|The Wiring is Not Right: Congenital Prosopagnosia|
Marlene Behrmann, Carnegie Mellon University
A puzzling disorder that has garnered recent attention from many researchers is congenital prosopagnosia (CP). This disorder refers to the lifelong difficulty in face recognition in individuals who do not have any obvious neurological damage. Moreover, these individuals have normal sensory vision and normal intelligence. The underlying basis of this disorder remains to be determined. Using diffusion tensor imaging and tractography, a novel technique for mapping out the white matter tracts in individuals, Thomas and colleagues from Marlene Behrmann's laboratory at Carnegie Mellon University discovered that a disruption in structural connectivity in ventral occipito–temporal cortex may be the neurobiological basis for CP. Of importance, there was a reduction in both macrostructural (such as the density and volume) of these tracts as well as the miscrostructural (how coherently the fibers run together) in these tracts. There was also a correlation between the extent of the behavioral impairment and the reduction in the one tract, the inferior longitudinal fasciculus (ILF), in the right hemisphere (see Figure below). The investigators, affiliated with the Temporal Dynamics Learning Center funded by NSF, found
no differences between the CP and their matched controls when they mapped out other fiber tracts in their brains, suggesting that this is not a general problem of the brain. Instead, the problem appears restricted to the tracts that are part of a circuit that underlies face recognition. These findings suggest that it is critical that the white matter fibers in ventral occipito–temporal cortex are intact to support the distributed cortical network that subserves normal face processing. This result is intriguing and suggests a possible model for other neurodevelopmental disorders. Also, given that CP runs in families and is hereditary, the compromised neural circuit may reflect an underlying genetic disorder and so this offer us the promise of uncovering the genetic basis for CP and other related disorders.
Diffusion tensor tractography results of the inferior longitudinal fasciculus (ILF) and the inferior fronto-occipital fasciculus (IFOF) in (a) a single individual with congenital prosopagnosia and (b) a matched control participant. Note the paucity of both fiber tracts in the former compared with the latter, however, only the reduction in the right ILF was correlated with the decrement in the behavioral recognition of faces (from Thomas et al., 2008).
Reference: Thomas, C., Avidan, G., Humphreys, K., Jung, K. J. , Gao, F. and Behrmann, M. (2008). Reduction in white matter connectivity in ventral temporal cortex may account for face processing impairment in congenital prosopagnosia, Nature Neuroscience, Advance online publication.