Letter matching within and between the disconnected hemispheres

Three patients with complete cerebral commissurotomy from the California series were given two letter-matching tasks, one requiring physical identity and the other requiring nominal identity. The pairs of letters were presented unilaterally to each disconnected hemisphere or bilaterally, with each hemisphere receiving one of the letters to be compared. The disconnected hemispheres of all three patients showed good performance in the unilateral conditions, even when visual field and response hand were crossed. The crossed visual field-hand conditions resulted in both slower and less accurate responses. Only N.G. was able to cross-compare letters in the bilateral condition and only for physical identity. The results qualify previous reports that higher-level information can transfer subcallosally while visual information cannot.