On 2017 Jul 03, P Jesper Sjöström commented:

Thanks for your interest in our work. I would like to make the following points:

1. We did not actually say 'no connections in mature cortex' -- that quote is certainly not lifted from Mizusaki et al Nat Neurosci 2016. We said "In fact, it was recently reported that, surprisingly, pyramidal cells in visual cortex of mature animals do not seem to interconnect at all, neither bidirectionally nor unidirectionally12," where 12 refers to Jiang et al. We thus say that Jiang et al report that PCs do not seem to interconnect, we do not say that there are no PC-PC connections in mature cortex. What Jiang et al state and what our opinion about that statement is, those are different things.

2. The intention of that passage in Mizusaki et al is to point out that Brunel is using my data from Song et al as a gold standard, but this may or may not be appropriate, since my connectivity data was acquired from a developmental snapshot in time (just after eye opening, typically postnatal day 14-16), whereas Brunel is in fact focussing on the functioning of the mature brain, when circuits are wired up. Our intention was thus to acknowledge that my own data need not be the ground truth, and this has important implications for the validity of the Brunel study. The Tolias study provides an alternative view: "the most compelling and consistent difference across experiments is the age of the animals tested, suggesting that mature cortical circuits are not identical to developing circuits." Such a developmental difference would important in the context of the Brunel study. Again, this is not necessarily my opinion, but as scientists, we have to acknowledge this possibility.

3. In the Tolias study, they report in Fig S14 that they found precisely zero L5 PC-PC connections even after 150 attempts, which is in stark contrast to my connectivity data presented in Song et al. Indeed, if you do a Chi-squared test for 931/8050 versus 0/150, you will find that this is a highly significant difference. We can debate the accuracy of the Tolias measurement (like they do in Barth et al Science 2016 353:1108, as you point out), but if we do so, we should also debate the accuracy of my measurements in juveniles, as presented in Song et al. While it is true that my data in Song et al is more in line with e.g. Thomson et al Cereb Cortex 2002 than with Jiang et al, the key point in the context of Brunel's theoretical study is that the ground truth is not necessarily well established.

In summary, I certainly believe in my own connectivity data set, and I think Brunel's study provides a very compelling theoretical framework for explaining such connectivity patterns, but I feel obliged to point out a few possible caveats associated with my connectivity data set. Jiang et al provide one such key caveat. I hope this clarifies somewhat.

On 2017 Jun 27, Gabriele Scheler commented:

There is a technical comment from Barthetal2016 in Science which sums this up, and gives more detail as to why the connectivity in Xiangetal2015 is underestimated.

On 2017 Jun 27, Gabriele Scheler commented:

Very surprisingly, Mizusaki etal report: " In fact, it was recently reported that, surprisingly, pyramidal cells in visual cortex of mature animals do not seem to interconnect at all, neither bidirectionally nor unidirectionally (12)." The reference (12) is to Xiangetal2015, where they state: "Finally, the connectivity among mature pyramidal neurons, particularly among L5 pyramidal neurons, was much lower than the connectivity among pyramidal neurons within the same range of intersoma distance in juvenile slices [figs. S13B (average, 91 ± 4μm) and S14 and supplementary text]". The percentages noted in the table S14 range from 4,8% to 0% for ~150 connections tested. Quite clearly, the percentages are low, but to summarize this as 'no connections in mature cortex' does not seem to be adequate. Note that the number of connections tested is not very high and that "evoking unitary excitatory or inhibitory postsynaptic potentials [uE(I)PSPs] on postsynaptic neurons with brief depolarizing current pulses applied in presynaptic neurons" may not be sufficient to map all synaptic connections.

On 2017 Jun 27, Gabriele Scheler commented:

Very surprisingly, Mizusaki etal report: " In fact, it was recently reported that, surprisingly, pyramidal cells in visual cortex of mature animals do not seem to interconnect at all, neither bidirectionally nor unidirectionally (12)." The reference (12) is to Xiangetal2015, where they state: "Finally, the connectivity among mature pyramidal neurons, particularly among L5 pyramidal neurons, was much lower than the connectivity among pyramidal neurons within the same range of intersoma distance in juvenile slices [figs. S13B (average, 91 ± 4μm) and S14 and supplementary text]". The percentages noted in the table S14 range from 4,8% to 0% for ~150 connections tested. Quite clearly, the percentages are low, but to summarize this as 'no connections in mature cortex' does not seem to be adequate. Note that the number of connections tested is not very high and that "evoking unitary excitatory or inhibitory postsynaptic potentials [uE(I)PSPs] on postsynaptic neurons with brief depolarizing current pulses applied in presynaptic neurons" may not be sufficient to map all synaptic connections.

On 2017 Jun 27, Gabriele Scheler commented:

There is a technical comment from Barthetal2016 in Science which sums this up, and gives more detail as to why the connectivity in Xiangetal2015 is underestimated.

On 2017 Jul 03, P Jesper Sjöström commented:

Thanks for your interest in our work. I would like to make the following points:

1. We did not actually say 'no connections in mature cortex' -- that quote is certainly not lifted from Mizusaki et al Nat Neurosci 2016. We said "In fact, it was recently reported that, surprisingly, pyramidal cells in visual cortex of mature animals do not seem to interconnect at all, neither bidirectionally nor unidirectionally12," where 12 refers to Jiang et al. We thus say that Jiang et al report that PCs do not seem to interconnect, we do not say that there are no PC-PC connections in mature cortex. What Jiang et al state and what our opinion about that statement is, those are different things.

2. The intention of that passage in Mizusaki et al is to point out that Brunel is using my data from Song et al as a gold standard, but this may or may not be appropriate, since my connectivity data was acquired from a developmental snapshot in time (just after eye opening, typically postnatal day 14-16), whereas Brunel is in fact focussing on the functioning of the mature brain, when circuits are wired up. Our intention was thus to acknowledge that my own data need not be the ground truth, and this has important implications for the validity of the Brunel study. The Tolias study provides an alternative view: "the most compelling and consistent difference across experiments is the age of the animals tested, suggesting that mature cortical circuits are not identical to developing circuits." Such a developmental difference would important in the context of the Brunel study. Again, this is not necessarily my opinion, but as scientists, we have to acknowledge this possibility.

3. In the Tolias study, they report in Fig S14 that they found precisely zero L5 PC-PC connections even after 150 attempts, which is in stark contrast to my connectivity data presented in Song et al. Indeed, if you do a Chi-squared test for 931/8050 versus 0/150, you will find that this is a highly significant difference. We can debate the accuracy of the Tolias measurement (like they do in Barth et al Science 2016 353:1108, as you point out), but if we do so, we should also debate the accuracy of my measurements in juveniles, as presented in Song et al. While it is true that my data in Song et al is more in line with e.g. Thomson et al Cereb Cortex 2002 than with Jiang et al, the key point in the context of Brunel's theoretical study is that the ground truth is not necessarily well established.

In summary, I certainly believe in my own connectivity data set, and I think Brunel's study provides a very compelling theoretical framework for explaining such connectivity patterns, but I feel obliged to point out a few possible caveats associated with my connectivity data set. Jiang et al provide one such key caveat. I hope this clarifies somewhat.