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Department of Biology, Washington University, St. Louis, Missouri
Submitted 4 December 2006; accepted in final form 11 March 2007
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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; Herbert et al. 1991; Malmierca et al. 1996). Focal electric stimulation of the auditory cortex (AC) facilitates or inhibits the auditory responses of collicular neurons (Jen et al. 1998; Zhou and Jen 2000) and not only sharpens but also shifts their frequency-tuning curves (Yan and Suga 1998; Zhang and Suga 2000). The best frequency (BF) shifts accompanied with the shifts of the frequency-tuning curves are specific and systematic according to the relationship in BF between the recorded collicular and stimulated cortical neurons. BF shifts, which result in reorganization of the frequency map, occur not only in the inferior colliculus (IC) but also in the medial geniculate body (MGB) (He 1997; Zhang and Suga 2000; Zhang et al. 1997), AC (Chowdhury and Suga 2000; Ma and Suga 2001; Sakai and Suga 2001, 2002), and cochlea (Xiao and Suga 2002).
The central auditory system creates physiologically distinct types of subcortical neurons that are different from peripheral neurons in response properties (e.g., Covey and Casseday 1999; Suga 1984, 1990). The response properties of all these subcortical neurons can presumably be changed by electric stimulation of the AC through nerve fibers descending from the AC. That is, they are subject to corticofugal modulation (Suga and Ma 2003) not only in the frequency (Yan and Suga 1998; Zhang and Suga 1997) but also in the amplitude (Jen and Zhou 2003; Yan and Ehret 2002), time (Ma and Suga 2001; Xiao and Suga 2004; Yan and Suga 1996), and spatial (Zhou and Jen 2005) domains. In the big brown bat, we previously reported that electric stimulation of cortical duration-tuned neurons evoked systematic shifts of the duration- and frequency-tuning curves of collicular duration-tuned neurons (Ma and Suga 2001). However, we did not study corticofugal modulation of collicular duration-tuned neurons in the amplitude domain.
Corticofugal modulation of collicular neurons in the amplitude domain has already been studied in the big brown bat (Jen and Zhou 2003) and house mouse (Yan and Ehret 2002). However, how single neurons corticofugally modulated in the amplitude, frequency, and time domains had not been studied. There was a possibility that different neurons were corticofugally modulated in the amplitude, frequency, or time domain. Therefore the aim of our current study is to demonstrate that corticofugal modulation of single neurons simultaneously occurs in all of these three domains characterizing a sound. We (Ma and Suga 2001) already demonstrated that duration-tuned neurons were simultaneously corticofugally modulated in both the frequency and time (duration) domains. Therefore we chose the duration-tuned neurons to demonstrate that they were also modulated in the amplitude domain.
If we studied corticofugal modulation of duration-tuned neurons only in the amplitude domain, one might consider that our work was incomplete because simultaneous modulation in the other domains was not demonstrated. Furthermore, the corticofugal modulation of responses to tone bursts in the amplitude domain greatly depends on the modulation in the frequency domain, as shown by our current study. Therefore we particularly studied corticofugal modulation of duration-tuned neurons in the frequency and amplitude domains. We minimize the description of the modulation in the frequency and time domains in our current paper and focus on our new findings on corticofugal modulation in the amplitude domain.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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. Therefore only the essential portions of these methods are summarized in the following text.
Fourteen adult big brown bats (Eptesicus fuscus) were used for the current experiments. Under neuroleptanalgesia (Innovar 4.08 mg/kg body wt), a 1.5-cm-long metal post was glued on the dorsal surface of the bat's skull. Physiological experiments were started 3–4 days after the surgery. The awake animal was placed in a polyethylene-foam body mold that was hung with an elastic band at the center of a soundproof room maintained at 31°C. The metal post glued on the skull was fixed to a metal rod with set screws to immobilize the animal's head, and the head was adjusted to face directly at a loudspeaker located 74 cm away. Holes (50–100 µm diam) were made in the skull covering the AC and IC. Through these holes, tungsten-wire electrodes (6–8 µm tip diameter, 1 M
impedance) for recording the action potentials of cortical and collicular neurons or for electrically stimulating cortical neurons were inserted into the primary AC (AI) or the central nucleus of the IC (ICc). The protocol for this research was approved by the animal studies committee of Washington University in St. Louis.
Acoustic stimulation
An acoustic stimulus was a 4.0-ms tone burst with a 0.5-ms rise-decay time, unless otherwise described, that was delivered at a rate of 4/s with a leaf tweeter. Its frequency and amplitude were varied manually or computer-controlled. The amplitude was calibrated with a Bruel and Kajael microphone placed at the bat's head and was expressed in decibels in sound pressure level (dB SPL).
The BF and minimum threshold (MT) of collicular or cortical neurons were first measured audiovisually. [A frequency tuning curve is based on many thresholds measured as a function of frequency. The threshold at the BF is called the MT (Suga 1997).] To obtain the frequency tuning curve of a single collicular neuron or multiple cortical neurons, the frequency and amplitude of the tone burst were varied at random by a computer. This computer-controlled frequency-amplitude scan consisted of 20 different frequencies at 0.5- or 1-kHz steps and 16 different amplitudes at 5-dB steps. An identical frequency-amplitude scan was delivered 10 times. To obtain the frequency-response curve, however, the amplitude of a tone burst was fixed at 10 dB above the MT of a given neuron, and its frequency was randomly varied by a computer across the BF of the neuron in 0.3- or 0.5-kHz steps. This computer-controlled frequency scan consisted of 21 250-ms-long time blocks. In the 21st (last) block, no stimulus was presented to count background discharges. An identical frequency scan was delivered 50 times.
The duration-tuning curve of a single collicular or cortical neuron was first audiovisually measured by changing the duration and amplitude of a tone burst set at the BF of a given neuron. The rise-decay time of tone bursts shorter than 5 ms was set at 0.1 ms. Then the amplitude of a tone burst was fixed where the duration-response curve was sharpest. On the average, this amplitude was 18 ± 4.1 dB (n = 58) above the MT of a given neuron. The duration of the tone bursts was randomly varied by a computer across the best duration (BDu) by 0.5- or 1.0-ms steps between 0.0 and 10 ms. This computer-controlled duration scan consisted of 13 250-ms-long time blocks. In the 13th (last) block, no stimulus was presented to count background discharges. An identical duration scan was delivered 50 times.
The amplitude-response curve of a single collicular or cortical neuron was measured by changing the amplitude of a tone burst set at the BF and BDu of a given neuron. The amplitude was randomly changed by a computer in 5-dB steps from 5 to 80 dB SPL. This computer-controlled amplitude scan consisted of 16 blocks. An identical amplitude scan was delivered 50 times.
Electric stimulation
To study the effect of electric stimulation of duration-tuned cortical neurons on a duration-tuned collicular neuron, a 6.2-ms-long train of four monophasic electric pulses (100-nA constant current, 0.2-ms duration, 2.0-ms interval) was delivered to the cortical neurons at a rate of 10/s for 30 min through a pair of tungsten-wire electrodes inserted orthogonally into the AC and placed at depths between 400 and 800 µm, i.e., in cortical layers III-VI. The tips of the paired electrodes were 6–8 µm in diameter and were separated by 
150 µm, one proximal to the other. It was estimated that such electric stimulation activates cortical neurons within a 60-µm radius in the plane orthogonal to the cortical columns (Yan and Suga 1996). These paired electrodes were first connected with a preamplifier for recording action potentials of multiple neurons at depths between 400 and 800 µm. After the measurement of their BF and MT, the electrodes were connected with a stimulus isolator to electrically stimulate them.
Data acquisition and processing
The ICc is huge and the ICd (dorsal division of the IC) is very thin in the big brown bat. In dorsoventral electrode penetrations into the ICc, BFs systematically increased because the ICc is tonotopically organized. The BFs of duration-tuned neurons were recorded at depths between 500 and 1,300 µm and their BFs were between 15 and 48 kHz.
The responses of a single collicular neuron to tone bursts in the identical amplitude, duration, and/or frequency scans repeated 50 times were recorded before and after cortical electric stimulation and were displayed as the arrays of poststimulus-time (PST) histograms. An action potential was stored on the screen of a digital storage oscilloscope at the beginning of the data acquisition and was used as a template to compare with action potentials discharged during data acquisition. The data acquisition was continued as long as action potentials visually matched the template. Data obtained before and after cortical electric stimulation were stored on a computer hard drive and were used for off-line analysis.
Off-line data processing included plotting amplitude, frequency, and duration response curves as well as frequency-tuning curves with the arrays of PST cumulative histograms displaying the responses of a collicular neuron to an identical scan repeated 50 times. The magnitude of responses to tone bursts was expressed by the number of impulses per 50 stimuli after subtracting background discharges counted in the last block of the scan.
The following criteria were used for a change in the amplitude, frequency, and duration response curves (i.e., changes in the MT, BF, and BDu) of a collicular neuron evoked by cortical electric stimulation. If the change did not recover by >50%, the data were excluded from the analysis. In stable, long recording conditions, the change recovered by >80% in 24 of the 26 neurons studied with the remaining two being excluded. This recovery itself helped prove that the change was significant. The paired t-test was used to test the difference between the responses obtained before and after the electric stimulation.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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When the stimulated cortical neurons and a recorded collicular neuron were matched in BF and BDu, the frequency- and duration-tuning curves of these collicular neurons (n = 6) became sharper after cortical electric stimulation. When they were not matched in BF and BDu, the frequency- and duration-tuning curves became sharper and their BFs and BDus shifted toward the BF and BDu of the stimulated neurons as reported by Ma and Suga (2001). If their MTs were unmatched in addition to their BFs and BDus, their MTs became higher or did not change.
In Fig. 1, the collicular neuron was tuned to 28.0 kHz (A1), 6.0-ms duration (B1) and 65 dB SPL (C1). When cortical neurons tuned to 28.0 kHz, 6.0-ms duration and 55 dB SPL were stimulated, the response of the collicular neuron was augmented at the 28.0 kHz (BF) and 6.0-ms duration (BDu), but suppressed on one or both sides of these (Fig. 1, A2 and B2). Its BF and BDu did not shift, but its amplitude tuning shifted from 65 dB SPL to 55 dB SPL (Fig. 1, C2). All these changes recovered 45 min after the electric stimulation (Fig. 1, A4, B4, and C4).
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In our previous work (Ma and Suga 2001) on corticofugal modulation of duration-tuned neurons, the modulation in the frequency domain was mainly described in terms of BF shifts, although the changes in response properties other than BF shifts (i.e., changes in MT, dynamic range in amplitude coding, response magnitude and sharpness of frequency-tuning curves) were also associated with the changes in the frequency-tuning curve. Therefore we first show variations in the corticofugal modulation of frequency-tuning curves and then the changes in the MT etc. (as listed in the preceding text) that occurred in the 48 neurons.
In Fig. 3A, the collicular neuron matched with stimulated cortical neurons in BF (28.0 kHz) and BDu (3.0 ms) but was different in MT by 5 dB (35 dB SPL for the collicular neuron and 40 dB SPL for the cortical neurons). Electric stimulation of the cortical neurons evoked changes in the collicular frequency-tuning curve: the MT changed from 35 dB SPL (
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), Q–10 dB changed from 2.8 to 4.6, and Q–30 dB changed from 0.7 to 1.8. The BF at 28.0 kHz did not change. The frequency-tuning curve returned (i.e., recovered) to that in the control condition 45 min after the electric stimulation (...). In Fig. 3B, the collicular neuron was unmatched with stimulated cortical neurons in BF, BDu, and MT: 8.0 kHz higher, 2.0 ms longer, and 5 dB lower than the cortical ones. When the cortical neurons were electrically stimulated, the collicular BF, MT, and BDu measured with 2.0-ms tone bursts changed from 33.0 to 30.0 kHz, from 30 to 35 dB SPL, and from 4.0 to 2.5 ms. That is, the BF, MT and BDu all changed toward those of the stimulated neurons. The frequency-tuning curve became sharper ().
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). Changes in threshold
When the BFs of delay-tuned neurons shifted after the cortical electric stimulation, the shifted BFs (i.e., the new BFs) were usually associated with the new MTs. The amount of MT shifts (i.e., the differences between the MTs at the control, i.e., original, and shifted BFs) was related to the difference in the control BF between the recorded collicular and stimulated cortical neurons (Fig. 4A). Five BF-matched neurons decreased their MTs by 5–10 dB, and the remaining one did not change its MT (). On the other hand, 16 of the 18 BF-unmatched neurons increased their MTs by 5–10 dB and the remaining 2 decreased their MTs by 5 dB (, 
). The increase in MT was larger for the larger BF differences between the recorded and stimulated neurons. Of the 48 neurons, 24 did not show any MT shifts. Their BF differences were mostly >7 kHz (Fig. 4A, x).
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). In 16 of the 42 BF-unmatched collicular neurons, their MTs increased after cortical electric stimulation regardless of whether their MTs were lower or higher than the stimulated cortical MT. Two neurons of the 42 lowered their MTs after cortical electric stimulation (Fig. 4B, , ). The remaining 24 BF-unmatched neurons did not show MT shifts. Their MTs were mostly different by >7 dB from the stimulated cortical MT (Fig. 4B, x), and their BFs were mostly different by >7 kHz from the stimulated control BF (Fig. 4A, x). The direction of the MT shift was toward the stimulated MT in 4 BF-matched and 12 BF-unmatched neurons but away from it in 1 BF-matched and 5 BF-unmatched neurons. The tendency was that the larger the MT difference, the larger the MT shift.
When the BF of a BF-unmatched collicular neuron shifted after the cortical electric stimulation, its threshold at the control BF (i.e., at the original BF) usually became higher. Such an increase in the threshold at the control BF was related to the difference in control BF between the recorded and stimulated neurons. The larger the difference between the control BFs of the recorded and stimulated neurons, the larger the threshold increase at the control BF. The increase in the threshold at the control BF caused by the electric stimulation was as large as 50 dB (Fig. 4C, , ).
Figure 4D shows the changes in the threshold at the control BF as a function of the differences between the MTs at the shifted and control BFs. In BF-matched neurons, the larger the MT difference, the larger the threshold decrease (Fig. 4D, ). The decrease in threshold was not more than 10 dB. In BF-unmatched neurons, however, the change in the threshold increased 
50 dB as a function of the differences between the MTs at the shifted and control BFs (Fig. 4D, ).
Changes in the amplitude-response curve and dynamic range in coding stimulus amplitude
The amplitude-response curve was nonmonotonic in three collicular neurons and monotonic in the remaining neurons plateauing at 60–80 dB SPL. Cortical electric stimulation evoked changes in the threshold and amplitude (dB)-response (number of impulses/stimulus tone) function. The change in threshold at a control BF was small in BF-matched neurons but it could be very large in BF-unmatched neurons because of their BF shifts. The larger the BF shift, which is related to the BF difference between the recorded and stimulated neurons, the larger the threshold change (Fig. 4C). Therefore the change in the amplitude-response curve at the control BF was much larger in the BF-unmatched neurons than in the BF-matched neurons.
Figure 5A shows the amplitude-response curves of a BF-matched neuron at the control BF () measured before cortical electric stimulation and at the unshifted BF (). The electric stimulation evoked a 5-dB decrease in the MT and a shift in the amplitude-response curve toward a smaller amplitude. The response at the plateau and the dynamic range increased by 10% and 4 dB, respectively.
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) but was nonmonotonic () at the BFs. The MT and DR were 5 and 6 dB smaller than those at the BFc. The amplitude-response curve at BFc after the electric stimulation indicated that the threshold became 30 dB higher than that before the stimulation and the response at the peak became 62% smaller (
). The amplitude-response curves in Fig. 5, C and D, also show that the changes in the DR were associated with changes in the MT. The DRs in the amplitude-response curves at the control BFs ranged from 10 to 30 dB (20 ± 7.1 dB, n = 6) for cortical neurons and from 20 to 40 dB (32.5 ± 6.48 dB, n = 42) for collicular neurons (Fig. 6A). This difference is statistically significant (t-test, P < 0.05). When cortical neurons were electrically stimulated, the DR increased in three of the six BF-matched neurons, did not change in two, and decreased in the remaining one, whereas the DR decreased in 16 of the 18 BF-unmatched neurons and increased in the remaining 2. The amount of the DR change was not related to the amount of the difference in any of the DR, BF, and MT between the recorded and stimulated neurons (Fig. 6, B–D).
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The direction of the BF shift was related to the sharpening or broadening of a frequency-tuning curve. Figure 8 shows the distributions of quality factors (Q–10, –30, and –50 dB). After cortical electric stimulation, for example, the Q–10 dBs of 6 BF-matched neurons increased, whereas the Q–10 dBs of 18 BF-unmatched neurons increased (n = 5), decreased (n = 11), or did not change (n = 2). The tendency was that Q values became larger for the BF-unmatched neurons showing the BF shift toward the BF of stimulated neurons () but smaller for those showing the BF shift away from that ().
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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; Yan and Ehret 2002) and only one paper that dealt with corticofugal modulation of duration-tuned neurons (Ma and Suga 2001). Jen and Zhou (2003) worked on the big brown bat as we did. However, they studied "corticofugally inhibited" collicular neurons instead of duration-tuned collicular neurons. The cortical electric stimulation was much stronger in their work (5- to 50-µA, 0.1-ms pulses) than in ours (0.1-µA, 0.2-ms pulses). Yan and Ehret (2002) studied collicular neurons in the house mouse instead of the bat. Their cortical electric stimulation was 25 times (0.5-µA, 1.0-ms pulses) stronger than ours. Therefore these three works are different in methods, species, amount of electric stimulation, and the types of neurons studied. Accordingly, there are similarities and differences in the results between these three works. In the following, we will discuss MT, DR, response magnitude, BF, and the functional significance of corticofugal modulation, comparing the preceding three works. Change in the MT
Yan and Ehret (2002) measured the MTs before and after cortical electric stimulation only at the BF in the control condition, i.e., only at the original BF, and they found that the MT shifts of BF-matched neurons mostly occurred toward the MTs of stimulated cortical neurons and ranged between –12 and +18 dB. These MT shifts were proportional to the MT differences between the recorded collicular and stimulated cortical neurons. However, the MT shifts of BF-unmatched neurons, which were also measured only at the original BF, always increased (1–60 dB) regardless of whether the collicular MTs were lower or higher than those of the stimulated cortical neurons. The tendency was that the larger the MT difference, the larger the MT shift. (These MT shifts of BF-unmatched neurons were not "true" BF shifts, as explained later.)
In our work, the MTs of most BF-matched neurons decreased 5–10 dB, whereas the MT shifts of BF-unmatched neurons always increased. The tendency was that the larger the BF difference between the recorded and stimulated neurons, the larger the MT shift. Thus our data obtained from the bat were similar to those obtained from the mouse. However, the amount of the MT shifts of BF-unmatched neurons in the house mouse (60 dB) was much larger than that in the bat (10 dB).
Yan and Ehret (2002) described an increase in the thresholds of BF-unmatched neurons (60 dB) measured only at the original BF (i.e., the BF in the control condition) as the MT shift (hereafter the "so-called" MT shift). The BF-unmatched neuron usually shifts its BF after cortical electric stimulation (Ma and Suga 2001; Sakai and Suga 2001, 2002; Xiao and Suga 2002; Yan and Ehret 2001; Zhang et al. 1997). Then the threshold at its original BF measured after the electric stimulation is not the MT of the neuron anymore. The threshold at the shifted BF (i.e., the new BF) is the MT. Therefore we measured the MTs at the shifted BF and at the control BF (i.e., the original BF) to calculate the "true" MT shift and also measured the thresholds at the control BF before and after the cortical electric stimulation. The true MT shifts of the BF-unmatched neurons were small, not more than 10 dB. However, the threshold shifts (i.e., the so-called MT shifts) of the BF-unmatched neurons measured only at the control BFs were large, 50 dB, similar to those measured by Yan and Ehret (2002). The true MT shifts may be not so different between the big brown bat and house mouse.
Jen and Zhou (2003) measured the thresholds at a control BF before and after cortical electric stimulation as did Yan and Ehret (2002) so that the so-called MT shifts calculated by them were not true MT shifts. They did not process the data grouping into two: BF-matched and -unmatched neurons. In Jen and Zhou's work, the so-called MT shifts (4–25 dB) always occurred toward the MTs of stimulated cortical neurons. Therefore their data were similar to those of the BF-matched neurons in the house mouse but different from those of BF-unmatched neurons studied by the other two groups.
Change in the dynamic range
In Yan and Ehret's work (2002), the decrease in the DR was 50 dB for BF-matched neurons and 80 dB for BF-unmatched neurons. The DRs of BF-unmatched neurons were measured only at the control BF before and after cortical electric stimulation. Jen and Zhou (2003) also measured the DRs only at the control BF before and after cortical electric stimulation.
In our work, BF-matched neurons increased their DRs by 5 dB, whereas BF-unmatched neurons decreased their DRs by 12 dB when the DRs measured at their shifted and control BFs were compared with each other. Therefore the changes in DR were much smaller in the bat than in the house mouse.
Change in the response magnitude
According to Yan and Ehret (2002), the responses of BF-matched neurons to the tone bursts at the control BFs increased dramatically in some neurons but did not change on the average in all matched neurons studied. The responses of BF-unmatched neurons to the tone bursts at the control BFs always decreased. The mean decrease at the control BF was 80% at a 10-kHz BF difference between the recorded and stimulated neurons and 50% at an 18-kHz BF difference. The change in response magnitude at the shifted BF (i.e., the new BF) was not measured. In our experiments, the response magnitude of BF-matched neurons increased by 6.2 ± 4.6% (mean ± SD; n = 6) and that of BF-unmatched neurons decreased by 10.9 ± 8.8% (n = 42) at a shifted BF compared with that at a control BF. Therefore the change in response magnitude was much smaller in the bat than in the house mouse.
BF shifts
During the current studies, we obtained data that were not reported in our previous paper on corticofugal modulation of duration-tuned neurons in the frequency and time domains (Ma and Suga 2001). That is, we found that the BF shifts were centripetal (i.e., the shifts were toward the BF of stimulated neurons) for BF differences 8 kHz and centrifugal (i.e., the shifts were away from the BF of stimulated neurons) for BF differences between 8 and 12 kHz and that the frequency-tuning curves became narrower in BF-matched neurons but became broader or narrower, if they changed at all, in BF-unmatched neurons. It has been found that nonduration-tuned collicular (Ma and Suga 2001) and cortical (Ma and Suga 2004; Sakai and Suga 2002) neurons often shifted their BFs away from the stimulated neurons when the BF difference between the recorded and stimulated neurons was large.
According to Yan and Ehret (2001), the BF shifts of BF-unmatched neurons were predominantly centripetal for BF differences 18 kHz but were mostly centrifugal for BF differences between 18 and 30 kHz. In Jen and Zhou's work (2003), BF shifts were centripetal for collicular BFs lower than cortical BFs and centrifugal for collicular BFs higher than cortical BFs. The amount of these BF shifts was proportional to the BF differences between the recorded and stimulated neurons. Therefore our data were qualitatively the same as those obtained from the house mouse by Yan and Ehret (2001, 2002) but were different from those obtained from the bat by Jen and Zhou (2003). In the big brown bat (Ma and Suga 2004) and the Mongolian gerbil (Sakai and Suga 2002), the BF shifts of cortical auditory neurons are centripetal in the large area surrounding the electric stimulation site but are centrifugal in a narrow zone surrounding the area for centripetal BF shifts. Therefore the BF shifts of cortical and collicular neurons of the big brown bat are basically the same as those of the house mouse and the Mongolian gerbil.
Functional significance of corticofugal modulation
There are three important findings reported in our current and previous (2001) papers. 1) The auditory responses of the BF-matched neurons were corticofugally augmented and their MTs and DRs became lower and wider, respectively. Furthermore, as previously shown, matched neurons were corticofugally augmented and were sharpened in their frequency and duration-tuning. Therefore they became more suitable to responding to acoustic signals when the parameter values of the signals matched their response properties. 2) On the other hand, the auditory responses of the BF-unmatched neurons were corticofugally reduced and their MTs and DRs, respectively, became higher and narrower. Furthermore, as previously shown, their frequency and duration tuning curves were shifted toward those of the stimulated cortical neurons. Therefore they became less suitable to responding to acoustic signals when the parameter values did not match their response properties. 3) The BF and BDu shifts of the unmatched neurons caused an increase in the population of collicular neurons that were similar to the electrically stimulated cortical neurons, i.e., similar to the matched collicular neurons. These shifts simultaneously caused a decrease in the population of the BF-unmatched collicular neurons that were different from the electrically stimulated cortical neurons. Therefore the overall effect of the collicular changes, including the matched and unmatched neurons, improves the neuronal processing of auditory signals that frequently stimulate the auditory system and the contrast in the neural representation of the signals. The improved neural signals ascend from the IC to the AC through the auditory thalamus and contribute to the auditory signal processing in the AC. The behavioral changes related to the changes evoked by the corticofugal system remain to be explored.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Address for correspondence: N. Suga, Dept. of Biology, Washington University, 1 Brookings Dr., St. Louis, MO 63130 (E-mail: suga{at}biology.wustl.edu)
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) in A for the amplitude-response curve at the control BF (
and 
in A: cortical and collicular neurons, respectively. 
