Apparatus and stimuli

For the listen and draw task, the sounds were generated by means of Audacity software (http://audacity.sourceforge.net/). We used five sounds with different pitches: 100, 500, 1,000, 5,000, and 9,000 Hz. The duration of the auditory stimulus was 500 ms. Visual stimuli were generated in MATLAB, using the Psychophysics Toolbox version extensions. The monitor on which the participants had to draw was a 19-inch LCD color monitor (resolution 1,280 × 1,024).

For the size discrimination task, stimuli were displayed on a Barco monitor (resolution of 1,280 × 1,024 corresponding to 41° × 31.2° from subject viewing distance of 57 cm with a refresh rate of 100 Hz).

In the size discrimination task, visual stimuli were Craik-O'Brien-Cornsweet stimuli. The stimuli were constructed by high-pass Gaussian filters (with a 50% cutoff at spatial frequency of 0.5 cycles/°), with the advantage of eliciting localized activation of the visual cortex, limited to their edges (Perna et al., 2005; Boyaci et al., 2007; Pooresmaeili et al., 2013). All visual stimuli were displayed with an eccentricity of 10°. The test stimulus was always presented on the left of the screen, whereas the reference stimulus was presented with the same eccentricity on the right side. The adapter stimulus was always centered at the same eccentricity as the test stimulus. The size of the adapter and the reference stimuli were keep constant, at 10° and 5° respectively. The size of the test stimulus was determined by QUEST (Watson and Pelli, 1983), an adaptive algorithm which, based on the current estimation of the participant, estimates the best stimulus value to be presented in the subsequent trial. The range of variation of the test stimulus was between 0.5 to 1.5 times the size of reference stimulus. Therefore, the physical size of the test stimulus was always smaller than the adapter. The test and the reference stimuli were presented for 500 ms, while the adapter lasted 10 s with a top-up of 6 s. For the auditory conditions, we used two of the sounds used in the previous task: the lowest pitch (100 Hz) and the highest pitch (9,000 Hz).

In all tasks, sounds were heard through noise canceling headphones (Bose® QuietComfort 25 Acoustic Noise Cancelling Headphones).