Enhanced neural responses correlated with perceptual binding of color and motion.

When both color and motion direction of visual stimuli are alternated in physical synchrony at a relatively higher frequency (approximately 2 Hz), the changes in motion direction are perceived to be delayed. On the other hand, color and motion direction changes are perceived to be in phase when the motion direction changes precede the color changes by about 100 ms [Moutoussis, 1997]. In the present study, we utilized this phenomenon to investigate the neural mechanisms underlying the binding of color and motion based on the temporal synchrony. Magnetoencephalogram (MEG) was recorded for ten human subjects under the following four conditions: color change (color), motion direction change (motion), and simultaneous color and motion direction changes (color+motion) in perceptual synchrony (physical asynchrony) or in perceptual asynchrony (physical synchrony). The wavelet analysis was applied on these MEGs to study the neural responses in time-frequency domain. The interactions of color and motion responses, defined by [color+motion]-([color]+[motion]), were calculated in time-frequency domain for both perceptually synchronous and asynchronous conditions. The results showed significantly larger interactions at gamma band (30-35 Hz) under the condition of perceptual synchrony than under the condition of perceptual asynchrony, suggesting that synchronized neural responses at gamma band are related to the synchrony-based binding of visual attributes. This result is consistent with previous studies reporting the correlation of gamma band responses with perceptual grouping [Castelo-Branco, 2000] [Tallon-Baudry, 1996].