In the course of our experiments, I encountered a phenomenon that was not reported previously in the VS cell system. Because VS cells are considered to be spatial integrators, it is expected that large-field motion will drive the cells more effectively, resulting in stronger
Figure 3-13: Small-field selectivity in VS cell dendrites
(a) Small-field gratings evoke stronger calcium influx in the dendrite of a VS cell than full-field grating. ΔF/F images averaged over stimulus presentation for a large field (left) and a small-field (right) stimulus. Note that dendritic areas excited by the 20° grating are more strongly excited by it than by the full- field grating. (b) Normalized ΔF/F responses averaged in dendrites (red) and axon-terminals (blue) of VS cells over stimulus presentation for gratings varying in horizontal width.
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responses, than small-field motion. This is indeed the case for electrical responses (Haag et al., 1992; Borst et al., 1995), and when I compared full-field calcium responses to small-field, 20° gratings, this is also what I observed in the axon terminals. But when I measured the dendritic calcium responses, the result was reversed – the small-field gratings elicited larger responses than the full-field gratings (Figure 3-13a). I began an initial quantification of the spatial extent of this small-field selectivity by measuring the responses to gratings varying horizontally in size in 15 VS cells (3 VS2, 6 VS3, 5 VS4, 1 VS5, Figure 3-13b). Here the axon-terminal responses increase by close to 100% on average from the smallest grating to the full-field grating, whereas the dendritic responses unexpectedly dropped approximately 30%.
To check whether this small field selectivity can also be seen in the vertical dimension, I presented gratings that varied in size in the vertical dimension. To keep total luminance constant I presented gratings consisting of an integer number of cycles, from 1 to 4 cycles, at a
Figure 3-14: Vertical size dependence of dendrites and axons
Left: illustration of stimuli used – gratings presented were 1-4 integer number of cycles long in the vertical dimension and 10° or 20° wide. Right: dependence of dendritic and axon terminal responses on vertical size. Both dendrites and axon terminal responses grow with stimulus size.
97 width of 10° and 20°. As expected, responses grew with the size of the grating both for the axon terminal and the dendrite, reflecting the integration expected in the vertical dimension of the dendrite (Figure 3-14, same cells as above).This also served as a control for the dendritic calcium imaging, demonstrating that the imaging could register increase in response magnitude as a function of stimulus size.
As putative mechanisms for this small-field dendritic selectivity, I considered either a presynaptic pooling inhibitory mechanism acting isotropically along the horizontal dimension, or a form of presynaptic short-range lateral inhibition. Longer ranges of lateral inhibition, between medial VS cells and the VS1 cell were reported in previous studies (Haag and Borst, 2004; Haag and Borst, 2007), however the shorter range of the effect I describe here, as well as its manifestation in the dendritic compartment of VS2-5 cells, removed from the effect of inhibition coming from VS1 through the axo-axonal gap junctions, suggested that this is a different inhibitory mechanism than any described before.
To differentiate between these two possible mechanisms, I presented a stimulus consisting of the 20° grating that elicited the strongest dendritic response (the “main” grating) and simultaneously another 20° grating horizontally positioned at various distances either lateral or medial to the main grating (the “inhibitory” grating), both drifting in the preferred direction of the cells. If short-range lateral inhibition is the underlying mechanism, the expectation is that further-distanced inhibitory gratings should result in a smaller inhibition. However, if the mechanism is an isotropic pre-synaptic pooling mechanism, we should expect no effect of the position of the inhibitory grating on the inhibition it elicits in the dendrite.
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The results of this experiment are presented in Figure 3-15, for the same cells as above. As expected from the linear summation of the responses to the two gratings, the axon terminal responded stronger to both gratings drifting together. In the dendrite, I could not detect a statistically significant effect of the second stripe on the calcium response, neither when I separately analyzed effect of lateral and medial inhibitory gratings (Figure 3-15), nor when I pooled responses to the lateral and medial inhibitory gratings. However, there seems to be a tendency for a stronger effect of medium distances (50°-70°), and this can be seen in a lower mean and lower t-test p-values testing for a difference from responses to the main grating alone (p-values 0.2493, 0.6431, 0.7394, 0.0943, 0.1856, 0.1229 for 20°-70°, respectively). The statistical insignificance of these results may be a result of the small effect for a 40° grating relative to a 20° one (Figure 3-13b), but presenting a larger inhibitory grating and moving it
Figure 3-15: Effect of a lateral grating on dendritic and axonal responses
a. Schematic representation of the stimuli used; a main grating (red) is presented together with a horizontally shifted “inhibitory” grating. b. Effect of the inhibitory grating on the axon-terminal and dendritic responses, separated into medially positioned inhibitory gratings and laterally positioned inhibitory gratings. c. Same as b, lateral and medial positioned inhibitory gratings pooled
99 laterally and medially is not possible using the current stimulus presentation device. Clearly, though, more experimentation is needed in order to obtain a satisfactory description of the underlying mechanism.
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