An illusory transformation of optic flow fields without local motion interactions

Abstract

The focus of expansion (FOE) of a radial optic flow pattern that is transparantly overlapped by planar motion is perceptually displaced in the direction of the planar motion (Duffy and Wurtz, Vision Research, 1993). Recently, Royden and Conti (Vision Research, 2003) showed that a model of heading detection based on operators that measure local motion opponency can predict the direction and magnitude of the illusory shift when presented with overlapping planar and radial fields. These operators were modeled after cells found in the middle temporal visual area (MT). Here we show psychophysically that under conditions without local motion opponency the illusion becomes weaker but can still be observed. In our experiments, the planar and radial fields were not presented with overlap but separately to the left and right halfs of the visual field with a blank vertical strip of 15 degrees horizontal width in between. These conditions exclude an explanation of the illusory shift of the FOE based on motion opponency in MT cells because (a) there was no transparant motion in the stimulus, and (b) the receptive fields of MT cells do not cross the vertical midline of the visual field. We conclude that large field motion detectors which integrate motion from both halfs of the visual field play a role in shifting the perceived position of the FOE and that local motion interactions are not necessary for the optic flow illusion to occur.