Summation of certain enduring sequelae of cortical activation in the rabbit.

Abstract In 32 rabbits, carried under high cord transection and infiltration of sensitive areas with novocaine, steady potential (SP) was studied using transcortical recording from the visual cortex by calomel half-cells and a “chopper” system. From immediately adjoining silver-silver chloride wire electrodes the usual ECG was recorded. Strychnine in different concentrations was applied to the surface of the cortex in some animals; in others primary visual responses were elicited by single and repetitive stimuli at different intensities applied to the contralateral optic nerve and the homolateral geniculate nucleus. 1. 1. Relatively long enduring (1–4 sec.) after-effects detected by the chopper method commonly followed the strychnine spikes and primary visual responses. 2. 2. Clusters of strychnine spikes produced somewhat longer and higher amplitude after-effects than single ones, suggesting summation. Trains of 2–3 primary visual responses evoked by very brief electrical stimulation did not summate, however. 3. 3. Repetitive stimulation of optic nerve for 2–10 sec. led to complex summations. Early in experiments the predominant polarity of the summated after-effect resulting from repetitive stimulation of the nerve was negative regardless of stimulus strength. Later under identical conditions of stimulation but following any major long-lasting SP shift, the summated after-effect was positive. 4. 4. Repetitive stimulation of the lateral geniculate nucleus caused somewhat different changes. At a stimulus strength maximal for the primary visual response the negative summation was replaced by a positive one, which was frequently accompanied by paroxysm. As with the optic nerve, following any major SP shift the summated after-effect changed and was positive at any stimulus strength. At this time repetitive stimulation of the lateral geniculate nucleus at maximum for the primary visual response resulted in a marked positive shift in SP which was most always accompanied by a sustained after discharge in the ECG. As the after-discharge continued to increase in voltage, the surface-positivity became more marked, and the latter did not disappear in a negative shift until the termination of the paroxysm. This pattern of SP shift with paroxysm was observed many times, and the only exception we have observed relates to the occasional appearance of fast spiky paroxysm associated with negativity in SP. 5. 5. It was concluded that relatively long-enduring positive and negative after-effects of primary visual response and of strychnine spikes occur more or less concomitantly, and the dominant polarity as detected in the records is determined by the algebraic summation of the opposing disturbances. 6. 6. Although summation of positive after-effects may account for the accumulation of positivity during paroxysm, and the summation of the opposing after-effects for the subsequent negative change that develops rapidly as paroxysm disappears, significant exceptions occur suggesting the existence of yet undiscovered factors. One such exception occurs when significant positive shift precedes the development of paroxysm.