Last year in Atlanta, my abstract for the Society for Neuroscience (SfN) was chosen abstracts to be part of their press book release. Every year SfN choses 700 abstracts out of over 14, 000 to release to the media. Here is a copy of that abstract.
Volumetric analysis of the hippocampus in early blind subjects.
Society for Neuroscience 2006
Press Book Summary by Daniel-Robert Chebat and Maurice Ptito
This preliminary study is part of an ongoing research program concerned with the anatomo-functional re-organization of the brain resulting from sensory deprivation at birth. We report here that the hippocampus, a structure involved in learning and memory, is significantly reduced in volume compared to normal seeing controls. This reduction concerns mainly the posterior part of the right hippocampal formation.
This finding is novel and interesting because it supports the hypothesis that the hippocampus is involved in the formation of spatial visual maps of the environment. Since our subjects have been blind from birth, the posterior part of the hippocampus showed atrophy.
Previous studies carried out on blinds have shown that these subjects are not impaired on a spatial competence level and that they maintain the capacity to form spatial maps of the environment when using tactile or proprioceptive and/or auditory cues. The absence of vision however complicates the encoding of spatial maps due to the lack of readily available spatial information. Extensive navigational training in normal subjects induces plastic changes in the hippocampus. For example, London taxi drivers show a larger posterior hippocampus compared to controls!
This study emphasizes therefore the importance of vision in the development of brain structures and the role of the posterior part of the right hippocampus in navigational skills that involve visual cues. We have recently shown that the visual pathways of born blind subjects are largely atrophied (Schneider, Kupers and Ptito, 2006). The striate and extrastriate visual areas have a reduced volume and both afferent and efferent fibers are altered.
The question arises then on how do blind people move around in their environment and what are the cerebral structures involved? It is known that spatial representation of the environment can be encoded through other sensory cues such as somesthesis, touch, audition and proprioception. These maps are not exclusively carried out by the hippocampus itself but rather in tandem with other cortical regions. The anterior insula/ventrolateral prefrontal cortex (AI/VP) and parietal cortex (PC) are most likely candidates. AI/VP is associated with the coding of auditory cues and spontaneous route planning and PC is involved in the planning of movements through immediate space when no visual cues are available.
Although navigation requires visual cues for the formation of spatial maps that involves the right hippocampal formation, early blinds are still able to build novel spatial maps of the environment when using tactile, proprioceptive and/or auditory cues. We hypothetize therefore that these map formations are carried out outside of the hippocampus and probably involve the parietal cortex.
These results have never been published and to our knowledge nobody has previously shown that blindness leads to the atrophy of the right posterior hippocampus, a brain structure known to be involved in the formation of visual maps.
These data were collected from a rather large sample of blind subjects and seeing controls using a double blind protocol with two types of analysis from magnetic resonance images (MRI) : volumetric analysis through segmentation of the hippocampus and voxel-based morphometry (VBM). Both approaches yielded the same results.
What would be interesting to do next is Tensor Diffusion Imaging to highlight the nature of the connections (white matter) between the hippocampus and the other cortical areas and correlate anatomy and behavior through route learning.