Behavioural studies have revealed enhanced reactivity to visual events in the profoundly deaf. Here we examined the electrophysiological response to visual stimuli in deaf individuals and hearing controls during a speeded simple-detection task. After the appearance of a warning-signal (500ms), a visual target was randomly presented with either a short (500ms) or long ISI (1800ms), at central (3°) or peripheral (8°) locations with respect to fixation. Behaviourally, deaf were faster than hearing controls (on average 40ms), particularly for targets appearing at short ISI. In addition, controls responded more slowly for peripheral than central targets, whereas this difference did not emerged for the deaf.
The ERPs revealed sustained activation at occipito-parietal sites in the deaf, before any visual stimulation. Moreover, the C1 component in response to the warning-signal peaked earlier in deaf than controls. Deaf also displayed prolonged and ampler visual analysis in the second phase of the P1 component (showing a marked second peak), which in turn produced a delay of the N1 onset. A comparison of potential and SCD suggests that the second phase of the P1 component in the deaf has deep generators. In addition, while the P1 was ampler for central than peripheral targets in the controls, comparable P1 amplitude emerged in the deaf regardless of target location. Finally, CNV preceding targets at short ISI had a larger amplitude range in deaf than controls. These results show for the first time changes in the dynamic of the very early visual-evoked potentials (C1, P1) in deaf individuals. Because modulation of the late P1 complex has been recently linked to exogenous attention capture, these findings point to a key role of this attentional component in enhanced reactivity in the deaf.