Figure 2. Intensity-response V/logI) functions and thresholds of the electroretinogram responses to 0.3 s stimuli. In A and B, the V/logI curves of the ON transients (left) and OFF transients (right) are presented obtained in the wild-type flies (open symbols, dashed lines, n=10) and in the null mutant hclB^T2 (filled symbols, solid lines, n=10) under dark adaptation (DA, black squares) as well as under three levels of background illumination (4.66 log quanta s⁻¹ μm⁻², blue circles; 5.66 log quanta s⁻¹ μm⁻², green triangles; 6.66 log quanta s⁻¹ μm⁻², orange diamonds). In A, the response amplitude in mV versus log stimulus intensity I_t) is represented. The amplitude of both ON and OFF transients is increased in the hclB mutant, the effect of the mutation being more pronounced with respect to ON responses (two way analysis of variance [ANOVA], 10⁻¹⁵<p<0.05 for ON and OFF responses under different backgrounds). In B, the same functions are normalized to Vmax. The relative sensitivity of the ON and OFF transients, assessed by the I₅₀ points of the V/log I curves, is mostly not significantly different between the wild-type and hclB mutant flies. The steepness of the V/log I curves of the ON transients is higher in the hclB mutant and thus the ON-response dynamic range is narrowed (two way ANOVA, p=0.015). In C, the thresholds of the electroretinogram (ERG) ON (left) and OFF (right) transients are presented, obtained under dark adaptation (DA), as well as under three levels of background illumination. The thresholds are estimated using 0.5 mV criterion amplitude. The thresholds of the wild-type flies, (gray columns) and the null mutant hclB^T2 (pink columns) are compared. The thresholds of the hclB^T2 mutant transients are significantly lower (two way ANOVA, p=1.22×10⁻⁵ for ON responses; p=0.032 for OFF responses), indicating an increased absolute sensitivity of the mutant responses.