Disorders of Visual Attention: Cognitive Neuropsychology
Michael Posner (e.g. Posner & Petersen, 1990) proposed a theoretical frame-
work within which various disorders of visual attention in brain-damaged
patients can be understood. In essence, he argued that at least three separate
abilities are involved in visual attention:
.The ability todisengageattention from a given visual stimulus.
.The ability toshiftattention from one target stimulus to another.
.The ability toengageattention on a new visual stimulus.
Disengagment of Attention Problems with the disengagement of attention have
been studied in patients suffering from what is known asunilateral visual neglect.
The most common form of unilateral visual neglect is when patients with dam-
age to the right hemisphere neglect or ignore visual stimuli in the left side of
space. The problem is not simply one of being unable to see what is presented
to the affected side because such patients can also show neglect on tasks involv-
ing images rather than visual perception (Bisiach & Luzzati, 1978).
Posner, Walker, Friedrich, and Rafal (1984) carried out a study on patients
with unilateral visual neglect in which cues to the locations of forthcoming tar-
gets were presented. The patients generally coped reasonably well with this
task, even when the cue and the target were both presented to the impaired
visual field. However, there was one major exception: when the cue was pre-
sented to the unimpaired visual field and the target was presented to the
impaired visual field, the patients’ performance was extremely poor. These
findings suggest that the patients found it particularly difficult to disengage
their attention from visual stimuli presented to the unimpaired side of visual
space.
Patients with unilateral visual neglect have suffered damage to the parietal
region of the brain (Posner et al., 1984). A different kind of evidence that the
parietal area is important in attention was obtained by Petersen, Corbetta,
Miezin, and Shulman (1994), who made use of PET scans. They used a variety
of tasks, and discovered that there was generally considerable activation within
the parietal area when attention shifted from one spatial location to another.
Problems with disengaging attention are also found in patients suffering
fromsimultanagnosia. In this condition, only one object (out of two or more)
can be seen at any one time, even when the objects are close together in the
visual field. As most of these patients have full visual fields, it seems that the
attended visual object exerts a ‘‘hold’’ on attention that makes disengagement
difficult. However, there is evidence that neglected stimuli are processed to
some extent. For example, Coslett and Saffran (1991) observed strong effects of
semantic relatedness between two briefly presented words in a patient with
simultanagnosia.
Shifting of Attention Posner, Rafal, Choate, and Vaughan (1985) investigated
problems of shifting attention by studying patients suffering fromprogressive
supranuclear palsy. Such patients have damage to the midbrain. As a conse-
quence of this brain damage, they find it very difficult to make voluntary eye
movements, especially in the vertical direction. These patients were given the
task of responding to visual targets, and there were sometimes cues to the loca-
376 Michael W. Eysenck and Mark T. Keane