The influence of thalamic stimulus parameters on primary and augmenting cortical intracellular potentials

1. 1. Cortical surface and intracellular recording in neurons at depths of 0.5-1.5 mm was carried out in the precruciate cortex to study the ECoG and PSP patterns evoked by conditioning and test VL thalamic stimuli of graded intensity. 2. 2. During the surface augmenting response evoked by test stimuli at C-T intervals of 60-180 msec test PSPs were interacted with the declining IPSP evoked by the conditioning stimulus in most neurons. Quantitative changes were noted in the early PSP pattern of response, when compared to the PSPs evoked during the primary response. The changes consisted of reduction of amplitude of the initial EPSP complex, correlated with depression of the surface positive wave of the primary response pattern, marked augmentation of the secondary EPSP complex, correlated with the appearance of the surface positive wave of the augmenting response, and blocking of the IPSP which is generated at the same time as the secondary EPSP during the primary response. 3. 3. A late, tertiary, EPSP was found during the augmenting response, correlated with the surface negative wave of the response. 4. 4. Increasing the intensity of the conditioning stimulus increased the amplitude of the augmenting response and the magnitudes of the PSP changes associated with it. Increasing the intensity of the test stimulus reduced the amplitude of the augmenting response and returned the PSP pattern toward a primary response configuration. 5. 5. The results suggest the activation of two specific thalamo-cortical organizations or projections in different proportions during the early phases of evoked primary and augmenting responses.