Abstract
Psychophysical data indicates humans can discriminate visual scenes based on their skewness– the ratio of dark and bright patches within a visual scene. It was also shown that on a phenomenological level this skew discrimination is described by the so-called Blackshot mechanism, which accentuates strong negative contrasts within a scene. Here we demonstrate that the neuronal correlate of the Blackshot mechanism is the asymmetric gain of the cone phototransduction cascade, which is higher for strong negative contrasts than for strong positive contrasts. We recorded from goldfish cone photoreceptors and found that the asymmetry in the phototransduction gain leads to higher amplitude of the responses to negatively than to positively skewed light stimuli. This asymmetry in the amplitude was present in the photocurrent, voltage response and cone synaptic output. Additionally, we found that stimulus skewness leads to a subtle change in photoreceptor kinetics. For negatively skewed stimuli, the cone’s impulse response functions peak later than for positively skewed stimulus. However, stimulus skewness does not affect the cone’s overall integration time.
Significance statement Humans can discriminate visual scenes based on skewness – the relative prevalence of bright and dark patches within a scene. Here we show that this discrimination originates in the asymmetric gain function of the retinal cone photoreceptors. This gain is higher for the strong negative (dark patches) than for the strong positive (bright patches) contrasts. Thus, we show that cone photoreceptors do not simply relay visual stimuli to downstream circuitry, but also emphasize specific features of those stimuli.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Funding This work was supported by a ZonMW grant 91215062 (M.K.), a grant from Horizon 2020: “Switchboard” (M.K.), a grant of ODAS (M.K.) and a grant of the Foundation of Friends of the NIN (M.K.).