Positive and negative factors in movement control: a current review of Denny-Brown's hypothesis

In his extensive writings, Denny-Brown hypothesized that two competitive 'tropisms,' one positive (exploratory) and one negative (withdrawal) act to coordinate normal movements at all levels of the neuraxis. Lesions in particular areas of the central nervous system result in disequilibrium between these tropisms, leading to disorders of posture and movement, including involuntary movements. The tactile manifestations of unbalanced exploratory tropisms are grasping responses, whereas the complementary withdrawal tropisms are avoiding responses. In Denny-Brown's view, at the level of the cerebral cortex, grasping responses result from frontal lobe injury whereas avoiding responses result from parietal lobe lesions. In this report we review Denny-Brown's conceptions of positive and negative tropisms, their anatomical loci, and whether his hypothesis has merit in a contemporary approach to brain function. We find that Denny-Brown's view on the anatomical loci associated with these behaviors is incomplete, but that the idea of conflicting behavioral tendencies is valuable for understanding and managing some neurological and perhaps also psychiatric disorders. For example, his hypothesis offers an important perspective in understanding the paradoxical success of stereotaxic surgery to alleviate the symptoms of Parkinson's disease.