Patients with compression of the visual pathways at the optic chiasma by a pituitary adenoma have been tested for temporal integration of visual form. The task requires that a stimulus, consisting of parts of a digit, persist in the visual system until it can be integrated with a complementary successive stimulus to give a unitary percept of the digit. Four interstimulus intervals (ISIs) were tested (15,30,45, 60 ms) by means of tachistoscopic presentations lateralized to either visual hemifield of either eye. Patients did not differ significantly from control subjects in overall accuracy or speed of recognition of the digits. However, they were impaired in accuracy of recognition in the temporal hemifields (nasal hemiretinae) at ISIs of 15 and 30 ms but not at longer ISIs (45 and 60 ms). These results suggest that visual persistence can be dissociated into two components: one lasting less than 45 ms that is markedly impaired by compression of the visual pathways, and another one lasting up to 60 ms (and probably longer) that shows no impairment. The former type of persistence might be analogous to ''visible'' persistence described by Coltheart (1980) and be mediated by neurones with transient response properties typical of the magnocellular system. The latter might instead be analogous to a longer-lasting form of persistence subserved by the sustained responses of the parvocellular system.
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