A Theory of Cortical Map Formation in the Visual Brain

Abstract

There are about 3 million afferents going from retina to our primary visual cortex through the lateral geniculate nucleus (LGN) of the thalamus. The thalamic afferents form clusters in visual cortex based on the stimuli to which they respond. There are afferent clusters based on retinal spatial arrangement (retinotopy), eye of input (ocular dominance), orientation preference, and light and dark polarity (ON and OFF). The similarities in the pattern organization of afferents in different species suggests a general mapping rule in the development of primary visual cortex. It has been suggested that the thalamocortical pathway is the origin of these maps (Kremkow and Alonso, 2018), but there are still many open questions regarding the functionality, interrelations, and underlying developing mechanisms of these maps. In the first part of this study, I investigated the relationship between the maps of retinotopy and ocular dominance. I explored the underlying reason behind the diversity of ocular dominance columns patterns in the primary visual cortex of different species. I found that the irregularity in the morphology of ocular dominance columns could be explained by local variations in the retinotopic map of different animals. In the second part of the study, I propose a general theory of cortical map formation that provides a biologically plausible mechanism of map development. The main idea of the theory is that the organization of visual cortical maps in different species is determined by the sampling density of the afferents responding to the same point of visual space. As the number of afferents per visual point increases, the visual cortex becomes larger to accommodate the increased number of inputs. Consequently, the input afferents sort not only by spatial location but also by other stimulus features like eye of input and contrast polarity. I test my theory with a computational model that compares computer simulations with experimental data. The model has three main developing stages: 1- retina, 2- cortical subplate, and 3- mature cortex. The result of the model is consistent with a large body of experimental evidence in the literature and our electrophysiological measurements from cat primary visual cortex. The model also allows simulating the cortical map of different animals and could help to guide the implantation of cortical prosthesis in the future to cure blindness.