Lateralization of acoustic stimuli by budgerigars ( Melopsittacus undulatus )
Welch, Thomas Eric
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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. Animals such as the barn owl and cat have “specialized” auditory systems that have been extensively studied, contributing a great deal to our understanding of the processes underlying sound localization. Virtually nothing is known about the mechanisms of sound localization in many other species with relatively “unspecialized” auditory systems, including the budgerigar ( Melopsittacus undulatus ), a small Australian parrot. Although behavioral measures of sound localization from several species of birds in the free-field and physiological correlates to sound localization from different stages in the auditory pathway of many species have been made, no study investigating the behavioral sensitivity to individual sound localization cues has been conducted in birds. The abilities of budgerigars to localize, detect, and discriminate a variety of sounds in quiet or noisy conditions are well known, but this is the first study to equip birds with headphones so that their sensitivity to interaural time difference (ITD) and interaural level difference (ILD) cues can be measured. Threshold ITD and ILDs were measured behaviorally in budgerigars using a two-alternative (left/right) identification task and a repeating background discrimination task. The stimuli were pure tones (0.5-8 kHz) and broad-band noise. Budgerigars appear to be less sensitive than humans and cats, as expected, and more similar to rabbits and monkeys in their abilities to lateralize signals having interaural intensive or temporal disparities. 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 their head size.