Overview of the visual system of Tarsius.

Tarsiers, which are currently considered to constitute the sister group of anthropoid primates, exhibit a number of morphological specializations such as remarkably large eyes, big ears, long hind legs, and a nearly naked tail. Here we provide an overview of the current state of knowledge on the tarsier visual system and describe recent anatomical observations from our laboratory. Its large eyes notwithstanding, the most remarkable feature of the tarsier brain is the large size and distinct lamination of area V1. Based on the need of tarsier for optimal scotopic vision and acuity to detect small prey in low lighting conditions, tarsiers may have preserved a high level of visual acuity by enlarging V1 at the expense of other areas. The other classically described visual regions are present in tarsier, albeit many borders are not clearly distinct on histochemical or immunohistochemical preparations. Tarsiers also have a large number and unusual distributions of cones in the retina, with high numbers of M/L-cones in the central retina and S-cones surprisingly at the periphery, which may be sensitive to UV light and may be useful for prey detection. These adaptive specializations may together account for the unique nocturnal predatory requirements of tarsiers.