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 assumed to represent a decreased excitability of cortical networks and suppression of their behavioral-cognitive output. The blink reflex probe is a commonly used defensive electromyographic response whose amplitude 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 scalp locations. Acoustic startling noise bursts were delivered during conditions 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 surface potentials showed a widespread P300-like positivity with a central 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 visually. Startle reflexes again showed smaller magnitude during the NoGo condition, which evoked larger positivity at central sites in comparison to the Irrelevant condition. The relationship between positivity in the EEG and inhibited startle responses is in line with the hypothesis that positive EEG shifts reflect a state of cortical disfacilitation.