Audiofile stimulus processor

Stimuli were generated using MATLAB (Mathworks, Natick, MA, USA) and Psychtoolbox extensions (Brainard, 1997 (link); Pelli, 1997 (link)). The visual stimulus was a stationary white circle with a diameter of 3.5° presented on a grey background in the center of the screen. Auditory stimuli were pure tones of 1000 Hz created in MATLAB and presented binaurally at 75 dB (measured at the auditory source) via Sennheiser HD 280 pro headphones laid flat, directly under the center of the display, with earpieces facing the ceiling to approximate the same spatial location as the visual signal. Visual and auditory stimuli were delivered through a Display ++ system with a refresh rate of 120 Hz and an AudioFile stimulus processor, respectively (Cambridge Research Systems, Rochester, UK). For all experiments, participants sat in front of the display 60 cm away from the screen.

The visual stimulus was an isotropic two-dimensional Gaussian blob subtending 3.5˚ visual angle presented on a uniform grey background for 30 ms. The contrast of the visual stimulus was set to either near- or supra-threshold levels based on the participant’s detection threshold measured in during the estimation of unisensory detection thresholds (see below). The auditory stimulus was a 30 ms 1000 Hz tone presented at a dB level set at either near- or supra- individual detection threshold levels. All stimuli were generated using MATLAB (Mathworks, Natick, MA, USA) and Psychtoolbox extensions (Brainard, 1997 (link); Pelli, 1997 (link)). Auditory and visual stimuli were presented via an AudioFile Stimulus Processor and a Display++ system with a refresh rate of 120 Hz, respectively (Cambridge Research Systems, Rochester, UK) in order to preserve precise timing. Auditory stimuli were presented via a speaker (Fantech HellScream GS 201, Nepal) that was centered underneath the display to align with presentation of the visual stimuli.