Auditory stimulus duration effects on discrimination and brain activity

Conclusions

• 1. Electrical activity of the brain and conditioned-reflex behavior of cats were investigated during gradual shortening of the duration of stimuli from 3 to 0.015 sec. The progressively increasing difficulty and stress in stimulus analysis developing under these circumstances are revealed initially by changes in the EEG, and only later by disturbances in the animal's behavioral responses.

• 2. Three critical periods can be distinguished as the duration of the stimuli is reduced:

• a) shortening of the stimuli to 1 sec increases the degree of tonic activation of the background EEG and the electrical responses of the brain to positive and fine differential stimuli are prolonged; behavioral responses of most animals remain as before;

• b) with the change to a duration of 0.2–0.1 see, besides a further increase in the degree of EEG activation, marked disturbances of conditioned reflex activity arise; failure of fine differentiation and the appearance of after-inhibition;

• c) stimulus duration of 0.03–0.015 sec was critical: the highest level of tonic background EEG activation is achieved, the EEG desynchronization response becomes prolonged to all stimuli, including gross differential stimuli (1000 Hz). Individual animals can continue to differentiate the 1800 Hz tone at that duration, but only at the beginning of the experiment. Most cats remain capable only of 1000 Hz differentiation. In every case deep after-inhibition is regularly exhibited, and incorrect responses appeared with negative stimuli. As the experiments with the critical duration of acoustic stimulation continue, conditioned-reflex activity became disturbed and disorganized, and the animal developed neurotic behavior and trophic disturbances (loss of hair, eczema).

• 3. It is postulated that the decrease in stimulus duration is equivalent to some extent, to an increase in the fineness of differentiation. For stimuli of minimal duration (0.015 msec), the number of impulses reaching the brain is inadequate for the perfect fine analysis of the stimuli to determine their biological significance. This dependence of tonic and phasic EEG activation on the complexity of stimulus analysis indicates that, under the particular conditions used, the analyzer system of the brain modulates its activating mechanisms.