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1. We have investigated the conductivity of neural pathways in slabs of unanaesthetized, isolated, cerebral cortex, cut from the isolated forebrains of twenty-five cats.

2. Neurones within the isolated area were indirectly excited, either by a small electrode thrust into the subcortical white matter, or by remote stimulation of the pial surface. Sometimes a small electrode was employed for intracortical stimulation.

3. The response of single neurones to these stimuli was recorded with extracellular micropipettes. Submaximal stimuli produced a stochastic response which was measured from the post-stimulus histogram (PSH) and provided an estimate of the probability of discharge at various times after the stimulus.

4. The PSH often displayed several discrete humps of different latencies, indicating several pathways between stimulated and recording point. Conductivity measurements were usually restricted to the pathway of shortest latency.

5. The conductivity of a pathway was defined as C = xy/x, averaged over 1 or 2 min, where

x = frequency of afferent test volleys,

xy = frequency of response: i.e. of those action potentials contained within a well-defined hump of the PSH,

(y = frequency of all discharges of the recorded neurone).

6. The progress of conductivity was tested with some constant form of cortical stimulation, repeated at regular intervals of 1-5 sec. Reliable results were obtained for twenty-six pathways subjected to thirty-eight experiments.

7. Temporary alteration of x, y or xy (conditioning with x, y or xy), for a period of 6-25 min, often caused a subsequent change in the conductivity (C) of a pathway which sometimes attenuated with a time constant of about 10 min, but which could persist without detectable attenuation for 20-30 min.

8. Conditioning periods less than 6 min rarely produced changes in conductivity; alterations of conductivity were more likely to be caused by conditioning periods longer than 17 min, than by periods of 6-16 min.

9. Changes in conductivity were usually correlated

negatively with temporary changes in xy,

negatively with temporary changes in x,

positively with temporary changes in y.

10. Nineteen of twenty-six pathways tested showed properties consistent with the formula [Formula: see text], where

G₁ lies between -1·0 and -0·16, with mean value -0·50,

G₂ lies between 0 and 0·42, with mean value +0·12,

G₃ lies between -0·61 and 0 with mean value -0·38,

K is a coefficient which is usually different for each experiment.

Four out of twenty-six pathways so tested provided results which did not fit this formula; three out of twenty-six pathways did not give adequate information.