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Computing time from space in the Primate Posterior Parietal Cortex

Elizabeth Torres

Monday, November 24, 2008, 01:00pm - 02:00pm

Rutgers University, Department of Psychology, Center for Cognitive Science and Center for Computational Biomedicine Imag

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A fundamental question in psychology and neuroscience is how skills are acquired, and what happens in the brain when a new motor memory is formed. To understand the neural basis of skill learning, we investigated the role of the posterior parietal cortex in the learning of new movements'  temporal dynamics. We had untrained monkeys reach in the dark towards remembered spatial locations while avoiding the remembered locations of obstacles.  We compared familiar reaches along straight hand paths to the new obstacle-avoidance reaches along curved hand paths. During the early initiation of the new curved reaches the high variability of the distance traveled up to the hand's velocity peak contrasted with the low variability of the length of time that it took to complete this first segment. Such temporal consistency across all target locations naturally dissociated the spatial from the temporal components of the new motion's speed. Changes in the initial movement speed amounted to changes in the distance traveled by the hand during a reliable length of time. About a second prior to the start of the movements, while the subjects were planning ahead such spatial changes, we recorded the neural activity of cells in the reach region of the Posterior Parietal Cortex (PRR). We found that the planning of the learning process of this initial segment segregated the PRR neurons into two complementary types according to their mean response properties and to the distribution of their spike widths. During the planning period, broad spiking neurons were inhibited by the learning of the new initial speed while the narrow spiking neurons were excited. Over time, as learning resumed both types of cells usually returned to their original firing rates for the familiar reaches. Our work suggests that the learning of new speed profiles can be achieved through distance-based spatial computations in the PRR. Furthermore this area seems to be directly linked to the acquisition of a new reaching motor memory as the association between the distance to be traveled in the earliest segment of the reach and the temporal length of this impending segment.


Suggested readings:


1999 Moran DW and Schwartz AB

Motor Cortical Representation of Speed and Direction During Reaching">
Neurophysiol. 82:2676-2692 (1999) 


Churchland MM, Santhanam G, Shenoy KV (2006)

Preparatory activity in premotor and motor cortex reflects the speed of the upcoming reach."> 

Journal of Neurophysiology. 96:3130-3146. 


Cisek P (2006)

 Preparing for speed. Focus on: "Preparatory activity in premotor and motor cortex reflects the speed of the upcoming reach.""> 

Journal of Neurophysiology. 96:2842-2843.

Elizabeth Torres