For most of us, visual perception is crucial for spatial navigation. We rely on vision to find our way around, to position ourselves and localize objects within the surroundings, and to plan our trajectory on the basis of the layout of the environment. Blind people would therefore seem to be at a disadvantage. Unable to rely on vision, they depend instead upon different sorts of cues to form their representations of space. They rely, for example, on proprioception, which provides a sense of the location, movement and posture of one's own body through space, and on vestibular information regarding changes in the rotational movements of the head.
To compensate for their inability to understand their environment visually, blind persons also store large amounts of non-visual information regarding the spatial organization of their environment, and are more reliant on this information than are sighted individuals. A team of Canadian researchers therefore predicted that blind people would perform better on spatial navigation tasks than sighted individuals, and that they would also show differences in the size of the hippocampus, the region on the inner surface of the temporal lobe which is involved in spatial memory. Remarkably, the study, which is published in the November issue of the journal Brain, confirmed both of these predictions.
Madelaine Fortin of the Université de Montréal and her colleagues recruited 19 blind participants and 19 sighted controls for their study. All the participants were asked to perform 3 spatial navigation tasks. The first of these was carried out in a series of human-sized labyrinths, of increasing complexity, measuring 9 m x 15 m (such as the one below). They were guided through each of the labyrinths once by the experimenters, along a pre-determined path, and encouraged to explore the space as much as possible. They were then taken back to the starting point, and told to try to follow the same path, while trying to make as few navigational errors as possible. The control group were blindfolded during the task, so that all participants relied on an acquired sequence of information to find their way.
In the second task, the participants were taken to three different locations within the same labyrinth, and asked to point as precisely as possible towards either the starting position or the last pointing position. Finally, the participants performed a spatial layout task. This was conducted in a small room adjacent to the labyrinth, which was configured in one of two ways, using the same panels as before. The participants explored the layout of the room; immediately afterwards they explored 5 small-scale models of the room with their hands, and made to choose wich of them represented the room. This procedure was then repeated for the second, more complex layout.
The blind participants were found to perform significantly better than the sighted controls on all three tasks, and clearly had a superior ability to learn complex paths through novel and unfamiliar environments, making fewer navigational errors as they re-negotiated the labyrinths. These differences were strongly correlated with differences in brain structure. Volumetric MRI used to measure the volume of the head, body and tail portions of the hippocampus. This analysis revealed that the head of the hippocampus was significantly larger in the blind than in the sighted particpants - the head volume was an average of 2527 mm3 in the experimental group, compared to 2250 mm3 in the controls. The two groups showed no difference in the volume of the body or tail of the hippocampus.
There were also differences within the experimental group. Although all the blind participants performed better than the controls on teh three tasks, the 12 who had experienced loss of vision before 5 years of age (the "early blind" sugroup) performed better than those who had lost their vision after the age of 14 (the "late blind" subgroup). Similarly, the volume of the head of the hippocampus was larger in the early blind than in the late blind group. This is likely due to differences between the two subgroups in the brain's re-organizational capacity - the plastic changes in the brains of the late blind participants were more limited than those in the brains of the early blind subgroup.
In their studies of London taxi drivers, Eleanor Maguire and her colleagues found a strong correlation between the amount of experience as a cabbie and the size of the hippocampus - the longer one had worked as a cabbie, the larger was the volume of their hippocampus. These differences were found in the tail of the hippocampus. By contrast, this study showed a difference in the head portion, which has not previously been implicated in spatial navigation, but is instead thought to be involved in verbal memory.
This study provides the first clear evidence that there are structural differences between the brains of blind and sighted individuals, with significant increases in the volume of the hippocampus of the former. It also shows that the conceptualization of space, which normally requires visual information, can develop normally in the complete absence of vision, as a result of plasticity in the head of the hippocampus. The findings suggest something about the strategies used by the blind in navigation: during the learning phases of the labyrinth tasks, the blind participants may have stored path information in the form of verbal commands rather than learning a representation of the space based on their own position.
Related:
- Developmental topographagnosia
- Seeing with sound: The boy who echolocates
- Channelrhodopsin restores vision in blind mice
- Restoring sight to the blind with deep brain stimulation
Fortin, M. et al (2008). Wayfinding in the blind: larger hippocampal volume and supranormal spatial navigation. Brain 131: 2995-3005. DOI: 10.1093/brain/awn250
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