Check for updates Review Blindsight and Unconscious Vision: What They Teach Us about the Human Visual System Sara Ajina' and Holly Bridge' The Neuroscientist 2017, Vol. 23(5) 529-541 @ The Author(s) 2016 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/1073858416673817 journals.sagepub.com/home/nro ®SAGE Abstract Damage to the primary visual cortex removes the major input from the eyes to the brain, causing significant visual loss as patients are unable to perceive the side of the world contralateral to the damage. Some patients, however, retain the ability to detect visual information within this blind region; this is known as blindsight. By studying the visual pathways that underlie this residual vision in patients, we can uncover additional aspects of the human visual system that likely contribute to normal visual function but cannot be revealed under physiological conditions. In this review, we discuss the residual abilities and neural activity that have been described in blindsight and the implications of these findings for understanding the intact system. Keywords blindsight, visual system, primary visual cortex, hemianopia, ventral visual stream, dorsal visual stream Introduction The human visual system is a remarkable brain network that allows us to take the light input arriving at the eye and transform it into a colorful, rich experience that comes effortlessly. That this visual percept can remain stable with constant movement of the eyes, across light levels that vary by orders of magnitude and with chang- ing wavelengths of light is mindboggling. The complex- ity of computation is reflected in the considerable proportion of the human brain (between one-third and one-half) dedicated to processing this visual input (Sereno and Allman 1991). Given the vast quantity of visual information that reaches the brain, it is simply not possi- ble for each and every visual signal to elicit a conscious percept. Conscious vision teaches us about the very important primary visual pathways. However, the some- what more challenging investigation of non-conscious vision may also reveal properties of less dominant struc- tures and pathways that are still an essential component of the human visual system. This introductory section will lay out the major processing pathways of the human visual system before addressing the consequences of damage to this system, and what we can learn from study- ing them. At the retinal level, phototransduction by the rods and cones is the primary source of light sensitive input to the visual system. These signals go through several process- ing steps within the retinal network before the axons of the ganglion cells project along the optic nerve. Figure 1A shows the decussation of the optic nerve fibers at the optic chiasm in which the fibers from the temporal retina remain on the ipsilateral side of the brain, whereas those from the nasal retina decussate to form part of the contralateral optic tract. While the figure shows the major geniculo- striate projection to the primary visual cortex (V1), there are multiple targets innervated by optic tract fibers, laid out below. Pathways Projecting via the Lateral Geniculate Nucleus In humans and non-human primates,