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Cognitive performance and cortical reorganization in blindness

We are all familiar with the situation of visually searching for our keys haphazardly left somewhere. The obvious strategy is just to search in the most likely places. But if it is pitch dark, tactile search may be difficult and time consuming. You might try to spend more time recalling your moves in an effort to allocate your keys then simply search for them. Such situations are the everyday experience of a blind person. As another example, have you ever tried following the developments of a basketball game on the radio rather then watching it on TV. In such a case, you must rely on verbal rather than visual information. Blind people are therefore likely to depend more on memory in general, and on verbal memory in specific, to interact with the world. Indeed, some studies indicate that the congenitally blind have superior verbal memory abilities. Anecdotally, our ancestors must have been aware of this. In ancient times the blind often served as a 'living database' of the interpretations of the bible (which were passed from generation to generation by word of mouth), and a quotation of a biblical source by a blind person was considered the most reliable. As the Talmud says "The traditions cited by Rabbi Sheshet are not subject to doubt as he is a blind man" (Talmud Yerushalmi, Shabbat 1b).

How does this come about? Humans, like other primates, rely primarily on vision to direct their behavior. The areas devoted to vision constitute a substantial part of our brain (~25% of the primate brain). In the past, the prevailing thought was that loss of vision due to blindness renders these regions useless. Recently, though, evidence has been accumulating that the 'unemployed' occipital cortex, which usually subserves vision, may be utilized in the blind for other purposes. Neuroimaging techniques have indicated that the occipital cortex of congenitally blind people is active during Braille reading. Furthermore, transcranial magnetic stimulation of the occipital cortex disrupts the blind participants' ability to identify correctly Braille letters. These findings and others that followed led to the hypothesis that the occipital cortex of the blind is recruited for tactile information processing.

Yet Braille reading involves more than just fine tactile judgments, since any reading obviously engages cognitive (language related) facilities as well. We recently found using functional imaging (fMRI) in the congenitally blind, that extensive regions within the occipital cortex are activated not only during Braille reading, but also during performance of verbal memory tasks, such as recalling a list of abstract words. One of these activated regions was the primary visual cortex or V1, which is the central gateway for visual information processing in the normal human brain. In contrast, no such verbal memory related activation was found in V1 of the sighted control group. This V1 activation, unique to the blind, was mirrored by superior verbal memory skills of the blind as a group, compared to their sighted peers. As usual, significant variation exists among the blind in their verbal memory performance. But most important, the magnitude of V1 activation during the verbal–memory condition was highly correlated with the blind individual's abilities in a variety of verbal–memory tests, suggesting that the additional occipital activation may have a functional role. Thus, subjects that showed greater occipital activation (in its extent and magnitude), were usually the ones to score best in the verbal memory tests used. These correlations were most dramatic in V1, were specific to tasks involving verbal memory, and were not explained by variation in the subjects' intelligence quotient (IQ), or education.

We also found evidence for topographical specialization in the reorganized occipital cortex of the blind. While the more anterior regions of the occipital cortex show preference for the tactile Braille condition, V1 showed for conditions involving verbal memory. This gradient may reflect 'reverse hierarchical' organization. Visual processing in the sighted brain is typically described in hierarchical terms. Cortical responses in V1 are generally governed by the basic physical aspects of the stimulus (such as bar size, orientation or direction of motion). As one moves to higher visual processing regions, the stimulus eliciting the greatest response becomes more complex. The neurons in those areas show relative invariance to the object defining cue, size, position, viewpoint or even modality. This probably reflects a more abstract representation in these putative object related regions. Interestingly, in total absence of visual input this hierarchy seems to be reversed. Thus, early visual regions are active during more 'cognitive' functions (such as verbal memory) while the more anterior regions are more 'sensory' (showing robust tactile activation). It is currently unclear how this reorganization comes about. One possibility is that it might be based on existing anatomical connections between visual cortex and classical memory areas in the frontal cortex and temporal lobes. Some of these projections to the visual cortex are much more extensive in the newborn, but attenuate gradually. In congenital blindness, these feedback pathways might be enhanced due to the lack of competition from visual input, giving rise to the functional plasticity reported here.

To summarize, we have reasons to believe that the visual cortex may undergo a dramatic reorganization during the first years of life, to be recruited for high-level cognitive functions. There is some evidence that this plasticity is much more limited in people blinded at later stages. The exact cognitive components that are responsible for the occipital activation (memory?, language?) as well as the functional significance of this cortical reorganization, are still unknown. This study will potentially open a window for understanding cortical plasticity in brain systems, which is crucial for future treatment of neurodegenerative diseases. Furthermore, if we know more about how this cortical reorganization can take place, and stimulate it by proper training, we can possibly give blind people an advantage that will serve them throughout life.