The effect of stimulus complexity on discrimination responses.

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Abstract
A technique for quantifying stimulus aspects of discrimination tasks was presented. Visual stimuli were used that varied in horizontal spatial separation, vertical spatial separation, size, and bright-ness. The motor responses were right or left movements of a joy-stick control. Four groups of 24 Ss each, differentiated in terms of a variable referred to as stimulus-response compati-bility, received 48 acquisition trials. The stimulus conditions for these trials represented the least complex discrimination corresponding to a scale, value of stimulus complexity of 1 bit. Following the acquisition trials, Ss of each group received four blocks of 24 trials each. These blocks represected 4 levels of stimulus complexity corresponding to the scale values of 1, 2, 3, and 4 bits. The purpose of this phase was to determine the relationship between stimulus complexity and the proficiency measures, reaction time and errors, of the discrimination per-formance. The results of the analyses of the reaction time data showed that reaction time during acquisition decreased at ap-proximately the same rate and to the same extent for all groups. An asymptotic reaction time level was reached within the 48 acquisition trials. The over-all group reaction time levels were different. The group in which the horizontal spatial discrimination was associated with the right-left movements showed the lowest over-all reaction time. Next was the group in which the vertical spatial discrimination was associated with the right-left move-ments. The other 2 groups in which the size and brightness discriminations were associated with the right-left movements, had still higher mean reaction times, but did not differ significantly between themselves. Reaction time was found to Increase as the scale values of stimulus complexity increased. The relationship was essentially linear, but the exact nature of the function was somewhat different for the different groups. Analyses of the error data yielded similar results.
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