Selective auditory grouping by zebra finches: testing the iambic-trochaic law

Abstract

Humans have a strong tendency to spontaneously group visual or auditory stimuli together in larger patterns. One of these perceptual grouping biases is formulated as the iambic/trochaic law, where humans group successive tones alternating in pitch and intensity as trochees (high-low and loud-soft) and alternating in duration as iambs (short-long). The grouping of alternations in pitch and intensity into trochees is a human universal and is also present in one non-human animal species, rats. The perceptual grouping of sounds alternating in duration seems to be affected by native language in humans and has so far not been found among animals. In the current study, we explore to which extent these perceptual biases are present in a songbird, the zebra finch. Zebra finches were trained to discriminate between short strings of pure tones organized as iambs and as trochees. One group received tones that alternated in pitch, a second group heard tones alternating in duration, and for a third group, tones alternated in intensity. Those zebra finches that showed sustained correct discrimination were next tested with longer, ambiguous strings of alternating sounds. The zebra finches in the pitch condition categorized ambiguous strings of alternating tones as trochees, similar to humans. However, most of the zebra finches in the duration and intensity condition did not learn to discriminate between training stimuli organized as iambs and trochees. This study shows that the perceptual bias to group tones alternating in pitch as trochees is not specific to humans and rats, but may be more widespread among animals.

Keywords: Acoustic perception; Iambic/trochaic law (ITL); Perceptual bias; Zebra finches.

Fig. 1

Example of six training stimuli. In each image, the top part shows the intensity of the tones and the bottom half shows the frequency. Two stimuli have changes in pitch: one stimulus with two duplets with iambic stress (a) and one stimulus with two duplets with trochaic stress (b); two with changes in duration: duplets with iambic stress (c) and duplets with trochaic stress (d); and two with changes in intensity: duplets with iambic stress (e) and with trochaic stress (f)

Fig. 2

Proportions of correct responses to the iambic and the trochaic training sounds. Duration iambs are quadruplets with increased duration of the second and fourth tone. Duration trochees are quadruplets with increased duration of the first and third tone. In the same fashion, intensity iambs are quadruplets with an increased intensity of the second and fourth tone and intensity trochees have an increased intensity on the first and third tone. Finally, pitch iambs are quadruplets with increased frequency of the second and fourth tone and pitch trochees are quadruplets with increased frequency on the first and third tone. The lines show the average responses of the 8 zebra finches in the pitch condition and 12 zebra finches in both the duration and the intensity condition, organized in blocks of 1000 trials

Fig. 3

Proportions of responses to the training and test stimuli of the pitch condition. The dark grey bars show the proportions of pecks on the iambic key, and the light grey bars show the pecks on the trochaic key. The white bars show the proportion of trials to which the birds did not respond by pecking on a key. The bars show the averages of all 8 zebra finches, and the error bars show the SEM