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Head injury in the champanzee

Part 2: Spontaneous and evoked epidural potentials as indices of injury severity

Frank S. Letcher, Paul G. Corrao and Ayub K. Ommaya

✓ The effect of experimental head injury on the electroencephalogram (EEG) and cortical evoked response was studied in 11 awake, moderately restrained chimpanzees. As a base for comparison with injury effects, the waking and sleeping electroencephalogram (EEG) and the somatic and visual evoked responses (SER and VER) were investigated first. Unusually high voltage occipital waves and a propensity for photic driving characterized the EEG. Controlled blows to the occiput in 10 animals produced reversible depression of consciousness in only four. In those four, the EEG and SER were affected differently immediately after the injury; the SER showing marked suppression while the early EEG was unaffected. Recovery of the SER and of consciousness paralleled each other. With injuries causing prolonged or irreversible loss of consciousness, the later EEG showed depression or large amplitude slow waves, which became isoelectric if the blow was fatal.

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Head injury in the chimpanzee

Part 1: Biodynamics of traumatic unconsciousness

Ayub K. Ommaya, Paul Corrao and Frank S. Letcher

✓ Requirements for extrapolating head injury tolerance levels from subhuman primates to man necessitated data from a primate species with a brain weight between that of the rhesus monkey (100 gm) and man (1200 gm). Cerebral concussion (traumatic unconsciousness) in the chimpanzee (brain weight, 400 gm) is found to occur when the head experiences angular velocities exceeding 70 to 120 radians/ sec irrespective of how such head rotations are produced, i.e., whether indirectly by a whiplash mechanism or directly by impact to the occiput. The threshold for the production of skull fracture and visible brain lesions appears to be quite close to that found for traumatic unconsciousness. Associated neurological, cardiovascular, and pathological effects of head injury in this species are also described, and a new hypothesis for traumatic unconsciousness (cerebral concussion) is presented.