Fig. 7.

Neuronal loss occurs near the electrode tip. (a) Layer V neurons from an electrode-implanted animal (top) and a window-only animal (bottom) $\sim 2$ months and $\sim 6$ months postsurgery ($\text{scale bar}=200\mu \mathrm{m}$). (b) Sample traces show loss of neurons over time in an electrode-implanted animal (red) compared with neuronal count in a window-only control (black). Downward-pointing blue arrow denotes maximum cell number, whereas upward-pointing blue arrow denotes point at which cells drop below 50% of max. Linear regression is plotted as a gray line. (c) Animals with electrode implants reached 50% cell loss within $450\mu \mathrm{m}$ from the electrode tip by $210.5\pm 72\mathrm{dpi}$ (error bars show $\mathrm{mean}\pm \mathrm{SEM}$). When not significantly different from the electrode group (Wilcoxon rank sum test, $p=0.254$), 1 of 5 window-only animals still had not reached 50% cell loss (gray circle). (d) Representative images of regions encompassed by $<100\text{-}\mu \mathrm{m}$, 100- to $200\text{-}\mu \mathrm{m}$, 200- to $300\text{-}\mu \mathrm{m}$, and 300- to $400\text{-}\mu \mathrm{m}$ rings. ($\text{scale bar}=200\mu \mathrm{m}$). Linear regression of neuronal counts over time reveal a significant decrease in cells within $100\mu \mathrm{m}$ of an electrode compared with window-only animals ($p=0.016$) and a slight decrease within 100- to $200\text{-}\mu \mathrm{m}$ distances (n.s.). No decrease is seen at distances $>200\mu \mathrm{m}$. Animals with electrode implants reached 50% cell loss within $100\mu \mathrm{m}$ from the electrode tip by $137\pm 56\mathrm{dpi}$ ($p=0.016$), by $199\pm 50\mathrm{dpi}$ within 100 to $200\text{-}\mu \mathrm{m}$ distances (n.s.), and by $258\pm 100\mathrm{dpi}$ within 200 to $300\text{-}\mu \mathrm{m}$ distances (n.s.).