PURPOSE: The Pinna illusion is a striking example of the perception of complex motion (rotation and radiation) in the absence of physical motion. For example, upon physically approaching or receding from the Pinna-Brelstaff figure, the observer experiences vivid illusory counter rotation of the figure. This visual phenomenon of illusory rotary motion is a well-known example of integration of local cues to form a global percept. Using psychophysical tests and functional magnetic resonance imaging (fMRI) of visual cortices, we recently found that the Pinna-Brelstaff figure (illusory rotation) and a real physical rotation control stimulus both predominantly activated subarea MST in hMT+, each with a similar response intensity. However, the detailed neural mechanisms underlying the Pinna illusory rotation as well as radiation remain unknown.
METHODS: By manipulation of the physical characteristics of the Pinna-Brelstaff figure, we could generate different types of illusory complex motion: rotation, expansion and contraction (radiation). We first test the illusory effect in human and nonhuman primate psychophysically, and then performed single-unit recordings of MST and MT in awake macaques.
RESULTS: We found that up to two-thirds of MST neurons encode Pinna illusory complex motions, with similar tuning preferences to their corresponding real physical motions. A subset of MT neurons was found to encode the local motion signals with earlier response latency than MST neurons.
CONCLUSIONS: These results demonstrate that neurons in area MST but not MT respond equivalently and respectively to Pinna illusory and real rotations, expansions and contractions. These findings suggest that the representation of illusory and real complex motion fields in primate MST relies on a similar cascade of neural integrative mechanisms from earlier visual areas to generate a global motion perception.