Lateralization of acoustic signals by dichotically listening budgerigars (Melopsittacus undulatus)

Abstract

Sound localization allows humans and animals to determine the direction of objects to seek or avoid and indicates the appropriate position to direct visual attention. Interaural time differences (ITDs) and interaural level differences (ILDs) are two primary cues that humans use to localize or lateralize sound sources. There is limited information about behavioral cue sensitivity in animals, especially animals with poor sound localization acuity and small heads, like budgerigars. ITD and ILD thresholds were measured behaviorally in dichotically listening budgerigars equipped with headphones in an identification task. Budgerigars were less sensitive than humans and cats, and more similar to rabbits, barn owls, and monkeys, in their abilities to lateralize dichotic signals. Threshold ITDs were relatively constant for pure tones below 4 kHz, and were immeasurable at higher frequencies. Threshold ILDs were relatively constant over a wide range of frequencies, similar to humans. Thresholds in both experiments were best for broadband noise stimuli. These lateralization results are generally consistent with the free field localization abilities of these birds, and add support to the idea that budgerigars may be able to enhance their cues to directional hearing (e.g., via connected interaural pathways) beyond what would be expected based on head size.

Audition and Hemispheric Specialization in Songbirds and New Evidence from Australian Magpies

“…The puzzle is how birds with small heads can identify the direction of sounds [128][129][130]. A more recent study suggests that budgerigars may be able to localize pure tones as high as 4 kHz based solely on ITD information and that small birds generally may be able to enhance directional hearing by using the acoustic coupling of the middle ear cavities and so perform well above expectations [131]. In larger birds, one suspects that head turning, studied in the context of visual perception, may be useful to identify sounds, and these could reveal side biases.…”

“…The same method (in principle) has been successfully employed in studying the ability of budgerigars to identify cues of interaural time differences (ITDs) and interaural level differences (ILDs) by implanting headphones [131], a technique also used to test left-right identification of sounds [136]. Interestingly, in humans, ITD performance drops off markedly for frequencies above 1.5 kHz, but budgerigars maintained sensitivity up to 4 kHz.…”

The Avian Head Induces Cues for Sound Localization in Elevation

“…Localization errors were estimated based on either the phase spectrum of the HRIR (pHRIR), the magnitude spectrum of the HRIR (mHRIR) or both combined (HRIR). Parameterizing the model with behavioural data in birds [20], we estimated the probability of sound originating from a given direction to be perceived as originating from any other direction. The localization error was expressed as the average angular deviation between the true origin of the sound and the perceived origin.…”

“…This allows us to assess the contribution of the mHRIR, which includes IIDs, in reducing the errors in the vertical plane. The parameters used in calculating the localization errors are derived from behavioural experiments [20]. In the following we describe the model used to estimate the localization errors.…”

“…The diagonal elements of the matrix represent the additive noise on the intensities and phases. These values were deduced from behavioral experiments [20].…”