When visual metacognition fails: widespread anosognosia for visual deficits

Abstract

Anosognosia for visual deficits - cases where significant visual deficits go unnoticed - challenges the view that our own conscious experiences are what we know best. We review these widespread and striking failures of awareness. Anosognosia can occur with total blindness, visual abnormalities induced by brain lesions, and eye diseases. We show that anosognosia for visual deficits is surprisingly widespread. Building on previous accounts, we introduce a framework showing how apparently disparate forms of anosognosia fit together. The central idea is that, to notice a deficit, individuals need to form expectations about normal vision, compare expectations and visual input, and judge any mismatch at the metacognitive level. A failure in any of these three steps may lead to unawareness of visual deficits.

Keywords: anosognosia; brain lesions; perceptual decision-making; visual metacognition.

Figure 1.. Anosognosia for different visual deficits.

A depiction of the visual deficits in several visual disorders. The gray areas in the figure represent absence of input rather than an actual perception of gray. A) Normal perception of hot springs on a sunny day. B) Anton syndrome patients are completely blind. However, patients are unaware of their blindness and sometimes confabulate non-existent objects or deny their blindness. C) Hemianopic patients experience only half of their visual field. Patients with anosognosia for hemianopia fail to report anything different about their visual experiences after the deficit has occurred. D) Achromatopsia patients cannot identify colors and describe colored objects as gray. Individuals with anosognosia for achromatopsia can lack awareness of their color vision impairment. E) Patients with glaucoma can experience progressive loss of peripheral vision. Those with anosognosia for glaucoma are unable to recognize these deficits. F) In the advanced stage of age-related macular degeneration (AMD), individuals experience dense scotoma in their central vision. Subjects with anosognosia for AMD are unaware of this visual impairment.

Figure 2.. A unified framework for failing to notice visual deficits.

According to our proposed framework, awareness of visual deficits requires several steps. To begin with, due to a failure of the visual system, visual input becomes degraded or absent. When compared against expectations, a mismatch signal is generated, which is then integrated with prior beliefs about one’s own vision at the metacognitive level, resulting in an explicit insight into one’s visual deficit. This healthy flow of information that alerts us about visual deficits can fail at three levels. (1) Filling in and reality monitoring failures: Degraded or missing input can be filled in by internally generated signals, unrecognized as internally generated by the subject. This includes cases of Anton syndrome in which subjects provide complex descriptions of objects. (2) Failures to form expectations: The same lesions that compromise vision may disturb patients’ ability to form visual expectations about normal vision, for example in hemianopia and achromatopsia. In cases of gradual degeneration, such as glaucoma or AMD, visual priors may gradually update to reflect unhealthy vision. (3) Metacognitive limitations: General impairments of metacognitive systems might make subjects unable to update their metacognitive beliefs.