Perceptual decision making is formalized as the accumulation of (noisy) sensory evidence until a decision threshold is crossed. Much understanding has been gained from studies of monkey neurophysiology and human neuroimaging as well as by computational modeling but there are still fascinating open questions about mechanisms of perceptual decision making. Perceptual decisions can be made within a fraction of a second (~ several hundred milliseconds, measured as reaction time (RT) which is usually defined as the time from the onset of the stimulus till the response). However, what part of the highly variable RT is dedicated to the processing of the percept itself, leaving motor preparation, motor execution and sensory latency aside? This is the central problem that Shankar et al.(2011) and Stanford et al.(2010) investigated. In order to isolate this processing step, they developed a new task design, the compelled response paradigm. In this task a participant (here a monkey) is asked to saccade from a fixation point (located in the middle of the presentation screen) to one of two targets. A ‚Go’ signal informs the participant that a saccade has to be initiated (when the fixation point disappears). Importantly, the true color of the targets (which indicates the correct target location) is revealed only after a random delay after the Go signal occurred. The delay is under the control of the experimenter, thus one can now calculate raw and effective processing time, i.e. by subtracting the delay from RT to get the former, and thereby come up with metrics that represent available time for sensory processing. The authors combine behavioral analysis with single neuron recordings and computational modeling to dissociate perceptual evaluation from motor execution.
In summary, the authors establish an original method for estimating pure evidence integration. Furthermore, by using different variations of this task, they show motor and perceptual contributions to decision making.
Stanford, T., Shankar, S., Massoglia, D., Costello, M., & Salinas, E. (2010). Perceptual decision making in less than 30 milliseconds Nature Neuroscience, 13 (3), 379-385 DOI: 10.1038/nn.2485
Shankar, S., Massoglia, D., Zhu, D., Costello, M., Stanford, T., & Salinas, E. (2011). Tracking the Temporal Evolution of a Perceptual Judgment Using a Compelled-Response Task Journal of Neuroscience, 31 (23), 8406-8421 DOI: 10.1523/JNEUROSCI.1419-11.2011
Tags: compelled-response-task, Perceptual decision making, sensory processing
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