Eye position encoding in the macaque posterior parietal cortex
F. Bremmer, A. Pouget & K.-P. Hoffmann
European Journal of Neuroscience, 10:153-160, 1998
- in two previous studies, we had demonstrated the influence of eye position on neuronal discharges in the middle
temporal area, medial superior temporal area, lateral intraparietal area and area 7A of the awake monkey (Bremmer
et al., 1997a,b). Eye position effects also have been found in visual cortical areas V3A and V6 and even in the
premotor cortex and the supplementary eye field. These effects are generally discussed in light of a coordinate
transformation of visual signals into a non-retinocentric frame of reference. Neural network studies dealing with
the eye position effect succeeded in constructing such non-retinocentric representations by using model neurones
whose response characteristics resembled those of 'real' neurones. However, to our knowledge, response properties
of real neurones never acted as input into these neural networks. In the present study, we thus investigated whether,
theoretically, eye position could be estimated from the population discharge of the (previously) recorded neurones
and, if so, we intended to develop an encoding algorithm for the position of the eyes in the orbit. The optimal
linear estimator proved the capability of the ensemble activity for determining correctly eye position. We then
developed the so-called subpopulation encoding of eye position. This algorithm is based on the partition of the
ensemble of neurones into two pairs of subpopulations. Eye position is represented by the differences of activity
levels within each pair of subpopulations. Considering this result, encoding of the location of an object relative
to the head could easily be accomplished by combining eye position information with the intrinsic knowledge about
the retinal location of a visual stimulus. Taken together, these results show that throughout the monkey's visual
cortical system information is available which can be used in a fairly simple manner in order to generate a non-retinocentric
representation of visual information.
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