Blindsight and Unconscious Vision: What They Teach Us about the Human Visual System

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; dorsal visual stream; hemianopia; primary visual cortex; ventral visual stream; visual system.

Figure 1

(A) The major visual pathway from the eyes to the visual cortex and the reconfiguration at the optic chiasm. The right geniculostriate projection (red) is damaged and hence the lateral geniculate nucleus (LGN) is reduced in size relative to the left intact side (white). (B) Several visual pathways from the optic tract. The major pathway via the LGN to primary visual cortex (V1) is shown in green. The three main classes of retinal ganglion cell are indicated by the red-green (P-cells), gray (M-cells), and blue-yellow (non-M–non-P cells) lines. No assumptions are made about the origins of the connections indicated with the unfilled lines. Illustration in A courtesy of Betina Ip.

Figure 2

In the healthy visual system hMT+ and V1 show distinct response patterns to increasing the proportion of coherent motion. (A) In hMT+, blood oxygenation level–dependent (BOLD) signal change increases with increasing coherence, apart from an initial dip. A model describing this pattern shows a clear correlation with hMT+ activity in controls. In contrast, in patients with visual field loss due to V1 damage who are shown images inside their scotoma, hMT+ has no significant correlation with this control-derived model. (B) V1 in healthy controls shows a decrease in response to increasing motion coherence. When a model of this V1 pattern is generated, unsurprisingly, in control subjects V1 has a response significantly correlated to this pattern. However, in the blind field of patients, the only cortical area that shows this V1-like response is hMT+ (from Ajina and others 2015a).

Figure 3

V1 shows a linear response to increasing stimulus contrast in healthy control subjects. hMT+ shows this pattern in the damaged hemisphere of hemianopic patients, and to some extent in healthy controls. However, the strongest response in healthy hMT+ is to a logarithmic model, a pattern not seen in hemianopic patients (from Ajina and others 2015c).

Figure 4

Diffusion tractography illustrating three visual pathways in patients with blindsight (blindsight positive), without blindsight (blindsight negative), and healthy controls. In blindsight-positive patients, only the lateral geniculate nucleus (LGN)– hMT+ pathway showed consistently intact microstructure, suggesting this may be the route underlying any residual visual function (from Ajina and others 2015b).

Figure 5

Cortical responses in bilaterally hemianopic patient SBR. When presented with images of faces and houses, he shows very little occipital activation, with small signals in the fusiform and parahippocampal places areas respectively (upper row). In contrast, when generating images of faces and houses, there is extensive activation throughout the occipital and posterior parietal lobes. This indicates that while V1 is critical for perceiving such stimuli, it is not necessary for visual mental imagery (from Bridge and others 2012).