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  1. Author Response:

    Reviewer #1:

    The authors demonstrate deficits in perceptual tests related to fine-time perception in non-speech and speech sounds in a group of patients with stroke aphasia compared to a control group without a lesion. A subgroup of patients with deficits in spectrotemporal processing at a fine timescale have lesions mapped to the posterior STS, MTG and adjacent white matter. The area associated with deficits in spectrotemporal analysis with a fine timescale is then used as a seed for probabilistic fibre tractography based on diffusion MR. These results show connectivity of the functionally defined seed region with a number of areas including the cerebellum.

    The work is carefully done and I think interesting in demonstrating the cerebellar connections of the functionally defined region associated with deficits in fine temporal analysis that might be a basis for event representation at this temporal level.

    We appreciate the referee's evaluation and constructive feedback.

    Reviewer #2:

    Based on consideration of supportive evidence in the literature, the authors propose that a cerebellar-temporal lobe functional network plays a key role in auditory temporal processing. The precise parsing of temporal information is critical to understanding dynamic auditory processing and thus is an interesting area of study. Better understanding of how the cerebellum and temporal lobe may interact to achieve such parsing of the dynamic signal in a generative/predictive internal model is of clear interest to a broad readership. This idea is put to the test by first having individuals with lesions in the posterior portion of left temporal lobe perform speech perception and timing tasks and comparing performance with 12 healthy controls to establish the role of this region in tasks reliant on intact fine temporal processing. Typically, a lesion model will be helpful when a dissociation between structure and function can be demonstrated, and preferably this would be a double dissociation. Here, while lesions to auditory regions of the left temporal lobe are associated with impoverished performance on speech and temporal order tasks relative to a healthy control group, performance on comparably difficult auditory tasks that do not require good temporal discrimination is not tested to determine if there is such a dissociation. Given the extensive discussion of hypothesized different time sensitivities of right and left auditory cortices in the Introduction, patients with right homologous lesions might also have served as an interesting control and could have supported a double dissociation. In a second step to their study, a seed region was generated based on comparison of the lesion loci for the half of the patients who performed most poorly on the behavioral tasks to the other half, and this was used to explore anatomical tract connectivity of the seed region to the rest of the brain in the neuroimaging data from the healthy controls, with a focus on connections with the cerebellum. This approach to establishing that "temporo-cerebellar connectivity underlies timing constraints in audition" is unfortunately just not that convincing. The data are interesting, but taken alone they simply do not support such a conclusion. In the data, there is no clear functional link established or even hinted at between the temporal lobe and the cerebellum.

    We appreciate the referee's evaluation and constructive feedback. We address the raised concerns point by point below. We appreciate the concerns regarding our methodological choice and our interpretation of a functional link between the temporal lobe and the cerebellum. It certainly is more reasonable to derive a functional interpretation based on disconnection measured directly in patients’ DTI. However, if unavailable, indirect measures of disconnection can also be used to establish a functional link between a lesioned region and the networks associated with it. The rationale behind this is that it reflects an indirect estimation of the effect of a lesion on structural brain networks. To make this approach clearer, we have revised the manuscript accordingly. See revised manuscript pages 6 and 12:

    [...] Assessing connectivity in healthy participants based on lesion information is a relatively new method that measures structural disconnection in networks associated with given anatomical regions (Foulon et al., 2018). This allows for the indirect estimation of the lesion effect on structural brain networks. In this regard, it was shown that behavioral deficits can be explained similarly by local brain damage and indirectly measured disconnection (Salvalaggio et al., 2020). [...]

    [...] We next used the respective areas as seed regions for probabilistic fiber tractography in a healthy age-matched sample to visualize the underlying common connectivity pattern (see Methods). Thus, we indirectly explored the association between posterior superior temporal disconnection and processing of sound at short timescales. [...]

    We also changed the abstract and conclusion accordingly. See pages 2 and 15 of the revised manuscript.

    [...] Here we tested whether temporo-cerebellar disconnection is associated with the processing of sound at short timescales. [...]

    [...] The evidence we describe (i) shows that lesion-related deficits in spectrotemporal analysis occur in posterior temporal regions connected to the cerebellum [...].

    Reviewer #3:

    Stockert et al. investigate the cortico-cerebellar network underpinning rapid temporal auditory analysis. This study uses a well-defined group of stroke participants with mostly circumscribed lesions to the left posterior superior temporal lobe to motivate probabilistic tractography from cortical regions associated with verbal and non-verbal rapid auditory temporal analysis. Lesion-symptom mapping identifies a specific region of the posterior superior temporal sulcus and underlying white matter as statistically associated with impairment in rapid auditory temporal analysis. Tractography results demonstrate that these regions have high structural connectivity to wider regions of the left hemisphere cortical language network and ipsilateral and contralateral connectivity to postero-lateral cerebellum and dentate nucleus. It is interpreted that this cortico-cerebellar network is crucial to developing representations of fine auditory temporal structure.

    The conclusions of the paper are an interpretation which is based on integrating previous neuropsychology with the current tractography results and based on well-defined models in the motor domain. Such conclusions are not unreasonable but there is no direct (associative) evidence linking this network to the cognitive function of interest.

    Strengths:

    The paper integrates neuropsychology and neuroimaging methodologies to build a coherent picture which is more than the sum of its parts. The stroke group has well-defined and selected lesions which enable testing of the hypotheses put forward by the authors. The behavioural measures are sensitive and suitable to identify impairments in the behaviours of interest. There has been a detailed analysis of the behavioural speech perception data in the stroke group which largely, although perhaps not entirely, conforms to the asymmetric temporal sampling hypothesis. The lesion-symptom mapping approach is suitable for the nature of the population (small group with similar lesion distributions) and has allowed neuropsychologically guided tractography in the neurotypical population. This has clearly illustrated the complexity of the structural connectivity of the posterior superior temporal sulcus and underlying regions.

    Weaknesses:

    The selective nature of the stroke population - relatively small, chronic lesions - has resulted in only mild impairments for a small number of participants (6/12 participants). At the group level there is no difference between the stroke and neurotypical population on speech perception measures - group statistics do not reach one tailed significance. This reduces the certainty with which the regions identified are associated with the behaviour or interest. However, the results do conform to previous neuropsychology and lesion studies and it is likely that this lack of effect is due to low statistical power.

    Please refer to our response to the next point.

    All the stroke participants have a similar lesion distribution, and this makes lesion-symptom mapping challenging. For example, lesion data do not give an indication of the functional integrity of perilesional regions which can be reduced, even at the chronic stage, therefore the superior temporal sulcus may not be functioning effectively, even in the proportion of the group without lesions to this area. Lesion symptom mapping is more robust with a wider distribution of lesions and the inclusion of participants with lesions remote from the area of interest. Having said that, the behavioural measures appear sensitive enough to identify mild impairments and the authors, for good reason, wished to reduce the extension of lesion into primary auditory regions. As above, given the limited sample and homogeneous lesion, the lesion symptom mapping approach is reasonable.

    We agree that the small number of patients is a possible limitation to the study and add this point to the limitations section. See revised manuscript page 21.

    [...] First, the study population is relatively small and lesion symptom mapping is typically applied to larger populations with wider lesion distribution. Although careful selection of circumscribed lesions has the advantage of highlighting behavioral differences without confounding other deficits (e.g., primary auditory processing), it is possible that additional regions are involved in processing of sound at short timescales. However, tractography based on healthy participants makes it possible to indirectly obtain information (i.e., structural disconnection) about brain regions contributing to the investigated function. In addition, it is likely that the small number of patients might hamper the ability to detect statistically significant differences between the behavior of controls and patients. Nevertheless, we are confident that the current results align with the fact that the posterior superior temporal cortex contributes to the processing of sound at short timescales, as indicated by previous neuropsychological evidence and lesion studies (Boemio et al., 2005; Chedru, Bastard, and Efron, 1978; Efron, 1963; Robson, Grube, Lambon Ralph, Griffiths, & Sage, 2013; Swisher & Hirsh, 1972). Further studies should however test larger populations to replicate and extend this finding. [...]

    The authors suggest that the behavioural results conform to the asymmetric temporal sampling hypothesis in that only place of articulation discrimination impairments in the stroke group can be (just about) detected, whereas there were no significant stroke-neurotypical differences in other phonetic contrasts. It is not clear that the VOT differences associated with plosive voicing changes and the cues associated with place changes happen over fundamentally different time-scales and, therefore, it is important to further justify the interpretation of the data. In the future it will be helpful to have this level of analysis applied to individuals with lesions to the wider speech perception network to draw conclusions about the specificity of the impairment to these regions - for example, impairments in phoneme discrimination have been associated with frontal lobe lesions.

    It appears that voicing contrasts in which shorter and longer voice onset times result in the perception of a voiced or voiceless plosive (for example [t] and [d]) are encoded in both the temporal envelope and fine structure (Rosen 1992) of the speech signal that occur in time windows of 20-500 ms and <2 ms, respectively. In words an additional cue is the closure time, which can be further used to discriminate between voiced and voiceless plosives. However, place of articulation contrasts are exclusively encoded in the temporal fine structure (i.e., very quick transitions of the frequency spectrum, formant transitions). Even though for all contrasts shorter timescale information plays a role, somewhat redundant encoding is present for voice contrasts. Ultimately, place of articulation contrasts seem to be the most difficult to discriminate. In Figure 2D it is apparent that despite highest error rates for the place of articulation contrasts, several patients also showed impaired discrimination for voicing contrast when compared to healthy controls. We do agree with the referee that it would be interesting to also extend this level of analysis to individuals with lesions in the wider speech perception network in future work.

    The tractography results reveal a complex pattern of structural connectivity, including other regions associated with speech perception. The authors have a theoretical motivation to focus on the importance of the temporo-cerebellar pathway but there is no correlation evidence to link auditory temporal analysis to the integrity of this pathway in the neurotypical population. The non-verbal measures appear to be sufficiently sensitive for this type of analysis. This lack of association with behaviour makes it hard to draw conclusions about the functional role of this network.

    We appreciate the referee’s concerns about our interpretation of the functional link between the temporal lobe and the cerebellum regarding auditory temporal analysis. It certainly is more reasonable to derive a functional interpretation based on disconnection measured directly in patients DTI. However, if unavailable, indirect measures of disconnection can also be used to establish a functional link between a lesioned region and the networks associated with it. The rationale behind this is that it reflects an indirect estimation of the effect of a lesion on structural brain networks. To make this approach clearer, we have modified the manuscript as such. See revised manuscript pages 6 and 12:

    [...] Assessing connectivity in healthy participants based on lesion information is a relatively new method that measures structural disconnection in networks associated with given anatomical regions (Foulon et al., 2018). This allows for the indirect estimation of the effect of a lesion on structural brain networks. In this regard, it has been shown that behavioral deficits are explained to a similar extent by both the local damage and indirectly measured disconnection (Salvalaggio et al., 2020). [...]

    [...] We next used the respective areas as seed regions for probabilistic fiber tractography in a healthy age-matched sample to visualize the underlying common connectivity pattern (see Methods). Thus, we indirectly explored the association between posterior superior temporal disconnection and processing of sound at short timescales. [...]

    We also changed the abstract and conclusion accordingly. See revised manuscript pages 2 and 15.

    [...] Here we tested whether temporo-cerebellar disconnection is associated with processing of sound at short timescale. [...]

    [...] The evidence we describe (i) shows that lesion-related deficits in spectrotemporal analysis occur in posterior temporal regions connected to the cerebellum [...].

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  2. Evaluation Summary:

    The authors demonstrate deficits in perceptual tests related to fine-time perception in non-speech and speech sounds in a group of patients with stroke aphasia compared to a control group without a lesion. An area in left auditory cortex is defined that is essential for fine-time perception that is shown in a separate group of normal subjects to other areas including the cerebellum. The work in interesting in suggesting an anatomical basis for interaction between cortical and cerebellar system for perceptual timing.

    (This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 agreed to share their name with the authors.)

    Was this evaluation helpful?
  3. Reviewer #1 (Public Review):

    The authors demonstrate deficits in perceptual tests related to fine-time perception in non-speech and speech sounds in a group of patients with stroke aphasia compared to a control group without a lesion. A subgroup of patients with deficits in spectrotemporal processing at a fine timescale have lesions mapped to the posterior STS, MTG and adjacent white matter. The area associated with deficits in spectrotemporal analysis with a fine timescale is then used as a seed for probabilistic fibre tractography based on diffusion MR. These results show connectivity of the functionally defined seed region with a number of areas including the cerebellum.

    The work is carefully done and I think interesting in demonstrating the cerebellar connections of the functionally defined region associated with deficits in fine temporal analysis that might be a basis for event representation at this temporal level.

    Was this evaluation helpful?
  4. Reviewer #2 (Public Review):

    Based on consideration of supportive evidence in the literature, the authors propose that a cerebellar-temporal lobe functional network plays a key role in auditory temporal processing. The precise parsing of temporal information is critical to understanding dynamic auditory processing and thus is an interesting area of study. Better understanding of how the cerebellum and temporal lobe may interact to achieve such parsing of the dynamic signal in a generative/predictive internal model is of clear interest to a broad readership. This idea is put to the test by first having individuals with lesions in the posterior portion of left temporal lobe perform speech perception and timing tasks and comparing performance with 12 healthy controls to establish the role of this region in tasks reliant on intact fine temporal processing. Typically, a lesion model will be helpful when a dissociation between structure and function can be demonstrated, and preferably this would be a double dissociation. Here, while lesions to auditory regions of the left temporal lobe are associated with impoverished performance on speech and temporal order tasks relative to a healthy control group, performance on comparably difficult auditory tasks that do not require good temporal discrimination is not tested to determine if there is such a dissociation. Given the extensive discussion of hypothesized different time sensitivities of right and left auditory cortices in the Introduction, patients with right homologous lesions might also have served as an interesting control and could have supported a double dissociation. In a second step to their study, a seed region was generated based on comparison of the lesion loci for the half of the patients who performed most poorly on the behavioral tasks to the other half, and this was used to explore anatomical tract connectivity of the seed region to the rest of the brain in the neuroimaging data from the healthy controls, with a focus on connections with the cerebellum. This approach to establishing that "temporo-cerebellar connectivity underlies timing constraints in audition" is unfortunately just not that convincing. The data are interesting, but taken alone they simply do not support such a conclusion. In the data, there is no clear functional link established or even hinted at between the temporal lobe and the cerebellum.

    Was this evaluation helpful?
  5. Reviewer #3 (Public Review):

    Stockert et al. investigate the cortico-cerebellar network underpinning rapid temporal auditory analysis. This study uses a well-defined group of stroke participants with mostly circumscribed lesions to the left posterior superior temporal lobe to motivate probabilistic tractography from cortical regions associated with verbal and non-verbal rapid auditory temporal analysis. Lesion-symptom mapping identifies a specific region of the posterior superior temporal sulcus and underlying white matter as statistically associated with impairment in rapid auditory temporal analysis. Tractography results demonstrate that these regions have high structural connectivity to wider regions of the left hemisphere cortical language network and ipsilateral and contralateral connectivity to postero-lateral cerebellum and dentate nucleus. It is interpreted that this cortico-cerebellar network is crucial to developing representations of fine auditory temporal structure.

    The conclusions of the paper are an interpretation which is based on integrating previous neuropsychology with the current tractography results and based on well-defined models in the motor domain. Such conclusions are not unreasonable but there is no direct (associative) evidence linking this network to the cognitive function of interest.

    Strengths:

    The paper integrates neuropsychology and neuroimaging methodologies to build a coherent picture which is more than the sum of its parts. The stroke group has well-defined and selected lesions which enable testing of the hypotheses put forward by the authors. The behavioural measures are sensitive and suitable to identify impairments in the behaviours of interest. There has been a detailed analysis of the behavioural speech perception data in the stroke group which largely, although perhaps not entirely, conforms to the asymmetric temporal sampling hypothesis. The lesion-symptom mapping approach is suitable for the nature of the population (small group with similar lesion distributions) and has allowed neuropsychologically guided tractography in the neurotypical population. This has clearly illustrated the complexity of the structural connectivity of the posterior superior temporal sulcus and underlying regions.

    Weaknesses:

    The selective nature of the stroke population - relatively small, chronic lesions - has resulted in only mild impairments for a small number of participants (6/12 participants). At the group level there is no difference between the stroke and neurotypical population on speech perception measures - group statistics do not reach one tailed significance. This reduces the certainty with which the regions identified are associated with the behaviour or interest. However, the results do conform to previous neuropsychology and lesion studies and it is likely that this lack of effect is due to low statistical power.

    All the stroke participants have a similar lesion distribution, and this makes lesion-symptom mapping challenging. For example, lesion data do not give an indication of the functional integrity of perilesional regions which can be reduced, even at the chronic stage, therefore the superior temporal sulcus may not be functioning effectively, even in the proportion of the group without lesions to this area. Lesion symptom mapping is more robust with a wider distribution of lesions and the inclusion of participants with lesions remote from the area of interest. Having said that, the behavioural measures appear sensitive enough to identify mild impairments and the authors, for good reason, wished to reduce the extension of lesion into primary auditory regions. As above, given the limited sample and homogeneous lesion, the lesion symptom mapping approach is reasonable.

    The authors suggest that the behavioural results conform to the asymmetric temporal sampling hypothesis in that only place of articulation discrimination impairments in the stroke group can be (just about) detected, whereas there were no significant stroke-neurotypical differences in other phonetic contrasts. It is not clear that the VOT differences associated with plosive voicing changes and the cues associated with place changes happen over fundamentally different time-scales and, therefore, it is important to further justify the interpretation of the data. In the future it will be helpful to have this level of analysis applied to individuals with lesions to the wider speech perception network to draw conclusions about the specificity of the impairment to these regions - for example, impairments in phoneme discrimination have been associated with frontal lobe lesions.

    The tractography results reveal a complex pattern of structural connectivity, including other regions associated with speech perception. The authors have a theoretical motivation to focus on the importance of the temporo-cerebellar pathway but there is no correlation evidence to link auditory temporal analysis to the integrity of this pathway in the neurotypical population. The non-verbal measures appear to be sufficiently sensitive for this type of analysis. This lack of association with behaviour makes it hard to draw conclusions about the functional role of this network.

    Despite some limitations, this paper highlights the potential importance of a previously under-studied network supporting auditory perception. The paper is, therefore, impactful in motivating future research into the cerebellar contribution of auditory processing.

    Was this evaluation helpful?