Excitability cycles of direct cortical responses during elaboration of conditioned defense reflexes in dogs]

Direct cortical responses (DCRs) to paired stimuli were studied in chronic experiments in dogs during elaboration of classical and instrumental defensive conditioned reflexes. The DCRs were recorded with 20 to 250 ms intervals between stimuli. Paired and single electrical stimulations of the middle suprasylvian gyrus given with a frequency of one per second were used as conditioned stimuli and were reinforced in a similar way. During electrical cutaneous stimulation of the dog's paw and to an even greater extent during isolated action of the conditioned stimulus the initial negativity of the testing DCR became shorter and the degree of its depression diminished. In the case of a following period of facilitation, its degree became greater. It was higher at a distance of 4 to 5 mm from the point of stimulation than at a distance of 2 to 3 mm. During isolated action of the conditioned stimulus, the degree of facilitation was higher than at the period of the possible action of the unconditioned stimulus. The greatest shorterning of the DCR excitability cycle was observed immediately before and during the conditioned lifting of the dog's paw. Excitability cycles of DCR, and possibly of other evoked potentials as well, are a more sensitive indicator of the function state of the cerebral cortex than responses to single stimuli. For this reason it appears promising to use them in studying conditioned reflexes.