Role of mesial temporal lobe structures in sensory processing in humans: a prepulse modulation study in temporal lobe epilepsy

Abstract

Prepulse modulation (PPM) is an electrophysiological method which enables to assess sensory processing in vivo. Reflex responses may be facilitated or inhibited (prepulse inhibition, PPI) after a weak stimulus. Theoretically, in animal studies, the generator of PPI involves pedunculopontine nucleus which is modulated by various structures, including amygdala. We aimed to investigate whether or not there was a role of limbic structures in the generation of PPM in humans. For this purpose, we studied PPM of the blink reflex (BR) in 10 patients with mesial temporal lobe epilepsy (MTLE group) and in nine patients who had previously undergone amygdala resection for medically resistant MTLE (surgery group). A control group including 19 healthy volunteers was formed. Blink reflex, BR-PPM and BR excitability recovery were recorded in all participants. Two components of BR, first early ipsilateral component (R1) and second late bilateral components (R2 and R2c) were identified. All BR parameters after single stimulation were normal in all groups. Compared to healthy subjects, R2-PPI was more pronounced in the surgery group whereas there was a R2-PPI deficit in the MTLE group. R2-PPI deficit in the MTLE group was more prominent on the lesion side. Ipsilesional R1 facilitation was more evident at ISI of 100ms in both MTLE and surgery groups compared to healthy subjects. BR excitability recovery was not different between groups. MTLE in humans leads to a PPI deficit. Interestingly, removal of amygdala in humans with MTLE probably provides more efficient functioning of PPI network. Amygdala and hippocampus play roles in the human R2-PPI circuit. Modulation of R1 facilitation is unilateral whereas the modulation of R2-PPI is bilateral, though asymmetric.