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Abstract

The perceptual organization of visual patterns is thought to draw upon hard-wired constraints of the sensory apparatus as well as active extraretinal influences such as attention. Elaborate automatic or ‘passive’ processing of a retinal pattern is perhaps best illustrated by the selective responses of neurons to complex objects in animals that are fully unconscious due to anesthesia. More ‘active’ elements of vision might be represented by phenomena such as multistability, where the impression of a constant sensory pattern continually changes. The present study examined the expression of active and passive mechanisms of perceptual organization in monkeys that were in a state of ‘dissociation’ following the administration of ketamine. Monkeys were trained to sit in a primate chair and accept both intravenous catheters as well as intramuscular injections. By administrating low doses of ketamine, the animals could be brought from wakefulness into a state in which they remained motionless with their eyes wide open. Visual processing was assessed by measuring eye movements generated in response to a variety of visual stimuli. Within a range of drug concentrations, strong but irregular optokinetic nystagmus (OKN) was elicited by moving patterns. When stimuli were presented that normally give rise to multistable perception, OKN alternately reflected each of the competing patterns, suggesting that perceptual mechanisms are active even during diminished states of consciousness. When moving stimuli were presented monocularly, OKN became asymmetric, suggesting that ketamine decreases the cortical contribution to OKN generation in the brainstem.