223 — Monkeys use the rod‐dense retinal region rather than the fovea to visually fixate small targets in scotopic vision 

Spivak, Thier, & Barash (10.1101/290759)

Read on 31 March 2018
#monkey  #rhesus  #neuroscience  #fovea  #retina  #fixation  #scotopic-vision  #vision  #dark-adaptation  #cones  #rods 

Scotopic vision is the term for looking around an environment in the dark: While cones are very useful for color and high-light vision scenarios, they’re not particularly useful in the dark. Furthermore, the fovea of the retina — the high-resolution central region present in many simian primates and some reptiles, birds, and fish — contains almost exclusively the color-sensitive cone photoreceptors, with very few rods.

This is a bit troublesome, because it means that our tendency to focus on a target in order to see it better falls apart in low light (“scotopic conditions”). This is most obvious when looking at a dim colored light in a very dark room. Staring directly at the color makes it less visible, not more. (Astronomers are probably very familiar with this phenomenon, because dim stars are easier to see with peripheral vision than by looking directly at them.)

In this paper, Spivak, Thier, & Barash demonstrate that rhesus monkeys account for this limitation in their vision by slightly adjusting their eye movements and fixation. In scotopic conditions, instead of looking directly at a stimulus, rhesus monkeys instead looked just above the stimulus, placing the stimulus not in the foveal center of high cone density, but instead in the ring of high rod density just adjacent to it.