Chickens have excellent sound localization ability.

The mechanisms of sound localization are actively debated, especially which cues are predominately used and why. Our study provides behavioural data in chickens (Gallus gallus) and relates these to estimates of the perceived physical cues. Sound localization acuity was quantified as the minimum audible angle (MAA) in azimuth. Pure-tone MAA was 12.3, 9.3, 8.9 and 14.5 deg for frequencies of 500, 1000, 2000 and 4000 Hz, respectively. Broadband-noise MAA was 12.2 deg, which indicates excellent behavioural acuity. We determined 'external cues' from head-related transfer functions of chickens. These were used to derive 'internal cues', taking into account published data on the effect of the coupled middle ears. Our estimates of the internal cues indicate that chickens likely relied on interaural time difference cues alone at low frequencies of 500 and 1000 Hz, whereas at 2000 and 4000 Hz, interaural level differences may be the dominant cue.

The barn owls' Minimum Audible Angle.

Interaural time differences (ITD) and interaural level differences (ILD) are physical cues that enable the auditory system to pinpoint the position of a sound source in space. This ability is crucial for animal communication and predator-prey interactions. The barn owl has evolved an exceptional sense of hearing and shows abilities of sound localisation that outperform most other species. So far, behavioural studies in the barn owl often used reflexive responses to investigate aspects of sound localisation. Furthermore, they predominately probed the higher frequencies of the owl's hearing range (> 3 kHz). In the present study we used a Go/NoGo paradigm to measure the barn owl's behavioural sound localisation acuity (expressed as the Minimum Audible Angle, MAA) as a function of stimulus type (narrow-band noise centred at 500, 1000, 2000, 4000 and 8000 Hz, and broad-band noise) and sound source position. We found significant effects of both stimulus type and sound source position on the barn owls' MAA. The MAA improved with increasing stimulus frequency, from 14° at 500 Hz to 6° at 8000 Hz. The smallest MAA of 4° was found for broadband noise stimuli. Comparing different sound source positions revealed smaller MAAs for frontal compared to lateral stimulus presentation, irrespective of stimulus type. These results are consistent with both the known variations in physical ITDs and variation in the width of neural ITD tuning curves with azimuth and frequency. Physical and neural characteristics combine to result in better spatial acuity for frontal compared to lateral sounds and reduced localisation acuity at lower frequencies.

Barn owls have ageless ears.

We measured the auditory sensitivity of the barn owl (Tyto alba), using a behavioural Go/NoGo paradigm in two different age groups, one younger than 2 years (n = 4) and another more than 13 years of age (n = 3). In addition, we obtained thresholds from one individual aged 23 years, three times during its lifetime. For computing audiograms, we presented test frequencies of between 0.5 and 12 kHz, covering the hearing range of the barn owl. Average thresholds in quiet were below 0 dB sound pressure level (SPL) for frequencies between 1 and 10 kHz. The lowest mean threshold was -12.6 dB SPL at 8 kHz. Thresholds were the highest at 12 kHz, with a mean of 31.7 dB SPL. Test frequency had a significant effect on auditory threshold but age group had no significant effect. There was no significant interaction between age group and test frequency. Repeated threshold estimates over 21 years from a single individual showed only a slight increase in thresholds. We discuss the auditory sensitivity of barn owls with respect to other species and suggest that birds, which generally show a remarkable capacity for regeneration of hair cells in the basilar papilla, are naturally protected from presbycusis.

Moving Objects in the Barn Owl's Auditory World.

Barn owls are keen hunters of moving prey. They have evolved an auditory system with impressive anatomical and physiological specializations for localizing their prey. Here we present behavioural data on the owl's sensitivity for discriminating acoustic motion direction in azimuth that, for the first time, allow a direct comparison of neuronal and perceptual sensitivity for acoustic motion in the same model species. We trained two birds to report a change in motion direction within a series of repeating wideband noise stimuli. For any trial the starting point, motion direction, velocity (53-2400°/s), duration (30-225 ms) and angular range (12-72°) of the noise sweeps were randomized. Each test stimulus had a motion direction being opposite to that of the reference stimuli. Stimuli were presented in the frontal or the lateral auditory space. The angular extent of the motion had a large effect on the owl's discrimination sensitivity allowing a better discrimination for a larger angular range of the motion. In contrast, stimulus velocity or stimulus duration had a smaller, although significant effect. Overall there was no difference in the owls' behavioural performance between "inward" noise sweeps (moving from lateral to frontal) compared to "outward" noise sweeps (moving from frontal to lateral). The owls did, however, respond more often to stimuli with changing motion direction in the frontal compared to the lateral space. The results of the behavioural experiments are discussed in relation to the neuronal representation of motion cues in the barn owl auditory midbrain.

Effects of signal features and background noise on distance cue discrimination by a songbird.

During the transmission of acoustic signals, the spectral and temporal properties of the original signal are degraded, and with increasing distance more and more echo patterns are imposed. It is well known that these physical alterations provide useful cues to assess the distance of a sound source. Previous studies in birds have shown that birds employ the degree of degradation of a signal to estimate the distance of another singing male (referred to as ranging). Little is known about how acoustic masking by background noise interferes with ranging, and if the number of song elements and stimulus familiarity affect the ability to discriminate between degraded and undegraded signals. In this study we trained great tits (Parus major L.) to discriminate between signal variants in two background types, a silent condition and a condition consisting of a natural dawn chorus. We manipulated great tit song types to simulate patterns of reverberation and degradation equivalent to transmission distances of between 5 and 160 m. The birds' responses were significantly affected by the differences between the signal variants and by background type. In contrast, stimulus familiarity or their element number had no significant effect on signal discrimination. Although background type was a significant main effect with respect to the response latencies, the great tits' overall performance in the noisy dawn chorus was similar to the performance in silence.

Why longer song elements are easier to detect: threshold level-duration functions in the Great Tit and comparison with human data.

Our study estimates detection thresholds for tones of different durations and frequencies in Great Tits (Parus major) with operant procedures. We employ signals covering the duration and frequency range of communication signals of this species (40-1,010 ms; 2, 4, 6.3 kHz), and we measure threshold level-duration (TLD) function (relating threshold level to signal duration) in silence as well as under behaviorally relevant environmental noise conditions (urban noise, woodland noise). Detection thresholds decreased with increasing signal duration. Thresholds at any given duration were a function of signal frequency and were elevated in background noise, but the shape of Great Tit TLD functions was independent of signal frequency and background condition. To enable comparisons of our Great Tit data to those from other species, TLD functions were first fitted with a traditional leaky-integrator model. We then applied a probabilistic model to interpret the trade-off between signal amplitude and duration at threshold. Great Tit TLD functions exhibit features that are similar across species. The current results, however, cannot explain why Great Tits in noisy urban environments produce shorter song elements or faster songs than those in quieter woodland environments, as detection thresholds are lower for longer elements also under noisy conditions.

Auditory memory for temporal characteristics of sound.

This study evaluates auditory memory for variations in the rate of sinusoidal amplitude modulation (SAM) of noise bursts in the European starling (Sturnus vulgaris). To estimate the extent of the starling's auditory short-term memory store, a delayed non-matching-to-sample paradigm was applied. The birds were trained to discriminate between a series of identical "sample stimuli" and a single "test stimulus". The birds classified SAM rates of sample and test stimuli as being either the same or different. Memory performance of the birds was measured as the percentage of correct classifications. Auditory memory persistence time was estimated as a function of the delay between sample and test stimuli. Memory performance was significantly affected by the delay between sample and test and by the number of sample stimuli presented before the test stimulus, but was not affected by the difference in SAM rate between sample and test stimuli. The individuals' auditory memory persistence times varied between 2 and 13 s. The starlings' auditory memory persistence in the present study for signals varying in the temporal domain was significantly shorter compared to that of a previous study (Zokoll et al. in J Acoust Soc Am 121:2842, 2007) applying tonal stimuli varying in the spectral domain.

Auditory memory: a comparison between humans and starlings.

In this study, we compare the processing of acoustic signals in European starlings (Sturnus vulgaris) and in human listeners by observing the decay of short-term auditory memory in delayed non-matching-to-sample experiments. A series of identical "sample" stimuli and a final "test" stimulus were separated by variable delays (1 to 180.1 s). Subjects had to classify sample and test stimuli as being either the same or different. Test stimuli were pure tones that differed in a single signal feature, i.e., frequency, and song motifs that differed in multiple signal characteristics. We have tested several predictions concerning the memory performance of starlings and humans and we obtained the following outcome: (1) In contrast to our expectation, signal complexity had no effect. The overall analysis of the starling data did not show differences in memory performance for signals differing in single or multiple signal features. (2) Starling and human data supported the hypothesis that auditory memory impairs with increasing delay. This was also seen when interfering noise was added to the delay periods in an additional series with human subjects. (3) The starling data showed that the repetition of sample stimuli improved memory performance, compared to only a single presentation. Human memory performance, however, was similar for a single and for the repeated presentation of signals. (4) Differences in salience between sample and test stimuli were positively related to memory performance only for tonal stimuli but not for song motifs. Results are discussed with respect to a model based on signal detection theory and to requirements for the analysis of natural communication signals.

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].

Auditory short-term memory persistence for tonal signals in a songbird.

This paper presents an animal model for studying the persistence of auditory memory for tonal signals. Five European starlings (Sturnus vulgaris) were trained in a Go/NoGo delayed nonmatching-to-sample task to discriminate between a series of identical "sample stimuli" and a single "test stimulus." Frequencies of sample and test stimuli should be classified as being either the same or different. The performance of the birds is measured as the percentage of correct classifications. Three parameters were varied: The delay between sample and test stimuli, the number of sample stimuli presented before the test stimulus, and the salience of the difference between sample and test stimuli. Auditory memory persistence time was estimated as a function of the delay between the last sample stimulus and the test stimulus. The performance of the birds deteriorated with increasing delay before the test stimulus. Increasing the number of sample stimuli in the series of tones presented before the test stimulus improved performance. Performance was also better for more salient differences between sample and test stimuli. The individual auditory memory persistence time varied between 4 and 20 s.

Asymmetry of masking in the European starling: behavioural auditory thresholds.

Psychophysical studies of simultaneous masking with human observers exhibit an asymmetry in the amount of masking that depends on the relative bandwidths of signals and maskers. For noise bands up to the bandwidth of one auditory filter, masked auditory thresholds are considerably lower when the bandwidth of the signal exceeds that of the masker compared to the reversed condition. We investigate asymmetry of masking in an animal model, that will allow to study the mechanisms associated with the asymmetry of masking effect. European starlings (Sturnus vulgaris) were trained in a Go/NoGo paradigm to report the detection of a 500 ms noise signal centred in a 700 ms noise masker. Signals and maskers with centre frequencies of 2 kHz had bandwidths of 4 Hz or 256 Hz. Thresholds for detecting the 256 Hz wide-band signal in a 4 Hz narrow-band masker were considerably lower compared to detecting the 4 Hz narrow-band signal in a 256 Hz wide masker and compared to all other conditions. The asymmetry of masking in starlings was on average 15 and 17 dB for 40 and 70 dB SPL overall masker level, respectively. Our animal model thus proved perceptual abilities similar to human subjects. The results are discussed with respect to the importance of both intensity and temporal cues for signal detection.