Stimuli

Stimuli were recorded for each participant individually with binaural microphones (OKM II Classic Microphone; sampling rate = 44.1 kHz) placed in the ear canals of each participant. Our goal was to recreate a natural perception of space, so we were not concerned with isolating individual spatial cues (i.e., interaural timing or level differences) or with the effect of echoes. The recordings were done in a room of 95 m³, with walls and ceiling made of gypsum board and a wooden floor covered by a thin carpet. Participants sat in the center of an array with eight speakers spaced at intervals of 30° from negative to positive 105° (0° denotes the midline), ∼2.4 m distance from the participant. Participants mobilized their head in a chin rest and were instructed not to move for the duration of the recording session, during which each stimulus was presented once. Stimuli were 3-s-long amplitude-modulated (AM) broadband pink noise generated in MATLAB (The MathWorks). Broadband noise was used to mitigate potential effects of frequency information in the neural responses (Sollini et al., 2017). The AM rate was 6, 8, or 10 Hz. This amplitude modulation was either consistent for the duration of the stimulus or interrupted for 500 ms after 1500, 1750, or 2000 ms to form the “target.” Using Vizard Virtual Reality software (Worldviz, http://www.worldviz.com/vizard-virtual-reality-software), stimulus location was simulated via enhanced higher-order ambisonic spatialization to appear from five different locations: −90°, −45°, 0°, +45°, and +90°. Therefore, there were a total of 60 stimuli recorded for each participant (3 AM rates × 4 target versions × 5 locations).

After the recording session, MATLAB was used to combine individual stimulus recordings into pairs to form the experiment conditions (Fig. 1). Each pair consisted of a 6 Hz AM rate stimulus, herein referred to as sound 1, presented with either an 8 or 10 Hz AM rate stimulus, herein referred to as sound 2. Sound 1 occurred at −90°, −45°, +45°, or +90° and sound 2 at 0°, 90°, or 135° separation from sound 1 in the direction of the opposite hemifield. The spatial locations, separations, and AM rates of these stimuli were determined based on pilot testing showing that the sound locations could be discriminated easily and that the sounds were segregated easily when separated.

Experimental conditions. a–f, Participants listened to 6 different pairs of concurrent sounds. Pairs varied in the location of sound 1 (either 90° or 45° from the midline, shown by the position of the black circle in the left and right columns, respectively) and in the spatial separation between sound 1 and sound 2 (0°, 90°, or 135° contralateral to sound 1, shown by the position of the black cross in the top, middle, and bottom rows, respectively). Half of the participants were tested with sound 1 in the left hemisphere, as shown, and half in the right.