POSITIVE SHIFTS OF EVENT-RELATED POTENTIALS - A STATE OF CORTICAL DISFACILITATION AS REFLECTED BY THE STARTLE REFLEX PROBE

Cortical positivity as measured by slow event-related potentials is as sumed to represent a decreased excitability of cortical networks and s uppression of their behavioral-cognitive output. The blink reflex prob e is a commonly used defensive electromyographic response whose amplit ude was shown to be modulated by emotional and attentional orientation . It was used here as an indicator of cortico-subcortical excitation. In study 1, 33 healthy subjects took part in a continuous performance test (CPT). Event-related potentials were recorded from 15 standard sc alp locations. Acoustic startling noise bursts were delivered during c onditions that required either performance of prepared motor responses (Go), inhibition of prepared motor responses (NoGo), or had no motor significance (Irrelevant condition). During the NoGo condition, EEG su rface potentials showed a widespread P300-like positivity with a centr al maximum. Startle responses were inhibited during the NoGo condition as compared to the Irrelevant condition. In study 2 (21 subjects) the same format was used, except that the startle reflex was elicited vis ually. Startle reflexes again showed smaller magnitude during the NoGo condition, which evoked larger positivity at central sites in compari son to the Irrelevant condition. The relationship between positivity i n the EEG and inhibited startle responses is in line with the hypothes is that positive EEG shifts reflect a state of cortical disfacilitatio n.