike count analysis.
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RESULTS |
Single-unit recordings for free-field apparent auditory motion were obtained in 92 single units in the IC and 3 single units in the SC. The BFs of the IC units ranged from 60.15 to 63.98 kHz, and their response properties were consistent with the well-documented response properties of DPD neurons (Fuzessery and Pollak 1985; Grinnell 1970; O'Neill 1985; O'Neill et al. 1989; Pollak and Bodenhamer 1981; Zook et al. 1985). Although this range of BFs exceeds that typically found within the DPD of a given bat, it was due primarily to differences between the six bats included in this study, likely reflecting individual differences in their "resting" vocalization frequency and cochlear tuning. Histological processing was used to confirm SC recording sites. The SC units did not have markedly different response properties from IC units and therefore are included in the overall analysis.
All units were responsive to both stationary search stimuli and apparent auditory motion; there were no "motion specialized" neurons in our sample that responded exclusively to apparent motion. This result corroborates the findings in other studies of free-field auditory motion (e.g., Rauschecker and Harris 1989; Wagner and Takahashi 1990, 1992) and is in contrast to directionally selective visual unit, which respond poorly or not at all to stationary stimuli (Albright et al. 1984; Hubel and Weisel 1962; Maunsell and Van Essen 1983).
A significant difference in the response between the two directions of motion (a "directional effect") was observed in 65% (n = 62) of the 95 units tested. We could not assess responses to all possible conditions of apparent motion in each of the isolated units, and in many units, the directional response changed depending on the characteristics of the stimulus. However, we found three consistent types of directional effects across units and motion conditions. A directional effect could take the form of a shift in the receptive field location (RF shift), a difference in response magnitude (directional bias), or a combination of the two effects.
Apparent motion produces receptive field shifts
An example of a receptive field shift (RF shift) to horizontal motion is shown in Fig. 2A, top. For both directions of motion, this cell responded in a restricted portion of the frontal hemifield, from ~10°IL to ~50°CL, typical of the contralateral preference generally observed at the level of the IC (see Irvine 1992 for review). There was no directional bias in this unit: the mean response in the two directions of motion was not significantly different. However, although motion did not alter the speaker locations to which the cell responded or the overall firing rate of the cell, the shape of the receptive field clearly changed with motion direction.
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| FIG. 2.
Top: response at each speaker location for opposite horizontal motion directions, averaged across all trials in a pooled FFM set. In this and all subsequent motion graphs, the response to motion from points displayed on the left side of the x axis toward those on the right is shown by the curve with the filled symbols and from the points on the right side toward points on the left by the curve with open symbols. The dashed horizontal line represents the average background firing rate measured with no acoustic stimulus and subjected to the same temporal window as the other data for the set (when available). Bottom: same data normalized as described in the APPENDIX. Normalized curves represent the raw data for the linear combination. A: receptive field (RF) shift to horizontal apparent motion. Stimuli were 30-ms tone bursts at the unit's best frequency (BF), jumping horizontally across the speaker array at an interpulse interval (IPI) of 66.6 ms at 10 dB above the unit's minimum threshold. Mean response in the contralateral direction of motion (1.046 spikes/stimulus, filled symbols) was not significantly different from that in the ipsilateral direction (1.054 spikes/stimulus, open symbols). However, the normalized data shown at the bottom of the figure showed a significant RF shift. Similarity between the top and bottom graphs indicates that intersweep variability and directional magnitude differences had little effect. B: example of directional bias to horizontal FFM [directional index (DI) = 0.287, P 0.01]. Stimulus duration was 175 ms, the IPI was 200 ms, and the intensity was minimum threshold (MT) +10 dB. Note the similarity in the 2 curves after normalization (bottom) and the size of the error bars. Although some locations in the normalized data exhibit minor differences, there was not a significant shift between the 2 directional receptive fields. C: combined shift and directional bias to horizontal FFM. Normalization to sweep total (bottom) reduced but did not eliminate directionality. Stimulus configuration was 30-ms duration/66.6 ms IPI, MT +20 dB. Although a directional bias occurred in this set, not all directional effects could be attributed solely to changes in the overall response across all locations. Linear combination on the normalized data was statistically significant, indicating that in addition to the directional bias, there was also a significant RF shift in this set. Note that the response in the ipsilateral hemifield was below the spontaneous firing rate, indicating that this unit had inhibitory ipsilateral input [i.e., was an excitatory/inhibitory binaural response type (EI) unit].
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Consider the response at 40°CL azimuth to stimuli differing only in apparent motion direction. Motion toward the ipsilateral hemifield elicited ~2.7 spikes/stimulus, whereas motion toward the contralateral hemifield elicited only ~1.6 spikes/stimulus from this location, a decrease of ~40%. Motion also affected the response on the medial receptive field border, but at these locations, the response to ipsilateral motion was lower than contralateral motion. At 0° azimuth, motion toward the contralateral hemifield elicited more than twice the number of spikes than ipsilateral motion (1.85 vs. 0.88 spikes/stimulus). The net result of these changes is a lateral shift between the RFs measured in the two motion directions, manifested by local increases in the response at certain locations and decreases at others.
The location of the receptive field border in DPD cells is considered an important information bearing parameter for encoding sound location based on a population code (Fuzessery and Pollak 1985; Wenstrup et al. 1986). Accordingly, we used the points at which the response was 50% of the peak response (in either direction) to estimate RF border location and used the difference in border location due to motion direction to quantify the magnitude of the RF shift. We refer to the angle between shifted border locations as the "border displacement" in contrast to an "RF shift," which indicates a statistically significant linear combination. In Fig. 2A, the lateral RF border was 48.9°CL for ipsilateral motion and 41.3°CL for contralateral motion (linearly interpolating between points), resulting in a lateral border displacement of 7.6°. The medial borders were 6.1°CL for ipsilateral motion and 3.0°IL for contralateral motion, producing a medial border displacement of 9.1°.
Figure 2A, bottom, shows the same data after normalization. The high degree of correspondence between the raw data (top) and normalized data (bottom) is typical of motion effects that only involve RF shifts (i.e., without a directional bias). In this case, analysis using the linear combination technique on the normalized data showed that the difference in the receptive fields was statistically significant.
Apparent motion can produce a directional bias in response magnitude without a RF shift
Similar to previous reports of directional selectivity in the visual and auditory modalities, we found that opposing motion directions also could produce differences in overall response magnitude. Directional preference, the term commonly used for this effect, generally refers to a response in the preferred motion direction at least twice that in the nonpreferred direction (e.g., Felleman and Kaas 1984; Suzuki et al. 1990). To avoid terminological confusion, we use the term directional bias to refer to the smaller differences we observed.
The example in Fig. 2B shows the largest directional bias observed in this study. The mean response across all locations to contralateral motion (1.301 spikes/stimulus) was significantly different from the mean response to ipsilateral motion (0.927 spikes/stimulus). The normalized data Fig. 2B, bottom, show that the shape of the RF in the two directions of motion was very similar when intersweep variability and magnitude differences were blocked out of the data (cf. Fig. 2A, bottom).
To compare the strength of the directional bias observed here with that in other studies, a directionality index (DI) (Felleman and Kaas 1984; Suzuki et al. 1990; Wagner and Takahashi 1990) was adopted where
The response measure is the mean number of spikes per stimulus across all locations in a given motion direction. As the response in the two motion directions diverges, the value of the DI approaches 1.0, and directional preferences >2:1 would have DIs >0.5. For the example shown in Fig. 2B, the DI was equal to 0.287. Apparent motion sets with a significant directional bias had DIs ranging from 0.048 to 0.287 with a mean DI of 0.128.
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| FIG. 3.
A: medial vs. lateral border displacement for horizontal FFM. Border displacement is displayed regardless of direction of displacement. Units with open RFs are shown as having a lateral border displacement of 1.0. Because a directional bias also can produce border displacements (e.g., Fig. 2B), sets with a directional bias in the absence of a RF shift are included in this graph. B: average response magnitude for horizontal FFM sets with a directional bias. Values along the y axis represent the mean response across all locations to contralateral motion and the x axis represents the mean response for ipsilateral motion.
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We should note here that a directional bias in the absence of a RF shift also could change the location of RF borders due to simple spike count differences across all locations. For example, the directional bias shown in Fig. 2B produced a medial border displacement of 7.2° and a lateral border displacement of 4.4°.
Apparent motion can produce both a RF shift and a directional bias
A third type of directional effect, a combined RF shift/directional bias, also was observed in response to apparent motion stimuli. The example shown in Fig. 2C exhibited a small, but highly significant directional bias (DI = 0.19;P 0.0001). After normalization to sweep total (Fig. 2C, bottom) the resultant curves still appear different: blocking the directional bias out of the data did not eliminate the directionality as it did for those sets showing a directional bias only (Fig. 2B). Statistical analysis showed that, in addition to the directional bias, there was a significant RF shift after normalization. The combined effects shifted the medial borders of the RFs by 6.2°, and the lateral borders by 1.0°.
Magnitude of border displacement and directional bias
Medial RF border displacements were typically larger than lateral displacements for horizontal motion. Figure 3A shows the medial and lateral border displacement for all significant horizontal motion sets. Although horizontal motion typically revealed "closed" RFs (i.e., the RFs had borders on both the medial and lateral sides), a limited number of units had "open" RFs, which lacked a lateral border (this border was presumably beyond the azimuthal range of the speaker array). For sets with open RFs, the missing (lateral) border displacement is displayed in Fig. 3A as 1.0. Points above the solid line indicate larger medial than lateral border displacements and represent 80% of all significant horizontal motion sets with closed RFs. The mean and median border displacements for the medial border were 5.35°and 4.32° (range = 22.7°), whereas the mean and median lateral border displacements were 2.98° and 2.15°, respectively (range = 24.7°).
Directional bias, while significant, was small in magnitude. Fig. 3B shows the mean response across all locations for horizontal motion sets with a significant directional bias either alone or in combination with a RF shift. The solid line shows where the spike counts in both directions of motion would be identical, and the dashed lines show where the spike count ratio for the two directions of motion would equal 2:1. Although responses to the two directions of motion were significantly different for all points on the graph, this figure illustrates how low the DI values were in the FFM paradigm.
Shift direction
In most sets showing significant motion effects, the RF borders were displaced in a direction opposite to the direction of motion, i.e., toward the motion source. For example, in Fig. 2A, the medial RF border was shifted 9.1° toward the source of the motion and the lateral RF border was shifted 7.6° toward the source. The direction of the border displacements for all significant FFM sets with closed RFs are shown in Fig. 4. Positive values indicate displacements toward the source. The clustering of the points in the first quadrant for RF shift alone (Fig. 4A) and for combination shift/bias sets (Fig. 4B) shows that most sets with RF shifts had positive border displacements toward the motion source. For 88% of shift alone and 85% of combination shift/bias sets, entry into the RF from either direction displaced the border opposite the direction of motion. Directional bias-only sets (Fig. 4C) had a more even dispersion among the four quadrants of the graph, indicating that motion direction does not predict displacement direction as accurately as in sets with significant RF shifts. More than one-half of the points were in either the second or fourth quadrant of this graph, indicating mixed positive and negative displacements like those in Fig. 2B, where the lateral border displacement was away from the source. Mixed displacements were much less common in the shift and combination shift/bias sets, ~10 and 12%, respectively. In addition, the bias only sets had a high degree of displacement asymmetry, where the border displacement on one side of the receptive field was much larger than on the opposite side. The ratio of the two border displacements was ~10:1 in sets with a bias alone as compared with 3:1 for sets with RF shifts and 8:1 for combined shift/bias effects.
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| FIG. 4.
Border displacements for all sets with significant directional effects and closed RFs. A: shift only sets. B: combined shift and directional bias sets. C: directional bias only sets. Border displacement was calculated from the 50% cutoffs as described in the text. RF border displacement is positive where the borders are displaced toward the source (i.e., opposite the direction of motion). Because motion orientation could vary, the 2 border displacements in a closed RF were arbitrarily assigned to either the 1st border displacement and 2nd border displacement axis. Thus the set shown in Fig. 2B is represented in C at the point (7.2, 4.4). Points in the 1st quadrant have both RF border displacements toward the motion source. Those in the 3rd quadrant have both displacements away from the motion source.
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Displacement asymmetry may generate a directional bias
A large border displacement on one side of the RF in combination with a small or nonexistent border displacement on the other might produce a significant directional bias. More symmetric RF displacements such as those seen in the shift-only example in Fig. 2A would offset a larger response on one border for a given motion direction by a smaller response along the other border, equalizing the total response in the two directions of motion. In contrast, highly asymmetric border displacements (e.g., Fig. 2C) would cause a RF shift also to have a greater number of spikes in one direction of motion over the other, generating a directional bias.
Open RFs could be considered an extreme form of displacement asymmetry. For example, the horizontal motion response shown in Fig. 5A shows only a medial border displacement. According to our statistical analyses, there was both a significant shift and directional bias in this set (medial border displacement = 7.3°; DI = 0.1, P 0.01). However, the calculated directional bias might not have been significant had there been an opposing change in activity laterally to offset the change in response documented medially. Open RFs were observed in only 17% of shift-only sets, compared with 23% of combined shift/bias sets and 40% of the sets with a directional bias only, supporting the idea that a high degree of border displacement asymmetry gives rise to significant directional bias calculations.
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| FIG. 5.
Directional effects observed across multiple orientations of motion in a single cell. All sets were run at 30-ms duration, 66.6 ms IPI and MT +10 dB. A similar directional effect was observed for vertical and oblique orientations of motion as to horizontal FFM. Entry into the RF elicits a larger response and shifts the RF border toward the motion source. Inset: motion orientation. Sets run in the horizontal, left-oblique (upper IL to lower CL), and the vertical orientations (A, B, and D, respectively) showed significant RF shifts. Horizontal and the right oblique (lower IL to upper CL) orientations (A and C) exhibited a directional bias.
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Border displacement asymmetry in combination with a consistent displacement direction (Fig. 4, A and B), would explain the slightly larger response to contralateral motion in 82% of horizontal motion sets with a directional bias (Fig. 3B). In neurons with RFs in the contralateral hemifield, RF shifts toward the motion source produce a larger response to contralateral motion on the medial edge of the RF and a larger response to ipsilateral motion on the lateral edge of the RF (e.g., Fig. 2, A and C). Because displacement asymmetry favored the medial over the lateral border (Figs. 2C and 3A), the local directional bias was typically greater on the medial border than the lateral border. Thus contralateral motion should generate a greater overall response in sets with a directional bias, if that directional bias was caused primarily by asymmetric border displacements.
Motion in any orientation produces analogous shifts
The directional effects observed for vertical and oblique motion were qualitatively similar to the horizontal FFM effects already described. Significant directional effects were observed in 70% (n = 14) of the 20 units tested with vertical or oblique apparent motion orientations. Figure 5 shows the dynamic receptive fields for a cell tested with the four possible orientations of FFM centered at 30°CL, 10°e. As was the case for horizontal motion (Fig. 5A) sound moving in the vertical (Fig. 5D) and oblique orientations (Fig. 5, B and C) produced a significant directional effect that also displaced the RF borders toward the motion source.
The RF borders in this unit for the four cardinal motion orientations were determined using the 50% points and are shown in the polar plot in Fig. 6A, along with two other examples (Fig. 6, B and C). The solid lines and arrows show the RF borders for stimuli entering the RF and the dashed lines and open arrows show the borders for stimuli exiting the RF. Because sound moving in any orientation entirely through the RF would displace both RF borders toward the motion source, motion entering the RF from any point in space should effectively expand the RF borders, and motion exiting the RF should contract the RF borders.
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| FIG. 6.
Polar plots of the RF borders gathered across multiple orientations of motion for 3 single units. Origin of each graph represents the location of the speaker common to all orientations of motion and border locations are shown as angular distance from this point. Solid lines and arrows show the RF borders for motion entering the RF; the dashed lines and open arrow show the RF border locations for motion exiting the RF. RF borders for horizontal motion are shown on the horizontal axis, vertical motion is on the vertical axis, and oblique motion, along the two oblique axes. A: RF borders for the data shown in Fig. 5 (center of plot = 30°CL, 10°e). Note that this was an open RF and only the medial side of the RF was obtained for horizontal FFM. B: FFM was run at 30-ms duration/100 ms IPI, MT +10 dB in this example. Motion in all orientations produced a significant RF shift and horizontal FFM also produced a significant directional bias. C: FFM run at 30-ms duration/100 ms IPI, MT +10 dB. Horizontal and lower IL to upper CL orientations of motion exhibited significant shifts.
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Directional effects change with stimulus timing
We varied the temporal characteristics of apparent motion stimuli in some recordings and found an influence on both the probability and magnitude of directional effects. Stimulus duration, IPI, the silent gap between stimuli, and apparent motion velocity covary and no single variable was a simple predictor of the response to apparent motion. In general, however, more consistent and larger effects were obtained for motion with high apparent velocity and with short temporal gaps between individual locations in the stimulus.
Figure 7, A-D, shows the effect of IPI (and apparent velocity) using 30-ms duration stimuli on horizontal motion responses in a single unit. Apparent motion at 300 ms IPI showed no significant difference for the two motion directions. Shorter IPIs of 200, 100, and 66.6 ms, each gave rise to significant RF shifts as well as a small directional bias at 100 ms IPI (Fig. 7, B-D). The average of the RF border displacements increased progressively from 0.8° at 200 ms IPI (50°/s velocity) to 1.9° at 66.6 ms IPI (150°/s velocity).
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| FIG. 7.
Effect of IPI (i.e., apparent velocity) and stimulus duration on horizontal FFM directionality in a single unit. Inset: stimulus timing. Directional effects increase with smaller temporal gap between stimuli whether produced by decreasing IPI (A-D) or increasing duration (B and E). All sets are horizontal FFM at MT +10 dB. A significant RF shift was observed at IPIs of 200 ms or shorter (B-E). A directional bias (DI = 0.071) also was observed at 100 ms IPI (C) and to the long duration stimulus in (E) (DI = 0.083).
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Decreasing the IPI concomitantly decreased the silent gap between stimuli and increased the apparent velocity, although none of these factors were solely responsible for the stronger directionality observed in this unit. This is illustrated by the cell's response to an FFM stimulus with a longer duration (Fig. 7E). Even though the sets in Fig. 7, C and E, had similar the silent gaps between stimuli (70 and 60 ms, respectively), the average border displacement for the long duration/IPI condition (Fig. 7E, 5.33°) was more than three times that of the short-duration/IPI condition (Fig. 7C, 1.65°). Moreover, the sets in Fig. 7, B and E, had the same IPI and apparent velocity, but a much larger average border displacement was observed for the long duration/short gap condition (Fig. 7E, 5.33°; Fig. 7B, 0.8°).
Figure 8A shows that a longer stimulus duration in sets with 200 ms IPI had a higher probability of producing a significant directional effect. One possible explanation for this effect is that longer duration stimuli elicit a greater number of spikes in tonically active units (Fig. 8B) and make directional effects statistically more robust. Figure 8C shows the proportion of sets with a significant directional effect as a function of the average spike count per trial in those sets. The probability of obtaining a significant result increased dramatically with response magnitude, from 38% (average spike count 500 spikes/trial) to ~78% (average spike count >2,000 spikes/trial). While this strongly suggests that the higher spike counts associated with long duration stimuli contribute to the probability of obtaining a significant directional effect, it should be noted that the temporal gap between stimuli decreased at long stimulus durations in Fig. 8A. Thus short temporal gaps also are correlated with the higher likelihood of a directional effect.
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| FIG. 8.
Effect of stimulus duration on 200 ms IPI sets. A: percentage of sets with significant directional effects as a function of stimulus duration for FFM sets with 200 ms IPI. B: average number of spikes/trial in the same populations shown in A. C: effect of response magnitude on directionality. Probability of obtaining a significant directional effect in all FFM sets is shown as a function of mean spikes/trial.
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Figure 9A shows that motion stimuli with shorter IPIs (and higher apparent velocities) were also more likely to elicit significant directional effects. In this case, however, stimulus duration was held constant at 30 ms and IPI was variable. The probability of obtaining a significant directional effect increased with progressively shorter IPIs. However, the number of spikes/trial was relatively constant at the three IPIs (Fig. 9B), indicating that increased statistical power was not responsible for the higher probability of a significant result. In fact, we commonly observed a decrease in spike counts at shorter IPIs within individual units (e.g., Fig. 7, A-D), which should decrease the probability of obtaining a significant effect in an individual unit. Because silent gap covaried with IPI, the highest probability of obtaining a significant directional effect in Fig. 9A was not only at the shortest IPI but also at the shortest silent gap between stimuli.
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| FIG. 9.
Effect of IPI on 30-ms duration FFM sets. A: percentage of 30-ms duration FFM sets with a significant directional effect shown as a function of interpulse interval. Probability of obtaining a significant difference in opposite motion directions was highest at shorter IPIs. B: average response per trial did not increase across the same populations. C: effect of IPI on directional effect type for FFM sets with 30-ms duration as a function of interpulse interval. D: DI magnitude in 30-ms duration FFM sets with a significant directional bias as a function of IPI. There was not a significant difference between the 3 populations (one-way analysis of variance). E: decreasing the IPI to 66.6 ms significantly increased the size of the border displacements observed. Total border displacement measurement used here was the absolute value of the sum of displacements. Thus sets with positive shifts on one RF border and negative shifts on the other have a lower value.
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Common to both Figs. 8A and 9A is a higher probability of a directional effect with shorter gaps, suggesting that the duration of the silent gap between stimuli plays a role in the presence or absence of directional effects. This implies an interaction between the responses to individual tone bursts in the apparent motion sequence and suggests that decreasing the interval between these responses allows for greater interaction.
Although shorter IPIs were more likely to produce directional effects (Fig. 9A), IPI had little influence on the type of directional effect observed (Fig. 9C). Furthermore, the magnitude of the directional bias observed at shorter IPIs did not increase (Fig. 9D). However, the total border displacement for RF shift sets (Fig. 9E) did show a significant difference (one-way analysis of variance; P 0.01). A post hoc analysis (Student-Neuman-Keuls test) indicated that the border displacements at 66.6 ms IPI differed from those at both 200 and 100 ms (P 0.05).
Directional effects are generated by motion across, but not within, receptive field boundaries
Motion across the entire receptive field had a pronounced effect at the RF edges as demonstrated by the border displacement measurements (see preceding sections). We examined the response to motion in restricted portions of the RF and found that small arcs of motion across RF borders also could elicit directional effects as shown for two cells in Fig. 10. Figure 10, top, shows the response of these units to horizontal FFM moving between the edges of the speaker array from 60°CL to 60°IL. A positive border displacement is evident in both cases, although the effect of motion direction shown in Fig. 10B was not significant. Figure 10, bottom, shows the response of these same cells under identical stimulus conditions except that the arc of motion is constrained to the medial border of the RFs. Significant directional effects were observed in both of the partial-arc sets, showing that motion across the border can account for the difference in the response to opposite motion directions and that motion through the entire RF is not necessary to induce directional effects.
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| FIG. 10.
Top: response to horizontal apparent motion across the entire speaker array for two cells. Bottom: response of these same cells to motion constrained to the medial border of the RF. A: 30-ms duration/66.6 ms IPI at MT +10 dB. Full range motion produced a significant shift and directional bias. Small arc (bottom) exhibited a significant directional bias. B: 30-ms duration/100 ms IPI at MT +10 dB. Full range motion did not produce significant directional effects. Small arc of motion across the medial RF border (bottom) gave rise to both a significant shift and directional bias. Here, both directions of motion in the partial-arc condition were gathered under unidirectional conditions: the response to contralateral motion was obtained by repeatedly presenting contralateral sweeps through the selected speakers separated by 570 ms of silence (produced by inserting 5 "blank" stimulus presentations at the end of each sweep). Response to an equal number of ipsilateral sweeps was gathered in the same fashion. Results were analyzed identically to round-trip motion and found to have both a significant RF shift and directional bias, demonstrating that FFM directional effects were not an artifact of the continuous back-and-forth nature of the typical stimulus presentation.
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Figure 11 shows the effect of constraining the arc of motion within the RF of a single unit. A significant RF shift occurred with horizontal apparent motion through the entire array (Fig. 11A) as well as when the arc of motion included only speakers spanning the edges of the RF (Fig. 11B). However, when only those speakers eliciting a response within the RF were included (Fig. 11C), there was no longer a significant directional effect (although a nonsignificant trend of a smaller magnitude and in the same direction as the previous sets can be seen). Further constraining the arc of motion also failed to produce directional effects (Fig. 11D), showing that motion across RF borders was necessary to produce directional effects.
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| FIG. 11.
Constraining FFM within the RF decreases the influence of motion direction. A-D: horizontal FFM (30-ms duration/66.6 ms IPI, MT +10 dB) sets with progressively smaller arcs of motion within the RF of a single unit. A and B: significant RF shift; C and D: no significant directional effects.
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Response latency also shifts as a function of motion direction
While we have focused on spike counts to illustrate the influence of auditory motion on central auditory neurons, the timing of the neural response is also an important descriptor neural response properties (see Brugge 1992 for review). For example, we found that the response latency of single units to stationary stimuli changed with stimulus location (Fig. 12A). This can be explained by the acoustical effects of the head and pinna in combination with the influence of sound intensity on neural response latency. Although we presented stimuli at the same amplitude from all locations, the amplitude measured at the tympanum should vary in a location-dependent manner (e.g., Flynn and Elliot 1965; Musicant et al. 1990). Stimulus amplitudes at the tympanum are greatest when presented from the "acoustical axis" of the pinna, i.e., the location where the stimulus frequency is maximally amplified by the directional properties of the pinna. The amplitude at the tympanum is diminished at off-axis locations.
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| FIG. 12.
A: static horizontal RF ( ) and mean 1st spike latency within the RF ( ) for 1 of the 3 SC units. Stimuli were 150-ms duration/200 ms IPI, at MT +10 dB. Medial and lateral borders for the stationary RF were located at 5° IL and 51.4°CL, respectively. To facilitate comparison with the peristimulus time histograms (PSTs; below), an additional 6.5 ms was added to the actual neural latency to incorporate the acoustic travel time from the array to the tympanum and a prestimulus acquisition period within the PST (see B). B: PST histograms for horizontal FFM in the same unit. Stimuli were 150-ms duration tone bursts, at MT +10 dB, presented at an IPI of 200 ms in arcs of motion across the entire array between 60°IL and 60°CL. For clarity, only locations eliciting substantial responses (between 10°IL and 50°CL) have been included in this graph.  along the abscissa indicates stimulus onset, and because this unit responded only at the stimulus onset, only the first 30 ms of the response to 150-ms tone bursts are shown. , response to each location to contralateralmotion;  , response to ipsilateral motion;  ,overlapping bins. The response to contralateral motion across the array can be found by following the dark PSTs from top to bottom and response to ipsilateral motion by following the light PSTs from bottom to top. Dashed vertical lines show the temporal windows used for both spike count and latency analysis, from 13 to 23 ms in the PST. Differences in response magnitude and latency can be seen at each location within the RF. C: directional spike count function for the data set shown in B. For contralateral motion, the medial and lateral RF borders were 10.8° IL and 49.6°CL, respectively. For ipsilateral motion, the medial border was at 0° azimuth and the lateral border was at 52.3°CL. This set exhibited a medial border displacement of 10.8°, a lateral border displacement of 2.7°, and a directional bias with a DI = 0.164. D: mean first spike latency as a function of location and motion direction for the same data. Latency at points with low spike counts is uninformative due to high variability and is not shown. Note the close correspondence between the RF shift (C) and the latency shift (D).
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Because neural latency is inversely related to amplitude (e.g., Møller 1975), the latency to a free-field stimulus should also vary as a function of location. The unit shown in Fig. 12A had a minimum response latency at 30° azimuth, the acoustic axis for 60 kHz in Pteronotus (Fuzessery and Pollak 1985). Latency increased by ~1.5 ms at off-axis locations within the RF. Although acoustic path length differences from the different locations to the ear also would influence latency, the maximum acoustic latency difference produced by the head of Pteronotus is only ~0.03 ms for the entire range of the speaker array.
PST histograms for horizontal motion in the same unit (Fig. 12B) show similar characteristics as shown for stationary stimuli: spike counts were highest and latency shortest near the center of the RF, whereas response magnitude was smallest and latency longest at the edges of the RF. However, directionally dependent differences in spike count and latency also can be seen in this figure. Along the medial border of the RF (from 10°IL to 20°CL), motion toward the contralateral hemifield () elicited greater spike counts at a shorter latency. Along the lateral border (40-50°CL) the response to ipsilateral motion () had a shorter latency. In addition, the response to ipsilateral motion was greater than that to contralateral motion at 50°CL.
These relationships can be seen more clearly in Fig. 12, C and D, which shows spike count and latency curves for this unit. In general, stimulus locations eliciting greater spike counts (Fig. 12C) also exhibited the shortest neural latency (Fig. 12D). However, in concert with shifting spike count curves, we observed shifting response latencies as a function of motion direction. The effect of apparent motion on latency bears a conspicuous resemblance to the directional effects in the spike count data. Latency is shorter on entry into the receptive field than on exit (regardless of motion direction), the size of the shift is larger on the medial border than the lateral border, and the latency curves cross at the same location as the spike count curves. Moreover, the directional bias in the spike count data is matched by a similar bias in latency.
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DISCUSSION |
This study has documented shifting receptive fields and latency profiles in inferior colliculus neurons to auditory motion stimuli. These data illustrate a widespread and consistent influence of motion direction on ICC spatial response properties. Moreover, they demonstrate that the stable receptive fields commonly thought to be a computational substrate for spatial processing are altered by moving sou
Abstract
Introduction
Abstract
