Overall, the data showed no evidence of preservation or transient neuroprotection in
merkd mouse eyes receiving SES from an MPA device. All differences between active and control groups were most likely a result of insult to the retina after surgical injury from implantation. At week 1 after surgery, active eyes had significantly attenuated
Vmax compared with controls but did not show a significant decrease in overall maximal b-wave amplitude. It is possible that rod photoreceptors were more sensitive to injury from surgery, thus resulting in attenuated rod-driven responses, whereas mixed rod-cone responses to the brightest flash intensities were unaffected, remaining at control amplitudes. The OPs were also significantly attenuated in the active implanted eyes and remained below control levels until the third week after surgery. OPs are known to be among the most sensitive of ERG components to changes in the retina; they are often affected before changes in a- or b-waves can be seen.
47,51,52 Finally,
Vmax began degenerating from the first week tested, but postreceptoral sensitivity (
log σ) did not begin declining until the third week after surgery. The former,
Vmax , takes into account the amplification of a graded signal, which will reflect a change in the number of cells contributing to the signal. Sensitivity, on the other hand, is related to the kinetics of phototransduction. Because the
Mertk defect does not directly affect phototransduction, it is likely photoreceptors retain functionality until structural disintegration causes cell death. If this is the case, then the functional response from the photoreceptors is appropriate for the intensity of the stimulus; there are simply fewer cells responding. This could correspond to a change in maximal amplitude (
Vmax ) before a change in sensitivity (
log σ).