Age Differences in Response Speed as a Function of Controlled Variations of Stimulus Conditions: Evidence of a General Speed Factor

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Abstract
The purpose of this study was to gain evidence of the specific or general nature of the age change in speed of performance by comparing the performance of young and elderly subjects in a standardized experimental context which systematically varied the complexity of stimuli while keeping the response form constant. The subject''s task was to respond serially to 1 of 10 signal lights by pressing 1 of 10 buttons. There was always a predetermined association between lights and buttons. The subject turned off each of the 10 lights as rapidly as possible by pressing the correct button. A total of 22 experimental conditions were used ranging from simple movement and reaction times, through numbers, letters, colors, symbols, and word relationships of a predetermined nature. A total of 30 young subjects (age range 18 to 30 years) and 23 elderly subjects (60 to 80 years) were studied. Under all experimental conditions the elderly subjects were slower than the young. The largest age differences in speed both relative and absolute, appeared for the superordinate, co-ordinate, and part word relations, and the color and color symbol associations. The smallest differences were found for choice reaction time and adjective word relations. It is apparent from these results that age differences in speed of response are not limited to the simple motor aspects of tasks but involve to an even greater extent verbal process. Even if one were to dissociate the time required for simple reaction time from the response times for complex associations only about 10 to 15% of the age difference would be accounted for. Furthermore, it appears from the results that this is not a justifiable computation since the age differences involve processes common to any stimulus response association, including both relatively simple choice reactions and complex verbal relations. The results in general support the view that older subjects tend to show a characteristic slower response speed, whereas young adult subjects are more task-specific in their response speed. Although the spatial order of stimuli and responses was changed for each experimental condition, the 22 conditions were always performed in the same order. For this reason any adaptation of the subjects to the experiment might result in somewhat better performance of the later and more complex conditions. For reasons discussed such influences would tend to minimize the appearance of a general speed factor.