Transformation of evoked potentials of the rabbit visual cortex into wave-spike discharges

Abstract

Academy of Sciences of the USSR One of the phenomena used to analyze the subcortical "generator" of some normal and paroxysmal EEG rhythms is the electrographic correlate of petit real, i.e., a wavespike discharge with frequency of 3/sec. Models of this phenomenon so far developed have largely determined progress in this field, which has been particularly marked in recent years (37). Strictly speaking, however, none of these potentials can be accepted as the experimental equivalent of the true petit real potential. In fact, a wave-spike combination can be obtained by the classical method only with repetitive (3/sec) electrical stimulation of the intralaminar nuclei of the thalamus (29,32,37,39,40) or the mesencephalie reticular formation (49, 50) and, consequently, a response of this type is usually a rhythmic cortical response and not an epileptiform after-discharge (43).It has also been suggested that activity of this type arises as a component of the paroxysmal after-discharge in response to direct electrical stimulation of the cortical surface (43) or to its treatmentwithcertainpharmacological agents (30, 34). Meanwhile analysis of the investigations cited above makes it doubtful that these discharges are identical with the potential of petit real, and essential differences were in fact found in some of them (25, 44). Furthermore, thesephcnomena were shown to be local (31).Finally, existing models cannot be used to study how, and through disturbance of what mechanisms, a normal rhythm is transformed into a paroxysmal wave-spike discharge with frequency of 3/see. Admittedly, an attempt has been made to reproduce such a complex by modifying (administration of leptazol) the parameters of the normal evoked potential of the visual cortex (25), but in this case also repetitive photic stimulation was necessary to evoke a series of spike and wave complexes. Unfortunately, later workers have not tried to use this interesting approach to the study of true epilepsy.However, facts have now accumulated to justify resumption of work in this direction. Foremost among these are results indicating the fundamental similarity of some types of slow wave, including those evoked by photic and thalamic stimulation (14,24,37) and also the concept that late responses of neurons provides reactivation for producing repetitive waves (16, 22) and wave-spike cycles (40). METHOD#.xperiments were carried out on43 chinchilla rabbits. In the acute experiments the animals were immobilized with diplacin and transferred to artificial respiration. After preliminary infiltration of the points of fixationwithprocaine, the animals were secured in a type STM-3 stereotaxic apparatus. Chronic experiments were carried out on waking rabbits fixed to a frame limiting their movement. Photic stimuli (50 msec, 0.45-1.5 J) were applied from a Biofizpribor FFS-01 photophonostimulator located 20 cm from the animal's eye.In the acute experiments the potential was recorded from the exposed cortex by wick electrodes in a platinum loop. In the chronic experiments n...