Unilateral Neglect by Marshall John Robertson Ian

Author:Marshall, John, Robertson, Ian
Language: eng
Format: epub
Publisher: Taylor and Francis

Attention and Visual Neglect

When relying on vision, all animals need some kind of mechanism to tell them where to look next. A system where the eyes would move automatically to whatever is most (or perhaps least) salient in the peripheral visual field would be rather maladaptive. For instance, without control over spatial attention, it would be hopeless to try to keep one’s eyes on this sentence until it is finished before moving them to the person who just entered the room. Some mechanism is required that monitors areas of the visual field for potentially important information and other mechanisms to determine when and where the eyes move next or even if they move at all.

The most thorough investigations of the elementary operations of such a system have been reported by Posner and his colleagues (Posner & Petersen, 1990; Posner, Walker, Friedrich, & Rafal, 1984; Posner, Inhoff, Freidrich, & Cohen, 1987a; Posner, Walker, Friedrich, & Rafal, 1987b; Rafal et al., 1988; Rafal & Posner, 1987). In a simple visual task, they were able to break down covert spatial attention into move, engage and disengage operations, each associated with different neural structures in an overall network that guides visual spatial attention (Posner & Cohen, 1984). For the present purpose, the disengage operation is the most important because it is said to be altered in patients with neglect due to middle cerebral artery infarct (Morrow & Ratcliff, 1987), and in patients with extinction and/or more focal parietal lobe damage (Posner et al., 1984, 1987; Rafal, pers. comm.). The basic procedure is as follows. Three boxes appear horizontally across the midline on a computer screen, and the subjects fixate on the central box. A trial begins with one of the two peripheral boxes brightening for a short period of time. This is the cue. After a variable interval (usually 50–1000 msec), an asterisk appears in either peripheral box. The subjects’ task is to keep their eyes fixated on the central box and to press a button as soon as the asterisk is detected. Reaction time (RT) to the target in the cued location becomes faster as the interval between cue and target lengthens with an increasing delay in RT when the target appears in the uncued location (invalid location RT minus valid location RT). This difference between valid and invalid RT theoretically reflects the time to disengage covert attention from the cued location, move and engage on the target in the uncued location. Patients with parietal lobe damage detect the asterisk nearly as well in the contralesional and ipsilesional sides of space when it occurs in the cued location (nearly equal ability to move and engage attention to cued locations). However, the delay in responding to the asterisk is increased substantially when it occurs in the uncued location in the neglected field. Patients with parietal lobe damage show an abnormal contralesional delay. When attention must be disengaged from a location on the intact side, there is abnormal delay, and, as with clinical neglect, the delay is worse for right hemisphere groups than for left.

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