Impacts were applied to temporal regions of 23 subhuman primates, while controlling 1) impact velocity, 2) impactor elasticity and 3) impact stroke. Such impacts were repeatedly applied to test primates until they showed symptoms of brain concussion or other brain injuries. As a result, the range of head accelerations, in which brain concussion and pathological brain injuries such as subdural hemorrhage, brain stem hemorrhage, etc. were likely to occur, was clearly indicated on the coordinates represented by the averaged resultant head acceleration and its duration of impact.

This provided a clear indication of thresholds of severe pathological brain injuries as well as the threshold of brain concussion of subhuman primates against lateral impacts. Based on the experimental results using subhuman primates, the extrapolation was done of human head acceleration and its duration, and thresholds of human brain concussion and severe pathological brain injuries were deduced. From results thus obtained it was estimated that the tolerances to brain concussion and severe pathological brain injuries for humans and subhuman primates were higher for lateral impacts than those of frontal or occipital impacts and that the hemorrhage of callosal, cortical and subcortical was often involved at time of occurrence of brain concussion by lateral impact on the head. According to the above, it was concluded that it should be appropriate to determine the tolerance threshold of the lateral impact, using slight skull fracture, minor subarachnoid hemorrhage or other similar head injury for reference.