|Neurons in the Retina (Figure 1h Kim et al., 2008)|
A 2008 paper shows that this downward direction in these dendrites is no coincidence. These neurons are sensitive to visual input moving in a specific direction, the same direction that their dendrites are pointing. In other words, when a visual stimulation (such as a a bar or dot) is moving across the retina in the soma to dendrite direction, these neurons are most active. When the visual stimulation moves in the opposite direction, these neurons are the least active.
|Figure 2e, Kim et al., 2008|
This diagram shows the direction of the dendrites (green line), and the direction of movement which activates that neuron the most strongly (red line). This is just one example, but on average the dendrite direction and the preferred stimulus direction matched up for these neurons.
Because the lens of the eye functionally reverses the visual world, this means that since the dendrites of these neurons point down, the actually respond to upward motion.
"One outstanding question is why the mouse has invested so heavily in sensitivity to upward motion." (Kim et al., 2008)
This could lead to speculation on mouse evolution and why a mouse would need to be extra-sensitive to upward visual input. But I think that is a goose chase, just because there are cells in the retina whose form and function match nicely for upward motion, doesn't mean that mice are actually more sensitive to upward motion. (A motion detection behavioral test is necessary to make that claim, and I don't know of any done on mice)
In fact, the same group more recently (Kay et al., 2011) found that there are four similar classes of cell responsive to each of the four cardinal directions. These cells have some dendrite-direction correlation, but it is not as strong and clear cut as the upward sensitive cells specified in the 2008 paper.
What is particularly interesting is that, while the (J-RGCs) cells in the 2008 paper have such strong correlations between dendrite direction and stimulus direction sensitivity, the cells (BD-RGCs) described in the 2011 paper do not. From the discussion:
"The correspondence of dendritic asymmetry with preferred movement direction in BD-RGCs resembles that in J-RGCs, a far more strikingly asymmetric group of OFF-DSGCs that we described recently (Kim et al., 2008, 2010). We suspect, however, that the association differs in the two cases. Both J- and BD-RGCs include some cells whose arbors appear symmetric. The symmetric J-RGCs are not direction selective, supporting the idea that structure underlies function for these cells (Kim et al., 2008). In
contrast, structurally symmetric BD-RGCs are as direction selective as asymmetric ones, suggesting that for these cells structural asymmetry does not determine directional preference." (Kay et al., 2011)
In other words: Some cells are direction-sensitive without their dendrites being weighted to one side.
So the really exciting questions are: What are the molecular and cellular mechanisms that make these cells directionally sensitive, and is the dendritic orientation necessary for direction sensitivity? If an upward motion cell was somehow transplanted in the opposite orientation, would it become a downward motion cell?
I suspect that just as computational neuroscience helped us understand the dendrite-based frequency sensitivity in the bird brain, a computational model would help us understand how a cell could respond maximally to soma-dendrite directional motion.
Kim IJ, Zhang Y, Yamagata M, Meister M, & Sanes JR (2008). Molecular identification of a retinal cell type that responds to upward motion. Nature, 452 (7186), 478-82 PMID: 18368118
Kay JN, De la Huerta I, Kim IJ, Zhang Y, Yamagata M, Chu MW, Meister M, & Sanes JR (2011). Retinal ganglion cells with distinct directional preferences differ in molecular identity, structure, and central projections. The Journal of neuroscience : the official journal of the Society for Neuroscience, 31 (21), 7753-62 PMID: 21613488