Unconscious Processing in Neglect and Extinction

Abstract

Abstract Unilateral spatial neglect is a complex neurological disorder after unilateral brain damage, usually to the right hemisphere. It has many components, but is generally characterized by a lack of conscious perception, and of spatially directed action, for stimuli on the side of space contralateral to the brain damage (i.e. usually towards the left). Neglect patients not only fail to notice stimuli on the contralesional side of space, but often appear inadequate at exploring for them. They may ignore people approaching from the contralesional side; miss words or letters when reading; incorrectly take ipsilesional turns in a corridor or a street; eat food from only the ipsilesional side of a plate; and shave or make up only this side of their face (Mesulam, 1981; Heilman et al., 1997; Halligan and Marshall, 1998). On bedside clinical tests (Figure 7.1), neglect patients typically miss contralesional targets in cancellation tasks, deviate ipsilesionally on line bisection, and omit contralesional details when drawing or copying a picture. The phenomenon of perceptual ‘extinction’ is another common clinical sign (Figure 7.2); the patient can detect a contralesional stimulus when presented alone, yet remains unaware of the same stimulus when presented together with a concurrent stimulus on the ipsilesional side (Bender and Teuber, 1946; Rafal, 1994). Extinction often accompanies other signs of spatial neglect, but can be reported in isolation (Liu et al., 1992). Neglect and extinction can affect every sensory modality, although it is unclear whether this is invariably so. Besides deficits in perceptual awareness, spatial neglect can also involve lateralized biases in overt motor responses, with eye, head, and/or limb movements preferentially directed towards the ipsilesional side. Recent research has sought to examine whether such motor biases are simply the consequence of perceptual biases, or may instead have a separate origin (e.g. Bisiach et al., 1990; Mattingley et al., 1998).