Processing of first- and second-order motion signals by neurons in area MT of the macaque monkey

Lawrence P. O'Keefe, J. Anthony Movshon

Research output: Contribution to journalArticlepeer-review


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).

Original languageEnglish (US)
Pages (from-to)305-317
Number of pages13
JournalVisual neuroscience
Issue number2
StatePublished - Mar 1998


  • Area MT
  • Extrastriate cortex
  • Macaque
  • Motion
  • Physiology

ASJC Scopus subject areas

  • Physiology
  • Sensory Systems


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