Aktuelle Neurologie 2005; 32 - P524
DOI: 10.1055/s-2005-919555

Influence of intermittent photic stimulation on the temporal aspects of cortical silent period in subjects with and without epilepsy – a TMS based study

S Groppa 1, M Siniatchkin 1, H Siebner 1, U Stephani 1
  • 1Kiel

Objective: Transcranial magnetic stimulation (TMS) is a well established method for investigation of excitatory and inhibitory circuits of human cortex. Our study analysed temporal changes of intracortical inhibition expressed in the duration of the cortical silent period (CSP) over the motor cortex before and under intermittent photic stimulation (IPS).

Methods: CSPs were analysed during ten successive trials. CSP was produced by TMS (2 T Magstim 200) stimuli with 150% intensity of resting motor threshold in healthy subjects with (N=12) and without (N=14) photoparoxysmal response, and in patients with idiopatic generalized epilepsies also with (N=8) and without (N=7) photosensitivity. CSP was measured before IPS as well as during IPS at 18 and 50Hz. Time-dependent changes in the duration of the CSP were assessed on individual basis using a linear regression analysis. The regression coefficients were compared using a repeated-measures ANOVA (within-subjects factor „State“: baseline vs. 18Hz IPS and baseline vs. 50Hz IPS, and between-subjects factor „Group“).

Results: For 18Hz IPS, a significant interaction „State x Group“ (F (3, 37)=2.92; P=0.047) was found. The detailed analysis revealed differences in CSP changes during 18Hz IPS among groups with and without photoparoxysmal response. Especially the group of epilepsy patients with photoparoxysmal response showed pronounced reduction of CSP unter 18Hz IPS. For 50Hz IPS, the effect of „State“ was significant (F (1, 37)=4,65; P=0.038). All groups of subjects demonstrated a gradual reduction of the CSP duration from trial to trial under 50Hz which did not differ among groups.

Conclusion: The data demonstrate a reduction of CSP over time during IPS. These results could be interpreted in terms of a temporal decrease of intracortical inhibition during photic stimulation, which is differently expressed in subjects with and without photoparoxysmal re-sponse.