High choice probabilities are associated with high interneuronal correlations in MT (V5) of the awake behaving macaque

Variations in firing of single neurons in MT(V5) of awake, behaving macaques correlate highly with perceptual choice for the rotation of an ambiguous zero disparity cylinder (choice probability 0.67). Neuronal responses in MT(V5) are also correlated with direction discrimination in a random motion stimulus, but this choice probability is considerably smaller (0.56; Britten et al., 1996). In models of sensory discrimination (Shadlen et al.,1996) the size of the neuronal pool underlying a percept is determined by the size of the choice probability and the degree of interneuronal correlation. Thus, one explanation for the difference in observed choice probability would be a difference in interneuronal correlation. We recorded from single cells in two awake macaques whilst they viewed rotating cylinders portrayed by structure-from-motion with random dots interleaved with random motion stimuli. We estimated interneuronal correlation by comparing detrended single unit (SU) and multi unit (MU) activity trial by trial. Sites with the same SU and MU preference for disparity defined cylinder rotation showed a mean interneuronal correlation of 0.44 for the zero disparity cylinder (n=29). For the same sites, random motion produces a significantly smaller interneuronal correlation of only 0.22 (paired t-test on Fisher's z, p<0.01). This holds true if we restrict the analysis to sites that also have the same preference for direction of motion (n=24; paired t-test on Fisher's z, p<0.05). These data suggest interneuronal correlation is susceptible to experimental manipulation. The high interneuronal correlation might be caused by the difference in the two visual stimuli or by the difference in choice probabilities associated with them. Therefore, we assessed the correlation between choice probability and interneuronal correlation for the cylinder stimulus. Considering all sites with the same cylinder preference choice probability and interneuronal correlation were significantly correlated (r=0.36; n=40; paired t-test p<0.05). Thus, high choice probabilities are associated with high interneuronal correlations. Neuronal pool size in MT underlying perceptual choice could at least in principle be kept approximately unchanged across different motion and stereo stimuli.