Paced and unpaced serial response performance during two types of EEG activity

Abstract

A 15-year-old girl with temporal lobe epilepsy was tested daily on a continuous choice response task, with simultaneous EEG recording, in order to examine the effects of two types of bioelectric activity: generalised spike-wave (SW) and theta-alpha background (B). The child was required to press keys corresponding with random digits presented serially upon a screen. The digits were selected from ensemble sizes of 2, 4 or 8 and could be presented either at a rate determined by the experimenter (the paced condition) or at a rate determined by the child's own speed of response (the unpaced condition). Performance was measured under three conditions of pacing (1 digit per sec, 1 per 2 sec and 1 per 4 sec) and compared with the unpaced condition in relation to S-W and B activity. The child's unpaced rate of work was found to approximate to the intermediate paced condition, viz. 1 digit per 2 sec. Reaction times were significantly increased during S-W activity under both paced and unpaced conditions. Under paced conditions, significantly more errors were made during S-W than during B activity, but under unpaced conditions, comparatively few errors were made during S-W. The following interpretation of the data was adduced. On a paced task, reaction times which approach the inter-stimulus interval necessitate the subject storing information about one stimulus whilst a response is made to another. Where several successive reaction times are of increased duration, the load upon immediate memory plus the processing load will eventually exceed the subject's information-handling capacity and errors will be made. Any source of interference whose effect is to increase reaction time, will cause an increase in errors by increasing the probability that stimuli and responses overlap. It was thus hypothesised that the relationship between probability of error ( P E ) and reaction time ( RT) would be essentially similar for the two types of EEG activity: S-W would merely propagate a shift in the RT distribution. An analysis of P E in relation to RT was carried out for all stimulus-response pairs (from the 1 2 paced data), and for responses to “next-but-one” and “next-but-two” stimuli. The curves for S-W and B did not differ significantly from each other. This suggests that the causes of errors made on a paced task during S-W may not be qualitatively different from the causes of errors made during B activity. The child's poor performance on the paced task is a logical and necessary consequence of her slowness during SW activity. Clearly, unpaced performance is less vulnerable to the effects of paroxysmal disturbance than performance which is paced at an equivalent rate.