Recognizing structure in novel tunes: differences between human and rats.

A central feature in music is the hierarchical organization of its components. Musical pieces are not a simple concatenation of chords, but are characterized by rhythmic and harmonic structures. Here, we explore if sensitivity to music structure might emerge in the absence of any experience with musical stimuli. For this, we tested if rats detect the difference between structured and unstructured musical excerpts and compared their performance with that of humans. Structured melodies were excerpts of Mozart's sonatas. Unstructured melodies were created by the recombination of fragments of different sonatas. We trained listeners (both human participants and Long-Evans rats) with a set of structured and unstructured excerpts, and tested them with completely novel excerpts they had not heard before. After hundreds of training trials, rats were able to tell apart novel structured from unstructured melodies. Human listeners required only a few trials to reach better performance than rats. Interestingly, such performance was increased in humans when tonality changes were included, while it decreased to chance in rats. Our results suggest that, with enough training, rats might learn to discriminate acoustic differences differentiating hierarchical music structures from unstructured excerpts. More importantly, the results point toward species-specific adaptations on how tonality is processed.

Sensitivity to the sonority sequencing principle in rats (Rattus norvegicus).

Albeit diverse, human languages exhibit universal structures. A salient example is the syllable, an important structure of language acquisition. The structure of syllables is determined by the Sonority Sequencing Principle (SSP), a linguistic constraint according to which phoneme intensity must increase at onset, reaching a peak at nucleus (vowel), and decline at offset. Such structure generates an intensity pattern with an arch shape. In humans, sensitivity to restrictions imposed by the SSP on syllables appears at birth, raising questions about its emergence. We investigated the biological mechanisms at the foundations of the SSP, testing a nonhuman, non-vocal-learner species with the same language materials used with humans. Rats discriminated well-structured syllables (e.g., pras) from ill-structured ones (e.g., lbug) after being familiarized with syllabic structures conforming to the SSP. In contrast, we did not observe evidence that rats familiarized with syllables that violate such constraint discriminated at test. This research provides the first evidence of sensitivity to the SSP in a nonhuman species, which likely stems from evolutionary-ancient cross-species biological predispositions for natural acoustic patterns. Humans' early sensitivity to the SSP possibly emerges from general auditory processing that favors sounds depicting an arch-shaped envelope, common amongst animal vocalizations. Ancient sensory mechanisms, responsible for processing vocalizations in the wild, would constitute an entry-gate for human language acquisition.

Detecting surface changes in a familiar tune: exploring pitch, tempo and timbre.

Humans recognize a melody independently of whether it is played on a piano or a violin, faster or slower, or at higher or lower frequencies. Much of the way in which we engage with music relies in our ability to normalize across these surface changes. Despite the uniqueness of our music faculty, there is the possibility that key aspects in music processing emerge from general sensitivities already present in other species. Here we explore whether other animals react to surface changes in a tune. We familiarized the animals (Long-Evans rats) with the "Happy Birthday" tune on a piano. We then presented novel test items that included changes in pitch (higher and lower octave transpositions), tempo (double and half the speed) and timbre (violin and piccolo). While the rats responded differently to the familiar and the novel version of the tune when it was played on novel instruments, they did not respond differently to the original song and its novel versions that included octave transpositions and changes in tempo.

Arc-shaped pitch contours facilitate item recognition in non-human animals.

Acoustic changes linked to natural prosody are a key source of information about the organization of language. Both human infants and adults readily take advantage of such changes to discover and memorize linguistic patterns. Do they so because our brain is efficiently wired to specifically process linguistic stimuli? Or are we co-opting for language acquisition purposes more general principles that might be inherited from our animal ancestors? Here, we address this question by exploring if other species profit from prosody to better process acoustic sequences. More specifically, we test whether arc-shaped pitch contours defining natural prosody might facilitate item recognition and memorization in rats. In two experiments, we presented to the rats nonsense words with flat, natural, inverted and random prosodic contours. We observed that the animals correctly recognized the familiarization words only when arc-shaped pitch contours were implemented over them. Our results suggest that other species might also benefit from prosody for the memorization of items in a sequence. Such capacity seems to be rooted in general principles of how biological sounds are produced and processed.

Early neural responses underlie advantages for consonance over dissonance.

Consonant musical intervals tend to be more readily processed than dissonant intervals. In the present study, we explore the neural basis for this difference by registering how the brain responds after changes in consonance and dissonance, and how formal musical training modulates these responses. Event-related brain potentials (ERPs) were registered while participants were presented with sequences of consonant intervals interrupted by a dissonant interval, or sequences of dissonant intervals interrupted by a consonant interval. Participants were musicians and non-musicians. Our results show that brain responses triggered by changes in a consonant context differ from those triggered in a dissonant context. Changes in a sequence of consonant intervals are rapidly processed independently of musical expertise, as revealed by a change-related mismatch negativity (MMN, a component of the ERPs triggered by an odd stimulus in a sequence of stimuli) elicited in both musicians and non-musicians. In contrast, changes in a sequence of dissonant intervals elicited a late MMN only in participants with prolonged musical training. These different neural responses might form the basis for the processing advantages observed for consonance over dissonance and provide information about how formal musical training modulates them.

Processing advantages for consonance: A comparison between rats (Rattus norvegicus) and humans (Homo sapiens).

Consonance is a salient perceptual feature in harmonic music associated with pleasantness. Besides being deeply rooted in how we experience music, research suggests consonant intervals are more easily processed than dissonant intervals. In the present work we explore from a comparative perspective if such processing advantage extends to more complex tasks such as the detection of abstract rules. We ran experiments on rule learning over consonant and dissonant intervals with nonhuman animals and human participants. Results show differences across species regarding the extent to which they benefit from differences in consonance. Animals learn abstract rules with the same ease independently of whether they are implemented over consonant intervals (Experiment 1), dissonant intervals (Experiment 2), or over a combination of them (Experiment 3). Humans, on the contrary, learn an abstract rule better when it is implemented over consonant (Experiment 4) than over dissonant intervals (Experiment 5). Moreover, their performance improves when there is a mapping between abstract categories defining a rule and consonant and dissonant intervals (Experiments 6 and 7). Results suggest that for humans, consonance might be used as a perceptual anchor for other cognitive processes as to facilitate the detection of abstract patterns. Lacking extensive experience with harmonic stimuli, nonhuman animals tested here do not seem to benefit from a processing advantage for consonant intervals. (PsycINFO Database Record

The use of interval ratios in consonance perception by rats (Rattus norvegicus) and humans (Homo sapiens).

Traditionally, physical features in musical chords have been proposed to be at the root of consonance perception. Alternatively, recent studies suggest that different types of experience modulate some perceptual foundations for musical sounds. The present study tested whether the mechanisms involved in the perception of consonance are present in an animal with no extensive experience with harmonic stimuli and a relatively limited vocal repertoire. In Experiment 1, rats were trained to discriminate consonant from dissonant chords and tested to explore whether they could generalize such discrimination to novel chords. In Experiment 2, we tested if rats could discriminate between chords differing only in their interval ratios and generalize them to different octaves. To contrast the observed pattern of results, human adults were tested with the same stimuli in Experiment 3. Rats successfully discriminated across chords in both experiments, but they did not generalize to novel items in either Experiment 1 or Experiment 2. On the contrary, humans not only discriminated among both consonance-dissonance categories, and among sets of interval ratios, they also generalized their responses to novel items. These results suggest that experience with harmonic sounds may be required for the construction of categories among stimuli varying in frequency ratios. However, the discriminative capacity observed in rats suggests that at least some components of auditory processing needed to distinguish chords based on their interval ratios are shared across species.