Electrophysiological responses from the cochlea and inferior colliculus in guinea pigs after head injury

Electrical responses from the inferior colliculus (ER) and round window (CP) to acoustical stimuli were compared in control and experimental guinea pigs which had received a single blow to the freely movable head. After the blow to the head, all of the animals showed signs of commotio cerebri, and some of them died. The experimental animals were divided into two groups so that the physiologic responses to sound could be evaluated one day (early group) and seven days (intermediate group) following the head blow. The animals of the early group had elevated ER thresholds over the entire frequency range tested with an average threshold of 37.8 db. The animals of the intermediate group also had elevated ER thresholds which averaged 23.9 db. Input-output intensity function of ER for the experimental animals showed a recruitment-like phenomenon. The CP thresholds in the injured animals did not differ significantly from those in the control group. Damage to the VIIIth nerve and higher central pathways is very likely a more frequent cause of the otoneurologic manifestations following head injury than end organ damage.