Intracranial EEG study of brain structures affected by internal carotid injection of amobarbital

Hippocampal function, considered critical in memory processing, is supposedly tested in the intracarotid sodium amobarbital (ISA) procedure; however, since the hippocampus is not completely irrigated by the internal carotid artery, some believe the procedure may be invalid for memory testing. We quantified delta activity in intracerebral EEGs during ISA tests. There was increased delta in ipsilateral structures as follows: amygdala (6.4 minutes), anterior hippocampus (7.2), middle hippocampus (7.4), temporal neocortex (9.1), frontal lobe (8.4), central/parietal area (11.0), and occipital lobe (9.7). Contralateral structures usually (> 64%) showed increased delta lasting 4 to 5 minutes. The ipsilateral hippocampus had delta waves in over 90% of injections. We conclude that the hippocampus is clearly affected by the ISA injection. We argue that the slow waves may not be caused by a direct effect of the drug, but rather by a functional deafferentation due to the profound inactivation of structures surrounding the hippocampus. Similarly, slow waves contralateral to injection may be caused by sudden removal of neuronal input from the regions receiving the amobarbital.