Adult age differences in the rate of processing expectancy information

Cognitive Development, 2, 59-70 (1987)

Adult Age Differences in the Rate of Processing Expectancy Information William J. Hoyer and M. Elliott Familant Syracuse University

Two experiments were conducted using a four-choice visual search task to examine the influence of processing rate on adult age differences in the use of expectancy information. In the first experiment, older adults (M age = 71.6 years) showed benefits and costs, and younger adults (M age = 20.6 years) showed only costs when advance knowledge about the location of the imperative stimulus was derived from a priori probabilistic information. However, the older adults were unable to use expectancy information (a precue) which preceded the imperative stimulus by 250 ms, whereas young adults showed both benefits and costs of precue information. In Experiment 2, the interval between precue onset and imperative stimulus onset was varied (350, 500, 750, 1000, and 1250 ms) for younger (M age = 19.74 years) and older adults (M age = 69.16 years). Older adults used precue information efficiently only when the precue-imperative stimulus interval was 750 ms or longer. These findings suggest that age-related slowing of processing rate can affect the extent to which briefly presented precue expectancy information is used by older adults.

One of the reasons older adults may perform less efficiently compared to younger adults on a wide variety of information tasks is that they may be less able to use advance knowledge about the nature of information to be received (i.e., expectancy information). However, the findings are not consistent with regard to the effects of age on the use of expectancy information in target detection tasks. Although it has been reported that older adults have difficulty using expectancy information compared to youngcr adults when very brief warning intervals are used (Gottsdanker, 1980; Plude, Cerella, & P o o n , 1982; Rabbitt, 1964; Rabbitt & Vyas, 1980; Talland, 1964; Talland & Cairnie, 1961), no age differences are reported in some studies (e.g., Madden, 1985; Nissen & Corkin, 1984). Nissen and Corkin (1984) examined the effectiveness of attentional cueing in younger and older adults using relatively long (i.e., 2-s and 3-s) warning signal intervals, and found no age differences in the use of expectancy information. That is, This research was supported by NIA research grant 06041. We would like to express our gratefulness to the members and staff of the Wagon Wheel Senior Center, Syracuse, NY, and the Jewish CommunityCenter, Ft. Lauderdale, FI. Correspondence and requests for reprints should be addressed to William J. Hoyer, Department of Psychology,430 Huntington Hall, Syracuse University, Syracuse, NY 13244-2340. Manuscript reviewed July 28, 1986; revision accepted September 22, 1986

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William J. Hoyer and M. Elliott Familant

although older adults were generally slower, response times were longest when an imperative stimulus appeared in an unexpected location and shortest when an imperative stimulus appeared in an expected location for both age groups. Madden (1985) varied the interval between presentation of a precue and a display and found that both younger and older adults were able to benefit from precue information even at relatively short intervals (200 ms). There were no age differences in the benefits of expectancy information at 200 ms and 400 ms but older adults showed greater benefits than the young at 800 ms and 1000 ms in the Madden (1985, Experiment 1) study. The purpose of Madden's (1985) study was to examine age differences in the use of memory-driven attentional cueing. Earlier, Rabbitt (1979, 1982; Rabbitt & Vyas, 1980) had proposed that there are age reductions in the efficiency of memory-driven processes but that the ability to use data-driven processes remains relatively unchanged with aging. Madden's (1985) findings suggested that age reductions in speed of processing rather than a decline in memory-driven processing can account for adult age differences in the effects of attentional cueing in visual search. Memory-driven processing was presumably involved at all stimulus-onset-asynchony intervals (SOAs), because the cue itself was not the target most likely to appear in the upcoming display. In an earlier study, directly examining age differences in memory-driven and data-driven processing, Madden (1984) found no age differences in data-driven and memory-driven processing when an informative precue preceded the search display by l-s. In the datadriven condition, the precue was one of the targets and, in the memory-driven condition, the effective use of the precue required the retrieval of additional information regarding the current target. Performance benefits, which were defined by a decrease in search reaction time on the cued trials compared to the noncued trials, and performance costs, which were defined by an increase in search reaction time associated with the presentation of misleading advance information, were found for both young and older adults in both conditions. Although previous researchers have pointed to age-related slowing as an explanation for adult age differences in the use of briefly presented precue information, the findings are not clear regarding the effects of processing rate (i.e., the amount of information to be processed per time) on age differences in the use of expectancy information. The phenomenon of age-related slowing in the speed of information processing is one of the most reliable findings in developmental psychology (e.g., see Birren, Woods, & Williams, 1980; Cerella, 1985; Salthouse, 1985a, 1985b). The purpose of the present research was to investigate the extent to which age-related slowing of processing rate can account for age differences in the use of expectancy information. EXPERIMENT 1

In the first experiment, we examined age differences in the effectiveness of two kinds of expectancy information in reducing the amount of uncertainty about

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61

target location. The first is a priori probabilistic information, which is defined as advance information about the relative probability of occurrence or location of the imperative stimulus. The second is precue information, which is indicated by a precue stimulus presented prior to the occurrence of an imperative stimulus and which predicts the occurrence or location of a particular imperative with some degree of accuracy. The amount of uncertainty reduction afforded by a priori probabilistic information and by precue information can be defined in terms of information theory (Attneave, 1959; Shannon & Weaver, 1949). If, for example, each of four imperatives occurs equally often, the stimulus uncertainty, H(s), of each imperative is equal to H(s) = log2 1/.25 bits or two bits of information. When a particular imperative stimulus occurs more often than others, stimulus uncertainty is reduced. In one condition (unequal blocks) of this experiment, subjects were instructed that the imperative stimulus would appear in one of the four stimulus locations 85% of the time, and that the imperative stimulus would appear in each of the other locations on 5% of the trials. In this condition, the average amount of uncertainty associated with the relatively probable stimulus in .23 bits, and the average amount of uncertainty associated with the relatively improbable imperatives is 4.32 bits (by substitution in the above equation). In the second condition (equal blocks), a warning stimulus indicated that the imperative stimulus would appear in any one of the four target locations with equal probability. In the third condition (mixed blocks), a mixture of noncued trials and precue trials was given. On the precue trials, a precue stimulus accurately predicted the subsequent location of the imperative stimulus 100% of the time. On the noncued trials, a warning stimulus indicated that the imperative would appear in any one of the four target locations with equal probability. On the precue trials, the amount of uncertainty reduction provided by the precue was 2 bits. The precue preceded the imperative stimulus by 250 ms, requiring that 8 bits of information needed to be processed per second on these trials. It is hypothesized that both a priori probabilistic information (in the unequal blocks condition) and precue information (in the mixed blocks condition) should serve to reduce stimulus uncertainty and should lead to reductions in RT. However, age differences in processing rate or channel capacity, defined as the amount of information that can be transmitted or processed per unit time (e.g., bits per second), will affect the use of precue information, and it is expected that older adults will be at a disadvantage in the mixed blocks condition for this reason. Method

Subjects. The participants were 12 young adults (M age = 20.6 years, SD = 2. I) who were drawn from the.Syracuse University Psychology Department

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William I. Hoyer and M. Elliott Familant

subject pool, and 12 community-dwelling older adults (M age 71.6 years, SD = 8.8). There were equal numbers of men and women within each age group. The young adults received course credit for participation, and the older adults were paid volunteers from a local senior center. The young adults had an average of 14.3 years (SD = .83) of education, and the older adults had an average of 13.75 years (SD = 1.5) of schooling. Mean Wechsler Adult Intelligence Scale (WAIS) vocabulary scores for the young and elderly adults were 48.5 (SD = 10.98) and 47.5 (SD = 8.79), respectively; this difference was not significant, t (22) = .31, p > .75. The older adults had an average forward digit span score of 6.5 (SD = 1.17) compared to a score of 6.92 (SD = 1.08) for the young adults, and this difference was not significant, t (22) = .91, p > .3. The average backward digit span scores for the young adults and older adults were 5.3 (SD = 1.56) and 4.4 (SD = 1.3), respectively, and this difference was.also not significant, t (22) = 1.56, p > . 1. All subjects showed visual acuity (corrected) of 20/40 or better, as measured by a portable visual screener, and all participants performed with 100% accuracy on a simple detection and familiarization test presented on the monitor screen. In addition, all subjects were in "very g o o d " or "excellent" health, based on self-report.

Materials and Procedure. Displays for the experiment were generated by an Apple lie microcomputer and shown on a black/white high-resolution monitor. Three types of displays were presented consecutively. Display 1 consisted of an asterisk in the center of the screen with four hollow l-cm x l-cm boxes below it. The centers of the boxes were aligned on an imaginary horizontal line that was 6 cm below the asterisk. The centers of the two inner boxes were located 2.5 cm to the right and left of the center of this line, and the centers of the two outer boxes were located 7.5 cm to the right and left of the center of this line. Display 2 consisted of a single character located in the center of the screen. The character was either a + or one of the d i g i t s - - l , 2, 3, or 4. Characters were .6 cm in height. Display 3 was the same as Display 1 with the exception that one of the four boxes was now distinctly and solidly filled in. The keyboard letters V, B, N, and M were marked with black tape and were designated as response buttons 1, 2, 3, and 4, respectively. Each of the four boxes on the screen corresponded to one of the four response buttons, that is, from left to right. Participants were seated approximately 45 cm from the display screen with the center of the screen at eye level. First, Display 1 was presented, and participants were told that whenever a box on the screen was filled, they were to respond by pressing the corresponding button. Both speed and accuracy were stressed in the instructions. Subjects were given two sessions of testing separated by about a 45-min rest period, the purpose of the first session being to familiarize the participants with the demands of the three experimental conditions. In each of the sessions, the conditions (mixed blocks, unequal blocks, and equal blocks) were presented in a counterbalanced order across subjects. There were 100 trials

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63

in each condition, and each trial consisted of Display 1. appearing for 500 ms, Display 2 appearing for 250 ms, and then Display 3 appearing until the subject made a response. At the beginning of each new block of 100 trials, subjects were given instructions specific to that condition. In the mixed blocks condition, Display 2 consisted of either a + or one of the digits-- 1, 2, 3, or 4; each of these five characters appeared on 20% of the trials. If the + appeared, any one of the four boxes was equally likely to be filled when Display 3 appeared. If a 1, 2, 3, or 4 appeared, the corresponding box was filled when Display 3 appeared. That is, a digit was a valid cue for its corresponding box on 100% of the trials. Display 2 consisted of a + in both the unequal and equal blocks conditions. In the unequal blocks condition, subjects were instructed beforehand that one of the boxes would be filled on 85% of the trials, and that each of the other boxes would be filled on 5% of the trials in Display 3. That is, subjects were told specifically which one of the boxes would be filled on 85% of the trials. In the equal blocks condition, each box was filled on 25% of the trials, and the subjects were so instructed. Results and Discussion Responses in Session 1 were taken as practice. Subjects' mean RT for correct responses were computed for each condition in Session 2. In order to reduce variability in the reaction time (RT) distributions, individual RTs that exceeded each individual's original mean (for each cell of the design) by plus or minus two standard deviations were excluded from analysis. In all conditions, RT data from sequential repetitions of a particular box were excluded, such that only the first RT in a sequence of responses to the same stimulus box was used for analysis. This was done to avoid confounding attributable to stimulus repetition effects (e.g., see Rabbitt, 1981, 1982; Rabbitt & Vyas, 1980). In all conditions, the error rate was less than 3%, and this rate was judged to be too small to analyze meaningfully. RT data from the unequal and equal blocks conditions were then combined in a 2 (age) x 3 (15%, 25%, or 85% appearance) mixed factorial analysis of variance (ANOVA) on RT. All post-hoc analyses were computed using a Dunn's multiple comparison statistic (Kirk, 1982). There were significant effects of age, F (1, 22) = 52.56, p < .0001; target appearance, F (2,44) = 70.61, p < .0001; and the interaction between age and target appearance, F (2, 44) = 12.52, p < .0001. Mean RT as a function of percentage of target appearance and age are reported in Table 1. In order to determine the source of the interactions, the percentage of target appearance variable was examined at each level of age. This factor was significant both for the older adults, F (2, 44) = 40.36, p < .0001, and for the younger adults, F (2, 44) = 50.68, p < .0001. Post-hoc analyses revealed that older adults developed an attentional set to respond to a priori probabilistic information, as shown by a significant difference between RT to the target which appeared on 85% of the trials in the unequal

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William J- Hoyer and M. Elliott Familant

Table 1. Mean RT (in ms) in the Unequal and Equal Blocks Conditions as a Function of Age and Percent of Target Appearance Percent Target Appearance Age Old Young

15

25

85

811 469

626 397

520 350

Note. The 15% target appearance condition shows combined RT to imperative stimuli which appeared in either of the three low probability (5%) boxes in the unequal blocks condition.

blocks condition, and RT to the targets which appeared on 25% of the trials in the equal blocks condition, tD = 7.49, p < .0001. Older adults also exhibited significant costs as measured by the difference in RT to the target which appeared on 15% of the trials (in the three 5% boxes) in the unequal blocks condition and RT to the targets which appeared on 25% of the trials in the equal blocks condition, tD = 4.31, p < .0004. Consistent with the findings of Posner, Snyder, & Davidson (1980), there was some evidence that young adults developed an attentional set to respond to a priori probabilistic information, as evidenced by significant costs, tD = 2.92, p < .02, but no significant benefits, tD = 1.92, p < .24. Because the difference between RT in the 85% and the 25% conditions was only 50 ms for the young adults and in the direction of suggesting a benefit of a priori probabilistic information, it is possible that a floor effect mimimized the demonstration of benefits for the younger adults. Data from the mixed blocks condition were combined across the four precues (i.e., l, 2, 3, and 4) and compared to the no precue (i.e., + ) situation, and then subjected to a 2 (age) x 2 (presence vs. absence of precue) mixed factorial ANOVA for RT. These data are presented in Table 2. There were significant effects of age, F (1, 22) = 60.23, p < .0001; presence of precue, F (l, 22) = 38.97, p < .0001; and the interaction of age and presence of the precue, F (1, 22) = 33.03, p < .0001. In order to determine the source of the interaction, the presence of the precue condition was examined at each level of age. For the younger adults, the effect of precue was significant, F (l, 22) = 55.72, p < .0001. RT was faster when preceded by a valid precue than when preceded by a neutral cue. However, for the older adults, the type of precue was not significant, F (1, 22) = .17, p > .60. Expectancy information serves to reduce RT because it allows individuals to make a detectional or a locational decision prior to stimulus appearance. It was found that older adults can effectively use expectancy information that is avail-

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Table 2. Mean RT (in ms) in the Mixed Blocks Condition as a Function of Age and Type of Precue (Valid or Neutral) Type of Precue

Age Old Young

Neutral

Valid

630 455

62 I 242

Note. RTs greater than plus or minus 2 SDs removed.

are

able a priori. Comparing the unequal and equal blocks conditions, the elderly subjects detected the imperative stimulus faster when expectancy information was available than when no expectancy information was provided. Furthermore, older adults exhibited significant costs in that RT to unexpected targets in the unequal blocks condition was significantly slower than RT to nonexpected targets in the equal blocks condition. However, consistent with previous findings (e.g., Plude et al., 1982; Rabbitt, 1964; Talland & Cairnie, 1961), it was found that older adults have difficulty using informative precues presented just prior to target appearance, compared to younger adults. That younger adults obtained costs but no benefits from probabilistic expectancy information, and that they did benefit from precue information replicated Posner's findings (Posner, Nissen, & Ogden, 1978; Posner et al., 1980). One possible explanation for the age differences found with regard to the effects of attentional precueing is that it simply takes older adults longer to use expectancy information. EXPERIMENT 2

Both types of expectancy information studied in the first experiment, a priori probabilistic information and precue information, were expected to reduce uncertainty (or entropy) and lead to reductions in RT. However, the older adults benefitted from the a priori probabilistic information but not from the precue expectancy information. The a priori probabilistic condition and the precueing condition differed in that relatively rapid processing was required when expectancy information was based on a precue. In order for expectancy information to be beneficial to the processing of an imperative stimulus, it must be available at the time of imperative stimulus onset. With the exception of the Madden (1985) study, it is reasonable to hypothesize on the basis of previous work that the SOA of 250 ms, which was used in Experiment l, was less optimal for the older adults than for the younger adults. Information-theory analysis provides a framework for relating changes in processing rate to a specific, quantifiable characteristic of stimuli (i.e., stimulus uncertainty or entropy). Many findings of age-related decline in information

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William I. Hoyer and M. Elliott Familant

processing have been attributed to a slowing of information-processing rate (e.g., see Salthouse, 1985b), yet only a few investigators have examined adult age differences in the effects on performance of the amount of information to be processed per time. Abramson (1963), MacRae (1970), and others have used the term channel capacity to refer to the amount of information that can be processed per time (i.e., in bits per second). This notion is particularly important in developmental research, because it has been reported that older adults are more slowed under conditions of greater entropy (stimulus uncertainty) than are younger adults (Familant, Hoyer, & Montaglione, 1984; Suci, Davidoff, & Surwillo, 1960; Welford, 1958). Familant et al. (1984) varied the amount of information associated with the precue in a visual search task, and found that older adults required more time than younger adults to process information as the amount of stimulus uncertainty increased. Suci et al. (1960) measured adult age differences in RT to a stimulus of one light-off in subsets of one light (0 bits), two lights (1.00 bit), three lights (1.58 bits), and four lights (2.00 bits), and found that age differences in reaction time increased as a function of greater amounts of stimulus uncertainty. Welford (1958) reported that age decrement in RT increased as task complexity increased in a variety of tasks (see also Cerella, Poon, & Williams, 1980). In the present experiment, as in the mixed blocks condition of Experiment 1, the amount of information that the precue supplied was defined as a function of the amount of uncertainty reduction it provided about which of the four stimulus locations would receive the imperative stimulus. Because each of the four locations held the imperative equally often, the amount of uncertainty, H(s), of each imperative location was equal to two bits. Although young adults were capable of processing two bits of information in 250 ms (or 8 bits per second), older adults seem less able to process that amount of information in that amount of time. It is our contention that adult age differences, in the time it takes to process the precue, may have accounted for the differences in the use of precue expectancy information obtained in Experiment 1. In the second experiment, adult age differences in the rate of processing precue information were investigated by varying amount of time available to process the precue, holding constant the information value of the precue. It was expected that older adults will require a longer SOA to show the beneficial effects of precue expectancy information. In addition, we examined the effects of practice on age differences in use of precue information by giving two sessions (replications) on separate days in this experiment. Note that, in Experiment 1, a session replication was given on the same day, separated by a 45-min rest interval.

Method

Subjects. Twelve young adults (M age = 19.74 years) and 12 older adults (M age = 69.16 years) took part in this study. None of the subjects in Experi-

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ment 1 participated in this experiment. The older adults were paid volunteers drawn from the roster of a senior center, and the young adult subjects were college students, who received class credit for participation. There were equal numbers of men and women in each age group. The groups did not differ in terms of self-rated health or number of years of education. Young adults had an average of 13.1 years of education, and the older adults had an average of 13.6 years of schooling. Also, there were no differences between the groups on measures of digit span performance, or WAIS vocabulary. The older adults had an average forward digit span score of 6.08, compared to 6.66 for the younger adults. The average backward digit span scores were 4.83 and 5.08, respectively, for the old and young adults. Mean WAIS vocabulary scores for the young and elderly adults were 46.91 and 49.08, respectively. Subjects were screened for visual acuity (20/40 or better, corrected) using a portable visual screener, and all subjects reported 100% accuracy on a visual detection and familiarization task presented on the CRT monitor.

Materials and Procedure. The materials, procedure, and design of this experiment were the same as in the mixed blocks condition of Experiment 1, except that the following SOAs (between precue onset and target appearance) were used: 350, 500, 750, 1000, and 1250 ms. There were 40 valid trials and 40 neutral trials at each SOA. Each session consisted of two blocks of 400 trials, and each subject participated in two such sessions separated by 1 to 5 days (M = 2.8). Results and Discussion Mean RTs to the imperative stimuli were computed for each subject at each SOA, preceded by each type of precue, in each session. After removal of "outliers" (scores greater than two standard deviations away from the cell mean for each subject), the data were submitted to a 2 (age) x 5 (SOA) x 2 (precue) x 2 (session) ANOVA for a mixed design. Post-hoe analyses were computed using a Dunn multiple comparison statistic and were evaluated at the .05 level of significance. There were main effects of age, F (1, 22) = 131.89, p < .0001; SOA, F (4, 88) = 42.45, p < .0001; and session, F (1, 22) = 44.61, p < .0001. There was also a quadruple interaction involving all four factors, F (4, 88) = 2.84, p < .03. To determine the source of this four-way interaction, the effect of precue was investigated at each combination of age, SOA, and session. A clear pattern emerged from this analysis, and the data are reported in Table 3. For the young adults, the effects of precue were significant at every SOA in both sessions. As expected, responses of the young adults were faster when preceded by a valid precue than when preceded by a neutral precue. However, for the older adults, the beneficial effects of the precue were significant only in the second session and only when the SOA was 750 ms or greater.

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William l- Hoyer and M. Elliott Familant Table 3. Mean RT (in ms) as a Function of Age, SOA, Type of Precue (Neutral or Valid), and Sessions in Experiment 2 Session 1 Young

Old

SOA

Neutral

Valid

Neutral

Valid

350 500 750 1000 1250

488 462 438 435 448

405 345 294 277 264

650 640 647 641 653

657 651 633 629 630

Session 2 Young

350 500 750 1000 1250

Old

Neutral

Valid

Neutral

Valid

434 405 410 406 417

366 300 262 244 233

642 636 633 642 639

619 622 585 564 560

GENERAL DISCUSSION Adult age differences in the use of expectancy information are affected by agerelated slowing of processing rate. In terms of information theory, the amount of information afforded by a precue in a search task is a function o f the amount of uncertainty reduction it provides about target location. In the mixed blocks condition in both experiments, the valid precue fully reduced uncertainty about target location. Its value in terms of uncertainty (or entropy) reduction was defined as 2 bits of information. In Experiment 1, because 2 bits of information needed to be processed in 250 ms, the procedure required that 8 bits o f information be processed per second, which may have been simply too fast for the older participants (e.g., see Salthouse, 1985b). In Experiment 2, it was demonstrated that older adults were able to use the precue as an aid to imperative stimulus location when given sufficient time to process it. That is, older adults benefitted from precue information when the S O A interval was 750 ms or greater (i.e., 2.7 bits per second), and younger adults showed a benefit of precue information at the 350 ms S O A (i.e., 5.7 bits per second). Note, however, that for the older adults the difference between the cued and neutral trials at the longest S O A s is about as great as it is for the younger adults at the shortest SOA. It remains to be

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determined why the older adults did not derive as much absolute benefit from the precue information as the young, even at long SOAs. Finally, it is suggested, based on the statistically significant interaction of SOA with sessions in Experiment 2, that practice factors may attenuate agerelated slowing in processing rate. Because it is unlikely that neural conduction rates are directly affected by the amount of practice used in this study, it seems that practice or training may lead to faster processing in older adults by functionally reducing the amount of transmitted information necessary for efficient performance. REFERENCES Abramson, N. (1963). Information theory and coding. New York: McGraw-Hill. Attneave, F. (1959). Applications of information theory to psychology: A summary of basic concepts, methods, and results. New York: Holt. Birren, J. E., Woods, A. M., & Williams, M. V. (1980). Behavioral slowing with age: Causes, organization, and consequences. In L. W. Pooh (Ed.), Aging in the 1980's: Psychological issues (pp. 293-308). Washington, DC: American Psychological Association. Cerella, J. (1985). Information processing rates in the elderly. Psychological Bulletin, 98, 67-83. Cerella, J., Poon, L. W., & Williams, D. M. (1980). Age and the complexity hypothesis. In L. W. Poon (Ed.), Aging in the 1980s: Psychological issues (pp. 332-340). Washington, DC: American Psychological Association. Familant, M. E., Hoyer, W. J., & Montaglione, C. J. (1984). Aging, entropy, and the use ofprecue information. Paper presented at the meeting of the American Psychological Association, Toronto. Gottsdanker, R. (1980). Aging and the use of advance probability information. Journal of Motor Behavior. 12. 133-143. Kirk, R. E. (1982). Experimental design: Procedures for the behavioral sciences (2nd ed.). Belmont, CA: Wadsworth. MacRae, A. W. (1970). Channel capacity in absolute judgment tasks: An artifact of information bias? Psychological Bulletin, 73. I 12-121. Madden, D. J. (1984). Data-driven and memory-driven selective attention in visual search. Journal of Gerontology, 39, 72-78. Madden, D. J. (1985). Adult age differences in memory-driven selective attention. Developmental Psychology, 21, 655-665. Nissen, M. J., & Corkin, S. (1984). Effectiveness of attentional cueing in older and younger adults. Journal of Gerontology. 40. 185-191. Plude, D. J., Cerella, J., &Poon, L. W. (1982). Shifting visual attention: An analysis of age effects. Paper presented at the meeting of the American Psychological Association, Washington, DC. Posner, M. I., Nissen, M. J., & Ogden, W. C. (1978). Attended and unattended processing modes: The role of set for spatial location. In H. L. Pick & E. Saltzman (Eds.), Modes of perceiving and processing information (pp. 137-157). Hillsdale, N J: Erlbaum. Posner, M. 1., Snyder, C. R., & Davidson, B. J. (1980). Attention and the detection of signals. Journal of Experimental Psychology: General. 109. 160-174. Rabbitt, P. M. A. (1964). Set and age in a choice response task. Journal of Geromology, 19, 307312. Rabbitt, P. M. A. (1979). Some experiments and a model for changes in attentional selectivity with old age. In F. Hoffmeister & C. Muller (Eds.), Brain function in old age: Evaluation of changes and disorders. New York: Springer-Verlag.

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Rabbitt, P. M. A. (1982). How do older people know what to do next? In F. 1. M. Craik & S. Trehub (Eds.), Aging and cognitive processes (pp. 79-98). New York: Plenum. Rabbitt, P. M. A., & Vyas, S. M. (1980). Selective anticipation for events in old age. Journal of Gerontology, 35, 913-919. Salthouse. T. A. (1985a). A theory of cognitive aging. Amsterdam: North Holland. Salthouse, T. A. 1985b). Speed of behavior and its implications for cognition. In J. E. Birren & K. W. Schaie (Eds.), Handbook of the psychology of aging (2nd ed., pp. 400-426). New York: Van Nostrand Reinhold. Shannon, C. E., & Weaver, W. (1949). The mathematical theory of communication. Urbana, IL: University of Illinois Press. Suci, G. J., Davidoff, M. D., & Surwillo, W. W. (1960). Reaction time as a function of stimulus information. Journal of Experimental Psychology, 60, 242-244. Talland, G. A. (1964). The effect of warning signals on reaction time in youth and old age. Journal of Gerontology, 19, 31-38. Talland, G. A., & Cairnie, J. (1961). Aging effects on simple, disjunctive, and alerted finger reaction time. Journal of Gerontology, 16, 370-374. Welford, A. T. (1958). Aging and human skill. London: Oxford University Press.