Area MT/V5 of macaque visual cortex contains a high proportion of directionally selective neurons and is thought to play an important role in motion perception. Psychophysical and physiological evidence indicates that motion processing in primate visual cortex occurs in at least two stages. The first stage, most likely located in primary visual cortex (V1), encodes the orientation and spatial frequency information in a local region of space. It is unable to decode more complicated motion in the stimulus, such as that created by overlaying two moving objects. In the second stage, inputs from the first stage are combined to extract the true direction and speed of a moving stimulus. Several lines of research have pointed to area MT as a candidate for this second stage of motion processing.
The use of plaid stimuli, made by combining two sinusoidal gratings with different orientations, has proved useful in probing the circuitry involved in these putative stages of motion processing. Although the motion of one grating behind an aperture is ambiguous, the true direction and speed of a plaid can be correctly determined by using the additional information gained from the intersections of the two gratings. One way to use this information is by computing an ``intersection of constraints'' (Adelson and Movshon, 1982).
Cells in primary visual cortex of cats and monkeys have long been known to be selective for the orientation and spatial frequency of drifting gratings (Movshon et al., 1978a; DeValois et al., 1982; Movshon et al., 1978b). When confronted with a plaid stimulus, these neurons only signal the direction of motion of the component gratings, and not the true direction of the pattern (Movshon and Newsome, 1996; Movshon et al., 1985). However, while some cells in area MT behave similarly to those in V1, others respond to the true direction of motion of the plaid stimulus (Rodman and Albright, 1989; Movshon et al., 1985). The former are termed component direction-selective (CDS) and the latter are pattern direction-selective (PDS).
The above studies have primarily been concerned with determining the steady-state responses of neurons over periods of several seconds. Recently, there has been mounting evidence from both psychophysics (Masson and Castet, 2002; Lorenceau et al., 1993) and physiology (Pack and Born, 2001; Li et al., 2001) that the perception of two-dimensional motion is a process that evolves dynamically over periods of tens to hundreds of milliseconds. However, because the physiological data collected so far have used stimuli composed of small line segments, it is uncertain how these findings relate to previous results in area MT using plaids. We therefore undertook a study of the time course of response of neurons to plaid and grating stimuli in area MT of anesthetized macaque monkeys.