Egocentric Distance Encoding in the Posterior Parietal Cortex

Citation

Bhattacharyya, Rajan (2009) Egocentric Distance Encoding in the Posterior Parietal Cortex. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/FBNQ-0P48. https://resolver.caltech.edu/CaltechETD:etd-12062008-112215

Abstract

Previous studies have shown that the parietal reach region (PRR) encodes the two dimensional location of frontoparallel reach targets in an eye centered reference frame in early movement plans (Batista 1999; Snyder et al. 2000). Performing a visually guided reach initially requires the ability to perceive the depth of a target in three dimensional space. Beyond that initial perception, however, reach plans may represent the egocentric distance of the target in different ways. To investigate how a reach target is represented in three dimensions, recordings were made of the spiking activity of PRR neurons from two rhesus macaques trained to fixate and perform memory reaches to targets at different depths. Reach and fixation targets were configured to explore whether neural activity directly reflects egocentric distance as the amplitude of the required motor command, which is the absolute depth of the target, or rather the relative depth of the target with reference to fixation depth.

This thesis shows that planning activity in PRR represents the depth of the reach target as a function of disparity and fixation depth, the spatial parameters important for encoding the egocentric distance of a reach goal in an eye centered reference frame. Most PRR neurons were found to be sensitive to the disparity of a reach target (82%), and vergence angle (74%) which determines fixation depth. Most PRR neurons carry both disparity and vergence angle signals, and comparisons of several modulation indices and the information carried in firing rates for each variable show a single homogenous PRR population encodes egocentric distance. The strength of modulation by disparity was maintained across vergence angle, where vergence angle gain modulates disparity tuning while preserving the location of peak tuning features in PRR neurons, which allows the absolute depth of the reach target to be decoded from the population activity. Neural activity in PRR shows a wide range of sensitivity to both target disparity and fixation depth that has never been previously tested in a reach task. The results demonstrate a specific role for PRR neurons in supporting eye-hand coordination when decoupling the effector from the point of gaze.

Thesis Files