A theory is presented which suggests that reinforcement evolved as a mechanism to insure species-typical responses to appropriate stimuli. Evidence is reviewed which suggests that such response sequences are organized in the brainstem of the mammal. The sequences are categorized as constituting approach to or withdrawal from a stimulusobject, and the brain stimulation literature is reviewed to demonstrate a relationship between these categories of behavior and the positive and negative rewarding effects of electrical stimulation, respectively. Reinforcement is identified as facilitation of activity in the neural systems which mediate the species-specific consummatory acts, and various anatomical and behavioral implications are discussed in some detail.

Following 4 unilateral contractions of hand muscles, subjects told stories about pictures from the T.A.T. A propositional analysis of the stories showed that the emotional tone of stories told following left hand contractions were more negative than those told after right hand contractions. These results are comparable to those reported following unilateral facial contractions and are consistent with the arousal of the emotional properties of the hemisphere contralateral to the contractions. When the stories were compared to ones told in a control, no contraction condition, it was found that the differences between the left and right contractions were attributable to the effects of either one or the other depending on the control condition responses to the pictures. The results therefore provide evidence for both left and right hemisphere involvement in emotion.

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Contractions of the left hand and of the left side of the lower third of the face induce negative emotional states whereas right-sided contractions induce positive states. Contractions also have mood-congruent influences on perception. This article reports that contractions affect behavior as well. Persistence in attempting to solve insoluble problems is greater following right-sided contractions than following left-sided contractions. This effect is unrelated to dominance of the contracted muscles because right-handed individuals tend to be left-face dominant. Results support the hypothesis that unilateral contractions activate the contralateral cerebral hemispheres and arouse the hemispheres' respective functions with regard to emotion and behavior.

In a series of three studies with right handed subjects, left side pain is tolerated less well than right side pain with a cold pressor, and results in greater emotional disturbance, both with a cold pressor and in chronic pain patients. In the second study where comparisons are made with non-stimulated controls, acute left side pain results in higher state anxiety scores than controls; right side pain and control groups are comparable. The differences between the reactions to left and right side pain are consistent with activation of the emotional properties of the hemisphere contralateral to the painful stimulation.