Interstimulus Interval Conditions Research Articles

Selective attention and “habituation” of the auditory averaged evoked response in humans

The study examined the effects of direction of attention and interstimulus interval on the rate of amplitude decrement over time of the vertex auditory evoked response in humans. Short-term or stimulus-by-stimulus changes were studied by averaging across 24 discretely presented trains each of 10 click stimuli. Long-term or train-by-train changes were studied by averaging over successive pairs of trains. When attending to the clicks, the subject performed a reaction-time (RT) task with the click as stimulus; when ignoring the clicks he performed RTs to a visual stimulus. Both RT tasks were performed with irregular interstimulus intervals of 2.4 – 3.6 and 8 – 12 s, each of the 8 subjects being thus studied on four occasions. Attending to the stimuli and long interstimulus intervals enhanced the P 1-N 1-P 2 components. Amplitude decreased over time both for stimulus-by-stimulus and train-by-train averages. Somewhat unexpectedly, there was a slight but significant tendency towards a steeper slope for the attending and long interstimulus interval conditions, probably following on the larger initial responses for these conditions.

EFFECTS OF INTERSTIMULUS INTERVAL AND PREDICTABILITY OF EVENT STRUCTURE ON SERIAL REACTION TIME

A mean interstimulus interval (ISI) of 10 sec. resulted in significantly‐shorter RTs to a single light stimulus than a 30 sec. average ISI. During a second workperiod the groups performing under each of the two ISI conditions were divided into four subgroups given 0, 25, 50 or 75% interspersion of an additional stimulus (sound). With short ISIs the condition of maximum predictability of event structure (light stimulus only) gave the shortest average RTs. With lessened predictability (increased proportion of the sound stimulus) there was a slight monotonous increase in RT. With long ISIs the same relationship was curvilinear, i.e. 25% interspersion of sound stimulus was most effective in preserving vigilance.