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Mehrdad Jazayeri, Pascal Wallisch, J. Anthony Movshon; Responses of macaque MT neurons to multi-stable moving patterns. Journal of Vision 2010;10(7):816. doi: 10.1167/10.7.816.
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Neurons in area MT are sensitive to the direction of motion of gratings and of plaids made by summing two gratings moving in different directions. MT component-direction-selective (CDS) neurons respond independently to the gratings of a plaid, while pattern-direction-selective (PDS) neurons combine component information to respond selectively to plaids that move in the direction preferred by single gratings. Adding a third moving grating creates a multistable “triplaid”, which alternates perceptually among different groupings of gratings and plaids. To examine how this multistable motion percept might relate to the activity of CDS and PDS neurons, we measured the activity of 77 MT neurons in anaesthetized macaques to triplaid stimuli in which three identical moving gratings whose directions were separated by 120 deg were introduced successively going from a grating (320 ms) to a plaid (320 ms) to a triplaid (1280 ms). CDS and PDS neurons – selected based on their responses to gratings and plaids – responded strikingly differently to triplaids. CDS neurons maintained their tuning properties for more than 1 s, but PDS neurons were slowly and progressively suppressed and lost their direction tuning properties altogether after 0.3–0.6 s. PDS but not CDS responses to triplaids also depended on the order in which the three components were introduced. We wondered whether these effects might be due to anesthesia and therefore repeated the experiment in area MT of an awake macaque performing a fixation task. Responses to the onset of individual gratings were more transient in the awake macaque than under anesthesia, but the sustained suppression of PDS responses persisted in both conditions. We attribute the differences between CDS and PDS response properties to an opponent suppression that is more potent in PDS cells, and discuss how area MT might contribute to the multistable perception of direction in moving triplaids.
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