THE EFFECT OF INTER-STIMULUS INTERVAL ON SOMATOSENSORY PROCESSING: A LORETA STUDY

The temporal intervals of sensory inputs fundamentally alter how the brain processes information, yet the specific cortical generators underlying the effects of fixed temporal spacing have not been fully characterized. This study investigated the source localizations of somatosensory-evoked potentials (SEPs) to determine how inter-stimulus interval (ISI) modulates cortical activations. Twenty-four healthy adults (10 F, M_age = 23.46 ± 3.89 years) participated in the study. Non-painful tactile stimuli were delivered 40 times to the right index fingertip across three separate sessions with fixed ISIs of 2, 4, and 8 seconds. Continuous EEG data were segmented into 3000 ms epochs (−1000 to +2000 ms from stimulus onset), and averaged. Source estimation was performed using LORETA on segmented and averaged EEG epochs, specifically targeting maximal source activation within the 100–600 ms time window. Results revealed distinct cortical activation patterns across ISI sessions. The short interval (ISI 2s) was primarily associated with frontal regions (Superior Frontal Gyrus, BA6), while the long interval (ISI 8s) showed localization to parietal networks (Inferior Parietal Lobule, BA40). In contrast, activity for the intermediate ISI (4s) was localized to sensory-perceptual regions (e.g., Cuneus BA17, Temporal Lobe). These findings suggest that while short and long intervals appear to engage frontal control and parietal attention networks respectively, a 4-second interval may represent an optimal temporal window for efficient sensory cortical processing.