Comodulation detection differences in the hooded crow (Corvus corone cornix), with direct comparison to human subjects

Detection of modulated tones in modulated noise by non-human primates

In natural environments, many sounds are amplitude-modulated. Amplitude modulation is thought to be a signal that aids auditory object formation. A previous study of the detection of signals in noise found that when tones or noise were amplitude-modulated, the noise was a less effective masker, and detection thresholds for tones in noise were lowered. These results suggest that the detection of modulated signals in modulated noise would be enhanced. This paper describes the results of experiments investigating how detection is modified when both signal and noise were amplitude-modulated. Two monkeys (Macaca mulatta) were trained to detect amplitude-modulated tones in continuous, amplitude-modulated broadband noise. When the phase difference of otherwise similarly amplitude-modulated tones and noise were varied, detection thresholds were highest when the modulations were in phase and lowest when the modulations were anti-phase. When the depth of the modulation of tones or noise was varied, detection thresholds decreased if the modulations were anti-phase. When the modulations were in phase, increasing the depth of tone modulation caused an increase in tone detection thresholds, but increasing depth of noise modulations did not affect tone detection thresholds. Changing the modulation frequency of tone or noise caused changes in threshold that saturated at modulation frequencies higher than 20 Hz; thresholds decreased when the tone and noise modulations were in phase and decreased when they were anti-phase. The relationship between reaction times and tone level were not modified by manipulations to the nature of temporal variations in the signal or noise. The changes in behavioral threshold were consistent with a model where the brain subtracted noise from signal. These results suggest that the parameters of the modulation of signals and maskers heavily influence detection in very predictable ways. These results are consistent with some results in humans and avians and form the baseline for neurophysiological studies of mechanisms of detection in noise.

Treefrogs as animal models for research on auditory scene analysis and the cocktail party problem

The perceptual analysis of acoustic scenes involves binding together sounds from the same source and separating them from other sounds in the environment. In large social groups, listeners experience increased difficulty performing these tasks due to high noise levels and interference from the concurrent signals of multiple individuals. While a substantial body of literature on these issues pertains to human hearing and speech communication, few studies have investigated how nonhuman animals may be evolutionarily adapted to solve biologically analogous communication problems. Here, I review recent and ongoing work aimed at testing hypotheses about perceptual mechanisms that enable treefrogs in the genus Hyla to communicate vocally in noisy, multi-source social environments. After briefly introducing the genus and the methods used to study hearing in frogs, I outline several functional constraints on communication posed by the acoustic environment of breeding "choruses". Then, I review studies of sound source perception aimed at uncovering how treefrog listeners may be adapted to cope with these constraints. Specifically, this review covers research on the acoustic cues used in sequential and simultaneous auditory grouping, spatial release from masking, and dip listening. Throughout the paper, I attempt to illustrate how broad-scale, comparative studies of carefully considered animal models may ultimately reveal an evolutionary diversity of underlying mechanisms for solving cocktail-party-like problems in communication.

Pulse-number discrimination by Cope's gray treefrog (Hyla chrysoscelis) in modulated and unmodulated noise

In Cope's gray treefrog (Hyla chrysoscelis), thresholds for recognizing conspecific calls are lower in temporally modulated noise backgrounds compared with unmodulated noise. The effect of modulated noise on discrimination among different conspecific calls is unknown. In quiet, females prefer calls with relatively more pulses. This study tested the hypotheses that noise impairs selectivity for longer calls and that processes akin to dip listening in modulated noise can ameliorate this impairment. In two-stimulus choice tests, female subjects were allowed to choose between an average-length call and a shorter or longer alternative. Tests were replicated at two signal levels in quiet and in the presence of chorus-shaped noise that was unmodulated, modulated by a sinusoid, or modulated by envelopes resembling natural choruses. When subjects showed a preference, it was always for the relatively longer call. Noise reduced preferences for longer calls, but the magnitude of this reduction was unrelated to whether the noise envelope was modulated or unmodulated. Together, the results are inconsistent with the hypothesis that dip listening improves a female gray treefrog's ability to select longer calls in modulated compared with unmodulated noise.

Dip listening or modulation masking? Call recognition by green treefrogs (Hyla cinerea) in temporally fluctuating noise

Despite the importance of perceptually separating signals from background noise, we still know little about how nonhuman animals solve this problem. Dip listening, an ability to catch meaningful 'acoustic glimpses' of a target signal when fluctuating background noise levels momentarily drop, constitutes one possible solution. Amplitude-modulated noises, however, can sometimes impair signal recognition through a process known as modulation masking. We asked whether fluctuating noise simulating a breeding chorus affects the ability of female green treefrogs (Hyla cinerea) to recognize male advertisement calls. Our analysis of recordings of the sounds of green treefrog choruses reveal that their levels fluctuate primarily at rates below 10 Hz. In laboratory phonotaxis tests, we found no evidence for dip listening or modulation masking. Mean signal recognition thresholds in the presence of fluctuating chorus-like noises were never statistically different from those in the presence of a non-fluctuating control. An analysis of statistical effects sizes indicates that masker fluctuation rates, and the presence versus absence of fluctuations, had negligible effects on subject behavior. Together, our results suggest that females listening in natural settings should receive no benefits, nor experience any additional constraints, as a result of level fluctuations in the soundscape of green treefrog choruses.

Receiver psychology turns 20: is it time for a broader approach?

Twenty years ago, a new conceptual paradigm known as 'receiver psychology' was introduced to explain the evolution of animal communication systems. This paradigm advanced the idea that psychological processes in the receiver's nervous system influence a signal's detectability, discriminability and memorability, and thereby serve as powerful sources of selection shaping signal design. While advancing our understanding of signal diversity, more recent studies make clear that receiver psychology, as a paradigm, has been structured too narrowly and does not incorporate many of the perceptual and cognitive processes of signal reception that operate between sensory transduction and a receiver's response. Consequently, the past two decades of research on receiver psychology have emphasized considerations of signal evolution but failed to ask key questions about the mechanisms of signal reception and their evolution. The primary aim of this essay is to advocate for a broader receiver psychology paradigm that more explicitly includes a research focus on receivers' psychological landscapes. We review recent experimental studies of hearing and sound communication to illustrate how considerations of several general perceptual and cognitive processes will facilitate future research on animal signalling systems. We also emphasize how a rigorous comparative approach to receiver psychology is critical to explicating the full range of perceptual and cognitive processes involved in receiving and responding to signals.

Enhanced signal detectability in comodulated noise introduced by compression

Many examples of natural noise show common amplitude modulations at different frequency regions. This kind of noise has been termed comodulated noise and is widely examined in hearing research, where an enhanced detectability of pure tones and narrow noise bands in comodulated noise compared to unmodulated noise is well known as the CMR or CDD effects, respectively. Here it is shown that only one signal processing step, a compressive nonlinearity motivated by the peripheral auditory system, is sufficient to explain a considerable contribution to these effects. Using an analytical approach, the influence of compression on the detectability of periodic and narrow band signals in the presence of unmodulated and comodulated noise is investigated. This theoretical treatment allows for identifying the mechanism leading to improved signal detection. The compressive nonlinearity constitutes an adaptive gain which selectively boosts a stimulus during time spans of inherently increased signal-to-noise ratio and attenuates it during time spans dominated by noise. On average, these time spans are more pronounced in stimuli with comodulated noise than with unmodulated noise, thus giving rise to the observed CMR and CDD effects.

The cocktail party problem: what is it? How can it be solved? And why should animal behaviorists study it?

Animals often use acoustic signals to communicate in groups or social aggregations in which multiple individuals signal within a receiver's hearing range. Consequently, receivers face challenges related to acoustic interference and auditory masking that are not unlike the human cocktail party problem, which refers to the problem of perceiving speech in noisy social settings. Understanding the sensory solutions to the cocktail party problem has been a goal of research on human hearing and speech communication for several decades. Despite a general interest in acoustic signaling in groups, animal behaviorists have devoted comparatively less attention toward understanding how animals solve problems equivalent to the human cocktail party problem. After illustrating how humans and nonhuman animals experience and overcome similar perceptual challenges in cocktail-party-like social environments, this article reviews previous psychophysical and physiological studies of humans and nonhuman animals to describe how the cocktail party problem can be solved. This review also outlines several basic and applied benefits that could result from studies of the cocktail party problem in the context of animal acoustic communication.

Peripheral and central aspects of auditory across-frequency processing

Many natural sounds such as, e.g., speech show common level fluctuations across frequency. It is generally assumed that the auditory system uses this spectro-temporal information to group the frequency components into auditory objects although the exact physiological mechanism is still not fully understood. The aim of the present study is to disentangle the relative contribution of peripheral and central aspects of this across-frequency processing using psychophysical experiments and modelling. The study focuses on two different psychophysical phenomena which are thought to be related to the ability to compare information across frequency: comodulation masking release (CMR), i.e., a release from masking of a sinusoidal signal due to the addition of a comodulated off-frequency masker component to the masker component at the signal frequency, and comodulation detection difference (CDD), i.e., the reduced ability of the auditory system to detect a masked signal if masker and signal share the same envelope. The comparison between model predictions and experimental results indicates that a considerable amount of these effects can be accounted for by peripheral processing alone. This is confirmed by experimental results with confounding across-frequency information about the grouping of the different frequencies into auditory objects.

Detecting modulated signals in modulated noise: (I) behavioural auditory thresholds in a songbird

Most signals from the auditory world have temporal patterns of amplitude modulation that either emanate from the signal source or result from environmental interference (e.g. air turbulence). To investigate mechanisms associated with the segregation and processing of amplitude-modulated signals, we trained European starlings (Sturnus vulgaris) to detect a signal noise band embedded in several flanking noise bands (FBs). We manipulated the envelope correlation between the signal and FBs, the onset synchrony between signal and FBs (0 or100 ms), signal duration (60 or 400 ms) and the spectrum level of the FBs (15 or 50 dB). The lowest signal-detection thresholds were found when the envelopes of the FBs were correlated with each other but different from the signal envelope (the 'co-uncorrelated' condition). Detection thresholds were on average 7 dB higher when both the signal and the FBs had correlated envelopes (the 'all correlated' condition). Thresholds were even higher when the envelopes of all noise bands were independent (the 'all uncorrelated' condition). The difference in detection thresholds between the co-uncorrelated and the all correlated conditions is termed 'comodulation detection difference' (CDD). Differences in signal duration and masker level had significant effects on detection threshold, but not on CDD magnitudes; differences in onset synchrony had no effects. We compare data from starlings with those from previous psychoacoustic studies of humans, and discuss possible mechanisms on which these perceptual effects may rely. Our behavioural data are the reference for a companion study investigating CDD at the neuronal level of the starling [M.A. Bee et al. (2007) Eur. J. Neurosci., 26, 1979-1994].