Attentional systems select information to process at serial bottle- necks where it is no longer possible to do things in parallel.
Auditory Attention
Some of the early research on attention was concerned with auditory attention. Much of this research centered on the dichotic listening task. In a typical di- chotic listening experiment, participants wear a set of headphones. They hear two messages at the same time, one in each ear, and are asked to “shadow” one of the two messages (i.e., repeat back the words from that message only). Most participants are able to attend to one message and tune out the other. Psychologists (e.g., Cherry, 1953; Moray, 1959) have discovered that very little information about the unattended message is processed in a dichotic listening task. All that participants can report about the unattended message is whether it was a human voice or a noise; whether the human voice was male or female; and whether the sex of the speaker changed during the test. They cannot tell what language was spoken or remember any of the words, even if the same word was repeated over and over again. An analogy is often made between performing this task and being at a party, where a guest tunes in to one message (a conversation) and filters out others. This is an example of goal- directed processing—the listener selects the message to be processed. However, to return to the distinction between goal-directed and stimulus-driven processing, important stimulus information can disrupt our goals. We have probably all experienced the situation in which we are listening intently to one person and hear our name mentioned by someone else. It is very hard in this situation to keep your attention on what the original speaker is saying.
■ Broadbent’s filter model proposes that we use physical features, such as ear or pitch, to select one message to process, but it has been shown that people can also use the meaning of the message as the basis for selection.
Visual Attention
The bottleneck in visual information processing is even more apparent than the one in auditory information processing. The retina varies in acuity, with the greatest acuity in a very small area called the fovea. Although the human eye registers a large part of the visual field, the fovea reg- isters only a small fraction of that field. Thus, in choosing where to focus our vision, we also choose to devote our most powerful visual processing resources to a particular part of the visual field, and we limit the resources allocated to processing other parts of the field. Usually, we are attending to that part of the visual field on which we are focusing. For instance, as we read, we move our eyes so that we are fixating the words we are attending to.
The focus of visual attention is not always identical with the part of the visual field being processed by the fovea, however. People can be instructed to fixate on one part of the visual field (making that part the focus of the fovea) while attend- ing to another, nonfoveal region of the visual field.2 In one experiment, Posner, Nissen, and Ogden (1978) had par- ticipants focus on a constant point and then presented them with a stimulus 7° to the left or the right of the fixation point. In some trials, participants were told on which side the stim- ulus was likely to occur; in other trials, there was no such warning. The warning was correct 80% of the time, but 20% of the time the stimulus appeared on the unexpected side. The researchers monitored eye movements and included only those trials in which the eyes had stayed on the fixation point. Figure 3.6 shows the time required to judge the stimu- lus if it appeared in the expected location (80% of the time), if the participant had not been given a neutral cue (50% of the time on both sides), and if it appeared in the unexpected location (20% of the time). Participants were faster when the stimulus appeared in the expected location and slower when it appeared in the unexpected location. Thus, they were able to shift their attention from where their eyes were fixated.
Posner, Snyder, and Davidson (1980) found that peo- ple can attend to regions of the visual field as far as 24° from the fovea. Although visual attention can be moved without accompanying eye movements, people usually do move their eyes, so that the fovea processes the portion of the visual field to which they are attending. Posner (1988) pointed out that successful control of eye movements requires us to attend to places outside the fovea. That is, we must attend to and identify an interesting nonfoveal region so that we can guide our eyes to fixate on that region to achieve the greatest acuity in processing it. Thus, a shift of attention often precedes the corresponding eye movement.
To process a complex visual scene, we must move our attention around in the visual field to track the visual information. This process is like shadowing a conversation. Neisser and Becklen (1975) performed the visual analog of the audi- tory shadowing task. They had participants observe two videotapes superimposed over each other. One was of two people playing a hand-slapping game; the other was of some people playing a basketball game. They were instructed to pay attention to one of the two films and to watch for odd events such as the two players in the hand-slapping game pausing and shaking hands. Participants were able to monitor one film successfully and reported filtering out the other. When asked to monitor both films for odd events, the participants experienced great difficulty and missed many of the critical events.
As Neisser and Becklen (1975) noted, this situ- ation involved an interesting combination of the use of physical cues and the use of content cues. Partici- pants moved their eyes and focused their attention in such a way that the critical aspects of the monitored event fell on their fovea and the center of their atten- tive spotlight. The only way they could know where to move their eyes to focus on a critical event was by making reference to the content of the event. Thus, the content of the event facilitated their processing of the film, which in turn facilitated extracting the content. Examples of the overlapping stimuli used in an experiment by O’Craven, Downing, and Kanwisher (1999) to study the neural conse- quences of attending to one object or the other. Partici- pants in their experiment saw a series of pictures that consisted of faces superimposed on houses. They were instructed to look for either repetition of the same face in the series or repetition of the same house. Recall from Chapter 2 that there is a region of the temporal cortex, the fusiform face area, which becomes active when people are observing faces. There is another area within the temporal cor- tex, the parahippocampal place area, that becomes more active when people are observing places. What is special about these pictures is that they consisted of both faces and places. Which region would become active—the fusiform face area or the parahippocampal place area? As the reader might suspect, the answer de- pended on what the participant was attending to. When participants were looking for repetition of faces, the fusiform face area became more active; when they were looking for repetition of places, the parahippocampal place area became more active. Attention determined which region of the temporal cortex was engaged in the processing of the stimulus.
■ People can focus their attention on parts of the visual field and move their focus of attention to process what they are interested in.
■ When people attend to a particular spatial location, there is greater neural processing in portions of the visual cortex correspond- ing to that location.
■It is necessary to search through a visual array for an object only when a unique visual feature does not distinguish that object.
Object-Based Attention
So far we have talked about space-based attention, where people allocate their attention to a region of space. There is also evidence, for object-based attention, where people focus their attention on particular objects rather than regions of space. An experiment by Behrmann, Zemel, and Mozer
(1998) is an example of research demonstrating that people sometimes find it easier to attend to an object than to a location. some of the stimuli used in the experiment, in which participants were asked to judge whether the numbers of bumps on the two ends of objects were the same. The left column shows instances in which the numbers of bumps were the same, the right column instances in which the numbers were not the same. Participants made these judgments faster when the bumps were on the same ob- ject than when they were on different objects (middle row). This result occurred despite the fact that when the bumps were on different objects, they were located closer together, which should have facilitated judgment if attention were space based. Behrmann et al. argue that participants shifted attention to one object at a time rather than one location at a time. There- fore, judgments were faster when the bumps were all on the same object because participants did not need to shift their attention between objects. Using a variant of the paradigm. Chen and Cave (2008) either presented the stimu- lus for 1 s or for just 0.12 s. The advantage of the within-object effect disappeared when the stimulus was present for only the brief period. This indicates that it takes time for object-based attention to develop.
Other evidence for object-centered attention involves a phenomenon called inhibition of return. Research indicates that if we have looked at a par- ticular region of space, we find it a little harder to return our attention to that region. If we move our eyes to location A and then to location B, we are slower to return our eyes to location A than to some new location C. This is also true when we move our attention without moving our eyes (Posner, Rafal, Chaote, & Vaughn, 1985). This phenomenon confers an advantage in some situations: If we are searching for something and have already looked at a location, we would prefer our visual system to find other locations to look at rather than return to an already searched location.
Tipper, Driver, and Weaver (1991) performed one demonstration of the inhibition of return that also provided evidence for object-based attention. In their experiments, participants viewed three squares in a frame, similar to what is shown in each part. In one condition, the squares did not move (unlike the moving condition. The participants’ attention was drawn to one of the outer squares when the experimenters made it flicker, and then, 200 ms later, attention was drawn back to the center square when that square flickered. A probe stimulus was then presented in one of the two outer positions, and par- ticipants were instructed to press a key indicating that they had seen the probe. On average, they took 420 ms to see the probe when it occurred at the outer square that had not flickered and 460 ms when it occurred at the outer square that had flickered. This 40-ms advantage is an example of a spatially defined in- hibition of return. People are slower to move their attention to a location where it has already been.
■ Visual attention can be directed either toward objects independent of their location or toward locations independent of what objects are present.
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