Supplementary Notes

Description of the Model.

We assume that a layer of phrase-selective units arises from innate and experience- dependent forces (Suppl. Figure). These cells then provide inputs to a second layer, wherein units respond selectively for particular pairs of phrases. The development of these combination-sensitive units could arise from direct instructive or selective processes; in the former only AB and BC detectors are formed, whereas in the selective scheme many or all combinations are represented initially, but primarily AB and BC types are maintained. These sensory units, representing the acquired template, could be activated by auditory feedback from self-generated vocalizations and provide reinforcement for “correct” motor patterns. On the motor side, as phrase generators develop, combinatorial processes give rise to a layer of units that generate phrase pairs and a subsequent layer that generates sequences of 3 phrases. Only the sequence ABC is reinforced maximally at each phrase linkage and, therefore, is maintained. The beginning of the song can be determined as the pair containing a phrase that is never in the second position of any of the tutoring pairs. Each phrase occurs only once in the final ‘song’ i.e., ABC instead of ABBC because BB detectors were not formed/maintained during tutoring, resulting in less reinforcement at this position. The end of the song is determined by a syllable that never occurs in the first position of a tutor pair. This model, presented here for 3 phrases, can be extended to explain the development of longer phrase sequences. The acquired template in this model minimally consists of a population of combination-sensitive neurons, i.e., neurons that respond best when a particular pair of phrases is presented in a specific order. Supplementary Figure legend:

Model of the development of a song consisting of three phrases: whistles (A), note complexes (B), and a buzz (C). Units that respond selectively to (sensory) or generate (motor) each phrase or pair of phrases are denoted by the corresponding letters. The strength of representation of sensory or motor units is indicated by the size of these letters. On the sensory side, units at the first level selectively respond to phrases A, B or C. These hypothetical cells are assumed to arise from a combination of innate and experience-dependent (tutoring) processes; because the focus of this model and study is on how phrase order is established, details of this development are not specified. Tutoring with pairs of phrases then either directly promotes the formation of units that respond selectively to AB or BC phrase pairs (instructive process), or maintains these units and ‘down-regulates’ (red) those selective for other phrase pairs (selective process). On the motor side, three levels of motor command are provided: production of single phrases, phrase pairs and triplets. A diversity of phrase combinations, as is shown, was observed during song development in this study. Motor patterns (phrase pairings) are then reinforced (blue arrows) to the extent that they excite, via auditory feedback, sensory units that are selective for phrase pairings that were present in the tutoring ensembles. Only the sequence ABC is reinforced at both phrase linkage positions. Because this sequence also is maximally reinforced, the motor instructions for generating this pattern are retained at the end of song development.