Receptive Field Characterization in MSTd Neurons

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The dorsal region of the medial superior temporal area (MSTd) in primate extrastriate visual cortex is reported to play a major role in the encoding and perception of optic flow stimuli, i.e. large-scale motion patterns on the retina created by the movement of the visual environment relative to the organism. Correspondingly, MSTd neurons show tuned responses to the direction of linear motion stimuli, as well as to the direction of spiral space motion stimuli (Graziano et al., 1994), i.e. complex motion patterns that include expansion, contraction, rotation, and their mixtures, arranged in a continuous circular dimension. In addition, MSTd cells have been reported to be position-invariant in their responses to spiral motion stimuli. Here we describe a study aimed at investigating the motion patterns MSTd neurons are most responsive to, unconstrained by the two limited sets of motion type dimensions mentioned above. We used reverse correlation, a linear method that has been successfully used to characterize receptive fields in V1 and MT. Our reverse correlation stimuli were large complex random dot patterns, formed by the smooth variation of local dot direction and speed between a virtual grid of positions in the stimulus where the local parameters were chosen randomly every 100ms from all possible linear directions and a large range of speeds. We investigated whether the reverse correlation method can be successfully used in MSTd to recover structured maps of receptive fields, and whether the motion patterns resulting from such analysis provide an appropriate description of the specific motion preferences of individual MSTd neurons, compared to the simple assumption of linear and/or spiral direction tuning. We also determined the position dependency of MSTd responses to spiral motion patterns. We recorded from 181 single MSTd cells in three rhesus monkeys, trained to foveate a fixation point. For around 25% of the 150 cells that underwent the reverse correlation task, analysis recovered in varying degrees significantly structured receptive field maps. The recovered maps show a dominance of preference to linear motion to varying degrees across cells, yet the neural responses to spiral and linear motion patterns was significantly correlated with their motion similarity to the reverse correlation maps in 54% to 68% of those cells. Almost all of the cells showed position invariant responses to spiral motion patterns. Our results indicate that reverse correlation can be applied successfully in area MSTd, although the resulting maps might be better able to explain linear motion preferences of the cell rather than more complex motion patterns. Our findings of position invariance are in line with previous evidence from literature (Graziano et al., 1994), and suggest that more studies are needed to clarify how MSTd neurons can be position invariant to optic flow stimuli and still be able to individually encode heading direction (Duffy & Wurtz, 1995; Orban, 2008).