The scotopic band: primate detailed scotopic vision and perceptual uncertainty

Primates perceive detailed images in photopic (daylight) vision via a preferred-processing, high-acuity pathway, made up of a small photopic center (the fovea), and dedicated eye movements, whose function is to shift images of target objects to the fovea and keep them while foveal cones sense the image. Thus, the preferred processing pathway processes images serially, not in parallel. No such pathway is known for scotopic (night-light) vision or mesopic (twilight) vision - though details are informative in dim light, even vital; it remains unclear whether and how scotopic vision of details is accomplished. Here we show that primates do have a scotopic preferred-processing pathway. It consists of a scotopic center, located on a scotopic band, and eye movements that shift target images not to the fovea but directly to the scotopic center. In contrast to the stationary fovea, the scotopic center can relocate over the scotopic band. The scotopic center relocates to match changing visual conditions and contextual needs. Ambient light intensity is mapped monotonically onto the scotopic band, with mesopic vision at one end. The dimmer the light is, the more dorsal the scotopic center relocates on the scotopic band. Importantly, the scotopic center relocates (or set) not only in passive response to ambient light but also actively, driven by internal factors. In near-threshold conditions the scotopic center relocates to a more dorsal band location than in salient conditions. That the same relocation of the scotopic center can be instigated both by dimming the ambient light and by reducing the detectability of targets indicates that, at the core, the longitudinal axis of the scotopic band encodes the level of perceptual uncertainty.