Figure 3. Intensity-response V/logI) functions and thresholds of the electroretinogram responses to 2 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 amplitudes of both ON and OFF transients are increased in the hclB^T2 mutant (two way analysis of variance [ANOVA], 10⁻¹⁴<p<0.01 for ON and OFF responses under different backgrounds), the effect of the mutation being more pronounced with respect to the ON responses. In B, the same functions are normalized to Vmax. The relative sensitivity of the ON transients, obtained in hclB^T2 flies under dark adaptation as well as under the dimmest background, is decreased (the curves are shifted to the right, p<0.01), while no change in relative sensitivity is observed under the two brighter backgrounds. Thus, in the hclB mutant, the shift of the ON transient V/logI curve along the intensity axis during light adaptation occurs within a narrower stimulus intensity range. The ON transient V/logI curves in the light adapted mutant are steeper and the dynamic range is narrowed by about 1 log unit (two way ANOVA, p=0.0035). The relative sensitivity and the dynamic range of the OFF transients are not significantly changed. 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=2.34×10⁻⁴ for ON responses; p=0.026 for OFF responses) indicating an increased absolute sensitivity of the mutant responses.