Audio-visual synchrony detection under scotopic conditions

In cluttered visual displays, abrupt, uniquely synchronised audio-visual events capture attention. This study compares the temporal determinants of this ‘pip and pop’ phenomenon under photopic and scotopic conditions. In conjunction with prolonged dark adaptation, scotopic conditions were achieved using goggle-mounted neutral density filters. The primary task involved identifying the orientation of a target singleton, horizontally or vertically presented among 5, 9 or 13 distractor lines tilted (±4°: photopic; ±10°: scotopic) from horizontal and vertical. Target and distractor elements were each surrounded by a luminance-defined annulus modulating at 0.78 Hz, each with a unique temporal phase. On half of the trials an amplitude-modulating 500Hz tone was synchronised with the modulation of the target annulus and was absent on the remaining trials. The modulation profile of luminance and tone was sinusoidal or square. For sinusoidally modulating trials, the presence of the tone had no effect, with
highly serial search performance under both illumination conditions. For square-wave trials the presence of the tone dramatically improved search efficiency only under photopic conditions, with no tone-related improvement observed scotopically. This result persisted after advancing the tone in time to account for luminance-related lags in neural latency. Contrary to predictions based on the scotopic temporal channels literature, the effects of illumination on AV synchrony-driven visual search appear to result from a profound low-pass temporal filtering within the scotopic system.