Stable electrophysiological correlates of food-getting conditioned reflexes in dogs

Abstract

Previous work has shown that well-stabilized food-getting conditioned reflexes formed in response to stimulation of the nonspecifie thalamus in cats are expressed eleetrophysiologically as stable, late, surface-negative waves recorded at relevant points of the cortex immediately after recruiting responses (RRs) (9). These findings are in agreement with those obtained by Walter et al. (29) who showed in the same year in experiments on human subjects that during formation of conditioned-reflex connections, a prolonged surface-negative wave is recorded immediately after the evoked potentials. They called this the "contingent negative variation" (CNV). Admittedly, they were able to record this clearly only by means of an electronic averager. Subsequently, also by means of an averaging technique, similar late, surface-negative waves recorded immediately after the principal component of the evoked potentials were observed by Gasanov (2) in the auditory projection zone of cats with conditioned blinking reflexes to clicks. Sakhiulina et al. (11) also showed in experiments on cats, using low-frequency stimulation of a specific thatamic nucleus (VPL) or electrodermal stimulation as conditional stimulus, that conditioning is accompanied by appearance of a stable, late surface-negative wave recorded immediately after the evoked potential at each presentation of the conditional stimulus, and accompanied by external manifestation of the conditioned reflex. In these cases, however, late waves were more difficult to distinguish against the general cortical background activity than the late negative waves obtained after recruiting responses.Presumably this is because of the functional significance of the nonspecific thalamic system. Low-frequency stimulation of this system is accompanied by inhibition of background electrical activity, so that RRs are recorded particularly clearly. This is presumably why late components can be detected in a more pure, unmasked form. Probably another important factor relevant here is that afferent fibers of the nonspecific thalamus are connected through interneurons with all layers of the cortex and are particularly well represented in its superficial, plexfform layer (13, 20, 21), which is primarily responsible for surface-negative cortical potentials (7,15, 25, 28).For the foregoing reasons, in this investigation on dogs, conditioned reflexes also were produced in response to low-frequency stimulation of the nonspecific thalamus. The unconditional stimulus was a piece of meat which the dog obtained by a movement of one forelimb. Attention was concentrated mainly on the study of changes in the pattern of RRs at points in the cortical representation of the actively working limb, because according to Asratyan's theory, electrophysiological changes during conditionedreflex activity must be localized mainly at points of the cortex necessarily involved in the arc of that particular conditioned reflex. This theory has been confirmed by work in his laboratory (2, 3, 10, 11). METHODExperiments were car...