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Michael N Shadlen

Publications and source records attributed to Michael N Shadlen.

6 recordsLinked to original sources
PubMed2003

Representation of time by neurons in the posterior parietal cortex of the macaque.

The neural basis of time perception is unknown. Here we show that neurons in the posterior parietal cortex (area LIP) represent elapsed time relative to a remembered duration. We trained rhesus monkeys to report whether the duration of a test light was longer or shorter than a remembered "standard" (316 or 800 ms) by making an eye movement to one of two choice targets. While timing the test light, the responses of LIP neurons signaled changes in the monkey's perception of elapsed time. The variability of the neural responses explained the monkey's uncertainty about its temporal judgments. Thus, in addition to their role in spatial processing and sensorimotor integration, posterior parietal neurons encode signals related to the perception of time.

Action Potentials↗
PubMed2003

The influence of behavioral context on the representation of a perceptual decision in developing oculomotor commands.

To make decisions about sensory stimuli, the brain must weigh the evidence that supports or opposes the alternative interpretations. In the present study, we evaluated the hypothesis that a quantity reflecting the weight of sensory evidence is represented in brain circuits responsible for the behavioral response used to indicate the decision. We trained monkeys to decide the direction of random-dot motion and to indicate their decision with an eye movement to one of two choice targets. We interrupted decision formation with electrical microstimulation of the frontal eye field, causing an evoked eye movement that is influenced by ongoing oculomotor activity. For the "pro-saccade" version of the task, in which the correct target was at a known location in the direction of motion, the microstimulus-evoked eye movement reflected both the impending pro-saccadic response and the temporal accumulation of motion information used to select that response. In contrast, for the "colored-target" task, in which the correct target was of a particular color but at an unpredictable location, little ongoing oculomotor activity was evident. The results suggest that formation of the decision and formation of the behavioral response share a common level of neural organization, but only when the decision is associated with a specific, predictable movement.

Animals↗
PubMed2003

Microstimulation of visual cortex affects the speed of perceptual decisions.

Direction-selective neurons in the middle temporal visual area (MT) are crucially involved in motion perception, although it is not known exactly how the activity of these neurons is interpreted by the rest of the brain. Here we report that in a two-alternative task, the activity of MT neurons is interpreted as evidence for one direction and against the other. We measured the speed and accuracy of decisions as rhesus monkeys performed a direction-discrimination task. On half of the trials, we stimulated direction-selective neurons in area MT, thereby causing the monkeys to choose the neurons' preferred direction more often. Microstimulation quickened decisions in favor of the preferred direction and slowed decisions in favor of the opposite direction. Even on trials in which microstimulation did not induce a preferred direction choice, it still affected response times. Our findings suggest that during the formation of a decision, sensory evidence for competing propositions is compared and accumulates to a decision-making threshold.

Animals↗
PubMed2002

Response of neurons in the lateral intraparietal area during a combined visual discrimination reaction time task.

Decisions about the visual world can take time to form, especially when information is unreliable. We studied the neural correlate of gradual decision formation by recording activity from the lateral intraparietal cortex (area LIP) of rhesus monkeys during a combined motion-discrimination reaction-time task. Monkeys reported the direction of random-dot motion by making an eye movement to one of two peripheral choice targets, one of which was within the response field of the neuron. We varied the difficulty of the task and measured both the accuracy of direction discrimination and the time required to reach a decision. Both the accuracy and speed of decisions increased as a function of motion strength. During the period of decision formation, the epoch between onset of visual motion and the initiation of the eye movement response, LIP neurons underwent ramp-like changes in their discharge rate that predicted the monkey's decision. A steeper rise in spike rate was associated with stronger stimulus motion and shorter reaction times. The observations suggest that neurons in LIP integrate time-varying signals that originate in the extrastriate visual cortex, accumulating evidence for or against a specific behavioral response. A threshold level of LIP activity appears to mark the completion of the decision process and to govern the tradeoff between accuracy and speed of perception.

Action Potentials↗
PubMed2002

Banburismus and the brain: decoding the relationship between sensory stimuli, decisions, and reward.

This article relates a theoretical framework developed by British codebreakers in World War II to the neural computations thought to be responsible for forming categorical decisions about sensory stimuli. In both, a weight of evidence is computed and accumulated to support or oppose the alternative interpretations. A decision is reached when the evidence reaches a threshold value. In the codebreaking scheme, the threshold determined the speed and accuracy of the decision process. Here we propose that in the brain, the threshold may be controlled by neural circuits that calculate the rate of reward.

Animals↗