Persistent representation of juvenile experience in the adult songbird brain

Abstract

Juveniles sometimes learn behaviors that they cease to express as adults. Whether the adult brain retains a record of experiences associated with behaviors performed transiently during development remains unclear. We addressed this issue by studying neural representations of song in swamp sparrows, a species in which juveniles learn and practice many more songs than they retain in their adult vocal repertoire. We exposed juvenile swamp sparrows to a suite of tutor songs and confirmed that, although many tutor songs were imitated during development, not all copied songs were retained into adulthood. We then recorded extracellularly in the sensorimotor nucleus HVC in anesthetized sparrows to assess neuronal responsiveness to songs in the adult repertoire, tutor songs, and novel songs. Individual HVC neurons almost always responded to songs in the adult repertoire and commonly responded even more strongly to a tutor song. Effective tutor songs were not simply those that were acoustically similar to songs in the adult repertoire. Moreover, the strength of tutor song responses was unrelated to the number of times that the bird sang copies of those songs in juvenile or adult life. Notably, several neurons responded most strongly to a tutor song performed only rarely and transiently during juvenile life, or even to a tutor song for which we could find no evidence of ever having been copied. Thus, HVC neurons representing songs in the adult repertoire also appear to retain a lasting record of certain tutor songs, including those imitated only transiently.

Figure 1.

Swamp sparrows heard tutor songs only during juvenile development. A, During the sensitive period of their first year of life, swamp sparrows memorize the songs they later recall and perform as adults, and those songs are stored in memory until the birds begin to sing in the following year (Marler and Peters, 1981) (top). In this study (bottom), nestlings were collected from the wild at ∼5 d of age and tutored in the laboratory using 21 conspecific songs (“tutor” songs). Each bird's vocalizations were documented during sensorimotor song development (“subsong” and “plastic” song) and beyond vocal maturation (“crystallized” song). After each bird's auditory and vocal life history had been documented, adult birds were anesthetized and neural responses to song stimuli were recorded. B, Tutor songs consisted of trilled multinote syllables (open boxes; song duration, ∼2 s; truncated for clarity) that were distinct in their spectrotemporal structure.

Figure 2.

Juvenile swamp sparrows develop their adult song repertoire through a process of juvenile overproduction and attrition. A, Songs performed during development were classified as juvenile subsong (top), juvenile plastic song (middle), or adult crystallized song [bottom left, songs from the same bird depicted in each panel; developmental criteria defined by Marler and Peters (1982a)]. Because imitation of tutor songs could be discerned in plastic and crystallized songs (gray boxes indicate similar syllables), those songs were further characterized as plastic or crystallized performances of the bird's copy of the corresponding tutor song (bottom right). B, C, Vocal maturation was characterized by a reduction in the number of distinct song types that a bird performed each day (all 6 birds shown) (B) and an increasing proportion of crystallized songs in the bird's daily vocal output (C). The transition from the juvenile “plastic state” into the adult “crystallized state” was defined as the first day on which the bird sang >80% of its daily song output as crystallized songs (dotted horizontal line). This transition was typically rapid, and production of predominantly crystallized songs persisted after crystallization (all 6 birds shown). D, Each bird's auditory and vocal life history could be summarized as a Venn diagram. During development, each bird heard the same set of 21 tutor songs (outer circle), imitated only a portion of those tutor songs in the juvenile state (middle circle), and retained only a subset of those juvenile song types as its adult song repertoire (inner circle). All songs that were part of the adult repertoire were also performed during juvenile learning.

Figure 3.

HVC neurons respond robustly to songs in the adult repertoire. A, Electrophysiological activity was recorded extracellularly from individual HVC neurons in anesthetized adult male swamp sparrows (parasagittal schematic of the swamp sparrow brain showing the telencephalic nucleus HVC and its auditory inputs, collectively represented as a dotted arrow). B, Multiunit auditory responses (top) to presentation of song stimuli (bottom) was collected and sorted off-line into records of single-unit activity [WaveClus (Quiroga et al., 2004)] (see Materials and Methods). C, As reported previously for HVC neurons in anesthetized swamp sparrows (Mooney et al., 2001), HVC neurons responded robustly and selectively to songs in the adult repertoire, with strong responses to forward playback of those songs (FWD, top) but little or no response to reverse playback of the same songs (REV, bottom; peristimulus time histogram bin size, 10 ms; song stimuli shown as oscillograms). D, Across the population of all HVC neurons sampled in all birds, songs in the adult repertoire evoked stronger responses than the set of all conspecific tutor and novel songs [mean d′ value of Gaussian fit (solid line) = 0.48, p = 0.004, Wilcoxon's signed rank test, 20 cells, 6 birds; positive d′ values indicate a stronger response to the strongest adult song type (Mooney et al., 2001); the shaded region indicates d′ values between −0.7 and 0.7].

Figure 4.

Individual HVC neurons respond not only to songs in the adult repertoire but also to tutor songs. The same HVC neuron depicted in Figure 2B responded vigorously to both songs in the adult repertoire (A) and tutor songs that the bird had heard during development (B, left) but not to novel songs (B, right; numbers in the top right indicate the number of times the bird performed its copy of that tutor song and the percentage of those performances that occurred in the adult crystallized state). The robust representation of tutor songs, often the strongest response of a cell to any stimulus [e.g., top left peristimulus time histogram (PSTH) in B], could be evoked by tutor songs that the bird sang only during the plastic state of song development (top left PSTH; N = 4 lifetime performances; 0% in crystallized state). Notably, this response did not generalize across all songs that were sung in the juvenile state, as another tutor song that was also performed during juvenile song was not effective in activating the cell (third left PSTH; N = 2 songs in plastic state). C, This panel shows the complete response profile of the cell depicted in A and B for all adult, tutor, and novel song stimuli. The symbols indicate the role of the song in the bird's life history as in Figure 2D; the dotted vertical line separates responses to tutor and novel songs; filled symbols, significant response strengths; open symbols, responses that were not significant. D, Tutor songs commonly evoked responses that were significantly greater than the response of the same cell to songs in the adult repertoire (defined as outside of the shaded region; N = 9 song types, 9 cells, 5 birds), but no such response was observed for novel songs. This difference likely reflects the relevance of tutor songs in the bird's life history (symbols indicate the role of the song in the bird's life history as in Fig. 1E; dotted vertical line separates responses to tutor and novel songs; d′ < −0.7 indicates significantly stronger response to test stimulus; the shaded region indicates d′ values considered not significantly different from zero; range, −0.7 and 0.7; responses to 20 HVC neurons shown). E, There was no systematic difference in acoustic structure of tutor and novel songs, evident in the distribution of novel songs (bold italics) throughout this phenogram of the stimuli used in these experiments (difference in acoustic structure is related to the distance between songs, analysis performed using custom software [Luscinia, http://luscinia.sourceforge.net; (Lachlan et al., 2010)]; adult repertoire songs for each bird excluded).

Figure 5.

Although the HVC population response is biased toward stronger responses to songs in the adult repertoire, notable exceptions are evident in individual cells. The response of each cell to its strongest adult song type (see text) was used to normalize the responses of that cell to other stimuli. A–C, Responses to tutor songs exceeded the response to the strongest adult song type in 14 of 20 cells (6 birds), and the identity of those efficacious tutor songs varied across cells and birds (different birds shown in A–C; symbols indicate the role of the song in the bird's life history as in Fig. 1E; the dotted vertical lines separate responses to tutor and novel songs; filled symbols, significant response strengths; open symbols, responses that were not significant). D, Notably, one HVC neuron expressed no significant response to any song in the adult repertoire, yet that cell responded significantly to other stimuli and most strongly to a tutor song (data from same bird as in A). Two additional HVC neurons expressed significant decreases in their firing in response to adult songs, yet those cells also expressed strong increases in firing in response to tutor songs (2 birds) (supplemental Fig. 2, available at www.jneurosci.org as supplemental material). Together, these panels represent the full range of responses evident in HVC neurons.

Figure 6.

The strength of neural representation in HVC is not related to the number of times that the bird performed its copy of the song. We considered that the strength of neural representation could have been affected by the absolute number of times that a tutor song was produced at any stage of development. However, there was no relationship between d′ values and the number of times that a song was produced in the plastic state (p = 0.19) (A), the crystallized state (p = 0.75) (B), the total lifetime vocal output (p = 0.55) (C), or the ratio of the number of times the song was sung in the crystallized state versus the plastic state (D) (p = 0.89, Pearson's correlation; negative value indicates stronger response to tutor song; shaded area indicates responses with d′ values between −0.7 and 0.7; data shown for 20 neurons (6 birds) that expressed a significant response to one or more adult song types).

Figure 7.

Individual HVC neurons can represent multiple acoustically distinct song types. A, B, An individual HVC neuron could respond robustly to its strongest adult song type (A) but not at all to the tutor song that served as the model for the bird's performance of that strongest adult song type (B). C, Although the strongest adult song type for the cell shown in A and B (top) and the corresponding tutor song (bottom) were similar in their acoustic structure, gross acoustic similarity did not dictate that the same neuron responded to both songs [5 cells (5 birds) responded to the strongest adult song type but not to the corresponding tutor song]. D, Notably, the same cell as in A and B also responded to a tutor song that was acoustically distinct from the strongest adult song type and its corresponding tutor song. E, There was no relationship between the spectrographic cross-correlation coefficient indicating the accuracy of a bird's imitation of a particular tutor song and the strength of the response of a neuron to the bird's copy versus the tutor song itself (p = 0.84; Pearson's correlation; N = 17 cells, 6 birds; filled symbols, significant responses; open symbols, no significant response). Thus, gross similarity in the acoustic features of two stimuli did not dictate that an individual neuron would respond similarly to those stimuli, indicating that individual HVC neurons can represent multiple acoustically distinct song types.

Figure 8.

Responses of individual neurons to multiple song types are not explained by acoustic similarity on the scale of full syllables or individual notes. Spectrographic cross-correlation (Clark et al., 1987; Nowicki et al., 2002) was used to compare the acoustic structure of a representative syllable of each adult, tutor and novel song versus either the syllable of the strongest adult song type in each cell (A), or the syllable of the song type that evoked the strongest response in each cell, regardless of whether that song was part of the birds' adult repertoire (B). In each case, there was no difference between the cross-correlation scores of stimuli that did evoke a significant response (shaded regions in each bar) and the stimuli that did not evoke a significant response (open regions in each bar; A, p = 0.42; B, p = 0.67, Mann–Whitney U test; N = 20 cells, 6 birds). C–F, Because multiple song types could share an acoustic sequence that spanned only a subset of the song syllable, we also compared song structures at a per-note resolution. Comparing individual notes (C), two-note sequences (D), three-note sequences (E), and four-note sequences (F), there was no difference between the cross-correlation scores of the stimuli that did evoke a significant response (shaded regions in each bar) versus the stimuli that did not evoke a significant response (open regions in each bar; C, p = 0.28; D, p = 0.20; E, p = 0.61; F, p = 0.46, Mann–Whitney U test; N = 20 cells, 6 birds). Together, these analyses of acoustic structure, considering the same data at both a per-syllable and a per-note resolution, reveal that individual HVC neurons can represent multiple acoustically distinct song types.