This paper presents the results of an investigation using a new instrument for the study of brain damage in human subjects. The design of the instrument, the Continuous Performance Test (CPT) was based on certain electroencephalographic evidence which suggested that brain-damaged individuals should show inferior ability as compared with non-brain-damaged individuals on tasks requiring sustained attention or alertness.The waking EEGs of brain-damaged patients generally show either random bursts of hypersynchronous (high amplitude) activity intruding upon the normal activity of the brain, or a general hypersynchrony (3, 8). Hypersynchronous activity is also evident in 1 The authors would like to extend their appreciation to the following individuals and institutions for providing advice and/or subjects and testing facilities for this research: Mr. Samuel Greenhouse of the

If lias been repeatedly confirmed that frontal lobe lesions in monkeys produce the syndrome of a severe postoperative deficit in performance on delayed response-type tests but not on visual discrimination problems (5,6,13, 16). The thalamus of such animals invariably contains areas of homolateral retrograde degeneration in the lateral parvoccllular portion of nucleus mcdialis dorsalis (1,13,17). The question arises whether thalamic degeneration alone would result in the frontal lobe syndrome. Two studies suggest that, it might not. Chow (3) reported thai four monkeys with restricted electrolytic lesions in mcdialis dorsalis showed postoperative savings on a group of problems which included a delayed response lest. Freudenberg el al. (4) described one rhesus monkey with unilateral destruction of medialis dorsalis which demonstrated an intact "memory span" 15 days posfoperatively.The present study was designed to evaluate the effect of bilateral lesions in medialis dorsalis upon the spatial delayed response, spatial delayed alternation, and visual pattern discrimination performance of rhesus monkeys. MKTI1OI) Surgical ProceduresAnimals were anesthetized with 5 per cent Nembutal (0.8 cc/kg. body wt.) and secured firmly in the Horsley-Clarke slcreolaxic instrument, (Pfeiflcr modification). With aseptic technique a skin incision approximately 5 cm. in length was made across the mid-line about 1 cm. posterior to the vertex. Single bilateral burr holes were joined across the mid-line and enlarged to provide an opening approximately 2 cm. by 4 cm. A small semicircular dural flap was turned on each side.

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