Neuromagnetic evidence for hippocampal modulation of auditory processing.

The hippocampus is well known to be involved in memory, as well as in perceptual processing. To date, the electrophysiological process by which unilateral hippocampal lesions, such as hippocampal sclerosis (HS), modulate the auditory processing remains unknown. Auditory-evoked magnetic fields (AEFs) are valuable for evaluating auditory functions, because M100, a major component of AEFs, originates from auditory areas. Therefore, AEFs of mesial temporal lobe epilepsy (mTLE, n=17) with unilateral HS were compared with those of healthy (HC, n=17) and disease controls (n=9), thereby determining whether AEFs were indicative of hippocampal influences on the auditory processing. Monaural tone-burst stimuli were presented for each side, followed by analysis of M100 and a previously less characterized exogenous component (M400: 300-500ms). The frequency of acceptable M100 dipoles was significantly decreased in the HS side. Beam-forming-based source localization analysis also showed decreased activity of the auditory area, which corresponded to the inadequately estimated dipoles. M400 was found to be related to the medial temporal structure on the HS side. Volumetric analysis was also performed, focusing on the auditory-related areas (planum temporale, Heschl's gyrus, and superior temporal gyrus), as well as the hippocampus. M100 amplitudes positively correlated with hippocampal and planum temporale volumes in the HC group, whereas they negatively correlated with Heschl's gyrus volume in the mTLE group. Interestingly, significantly enhanced M400 component was observed in the HS side of the mTLE patients. In addition, the M400 component positively correlated with Heschl's gyrus volume and tended to positively correlate with disease duration. M400 was markedly diminished after hippocampal resection. Although volumetric analysis showed decreased hippocampal volume in the HS side, the planum temporale and Heschl's gyrus, the two major sources of M100, were preserved. These results suggested that HS significantly influenced AEFs. Therefore, we concluded that the hippocampus modulates auditory processing differently under normal conditions and in HS.

Thalamic hypometabolism on 18FDG-positron emission tomography in medial temporal lobe epilepsy.

OBJECTIVES: Degree of hypometabolism in the thalamus on (18)Fluorodeoxyglucose-positron emission tomography (FDG-PET) was compared with those of medial and lateral temporal lobes in patients with medial temporal lobe epilepsy (mTLE), and its relationship with post-operative seizure outcomes was investigated. METHODS: Twenty-six patients with mTLE who underwent anterior temporal lobectomy were included. Post-operatively, 13 patients became completely seizure-free and 13 showed residual seizure, regardless of frequency (five patients became almost seizure-free, six had rare seizures and two showed significant improvements). Degrees of hypometabolism in bilateral thalamus, ipsilateral medial and lateral temporal lobes were evaluated visually and semi-quantitatively by determining the asymmetry index (AI), a value indicating 100 x (ipsilateral - contralateral)/[1/2 x (ipsilateral + contralateral)] and the region-to-cerebral hemisphere ratio (R/C ratio) being the ratio between averaged counts in each area and those in the cerebral hemisphere of the same side. RESULTS: Hypometabolism in the medial temporal lobe was visually observed in all patients. Hypometabolism in the lateral temporal lobe was observed in 20 patients and was semi-quantitatively more prominent than that of the medial temporal lobe. Pathologically, hippocampal sclerosis and prominent astrogliosis of the lateral temporal lobe were present in all cases. However, while thalamic hypometabolism was visually observed in nine patients (in the ipsilateral side of four cases, contralateral side of three and on both sides of two), no significant thalamic hypometabolism was semi-quantitatively observed. No significant differences in metabolic rate in any area except for the lateral temporal lobe between seizure-free patients and residual seizure patients were seen semi-quantitatively. DISCUSSION: Data indicated that metabolism in the lateral temporal lobe of patients with mTLE significantly decreased and revealed pathologic glial changes. Thalamic hypometabolism was quite mild and did not correlate with post-operative seizure outcome.