PREreview of Goal-directed vocal planning in a songbird

Part 1. Summary 

The goal of this paper is to observe whether targeted vocal plasticity in the songbird is a result of goal directed vocal planning independent of sensory feedback experience. The authors first look into whether or not a bird can recover song syllables without practice, in terms of syllable pitch. They carry out experiments where they shift the bird’s pitch of song and then mute the birds for a duration of time, then unmute them to observe whether or not the pitch of their song changes from baseline. They find that after the muted state birds are not able to accurately recover their reinforced songs compared to birds undergoing normal song recovery, in unmanipulated control birds after removal of pitch altering reinforcement. They bring up the factor that invasive muting treatment might lead to decreased recovery potential. These findings dictate that either motor practice (i.e. physically singing) or sensory feedback (i.e. hearing a bird's own song), or a combination thereof, are required for song recovery. 

They specifically delve into whether motor experience, but not sensory experience, is what facilitates recovery. They gave birds slightly more song tutoring, and deafened them through bilateral cochlear removal, which does not suppress the physical action of singing, just removes sensory feedback through audition. They have feedback from somatosensory integration, and vibrational sensing. They find that there is negligible song recovery after practice. They bring up another potential confounding variable which is that birds might be overwhelmed by sudden deafening, so they made a third experimental group which were deafened, monitored for return to stable baseline song, then had their pitch manipulated through a light based mechanism. They were then monitored for pitch recovery, and the researchers discovered that this group was unable to recover to baseline pitch, suggesting that song recovery requires no change in sensory experience, which includes auditory feedback. 

They continue to conduct a set of experiments where they observe whether birds shift toward baseline when deprived of sensory experience. They hypothesize that hearing a mismatch in pitch fuels birds to plan to recover their pitch target, and this is carried out without feedback. They find that pitch reversion in deaf birds is initially faster than in hearing birds in the WNd (white noise deafened) group, while in the dLO (deafened then light out stimuli) pitch shifts further from baseline, but this change is no different from dC (deaf control) birds. They carry out control experiments to outrule circadian rhythm and time effects on recovery. They conclude by presenting the model that birds plan reduction of mismatch by targeting the mismatch, driven by sensory feedback. Without sensory feedback they are limited to the scope of previous song practice. 

Overall the authors provide strong reasoning for why they used their specific model organism, valid controls for each experimental data set, and touched on how confounding variables might play into their experiments by ruling them out as contributing factors. They are very thoughtful in their approach and funnel their experiments nicely.

Part 2. Detailed comments: 

1. Significance 

The authors specifically provide a significance statement directly under the abstract which underlines that zebra finches are able to make target-directed changes to song without the need for sensory feedback. This could be placed in the broader scope, in the context of how this relates to birdsong as a model mechanism as a whole. This implies that song (as a model for language) can be modulated in a goal directed manner without current sensory input. 

2. Observation 

I believe that the observations of the paper are mostly clear and relative to the points the authors make. Their logic is sound based on the evidence in each figure, and they order their experiments to narrow down variables very concisely. Confounding variables are discussed and even implemented in the data analysis with complementary figures to provide stronger validation of the authors’ conclusions. Most of the data is easily readable and understandable, though there are a few figures which are field specific, such as the syllable fluorograms in figure 1. However, they are not superfluous because there is adequate detailing of differences in experimental groups in these figures and they are also elaborated upon quite strongly. There is extensive statistical analysis described which allows us to interpret the results as significant. 

Figure 1: Recovery of pitch requires practice. This figure begins with an outline of three hypotheses for potential recovery pathways in 1A. Figure 1B displays timepoints for observation of recovery in early or late time points in muted versus control birds, and experimental outline. Figure 1C is an inclusion of potentially confounding data, i.e. spontaneous unmuting events. This is a necessary inclusion because they show their experimental errors did not affect the outcome of the experiment as a whole. 1D is a spectrogram, a visual representation of the song in different experimental groups. This may be difficult for the layman to understand and interpret but is commonplace in this field, so I don’t think it's superfluous to add this information. 1E is an elaboration of the previous image, and I think the line chart associated with it (1E) is a great visualization of comparison of pitch in a single syllable in different groups. 1F is a further elaboration where we see syllable variance from baseline. 1H is a bit chaotic to read in my opinion, but the basic message is that control birds return to baseline more regularly than WNm birds either in early or late time stages. Perhaps an average of the samples would make it more reader friendly than having all the interesting data lines. 1I is a better representation of the data in 1H, showing early vs. late recovery, but needs a better key to distinguish the data sets.

Figure 2: Recovery of pitch is impaired after deafening. 2A is again an outline of their experimental design, 2B displays the change in pitch during reinforcement and the subsequent inability to revert to baseline. 2C displays again a lack of recovery capability after deafening events without sensory feedback in this experiment, and 2D are violin plots of the same data. I would have included the control data for these experiments as a comparison, but the data only displays the WNd experimental group. 

Figure 3: Deaf birds do not recover pitch target after light induced mismatch. Figure 3 is very similar in structure to figure 2, where there is an outline of the experimental design (3A), followed by visualizations of lack of recovery capability (3B-D). Again I would include these data compared to a control. 

Figure 4: Target mismatch experience is necessary for revertive pitch change. 4A also begins with a layout of the experimental design. We then observe pitch changes in all the experimental groups (4B-D). 4E displays pitch changes in all groups in the early vs. late recovery period. I like this data because all groups are easily comparable on one plot. 4F (typo as 4C in the figure description) rules out time post-deafening as a confounding factor. I like that they implement this data directly in the article. 

Figure 5: A visualization of the goal-directed planning pathway of directing vocal change. This is a very strong schematic that outlines the proposed outcome of the amalgamation of their data. 

3. Interpretation 

I believe the inferences are supported by adequate observation. There are also multiple visualization modalities for many of the data sets that are offered, making the point of each data set more understandable. We see that they started their experiments broad, observing that recovery of pitch requires practice in figure 1. They then narrow their variables to different modalities of altering pitch to observe whether recovery of song is due to sensory feedback or motor practice. My one question is why the birds in figure 4A were treated differently (i.e. WNd vs. dLO treatment groups) prior to being given either mismatch experience or no mismatch experience. I would have done this experiment in WNd birds and dLO birds separately, with each reinforcement group being given both mismatch and no mismatch experience. This way we can control which variable is giving us the revertive pitch change outcome, be it the reinforcement modality or the mismatch experience. I wonder if there is a possibility to give WNd birds either mismatch or no mismatch experience even though they are deafened at the same time, or give the dLO birds pretreatment with either mismatch or no mismatch experience prior to the deafening process to compare outcomes in the same treatment groups. 

I like in their discussion section how they relate their findings to humans and propose models for comparing their findings here to future directions in human learning and speech. The authors are also extremely specific in the materials and methods section with time points and descriptive wording, leading me to believe that these results would be robust and reproducible.

4. Clarity 

I think the manuscript is very easy to read and has a very good flow. They start with a broad question and go on to narrow it variable by variable. They start with the question of how song is recovered, and then go on to isolate whether this is a result of sensory feedback or motor practice, with clear logic on the formulation of their hypotheses throughout. The figures integrate fantastically into the article by providing detailed descriptions of hypotheses explored in each experiment. They end the article with a schematic of their proposed mechanism of song recovery (Fig 5), that song recovery is the result of goal directed vocal change. There is one figure (Fig 1I) that I think could use a better key on which group is represented by which color, but we can infer this from the previous panel if anything, and I also think figure 1H might benefit from a little bit of data averaging to make it more readable, because the high intersection of lines and data points makes it a bit difficult to follow and interpret. Also, there is one typo in the labeling of the figures (4F labeled as 4C) but otherwise the paper was very clear, had logical flow, and provided convincing evidence to drive their claim.

Competing interests

The author declares that they have no competing interests.