ESTIMATES OF THE NET EXCITATORY CURRENTS EVOKED BY VISUAL-STIMULATIONOF IDENTIFIED NEURONS IN CAT VISUAL-CORTEX

The action potential discharge response of single neurons to both visu al stimulation and injections of current were obtained during intracel lular recordings in cat visual cortex in order to estimate the net exc itatory current arriving at the soma during visual stimulation. Of 45 neurons recorded intracellularly, 19 pyramidal neurons and one basket cell were labelled with horseradish peroxidase. The discharge of all n eurons adapted to constant current. For 40 neurons, a single exponenti al provided a good fit to the adapting discharge (r(2) = 0.73 +/- 0.03 ) for all current intensities. Superficial layer neurons were signific antly faster adapting [P < 0.001, mean (+/- SEM) time constant of adap tation = 11.5 +/- 1.3 ms; n = 20] than deep layer neurons (mean time c onstant of adaptation = 51.4 +/- 6.4 ms; n = 10). The percentage adapt ation of the spike frequency, %(peak - adapted rate)/peak, was determi ned from the fitted exponential. Superficial layer neurons adapted sig nificantly more strongly (P < 0.01, mean = 67 +/- 3%) than deep layer neurons (mean = 51 +/- 5%). The mean firing frequency in response to a current step of 320 ms duration had a linear relationship to the ampl itude of the injected current (slope 66 spikes/s/nA; origin zero, mean r(2) = 0.94; n = 33). This relationship provided a means of estimatin g the net peak excitatory current generated by Visual stimuli. The est imated mean peak somatic current during the passage of a bar across th e receptive field was 1.1 nA and the average current for the duration of the visually evoked discharge was 0.64 nA (n = 17). The transfer re sponse of real and model neurons was obtained by differentiating the d ischarge response to a step input current and was then used to predict the output of the neuron following an arbitrary input. When these tra nsfer responses were convolved with known input signals in model neuro ns, the predicted output was close to the simulated response of the mo del neuron to the same input waveforms. The transfer response was calc ulated for eight real neurons. Estimates of the net excitatory current arriving at the soma during visual stimulation was obtained by deconv olution. The mean peak somatic current for these neurons was 0.62 nA.