Motion energy versus position tracking: spatial, temporal, and chromatic parameters

The fundamental question in motion perception is whether motion is an inter pretation imposed on an object or feature perceived at separate positions a t sequential instants, or whether it is the response of direction-sensitive detectors that can extract the motion-energy in the stimulus, i.e. the ori entation of spatio-temporal energy. To answer this question we constructed stimuli whose position changed in one direction while the motion energy con tained in the same spatial frequency moved in the same or the opposite dire ction (by superimposing moving sinusoidal gratings on stationary gratings o f the same spatial frequency and orientation). In every case tested (0.25-2 5 Hz temporal frequency; 0.25-1.0 cyc/deg spatial frequency; achromatic and equiluminant contrast), the perceived direction of motion was in the direc tion of motion energy, indicating the existence of neurons which compute mo tion direction without explicitly computing spatial position. The measureme nts also confirmed that motion-energy computations can be modeled as separa ble in spatial and temporal frequency. (C) 2000 Elsevier Science Ltd. All r ights reserved.