E.F. Vastola

Visual radiation activity during a cortical penicillin discharge

The electrical behavior of 274 principal cells of the dorsal nucleus of the lateral geniculate body of cat has been studied during the discharge of a penicillin focus in the primary visual cortex. In 17% of the cells, spike activity was observed during the penicillin discharge. The activity consisted of 1–40 spikes at frequencies as high as 500/sec. When the penicillin discharge initiated a prolonged seizure, activity in the principal cells continued throughout the seizures at rates from 100–200/sec. The temporal properties of the activity and the detailed structure of single spikes studied with glass microelectrodes in the geniculate suggest that this activity is antidromic. Stimulation of the visual radiation at its origin in the geniculate at various times during a penicillin discharge revealed marked reduction in amplitude of the population response recorded at the terminus of the radiation in the cortex. The degree of reduction followed approximately the time course of the penicillin discharge and was great enough to suggest two additive etiologies, viz., refractoriness or occlusion during the penicillin discharge and depolarization of the radiation fiber terminals.

Binocular inhibition in the lateral geniculate body

Abstract Afterpositivity elicited in the dorsal nucleus of the lateral geniculate body of cats by stimulation of either of the optic nerves was found to be closely associated with depression of the postsynaptic spike and afterpositivity elicited by a test stimulus to the other optic nerve. The radiation spike was found to be more vulnerable to the inhibitory effect of afterpositivity than the postsynaptic spike recorded in the lateral geniculate body (LGD). Anatomical and electrophysiological evidence is adduced in support of the hypothesis that much of the afterpositivity is not a positive afterpotential but a hyperpolarizing postsynaptic potential propagated among principal geniculate neurons by neurons of the short axon type which are activated by recurrent collaterals from radiation axons.

Suppression by anticonvulsants of focal electrical seizures in the neocortex

Abstract 1. 1. A description is given of focal electrical seizures elicited in the visual cortex of essentially unanaesthetized cats by direct and restricted repetitive electrical stimulation of the same region of cortex. 2. 2. These seizures were markedly depressed by Dilantin and phenobarbital in doses of the order found to be effective in terminating status epilepticus in man. Nembutal in similarly small doses exhibited approximately the same action. Tridione had no significant effects. 3. 3. It is suggested that this technique might be of value in predicting the clinical action of new anticonvulsants.

Unit signs of visual cortex modulation by the lateral geniculate body.

Abstract In unanesthetized cats with intercollicular transection of the brain-stem, significant positive correlation was found between the amplitude of the population response in striate cortex to stimulation of the visual radiation and the number of spikes discharged in the preceding second by spontaneously active units in the dorsal nucleus of the lateral geniculate body. Correlation was usually highest for units discharging in the 250 msec interval immediately prior to stimulation but in some cases correlation was found with activity in the interval 500–1000 msec prior to stimulation. Spontaneously active units were not found in those preparations which did not exhibit variable amplitude of the cortical response. In conjunction with the observations of an earlier report (Rosen and Vastola 1962), these results strongly suggest that the variation in amplitude of the cortical response under our experimental conditions is determined in the main by modulatory activity in the geniculate body which is carried to the cortex by the visual radiation. It also seems probable that this geniculo-cortical system mediates some of the modulatory influences upon the cortex of reticular and thalamic activity during other experimental conditions, e.g. , arousal and alerting.

Conduction velocities in single fibers of the visual radiation

Abstract Antidromic activation of a unit in the dorsal nucleus of the lateral geniculate body is defined as a response which follows stimulation in the striate cortex with a latency less than 0.8 msec or which exhibits both of the following characteristics: constant latency throughout the period of observation for all suprathreshold stimulus intensities; and continuous, inverse relationship between the latency of the response to the second of two equal stimuli and the time interval between the two stimuli at small values of the interval. Conduction velocities of antidromic spike propagation in seventy-three such units were found to occupy the range between 8 and 100 m/sec with relatively dense distributions at 18, 33 to 40, and 67 m/sec. Reasons are given for the conclusion that most, if not all, of these units projected to the striate cortex in the posterior portion of the lateral gyrus. The population response predicted by the observations of this sample agrees well with those recorded by macroelectrodes for the cases of both antidromic and orthodromic propagation.

Antidromic activity in visual radiation during evoked cortical responses

1. Antidromic stimulation of the visual radiation of cats has been used to investigate the possibility that some of the activity in principal geniculate cells following an optic tract stimulus is antidromic. Single spikes were selected from two classes of poststimulus activity to condition the antidromic test spike--the undoubted orthodromic postsynaptic spike and the later spikes, occurring up to 100 ms after the optic tract stimulus. 2. In 15 of 39 cells the minimum antidromic activation times and the minimum spike-spike intervals were found to be shorter and latencies for antidromic stimulation were longer when the conditioning spikes belonged the class of late poststimulus activity. The differences are in accord with the assumption that some of the conditioning spikes were antidromic. 3. Test spikes were frequently found to have long and variable latency when the conditioning spike occurred more than 45 ms after the optic tract stimulus. Possible reasons are briefly discussed. 4. It is suggested that antidromic activity may occur in conditions of the cortex that are more physiological than those associated with a penicillin-induced seizure focus. Some possible mechanisms and functional significance are briefly discussed.

Geniculate interactions with a penicillin discharge in visual cortex.

Abstract Spontaneous activity of principal cells in the dorsal nucleus of the lateral geniculate body was studied 1 sec before and 1 sec after penicillin discharges in the striate cortex. Many cells exhibited increased activity prior to the penicillin discharge suggesting that corticopetal activity from the geniculate is capable of triggering the penicillin discharge. Many cells also exhibited increased activity for as long as 1 sec after the discharge. Some cells displayed both phenomena. Two patterns of transient depression were also observed in the postdischarge period. The latter phenomena are believed to be related to the inhibitory process associated with after-positivity which follows orthodromic or antidromic stimulation of the geniculate body.

Afterpositivity in the lateral geniculate body during repetitive stimulation

Abstract Afterpositivity elicited in the dorsal nucleus of the lateral geniculate body of unanesthetized cats by a single shock to the optic nerve has been described previously. The experiments to be described below were performed in order to study afterpositivity under a condition of repetitive stimulation which resembled to some degree the normal state. Repetitive stimulation was applied to the optic nerve and to the visual radiation fibers. Responses were recorded in the lateral geniculate body with a bipolar capillary electrode connected to the amplifier through calomel half-cells. Afterpositivity induced by the onset of both orthodromic and antidromic high-frequency stimulation was found to be augmented compared with the response to a single stimulus. It was also found that afterpositivity is elicited by sudden increases in stimulation frequency, and that, for a given final frequency, its amplitude decreases with increasing initial frequency. It is suggested that this phenomenon is the electrical sign of a process which plays a significant role in the determination of thresholds for perception of changes in light intensity.