2.4 Task
A visual illustration of the audio-visual speeded reaction time task (AVSRT) task is provided in Figure 1. Participants were instructed to press a button with their right thumb as quickly as possible when the onset to any auditory, visual, or multisensory (audio-visual) cue was detected. The auditory cue (A) consisted of a 1000 Hz tone presented for 60 ms binaurally (75 dB sound pressure level, rise/fall time 5 ms). The visual cue (V) consisted of a red sphere presented directly above a fixation cross which subtended 1.5 degrees both vertically and horizontally. The multisensory audio-visual cue (AV) consisted of both A and V cues presented simultaneously. Auditory stimuli were presented using a centrally located speaker (JBL Duet Speaker System, Hartman Multimedia) and visual stimuli were presented on a flat panel LCD monitor (Dell Ultrasharp 1704FTP). To reduce predictability of cue onset, a jittered interstimulus interval (ISI) was used. Stimulus onset varied randomly according to a uniform distribution between 1000 and 3000 ms following the previous stimulus. The full task consisted of 1000 trials presented across 10 task blocks. In the current analysis, all participants completed a minimum of 5 valid blocks. For a small subset of participants, individual blocks were removed from analysis based on experimenter notes indicating participant non-compliance. On average, each participant completed 972 ± 58 trials.
Button presses were recorded on a response pad (Logitech Wingman Precision Gamepad) with the same response key used for all three stimulus types. All aspects of task presentation were controlled using Presentation software (Neurobehavioral Systems Inc., Berkeley, CA). 8-bit, analog triggers indicating stimulus latency were sent from the PC acquisition computer by Presentation software in response to the onset of a cue or when the response button was depressed.