Evidence for an early motion system which integrates information from the two eyes

In one type of cyclopean motion stimulus one eye views a counterphase flickering grating while the other eye views the same pattern in spatio-temporal quadrature. Algebraic summation of the two image sequences results in a drifting grating. Upon binocular (cyclopean) combination of the two patterns a drifting grating is perceived even though neither monocular pattern is moving. While this appears to support the position that the motion system is binocular, it has been suggested that such demonstrations involve higher level feature tracking rather than early motion system activation. The perceived direction of motion could result from the tracking of features after neural summation of left and right eye images. However, by adding a static, in-phase, pedestal grating to the left and right eye flickering test gratings, the direction information based on feature tracking is removed while leaving the motion energy information unchanged. We have found that when such stimuli are presented for several seconds, direction discrimination performance is significantly better than chance for pedestal grating contrasts several times the test grating contrast. Therefore, in the absence of a feature tracking cue, the direction of motion is identified using a binocular motion energy mechanism. The results do not exclude the existence of a binocular feature tracking system. Both systems are likely to exist.