Psychophysical and rTMS Evidence for the Presence of Motion Opponency in Human V5

Abstract

Background: Motion sensitive cells within macaque V5, but not V1, exhibit motion opponency whereby their firing is suppressed by motion in their anti-preferred direction. fMRI studies indicate the presence of motion opponent mechanisms in human V5.

Objective/hypothesis: We tested two hypotheses. 1) Performance of a motion discrimination task would be poorer when stimuli were constructed from pairs of dots that moved in counter-phase vs. in-phase, because counter-phase dots would activate motion opponent mechanisms in V5. 2) Offline 1 Hz rTMS of V5 would impair discrimination performance for in-phase stimuli but not counter-phase stimuli, and the opposite effect would be found for rTMS of V1.

Methods: Stimuli were constructed from 100 dot pairs. Paired dots moved along a fixed motion axis either in counter-phase (motion opponent stimulus) or in-phase (non-opponent motion stimulus). Motion axis orientation discrimination thresholds were measured for each stimulus. Blocks of 300 trials were then presented at 85% correct threshold and discrimination accuracy was measured before and after 1 Hz offline rTMS of either V1 or V5. Subjects were 8 healthy adults.

Results: Discrimination thresholds were significantly larger (worse) for counter-phase than in-phase stimuli (p = 0.02). V5 rTMS mildly impaired discrimination accuracy for the in-phase dot stimuli (p = 0.02) but not the counter-phase dot stimuli. The opposite effect occurred for V1 rTMS (p = 0.05).

Conclusions: Opponent motion mechanisms are present within human V5 and activation of these mechanisms impairs motion discrimination. In addition, perception of the motion axis within opponent motion stimuli involves processing within V1.

Keywords: MT; Middle temporal area; Motion perception; Primary visual cortex; Repetitive transcranial magnetic stimulation; Visual cortex.

Figure 1

Schematic representations of the psychophysical stimuli. Left: Counter-phase dot twin pairs with a motion axis orientation counter-clockwise from vertical. The grey arrow represents a vertical motion axis orientation and the black arrows indicate the motion direction of each dot. Right: Schematic examples of in-phase and counter-phase twin pairs (two twin pairs per panel). Black arrows indicate the motion direction of each dot.

Figure 2

Psychometric functions for each participant for both the in-phase (closed circles) and counter-phase (open circles) dot stimuli. The majority of participants exhibited greater accuracy for the in-phase stimuli.

Figure 3

Left panel: Average psychometric functions for in-phase dots (filled symbols) and counterphase dots (open symbols). Accuracy was poorer for counter-phase dots. Error bars show ± 1 SEM. Note that error bars represent between subjects error whereas statistical significance represents within-subjects differences. Right panel: Individual motion axis discrimination thresholds corresponding to 85% correct accuracy for in-phase and counter-phase dots. Data points above the dashed unity line indicate larger (worse) thresholds for counter-phase dots than in-phase dots. The open symbols indicate thresholds that were extrapolated from the psychometric function fit.

Figure 4

The effect of rTMS over V1 and V5 on task performance. The top panel shows mean percent change from baseline for each condition. Open bars show data for the counter-phase dot stimulus and filled bars for the in-phase dot stimulus. Bars on the left are for V1 stimulation and bars on the right for V5 stimulation. Errors bars show ± 1 SEM and asterisks indicate a significant change from baseline. The lower panel shows raw percent correct scores of each participant for each condition. Each colour denotes a different participant. The dashed lines indicate overlapping data.