Some effects on performance of changes in perceptual load.

Abstract

An experiment was carried out using 12 practiced S''s in which the perceptual load was systematically varied. The aim of the experiment was to determine whether changes in serial performance as well as changes in score could be associated with the experimental variable. Increasing the number of pointers from 4 through 12 had the effect of increasing both the number and the duration of errors. Detailed performance records were scrutinized clinically, and a number of features thus isolated were subjected to conventional statistical treatment. It was found that: (1) Errors occurred when signals requiring action were bunched. It was pointed out that with load changes the errors arising from this source would change in certain skills, but not in all. (2) Errors occurred when S''s failed to determine the correct signal order. These errors occurred only when the interval between successive signals was short. Increasing the load had the effect of increasing the frequency of such errors, and of increasing the maximum interval between two signals likely to result in such error. A subsidiary experiment showed an association between the incidence of this type of error and a general shortage of time. (3) With load increases, errors were more likely to arise in event sequences of identical temporal structure, even when an essential element of the sequence was an event which occurred after the error. (4) Recovery from error took longer with greater loads. (5) Even under the condition of greatest stress, attention was apparently distributed strictly according to display demands rather than in accordance with either a spatial or a signal-frequency pattern. The results are discussed briefly in terms of their value as general criteria of changes in skilled performance under stress. A concept of load stress analogous to that of speed stress is postulated, and certain objections made from a consideration of eye movements are reviewed.