Neuronal and psychophysical sensitivity to motion signals in extrastriate area MST of the macaque monkey

Abstract

We recorded the responses of single neurons in extrastriate area MST while rhesus monkeys discriminated the direction of motion in a set of stochastic visual displays. By varying systematically the strength of a coherent motion signal within the visual display, we were able to measure simultaneously the monkeys' psychophysical thresholds for direction discrimination and the responses of single neurons to the same motion signals. Neuronal thresholds for reliably signaling the direction of motion in the visual display were calculated from the measured responses using a method based in signal detection theory. Neurons in MST were exquisitely sensitive to motion signals in the display, having thresholds for discriminating the direction of coherent motion that were, on average, equal to the psychophysical thresholds of the monkeys. For many MST neurons, the intensity of the response was correlated with the monkey's psychophysical judgements for repeated presentations of a given near-threshold stimulus; the monkey tended to choose the preferred direction of the neuron under study when that neuron responded more strongly to the stimulus. In both of these respects, MST neurons were indistinguishable from neurons in extrastriate area MT, a major source of afferent input to MST. In a second set of experiments, we found that both of these results held true in the face of pronounced manipulations of the visual stimulus. Severe reductions in stimulus size and speed, for example, compromised neuronal and psychophysical sensitivities by similar amounts so that the average neuronal and psychophysical thresholds remained approximately equal. In addition, the trial-to-trial covariation of neuronal response and perceptual decision was unaffected by our stimulus manipulations. Thus, MST neurons carry signals appropriate for supporting psychophysical performance on our task over an impressively wide range of stimulus configurations.

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

“…This could explain why CPs for motion discrimination are larger in MST (medial superior temporal area) than MT (Celebrini and Newsome, 1994;, and a similar metric devised for detection tasks is larger in VIP (ventral intraparietal area) than MT (Cook and Maunsell, 2002b). This might also account for why correlations between neuronal firing and choice are modest or absent in V1 (Leopold and Logothetis, 1996;Grunewald et al, 2002).…”

“…These observations may reflect a causal link between the activity of single neurons and sensory decisions. Several groups quantified these correlations with choice probabilities (CPs), allowing comparison across cortical areas (Celebrini and Newsome, 1994;Britten et al, 1996;Uka et al, 2005) and visual tasks Dodd et al, 2001;Uka and DeAngelis, 2004;Purushothaman and Bradley, 2005;Uka et al, 2005).…”

“…Several studies found systematic correlations between CP and the precision of task-relevant information (Celebrini and Newsome, 1994;Britten et al, 1996;Uka and DeAngelis, 2004). These observations are readily explained as the result of stochastic variation in neuronal firing contributing to the animals' decisions, if one assumes appropriate correlations between neurons in the pool .…”

Psychophysically measured task strategy for disparity discrimination is reflected in V2 neurons

“…One method that has been widely used to suggest that signals from particular neurons contribute to a given perceptual decision combines threshold psychophysics with single-unit recordings. A trialto-trial correlation between neuronal activity and perceptual judgment, which cannot be explained by the visual stimulus, is taken as evidence that neuronal signals contribute to a decision [1][2][3][4][5][6][7] . Such correlation, often quantified by 'choice probability', should then also depend on the task strategy employed by the subject 6 .…”

“…It is natural that subjects usually rely on those signals that are most suitable statistically for the task at hand. Several studies have found that neurons carrying higher precision signals for a given task show stronger correlation with perceptual judgment 2,3,5,8,9 .…”