resting activation in institutionalized defectives comes from studies of skin conductance; however, there is also some support for this from experiments on eye movements and heart rate. There is nevertheless a need to broaden the basis for this conclusion by testing it with other measures and to ascertain whether it results from level of nervous system activity or from some peripheral mechanism.
Although the evidence from studies of basal level is rather limited results regarding responsiveness to short-duration stimulation have been found with a larger group of measures. In general, defectives are less responsive to stimuli than are normals. This conclusion is based on measures of GSR, EEG, phosphene thresholds, change of pulse rate, and vascular response. Although defectives appear less responsive to stimuli, there is contradictory evidence from a measure of respiration rate and from a study of the startle response. Moreover, when differences in basal level or response amplitude do exist between defectives and normals, the differences are very small and there is a marked degree of overlap between the groups. Thus responsiveness is hardly diagnostic of mental deficiency.
There is some evidence to support the notion that mental defectives are less responsive to stimuli of short duration than are normals, and there is reason to believe that such a defect in responsiveness is related to the diffuse brain abnormality found in this group. Whether lowered responsiveness is a function of abnormality in the reticular formation of the midbrain or whether it is a reflection of more extensive neuropathology may be a question that can only be answered by subjecting infrahuman animals to localized experimental lesions.
Should a concept of lowered responsiveness in the mentally deficient be established more firmly, it could have implications for analyses of defects in more complex behavioral functions. It may be, for instance, that the slower reaction time of the mentally deficient is related to a generally low responsiveness to short-duration stimuli. In learning experiments, certain of the mentally deficient may differ from normals, not so much in the rate at which they learn, but in their initial and final level of response to the learning material ( Berkson, 1960b; Franks & Franks, 1960).
However, it is clear from the experiments that have been done that defectives are not always less responsive to short-duration stimuli than are normals. In some instances, they have been shown to give greater responses. Future studies would do well to clarify the conditions under which a concept of "lower responsiveness" of defectives holds and when it must be qualified.
Another issue concerns the response level of defectives when presented with a relatively unchanging environment. In this instance, the results of experiments appear to vary with the group of defectives studied. It is possible that results with resting measures depend on the nature of the environment as well as the group.
A study of the relationship between basal or prestimulus levels to the