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The precedence effect encompasses several phenomena that influence sound perception and localization when multiple sound sources arrive at the listener in close succession (Brown et al. 2015; Litovsky et al. 1999). For a range of delays between a leading (i.e., first arriving) and lagging (i.e., later arriving) stimulus that arrive from different directions, receivers tend to perceive only a single sound source, a phenomenon known as fusion. When leading and lagging sounds are presented simultaneously or at extremely short delays, a single fused sound is perceived, but the sound’s location is perceived to be somewhere in between the actual location of the two sound sources, a phenomenon known as summing localization. At longer delays, receivers tend to perceive this fused sound as originating from the location of the leading sound source, a phenomenon known as localization dominance. Finally, as the delay between leading and lagging sounds increases beyond what is termed the echo threshold, receivers perceive both the leading and lagging sounds as separate images. Leading sounds can not only affect sound localization but also the ability of receivers to discriminate characteristics of the lagging sound, a phenomenon known as lag discrimination suppression.
Functional Significance of the Precedence Effect
The precedence effect in humans is generally thought to be a directional hearing mechanism that allows for the perception and localization of sounds in reverberant environments (Wallach et al. 1949). Fusion and localization dominance may reduce the influence of echoes arriving somewhat later and from different directions than sound originating from the source. Psychophysical studies of the various phenomena comprising the precedence effect in humans have shown that the specific time delays at which each phenomenon takes place vary with factors including stimulus duration, frequency, and interstimulus angle, whether stimuli are presented in the azimuthal or median sagittal plane, subject age, previous experience, and hearing capabilities (Litovsky et al. 1999).
The precedence effect also appears to be widespread in animal hearing, although its ecological relevance is less clear. In certain environments, reverberations may be as problematic for animals as they are for humans in buildings. However, in many other cases, reverberation is probably a less serious problem for sound localization than the presence of noise from both abiotic sources such as wind and flowing water and that produced by other signaling animals, as well as general sound attenuation and degradation through the environment. Nevertheless, there is conclusive evidence for one or more of the phenomena associated with the precedence effect in such disparate taxa as crickets, flies, frogs, birds, and mammals (Lee et al. 2009). Furthermore, in many cases, the time delays at which localization dominance and echo thresholds occur are similar in human and nonhuman animals. Precedence effects may be a general adaptation for auditory scene analysis and directional hearing in noisy and complex acoustic environments, which is a fundamental problem in animal communication.
Methods for Studying the Precedence Effect
In humans, the precedence effect is most often studied by presenting subjects with acoustic stimuli using either headphones or loudspeakers that vary in their relative timing or another relevant characteristic, and having the subjects report on various aspects of their perception of the stimuli (e.g., for fusion and localization dominance, whether one or two stimuli were perceived and where the stimulus was perceived to have originated, respectively). In nonhuman animals, the subject’s perception cannot be reported verbally, and most studies rely on either conditioning protocols or on responses that are an important component of the species’ natural behavioral repertoire (e.g., phonotaxis toward mate attraction signals, predator-avoidance responses, and orientation toward prey sounds). Animal studies are also used in neurophysiological investigations of the precedence effect. Indeed, correlates of the precedence effect have been found in most areas of the ascending auditory system, although certain precedence effect phenomena may only arise in more central processing centers. The inferior colliculus has been proposed to be a particularly important brain region for the generation of the precedence effect in mammals (Tollin et al. 2004), but the precedence effect is much more taxonomically widespread and may involve different neural mechanisms in other taxa, particularly insects.
The Precedence Effect and Preferences for Leading Signals in Mate Choice
In behavioral ecology studies, the term “precedence effect” is also used in a somewhat looser manner to refer to the observation that in mate choice, choosing individuals are often attracted to the first arriving of two mate attraction signals (Greenfield 2005). These leader preferences are behaviorally analogous to some aspects of the psychophysical precedence effect because they result in the animal orienting toward the leading of two signals, often with no apparent influence from the lagging signal. However, in only a few cases has it been demonstrated that the precedence effect (sensu stricto, as defined in the Introduction) is the mechanistic basis for such leader preferences (Marshall and Gerhardt 2010). Leader preferences could also arise because leading signals suffer from less masking. Furthermore, even if both signals are equally well-perceived as separate signals, a leader preference could be expressed if leading signalers are of higher quality and are therefore preferred as mates. The delays over which leading signals are preferred to lagging signals in mate choice are often much longer than echo thresholds measured with psychophysical protocols in the laboratory, although these two measurements are rarely carried out on the same species. It is therefore unclear if the precedence effect can account for leader preferences, although neither can it be ruled out because most psychophysical studies use simplified click stimuli, whereas most mate selection involves much more complex signals; echo thresholds depend strongly on stimulus characteristics.
In any event, it has been argued that precedence effects may be a general adaptation to localize an individual signaler among many competing signals, and this is a common situation faced by animals choosing mates. Comparative studies could reveal if the time course of various precedence effect phenomena varies across species and if this relates to the challenges associated with mate localization. Furthermore, regardless of the mechanistic basis, leader preferences appear to be an important mechanism behind the striking temporal patterning observed in many signaling assemblages. Such assemblages are often characterized by highly precise synchrony or alternation of signals and leader preferences, perhaps caused by the precedence effect, selected for competitive signal timing strategies that generate temporally structured choruses as epiphenomena (Greenfield 2005).
Precedence Effects and Perception in Complex Natural Environments
The precedence effect is just one of many mechanisms facilitating the localization and discrimination of signals in complex environments, and recent studies suggest that these mechanisms likely interact. Most of these studies have been carried out in humans, but the same phenomena are likely to be found in nonhuman animals facing similar ecological challenges. First, the precedence effect has recently been demonstrated in humans to be affected by visual input. Cross-modal integration of visual and acoustic stimuli can alter the localization of leading and lagging acoustic signals depending on which of these stimuli is in spatial proximity to the visual stimulus (Bishop et al. 2011). This phenomenon has only been tested in one nonhuman animal, the gray tree frog Hyla versicolor, which experiences the precedence effect but for which there was no evidence of cross-modal integration of visual and acoustic stimuli (Reichert et al. 2016). Interestingly, orientation toward leading signals has also been demonstrated in the visual modality alone: female fiddler crabs evaluate males’ claw waving displays and prefer males whose waves lead those of other nearby individuals (Reaney et al. 2008). Many animals evaluate multimodal signaling displays containing both visual and acoustic information, and it would therefore not be surprising if both modalities contributed to localization, potentially via the precedence effect. Second, the precedence effect in humans is strongly affected by recent experience, and the strength of the precedence effect in the perception of any given stimulus can be built up by previous experience with the same stimulus (Litovsky et al. 1999). This buildup, however, can be broken down if the spatial characteristics of the stimuli are suddenly altered. Both effects are likely to be important for animals evaluating signals composed of repetitions of single signal elements. Third, the precedence effect is strongly affected by other characteristics of signals besides their relative timing. Some of these other characteristics, in particular signal duration and frequency, as well as spatial separation, are likely to vary between populations and species, which may result in corresponding variation in the parameters and general importance of the precedence effect.
The precedence effect is widespread in animals and is an important mechanism for sound localization in complex environments. In humans, this is particularly important in indoor, reverberant environments. In nonhuman animals, echo suppression may also be an important function of the precedence effect, although the adaptive significance of precedence effects and their relative importance in various auditory perception and processing tasks is poorly understood in most species. One potentially important function of the precedence effect is to facilitate localization of a signaler in the midst of multiple other individuals signaling in close temporal proximity. The precedence effect may therefore play an important role in sexual selection resulting in receiver preferences for leading signals, although in only a few cases has it been confirmed that the precedence effect is indeed the mechanism responsible for these preferences.