Abstract
Over the last 50 years, studies of receptive fields in the early
mammalian visual system have identified many classes of response
properties in brain areas such as retina, lateral geniculate nucleus
(LGN), and primary visual cortex (V1). Recently, there has been
significant interest in understanding the cellular and network mechanisms
that underlie these visual responses and their functional architecture.
Small mammals like rodents offer many advantages for such studies, because
they are appropriate for a wide variety of experimental techniques.
However, the traditional rodent models, mice and rats, do not rely heavily
on vision and have small visual brain areas. Squirrels are highly visual
rodents that may be excellent model preparations for understanding
mechanisms of function and disease in the human visual system. They use
vision for navigating in their environment, predator avoidance, and
foraging for food. Visual brain areas such as LGN, V1, superior
colliculus, and pulvinar are particularly large and well elaborated in the
squirrel, and the squirrel has several extrastriate cortical areas lateral
to V1. Unlike many mammals, most squirrel species are diurnal with
cone-dominated retinas, similar to the primate fovea, and have excellent
dichromatic color vision that is mediated by green and blue cones. Owing
to their larger size, squirrels are physiologically more robust than mice
and rats under anesthesia, and some hibernating species are particularly
tolerant of hypoxia that occurs during procedures such as brain slicing.
Finally, many basic anatomical and physiological properties in the early
visual system of squirrel have now been described, permitting
investigations of cellular mechanisms. In this article, we review four
decades of anatomical, behavioral, and physiological studies in squirrel
and make comparisons with other species.