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Dissociating the spatial signal in area LIP from evidence accumulation

The lateral intraparietal area (LIP) contains spatially selective neurons that help guide eye movements and, according to numerous studies, do so by accumulating sensory evidence in favor of one choice (e.g., look left) or another (look right). To examine this functional link, we trained two monkeys on an urgent version of the random-dot motion (RDM) discrimination task, a task with which the evolution of both the recorded neuronal activity and the subject's choice can be tracked millisecond by millisecond. We found that while choice accuracy increased steeply with increasing sensory evidence (motion viewing, in our case), at the same time, the LIP selection signal became progressively weaker, as if it hindered performance. Furthermore, comparison to a different urgent task in which the relevant sensory evidence was found at the choice targets (rather than near fixation) showed that the RDM result was not due to time pressure alone, but rather to the specific demands placed on the spatial allocation of attention. The results suggest that the ramping activity in area LIP traditionally interpreted as evidence accumulation may correspond to a slow, post-decision shift of spatial attention from one location (where the dots are) to another (where the chosen target is).

The graph shows two curves as functions of processing time, which is the cue viewing time in each trial of the task. The black trace shows the combined performance of the two monkey subjects (percent correct), whereas the brown curve shows the differential activity in area LIP (SROC), which measures how strongly LIP neurons point to one choice location (into the response field) versus another (away from the response field). In this task, the performance of the monkeys increases with increasing sensory evidence while the spatial signal in LIP becomes weaker.

See a short slide show describing the main findings.

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