Neural Activity in Cortical Area V4 Underlies Fine Disparity Discrimination

Abstract

Primates are capable of discriminating depth with remarkable precision using binocular disparity. Neurons in area V4 are selective for relative disparity, which is the crucial visual cue for discrimination of fine disparity. Here, we investigated the contribution of V4 neurons to fine disparity discrimination. Monkeys discriminated whether the center disk of a dynamic random-dot stereogram was in front of or behind its surrounding annulus. We first behaviorally tested the reference frame of the disparity representation used for performing this task. After learning the task with a set of surround disparities, the monkey generalized its responses to untrained surround disparities, indicating that the perceptual decisions were generated from a disparity representation in a relative frame of reference. We then recorded single-unit responses from V4 while the monkeys performed the task. On average, neuronal thresholds were higher than the behavioral thresholds. The most sensitive neurons reached thresholds as low as the psychophysical thresholds. For subthreshold disparities, the monkeys made frequent errors. The variable decisions were predictable from the fluctuation in the neuronal responses. The predictions were based on a decision model in which each V4 neuron transmits the evidence for the disparity it prefers. We finally altered the disparity representation artificially by means of microstimulation to V4. The decisions were systematically biased when microstimulation boosted the V4 responses. The bias was toward the direction predicted from the decision model. We suggest that disparity signals carried by V4 neurons underlie precise discrimination of fine stereoscopic depth.

Neuronal thresholds and choice-related activity of otolith afferent fibers during heading perception

“…Although it might be surprising to some readers, this observation is consistent with studies that involve fine discrimination tasks in other sensory systems, where perceptual threshold may be set by the most sensitive neurons available (4). For example, similar findings have been reported for orientation discrimination in primary visual cortex (V1) (31), fine direction discrimination in the middle temporal (MT) visual area (7), and fine binocular disparity discrimination in areas V4 (32) and V1 (33). In contrast, studies that have compared neuronal and psychophysical performance in coarse discrimination or amplitude discrimination tasks have often reported that many single neurons are more sensitive than the animal (5, 6, 9, but also ref.…”

Binocular depth processing in the ventral visual pathway

“…This was further evidenced by the findings from a study by Shiozaki et al [67], in which the authors examined how V4 activity relates to behaviour in a fine disparity discrimination task, which was based on relative rather than absolute disparity signals. They found that the information encoded by V4 neurons was nearly sufficient to support behaviour during relativedepth discrimination.…”

Weighted parallel contributions of binocular correlation and match signals to conscious perception of depth

“…The chance-level performance is 50%, and a correct performance rate below 50% indicates reversed depth. We assumed a readout mechanism suitable for near/far discrimination; the sensory outputs of the matching or correlation computations are compared between their near and far detectors, and whichever detector has the larger response determines the choice [23][24][25]36]. Through this readout mechanism, the correlation computation predicts that a negative correlation between the paired images leads to reversed depth perception because the response balance is reversed between near and far detectors.…”

“…Monkeys are a useful model animal to study stereoscopic depth perception because they perceive depth in a similar way to humans [7,8,36,53]. Classically, the neural processing of binocular disparity was ascribed to the dorsal (occipitoparietal) pathways [54,55].…”