Abstract
Purpose :
Asymmetries in the morphology and function of ON and OFF pathways may optimize the processing of natural images. However, we do not know whether freely behaving mice respond differentially to light increments and decrements. Here we apply an operant behavioral assay to investigate sensitivity of mice to ON and OFF stimuli. Our hypothesis is that the relative preference of mice for ON vs OFF stimuli depends on 1) the mean illumination level, 2) the temporal frequency of the periodic stimulus waveform and 3) the kinetics of the photoreceptor responses.
Methods :
We applied a forced-choice operant behavioral assay to determine the sensitivity of mice to ON and OFF sawtooth waveforms with rapid luminance increments and decrements. We used contrast frequencies of 3, 6 and 12 Hz and mean retinal illuminances of 10, 400, and 9000 R*/rod/sec. We utilized GNAT2 KO animals that have no cone responses and R9AP95 transgenic mice which have accelerated rod photoresponse recovery kinetics. To quantify the asymmetries in the detection, we calculated the sensitivity ratio (AR, ON : OFF) . Statistical analysis used 2 Way-ANOVA, p = 0.05.
Results :
At mesopic levels (9000 R*/rod/sec), WT mice (n = 7, 3-6 replicate measurements) had AR=1.2±0.1 at 3 Hz, 1.0±0.1 at 6 Hz and 0.7±0.1 at 12 Hz, consistent with a shift in preference for fast light decrements with faster frequencies. At scotopic illumination levels (10 R*/rod/sec) and 3 Hz stimuli, AR=0.8±0.1, a slight bias for the OFF decrement. The AR of GNAT2 KO mice (n = 5, 3-6 replicate measurements) at 9000 R*/rod/sec and 3 Hz was 1.8±0.1, suggesting stronger preference for increments than WT mice. GNAT2 KO and WT mice exhibited same ratio values at all other frequencies and illumination levels. The AR of R9AP95 mice (n = 4, 3-6 replications) was similar to WT except at scotopic levels (10R*/rod/sec) and 3Hz with AR = 1.1±0.1.
Conclusions :
Mice show a preference for either rapid ON or OFF waveforms depending on stimulus mean illuminance and frequency. Faster rod photoresponse kinetics bias the response sensitivity towards ON waveforms.
This is a 2020 ARVO Annual Meeting abstract.