By applying electrical stimulation, retinal prostheses aim to bypass dysfunctional photoreceptors and directly activate retinal ganglion cells (RGCs). Therefore, effective modulation of RGCs is the key step in the development of retinal prostheses. Six cynomolgus monkeys were used: three as a control and the other three as an N-methyl-N-nitrosourea (MNU)-induced retinal degeneration model. The retinal recording was performed using the 8x8 multi-electrode array. Figure shows the electrical stimulation pulses (a), raster plots of RGC spikes (b), and the modulation efficiency score (MES) (c). The number of stimulation conditions resulting in significantly higher post-stimulation firing rates compared to pre-stimulus firing rates is defined as the MES. We investigated the relationship between the variables and the MES in normal and degenerated primate RGCs. External variables such as duration and inter-electrode distance and internal variables such as average firing rates and statistics [mean, standard deviation, and coefficient of variation (CV)] of inter-spike intervals (ISIs) of spontaneous spikes were used. External variables have similar effects on MES in normal and degenerated RGCs; longer stimulation duration and shorter inter-electrode distance resulted in higher MES. In contrast, internal variables affect MES differently in normal and degenerated RGCs; in normal RGCs, they were not related to the MES, while in degenerated RGCs, the mean ISIs were positively correlated with the MES, and the CV of ISIs was negatively correlated with the MES. This finding could be used for the optimal selection of stimulation channels and practical calibration methods for higher efficiency when testing retinal prostheses.