All sensory information is encoded in neural spike trains. It is unknown how the brain utilizes this neural code to drive behavior. Here, we unravel the decoding rules of the brain at the most elementary level by linking behavioral decisions to retinal output signals in a single-photon detection task. A transgenic mouse line allowed us to separate the two primary retinal outputs, ON and OFF pathways, carrying information about photon absorptions as increases and decreases in spiking, respectively. We measured the sensitivity limit of rods and the most sensitive ON and OFF ganglion cells and correlated these results with visually guided behavior using markerless head and eye tracking. We show that behavior relies only on the ON pathway even when the OFF pathway would allow higher sensitivity. Paradoxically, behavior does not rely on the spike code with maximal information but instead relies on a decoding strategy based on increases in spiking.

Highlights d Diurnal effects on visual sensitivity were studied from the retina to behavior d Visually guided behavior of mice reaches the highest sensitivity at night d The sensitivity limit of retinal ganglion cells does not show diurnal differences d Mice use a better search strategy for visual cues at night

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SUMMARYPerception of light in darkness requires no more than a handful of photons and this remarkable behavioral performance can be directly linked to a particular retinal circuit – the retinal ON pathway. However, the neural limits to shadow detection in very dim light have remained unresolved. Here, we unravel the neural mechanisms that determine the sensitivity of mice to the dimmest light decrements by measuring signals from the most sensitive ON and OFF retinal ganglion cell types and by correlating their signals with visually guided behavior. We show that mice can detect shadows when only a few photon absorptions are missing among thousands of rods. Behavioral detection of “quantal” shadows relies only on the retinal OFF pathway and is limited by noise and losses of single-photon signals in retinal processing. Thus, in the dim-light regime, light increments and decrements are encoded separately via the ON and OFF retinal pathways, respectively.