The optimal spot size, which evoked the maximum discharge, was selected to determine the luminance threshold (
Fig. 2B). This stimulus was used in conjunction with at least six neutral density filters that were placed in the optic path to measure threshold irradiance. At 3.4 log photons/μm
2/s, cell 217-C1 showed vibrant discharges (
Fig. 4A1). However, reducing the irradiance approximately 10-fold (2.4 log photons/μm
2/s) yielded a substantial reduction in firing rate (
Fig. 4A3). The impulse rates of cell 122-C2 (
Fig. 4B1) and cell 115-C1 (
Fig. 4C1) were also strongly modulated by testing spots, and reduced irradiance levels substantially decreased discharge rates (
Figs. 4B3, 4C3). Response profiles are shown (
Fig. 4A5). This cell in the control retina exhibited a substantially reduced response magnitude at all tested irradiance levels. It could therefore be driven by a wide range of visual stimulus intensity, whereas extended maintenance in the recording chamber (more than 3 hours) resulted in the production of responses only to high-intensity visual stimuli. As summarized in
Table 2, this response pattern was confirmed in most recorded cells in later experiments. In contrast, after antioxidants were delivered to the perfusion system, both cells exhibited substantially elevated firing rates at both irradiance levels. The irradiance response profiles of two additional cells are shown in
Figures 4B5 (122-C2) and 4C5 (115-C1), respectively. It is evident that cells exhibited substantially elevated irradiance thresholds without antioxidant treatment (
Fig. 4A5), that ALA treatment reduced the irradiance threshold to the basal level (
Fig. 4B5), and that TUDCA substantially reduced the irradiance threshold of the cell to a point surpassing its baseline threshold (
Fig. 4C5). We observed a statistically significant elevation in the threshold irradiance in cells recorded after prolonged maintenance in the recording chamber, compared to normal control cells (
Fig. 4D; paired
t-test;
P = 0.029, SEM = 0.082,
n = 6). However, as shown in
Figure 1B and
Table 2, the ALA treatment reduced the irradiance threshold (
Fig. 4D; paired
t-test;
P < 0.459, SEM = 0.075,
n = 7), and the TUDCA treatment greatly elevated the irradiance sensitivity of the recorded cells (
Fig. 4D; paired
t-test;
P < 0.001, SEM = 0.0258,
n = 12). This trend of the irradiance threshold suppression by the TUDCA is shown in
Figure 1D. The difference of average threshold irradiance (Δ threshold irradiance) was significantly reduced after both ALA (
Fig. 4E; ANOVA,
P = 0.019) and TUDCA (
Fig. 4E; ANOVA,
P < 0.001) treatments. There was, however, no statistically significant difference between ALA and TUDCA treated cells (
Fig. 4E; ANOVA,
P = 0.177).