Chilean brush tailed mouse (Octodon degus): a diurnal precocial rodent as a new model to study visual receptive field properties of superior colliculus neurons

Lab rodent species commonly used to study the visual system and its development (hamsters, rats, and mice) are crepuscular/nocturnal, altricial, and possess simpler visual systems than carnivores and primates. To widen the spectra of studied species, here we introduce an alternative model, the Chilean degu (Octodon degus). This diurnal, precocial Caviomorph rodent has a cone enriched, well-structured retina, and well-developed central visual projections. To assess degus' visual physiological properties, we characterized the visual responses and receptive field (RF) properties of isolated neurons in the superficial layers of the superior colliculus (sSC). To facilitate comparison with studies in other rodent species, we used four types of stimuli: (1) a moving white square, (2) sinusoidal gratings, (3) an expanding black circle (looming), and (4) a stationary black circle. We found that as in other mammalian species RF size increases from superficial to deeper SC layers. Compared to other lab rodents, degus sSC neurons had smaller RF sizes and displayed a broader range of spatial frequency (SF) tunings, including neurons tuned to high SF (up to 0.24 cycles/degree). Also, unlike other rodents approximately half of sSC neurons exhibited linear responses to contrast. Additionally, sSC units showed transient ON-OFF responses to stationary stimuli but increased their firing rates as a looming object increased in size. Our results suggest that degus have higher visual acuity, higher SF tuning, and lower contrast sensitivity than commonly used nocturnal lab rodents, positioning degus as a well-suited species for studies of diurnal vision that are more relevant to humans.