Receptive field properties

Different cell types often show distinctive spatio-temporal dynamics. With a spatio-temporal white noise stimulus a cell’s area of spatial integration, the re-ceptive field (RF), and dynamics of temporal integration can be determined by reversely correlating stimulus and response (see Sect. 3.2 for details). These properties can be indicators of the cells’ morphology and function. For example, the receptive field size is usually strongly correlated with the size of the cell’s dendritic field (Yang and Masland, 1994). Furthermore, the temporal dynamics indicate the characteristic temporal frequency range that a cell can encode. Cells with fast temporal dynamics can encode higher temporal frequencies.

The example in figure 4.3a shows the receptive field contours of standard DS (magenta), standard OMS (blue) and OMS-DS cells (green) within a small retinal patch where standard DS cells had much larger receptive fields than standard OMS and OMS-DS cells. The contours were obtained from a two-dimensional Gaussian fit of the spatial component of the spatio-temporal STA at 1.5 standard deviations (Sect. 3.2). In the pooled data from 16 retinas (Fig. 4.3c), standard OMS and OMS-DS cells were among the smallest, with receptive field diameters around 290±80µm. Standard DS cells had very large receptive fields (390±90µm) which would allow them to integrate motion within a large area and might be useful for capturing information about global background motion. Small receptive fields, as of the standard OMS and OMS-DS cells observed here, usually result in a higher density of cells of the same type which would provide a higher spatial resolution and might be important for detecting small moving objects.

First Peak Latency (ms)

Stand. DS OMS-DSStand. OMS

**

n. s.

***

0 100 200 300

RF Diameter (µm)

Stand. DS OMS-DSStand. OMS

******^{n. s.}

0 300 600 900

100 ms 300 µm

d c

b a

Figure 4.3 OMS-DS cells have small receptive fields like OMS cells and long response latencies as standard DS cells. a-bReceptive field contours (a) and temporal filters (b) of standard DS (magenta), OMS (blue) and OMS-DS cells (green) from a single retina. A negative deflection of the temporal filter signifies an average darkening of the screen before spiking. c-dBox plots of receptive field diameters (c) and first peak latencies (d) of 134 standard DS (magenta), 186 OMS (blue), 18 OMS-DS (green) and 852 unspecified cells (gray) from 16 experiments. Upper and lower box edges indicate interquartile range (IQR) between the 25th and 75th percentiles, respectively. Central box line indicates the median. Whiskers are extended to the most extreme values which are within1.5·IQR from the edges of the box, outliers are marked with bullets. Gray bar in the background indicates population IQR with population median (dashed line). cRF diameters of standard OMS and OMS-DS cells were not significantly different from each other (n.s.) but significantly different from RF diameters of standard DS cells (p <0.005).

d First peak latencies of standard DS and OMS-DS cells were significantly slower than latencies of OMS cells (p <0.01). Standard DS and OMS-DS cells had similar first peak latencies (n.s.).

Receptive field properties

To characterize the temporal integration of the cells, one usually considers the timing of the first peak of the temporal filters (Fig. 4.3b), thefirst peak latency.

This measure provides an estimate for how fast the cells respond to contrast changes. Standard DS and OMS-DS cells were significantly slower with first peak latencies of114±27 msand 112±41 ms, respectively, than standard OMS cells (87±27 ms). Though, all three cell types were significantly slower than the pop-ulation average (Fig. 4.3d). The sign of the filter peaks (Fig. 4.3b) suggests that all three cell types responded to changes from bright to dark contrast within their receptive field center and could therefore be OFF types as indicated in the re-sponses to the drifting gratings before (Sect. 4.1a).

OMS-DS cell

c

−1 −0.5 0 0.5 1 Mean irradiance 6.33 mW/m²

−1 −0.5 0 0.5 1

Figure 4.4 Standard DS and OMS-DS cells are OFF-cells throughout photopic and high mesopic light levels, DS and OMS properties did not change. a Distri-bution of the On-off index at 6.33mW/m² mean irradiance. bResponses of a standard DS and an OMS-DS cell to 40% contrast steps from a mean luminance background at different light levels (mean irradiance0.63mW/m²−6.33mW/m²). c-dDistributions of DSI and OMSI do not change significantly between low photopic (c) and high mesopic light levels (d).

As the direction-selective cells observed in mammals were usually ON or ON-OFF type, I tested if standard DS and OMS-DS cells were true OFF cells by exam-ining their responses to flash stimuli. I used alternating flashes of half a second of 40% bright or dark contrast with 1.5 s periods of mean background illumina-tion in between (Fig. 4.4a-b). I calculated an on-off index by subtracting the

responses to dark contrast from the responses to the bright contrast flashes and normalizing by the total spike count. A negative index signified an OFF preference while a positive index showed an ON preference. By this measure, standard DS and OMS-DS cells were clearly OFF-cells as most cells in the salamander retina (Fig. 4.4a). But responses to the bright contrast might also be suppressed at high light levels. Therefore, I tested the flash responses for several lower light levels, starting from the usual low photopic range and going down to the high mesopic range (Fig. 4.4b). Throughout light levels standard DS and OMS-DS cells only responded to the dark contrast but with slightly changing spike patterns.

The distributions of the DSI and OMSI were also stable across different light lev-els (Fig. 4.4c-d). This demonstrates that direction-selectivity and object-motion-sensitivity seem to be fairly robust properties of retinal ganglion cells in the sala-mander, not depending on ambient light levels.

Standard OMS cells were more diverse in their responses to contrast flashes (Fig. 4.4a). Although most of them were OFF cells, the distribution of the on-off index indicated that they also comprised ON and ON-OFF types. Whether these ON and ON-OFF types were subtypes of the standard OMS cells or whether they were non-OMS cells which were erroneously classified as standard OMS cells by their high OMS index, still needs to be illuminated but will not be objective of this work. In the following, I will focus on the properties of standard DS and OMS-DS cells and will provide evidence that standard OMS-DS and OMS-OMS-DS cells could be different cell types.