Elderly adults do not perform as well as young adults on complex tasks. Elderly adults' poorer performance may be partly due to an age-related increase in the occurrence of illusory conjunctions. To investigate this possibility, this research is designed to examine the relationship between attention and illusory conjunctions in young and old adult performance. Experiment 1 is modeled after Cohen and Ivry (1989; Experiment 3) and requires participants to perform concurrent digit-matching and letter identification tasks. The digit-matching task manipulates the spread of attention, i.e., narrow vs. wide; and the letter identification task provides opportunity for illusory conjunctions, because both a target and non-target letter differing in color and identity appear in the display. The results suggest that selective attention affects the formation of illusory conjunctions in young but not elderly adults.
In young adults illusory conjunctions are more likely to be formed within the attentional window. The elderly are just as likely to form illusory conjunctions inside and outside the attentional window. Because the design of Experiment 1 requires the participants to identify two properties of the target letter simultaneously (i.e., the subject must determine the color and the shape of the target letter) this experiment is a dual property experiment. Since elderly performance often suffers when required to complete simultaneous tasks (Craik, 1977; Hartley, 1992; McDowd & Shaw, 2000), it is possible that an agedifference in the occurrence of illusory conjunctions in Experiment 1 was due to age differences in ability to handle dual task performance. Experiment 2 was used to investigate this possibility. Thus, Experiment 2 consisted of two conditions. In the dual property condition, the participants were required to determine both the color and the identity of the target letter. In the single property condition, the subject only reported the color of the target letter. Experiment 2 demonstrated that the results of Experiment 1 were not due to the dual property nature of Experiment 1. The pattern of illusory conjunctions was similar whether the requirements of the task were to identify one or two properties of the target letter.
Attentional Theory and Illusory Conjunction
There has been considerable research on the role of attention in the production of illusory conjunctions in young adults. This research has been prompted by twostage theories of object perception. These theories have driven empirical research for over two decades (Feature Integration Theory; Guided Search; Treisman & Gelade, 1980; Treisman & Gormican, 1988; Treisman & Souther, 1985; Wolfe, 1994, 1998; Wolfe, Cave, & Franzel, 1989; Wolfe, Klempen, & Dahlen, 2000). According to these theories perceptual processing occurs in two stages: (1) an early feature detection stage, and (2) a late feature combination stage. The early theory, Feature Integration Theory, (Treisman, Sykes, & Gelade, 1977; Treisman & Gelade, 1980; see Quinlan, 2003 for a review of Feature Integration Theory) posits that in the first (non-attentional stage) features are free-floating. Only in the second attentional stage are features combined into objects. An erroneous combination of features into objects presumably results when free-floating features are inadvertently combined.
More recent research has demonstrated that a number of predictions concerning illusory conjunctions of the earlier two-stage theories do not always hold true because a free floating features model assumes that the features that are erroneously combined are combined at random. Evidence suggests that illusory conjunctions do not occur randomly -- as described by Treisman -- but as the result of guesses based on the location of the search object (Cohen & Ivry, 1989; Ashby, Prinzmetal, Ivry, & Maddox, 1996; Hazeltine, Prinzmetal, & Elliot 1997). That is, the search is perceptually guided. For example, suppose a subject searched for a target (a red "X") in a display consisting of two non-targets (a green "X" and a red "O"), and all the features were perceived, but not in the correct location. Then the perceptual system, in an effort to determine how the features combine, might use a simple decision rule. It might combine the features located closest to each other (for example, combine the shape "X" with the color red to erroneously perceive the illusory conjunction, a red "X"), and report that the target was present when it was not. Multinomial modeling was used to compare models that describe the occurrence of illusory conjunctions.
There is also some evidence that suggests that illusory conjunctions are observed regardless of whether attention is diverted or not (Treisman & Schmidt, 1982). Prinzmetal et al. (Prinzmetal, Henderson, & Ivry, 1995) employed an attentiondiverting task and found that it did not affect incidence of illusory conjunctions. Attention was diverted by visually presenting digits at fixation and requiring the participant to respond every time a A0@ appeared. Thus attention did not seem to be an essential component in producing illusory conjunctions.
Aging and Attention
There is considerable research in the aging literature devoted to the effects of attention on elderly adult performance (McDowd & Shaw, 2000; Hartley, 1992; Salthouse, 1991). Attention is a multidimensional construct. In this dissertation, the focus is on two functionally different dimensions of attention: selectivity and divided attention (see McDowd & Shaw, 2000 for a functional analysis of attention). Selective attention is the ability to select some stimuli and ignore other stimuli. Divided attention is the ability to divide attention between two or more tasks. In Experiment 1 of this dissertation the effects of selective attention on aging and illusory conjunctions is examined. In Experiment 2 the effects of divided attention on aging and illusory conjunctions is examined.
Selective Attention and Aging
Selective attention is the ability to attend to some stimuli and ignore other stimuli. Most selective attention experiments require participants to do a task that requires attending to a target and ignoring distractors. Rabbitt in 1965 was the first to do this type of experiment with the elderly. Rabbitt found that in a search of a target among a field of a varying number of distractors that there was an age difference in performance, and the age difference increased as the number of distractors increased. That is, there is a greater cost to elderly performance the greater the selective attention demands of an experiment. A review of the selective attention and aging literature goes beyond the scope of this paper (see McDowd & Shaw, 2000; Hartley, 1992; Salthouse, 1991). The following section, on aging and illusory conjunctions, includes a sample of selective attention experiments that compare age differences and that suggest that performance may be influenced by the formation of illusory conjunctions.
In conclusion, although elderly adults= performance is often compromised in visual search tasks that require selective attention, the mechanism behind this phenomenon is not known. However, elderly performance is often affected when a conjunction search is required and illusory conjunctions are possible in conjunction searches. A number of different lines of research suggest that it is possible that illusory conjunctions may be partly responsible for this phenomenon. In Experiment 1 of this dissertation the effects of varying selective attention on the formation of illusory conjunctions in the young and older adults is examined. The attentional window is adjusted and the incidence of illusory conjunction formation inside and outside the attentional window is measured. If the attentional window is successfully adjusted then display items inside the attentional window are selected and display items that fall outside the attentional window are ignored.
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Dissertation Done by Lisa Jean Murphy, University of Maryland