Focused Visual Attention

Over the past 25 years, most researchers have studied visual rather than auditory attention. Why is this? Probably the main reason is that it is generally easier to control the presentation times of visual stimuli than of auditory stimuli.

Some of the issues we will be discussing in this section of the chapter have been considered from the cognitive neuropsychological perspective. Three attentional disorders have been studied fairly thoroughly: neglect; extinction; and Balint's syndrome (see Driver, 1998, for a review). Neglect is typically found after brain damage in the right parietal lobe, and is often the result of a stroke. Neglect patients with right-hemisphere damage do not notice, or fail to respond to, objects presented to their left (or contralesional) side. For example, when neglect patients draw an object or copy a drawing, they typically leave out everything on the left side of it. According to Driver (1998, p. 308), "The essential problem in neglect may be that while the patient can, in principle, look or attend toward the contralesional side, they usually fail to do so spontaneously." In addition, neglect patients can also show neglect on tasks involving images rather than visual perception (Bisiach & Luzzati, 1978). It is important to note that "neglect is not a single disorder but a range of disorders which can occur in varying degrees within any patient" (Parkin, 1996, p. 91).

It might be thought that neglect occurs because stimuli on one side of the visual field are not processed perceptually. However, most of the evidence indicates that that is not typically the case. For example, Marshall and Halligan (1988) presented a neglect patient with two drawings of a house that were identical, except that the house presented to the left visual field had flames coming out of one of its windows. The patient was unable to report any differences between the two drawings, but indicated that she would prefer to live in the house on the right.

How can we explain neglect? According to Parkin (1996, p. 108), "At the moment the most convincing class of theories concerning neglect are those that propose some form of attentional deficit. Essentially these theories suggest that there is an imbalance in the amount of attention allocated to left and right.. .However, the idea that a single theory of neglect will emerge is highly unlikely because of the diversity of defects being discovered." Posner's attentional theory of neglect is discussed later.

Extinction is a phenomenon frequently found in neglect patients. A single stimulus on either side of the visual field can be judged normally. However, when two stimuli are presented together, the one farther towards the side of the visual field away from the damage tends to go undetected. Some patients only show extinction when the two objects presented simultaneously are the same.

Balint 's syndrome is associated with lesions in both hemispheres involving the posterior parietal lobe or parieto-occipital junction. It is characterised by various attentional problems. These include fixed gazing, gross misreaching for objects, and simultanagnosia, in which only one object can be attended to at a time. As Martin (1998, p. 228) noted, "A patient with Balint's syndrome might focus quite narrowly on the tip of a cigarette in his or her mouth and be unable to see a match offered a short distance away."

Convincing evidence that Balint's patients can only attend to one object at a time was reported by Humphreys and Riddoch (1993). When Balint's patients were presented with a mixture of red and green circles, they were generally unable to report seeing both colours. Presumably this happened because the patients could only attend to a single circle at a time. However, when the red and green circles were joined by lines (so that each object contained red and green), the patients' performance was much better.

Spotlight or zoom lens?

According to Pashler (1998, p. 4), "the findings with visual stimuli have closely paralleled those with auditory stimuli". This similarity is clear when we consider research on focused attention. In some ways, focused visual attention resembles a spotlight. Everything within a fairly small region of the visual field can be seen clearly, but it is much harder to see anything not falling within the beam of the attentional spotlight. Attention can be shifted by moving the spotlight, and the simplest assumption is that the attentional spotlight moves at a constant rate (see Yantis, 1998). A more complex view of focused visual attention was put forward by Eriksen and St. James (1986). According to their zoom-lens model, attention is directed to a given region of the visual field. However, the area of focal attention can be increased or decreased in line with task demands.

Posner (1980) favoured the spotlight notion. He argued that there can be covert attention, in which the attentional spotlight shifts to a different spatial location in the absence of an eye movement. In his studies, the participants responded as rapidly as possible when they detected the onset of a light. Shortly before the onset of the light, they were presented with a central cue (arrow pointing to the left or right) or a peripheral cue (brief illumination of a box outline). These cues were mostly valid (i.e., they indicated where the target light would appear), but sometimes they were invalid (i.e., they provided misleading information about the location of the target light).

Posner's (1980) key findings were that valid cues produced faster responding to light onset than did neutral cues (a central cross), whereas invalid cues produced slower responding than neutral cues. The findings were comparable for central and peripheral cues, and were obtained in the absence of eye movements. When the cues were valid on only a small fraction of trials, they were ignored when they were central cues but affected performance when they were peripheral cues. These findings led Posner (1980) to distinguish between two systems:

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