Society for Neuroscience 2006

Role of macaque area V4 in stereoacuity discrimination: neuronal performance and choice probability

Hiroshi Shiozaki, Seiji Tanabe, Takahiro Doi, Ichiro Fujita
Graduate School of Frontier Biosciences, Osaka University, Toyonaka, Osaka, Japan

Neurons selective for binocular disparity have been found in various areas along the primate cortical visual pathway. In area V4, the neural representation of stereoscopic depth accounts for perceived depth in ways the disparity energy computation in V1 fails to. Yet the putative role of V4 neurons in perceptual decisions about stereoscopic depth has not been directly tested. We addressed the relationship between neuronal activity of V4 and behavioral choice in a stereoacuity task. One monkey (Macaca fuscata) was trained to report whether the center region of a random-dot stereogram was seen slightly nearer or farther than its immediate surround. The disparity applied in the stereogram varied from trial to trial so as to span the monkeyfs psychophysical threshold. We recorded single-cell activity in V4 while the monkey performed this task. Neuronal responses of V4 cells during the viewing period were correlated with the subsequent monkey's choice on a trial-by-trial basis. We quantified this correlation as a choice probability (CP), which is the probability that an ideal observer listening to the neuronal activity can correctly predict the upcoming choice that the monkey makes. Over one third of the cells (13/34) showed a significant CP, and the average CP (0.56) is significantly higher than chance. Choice-related activity was apparent from the beginning of visual response, suggesting that this activity did not reflect feedback signals from downstream decision-making mechanisms. Using the same set of data, we compared the neuronal and psychophysical thresholds. The neuronal threshold of most V4 neurons was higher than the psychophysical threshold. Cells with low thresholds had high choice probabilities. Decision-making mechanisms may selectively read-out a subpopulation of V4 cells that exhibit high signal-to-noise ratio in stereoacuity discrimination.

Supported by grants from MEXT (17022025) and the Takeda Science Foundation.