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 used to study the visual system and its development (hamsters, rats, and mice) are 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), a diurnal, precocial Caviomorph rodent with a cone enriched, well-structured retina, and well-developed central visual projections. To assess degu 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. Interestingly, compared to other lab rodents, degus have smaller RF sizes, likely indicating higher acuity. sSC neurons displayed spatial frequency tuning to grating stimuli from 0.08 to 0.24 cycles/degree. Additionally, neurons from sSC showed transient ON, OFF, or ON-OFF responses to stationary stimuli but increased their firing rates as a looming object increased in size. Our results suggests that degus have higher visual acuity, higher frequency tuning, and lower contrast sensitivity than commonly used nocturnal lab rodents, positioning degus as a well-suited model for studies of diurnal vision that are more relevant to humans.