Lights can reverse illusory directional hearing

Adding brief flashes of light to a train of auditory clicks [R. Hari, Illusory directional hearing in humans, Neurosci. Lett. 189 (1995) 29-30] can alter the sounds perceived location within the head. In an experimental procedure adopted from Hari [R. Hari, Illusory directional hearing in humans, Neurosci. Lett. 189 (1995) 29-30], 16 observers listened over headphones to 8 binaural clicks (i.e., 4 left-ear leading followed by 4 right-ear leading) separated by one of three ISIs (8, 64 and 120 ms), then reported the perceived location of each click-pair within the head. Flashing a light rightward across a CRT screen during temporally coincident click-pairs made observers report the location of the sounds in roughly equally spaced steps from left-to-right through the head. In contrast, light flashes originating on the right of the CRT and moving leftward reversed the perceived location of the clicks, so that the sound appeared to originate on the right side of the head and shift leftward. These effects were diminished when the first four lights were all presented on one side of the CRT and the last four lights were all presented on the other side of the CRT. This multimodal phenomenon occurs although the light was perceived external to the head while the sounds presented over headphones were perceived within the head.