Spatiotemporal factors influence sound-source segregation in localization behavior

To program a goal-directed response in the presence of acoustic reflections, the audio-motor system should suppress the detection of time-delayed sources. We here examined the effects of spatial separation and inter-stimulus delay on the ability of human listeners to localize a pair of broadband sounds in the horizontal plane. Participants indicated how many sounds were heard and where these were perceived by making one or two head-orienting localization responses. Results suggest that perceptual fusion of the two sounds depends on delay, as well as spatial separation. Leading and lagging stimuli in close spatial proximity required longer stimulus delays (fusion echo thresholds) to be robustly perceptually separated than those further apart. Whenever participants heard one sound, their localization responses for synchronous sounds were oriented to a weighted average of both source locations. For short delays, responses were directed towards the leading stimulus location. Increasing spatial separation enhanced this effect. For longer delays, responses were again directed towards a weighted average. When participants perceived two sounds, the first and the second response were directed to either of the leading and lagging source locations. Perceived locations were interchanged often in their temporal order (in ~40% of trials), and this behavior did not vary systematically over a large range of delays and spatial separations. We show that the percept of two sounds occurring requires sufficient spatiotemporal separation, after which localization can be performed with high accuracy. We propose that the percept of temporal order of two concurrent sounds results from a different process than localization, and discuss how dynamic lateral excitatory-inhibitory interactions within a spatial sensorimotor map could explain the findings.为了在存在声学反射的情况下生成目标导向响应，听觉运动系统 (audio-motor system) 需要抑制对延时声源的检测。本研究考察了空间间距与刺激间延时对人类听众在水平面定位一对宽带声音 (broadband sounds) 的能力的影响。被试通过做出单次或两次头部转向的定位反应，报告其所感知到的声音数量与声源位置。 研究结果表明，两种声音的知觉融合 (perceptual fusion) 同时依赖于延时与空间间距。空间邻近的先导刺激与滞后刺激，相较于间距较远的刺激，需要更长的刺激延时 (fusion echo thresholds) 才能被稳定地知觉为分离。 当被试仅感知到一个声音时，其对同步声音的定位反应会指向两个声源位置的加权平均值。当延时较短时，反应会指向先导刺激的位置；空间间距越大，这一效应越显著。而当延时较长时，定位反应则再次回归至加权平均位置。 当被试感知到两个声音时，第一次与第二次定位反应会分别指向先导与滞后声源的位置。约40%的试次中，被试报告的声源位置会出现时序互换，且这一行为在较大的延时与空间间距范围内均无系统性变化。 本研究表明，感知到两个声音需要足够的时空分离，在此之后定位任务可被高精度完成。本研究提出，同时呈现的两个声音的时序知觉源于与定位不同的加工过程，并讨论了空间感觉运动映射 (spatial sensorimotor map) 内的动态侧向兴奋-抑制交互作用如何能够解释本研究的实验发现。