The Precision of Single Neuron Responses in Cortical Area V1 during Stereoscopic Depth Judgments

Abstract

The performance of single neurons in cortical area V1 of alert macaque monkeys was compared against the animals' psychophysical performance during a binocular disparity discrimination task. Performance was assessed with stimuli that consisted of a patch of dynamic random dots, whose disparity varied from trial to trial, surrounded by an annulus of similar dots at a fixed disparity. On each trial, the animals indicated whether the depth of the central patch was in front of or behind the annulus. For each disparity of the center patch, neural performance was assessed by calculating the probability that the response of the neuron was greater or less than the response when the center disparity was the same as that of the annulus. Initially the animals performed the task simultaneously with the neural recording. However, the range of disparities used, which was appropriate for the neuronal recording, may have affected performance, because the thresholds were substantially lower (2.6x) when the psychophysical measurements were repeated later. Average neuronal thresholds were approximately 4x poorer than these behavioral thresholds, although the best neurons were marginally better than the animals' behavior. Thus, the well known precision of relative depth judgments can be supported with signals from a small number of V1 neurons. Interference with the relative depth information in the stimulus profoundly affected behavioral thresholds, which were approximately 10x poorer when the surround was absent or contained binocularly uncorrelated dots. In this case, single V1 neurons consistently outperform the observer: presumably here, psychophysical thresholds are limited by other factors (such as uncertainty about vergence eye position).

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.…”

Macaque V2 Neurons, But Not V1 Neurons, Show Choice-Related Activity

“…Before training for the task of the present study, both monkeys had been fully trained on a stereoacuity task, similar to that described previously (Prince et al, 2000), and had been psychophysical subjects in a study of stereoacuity as a function of interocular delay (Read and Cumming, 2005). The monkeys viewed the RDS stimulus while fixating on a 0.2 ϫ 0.2°fixation square.…”

“…The first explanation seems improbable: V1 is the likely source of disparity signals for neurons in V2. Furthermore, several limitations in stereoscopic performance (especially spatial and temporal resolution) seem to reflect processing in V1 (Prince et al, 2000;Nienborg et al, 2004Nienborg et al, , 2005. The second explanation would make it quite natural that CPs are not significant in V1: One need only suppose that the top-down signal does not reach all the way back to V1.…”

Attentional Modulation of Behavioral Performance and Neuronal Responses in Middle Temporal and Ventral Intraparietal Areas of Macaque Monkey

“…First, the neuronal modulation that occurs when attention is directed to a stimulus (Spitzer et al, 1988) or when effort is increased (Spitzer and Richmond, 1991) is typically an enhancement, which is consistent with behavioral improvements. Second, a rough correspondence exists between behavioral performance and the ability of individual neurons to discriminate among or detect stimuli (Parker and Hawken, 1985;Barlow et al, 1987;Britten et al, 1992;Geisler and Albrecht, 1997;Prince et al, 2000). Because these studies are likely to have spanned a range of attentional states, it is possible that the relationship between neuronal activity and behavioral performance persists across different attentional conditions.…”