EYE POSITION ENCODING IN THE MACAQUE POSTERIOR PARIETAL CORTEX

In two previous studies, we had demonstrated the influence of eye posi tion on neuronal discharges in the middle temporal area, medial superi or 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 cor tex and the supplementary eye field. These effects are generally discu ssed in light of a coordinate transformation of visual signals into a non-retinocentric frame of reference. Neural network studies dealing w ith the eye position effect succeeded in constructing such non-retinoc entric representations by using model neurones whose response characte ristics 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, t heoretically, eye position could be estimated from the population disc harge of the (previously) recorded neurones and, if so, we intended to develop an encoding algorithm for the position of the eyes in the orb it. The optimal linear estimator proved the capability of the ensemble activity for determining correctly eye position. We then developed th e so-called subpopulation encoding of eye position. This algorithm is based on the partition of the ensemble of neurones into two pairs of s ubpopulations. Eye position is represented by the differences of activ ity levels within each pair of subpopulations. Considering this result , encoding of the location of an object relative to the head could eas ily be accomplished by combining eye position information with the int rinsic knowledge about the retinal location of a visual stimulus. Take n together, these results show that throughout the monkey's visual cor tical system information is available which can be used in a fairly si mple manner in order to generate a non-retinocentric representation of visual information.