A BIOPHYSICAL MODEL FOR THE DEVELOPMENTAL TIME-COURSE OF RETINAL ORIENTATION SELECTIVITY

A quantitative study of the time course of development of the percenta ge of orientationally selective and isotropic ganglion cells in turtle retina has recently been performed. This study revealed that as soon as ganglion cells start responding to light, a large percentage of the m are selective to the orientations of moving visual stimuli. This per centage decreases with age to reach a minimum around hatching, increas es dramatically after birth and finally, decreases again following the first month of life to reach adult level. Concomitantly, the percenta ge of cells responding isotropically to the orientation of elongated s timuli increases monotonically until about 30 days after birth, stabil izing afterwards. To account for both time courses, we propose a bioph ysical model implementing features ubiquitous to developing vertebrate retinas. These features include early dendritic and synaptic spatial polarization, dendritic growth, and waves of activity generated sponta neously or by visual stimulation sweeping across the inner plexiform l ayer (IPL). The model also assumes a physiologically plausible Hebbian rule, which includes long-term potentiation and depression. Computer simulations of this model yield good fits of the data. The quality of these fits confirms and extends results from an earlier model using co mputationally-simple mechanisms, which suggested that early dendritic polarization might be the seed for mature orientation selectivity. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.