Copyright 1992 by the American Psychological Association, Inc. 0882-7974/92/J3.00

Psychology and Aging 1992, Vol. 7. No. 3, 453-465

Aging and Shifts of Visual Spatial Attention William I Hoyer Syracuse University

Charles L. Folk

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Villanova University

Three experiments examined adult age differences in the efficiency of endogenous (voluntary) and exogenous (involuntary) attention shifts. Younger and older subjects performed a spatial cuing task in which abruptly onset peripheral cues (Experiment 1) or central, symbolic cues (Experiments 2 and 3) were presented before a target stimulus at intervals ranging from 50 to 250 ms. With peripheral cues, the magnitude of cuing effects was at least as great for older as for younger adults and followed a similar time course. Similar results were obtained with symbolic cues, although cuing effects for older adults varied with cue difficulty. The results suggest that cue encoding may decline with advancing age but that the efficiency of the shift process is preserved.

The performance of many types of cognitive tasks requires the selective allocation of a limited supply of attentional resources. In the case of visual search and target identification, the selective allocation of attention is often assumed to be spatial in nature, involving "shifts" in the distribution of resources across display locations and elements (Bergen & Julesz, 1983; Eriksen & Hoffman, 1972; Posner, 1980; Shulman, Remington, & McLean, 1979; Treisman & Gelade, 1980). An important issue with regard to shifting the locus of spatial attention is the extent to which the efficiency of this process varies with increasing age. It is well established that there are age-related declines in visual search and target identification performance (e.g., see Hartley, in press; Madden, 1983, in press; Madden & Plude, in press; Owsley, Ball, Sloane, Roenker, & Bruni, 1991; Plude & Doussard-Roosevelt, 1989; Plude & Hoyer, 1981; 1986; Plude, Hoyer, & Lazar, 1982; Rabbitt, 1965; Scialfa & Kline, 1988; Scialfa, Kline, & Lyman, 1987). This age decrement may be due to a decline in the efficiency or speed with which attention is shifted to spatial locations in the display (Plude, Cerella, & Poon, 1982). That older adults produce steeper slopes for response time and display size functions than younger adults in visual search is consistent with this hypothesis, although other explanations are possible (Plude & Doussard-Roosevelt, 1989). Standard visual search tasks, however, do not provide a direct measure of the occurrence and efficiency of shifts in spatial attention. An experimental paradigm that has been used to directly assess the nature of shifts of attention is the spatial cuing task. In the typical spatial cuing experiment, subjects are provided with advance information (i.e., a cue) regarding the This research was supported in part by funds from National Institute of Mental Health Grant 1 R03 MH 45008 to Charles L. Folk and in part by National Institute on Aging Grant AG 06041 to William J. Hoyer. Portions of this research were presented at the annual meeting of the Psychonomic Society, November 1990. We thank Alan Hartley, David Madden, and Dana Plude for helpful comments and suggestions concerning this work. Correspondence concerning this article should be addressed to Charles L. Folk, Department of Psychology, Villanova University, Villanova, Pennsylvania 19085. 453

spatial location of a subsequent target stimulus. The predictive validity of the cue is varied such that on some trials the cue indicates the target location (valid cue), and on other trials the cue indicates a nontarget location (invalid cue). Benefits and costs in performance associated with valid and invalid cues, respectively, are assumed to index the occurrence of shifts in spatial attention. Variation in the magnitude of benefits and costs as a function of the stimulus onset asynchrony (SOA) between cue and target are assumed to index the time course (speed) of attentional shifts (Muller & Findlay, 1987; Muller & Rabbitt, 1989; Posner, 1980; Remington, Johnston, & Yantis, 1992; Remington & Pierce, 1980; Shulman et al, 1979). Spatial cuing experiments with young adults have yielded evidence for two distinct mechanisms underlying shifts in spatial attention (Jonides, 1980; Mttller & Humphreys, 1991; Muller & Rabbitt, 1989). Attention shifts have been shown to result from voluntary action in response to centrally presented, symbolic spatial cues (e.g., arrows) or from an involuntary or "automatic" action elicited by peripherally presented, transient spatial cues (e.g., flashes of light). These two modes of attention allocation have been referred to as endogenous and exogenous control, respectively (Briand & Klein, 1987; Posner, 1980). Relative to endogenous attention shifts, exogenous shifts have been shown to develop more rapidly, yield larger cuing effects, and remain resistant to variations in concurrent processing load (Jonides, 1980; Muller & Rabbitt, 1989). How might age affect the efficiency of endogenous and exogenous shifts of attention? A number of researchers have suggested that, in general, age-related decrements in cognitive performance are limited to those tasks that require controlled or effortful processing; automatic tasks are left relatively unaffected (Craik & Byrd, 1982; Hasher & Zacks, 1979; Hoyer & Plude, 1980,1982; Plude et al., 1983; Rabbitt, 1979; Rabbitt & Vyas, 1980). Thus, if exogenous attention allocation proceeds automatically and endogenous allocation requires voluntary or controlled processing, then one might expect the efficiency of exogenous shifts of attention to be preserved and endogenous shifts to show a decline with increasing age. There have been only a handful of published studies that have directly addressed age-related effects in the time course of

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CHARLES L. FOLK AND WILLIAM J. HOYER

attention allocation using the spatial cuing paradigm (Hartley, Kieley, & Slabach, 1990; Hoyer & Familant, 1987; Madden, 1990; Nissen & Corkin, 1985). Studies using central, symbolic cues requiring the voluntary (endogenous) allocation of attention have yielded mixed results. Nissen and Corkin (1985) found that although overall response time to identify a target was greater for older than for younger adults, the magnitude of cuing effects for the older group was just as large as for the younger group. However, because the cue-target SOAs were long and coarsely sampled (i.e., 2 s and 3 s), this study is not analytic of age-related time course effects. Hoyer and Familant (1987, Experiment 2) also found that at long SOAs (i.e., longer than 750 ms) older adults were just as efficient as younger adults at using a central, symbolic cue to improve target identification performance. At SOAs shorter than 750 ms, however, only younger adults showed benefits in performance associated with a symbolic cue. These results suggested an age deficit in the efficiency of endogenous attention shifts. In contrast, Hartley et al. (1990, Experiment 2) found that age had little or no effect on the magnitude and time course of costs and benefits generated by centrally presented symbolic cuing arrows, even at SOAs shorter than 500 ms. Experiments using peripheral cues have also yielded mixed results. Madden (1990) presented cues consisting of the peripheral, abrupt onset of a cathode-ray tube (CRT) screen cursor that always appeared in the position of the subsequent target. Older subjects showed smaller cuing effects than younger subjects when noise letters were present in the display. Moreover, the development of costs and benefits proceeded more slowly for older adults compared with younger adults, suggesting an age-related decline in the efficiency of exogenous attention allocation. These results, however, are difficult to interpret because no control condition was included (e.g., an "invalid" cue condition providing invalid advance information concerning the location of the target) to ascertain that any benefits in performance with SOA were due to the spatial information provided by the cue and not, for example, to general alerting effects (Posner, 1980). Moreover, the cues in the experiment were 100% valid; they indicated the correct location of the target on every trial. Thus, it is difficult to tell whether the obtained cuing effects were due to automatic shifts of attention or to voluntary shifts. Hartley et al. (1990, Experiment 3) cued younger and older subjects with an abruptly onset bar marker that was valid with respect to target location on 75% of the trials and invalid on 25% of the trials. Four cue-target SOAs were used ranging from 100-400 ms. The authors found no age differences in the magnitude or time course of costs and benefits, suggesting that the efficiency of exogenous shifts of attention are indeed preserved with increasing age. As the authors pointed out, however, the use of only four SOAs varying by 100 ms may have been insufficient for detecting age-related differences in the time course of attention shifts. More important, recent studies with younger adults have shown that voluntary or endogenous attention control can modulate or supersede exogenous shifts of attention (Muller & Humphreys, 1991; Warner, Juola, & Koshino, 1990; Yantis & Jonides, 1990). Thus, given that the cue had a high validity, it is possible that the cuing effects demonstrated by Hartley et al. (1990, Experiment 3) reflected endogenous shifts

of attention to the peripheral bar marker or some combination of endogenous and exogenous processes. Although there is a theoretical basis for expecting the selective preservation of exogenous attention shifts with increasing age, the existing literature on the effects of aging on endogenous and exogenous control of spatial attention does not provide clear, unambiguous evidence either way. Experiment 1 was conducted to provide a direct and sensitive test of whether exogenous (i.e., involuntary) shifts of visual attention are preserved in older adults.

Experiment 1 Young and elderly adults participated in a variation of the standard spatial cuing task adapted from a procedure reported by Remington et al. (1992). A peripheral cue consisting of the abrupt onset of four, highly salient, bright circles was flashed briefly around one of four boxes in which a target character could subsequently appear. To provide a sensitive measure of the time course of attention shifts, cuing effects were measured at five cue-target SOAs, ranging from 50-250 ms. The cue was either valid, invalid, or neutral (i.e., no cue was presented) with respect to predicting the target location. Shifts in attention, and their time course, were inferred from "benefits" and "costs" in response time (relative to the no cue control) for the valid and invalid conditions, respectively, at each SOA. To ensure that any shifts of attention were truly exogenous the predictive validity of the cue was varied across blocks of trials (i.e., the cue was either 100% valid or 100% invalid with respect to the exact target location). Thus, the critical measure of involuntary shifts of attention is performance in the 100% invalid cue condition. Because, in this condition, subjects were fully aware that the cue location did not predict the target location, it was assumed that subjects would withhold any shifts of attention to the cue if possible. Thus, any observed costs, relative to the neutral condition, were assumed to reflect the inability of subjects to withhold an attentional shift to the location of the cue. In other words, significant costs in this condition are a strong measure of involuntary attention shifts (Remington et al., 1992).1 It should be noted that blocking cue validity compromises the interpretation of any benefits obtained in the 100% valid condition. Because subjects are fully aware that the target will appear in the cued location, any benefits obtained may result from voluntary, endogenous allocation. Nonetheless, this condition provides a necessary control for the interpretation of the 100% invalid condition. A peripheral cue may produce costs that are unrelated to the spatial attention effects, such as a more generalized disruption of "central" processing. Thus, to estab1

It has been argued that in the spatial cuing paradigm, blocking cue validity can lead to strategic differences in the treatment of precues, which can confound the interpretation of costs and benefits (Eriksen & Yeh, 1985; Jonides & Mack, 1984). In the present experiments, however, inducing strategy differences across conditions was an explicit goal. Specifically, the 100% invalid condition was used ta induce a strategy of completely ignoring invalid cues, allowing a strong test of whether any costs associated with such cues represent truly involuntary (exogenous) shifts of attention.

AGING AND VISUAL ATTENTION

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lish that any costs produced by an invalid cue are due to shifts in spatial attention, it is necessary to show that a valid cue, which varies from an invalid cue only with respect to its spatial relationship to the target, does not produce a similar cost (Remington et al., 1992). If the occurrence and efficiency of exogenous, involuntary shifts of attention are preserved with increasing age, then both young and old subjects should show significant costs associated with a 100% invalid cue, accompanied by a lack of costs (or even benefits) with a valid cue. Furthermore, these cuing effects should develop at least as early (in terms of cue-target SOA) for older adults as for younger adults. Method Subjects. Eighteen young adults (14 men and 4 women) between the ages of 18 and 21 years (M = 18.7, SD = 0.8) and 18 elderly adults (7 men and 11 women) between the ages of 64 and 82 years (M = 71.2, SD = 4.9) participated in this experiment. Five subjects (4 elderly and 1 young) were replaced because their error proportions in the no cue control condition were greater than 15%. The young adults were recruited from the subject pool at the Syracuse University Psychology Department. All participated to partially fulfill a course requirement. All had normal or corrected-to-normal visual acuity of 20/30 or better (M = 20/22.5) as measured by a Rosenbaum pocket vision screener at a distance of 14 in. Mean years of education for young adults was 13.2 (SD =1.2), and the mean raw score on the Wechsler Adult Intelligence Scale (WAIS) Digit Symbol test was 71.6 (SD = 12.2). All young subjects reported themselves to be in good or excellent health. The elderly adults were community-dwelling volunteers, recruited from the membership of a senior center, and they were paid a modest honorarium for completing the testing session. All had normal or corrected-to-normal visual acuity of 20/30 or better (M = 20/28). Mean years of education was 12.44 (SD = 1.2), and mean raw score on the WAIS Digit Symbol test was 44.9 (SD = 6.8). All elderly subjects reported themselves to be in good or excellent health. Apparatus. Stimuli were presented on a Princeton Graphics SR-12 monitor controlled by a Zenith Data Systems microcomputer equipped with a Sigma Designs, Color 400 EGA high-resolution graphics board. Display presentation was synchronized with the 16.7 ms refresh rate of the monitor. All reported time intervals are integer multiples of this refresh rate. Stimuli. Displays were of three basic types: fixation display, cue display, and target display. Examples of each of these display types for the invalid cue condition are shown in row A of Figure 1. The fixation display consisted of five square boxes measuring 1.15° (visual angle) on a side from a viewing distance of approximately 40 cm. Four boxes were located at the vertices of an imaginary diamond centered on the fifth box with a center-to-vertex distance of 4.7°. The boxes were light gray (IBM color designation 8) against the black CRT screen. Cue displays consisted of the fixation display with the addition of four small hollow circles, each subtending .36° (visual angle) around one of the five boxes. The circles were placed such that each was centered approximately .3° (visual angle) peripheral to its respective side of the box. The circles were high-contrast white (IBM color designation 15) against the black CRT screen. Target displays consisted of the fixation display with a single "X" or an "=" centered in one of the four outer boxes. This target character was high-contrast white (IBM color designation 15) and subtended .57° visual angle. Design. Three within-subjects cue conditions were created by varying the spatial relationship between the cue and the target. In the valid

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cue condition, the target character always appeared in the box marked by the cue. In the invalid cue condition, the target always appeared in one of the three boxes not marked by the cue (see Figure 1). In the no cue condition, no cue was presented around any of the boxes, that is, the fixation display was substituted for the cue display. This condition served as the control for the invalid and valid cue conditions. The three cue conditions were crossed with five different cue-target SOAs measured from cue onset: 50,100,150, 200, and 250 ms. Each of the three cue conditions was presented to each subject in separate blocks of trials. Therefore, in the invalid cue condition the box indicated by the cue was 100% invalid with respect to the subsequent target location whereas in the valid cue condition it was 100% valid. As such, subjects were fully aware, prior to each trial, of the nature of the spatial relationship between the cue and target. Each cue condition consisted of one block of 120 trials. Subjects were allowed to rest halfway through each condition. Condition order was varied across subjects, and subjects were randomly assigned to orders. Within each block, each of the two possible targets (X or =) appeared equally often in each of the four possible locations at each of the five SOAs. Cue positions in the invalid cue condition were chosen with the additional constraint of appearing equally often in each of the three possible nontarget locations for each possible target location and SOA. Fifteen practice trials were presented at the beginning of each condition. In an effort to reduce variability in the data, a "buffer" trial, chosen randomly from the set of possible trial parameters for that condition, was inserted after any error trial. Response times to practice and buffer trials were not included in the data analysis. Procedure. Each subject participated in a single session that lasted about 1 hr. Subjects were tested in a dimly lit room and were seated such that viewing distance to the CRT screen was approximately 40 cm. They were given both written and oral descriptions of the stimuli and task, including demonstrations on the computer of the types of stimuli they would encounter. They were fully informed of the blocked structure of the experiment and were encouraged to take advantage of their knowledge of the relationship between cue and target positions. For example, subjects were explicitly told that the best strategy in the 100% invalid cue condition was to "ignore the cue if possible." Subjects were instructed to respond "as quickly as you can while making as few errors as possible." Maintaining fixation on the center box in the display was heavily stressed. Subjects were told that because the trial events would be occurring very rapidly, attempting to make eye movements would ultimately impair performance. When the experimenter was satisfied that subjects understood the task and nature of the different conditions, the subject was asked to initiate the experiment when ready. Each block began with the presentation of a screen indicating which of the three cue conditions (invalid, valid, or no cue) was about to begin. The subject then pressed a key to begin the sequence of trials in that condition. A message appeared at the end of each block instructing the subject to rest before beginning the next block. The subject then pressed a key when he or she was ready to continue. The sequence of events on a given trial was as follows. First, the fixation display appeared for 500 ms. The center box then blinked off for 100 ms, which served as a warning signal to subjects that the trial was beginning. The fixation display then remained on for a foreperiod interval randomly chosen from 1000, 1100, 1200, 1300, or 1400 ms. Following the foreperiod interval, the cue appeared for 50 ms (except in the no cue condition, in which the fixation display remained on during the 50-ms cue interval). After a variable interstimulus interval (ISI) ranging from 0-200 ms (which yielded the five SOAs reported above), the target display appeared for 100 ms. Phenomenally, the five display boxes appeared to remain on the screen throughout each trial

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456

CHARLES L. FOLK AND WILLIAM J. HOYER FIXATION DISPLAY

CUE DISPLAY

FIXATION DISPLAY

TARGET DISPLAY

1000-1400 mt

50ms

0-200 ms

100ms

Figure 1. Examples of displays and sequence of events for an invalid trial in Experiments 1 (A), 2 (B) and 3 (C). (These figures are representations intended to clarify the experimental design; they are not necessarily to scale. See text for actual measurements.)

(except for the warning blink of the center box). It is important to note that the total SOA between cue onset and target onset was no more than 250 ms, which is close to the lower limits for the time required by young adults to initiate a saccade in similar tasks (Colgate, Hoffman, & Eriksen, 1973). Thus, although eye position was not explicitly monitored, eye movements to the cue or other locations were unlikely, allowing any effect of cue validity to be attributed to attentional, not acuity, factors. Subjects made a speeded, forced-choice response to the target display by pressing one of two labeled keys on the keyboard. If the target was an =, subjects were instructed to press the left key with their left index finger. If the target was an X, subjects were instructed to press the right key with their right index finger. A 500 ms, 1000 Hz tone was sounded by the computer if subjects made an incorrect response. If a response was not made within 1500 ms, an error was scored and the trial sequence continued. Following the subject's response, an intertrial interval of 500 ms elapsed before the center box blinked to indicate initiation of the next trial. Response time and error status for each trial were measured and recorded by the computer. Response time was measured from the on-

set of the target display until a response was made. Anticipatory responses, defined as response times less than 200 ms, were excluded from the analysis. These responses occurred on 3.5% and 2.9% of the trials for younger and older adults, respectively.

Results Response times. Mean correct response times for each cue condition at each SOA pooled across target identity and condition order are shown for young and old subjects in Figure 2. The means were entered into a mixed analysis of variance (ANOV\) with age group and condition order as between-subjects factors and cue condition and SOA as within-subjects factors. The main effect of condition order was not significant nor did it interact with any of the other variables. All other main effects were significant. Older adults were 160 ms slower, on average, than younger adults, F(l,30) = 63.67(A«t=54,501))p CO

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o CO

Young 400

300

50

100

150

200

250

Stimulus Onset Asynchrony (ms) Figure 4. Average mean response times for young and old subjects at each combination of cue condition and stimulus onset asynchrony in Experiment 3.

tion order are shown for young and old subjects in Figure 4. A mixed ANOVA was conducted on the mean response times with age group and condition order as between-subjects factors and cue condition and SOA as within-subjects factors. The main effect of condition order was not significant nor did it enter into interactions with any other variables. All other main effects were significant: for age, F(l, 28) = 9.67 (MS, = 161, 278), p < .005; for SOA, F(4,112) = 8.78 (MS, = 979), p < .0001; for cue condition, F(2, 56) = 52.15 (M$ = 5, 218), p < .0001. For both age groups, the effect of cue condition varied as a function of SOA as indicated by a significant SOA X Cue Condition interaction, F(8, 224) = 8.42 (MS. = 1, 320), p < .0001. The nature of these effects were not influenced by age, however, as the three-way interaction between age, cue condition, and SOA did not reach significance, F(8, 224) = 1.35 (MSt = 1, 320), p > .20. Simple effects analyses of the Cue X SOA interaction revealed a significant effect of cue condition at each level of SOA (p < .0001 in all cases). The existence of costs and benefits at each SOA was assessed using the Dunnett test. The results are presented in Table 3. Invalid cues produced significant costs at all SOAs and valid cues produced significant benefits at 200-ms and 250-ms SOAs. Cue condition also interacted with age, F(2, 56) = 52.15 (MSe = 5, 218), p < .0001. Specifically, older adults showed a larger cuing effect (529, 539, and 626 ms for valid, no cue, and invalid conditions, respectively) than young adults (433, 435, and 484 ms, respectively). Error proportions. Overall error rate averaged 3.9%. Mean error proportions by condition for each age group are shown in Table 2. A mixed ANOVA with age group as the between-sub-

Discussion These results suggested that, given simple symbolic cues such as those used in the present experiment, older adults are just as efficient as younger adults at voluntarily shifting attention to the cued location. As is evident in Figure 4, both age groups showed differential effects of valid and invalid cues with increasing SOAs, a fact confirmed by the interaction between cue condition and SOA. Post-hoc analyses of this interaction indicated that with increasing SOAs, invalid cues were associated with significant costs in response time and valid cues were associated with significant benefits. The lack of an Age X Cue Condition X SOA interaction suggested that the time course of these cuing effects did not vary with increasing age. It is important to point out, however, that a post-hoc analysis revealed that the power to detect this interaction was quite low (.25) in the present experiment. Therefore, we cannot rule out the possibility that an age-related difference in the time course of cuing effects exists. Inspection of Figure 4 suggests that if such a difference existed, it would be in the direction of a speedier time course for older adults (the valid and invalid functions for older adults diverge more rapidly than for younger adults). Thus, the conservative conclusion is that there are no age differences in the efficiency of the attention shifting operation. These results also support the hypothesis that older adults' performance with symbolic cues in Experiment 2 reflect a decline in the efficiency of cue encoding with advancing age. The significant Age X Cue Condition interaction provides evidence that with the symbolic cues used in the present experiment, older adults may be more sensitive to spatial cuing than young adults. Although this result is very different from Experiment 2, it is consistent with the recent findings of Hartley et al.

Table 3 Mean Costs (Invalid Cue-No Cue) and Benefits (Valid Cue-No Cue) in ms as a Function of Stimulus Onset Asynchrony (SOA) in Experiment 3 SOA

Difference measure

50

100

150

200

250

Invalid cue-no cue Valid cue-no cue

56* 32*

66* 9

71* -6

85* -30*

66* -31*

Note. Positive numbers reflect a cost and negative numbers reflect a benefit. * p < .05.

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AGING AND VISUAL ATTENTION

(1990, Experiment 2), who also found larger costs and benefits for older subjects in a symbolic cuing task. The larger cuing effects for older adults in the present experiment are also consistent with the larger benefits found for older adults with 100% valid cues in Experiment 1. The present results suggest that the benefits obtained for older adults in Experiment 1 were indeed the result of a voluntary process that may be more efficient than the similar process in younger adults. The dramatic difference in performance for older adults between Experiment 3 and Experiment 2 can be attributed to changes in the nature of the symbolic cue, because, in other respects, the two experiments were identical. To provide a direct statistical test of the hypothesis that older adults' performance varied directly with the nature of the cue, the data for the older adults in both experiments were entered into an mixed ANOVA with cue validity and SOA as within-subjects factors and experiment (2 vs. 3) as a between-subjects factor. The experiment factor entered into a two-way interaction with cue validity, F(2, 64) = 6.22 (MS, = 8, 609), p < .01, as well as a three-way interaction with cue validity and SOA, F(&, 256) = 2.48 (MS{. = 1, 504), p < .02. These interactions indicated that the pattern of cuing effects for older adults varied with the nature of the cue across the two experiments.5 A similar analysis carried out on the data for young adults yielded no significant effects involving the experiment factor. Thus, the findings of Experiment 3 and Experiment 2 suggest that there are age differences in the efficiency with which symbolic cues are encoded, but that the efficiency of the shift process itself is preserved with advancing age. These findings may account, at least in part, for the inconsistency observed in previous studies on the effects of age on voluntary attention shifts at short SOAs (e.g., Hoyer & Familiant, 1987; Hartley et al., 1990). Specifically, differences in the pattern of cuing effects for older adults across studies may simply reflect differences in the difficulty of cue encoding.

General Discussion This series of experiments was motivated by two independent hypotheses: (a) that two separable mechanisms, one voluntary or controlled and another involuntary or automatic, underlie the allocation of spatial attention (Jonides, 1980; Muller & Rabbitt, 1989; Posner, 1980; Yantis & Jonides, 1984), and (b) that aging selectively impairs voluntary or controlled processes while leaving automatic processes intact (Craik & Byrd, 1982; Hasher & Zacks, 1979; Rabbitt, 1979; Rabbitt & Vyas, 1980). The specific aim of the present studies was to provide a sensitive test of the hypothesis that exogenous (involuntary) shifts of spatial attention are selectively preserved with advancing age. The results of Experiment 1 clearly showed that the existence and efficiency of such shifts are preserved in older adults. Cues that were 100% invalid produced significant costs for older adults at SOAs comparable to those at which younger adults showed similar costs. Assuming these costs reflect shifts in the spatial distribution of attention, this is the first unambiguous demonstration of the preservation of exogenous shifts of attention in older adults. The results of Experiments 2 and 3, however, suggest that the efficiency of voluntary shifts of attention is also preserved (and

463

perhaps enhanced) with advancing age, but that the efficiency of encoding symbolic cues may be subject to an age-related decline. Significant costs and benefits in response time as a function of cue validity were observed for older adults but were found to depend on the exact nature of the symbolic cues used. Cues that were larger and contained redundant spatial information produced significant cuing effects for older adults whereas smaller, nonredundant symbolic cues did not. In contrast, variations in the nature of the cue had little effect on the performance of younger adults. The results of the present studies do not provide strong support for either of the hypotheses on which the studies were based. First, the results are not consistent with the hypothesis that voluntary processes are impaired with increasing age while automatic processes are spared (see also Fisk & Rogers, 1991). In Experiment 3 there was no evidence of age differences in the efficiency of the voluntary processes involved in shifting attention across space. Second, the results do not provide strong evidence for the existence of two separable mechanisms underlying the allocation of spatial attention. Specifically, there was little evidence to suggest an age-specific dissociation between the efficiency of voluntary and involuntary attention shifts. The lack of an age-related dissociation does not necessarily rule out a two-mechanism model; it may simply indicate that both mechanisms are unaffected by aging. One might argue, however, that a single, voluntary mechanism, in which the apparent efficiency varies with the amount of effort required to process the cues, provides the most parsimonious account of the pattern of results across the three experiments. This suggestion is in line with recent arguments that suggest a single mechanism may account for many observed age differences (e.g., Cerella, 1991). On the basis of this argument, all allocation is voluntary and variations in the magnitude and time course of cuing effects for older adults are simply a function of the difficulty of cue processing, with the peripheral cues of Experiment 1 requiring the least effort and the symbolic cues of Experiment 2 requiring the most effort. This argument cannot, however, explain the observed costs for invalid cues in Experiment 1. Because these cues were 100% invalid, there would have been no impetus to voluntarily shift attention to the cued location and thus no costs in performance. That such costs were obtained with both younger and older adults suggests that some form of involuntary attention allocation mechanism does exist. This conclusion, however, is based on the assumption that costs for 100% invalid cues do indeed reflect shifts in the spatial distribution of attention. It is possible that these costs may not be a function of the spatial aspects of the cue, but may simply reflect the central effort or processing required to inhibit or suppress a shift of attention to the irrelevant cue. Recently, investigators have suggested that older adults suffer 5

As a check on the comparability of the older samples across the two experiments, an additional analysis was conducted that took advantage of the fact that the no cue condition was procedurally identical across the two experiments. The data for older adults in this condition were subjected to a simple interaction comparison involving the experiment and SOA factors. Neither factor produced a main effect nor did they interact (for all, F < 1), suggesting that the two samples were indeed comparable.

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CHARLES L. FOLK AND WILLIAM J. HOYER

from a reduction in the efficiency of inhibitory processes (Hasher, Stoltzfus, Zacks, & Rypma, 1991; McDowd & OseasKreger, 1991; Tipper, 1991). Although the results of the present experiments cannot distinguish between a spatial-shift versus a shift-suppression account of the costs in Experiment 1, previous studies of peripheral cuing suggest that the former interpretation is the more plausible one. Posner and his colleagues, for example, have shown that irrelevant (uninformative) peripheral cues, similarto those used in Experiment 1, produced inhibitory effects on information presented at least 300 ms after cue onset, independent of the typical costs and benefits seen at earlier SOAs (Posner & Cohen, 1984; Posner, Cohen, Choate, Hockey, & Maylor, 1984). This effect, termed inhibition of return, does not occur under voluntary cuing conditions and is specific to the spatial location of the irrelevant peripheral cue. Such results are not consistent with a shift-suppression interpretation of costs for invalid peripheral cues. Clearly, further research is needed to determine whether central, nonspatial factors play any role in generating costs for the 100% invalid peripheral cues used in Experiment 1. The results of the present experiments raise several additional issues for future research. One concerns the apparent enhancement of cuing effects with increasing age. Does this effect indicate that voluntary shifts of attention, at least in response to simple symbolic cues, become more efficient with increasing age? If so, it would be important to investigate possible underlying functional architectures that could support such an age-related enhancement. Another aspect of the present study that deserves further investigation is the effect of cue difficulty on the apparent efficiency of voluntary attention shifts in older adults. Although Experiments 2 and 3 suggest that cue difficulty effects older adults' performance, it would be interesting in future experiments to manipulate this factor within subjects to provide the strongest test of this effect. Moreover, it is not clear at this time exactly what aspects of the cue are associated with cue difficulty. The cues used in Experiment 3 were both larger and more redundant than those used in Experiment 2. Thus, the inferred decline in the efficiency of cue processing could reflect age-related changes in sensory and cognitive processes. Future research involving the systematic manipulation of various aspects of symbolic cues is needed to isolate the age-sensitive components of cue processing.

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