On 2026-03-17 18:12:11, user markus wrote:
This is a review of the preprint Connectome of a human foveal retina, by Kim, Y.J., O. Packer, T. Macrina, A. Pollreisz, C.A. Curcio, K. Lee, N. Kemnitz, D. Ih, T. Nguyen, R. Lu, S. Popovych, A. Halageri, J.A. Bae, J. Strout, S. Gerhard, R.G. Smith, P.R. Martin, U. Grünert, and D.M. Dacey. 2026. https://www.biorxiv.org/content/10.1101/2025.04.05.647403v5 , version of Jan 28, 2026.
This report describes a remarkable new data set, which is being made available for public use: The connectomic reconstruction of the neuronal network in a small volume of a human fovea, based on 3D electron microscopy.
The fovea is the very central portion of our retina, responsible for all our high-acuity vision at the center of gaze. In some sense, this is the part of the retina most precious to us, and an inordinately large fraction of the visual brain is busy processing signals from the fovea. Much earlier research has focused on retinas of model animal species, in particular the closely related macaque retina. This is the first time a full sample of neurons, processes, and synapses has been assembled from the human fovea. I expect that many interesting discoveries will come from exploring this data set.
In addition to describing the process used in the connectome assembly, this article also reports some early results of the analysis. Topics include the electrical coupling between cones of different spectral sensitivity, the connections of horizontal cells to different cone types, the connectivity of the AII amacrine cells, and the number of ganglion cell pathways that encode the foveal output. My review and suggestions here focus on this part of the article reporting scientific claims.
The data set<br />
Is this the same data set as used for the reports in Kim 2023 (ref 44) and Kim 2024 (ref 106)? If so, please state that early on, and explain what are the methods enhancements since then.
Some of the results overlap with the Kim 2023 report: Fig 2 has a close relative in Kim 2023 Figs S2 and S3. Fig 7 top shows some of the same things as Kim 2023 Fig S8.
Line 124 "including those related to processes extending into or through the volume but not linked to cell bodies": It would be useful to state, for each cell type, how many complete neurons are in the 180 x 180 um EM volume vs those with only partial dendritic fields (e.g. Fig 6H).
Line 330 "Moreover, all cell types in the HFseg1 volume are identified": Meaning? Presumably not every process is identified.
Coupling of S to LM cones<br />
Line 127: "17 cone pedicles were identified as short wavelength-sensitive (S) cones (Fig. 2A)(44)." Please state how this was achieved. Ref 44 says "A clear population of likely S cone pedicles was not conspicuous in the human retina". Identifying these is essential for the subsequent claims of cone coupling.
Line 221 "We found abundant S-LM cone contacts": The authors already reported this in ref 44. Please emphasize what is new here.
Model of the S-cone spectral sensitivity<br />
The coupling between S and LM cones is a puzzle. In other retinas, biology goes to some trouble to avoid it. The principle extends to amphibians, in fact it was first reported in turtle (Detwiler 1979). The unexpected departure in human retina has been reported and debated for some time. One would hope that the modeling effort here would shed additional light on the issue.
Line 412: "A network model of this connectivity showed that ... mixing S and LM cone signals can alter the action spectrum of the S cone light response." This much was obvious from the outset and didn't require any modeling. *How much* does it alter the action spectrum, and does that explain any puzzles from human color vision? Given all the quantitative detail that went into the model, one would like to see a quantitative interpretation of the result.
Line 421: "Coupling among the S, L, and M cones prior to this connection may serve to preserve the smaller S cone signal within the H1 cell network." The meaning here is unclear. How does mixing in other signals serve to preserve the S cone signal?
Constraints on the number of ganglion cell pathways<br />
The authors report finding "only 11 visual pathways, with 5 high-density RGC pathways accounting for over 95% of foveal output to the brain: a dramatic contrast to the 40+ ganglion cell types recognized in mouse retina." (line 50). This difference between human fovea and mouse may well be correct, but the present study does little to support the claim. The volume included 599 RGCs (line 141). If we subtract the obvious midgets and parasols, that leaves only 37 RGCs. In the mouse, many of the RGC types have a prevalence of less than 1%. Under a Poisson distribution, there is therefore a good probability of missing such rare types within the relatively small sample here.
The complementary analysis of the "vertical connectome" (line 294ff) rests on the assumption that every cone must ultimately have an excitatory pathway to every ganglion cell. That is an assumption, not a natural law. If the probability of connection is less than 1, then tracing the output of just 3 LM cones may well miss some ganglion cells (line 319).
That said, the transcriptomic analysis of human fovea in Yan 2020 poses much more stringent limits on the existence of rare RGC types, because it included 11404 RGCs. The present work is consistent with the idea that there are only 11 or 12 pathways in human fovea, but doesn't strengthen the evidence much.
Synaptic circuitry of AII cells<br />
The textbook picture is that the AII cell feeds signals from rod bipolars into cone on and off bipolars, with opposite sign, via gap junctions and glycinergic synapses respectively. When rod signals aren't available, e.g. at very high light levels, the cone on bipolar excites the AII through the gap junctions, which leads to crossover inhibition of the cone off bipolar cells.
The current report (line 282ff) adds at least two non-canonical connections: (1) ribbon synapses from cone off bipolars and (2) ribbon synapses from cone on bipolars, both presumably glutamatergic and excitatory. Both these components have been described before in the mammalian retina. However, in the presence of the overwhelming input from rod bipolars, they were seen as a minor distraction; for example Strettoi 2018 say that the large majority of inputs in the On sublamina is from rod BCs, with only 23% from cone BCs. This raises a few questions about the present data set from human fovea:
What fraction of the ribbon synapses in the on sublamina are from rod bipolars? Clearly there are some rod bipolars in the data set (Fig 4B), but no results are presented about them. One would like to hear more.
The AII in Fig 9 gets more synapses from Off bipolars than from On bipolars. So should one expect the human AII cell to have a net Off response, unlike those in other retinas? Or is it conceivable that the postsynaptic response at Off bipolar synapses is sign-inverting?
Line 290 "to provide an inhibitory pathway from ON to OFF bipolar cells under photopic conditions". But also an excitatory pathway from OFF to ON bipolars? How can one reconcile that with the idea of "crossover inhibition"?<br />
Seeing how intensely the AII cell has been studied across species, it would be remarkable if it plays a very different functional role in the human fovea. More detail on this would be useful.
Abstract<br />
As it stands, I feel the abstract does not optimally highlight the unique contributions of the report. The biophysical model is new, but in the present form doesn't add new insight (see above). The small number of visual pathways compared to the mouse retina has been reported elsewhere with stronger numerical evidence (see above). There is reference to "novel synaptic pathways" and "distinctive features of human neural circuitry"; perhaps these could be spelled out? For example the cone connectivity of H1 vs H2 cells seems to be a new result (line 255ff). Similarly the observation of the "monopolar bipolar" cells previously seen in mouse retina (Fig 4B, line 270ff).
Other things<br />
Where is "SI Appendix" and Tables 1-4?<br />
Line 121 "We were therefore able to use the original segmentation": Meaning unclear.<br />
Line 233 "We used a fixed ratio of L to M cones of ~1.7:1": What motivated that choice? Empirically that ratio varies a lot across humans. How much does it matter, i.e. what would be the effect on the final modeling results of different ratios?<br />
Line 291 "while at the same time suggesting specialized synaptic alterations for the AII cells in the human fovea in relation to the midget circuit": Unclear to what this refers.<br />
Line 335 "The HFseg1 volume permits a comparison of the two approaches": Which two?<br />
Line 422 "This in turn may be critical for the unique requirements of human color vision": Unclear. What are those unique requirements? Something imposed by the ecology of humans vs macaques?<br />
Line 521 "White arrowhead shows branching processes of unknown significance arising from the cell body": These look remarkable. Other RGC types don't do that? Is there functional circuitry within the RGC layer? What is at the end of those processes? Gap junctions? This seems worth following up.<br />
Line 198 "Vesicle clouds were detected with high sensitivity": Please explain "vesicle cloud"? What is its functional meaning?<br />
Line 740 "Vesicle cloud detection and conventional synapse assignment in the IPL." These methods seem rather ad hoc. What is the ground truth here? How did the human annotators distinguish "vesicle clouds"?<br />
Line 620 "as previously described1.": Meaning of the superscript 1?<br />
Line 839 "La rètine des vertèbrès": Please check the direction of those accents.<br />
Line 846 "et al.": Now that we're not killing trees anymore for publications, there's no cost to listing all the authors of a cited paper.
- Markus Meister, meister@caltech.edu