PROCESSING OF FIRST-ORDER AND 2ND-ORDER MOTION SIGNALS BY NEURONS IN AREA MT OF THE MACAQUE MONKEY

Extrastriate cortical area MT is thought to process behaviorally impor tant visual motion signals. Psychophysical studies suggest that visual motion signals may be analyzed by multiple mechanisms, a ''first-orde r'' one based on luminance. and a ''second-order'' one based upon high er level cues (e.g. contrast, flicker). Second-order motion is visible to human observers, but should be invisible to first-order motion sen sors. To learn if area MT is involved in the analysis of second-order motion, we measured responses to first-and second-order gratings of si ngle neurons in area MT (and in one experiment, in area V1) in anesthe tized, paralyzed macaque monkeys. For each neuron, we measured directi onal and spatio-temporal tuning with conventional first-order gratings and with second-order gratings created by spatial modulation of the f licker rate of a random texture. A minority of MT and V1 neurons exhib ited significant selectivity for direction or orientation of second-or der gratings. In nearly all cells, response to second-order motion was weaker than response to first-order motion. MT cells with significant selectivity for second-order motion tended to be more responsive and more sensitive to luminance contrast, but were in other respects simil ar to the remaining MT neurons, they did not appear to represent a dis tinct subpopulation. For those cells selective for second-order motion , we found a correlation between the preferred directions of first-and second-order motion, and weak correlations in preferred spatial frequ ency. These cells preferred lower temporal frequencies for second-orde r motion than for first-order motion. A small proportion of MT cells s eemed to remain selective and responsive for second-order motion. None of our small sample of V1 cells did. Cells in this small population, but not others, may perform ''form-cue invariant'' motion processing ( Albright, 1992).