Apparent motion speed dependence on contrast and orientation: Evidence from MEG

Abstract

The perceived speed of fast (>40°/s.) apparent motion of a Gabor patch whose orientation is aligned with the motion axis is much faster than when it is orthogonal to it. Electrophysiological recordings in cat and modeling (Lorenceau & al., 2002) suggest that this perceptual bias is explained by the phase advance in the firing of V1 neurons resulting from synaptic summation of horizontal and feed-forward inputs, whose responses are read-out by MT speed selective cells. Using the same paradigm in humans, we present additional psychophysical and MEG experiments showing that: 1. At high physical speeds, perceived speed is faster at low (20%) than at high (50%) Gabor contrast, a result at odd with previous reports of lower perceived speed at low contrasts (Thompson, 1982). 2. MEG data show that: i. the latencies of the first peak responses (∼90 ms.) are longer (by ∼20 ms.) at low as compared to high contrast. At these latencies, response amplitude is independent of Gabor orientation; ii. At longer latencies (∼160 ms.) and low stimulus contrast, the peak response amplitude is larger for a Gabor aligned to the motion axis than for a Gabor orthogonal to it, an effect not seen at high contrast. As predicted by our model, perceived speed of fast apparent motion sequences is biased in a contrast dependent way by the orientation of a target relative to its motion path, a finding well correlated to the recorded MEG activity.