Prominent among the many bizarre consequences of taking LSD are visual hallucinations and altered visual perceptions. So in investigating LSD's remarkable activity, it makes sense to look at the nerve cells of the visual cortex, and see how LSD affects their responses. David Rose and Professor Gabriel Horn at Bristol University have done this in the cat brain, and find a complex range of effects. (Experimental Brain Research, vol 27, p 71). Cats are good animals to study, for one thing because their visual systems are better understood than any other animals', and for another because LSD has suitably bizarre effects on their behaviour, albeit in larger doses weight for weight than a man needs. 100 mcg of LSD not only makes a cat more active and playful, but also induces new activities, like flicking a paw, making half attempts at grooming, or, occasionally, reacting to non-existent objects as if hallucinating. The cats Rose and Horn used were anaesthetised, and the experimenters recorded signals from individual nerve cells in the cortex when the cats were given 200 mcg of LSD. These cells res pond to a bright bar in a particular orientation moving across the field of view. With the bar at the preferred angle, LSD usually decreased the res ponses, particularly in the smaller cats for whom the dose was effectively larger. But some nerve cells became more r esponsive, and others were un affected. There were also various effects with bars set at wrong angles, so that the receptive patterns of the nerve cells sometimes changed slightly. The results are not simple to interpret, since with the relatively high doses used, awake cats respond less than with lower doses. It seems likely that all the changes resect altered input to the cortex from further down the visual pathways, in particular from the cells of the lateral geniculate nucleus, which respond in the same ways to LSD although their recep - tive fields are simpler. These in turn seem to be controlled by altered signals from elsewhere in the brain. The only place so far found to be directly affected by LSD is in the brainstem, and effects down there might possibly be the ulti mate source of the weird disturbances in the visual cortex. Be that as it may, the overall effects in the cat's visual cortex–the slight changes in receptive fields, and the dif - ferent behaviour of different cells– could well be the neurological equiva lents of the alterations in subjective perception reported by humans.