One of the longstanding puzzles of brain development is why, in some cases, individuals with developmental disabilities sometimes show enhanced function, rather than a more typical loss of cognitive function. In the case of Williams Syndrome – which is caused by a hemizygous deletion of a cluster of about 25 genes on 7q11.23 – children show a mild form of mental retardation but also a notable increase in gregarious and social behavior. How might a genetic deletion lead to a gain of function ? In a recent paper by Sarpal and colleagues [doi:10.1093/cercor/bhn004], they explore the role of the visual cortex and its role in feeding and filtering information to emotional regions of the brain.
From its receipt of visual information from the eyes – say perhaps, you’re looking at someone’s face, the primary visual cortex parses information into 2 separate streams – a dorsal stream which is good at processing “where” information related to location; and a ventral stream which is good at processing “what”information related to identity and recognition – and moreover, provides inputs to the prefrontal and amygdala (brain regions which are important for social behaviors). What if the genes deleted in Williams Syndrome altered the development of a part of visual cortex that participates in early visual processing to alter the relative balance of dorsal to ventral processing ? Might it result in a an individual who was better than usual at processing objects (faces) and also showing related emotional traits ? Indeed, this has been a longstanding hypothesis that has since been supported by findings that show relatively intact ventral stream processing but disrupted dorsal stream processing.
In their current paper, Sarpal and colleagues measured brain activity as well as correlations of activity (connectivity) between brain regions as patients with WS passively viewed visual objects (faces and houses). They report that connections from early visual processing areas (fusiform and parahippocampal gyrus) in WS are actually weaker to the frontal cortex and amygdala. Since activation of the frontal cortex and amygdala are associated with inhibition and fear, it may be case that the weaker connections from early visual areas to these regions gives rise to the type of gregarious and prosocial (a lack of fear and inhibition) behavior seen in WS. In further pinpointing where in the brain the genes for WS might be causing a developmental change, the authors point to the ventral lip of the collateral sulcus, an area situated between the fusiform and parahippocampal gyri. This may be the spot to more closely examine the role of genes such as LIMK1 – a gene that participates in the function of the actin cytoskeleton (an important process in synaptic formation).
This lecture by V.S. Ramachandran covers some of these pathways with respect to Capgras Syndrome.