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Cortical Blindness (Plus Anton-Babinski Syndrome, Blindsight, & Riddoch Syndrome)
Published in Alexander R. Toftness, Incredible Consequences of Brain Injury, 2023
In blindsight, a person loses the ability to consciously use visual information, but they can still use it nonconsciously. In other words, if you ask them whether they are blind, they will say yes, but if you test their blindness, they will perform better than chance on tasks where they have to visually identify things like object position and movement, and they might report feelings similar to seeing that are not seeing (Overgaard, 2011). In blindsight, people can often successfully detect color and orientations of lines, and sometimes they can detect shapes, but they generally cannot detect more complex things such as identities of faces (Ajina & Bridge, 2017). These scenes can be remarkable—for example, cortically blind children with blindsight being fed by caregivers can detect a spoon approaching their mouths if the spoon is moved in a particular way (Boyle et al., 2005).
Cortical Visual Loss
Published in Vivek Lal, A Clinical Approach to Neuro-Ophthalmic Disorders, 2023
On the other hand, some studies have found that patients with unilateral hemianopia or cerebral blindness may still possess some residual visual function in their blind area, of which they may be unaware (42). This “blindsight” has been shown by many techniques, such as pointing or making saccades to a visual target in the blind field, by better-than-chance guesses about the movement, form or color of objects, or even by showing that how hemianopic patient respond to visible targets is influenced by targets in the blind hemifield (43, 44). One taxonomy groups these phenomena into three clusters (45). “Action blindsight” includes the localization of targets, “attention blindsight” refers to orienting, inhibition of return and motion perception, and “agnosopsia” encompasses abilities to report on object properties such as form and color. These may have different anatomic explanations (45). Action blindsight may involve a subcortical pathway from the retina to superior colliculus (46, 47). Residual motion perception may involve retinotectal pathways that project via the pulvinar to extrastriate cortical regions like V5. Perception of color and form may be mediated by direct projections from surviving lateral geniculate neurons to extrastriate cortex (48). Blindsight can only be demonstrated convincingly in a laboratory, with careful control for light scatter (49, 50). While most blindsight studies have single cases or small numbers of subjects, larger samples indicate that blindsight is rare (51–53). It is not yet proven that blindsight benefits the patients who show it.
Disruptions in physical substrates of vision following traumatic brain injury
Published in Mark J. Ashley, David A. Hovda, Traumatic Brain Injury, 2017
Once the brain receives the visual representation, it can then attend to it in that hemispace. Disorders of attention vary in regards to the network involved, i.e., the attention arousing, orienting, or selective attentional network. The reticular activating input to the thalamus and cortex is necessary to be awake and for conscious awareness. Disruptions in the reticulofrontal pathway result in decreased attention and impaired speed of processing. This system is fundamental for all cognition, essentially serving as an “ignition switch.” The orientation network is responsible for preattention or the screening or selecting out of irrelevant signals as there are limits of the brain’s attentional capacity. This is an automatic subconscious process carried out primarily in the pulvinar. Blindsight is an example of the subconscious ability to orient to visual stimuli. In patients with a unilateral occipital lesion who demonstrate a contralateral homonymous hemianopsia, they are still able to subconsciously discriminate some form, color, or orientation in that hemifield. The selective attentional system is a cortical system preparing for the conscious visual awareness picking out relevant information from distractors in vision. The disruption of this system results in visual hemispatial neglect, a disorder of higher visual processing.
Neurons Embodied in a Virtual World: Evidence for Organoid Ethics?
Published in AJOB Neuroscience, 2022
Brett J. Kagan, Daniela Duc, Ian Stevens, Frederic Gilbert
Foremost, it is important to distinguish between key states which organisms may possess. Sawai et al. (2022) focus their discussion on phenomenological consciousness, characterized by what-it’s-likeness, which they use interchangeably with the term sentient. While colloquially the terms are exchangeable, it is imprecise and may lead to some conceptual conflations and to the wrong ethical conclusion. For the purpose of this commentary the term consciousness is used in line with phenomenological consciousness as per Sawai et al. (2022) Sentience on the other hand has been formally defined as “responsive to sensory impressions” (Friston, Wiese, and Hobson 2020). While the two are intuitively related and would typically coexist, it is possible to imagine states where they may present exclusively. For example, Type 1 Blindsight patients present with visual sentience—where they can receive visual information, process that information, then act upon it—while reporting no conscious experience of the stimulus. Accordingly, the definition of consciousness needs to be better defined regarding organoids and in vitro neuronal tissues. Such definitions need to include all aspects of consciousness to measure and determine its presence in tissues (engineered or other). Specific definitions would consider other aspects of consciousness for clearer and objective classifications, such as first-person introspective, qualitative character, phenomenal, subjectivity, perspectival, intentionality and transparency, unity and importantly, dynamic flow (Van Gulick 2021).
Dr Gordon Plant’s Festschrift Tidings
Published in Neuro-Ophthalmology, 2022
Sui H. Wong, Susan Mollan, Simon J Hickman, Stephen Madill, Luke Bennetto, Sarah Cooper
Professor Holly Bridge talked about ‘Investigating residual pathways in homonymous hemianopia’. She explained research work into the phenomenon of blindsight with functional MRI had revealed activation in the visual motion cortex (MT) in the absence of any V1 activity. Professor Bridge’s further elegant clinical and radiological studies suggest that the phenomenon of blindsight is dependent upon preservation of a direct pathway between the LGN and MT. In the discussion section Professor Bridge explained the potential reasons for the LGN MT pathway in health – before Dr Plant revealed that MT allows us to play cricket.
The Unconscious: Theory, Research and Clinical Implications
Published in Psychiatry, 2023
Researchers studying implicit memory, perception, learning, and brain damage found evidence for unconscious processes. Studies by Schacter (1987) and Reber (1993) revealed the evidence for implicit memory. Seger’s studies (1994) demonstrated the role of implicit learning. Studies of brain-damaged individuals by Gazzaniga (1970) and research into blindsight (Weiskrantz, 1986) showed that unconscious processes are normal, how the brain operates.