Failures of visual analysis: scotoma, agnosia, and neglect

Abstract

This chapter deals with different types of failure to analyse the visual world, starting with the complete loss of vision, blindness, as well as blindness for circumscribed parts of the visual field, scotomata. It also reports some syndromes characterized by difficulties in perception of and discrimination between different domains or submodalities of vision, while leaving most others intact, such as acquired inability to assess visual motion, or acquired colour blindness caused by cerebral damage (achromatopsia). It then provides an overview of the most astonishing examples of failed visual perception: simultanagnosia; neglect; and Balint's syndrome. These syndromes share the phenomenon that conscious perception of objects in (parts of) the peripheral visual field is absent in spite of intact stimulation of the primary visual cortex by these objects. It also attempts to fill in some gaps of knowledge by means of speculation to produce a consistent taxonomy of symptoms encountered in visual neuropsychology. It combines the theoretical background with clinical findings and with a description of at least some of the methods used to diagnose patients suffering from failures of visual analysis.

Can Anosognosia Vindicate Traditionalism about Self-Deception?

“…Marshall and Halligan then proceeded to ask the patient which house they would prefer to live in and reliably chose the house that was not burning. Note, however, that even if this result were undisputed (Fahle, 2003), Levy doesn't offer grounds for accepting evidence from neglect to apply to anosognosia across the board. Nevertheless, even if we were to accept the evidence of implicit processing from neglect patients to transfer to anosognosia patients, Levy again gives us no reason to infer that that the patient believes the relevant proposition from evidence that establishes only (if at all) what he himself terms 'the lowest degree of personal availability'-that of cases of blindsight, in which patients cannot use visual information from their blind field in everyday life, but are able to use it to guess above chance in forced-situations (Weiskrantz, 1986).…”

Die zentrale Sehfunktion

“…[5]). Störungen auf dieser Verarbeitungsebene können unter dem Begriff "Indiskriminationen" zusammengefasst werden [3].…”

“…Dabei wird untersucht, wie stark sich ein isolierter stationärer Punkt während einer Sakkade zu bewegen scheint (vgl. [3] Wir haben uns oben klar gemacht, dass die Rezeptoren und Ganglienzellen der Netzhaut jeweils nur einen kleinen lokalen Bereich der Außenwelt abbilden, und das Gleiche gilt für die Neurone der primären visuellen Hirnrinde, Area 17 nach Brodmann. Diese neuronalen Elemente können Kontraste signalisieren, wie sie an den Kanten von Objekten auftreten, also deren Begrenzungslinien.…”

“…Ich kann hier nicht auf produktive Symptome wie Illusionen oder Halluzinationen sowie auf Stö-rungen der visuellen Aufmerksamkeit wie Neglekt oder Bµlint-Holmes-Syndrom eingehen, da sich Letztere insbesondere auf die Gesichtfeldperipherie auswirken (vgl. [3]). …”

Visual search training in occupational therapy – an example of expert practice in community-based stroke rehabilitation

“…Visual field loss results from disruption to the visual pathways causing part of the field of view to be lost from both eyes, so that the person is unable to see part of a scene on one side. The side and size of the visual field deficit depends on the site in the visual pathway that is interrupted by stroke (Fahle, 2003). Loss of half the field is known as homonymous hemianopia (HH).…”