Article Figures & Data
Figures
- Fig. 1.
The geometry of the display (A) and timing of events (B) in a 2-alternative, forced-choice direction-discrimination task. Three hundred milliseconds after the monkey foveated a fixation point, 2 saccade targets were illuminated. Five hundred to 900 ms later, a stochastic motion stimulus was shown at the center of gaze for 2 s and was followed by a delay period lasting from 1 to 1.5 s. After the delay period, the fixation point was extinguished, whereupon the monkey had 500 ms to shift its gaze to the target in the direction of stimulus motion. For each cell studied, 1 of the saccade targets (T1) was positioned inside the movement field and the other (T2) was positioned outside.
- Fig. 2.
Direction tuning measurement for visual responses of a single superior colliculus (SC) neuron. A: the motion stimulus (disk) subtended 7° of visual angle and was presented at the center of gaze. Electrical stimulation at the recording site (25 μA, 500 Hz for 130 ms) evoked 10° amplitude right-downward saccades (arrow).B: responses of an SC neuron at this stimulation site to 8 directions of visual motion. The positions of the spike rasters correspond to the 8 directions of motion. Vertical bars in the rasters delineate 2-s-long stimulus presentation intervals. The polar plot shows mean responses recorded during this interval. The small circle at the origin indicates baseline activity level.
- Fig. 3.
Scatterplot of preferred visual direction against the direction of electrically evoked saccades. Data are shown for 22 direction-selective neurons; *, the data point for the cell in Fig. 2. In this coordinate system 0° is rightward (experiments in the left SC), 90° is upward, and 180° is leftward (experiments in the right SC).
- Fig. 4.
Direction tuning of a single SC neuron for stimuli positioned at 5 locations in the visual field (gray disks). Direction tuning curves measured at each retinal location are displayed at corresponding locations in the figure. Dashed circles represent baseline responses and identify the origin of each tuning curve. Electrical stimulation at the site evoked stereotyped saccades (arrow). At the center of gaze, this cell preferred motion flowing toward the movement field; the preferred direction was roughly consistent at all locations tested.
- Fig. 5.
Receptive field boundaries estimated from responses to the motion stimulus for 5 direction-selective SC neurons. Hand-drawn curves encompass all tested locations at which direction tuning attained statistical significance (ANOVA: P < 0.05). Solid curves indicate boundaries between direction-tuned and non-direction-tuned locations. Dashed curves encompass eccentric direction-tuned locations and indicate that more eccentric locations were not tested. Data from 3 cells recorded from the right SC have been flipped about the vertical axis so that, for all neurons shown here, the right side of the figure is contralateral and left is ipsilateral.
- Fig. 6.
Direction tuning of a single SC neuron during delayed saccades.A: 300 ms after the monkey achieved visual fixation, a single saccade target appeared. Eight hundred milliseconds later the motion stimulus was presented for 2,000 ms. The monkey received a liquid reward for making a saccade to this target within 500 ms of fixation point offset. B: the geometry of the display was adjusted so that the target (black disk) appeared at a location remote from the end points of electrically evoked saccades (arrow).C: direction tuning curves measured during passive-fixation (dashed) and delayed-saccade (solid) tasks were very similar. The circle at the origin of the tuning curves indicates the baseline firing rate.
- Fig. 7.
Scatterplots of fitted parameters for direction tuning data measured during passive fixation (abscissa) and delayed-saccade (ordinate) trials. Corresponding symbols from Eq. 1 are as follows: μ, preferred direction; A, tuning curve amplitude;B, baseline; ς, tuning width.
- Fig. 8.
Direction-selective response that persisted through saccade initiation.A: we measured direction tuning immediately prior to saccades directed to a visual target outside of the movement field (arrow). B: all trials are aligned to the time of saccade initiation (vertical bar). This cell responded to motion in 1 direction [left spike raster, solid peristimulus time histogram (PSTH)] but not the other (right spike raster, dashed PSTH) until, and slightly after, saccade initiation. Bin width = 30 ms.
- Fig. 9.
A direction-selective response that ceased shortly before saccade initiation. Conventions as in Fig. 8 B.
- Fig. 10.
Population responses to 2 opposed directions of stimulus motion. Responses were averaged over the 15 neurons tested in the persistent motion task. All trials were aligned to saccade initiation. Motion flowing toward the movement field (thick line) elicited greater responses than motion in the opposite direction (thin line) at all time points up to and including the time of the saccade. Dashed lines indicate ±1 SE of the mean.
- Fig. 11.
Responses of a single SC neuron to interrupted and smooth motion stimuli. A: interrupted stimuli (left) alternated at 5 Hz between 3°/s motion in the preferred direction and stationary dots (0°/s). Smooth stimuli (right) moved constantly at 3°/s in the preferred direction.B: neural responses to interrupted (left) and smooth (right) motion stimuli. C: PSTHs of neural activity in B. D: histograms of the times of fixational saccades during interrupted and smooth motion. E: power spectra computed from the response PSTHs in C.
- Fig. 12.
Scatterplot of relative 5-Hz power in the neural responses to interrupted and smooth motion. Seven of 8 cells exhibited significantly more 5-Hz modulation in the response to interrupted motion (filled symbols, permutation test: P < 0.001). One cell did not show this effect (P > 0.4, open symbol). Asterisk indicates the cell whose responses are illustrated in Fig. 11. Note that the absolute percentages differ in Figs. 12 and 11 Ebecause “relative power” was used in Fig. 12 (see text).
- Fig. 13.
Histogram of direction tuning indices from 96 choice-predicting neurons. A total of 44 significantly direction-selective neurons are represented by filled bars (permutation test: P < 0.05). The remaining 52 cells were not significantly direction selective (open bars).
- Fig. 14.
Firing rate means and standard errors during the stimulus presentation, delay period, and perisaccadic interval for the population of direction-selective cells (open bars) and the population of non-direction-selective cells (filled bars).
- Fig. 15.
Average fixational saccade-triggered responses across the population of direction-selective cells (A) and non-direction-selective cells (B). The gray band indicates ±1 SE. Fixational saccades were identified from the time of target onset until fixation point offset. Neuronal responses were aligned on the initiation of fixational saccades and averaged across trials and then across cells. Fixational saccades caused a transient, biphasic modulation of firing rate that was substantially larger in amplitude in the non-direction-selective population. The initial negative-going phase (arrows) is too rapid to have been a visual response and may reflect intracollicular inhibition that is time locked to saccade execution. C: histogram of saccade-triggered response modulation indices for direction-selective cells (open bars) and non-direction-selective cells (filled bars). Triangles indicate median index values.
- Fig. 16.
Average response to target onset across the population of direction-selective cells (A) and non-direction-selective cells (B). Gray band indicates ±1 SE. C: histogram of target response indices for direction-selective cells (open bars) and non-direction-selective cells (filled bars). Triangles indicate median index values.
Tables
- Table 1.
Numbers of cells studied
Studied Quantitatively in Direction Discrimination Not Studied Quantitatively in Direction Discrimination Directional cells 44 (of 96) 23 Number of directional cells studied in particular control conditions Delayed saccades 8 14 Multiple stimulus locations 11 12 Persistent motion 6 9 Interrupted motion 3 4
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