Mean reaction time to the probe as a function of probe position. The probe was presented at the time that a letter string would have been presented. Data from LaBerge (1983).

1. An endogenous system, which is controlled by the participant's intentions and is involved when central cues are presented.

2. An exogenous system, which automatically shifts attention and is involved when peripheral cues are presented.

Some evidence does not support the spotlight notion. Kwak, Dagenbach, and Egeth (1991) presented their participants with two letters at a time, and asked them to decide whether they were the same. The decision times were the same whether the letters were close together or far apart. This is inconsistent with the notion that visual attention is like a spotlight moving at a given rate.

Evidence in favour of the zoom-lens model was reported by LaBerge (1983). Five-letter words were presented. A probe requiring rapid response was occasionally presented instead of, or immediately after, the word. The probe could appear in the spatial position of any of the five letters of the word. In one condition, an attempt was made to focus the participants' attention on the middle letter of the five-letter word by asking them to categorise that letter. In another condition, the participants were required to categorise the entire word. It was expected that this would lead the participants to adopt a broader attentional beam.

The findings on speed of detection of the probe are shown in Figure 5.4. LaBerge (1983) assumed that the probe was responded to faster when it fell within the central attentional beam than when it did not. On this assumption, the attentional spotlight can have either a very narrow (letter task) or rather broad (word task) beam.

Eriksen and St. James (1986) also obtained support for the zoom-lens model. Their participants performed a task on a target stimulus whose location was indicated beforehand. Performance was impaired by the presence of distracting visual stimuli. However, the area over which interference effects were found was less when the participants had longer forewarning of the target stimulus. Presumably visual attention zoomed in more precisely on the area around the target stimulus over time. Evaluation

As the zoom-lens model predicts, the size of the visual field within focal attention can vary substantially. However, focused visual attention is more complex than is implied by the model. For example, consider a study by Juola, Bowhuis, Cooper, and Warner (1991). A target letter (L or R) which had to be identified was presented in one of three rings having the same centre: an inner, a middle, and an outer ring. The participants fixated the centre of the display, and were given a cue that mostly provided accurate information as to the ring in which the target would be presented. If visual attention is like a spotlight or zoom lens, speed and accuracy of performance would be greatest for targets presented in the inner ring. In fact, performance was best when the target appeared in the ring that had been cued. This suggests that visual attention can be allocated in an O-shaped pattern to include only the outer or the middle ring.

There is a more fundamental objection to the spotlight and zoom-lens models. It is assumed within both models that visual attention is directed towards a given region in the visual field. However, visual attention is often directed to objects rather than to a particular region. Consider, for example, a study by Neisser and Becklen (1975). They superimposed two moving scenes on top of each other. Their participants could easily attend to one scene while ignoring the other. These findings suggest that objects within the visual environment can be the main focus of attention.

According to the spotlight approach, it might be expected that visual attention in patients with neglect and extinction would be limited only in area. However, this is not so. Marshall and Halligan (1994) presented a patient with neglect in the left visual field with ambiguous displays, each of which could be seen as a black shape against a white background or as a white shape on a black background. There was a jagged edge dividing the two shapes at the centre of each display. The patient was able to copy this jagged edge when asked to draw the shape on the left side of the display, but could not copy exactly the same edge when asked to draw the shape on the right side. Thus, the patient attended to objects rather than simply to a region of visual space.

Ward, Goodrich, and Driver (1994) studied two patients with extinction in the left visual field. Two stimuli were presented at once, and they either formed a good perceptual group (e.g., "[and]") or they did not (e.g., "[and o"). The patients were much better at detecting the stimuli on the left side of the visual field when they belonged to a good perceptual group. Thus, visual attention in extinction patients is affected by grouping factors as well as by location.

What conclusion can we draw from studies such as those of Marshall and Halligan (1994) and Ward et al. (1994)? According to Driver (1998, p. 315), "The spatial extent of both normal and pathological [abnormal or diseased] attention is substantially modulated by grouping processes. Clearly, human covert attention is rather more sophisticated than a simple 'spotlight' metaphor implies."

Unattended visual stimuli

There is generally rather limited processing of unattended auditory stimuli. What happens to unattended visual stimuli? Neurophysiological evidence suggests there is reduced processing of such stimuli. Luck (1998) discussed several studies in which the participants fixated a central point while attending to the left or the right visual field. A rapid succession of bars was presented to both fields, and the task involved detecting targets (smaller bars) in the attended visual field. Event-related potentials (ERPs; see Chapter 1) are larger to attended than to unattended stimuli. The ERPs to the two types of stimuli begin to differ with the first positive wave (P1), which starts about 75 milliseconds after stimulus onset.

Heinze et al. (1994) used a similar procedure to the one just described, and obtained PET scans as well as ERPs. They replicated the greater P1 to attended than to unattended visual stimuli. However, according to Luck (1998, p. 274), their key finding was that, "visual attention influences sensory processing in extrastriate visual cortex within 100 ms of stimulus onset, consistent with early-selection models of attention."

Evidence suggesting that there is very little processing of unattended visual stimuli was reported by Francolini and Egeth (1980). Circular arrays of red and black letters or numerals were presented, and the task was to count the number of red items and to ignore the black items. Performance speed was reduced when the red items consisted of numerals conflicting with the answer, but there was no interference effect from the black items. These findings suggest there was little or no processing of the to-be-ignored black items.

The findings of Driver and Tipper (1989) contradicted this conclusion. They used the same task as Francolini and Egeth (1980), but focused on whether conflicting numerical values had been presented on the previous trial. There was an interference effect, and it was of the same size from red and black items. The finding that performance on any given trials was affected by the numerical values of to-be-ignored items from the previous trial means those items must have been processed. This is the phenomenon of negative priming. In this phenomenon, the processing of a target stimulus is inhibited if that stimulus or one very similar to it was an unattended or distracting stimulus on the previous trial.

Further evidence that there is often more processing of unattended visual stimuli than initially seems to be the case has been reported with neglect patients. McGlinchey-Berroth et al. (1993) asked such patients to decide which of two drawings matched a drawing presented immediately beforehand to the left or the right visual field. Neglect patients performed well when the initial drawing was presented to the right visual field, but at chance level when it was presented to the left visual field (see Figure 5.5). The latter finding suggests that stimuli in the left visual field were not processed. However, a very different conclusion emerged from a second study, in which neglect patients had to decide whether letter strings formed words. Decision times were faster on "yes" trials when the letter string was preceded by a semantically related object rather than an unrelated object. This effect was the same size regardless of whether the object was presented to the left or the right visual field (see Figure 5.5), indicating that there is some processing of left-field stimuli by neglect patients.

Section summary

Neurophysiological evidence suggests there is reduced processing of unattended visual stimuli. The fact that processing of, and responding to, attended visual stimuli is often unaffected by unattended stimuli suggests there is very little processing of such stimuli. However, when sensitive measures are used, there is strong evidence for some processing of the meaning of unattended stimuli by normals and by neglect patients. For example, normals exhibit a phenomenon known as negative priming.

Visual search

One of the main ways we use focused visual attention in our everyday lives is in visual search (see Chapter 3). For example, we search through the books in a library looking for the one we want, or we look for a friend in a crowded room. An attempt to study the processes involved has been made by using visual search tasks. The participants are presented with a visual display containing a variable number of items (the set or display size). A target (e.g., red G) is presented on half the trials, and the task is to decide as rapidly as possible whether the target is present in the display. Theory and research on this task are discussed next.

Feature integration theory

Effects of prior presentation of a drawing to the left or right visual field on matching performance and lexical decision in neglect patients. Data from McGlinchey-Berroth et al. (1993).

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