On Habit Interference from Cortical Stimulation

Abstract

Electrical stimulation of the cerebral cortex has been shown to produce a deficit in hunger-motivated lever-pressing behavior (Burns & Mogenson, 1958, 1961). The effect was greater during acquisition than during performance of the habit. Presumably an established habit is less vulnerable to the effects of neural interference than one that is being acquired. The present experiment was performed in order to investigate further the interference effects of cortical sti~nulation on performance. Ten male Wistar rats, with bipolar electrodes placed stereotaxically in visual and somatosensory cortex and later verified histologically, were tesced in a Skinner box. The Skinner box was 24 in. long with the lever at one end and the food cup at the other end so that Ss had to run back and forth to obtain food reinforcement. Using a switch-back design half of the Ss were stimulated each day with I-sec. trains of cortical stimulation (60 cycle sine wave, 2-6 v) during the 20-min. test session which followed 23 hr. of food deprivation. The cortical stimulation was delivered automatically every 15 sec. The mean lever-pressing rate was 120.9 responses/20 min. for the two non-stimulation days and 112.2 responses/20 min. for the two stimulation days. This small, but statistically significant, performance decrement (t = 2.85, p < .02) confirms the earlier findings based on a simpler behavioral sequence (Burns & Mogenson, 1961). If the cortical stimulation occurred while Ss were running, an arrest reaction, similar to that reported by Fangel and Kaada (1960), was often observed. This effect was not observed if the cortical stimulation occurred during lever pressing or eating. The results agree with those of a number of other studies in which performance of learned responses was disturbed by brain stimulation (e.g., Chiles, 1954). It appears that cortical stimulation can block or disrupt the delicate neural adjustments required in the serial ordering of behavior.