The effects of preceding lead-alone and lag-alone click trains on the buildup of echo suppression.

Spatial perception in echoic environments is influenced by recent acoustic history. For instance, echo suppression becomes more effective or "builds up" with repeated exposure to echoes having a consistent acoustic relationship to a temporally leading sound. Four experiments were conducted to investigate how buildup is affected by prior exposure to unpaired lead-alone or lag-alone click trains. Unpaired trains preceded lead-lag click trains designed to evoke and assay buildup. Listeners reported how many sounds they heard from the echo hemifield during the lead-lag trains. Stimuli were presented in free field (experiments 1 and 4) or dichotically through earphones (experiments 2 and 3). In experiment 1, listeners reported more echoes following a lead-alone train compared to a period of silence. In contrast, listeners reported fewer echoes following a lag-alone train; similar results were observed with earphones. Interestingly, the effects of lag-alone click trains on buildup were qualitatively different when compared to a no-conditioner trial type in experiment 4. Finally, experiment 3 demonstrated that the effects of preceding click trains on buildup cannot be explained by a change in counting strategy or perceived click salience. Together, these findings demonstrate that echo suppression is affected by prior exposure to unpaired stimuli.

Neural time course of visually enhanced echo suppression.

Auditory spatial perception plays a critical role in day-to-day communication. For instance, listeners utilize acoustic spatial information to segregate individual talkers into distinct auditory "streams" to improve speech intelligibility. However, spatial localization is an exceedingly difficult task in everyday listening environments with numerous distracting echoes from nearby surfaces, such as walls. Listeners' brains overcome this unique challenge by relying on acoustic timing and, quite surprisingly, visual spatial information to suppress short-latency (1-10 ms) echoes through a process known as "the precedence effect" or "echo suppression." In the present study, we employed electroencephalography (EEG) to investigate the neural time course of echo suppression both with and without the aid of coincident visual stimulation in human listeners. We find that echo suppression is a multistage process initialized during the auditory N1 (70-100 ms) and followed by space-specific suppression mechanisms from 150 to 250 ms. Additionally, we find a robust correlate of listeners' spatial perception (i.e., suppressing or not suppressing the echo) over central electrode sites from 300 to 500 ms. Contrary to our hypothesis, vision's powerful contribution to echo suppression occurs late in processing (250-400 ms), suggesting that vision contributes primarily during late sensory or decision making processes. Together, our findings support growing evidence that echo suppression is a slow, progressive mechanism modifiable by visual influences during late sensory and decision making stages. Furthermore, our findings suggest that audiovisual interactions are not limited to early, sensory-level modulations but extend well into late stages of cortical processing.

Spatial attention modulates the precedence effect.

Communication and navigation in real environments rely heavily on the ability to distinguish objects in acoustic space. However, auditory spatial information is often corrupted by conflicting cues and noise such as acoustic reflections. Fortunately the brain can apply mechanisms at multiple levels to emphasize target information and mitigate such interference. In a rapid phenomenon known as the precedence effect, reflections are perceptually fused with the veridical primary sound. The brain can also use spatial attention to highlight a target sound at the expense of distracters. Although attention has been shown to modulate many auditory perceptual phenomena, rarely does it alter how acoustic energy is first parsed into objects, as with the precedence effect. This brief report suggests that both endogenous (voluntary) and exogenous (stimulus-driven) spatial attention have a profound influence on the precedence effect depending on where they are oriented. Moreover, we observed that both types of attention could enhance perceptual fusion while only exogenous attention could hinder it. These results demonstrate that attention, by altering how auditory objects are formed, guides the basic perceptual organization of our acoustic environment.

Visual influences on echo suppression.

Locating sounds in realistic scenes is challenging because of distracting echoes and coarse spatial acoustic estimates. Fortunately, listeners can improve performance through several compensatory mechanisms. For instance, their brains perceptually suppress short latency (1-10 ms) echoes by constructing a representation of the acoustic environment in a process called the precedence effect. This remarkable ability depends on the spatial and spectral relationship between the first or precedent sound wave and subsequent echoes. In addition to using acoustics alone, the brain also improves sound localization by incorporating spatially precise visual information. Specifically, vision refines auditory spatial receptive fields and can capture auditory perception such that sound is localized toward a coincident visual stimulus. Although visual cues and the precedence effect are each known to improve performance independently, it is not clear whether these mechanisms can cooperate or interfere with each other. Here we demonstrate that echo suppression is enhanced when visual information spatially and temporally coincides with the precedent wave. Conversely, echo suppression is inhibited when vision coincides with the echo. These data show that echo suppression is a fundamentally multisensory process in everyday environments, where vision modulates even this largely automatic auditory mechanism to organize a coherent spatial experience.