title: Processing of first- and second-order motion signals by neurons in area MT of the macaque monkey.
abstract: Extrastriate cortical area MT is thought to process behaviorally important visual motion signals. Psychophysical studies suggest that visual motion signals may be analyzed by multiple mechanisms, a "first-order" one based on luminance, and a "second-order" one based upon higher level cues (e.g. contrast, flicker). Second-order motion is visible to human observers, but should be invisible to first-order motion sensors. To learn if area MT is involved in the analysis of second-order motion, we measured responses to first- and second-order gratings of single neurons in area MT (and in one experiment, in area V1) in anesthetized, paralyzed macaque monkeys. For each neuron, we measured directional and spatio-temporal tuning with conventional first-order gratings and with second-order gratings created by spatial modulation of the flicker rate of a random texture. A minority of MT and V1 neurons exhibited significant selectivity for direction or orientation of second-order 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 similar to the remaining MT neurons; they did not appear to represent a distinct 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 frequency. These cells preferred lower temporal frequencies for second-order motion than for first-order motion. A small proportion of MT cells seemed 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).
authors.affiliation: Howard Hughes Medical Institute and Center for Neural Science, New York University, New York 10003-6621, USA.
authors.forename: L P
authors.initials: LP
authors.lastname: O'Keefe
authors.forename: J A
authors.initials: JA
authors.lastname: Movshon
mesh_terms: D000818:Animals; D008252:Macaca fascicularis; D009039:Motion Perception / Q000502:physiology*; D009474:Neurons / Q000502:physiology*; D010775:Photic Stimulation / Q000379:methods; D015398:Signal Transduction / Q000502:physiology*; D013998:Time Perception / Q000502:physiology; D014793:Visual Cortex / Q000166:cytology / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*