Decision-related activity in V2 for a fine disparity discrimination task.

The stereoscopic depth of a target can be evaluated relative to the fixation plane (absolute disparity) or relative to a background (relative disparity). Fine disparity discrimination is thought to rely on relative disparity judgments (Prince et al., 2000). V2 neurons are the first in the visual processing hierarchy to show selectivity for relative disparity (Thomas et al., 2002), and have previously been found to have decision-related activity for coarse disparity discrimination tasks (Nienborg and Cumming, 2006, 2009). Here, we asked whether V2 neurons also show decision-related activity in a fine disparity discrimination task. We recorded extracellular single unit activity while two macaques performed fine disparity discrimination. The stimuli were random dot stereograms with a disparity defined central disk and a surrounding annulus. The animals had to discriminate whether the disk was protruding or receding relative to the surrounding annulus. The neurons discriminated the stimuli reliably although neuronal discrimination performance compared to that of the animals was lower on average (mean neurometric to psychometric threshold ratios: 2.61). These results suggest a perceptual role of V2 neurons for fine disparity discrimination. The V2 units also had decision-related activity, quantified as choice probabilities (mean=0.57, significantly different from 0.5 chance level, p< 0.01, n=63). Interestingly, neuronal selectivity for relative disparity was not correlated with decision-related activity (r=0.014, p=0.93). In a subset of units (n=28) we measured decision-related activity for the coarse and fine disparity discrimination task and found a significant correlation (r=0.4, p< 0.05). Choice probabilities have been shown to depend on the structure of interneuronal correlations (Shadlen et al. 1996, Gu et al. 2008). This could explain that choice probabilities in the fine task were independent of their selectivity for this task. The correlation of the choice probabilities between tasks suggests a surprisingly stable structure of interneuronal correlations between the two tasks. Meeting abstract presented at VSS 2015.