The effect of thalamic lesions upon delayed response-type tests in the rhesus monkey.

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Abstract
By means of the Horsley-Clarke stereo-taxic instrument, electrolytic lesions were placed in the thalamus of 20 monkeys, destroying up to 80% of medialis dorsalis bilaterally and adjacent areas to a lesser extent. Eleven subjects received pre- and postoperative training to criterion on either spatial delayed response or on spatial delayed alternation and a visual pattern discrimination. Nine subjects received such training to criterion only postoperatively. At the end of postoperative training all animals were sacrificed, the brains sectioned, and the lesions reconstructed. All subjects met a postoperative criterion of 85 correct responses in 100 trials on either delayed response or delayed alternation and visual discrimination problems. Animals trained preoperatively on delayed alternation and visual discrimination showed significant savings. Postoperative initial learning of delayed alternation and visual discrimination was not significantly slower than pre-operative learning. Postoperative initial learning of delayed response required no more trials than preoperative learning. It is concluded that the inability of frontal-damaged animals to perform correctly on delayed response-type tests is not attributable primarily to a loss of cells in medialis dorsalis.