Psychosomatic Medicine 23:138-152 (1961)
© 1961 American Psychosomatic Society

Short- and Long-Range Effects on the Rat of a Fear-Provoking Stimulus

¹ National Institute of Mental Health, Public Health Service, Bethesda, Md.

An electrical traumatic stimulus given to rats had both short- and long-range effects. The short-range (4-wk.) effect was marked on weight gain, activity, and defecation; marked but brief on certain social behaviors; and less marked on urination and vocalization. Weight gain of control rats (interpreted as a social effect) was also affected. The long-range (10-14 mo. after trauma) results were suggestive of a residual effect of trauma. Other related experiments of one of us¹⁷ showed a clear residual effect in the form of a reduced activity at 5, 11, and 12 mo. after trauma. Retraumatization in both experiments obscured this residual effect and did not indicate that rats previously traumatized were more vulnerable to retraumatization.

The fact that weight was affected indicates that the emotional disturbance created by the trauma was present in the home cages, away from the experimental situation, and therefore had become an on-going, internal process,⁶ as opposed to one dependent on the presence of cues closely associated with the trauma.

During contact with an untraumatized rat and for a short period thereafter, the experimental rats approached normal social behavior and normal levels of activity. This indicated that social behavior and activity influenced by social contact, temporarily at least, tended to escape the disturbed emotional state, but the groups retained their relative positions. Novel stimuli, including unfamiliar persons, caused a sharp decrease in activity of both traumatized and control rats, but here again the groups retained their relative positions: control, highest; moderate trauma, less; and severe trauma, least activity. A familiar person returned activity to the previous levels. That their relative positions were retained in spite of fluctuations caused by novel and familiar stimuli also suggests the presence of a temporarily built-in, on-going process (short-acting autokinesis⁶) as opposed to one entirely dependent for its manifestation upon specific, learned cues. We interpret this to mean that the trauma caused changes in the central nervous system that led to a lowered threshold for most stimuli, thus allowing for many kinds of stimuli to act as perpetuators of the disturbed emotional state. When the boundaries of stimulus categories are leaped, we see this as a more profound process than mere stimulus generalization.

At age 9 wk. trauma caused an activity decrease lasting 4-5 wk; at age 14 mo. trauma caused a permanent drop in activity level for both previously traumatized rats and rats who had never before had trauma; thus there was no evidence in this experiment that prior trauma increased later vulnerability. The fact that there was no recovery from this trauma, the first trauma for the control group, is attributed to an age factor.

Coefficients of correlation indicated high individual consistency for vocalization and urination. Weight and activity rankings were well maintained for 3-6 wk. periods and diminished thereafter. In all measures certain individuals stood out as very consistent throughout the experiments. Activity and defecation were highly correlated negatively.