Figure 8.

RGC responses to glutamate injected through any single-microport of the multiport synapse chip device are spatially localized. (a) A color map displaying the spatial spread of responsive somal RGCs (warmer colors representing larger numbers of responsive cells) to glutamate injected through a representative microport on the left edge of the device (indicated by the small gray square in the inset shown at the upper right corner). The RGC response plot is overlaid with the locations of the device microports (the single active microport represented by a yellow circle and all other inactive microports represented by red circles) and the 60 electrodes of the MEA (the black circles at the intersections of the 8×8 dashed grid lines, with the exception of the four corner intersections, represent the locations of the 60 electrodes). As can be clearly discerned via visual inspection of the plot, RGC responses to glutamate injected through this representative microport were spatially localized with the highest density of responsive cells located near the center of the MEA electrode located directly under this injection site. The median spread of the glutamate-responsive RGCs under this injection site was found to be 400 μm with lower and upper quartiles of 200 and 630 μm, respectively. (b) The average spatial distribution of RGCs responding to glutamate injections through individual microports plotted as a function of the radial distances of the RGCs from the center of the electrode under a typical injection site. The plot shows that a relatively high proportion of glutamate-responsive cells were located at or very near the electrode under the typical injection site but a small number of these glutamate-responsive cells were located as far as 1200 μm (which is within the farthest span of ~1720 μm between the corner electrodes of the MEA) from the same electrode. The solid and the dashed red vertical lines indicate the median and the lower/upper quartiles of the spread of the glutamate-responsive cells, respectively.