Mesencephalic representations of recent experience influence decision making

Essential revisions:

1) However, the reviewers also brought several major concerns. Primary among these was questions about the interpretability of the optogenetic results, given the modest effect size, apparent lack of certain controls, and other issues. If and how those results should be presented in a revised manuscript should be considered carefully. We are concerned that doing the optogenetic work to the standard needed for publication in eLife will require significant new experiments, and it is against eLife policy to require substantial additional new experiments, although some authors take this on to make a paper as strong as it can be. So there are two options we see at this time: delete the optogenetic work or do the additional work needed to strengthen the optogenetics, and thus the paper. We encourage you to contact Josh Gold directly if you need further discussion or guidance on this point. Please note that if you choose to delete the optogenetic experiments, the other proposed revisions become crucial to enhance the depth and rigor of the data.

We thank the Editors for their evaluation of the optogenetics experiments and for suggesting these two options. We recognize that much more work could be done to strengthen the optogenetic results such that they would be suitable for publication as part of this manuscript. However, we agree that doing so would require substantial new experiments. While these are worthwhile experiments and we are confident that they would yield interesting data, performing the necessary experimental work is not feasible at this time. We have therefore decided to delete the optogenetic experiments from this manuscript. Thus, we do not address the optogenetics-related comments here, but we of course address all other concerns and have made every effort to enhance the depth and rigor of the data. Throughout the revised manuscript, we have removed text related to the optogenetics experiments from the Abstract, Introduction, Methods, Results, Discussion and Figures.

2) The muscimol experiment in Figure 1 seems to be disconnected from the rest of the paper. It was not clear until the end of the paper that the authors tried to make a conclusion that this manipulation affected the effect of previous choice. However, as discussed above, this conclusion is based largely on the optogenetic results, which are not convincing.

The intent of our muscimol experiments was to show that unilaterally inactivating PPTg affects behavior, demonstrating a causal relationship between PPTg activity and behavior. In the revised manuscript, we have reorganized our presentation of the muscimol experiments such that they are all now at the end of the Results (Revised Figure 5; Results, last two paragraphs) instead of being split between Original Figure 1B, C (direct effect of inactivation on behavioral choice; now Revised Figure 5A, B) and Original Figure 6D (effect of inactivation on the influence on behavior of the previous choice; now Revised Figure 5C). Now that we have deleted the optogenetics experiments, we believe that this organization provides the most natural flow for the manuscript: Revised Figures 2–4 demonstrate correlation, and Revised Figure 5 demonstrates causality, between PPTg activity and behavior.

3) Figure 4: it would be helpful to include a panel showing the population data as not just fractions of significant units (as in panels B and C), but also as the mean{ ± }SEM coefficient estimates (as in Figure 4A), to get a better sense of the magnitude of the effects across the population.

In Revised Figure 3 (adapted from Original Figure 4), we now include two new panels that show mean ± SEM coefficient estimates across the population for choice (Revised Figure 3B) and outcome (Revised Figure 3D). Interestingly, displaying the data this way reveals that the preferred previous trial choice of many neurons briefly inverted immediately preceding movement initiation (Revised Figure 3B), consistent with the dip in the fraction of significant neurons at this time (Revised Figure 3C). We note this observation in the revised manuscript (subsection “Influence of previous trials on PPTg activity”, last paragraph).

4) A number of important experimental and analysis details should be included. For example, were neural data from animals M1 and M2 included, despite the fact that neither had behavior that was influenced by the prior trial? This should be explained and justified better.

We clarify in the revised manuscript (subsection “Influence of previous trials on PPTg activity”, first paragraph) that neural data (shown in Revised Figures 2, 3, and 4) were included from all mice for which behavioral data were included (shown in Revised Figure 1). Thus, data from M1 and M2 were included, even though these mice did not exhibit behavior influenced by the previous trial (Revised Figure 1E). Indeed, we took advantage of the absence of a behavioral effect in these mice to show that the relationship between firing rate and current choice (Revised Figure 4A, B) was not indirectly due to the fact that both of these variables depend on previous choice (Firing rate: Revised Figure 2; Current choice: Revised Figure 1): Even for M1 and M2, in which current choice did not depend on previous choice, more neurons exhibited a positive than negative relationship between firing rate and current choice (Revised Figure 4C; subsection “Influence of PPTg representations of previous trials on behavior”, first paragraph). In the revised manuscript, we have edited our conclusion from this analysis to “These results suggest that the representation of the choice on the previous trial (Figure 2) can directly influence the choice made by the mouse on the current trial”, and have likewise edited text in the Discussion to “Together, our results demonstrate that the PPTg encodes representations of recent experience that can contribute to action selection”.

Finally, in the revised manuscript we speculate that these mice exhibited a neural, but not a behavioral, effect because the PPTg provides information about the previous trial to downstream regions (e.g., the superior colliculus), but that “downstream circuits do not necessarily utilize this information, given that it is represented in PPTg activity even in the absence of a behavioral dependence on previous choice (Figure 1D)” (Discussion).

Moreover, the authors should put in the main text: how many trials were run per session, how many sessions were run per animal, and per condition, how many animals were included in every given analysis, and the total number of single units analyzed.

In the revised manuscript we have included substantial new experimental details: a) For the behavioral and recording experiments (Revised Figures 1–4), each of the 7 mice performed 12 sessions (subsection “Influence of previous trials on behavior”, second paragraph). b) In these sessions, mice performed 451 ± 115 trials per session (mean ± STD; in the aforementioned paragraph). c) For the muscimol experiments (Revised Figure 5), each of the 3 mice performed 10 sessions per condition [saline (pre), muscimol, and saline (post)], for 30 total sessions per mouse (subsection “Influence of PPTg representations of previous trials on behavior”, second paragraph). d) In these sessions, mice performed 424 ± 105 trials per session (mean ± STD; in the aforementioned paragraph). e) We recorded from 506 well-isolated PPTg neurons (subsection “Influence of previous trials on PPTg activity”, first paragraph), data from all of which are included in Revised Figure 2. Revised Figures 3 and 4 include data from the 378 neurons that exhibited a significant preference for choice on the previous trial (corresponding to the black bars in Revised Figure 2G; subsection “Influence of previous trials on PPTg activity”, last paragraph and subsection “Influence of PPTg representations of previous trials on behavior”, first paragraph.