[Dynamics of neuronal reactions during elaboration of a conditioned defensive reflex to sound]

When elaborating and realizing defensive conditioned reflexes in cat cortex areas 3 and 4 series of phases was distinguished in the neuronal reactions: initial response, short-lasting depression of activity, early and late after-discharges, activation after sound cessation. The initial response arose or was augmented during the beginning of the reflex elaboration (initial 5-15 stimuli combinations). After-discharges were relatively stabilized to the 20th-40th combinations. Conditioned motor reactions occurred 100-400 ms after the onset of the sound. Movements usually coincided with early after-discharges of cortical neurons during both real conditioned reflexes and initial period of trace reflexes. Differential neuronal reactions were elaborated with the same efficacy as positive ones. The conditoional stimulus reinforcement plays an important role not only in increasing neuronal excitability (which is significant for acquiring polysensory properties), but also in modifying after-discharges.