Oxyopia Abstract

April 4, 2008
Friday, 4:00 PM
489 Minor Hall

Bruce Cumming, MD, PhD
Disparity Section, Laboratory of Sensorimotor Research, National Eye Institute, Bethesda, MD
Host: Ralph Freeman

Title

Where do correlations between neuronal activity and sensory decisions originate?

Abstract

Simultaneous use of single unit recording and threshold psychophysics has revealed correlations between perceptual choice and firing rate, that cannot be explained by the visual stimulus (Choice Probability, CP). Quantitative modeling studies have explained the observed magnitudes with a bottom-up scheme, in which CP reflect an effect of random fluctuations in firing rate upon choice. In order to test this interpretation further, we measured CP using a stimulus which simultaneously allowed the use of white noise analysis to infer how fluctuations in the stimulus content affected (1) neuronal activity and (2) psychophysical choices. Two monkeys performed a disparity identification task while we recorded the activity of disparity selective neurons in V2. The stimulus was a random dot stereogram in which the disparity was chosen at random (from a discrete distribution) for each 10ms video frame. Signal was added by increasing the probability with which one disparity was presented on a given frame. Examining the distribution of disparities associated with near and far choices ("psychophysical reverse correlation") revealed that the animals' choices were primarily determined by events early in the trials. CP showed no attenuation as the trial progressed. The presence of a CP at times when the animal is not using information about the stimulus is at odds with a purely bottom-up scheme, and indicates top-down involvement. We examined neuronal responses for evidence of this.

Calculating the mean response following one video frame, for each disparity, yields disparity response functions. These were calculated separately according to the choice reported at the end of the trial. Trials (with no added signal) on which animals report the preferred disparity have higher mean firing rates - as expected from earlier observations on CP. The disparity response functions reveal that this mainly reflects an increase in the gain of the neuronal response to disparity, on trials where the animal chooses that neurons preferred disparity. We have been unable to generate such large gain changes in (bottom-up) simulations where the pooled response of a neuronal population determines choice. These gain changes resemble the effects of spatial or feature-based attention that have been reported by others. This suggests that a significant component of the CP in this task reflects a top-down process similar to feature-based attention.

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