Visual temporal order judgment in profoundly deaf individuals

We investigated temporal processing in profoundly deaf individuals by testing their ability to make temporal order judgments (TOJs) for pairs of visual stimuli presented at central or peripheral visual eccentricities. Ten profoundly deaf participants judged which of the two visual stimuli appearing on opposite sides of central fixation was delivered first. Stimuli were presented either symmetrically, at central or peripheral locations, or asymmetrically (i.e. one central and the other peripheral) at varying stimulus onset asynchronies (SOAs) using the method of constant stimuli. Two groups of hearing controls were also tested in this task: 10 hearing controls auditory-deprived during testing and 12 hearing controls who were not subjected to any deprivation procedure. Temporal order thresholds (i.e. just noticeable differences) and points of subjective simultaneity for the two visual stimuli did not differ between groups. However, faster discrimination responses were systematically observed in the deaf than in either group of hearing controls, especially when the first of the two stimuli appeared at peripheral locations. Contrary to some previous findings, our results show that a life-long auditory deprivation does not alter temporal processing abilities in the millisecond range. In fact, we show that deaf participants obtain similar temporal thresholds to hearing controls, while also responding much faster. This enhanced reactivity is documented here for the first time in the context of a temporal processing task, and we suggest it may constitute a critical aspect of the functional changes occurring as a consequence of profound deafness.