Hallucinogenic drugs, such as LSD, Mescaline, delta 1-THC, can exert profound effects upon visual perception. We propose to study the cortical mechanism of action of these abused drugs. Neurons in the visual cortex of the cat and the monkey respond selectively to particular aspects or features of an incoming stimulus pattern and may act as part of an active filtering system, emphasizing some features of a pattern while suppressing others. Many of the perceptual abnormalities induced by hallucinogenic drugs appear to involve disturbances in the process of attending selectively to the visual world, either by alteration of the elements attended to, or their relationship to one another. We propose, therefore, to determine whether hallucinogenic drugs (LSD, Mescaline, delta 1-THC) can perturb the response characteristics of single neurons in the visual cortex of the cat, or whether they can alter the interactions between these cells. We propose to use: (1) conventional electrophysiological techniques to study drug-induced disturbances in the response properties of single neurons in the cat's visual cortex, (2) chronically implanted arrays of recording microelectrodes to neurons, (3) a novel procedure we have developed to assay modifications in intracortical interactions produced by administration of these abused drugs. These studies should provide some insight into the neuronal mechanisms underlying the hallucinogenic effects of abused drugs.