Lack of habituation in somatosensory cortex but not in visual cortex of ALS patients

Amyotrophic lateral sclerosis (ALS) is a multisystem degenerative disease with extra-motor components. In ALS, there is also hyperexcitability of extra-motor areas. Habituation is defined as ''a response decrement" caused by repeated stimulations. Studies on evoked potential habituation can be conducted to detect cortical excitability. This study aimed to explore lack of habituation in non-motor cortical structures in ALS.

Twenty-one ALS patients and 14 controls were enrolled. Recordings were obtained in 3 and 10 consecutive blocks (each containing 100 responses) during right median somatosensory evoked potential (SEP) and bilateral visual evoked potential (VEP), respectively. "Habituation" and "lack of habituation" were defined as the amount of increase or decrease in the average N20 or N75-P100 amplitude of the last blocks compared to the first blocks, respectively. Comparative analyses were performed between patient and control groups, as well as the first and last block within groups.

Paired sample t-test showed that in control group N20 peak amplitude of last blocks were significantly lower than first block values (p = 0.025) that indicate the physiological habituation as expected. On the other hand, there was not such a difference in ALS group (p = 0.239) which indicated lack of habituation.

Our study results suggest somatosensory hyperexcitability in line with cortical reorganization in ALS patients.