All experimental procedures were approved in advance by the Institutional Animal Care and Use Committee of the University of California, San Francisco and conformed to federal guidelines. Two male rhesus monkeys (Macaca mulatta
, 7−12 kg) served as subjects, and their eye movements were recorded with the magnetic search coil method. Surgical and training procedures to prepare monkeys for the study have been described previously43,44
. Both monkeys were well trained on pursuit tasks but naive to pursuit learning experiments. Experiments were conducted approximately five times per week and lasted 2−5 hours. All experiments were conducted in a nearly dark room.
Visual stimuli were presented on an analog oscilloscope (Hewlett Packard 1304A) with a refresh rate of 250 Hz. The display was positioned 30 cm from the monkey and subtended 41° horizontal × 34° vertical of visual angle. Target position and velocity were specified through the user interface on a DEC Alpha UNIX workstation and were controlled by the outputs of two 16-bit digital-to-analog converters on a digital signal processing board in a PC.
A small bright spot was presented as a pursuit target in individual trials. Each trial began with a fixation interval of a duration that was randomized between 500 and 1,000 ms. The target then underwent ‘step-ramp’ motion to minimize the occurrence of catch-up saccades. The target stepped away from the fixation point by 2.5° (Monkey Q) or 3.0° (Monkey E) and simultaneously began moving at 20° s−1 towards the fixation point. Monkeys were rewarded with a drop of fluid if they kept eye position close to target position throughout the trial (see below). They completed approximately 1,500 pursuit trials, plus several hundred receptive field-mapping trials, in each daily experiment.
In some experiments, we also presented a visual background that consisted of randomly placed dots at an average density of 0.09 dots/deg2. The dots were one-sixteenth as bright as the pursuit target. To avoid spatial overlap of the target and the background, dots were visible within two separate invisible apertures of 38° × 14°, one above and one below the horizontally-moving pursuit target. Thus, the pursuit target was centered vertically on a 6°-tall horizontal stripe in which background dots never appeared. The stripe between the two apertures was only slightly taller than the average separation between dots, thereby minimizing the interruption of the background pattern while preventing the background from introducing ambiguity as to the task requirements by overlapping the path traversed by the target. The placement of individual dots within the background pattern was varied randomly on a trial-by-trial basis. Motion of the visual background consisted of 100% coherent vertical motion of the dots at 20° s−1 within the stationary, virtual apertures.
In a subset of our experiments we stabilized targets with respect to the moving eye45
to eliminate image motion resulting from eye motion relative to the target on learning trials. The position of the eye was sampled every millisecond and fed back to the computer so that it could be used to drive target motion in real time. We used the monkeys’ eye movements in response to small position and velocity errors to verify the accuracy of the calibration of the eye coil45
and ensure accurate image stabilization. Comparison of the eye movements evoked by target motion with and without target stabilization showed neither the saccades nor the smooth eye accelerations that would have been predicted by systematic errors in stabilization. In a subset of our experiments we selectively stabilized the target only along the (horizontal or vertical) learning axis and not along the orthogonal pursuit axis. Our results did not depend on whether the target was stabilized along both or just one of the axes.
MT recording and microstimulation
We recorded in and stimulated MT in three hemispheres of two monkeys (left and right hemispheres of Monkey Q, left only in Monkey E). We used tungsten microelectrodes with impedances from 800 KΩ to 1.2 MΩ measured at 1 kHz (Frederick Haer). Area MT was identified as described previously44
based on stereotaxic coordinates, directional response properties of MT neurons, receptive field sizes, retinotopic organization, and activity recorded in surrounding cortical areas. We used a vertical approach to MT, so that we usually were able to identify the middle superior temporal area (MST) and the lumen of the superior temporal sulcus before the electrode entered MT. Once we understood the topographical organization of MT in each hemisphere, we attempted to find sites with receptive field locations in the central 10° of the visual field.
Once we entered MT, we searched for a site where the multiunit activity indicated that nearby neurons shared a common direction preference for the motion (usually at 16° s−1) of dots that were placed randomly within a 38° × 30° aperture in the visual hemifield contralateral to the recording site. Only strongly direction-selective sites with reasonably Gaussian direction tuning curves were selected for further experimentation. Once a potential stimulation site had been identified we characterized its direction and speed preferences for motion of patches of dots, as well as its spatial receptive field location and size, its responses to motion of pursuit targets and background textures, and the effects of microstimulation on eye movements during ongoing pursuit.
We delivered biphasic current pulses under the control of a Grass S88 stimulator. Pulses had intensities of 30−50 μA and durations of 0.2 ms, and occurred at a rate of 200 Hz in trains of 300-ms duration. In agreement with the previous finding46
that small changes in electrode position away from the center of a direction column can reduce the effectiveness of microstimulation in MT, we found that microstimulation at sites with weak or ambiguous direction tuning profiles usually failed to elicit smooth eye movements even during ongoing pursuit. However, stimulation nearly always evoked a directional eye movement at the relatively low currents we used, if performed at sites with strong multiunit responses to motion and clear direction tuning. In contrast to previous studies in which stimulation coincided with the onset of pursuit15,47
, the stimulation-evoked eye movements we observed during pursuit maintenance were consistently towards (and not away from) the preferred direction of the stimulation site. Because we required the motion signal injected through stimulation to be as directional as possible, we abandoned sites at which microstimulation applied during ongoing pursuit failed to produce a smooth eye movement. The presence of a stimulation-evoked eye movement also allowed us to monitor the continued effectiveness of the stimulation site during long experiments.
The learning experiments were similar in design to previous studies on pursuit learning6,7,27
and consisted of two blocks of trials: a baseline block and a learning block. Each block consisted of one or more trial types presented in varying ratios in pseudorandom order. The baseline block consisted of probe trials in which the target moved along the pursuit axis without any additional experimental manipulations. Before proceeding to a learning block of trials, we confirmed the consistency and stability of learning axis eye velocities on baseline probe trials. If the eye velocities were not stable across different fractions of the baseline block, we terminated the experiment for that day to avoid contamination of our data from fluctuations caused by poor pursuit performance. In up to 10% of the trials in the baseline block, microstimulation was applied in MT starting 200 or 250 ms after the onset of target motion. The learning block contained at least 80% learning trials and 20% or fewer probe trials. Learning trials always consisted of pursuit target motion identical to that of probe trials. In addition, either microstimulation in MT or motion of a visual background was delivered for 300 ms beginning at a fixed interval (100−250 ms, usually 200 ms) after the onset of target motion. In some experiments, the target was stabilized on learning trials during the 300 ms segment when microstimulation or visual background motion was delivered. Probe trials in the learning block were identical to probe trials in the baseline block.
Monkeys were generally required to keep their eyes within a 3° window around the pursuit target to complete a trial and receive a fluid reward. Our experimental manipulations caused deviations in eye position of less than 1° and therefore should not have affected the monkey's likelihood of successfully completing the trial. However, to be absolutely certain that microstimulation, background motion or learning did not affect reward probabilities, we opened the fixation window along the learning axis to 5° during the segment of the learning trials in which microstimulation was applied or the visual background moved and during the corresponding segments on probe trials. In an attempt to eliminate potential confusion between the target and the dots in the visual background experiment, we also conducted the background experiments with the fixation window in only the learning axis narrowed to as little as 1°. Learning was unaffected by these changes in fixation requirements; we obtained excellent task performance and clear, bidirectional learning with the fixation window either widened or narrowed along the learning axis.
Details of our data acquisition have been published before7,44
. Data from any given trial were used for analysis only if the trial had been completed successfully by the monkey. Saccades were marked by hand using an interactive computer program, and the portions of the eye velocity traces corresponding to saccades were treated as missing data. If a saccade occurred during the segment corresponding to microstimulation or visual background motion, then the entire trial was excluded from analysis. Due to the small size of the learned eye movements, we aligned responses to identical stimuli on the onset of target motion or the time of microstimulation or visual background motion and computed time averages of eye velocity. Responses to MT stimulation were isolated by subtracting the eye velocity along the learning axis on trials in which microstimulation was not delivered from that when stimulation occurred. Learning was assessed by computing the difference between averaged eye velocity in the learning axis for probe trials near the end of the learning block and those in the baseline block. No attempt was made to assess the amount of learning until ~100 learning trials had been completed.