Vision is so important to humans that it is difficult to imagine how we can produce a conscious model of the world without it. And what is done with the third of the cortex that is involved in vision when it is idle. Kupers and others (see citation) have been comparing fMRI scans using congenitally blind, blind that were once sighted, sighted and blindfolded sighted individuals.
How do individuals who never had any visual experience since birth form a conscious representation of a world that they have never seen? How do their brains behave? What happens to vision-devoted brain structures in individuals who are born deprived of sight or who lose vision at different ages? To what extent is visual experience truly necessary for the brain to develop its functional architecture? What does the study of blind individuals teach us about the functional organization of the sighted brain in physiological conditions?
It is known that the cortex has capacity for plasticity and reorganization when input from a sense is lost. Other senses will use the spare cortical areas. Studies have shown that there are changes in the grey matter, the white matter under it, and the cell metabolism during this reorganization. In the blind, the occipital cortex (visual cortex) becomes involved in other senses and in a variety of cognitive functions including: lexical, semantic, phonological, attention, verbal memory, working memory.
A part of the cortex (extrastriate ventrotemporal cortex) is concerned with recognizing objects, a function that is very important to acquiring knowledge of the external world. Different categories of object give specific activity patterns in this region, termed object-form-topology. This processing relies heavily on vision. But blindfolded people who recognize an object from feel show very similar patterns to those that occur when they use sight. The patterns are supramodal – they do not depend on any particular sense.
The findings in the congenitally blind subjects are important also because they indicate that the development of topographically organized, category-related representations in the extrastriate visual cortex does not require visual experience. Experience with objects acquired through other sensory modalities appears to be sufficient to support the development of these patterns. Thus, at least to some extent, the visual cortex does not require vision to develop its functional architecture that makes it possible to acquire knowledge of the external world.
So the ventral ‘what’ pathway can process without vision. What about the dorsal ‘where’ pathway? Is spatial processing possible without vision? Yes, the dorsal pathway can use senses other than sight and does not require visual experience to develop. We process motion per se.
Both optic and tactile motion provide information about object form, position, orientation, consistency and movement, and also about the position and movement of the self in the environment.
And when they looked at mirror neurons, they found the same condition. Vision is not necessary for the development of a functional efficient mirror neuron system. This suggests that abstract representation of actions is also not tied to any particular sense.
The main hypothesis that we have put forward here is that the development of consciousness in the absence of vision is made possible through the supramodal nature of functional cortical organization. The more abstract representation of the concepts of objects, space, motion, gestures, and actions – in one term, awareness of the external world – is associated with regional brain activation patterns that are essentially similar in sighted and congenitally blind individuals. The morphological and/or functional differences that exist between the sighted and the blind brain are the consequence of the cross-modal plastic reorganization that mostly affects that part of the cortex that is not multimodal in nature.
What about the experience that results from the reorganization in the blind? It appears that the type of qualia is connected to the source of the input not the region that processes it.
The results of these TMS studies constitute the first direct demonstration that the subjective experience of activity in the visual cortex after sensory remapping is tactile, not visual. These findings provide new insights into the long-established scientific debate on cortical dominance or deference. What is the experience of a subject in whom areas of cortex receive input from sensory sources not normally projecting to those areas? Our studies suggest that the qualitative character of the subject’s experience is not determined by the area of cortex that is active (cortical dominance), but by the source of input to it (cortical deference). Our results are in line with evidence that sensory cortical areas receive input from multiple sensory modalities early in development.
Kupers, R., Pietrini, P., Ricciardi, E., & Ptito, M. (2011). The Nature of Consciousness in the Visually Deprived Brain Frontiers in Psychology, 2 DOI: 10.3389/fpsyg.2011.00019