Where and what visuospatial processing in adolescents with congenital hypothyroidism

Visuospatial processing is accomplished in distinct neuroanatomic pathways. One such pathway, known as the where pathway, involves a dorsal route thro ugh magnocellular thalamic cells to occipital and parietal cortices and con veys location and motion information. A second pathway, known as the It wha t pathway, involves a ventral route through parvocellular thalamic cells to occipital and temporal cortices and conveys color and form information. Th e where pathway is thought to he responsible for processing spatial relatio nships while the what pathway is responsible for object identification. Chi ldren with early-treated congenital hypothyroidism (CH) who exhibit selecti ve visuospatial deficits may provide a good model to study the differential development of these pathways. Because children with CH lacked thyroid hor mone at a time when needed by developing brain regions such as the parietal cortex, these children may be affected to a greater degree on tasks tappin g where but not what pathway processing. We tested this hypothesis via retr ospective analysis of their performance on 6 spatial tasks. Compared were 4 9 adolescents with CH and 49 matched control participants. On the basis of confirmatory factor analysis, tasks were assigned to either where or what p athway groupings. A repeated measures ANOVA showed the CH group was impaire d relative to a normal comparison group only on where pathway tasks. Regres sion analyses indicated that severity of early hypothyroidism was the stron gest predictor of M here pathway processing but had no effect on what pathw ay tasks. It is concluded that thyroid hormone is required during late gest ation and early life for the normal development of the where aspects of vis uospatial processing.