Behavioral assessment of visual deficits in the mutant.

(tremor, ataxia, immobility, epilepsy, paralysis) mutants show a significant increase in myelin thickness from 10 to 30 days of age but then demonstrate a decrease in myelin thickness from 1 to 6 months. The severity of the demyelination in the optic nerve suggests that visual deficits may exist in the mutants. Animals were trained on a discrimination task, in which responses to a light stimulus (the S period) were reinforced on a fixed ratio (FR)-1 schedule, and responses in the absence of the light stimulus (the S period) were not reinforced. Following training, the light intensity presented during the S period was gradually reduced between sessions until -6.0 candela/m was reached. Both groups of animals - mutants and control Sprague Dawley rats - successfully recognized and responded in the presence of the stimulus near perfectly by the final day of training, suggesting that mutants demonstrated normal learning, at least under this paradigm. Despite the severe demyelination of the optic nerve, no visual deficits were detected as both groups of animals performed similarly as the light intensity decreased. Though the myelin loss of the optic nerve may have negatively affected signal transduction, this did not result in an increase in visual threshold.