INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

There is now substantial evidence implicating an area that we call the "frontal pursuit area" (FPA) in smooth pursuit eye movements. The FPA has also been called the "smooth eye movement part of the frontal eye fields" and lies just caudal to and below the part of the frontal eye fields that is involved in saccadic eye movements (e.g., Gottlieb et al. 1993, 1994). Physiological evidence of its importance for pursuit comes from lesions (Keating 1991, 1993; Lynch 1987; MacAvoy et al. 1991; Shi et al. 1998), electrical stimulation (Gottlieb et al. 1993, 1994; Tian and Lynch 1996a), imaging (Berman et al. 1999; Petit and Haxby 1999; Petit et al. 1997), and unit recording studies (Fukushima et al. 2000; Gottlieb et al. 1994; MacAvoy et al. 1991; Tanaka and Fukushima 1998).

The anatomical connections of the FPA are also appropriate for a role in pursuit eye movements. The FPA and adjacent saccadic frontal eye fields have reciprocal connections with extrastriate visual motion areas such as the middle temporal area (MT) and the medial superior temporal area (MST) (e.g., Huerta et al. 1987; Leichnetz 1989; Stanton et al. 1988a, 1995; Tian and Lynch 1996b), both of which are thought to be important for pursuit. The FPA also sends strong outputs to the caudate nucleus (Cui et al. 2000; Yan et al. 1999) where a recent imaging study found a strong increase in the regional blood flow during pursuit (O'Driscoll et al. 2000). The descending output from the FPA and adjacent frontal eye fields projects to the cerebellum via the dorsomedial pontine nuclei and the nucleus reticularis tegmenti pontis (Huerta et al. 1986; Leichnetz 1989; Leichnetz et al. 1984; Stanton et al. 1988b; Yan et al. 1999), and there is a strong projection from the inferior frontal eye fields to the dorsolateral pontine nuclei (DLPN) in macaque monkeys (Leichnetz 1989). Despite the accumulation of the evidence that the FPA is involved in pursuit, it is not known whether the FPA has a unique role or if it simply provides a cortical representation of the eye-velocity command for pursuit. In the present study and an earlier report (Tanaka and Lisberger 2001), we have begun to search for possible unique functions of the FPA by analyzing the interaction among visual stimuli, pursuit eye movement, and electrical microstimulation in the FPA.

Our analysis of possible functions for the FPA has stemmed from behavioral studies showing that there are at least two different operations performed within the pursuit system (Goldreich et al. 1992; Grasse and Lisberger 1992; Keating and Pierre 1996; Krauzlis and Lisberger 1994; Krauzlis and Miles 1996a; Tanaka and Lisberger 2000). One operation converts visual motion into eye velocity, while the other provides an on-line "gain control" that regulates either or both of the strength of the visual-motor transmission for pursuit and the gain of the eye-velocity commands for pursuit. The most direct evidence for the on-line gain control was provided by the experiments of Schwartz and Lisberger (1994), who showed that eye movement responses were tiny if a brief target perturbation was presented during fixation but were large if the same perturbation was presented during the maintenance of pursuit. In addition, recent analyses have suggested that postsaccadic enhancement of pursuit is mediated by the same on-line gain control (Lisberger 1998) and that the gain control may be spatially selective in a way that causes target choice (Gardner and Lisberger 2001).

The neural loci of on-line gain control are not known. Because lesions made in MST or FPA cause deficits in the initiation and the maintenance of pursuit (references given in the preceding text) and because the pursuit-related neurons recorded from these areas carry both retinal and extra-retinal signals, it seems plausible that MST, FPA, or both participate in gain control (Grasse and Lisberger 1992; Keating and Pierre 1996; Schwartz and Lisberger 1994). We have now tested this hypothesis for the FPA with the use of electrical microstimulation. Our data show that activation of the FPA modulates the on-line gain of pursuit during both the initiation and the maintenance of pursuit.

Some of these results have been presented in a brief report (Tanaka and Lisberger 2001).

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Animal preparation

Data were collected from two male rhesus monkeys (Macaca mulatta, monkeys PCK and OLV), weighing 8-10 kg. All experimental protocols described in the following text were approved in advance by the Institutional Animal Care and Use Committee of the University of California, San Francisco. The procedures for animal preparation were similar to those described previously (Lisberger and Westbrook 1985). Monkeys were trained to sit in a primate chair and to fixate a spot of light to obtain liquid reward. Under isoflurane anesthesia and using sterile procedure, a head holder was installed over the skull using orthopedic stainless steel plates and dental acrylic. Within a few weeks after the first surgery, a coil of wire was implanted between the sclera and Tenon's capsule to record eye movements (Judge et al. 1980) under the same surgical condition. For all subsequent training and experiments, the implanted head holder was used to secure the animal's head to the ceiling of the primate chair so that horizontal and vertical eye position could be recorded.

After training in oculomotor tasks was complete, the monkeys participated in behavioral experiments that lasted for several months. A third surgery then was performed to implant a stainless steel cylinder over the right arcuate sulcus. We used a trephine to create holes in the skull centered at A26.5, L20 for monkey PCK and A26, L13 for monkey OLV. The cylinder was tilted 30-31° from the parasaggital plane to allow electrode penetrations through the dura roughly perpendicular to the cortical surface. The animals received analgesia with intramuscular injections of buprenorphine (Buprenex, 0.01 mg/kg) for 2-3 days following each surgery. Topical antibiotics were administered around the implants as necessary. The water intake was controlled daily, and the weight of the animal was checked before each experimental or training session.

Visual stimulus and behavioral tasks

The animals faced an analog oscilloscope (Hewlett Packard 1304A) that was located 28 cm from the eyes and subtended 42 × 36° of visual angle. Visual stimuli were presented on the oscilloscope at a refresh rate of 250 Hz under the control of a Pentium PC computer. A 0.2° diam, white spot with luminance of 3.8 cd/m² served as a visual stimulus. All experiments were carried out in a dark room. The horizontal and vertical eye-position signals were calibrated by having the animals fixate a stationary target at known visual angles. Thereafter targets and electrical stimulation were presented in individual trials. In each trial, eye position was compared with target position, and the animal was reinforced by drops of water or juice for maintaining eye position within a window that surrounded target position throughout the trial. The trial was aborted and followed by a newly selected trial if the monkey failed to maintain eye position within the specified window. Different window sizes were used at different times during different types of target motions (see following text).

Three behavioral paradigms were used: fixation task, saccade task, and pursuit task. In all tasks, only a single target was presented at any given time. Target motion could be in any of eight directions including the four cardinal directions and the four 45° oblique directions. Trials that presented target motion in different directions and those with or without electrical stimulation were interleaved randomly within a block.

FIXATION TASK. A stationary target appeared at the center of the screen for 2,500-3,000 ms. In some trials, the target remained stationary throughout the trial. In others, the stationary target underwent a brief perturbation 1,000-1,500 ms after its appearance. The perturbation velocity consisted of a single cycle of a 10-Hz sine wave with peak-to-peak velocity of 20°/s. This yielded a maximum position excursion of 0.32°. The monkeys were required to maintain eye position within 1° of target position in fixation trials and normally did not make any saccades.

SACCADE TASK. A target appeared at the center of the screen for a random duration of 1,000-1,500 ms and then jumped 4-16° and remained stationary for an additional 1,500 ms. The monkey was required to fixate within 1° while the target was stationary. The fixation window was suspended for 300-800 ms at the time of the target step, and the monkey was required to fixate within 4° when the requirements were reinstated.

PURSUIT TASK. A target appeared and remained stationary for a random duration of 1,000-1,500 ms. The target then executed a "step-ramp" of position (Rashbass 1961) consisting of 1-6° steps and ramps at 5-30°/s. Except for the experiments summarized in Figs. 13 and 14, the direction of target motion was opposite to the direction of target step, and the size of the step was adjusted so that the target crossed the position of fixation 200 ms after the onset of motion. As a consequence, monkeys initiated pursuit without catch-up saccades in most trials. When we examined the effects of electrical stimulation during the maintenance of pursuit, the fixation target appeared at an eccentric location on the screen before executing step-ramp motion. The initial position was adjusted so that the target would cross the center of the screen 600 ms after the onset of step-ramp motion. When we examined the effects of electrical stimulation on the initiation of pursuit or searched for the pursuit-related regions, the fixation target appeared at the center of the screen before executing step-ramp motion.

Physiological procedures

Tungsten microelectrodes (Frederick Haer and Company) were lowered through the intact dura with a manual hydraulic micromanipulator (Narishige MO-95) while the monkeys performed either the pursuit task or the saccade task. We searched for pursuit-related regions in the periarcuate cortex by recording unit activity and by testing the effects of electrical microstimulation on eye movements every ~500 µm along each penetration. A train of cathodal pulses (width, 0.2 ms) was delivered through the electrode as electrical stimulation. The current intensity was monitored by measuring the voltage across a serially connected 1-k resister and was maintained at 50 µA. When we examined the electrically elicited smooth eye movements or searched for the sites likely to be related to pursuit, the train of stimulation pulses was 75 or 375 ms in duration at 333 Hz. When we examined the effects of stimulation on the responses to brief perturbations of the target, the train of stimulation pulses was either 100 or 200 ms in duration at 100 or 200 Hz. All stimulation experiments were performed at sites believed to be located in the gray matter on the basis of the neuronal activity recorded at the site.

Data acquisition and analysis

Horizontal and vertical eye-velocity signals were obtained by passing the eye-position voltages through analog differentiators (6 dB/octave above 25 Hz). Data were digitized, sampled each channel at 1 kHz, and stored on disk for later analysis on a UNIX workstation. We initially reviewed the eye-position and -velocity traces for each trial on a video monitor, detected saccades visually, and marked them with a mouse-controlled cursor. Portions of the eye-velocity traces during saccades were not used for the subsequent analyses, which were done with Matlab (The MathWorks). Most analyses consisted of aligning the eye movement responses to identical stimuli on the onset of target motion and computing averages of horizontal and vertical eye velocity as a function of time. Points from individual eye-velocity traces were included in the average only if they did not occur during a saccade, so that there was a different number of samples in the average computed for each time point. Time points were drawn in our figures and used for further analysis only if the average had been constructed from at least five trials. In some cases, further analysis consisted of measuring the magnitude of the response from the average eye-velocity trace. In other instances, measurements were made directly from individual traces. Details are provided at the relevant places in RESULTS.

We assessed the effect of electrical stimulation by comparing the eye velocities from interleaved trials that had delivered identical target motions with and without electrical stimulation. First, we computed the "difference eye velocity," defined as the millisecond-by-millisecond difference between the average eye velocities obtained from trials that provided identical target motions with and without electrical stimulation. This yielded estimates of the horizontal and vertical eye velocity evoked by the electrical stimulation, which we define as _h[t] and _v[t]. Then, for a 100-ms interval after the onset of electrical stimulation, we computed the electrically evoked eye speed as . The maximum electrically evoked eye speed over the analysis interval was taken as an estimate of the response size. Finally, the direction of the electrically elicited eye movements was computed as tan¹(_v[t]/_h[t]) at the time of the maximum eye speed and expressed as a polar angle: rightward, upward, leftward and downward movements corresponded to 0, 90, 180, and 90°, respectively.

The latency of the electrically-elicited smooth eye movements was measured by applying a technique modified from Carl and Gellman (1987). We obtained baseline eye speed by calculating eye speed in individual trials and obtaining the mean and SD of eye speed in the stimulation trials for the 100-ms interval before the stimulation onset. Then a regression line was fitted to the average eye speed for the period from 5 ms before to 10 ms after the average eye speed exceeded 2 SDs of the mean of the baseline. The point where the regression line crossed the baseline mean was taken as the movement onset. The latency of saccades was measured from individual eye-position traces by determining the time when eye speed exceeded 100°/s. For this analysis only, horizontal and vertical eye velocity were estimated by computing regression slopes of every seven eye-position samples.

We have not yet obtained histological verification of stimulation sites because both animals are currently in use for other projects.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Selection of stimulation sites

We have analyzed the smooth eye movement responses elicited from 73 stimulation sites in two monkeys. Sites were identified by the presence of pursuit-related neurons, the positive effect of electrical stimulation, or both as illustrated in Fig. 1. The top panels show the activity of a pursuit-related neuron that discharged vigorously during rightward (ipsiversive) pursuit at 20°/s (Fig. 1A). It began to discharge ~90 ms after the onset of rightward target motion and showed strong sustained activity for the duration of rightward pursuit (Fig. 1A); its slight background discharge was suppressed during leftward pursuit (Fig. 1B). At the same site, a train of stimulation pulses (50 µA, 375 ms at 333 Hz) in the dark consistently elicited rightward smooth eye movements without eliciting saccades (Fig. 1C). The direction of the stimulation-evoked smooth eye movements was just upward from pure rightward (+2.8°). A shorter train of pulses (75 ms) with the same parameters evoked a brisk eye movement response when presented during sustained pursuit of rightward step-ramp target motion at 20°/s (Fig. 1D).

View larger version (42K): [in this window] [in a new window]   Fig. 1. Criteria for selection of stimulation sites. A and B: activity of a pursuit-related neuron with ipsiversive (rightward) directional preference during tracking of step-ramp target motion to the right (A) or left (B) at 20°/s. From top to bottom, traces are superimposed trials of horizontal eye position and horizontal eye velocity and superimposed rasters and spike density, aligned on the onset of target motion. Rapid deflections associated with small corrective saccades were removed from eye-velocity traces during the maintenance of pursuit. C and D: superimposed trials of the smooth eye movements evoked by electrical stimulation (50 µA, 333 Hz) in the dark (C) and during ongoing pursuit (D). The bold horizontal lines labeled "stim" show the duration of microstimulation.

In general, we found a coincidence of pursuit-related activity and electrically evoked smooth eye movements. In agreement with previous studies (Gottlieb et al. 1993, 1994), the smooth eye movement sites were located posterior to the sites where contraversive saccades were elicited by stimulation (Fig. 16).

Latency of smooth eye movements evoked by electrical stimulation

Figure 2 shows the distributions of the latency of the electrically evoked smooth eye movements when the monkeys fixated a central stationary target (A) or tracked a target moving at 20°/s (B). Latency averaged 25.6 ± 6.1 ms during fixation and 21.9 ± 3.3 ms during pursuit. Although the means were similar, a paired t-test revealed significant differences in latency between fixation and pursuit (P < 0.0001, df = 72). At individual sites, the latency of the evoked smooth eye movement during fixation minus that during pursuit ranged from 12.9 to 18.9 ms (mean, 3.7 ± 5.5 ms; median, 2.3 ms). For data obtained during pursuit, electrical stimulation was delivered during each of the eight pursuit directions (4 cardinal and 4 oblique directions), and latency was estimated from the direction that produced the largest electrically evoked eye speed. In 70 of 73 sites (96%), the smooth eye movements evoked during pursuit were largest during pursuit in the direction same as the eye movements evoked from that site during fixation. For one site, the elicited eye movements were too small to use the automated procedure to measure latency, so the latency was estimated visually.

View larger version (13K): [in this window] [in a new window]   Fig. 2. Distribution of latency of smooth eye movements elicited by electrical stimulation of the frontal pursuit area (FPA) during fixation (A) and during the maintenance of pursuit at 20°/s (B). Each histogram shows data from 73 stimulation sites.

Enhancement of smooth eye movements evoked by microstimulation during ongoing pursuit

We reported previously that the magnitude of the electrically elicited smooth eye movements was larger if stimulation was delivered during pursuit than during fixation (Tanaka and Lisberger 2001). We have now stimulated at more sites than were included in our earlier report and analyzed the data more extensively. The means of the peak electrically evoked eye speed were 7.1 ± 5.3°/s during fixation and 18.6 ± 8.1°/s during pursuit. These were statistically different (paired t-test, P < 0.0001, df = 72). The present section will provide new information about the enhancement of the response to electrical stimulation during ongoing pursuit.

EFFECTS OF PURSUIT SPEED AT THE TIME OF MICROSTIMULATION. The size of the electrically evoked smooth eye movement depended on the speed of pursuit at the time of stimulation. A typical effect is illustrated in Fig. 3A, which shows the time courses of the electrically evoked eye movements for stimulation during pursuit at different target velocities (solid traces) superimposed on averages of eye velocity for the equivalent time in the control trials without electrical stimulation (dashed traces). Stimulation of this site in the right FPA increased the speed of rightward pursuit and decreased the speed of leftward pursuit transiently. The magnitude of electrically evoked responses changed systematically as a function of ongoing eye velocity: larger responses for faster speeds of rightward target motion, indicated by positive values of target velocity. Since a previous study had shown that the direction and magnitude of elicited eye movements sometimes depended on eye position (Gottlieb et al. 1993), we contrived the target motions in this experiments so that microstimulation was delivered as the tracking target crossed the center of the screen: the target appeared 2, 4, 8, or 12° from the center of the screen and executed step-ramp motion with 1, 2, 4, or 6° steps away from this initial fixation position for target velocities of 5, 10, 20, or 30°/s, respectively.

View larger version (24K): [in this window] [in a new window]   Fig. 3. Effects of target velocity on the magnitude of electrically-elicited smooth eye movements. A: examples of the time courses of average horizontal eye velocity for stimulation with a 75-ms train of pulses at 333 Hz, 50 µA during tracking of different target velocities. Solid and dashed lines, data obtained with and without stimulation. Numbers at the beginning of traces indicate target velocity. The positive numbers indicate rightward target motion. Upward deflections of average traces indicate rightward eye movements. B: amplitudes of the responses to stimulation as a function of target velocity. The positive and negative values of target velocity indicate target motion in the direction of electrically elicited smooth eye movements (Same) or the opposite direction (Opposite). Each connected line indicates data from single stimulation site. The filled symbols connected by thick lines plot measurements made from the data in A.

View larger version (22K): [in this window] [in a new window]   Fig. 4. Absence of the effect of retinal events at the time of electrical stimulation on the magnitude of response. A, C, and E: responses to retinal velocity errors. B, D, and F: responses to retinal position errors. A and B: schematic drawings of experiments. A target moving at 20°/s changed velocity by 5°/s (A) or position by 3° (B) when it crossed the center of the screen. The thick bar labeled "stim" indicates the time of microstimulation. C and D: averages of angular eye velocity for stimulation (thick lines) or control (thin lines) trials. Traces are aligned on the onset of stimulation or equivalent time in the nonstimulation controls (triangles). Different line types indicate different directions of image velocity (C) or position (D) relative to ongoing target motion: dotted traces, control trials; solid traces, retinal stimulus in direction of target motion; long dashed traces: retinal stimulus in direction opposite to target motion. E and F: difference eye velocity, documenting the time courses of the difference in eye speed between the stimulation trial and nonstimulation control.

EFFECTS OF PURSUIT DIRECTION AT THE TIME OF MICROSTIMULATION. We applied electrical stimulation (75 ms, 333 Hz) in the FPA during fixation and during pursuit at a fixed speed of 20°/s in eight different directions, yielding traces like those shown around the perimeter of Fig. 5A, where the position of the traces indicates the direction of pursuit at the time of stimulation. To quantify the responses to microstimulation of the FPA during fixation and pursuit, we determined the time of the largest peak in the electrically evoked eye speed across all directions of target motion and measured the eye velocity in each trace at that time. In practice, the time of the peak response amplitude was nearly the same for all directions of pursuit, so this analysis yielded the effectively same results as would one based on measuring the peak response. In the polar plot at the center of Fig. 5A, we have plotted these estimates of eye velocity for both control trials and stimulation trials. Each vector starting at a control value (open symbols) and ending at a stimulation value (filled symbols) represents the evoked eye velocity for stimulation delivered during each direction of pursuit. The pair of connected symbols with the control value at the origin shows the responses during fixation with and without electrical stimulation. Inspection of Fig. 5A shows that the direction of the electrically evoked movement was largely independent of whether the monkey was fixating or pursuing at the time of stimulation and of the direction of pursuit. In contrast, for this site, the amplitude of the evoked eye velocity depended on the conditions at the time of stimulation. The response was larger during pursuit than during fixation, and was largest for stimulation during rightward (ipsiversive) pursuit.

View larger version (16K): [in this window] [in a new window]   Fig. 5. Effects of electrical stimulation of the FPA during pursuit in different directions. A: pairs of traces around the polar plot show horizontal eye velocity (H-vel) or vertical eye velocity (V-vel) during the maintenance of pursuit, positioned to coincide with the direction of pursuit. In each pair of traces, the solid and dashed traces show data for the stimulation trial and the nonstimulation control, respectively. The traces start 500 ms after the onset of target motion. For the stimulation trials, the train of stimulation pulses (duration, 75 ms) was delivered 600 ms after the onset of target motion. The polar plot at the center of A shows eye velocity 90 ms after stimulation onset (filled symbols) and at the equivalent time for the controls (open symbols). Data for the same target motion are connected by lines. B-D: examples of the stimulation-evoked responses for 3 other sites. Each set of 8 data points for the stimulation trials (filled symbols) or controls (open symbols) is connected by lines. The thick line starting at the origin of each polar plot is a vector that indicates the direction and amplitude of eye velocity elicited by stimulation during fixation.

View larger version (12K): [in this window] [in a new window]   Fig. 6. Patterns of eye velocity predicted from 3 different computations that might account for the interaction of ongoing pursuit and electrical stimulation. The dashed lines in each polar plot indicate control eye velocity during pursuit in 8 directions, and the solid lines connect eye velocity predicted in stimulation trials. The arrows starting from the origin of each polar plot illustrate the eye velocity evoked by electrical stimulation during fixation. Predictions for the interaction of ongoing pursuit and electrical stimulation are: vector average (A), vector summation (B), and vector summation with omni-directional enhancement of pursuit eye velocity (C).

View larger version (22K): [in this window] [in a new window]   Fig. 7. Quantitative analysis of the interaction between ongoing pursuit and the eye movements evoked by electrical stimulation for 44 sites. Data were fitted by the equation, S = r*C + dp, where r represents the ratio of pursuit gain and dp represents the directional component injected during pursuit. A: distribution of the gain term (r). B: polar plot where each line is a vector showing the direction and amplitude of the eye velocity caused by stimulation during pursuit (dp). C: relationship between the size of the eye movement evoked by stimulation during pursuit (dp) and the gain enhancement of ongoing pursuit (r). The correlation coefficient was 0.41. D: amplitude of the eye velocity caused by stimulation during the maintenance of pursuit (dp) compared with that during fixation (df). The regression line has a slope of 1.58, a y intercept of 1.84, and the correlation coefficient was 0.90. In C and D, each point shows data from a single stimulation site; filled symbols indicate measurements from the data shown in Fig. 5.

Further evidence that the directional signals and gain control are independent effects

The data in Figs. 5 and 7 are consistent with the idea that there are two different effects of microstimulation in the FPA. One is the injection of a signal that drives pursuit in a fixed direction and that is subject to ~1.58-fold enhancement during pursuit at 20°/s versus during fixation. The other is an enhancement in the speed of ongoing pursuit for all directions. These two effects would cause different directions of eye movements if stimulation was delivered during pursuit opposite to the direction of evoked eye movements during fixation. If they had different time courses, then they might yield a bidirectional modulation of eye velocity around the control data. This prediction was borne out at 12 of 73 stimulation sites. For example, Fig. 8 shows data for a site at which microstimulation during fixation elicited smooth eye movements to the right and upward (+41.4°). Stimulation during pursuit to the right and up (same as the eye movement evoked from fixation) increased the speed of ongoing pursuit transiently (Fig. 8A). The evoked eye velocity had a latency of ~25 ms, and its time course was monophasic. In contrast, the time course of the eye velocity evoked during pursuit in the opposite direction was biphasic (Fig. 8B). Initially, stimulation increased the speed of ongoing left and downward pursuit with a latency of ~20 ms. Later with a latency of 40 ms from the onset of stimulation the direction of eye acceleration reversed, and 80 ms after the onset of stimulation the speed of left and downward pursuit became less than control. The initial component is in the direction expected from an omni-directional increase in the speed of ongoing pursuit, while the later component is in the same direction as the eye movement evoked by microstimulation during fixation. The reversal seen in Fig. 8B and in 11 other sites would be expected if the latency of the directional component was longer than that of the gain enhancement, and the two components were comparable in size. At 49 other sites, we found more subtle evidence for oppositely directed directional component and gain enhancement: stimulation during pursuit in the opposite direction from the elicited eye movements evoked no change in eye velocity (n = 7) or a smaller response (n = 42) than did stimulation during pursuit in the direction of the directional signals. This would occur if the directional component was equal to or larger than the eye movement caused by the effects of stimulation on gain control, and these two components had similar latencies. We did not find any sign of conflict of the two components in the remaining 12 sites where stimulation decreased (n = 3) or increased (n = 5) the speed of pursuit for both directions, or injected fixed eye-velocity signals irrespective of ongoing pursuit directions (n = 4).

View larger version (20K): [in this window] [in a new window]   Fig. 8. Evidence for the presence of 2 components with different time courses resulting from FPA stimulation. Averages of eye velocity () are aligned on the onset of stimulation applied during the maintenance of pursuit. · · · , the mean plus 1 SD. - - -, average eye velocity in the nonstimulation control trials. Stimulation of this site during fixation elicited smooth eye movements in right-up direction. The  at the left of each panel indicate the direction of pursuit during stimulation. A: right and up. B: left and down.  in A also indicates the direction of the eye movement evoked by stimulation at this site during fixation. Stimulation was applied for 75 ms starting 600 ms after target motion onset. Target speed was 20°/s.

Enhancement of visual-motor gain during fixation by electrical stimulation in the FPA

We now revisit a phenomenon published in an earlier brief communication (Tanaka and Lisberger 2001) to provide additional quantitative evaluation of the phenomenon and to demonstrate temporal contingencies in the interaction of eye movements evoked by visual motion and electrical stimulation of the FPA. In the earlier publication, we used the experimental design outlined in Fig. 9 to show that electrical stimulation in the FPA enhanced the smooth eye velocity evoked by a brief target perturbation during fixation. Although Fig. 9 is very similar to Fig. 2 of our prior publication (Tanaka and Lisberger 2001), it shows data from a different stimulation site. Briefly, a fixation target appeared at the center of the screen for 1,000-1,500 ms and then underwent a brief oscillation consisting of a single cycle of a 10-Hz sine wave (±10°/s) with initial motion to the right, left, up (Fig. 9A), down (Fig. 9B), or in one of the four 45° oblique directions. The responses to the perturbation of target velocity were small in the absence of electrical stimulation (Fig. 9, C and D) and larger if microstimulation was applied in the FPA (200 ms, 200 Hz) at the onset of target motion (continuous traces in Fig. 9, E and F). Electrical stimulation alone at the same time in the fixation trials without target perturbations elicited small eye movements (2.2°/s) in the up-right direction (63°; dashed traces in Fig. 9, E and F). To estimate the response to the perturbations of target motion in conjunction with microstimulation, we computed the difference: eye velocity evoked by perturbations during microstimulation minus that evoked by stimulation alone. To reveal enhancement, the difference eye velocity (thick continuous traces in Fig. 9, G and H) was compared with the responses to perturbation in the absence of electrical stimulation (thin dotted traces in Fig. 9, G and H).

View larger version (19K): [in this window] [in a new window]   Fig. 9. Example data from a stimulation site showing the enhancement of the responses to perturbation of a stationary target. A and B: the central fixation target underwent a perturbation that caused velocity to undergo a single cycle of a 10 Hz, ±10°/s sine wave with the initial component upward (A) or downward (B). C and D: vertical eye velocity evoked by the perturbation without electrical stimulation. E and F: responses to the target perturbation in the presence of electrical stimulation. In E and F, the broken line shows the average of the responses to electrical stimulation during fixation of a stationary target. In C-F, the gray traces show the responses in all individual trials, and the black bold traces show the average eye velocity. G and H: comparison of the average vertical eye velocity to the perturbation in the presence (bold solid traces) and absence (fine dotted traces) of electrical stimulation. The bold horizontal lines labeled "stim" below G and H show the time of stimulation. I: polar plot showing the size and direction of responses to target motion in 8 different directions. The filled symbols indicate responses with electrical stimulation, whereas the open symbols indicate those without stimulation. Data were obtained from a different site than that shown in a similar figure in Tanaka and Lisberger (2001).

Microstimulation in the FPA caused enhancement of the response in the direction of the perturbations. In Fig. 9, G and H, for example, the enhanced response to the perturbation is in the direction of the original perturbation (Fig. 9, A and B) rather than in the direction of the smooth eye velocity evoked by microstimulation. To quantify the directionality of the enhancement, we measured the response to the initial half cycle of target perturbation by measuring the peak average eye velocity in the interval from 100 to 150 ms after the onset of the perturbation in each of eight directions. Figure 9I contains a polar plot of the peak responses for all eight directions of target perturbation. The responses to the perturbations were three times as large during microstimulation trials (filled symbols) as during the control trials (open symbols), while the directions of the responses were changed by an average of only 9.0 ± 5.5° (n = 8) at this site.

The enhancement of the responses to target perturbation was omni-directional for this and most of our stimulation sites. The directional preference of the enhancement for each given site was evaluated statistically in the previous report (Tanaka and Lisberger 2001). Briefly, the direction of the change in eye movements caused by stimulation was distributed uniformly for 30 of 33 sites (Rayleigh's test, P > 0.05). Furthermore, Mardia's circular-linear analysis showed that the direction and amplitude of the enhancement were correlated each other for only 6 of 33 stimulation sites (P < 0.05). In the present paper, we quantify the directionality of the change in perturbation responses in a different way using the same set of stimulation data published previously. Because a previous study in our laboratory has shown that the enhanced responses to a brief perturbation during the performance of pursuit is largest if the perturbation is along the axis of ongoing pursuit direction (Schwartz and Lisberger 1994), we also asked whether the enhancement of the responses to a brief perturbation during stimulation of the FPA is stronger along a certain axis.

For each stimulation site and direction of target perturbation, we obtained the enhanced eye velocity by subtracting the responses in control trials from those in stimulation trials. Figure 10A plots data from another stimulation site in which the responses to perturbation were enhanced for all motion directions with a strong directional preference. We then fitted an ellipse to the set of eight data points. We computed the axial bias index (I_ax) as
where a and b were the lengths of the major and the minor axes of the fitted ellipse, respectively. The directional bias (I_dr) was computed in a similar way as
where c and d were the lengths of vectors from the origin to the ends of the major axis of the fitted ellipse (c > d,  in Fig. 10A). We performed this analysis for 25 of 33 stimulation sites in which stimulation changed the gain of the responses to target perturbation by 0.1. For smaller changes in gain, the data were sufficiently noisy that it was difficult to have confidence in the parameters of the best-fit ellipses.

View larger version (15K): [in this window] [in a new window]   Fig. 10. Quantitative analyses showing the lack of selectivity of enhancement for the direction or axis of target perturbation during fixation. A: symbols plot the difference in responses to target motion between stimulation trials and controls for 8 directions of perturbation at a single stimulation site. , the best-fitting ellipse. , vectors from the origin to the 2 ends of the major axis of the ellipse. B: distribution of the directional bias defined as 1- d/c. C: distribution of the axial bias defined by 1- b/a, where a and b are the lengths of major and minor axes of the ellipse, respectively. The ordinates of B and C indicate number of stimulation sites.

Figure 10, B and C, shows distributions of these indexes. For both indexes, the value closer to unity indicates stronger preferences to a certain direction (I_dr) or axis (I_ax) of target motion. In general, both indices were <0.5 at the overwhelming majority of sites, indicating that the enhancement was not strongly specific to a certain direction or axis. The site summarized in Fig. 10A had one of the strongest biases in our sample: the values of I_dr and I_ax were 0.51 and 0.13, respectively. The site summarized in Fig. 9I had much less directional bias: the values of I_dr and I_ax were 0.05 and 0.17, respectively. The means for all 25 sites we analyzed were 0.29 ± 0.21 (I_dr) and 0.17 ± 0.09 (I_ax).

Comparison of effects of FPA stimulation on eye velocity during pursuit and on responses to perturbations during fixation

We have now documented three effects of microstimulation in the FPA on different components of pursuit: injection of a directional signal that is enhanced 1.58-fold during ongoing pursuit at 20°/s; enhancement of the gain of steady-state eye velocity in a nondirectional way; and enhancement of the smooth eye velocity evoked by a brief perturbation of target velocity during fixation. We documented the latter two effects in separate blocks at 33 stimulation sites, allowing direct comparison of the size of the two different forms of gain enhancement.

For each of these 33 sites, Fig. 11 plots the effect of microstimulation in the FPA on the gain of the responses to perturbations during fixation as a function of the effect on eye velocity during the maintenance of pursuit. For the responses to target perturbation during fixation, we measured the gain by using the same method as the previous study (Tanaka and Lisberger 2001). For both the control responses and the responses during stimulation, the gain was computed as , where A_E is the area within the polygon defined by each set of eight data points for different target directions (e.g., Fig. 9I) and A_T is the area within the polygon defined by the peaks of target velocity (always 282.84). We then estimated the effect of microstimulation as the gain during stimulation minus that during fixation without stimulation. For stimulation during the maintenance of pursuit, we used the same approach. For the control and stimulation responses, the gain was computed as , where A_E is the area within the polygon defined by each set of eight data points for different target directions (e.g., Fig. 5, B-D) and A_T is the area within the polygon defined by target velocity (always 1131.4). We then estimated the effect of microstimulation as the gain during stimulation minus that during pursuit without stimulation. The gain changes were smaller for the responses to perturbation than those observed during pursuit in most sites, and the correlation coefficient was 0.55 (n = 33, P < 0.001). The analysis in Fig. 11 uses a difference between the gains with and without stimulation. A similar analysis using the ratio of the gains with and without stimulation yielded the same basic result. The gain ratio for perturbations was now much larger than that for pursuit maintenance because the gains of the responses to perturbations in the absence of stimulation were very small, but the correlation coefficient was about the same (r = 0.48, P < 0.01).

View larger version (17K): [in this window] [in a new window]   Fig. 11. Relationship between the amount of stimulation-induced enhancement of eye velocity during pursuit (x axis) and during stimulation-induced enhancement of the response to a brief perturbation of target motion during fixation (y axis).  and , data from different monkeys. - - -, unity slope. Correlation coefficient was 0.55.

Time window for enhancement of visual-motor gain during fixation by electrical stimulation in the FPA

In the experiments reported so far, we used a 200-ms train of stimulation pulses to ensure the conjunction of the electrical stimulation and the 100-ms target perturbation. To determine the most effective time for stimulation, we next used a 100-ms train of stimulation pulses and varied the time between the onset of microstimulation and the 100-ms duration perturbations of target motion. Figure 12A illustrates the experimental design. As before, a single cycle of a 10 Hz, ±10°/s sine wave perturbed a central fixation target 1,000-1,500 ms after it appeared. Perturbations had a phase of 0° (continuous trace) or 180° (dashed trace) and a 100-ms train of 21 pulses was applied in the FPA before, after, or at the same time as the target motion onset. Figure 12B shows an example of the average eye velocities evoked by the two perturbations when the stimulation was applied at the onset of target motion, where the solid traces show the average responses for perturbations with a phase of 0 or 180° and the dotted traces indicate the SDs. The responses to the two perturbations diverged during the interval from 85 to 165 ms after the onset of the perturbation. To assess the response to the perturbations and reject the smooth eye velocity caused by the microstimulation, we computed the difference: eye velocity evoked by the 0° phase perturbation minus that evoked by the 180° phase perturbation.

View larger version (26K): [in this window] [in a new window]   Fig. 12. Effect of timing of stimulation in the FPA on the enhancement of the responses to target perturbations. A: schematic diagram showing the timing of the stimuli. Traces labeled "targ" show the 2 polarities of perturbation velocity along the horizontal meridian. Rectangles labeled "stim" show the timing of 100-ms-duration trains of 200-Hz stimulation pulses at different delays. The numbers to the right of each stimulus marker indicate the time of electrical stimulation relative to the onset of target perturbation in milliseconds. B: example of responses for synchronous perturbations and electrical stimulation. The solid traces show averages of horizontal eye velocity for 2 different polarities of perturbation aligned on the onset of target motion (triangle). The dotted traces indicate 1 SD. The 2 vertical dashed lines indicate the interval used to measure the magnitudes of responses. C: time courses of the responses to perturbation for different times between the onset of microstimulation and the perturbation. The traces are the differences of average eye velocity for the 2 perturbation polarities. Two vertical dashed lines indicate the 50-ms interval during which peaks of the responses were measured. The numbers to the left of the traces indicate the time between the onset of microstimulation and the perturbation. Trace labeled "NoStim" shows the response in the nonstimulation control. D: peak response amplitude as a function of stimulation timing. Each set of connected data points indicates single experiment. Data points connected by bold lines were obtained from the experiment shown in B and C.

The responses to the perturbation were largest when microstimulation was delivered either at the same time as or shortly after the onset of the perturbation. Figure 12C shows the difference eye velocity as a function of the time since the onset of the perturbation for different intervals between stimulation and target perturbation (numbers at the beginning of the traces). When the electrical stimulation was applied before the onset of target motion (negative values of interval between stimulation and target perturbation), responses to the perturbation were small and comparable to those in trials that presented perturbations in the absence of stimulation (trace labeled NoStim). When the stimulation was applied 0, 48, or 100 ms after the onset of target motion, however, the eye movement responses to the target perturbation were clearly enhanced.

To summarize the effect of the timing of electrical stimulation, we measured the peak of the difference eye velocity in the interval from 100 to 150 ms after target motion onset (delimited by vertical dashed lines in Fig. 12C). The effect of timing was similar in all seven sites we studied. As shown in Fig. 12D, the responses to the perturbations in this interval were maximal when the electrical stimulation was applied at the same time as the onset of target motion (0 on the x axis in Fig. 12D). The enhancement decreased as the electrical stimulation was applied later (positive values on the x axis in Fig. 12D) and was nearly absent when electrical stimulation was applied before the target motion (negative values on the x axis in Fig. 12D).

Superposition of the enhanced response and the control response revealed that the enhanced responses in Fig. 12C started 85, 100, and 130 ms after the target motion onset or 85, 52, and 30 ms after stimulation onset for trials with intervals of 0, 48, and 100 ms, respectively. The enhanced responses lasted 80, 120, and 120 ms, meaning that they terminated 65, 72, and 150 ms after the end of the perturbation for trials with intervals of 0, 48, and 100 ms, respectively. The time courses of the responses were similar at all seven sites we studied. It was curious that electrical stimulation would cause an enhancement that outlasted the normal response. With this paradox in mind, we attempted to fit the time course of the responses with a model that placed filters both upstream and downstream from the site of gain control. We were not successful.

Enhancement of the initiation of pursuit by electrical stimulation in the FPA

We have shown thus far that stimulation of the FPA enhances the visual-motor processing for pursuit, at least for the slightly contrived situation where we have probed the state of the pursuit system by presenting a brief perturbation of a stationary target. We now use the more-natural situation that is commonly used to analyze the gain of the pursuit system: the initiation of pursuit for a step-ramp of target position, providing a step of target velocity starting at different locations in the visual field and moving in different directions with respect to the position of fixation.

In the trials illustrated in Fig. 13, a target appeared 4° to the right (A and D) or left (B and C) and moved horizontally at 20°/s either toward (C and D) or away from (A and B) the position of fixation. For each site, we customized the trials used to study the initiation of pursuit so that target motion was along the axis of the smooth eye movements elicited by a train of high-frequency stimulation pulses (333 Hz), either in or opposite the direction of the evoked eye movements. As before, a 200-ms duration train of stimulation pulses, of either 100 Hz (11 sites) or 200 Hz (17 sites), was delivered for 200 ms at the onset of target motion (horizontal bars below the traces) in half of the randomly interleaved trials. The stimulus configuration and the general sequence of pursuit and saccades can be seen best in the position traces in the top two rows of Fig. 13. However, the smooth eye movements themselves can be appreciated better in the average eye-velocity traces shown in the bottom two rows.

View larger version (21K): [in this window] [in a new window]   Fig. 13. Effects of electrical stimulation on the initiation of pursuit. Top 2 rows: horizontal eye and target position aligned on the target motion onset for trials with (Stim) or without (Control) electrical stimulation. Third row: average eye velocity in 3 different task conditions. The dashed traces show responses elicited by stimulation without target motion and are the same for A through D. The fine, continuous traces show responses to target motion in the absence of stimulation. The bold traces show responses to target motion during microstimulation. Interruptions in traces indicate intervals when catch-up saccades were so frequent that data from fewer than 5 trials were available for averaging. Bottom row: comparison of the responses to target motion with or without stimulation. The bold trace shows the time course of the difference between eye velocity for target motion during stimulation and that during stimulation alone. The bold horizontal bars below each column indicate the times of stimulation. A: the target stepped and ramped rightward. B: the target stepped and ramped leftward. C: the target stepped leftward and ramped rightward. D: the target rightward and ramped leftward. Step amplitude was always 4° and ramp speed was always 20°/s.

Consider first the configuration in Fig. 13A, where the target stepped in the direction of the smooth eye movement evoked by electrical stimulation during fixation and then ramped away from the position of fixation. The third row of traces in Fig. 13 shows the eye velocities elicited in the presence and absence of electrical stimulation: electrical stimulation itself caused little smooth eye movement (dotted trace), while strongly enhancing the eye velocity at the initiation of pursuit (bold trace) relative to the eye velocity at the initiation of pursuit in the absence of stimulation (fine continuous trace). To view the eye velocity evoked by target motion without contamination from the direct effects of stimulation, we again computed the "difference eye velocity," defined as the point-by-point difference of the eye velocity caused by target motion in the presence of microstimulation and that evoked by stimulation alone (bold traces in 4th row of Fig. 13). Inspection of the four columns of Fig. 13 reveals that microstimulation in the FPA enhanced the initiation of pursuit for targets that moved either rightward (A) or leftward (B) away from the position of fixation. When the target moved toward the position of fixation, however, the FPA stimulation enhanced pursuit initiation only for target motion in the direction of the stimulation-evoked smooth eye movements (C).

We measured horizontal and vertical eye velocity 180 ms after target motion onset for the four combinations of target motion, and subtracted the mean of the responses to electrical stimulation alone. We then computed the component of the enhanced eye velocity that was along the axis of eye velocity during the initiation of pursuit without stimulation. The results for each combination of target step and ramp are summarized in Fig. 14, which plots component eye speed in stimulation trials versus that in nonstimulation controls. For target motion in the direction of the electrically evoked smooth eye movement, microstimulation enhanced the initiation of pursuit for target motion toward and away from the position of fixation (Fig. 14, A and C), and the effect was statistically significant in almost all sites (filled symbols, unpaired t-test, P&