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The Adjusting-Paced Serial Addition Test (Adjusting-PSAT): thresholds for speed of information processing as a function of stimulus modality and problem complexity
Archives of Clinical Neuropsychology 19 (2004) 131–143
Brief report
The Adjusting-Paced Serial Addition Test (Adjusting-PSAT): thresholds for speed of information processing as a function of stimulus modality and problem complexity夽 J. Royan a , T.N. Tombaugh b,c,∗ , L. Rees c,d , M. Francis e a
University of Victoria, Victoria, BC, Canada Carleton University, Ottawa, Ont., Canada ON K1S 5B6 c Ottawa Concussion Clinic, Carleton University, Rm B550 Loeb Bldg., 1125 Colonel By Drive, Ottawa, Ont., Canada ON K1S 5B6 d The Rehabilitation Centre, Ottawa, Ont., Canada e University of Auckland, Auckland, New Zealand b
Accepted 3 September 2002
Abstract A modified computer version of the PASAT (Adjusting-PSAT; Tombaugh, 1999) is described that measures speed of information processing and working memory by means of a temporal threshold rather than number of correct responses. This is accomplished by making the duration of the interval between numbers depend on the correctness of responding—a correct response decreases the interval between digits and an incorrect response increases the interval. Modality of presentation (visual and auditory) was factorially combined with problem difficulty (answers between 2–10 or 2–18). Performance of 60 healthy student volunteers on the Adjusting-PSAT was compared to that obtained on several traditional neuropsychological measures (Digit Span, Trail Making Test, and Symbol Digit Modality Test) and on a test of basic addition skills. The visual version of the test produced a lower threshold than did the auditory version, but problem difficulty did not produce a significant effect. Of the neuropsychological
夽 This article was presented in part at the 21 annual meeting of the National Academy of Neuropsychology, San Francisco, CA, November, 2001. This article was based in part on a thesis by Jodie Royan submitted to Carleton University in partial fulfillment of the requirements for a Master’s degree in Psychology. ∗ Corresponding author. Tel.: +1-613-520-2659; fax: +1-613-520-3667. E-mail address: [email protected] (T.N. Tombaugh).
0887-6177/$ – see front matter © 2002 National Academy of Neuropsychology. PII: S 0 8 8 7 - 6 1 7 7 ( 0 2 ) 0 0 2 1 6 - 0
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tests, Trails-B (TMT-B) was most highly correlated with thresholds. However, regression analyses revealed that math ability accounted for more variance than did TMT-B. The clinical implications of these finding are discussed. © 2002 National Academy of Neuropsychology. Published by Elsevier Science Ltd. All rights reserved. Keywords: Information processing; Stimulus intervals; PASAT
1. Introduction The Paced Serial Addition Test (PSAT) was originally developed by Sampson (1956) to examine temporal integration. The basic procedure involved presenting a series of single digit numbers and requiring the person to add the two most recent digits. Speed of information processing was manipulated by presenting the numbers at different rates on each of four trials. The interstimulus intervals (ISIs) were 2.4, 2.0, 1.6 and 1.2 s on Trials 1, 2, 3, and 4, respectively. The number of correct responses was calculated for each trial (maximum = 60) and then combined over all four trials for a total score. Originally, Sampson (1956) developed both a visual (PVSAT) and an auditory (PASAT) version of the test. However, as a result of Gronwall and Sampson’s (1974) pioneering research using the PASAT to assess the effects of traumatic brain injury (TBI), the auditory version is now used almost exclusively in neuropsychology. Currently, the PASAT is one of the more frequently used neuropsychological tests to measure attention and concentration (Gordon & Zillmer, 1997). Evidence of its popularity is witnessed by the fact that it was recommended as the primary test to evaluate neuropsychological functions in clinical trials for multiple sclerosis (Rudick et al., 1997). In spite of its popularity, the PASAT has several weaknesses. Perhaps the most important of these is that it does not determine precisely at what speed information processing actually breaks down. Stated slightly differently, the PASAT does not provide any temporal threshold measure as to when an individual is no longer able to consistently process information. This lack of precision seriously curtails its ability to assess the initial effects that various neurological insults have on speed of information processing and to track recovery from the injury. The lack of a threshold measure can be attributed, at least in part, to the technology that was available when the Gronwall and Sampson (1974) first used the PASAT clinically. At that time, the necessity of using an audiotape to present the digits required the presentation of a fixed series of 4 ISIs and reliance on the number of correct responses as the measure of performance. The present experiment describes a computerized test that combines the basic PSAT methodology with the psychophysical stair-step/titration procedure for determining thresholds. This procedure, called the Adjusting-Paced Serial Addition Test (Adjusting-PSAT; Tombaugh, 1999), makes the ISI contingent on the “correctness” of the response. That is, the interval between numbers increases by 20 ms whenever an incorrect response occurs and decreases by 20 ms whenever a correct response is made. The continuous adjustment of the ISI allows a temporal threshold to be determined where the speed of digit presentation outstrips the
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person’s ability to process information. The shortest interval where a correct response occurs represents this threshold. Making the duration of the interval depend on the correctness of responding eliminates another methodological criticism of the original PASAT. Snyder, Aniskiewicz, and Snyder (1993) observed that individuals frequently employ a “chunking” strategy to cope with the increased difficulty that occurs when presentation speed exceeds a person’s processing capabilities. Thus, at the shorter intervals an individual may abandon any attempt to add consecutive digits and adopt the “alternate answer” strategy of adding two numbers, skipping one, adding two numbers, skipping one etc. Often this type of strategy will produce a spuriously superior performance on the original PASAT. However, it will not substantially affect the threshold score on the Adjusting-PSAT. This is because the ISI associated with the correct responses will be offset by the increased ISI produced when no response is made on the next trial. In effect, the “alternate answer” strategy will produce a non-changing, oscillating threshold when calculated over a series of trials. A third advantage of the using the Adjusting-PSAT is that it greatly reduces or eliminates the problem of making an a priori decision about what ISI intervals should be employed. Currently, a variety of different ISIs are used in research applications and clinical practice. For example, Gronwall and Sampson (1974) originally used 4 ISIs (2.4, 2.0, 1.6, and 1.2 s) with TBI patients, Deary, Langan, Hepburn, and Frier (1991) selected 4 and 2 s ISIs to assess cognitive functioning in diabetic patients, and Rudick et al. (1997) recommend that a 3-s ISI be used for clinical trials with multiple sclerosis patients. The selection of the most appropriate ISI is dictated primarily by the specific level of cognitive processing that occurs with different patients, and is based on the expected performance of the “average” patient. In many cases, however, the interval that is selected is insensitive to the level of cognitive functioning of individual patients because it is either too long or too short. This problem is largely eliminated in the Adjusting-PSAT because the threshold is individually determined regardless of the initial value. That is, if the initial ISI is too “easy” subsequent responding will progressively decrease it and if it is too “hard” subsequent responding will progressively increase it. Although the procedure used in the adjusting procedure may overcome some of the shortcomings of the traditional PASAT, it does not directly address two other fundamental questions. The first question concerns the degree that mathematical ability affects performance. There is considerable evidence to suggest that the traditional PASAT is affected by mathematical ability (Chronicle & MacGregor, 1998; Crawford, Obonsawin, & Allan, 1998; Egan, 1988; Hiscock, Caroselli, & Kimball, 1998; Sherman, Strauss, & Spellacy, 1997). Most of the evidence to support this relationship has been derived from experiments that have correlated different measures of mathematical ability with performance. However, the construct of mathematical ability is broad and encompasses many dimensions and skills that may not be directly relevant for Serial Addition Tests. The skill that appears to be most directly related to Serial Addition Tests is the ability to solve basic arithmetic problems. It is this ability that is measured in the present experiment. The basic measurement of arithmetic skills does not, however, provide any practical solution as to how to counteract the effects of arithmetic ability if, in fact, it is shown to be biasing factor. Johnson, Roethig-Johnston, and Middleton (1988) proposed one solution to this problem when they created a children’s version
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of the PASAT. They reduced the complexity of the answers by restricting them to a sum that ranged between 2 and 10 rather than between 2 and 18 as occurs in the adult version. A similar approach is taken in the present study, where the difficulty of the Adjusting-PSAT was manipulated by using two number sequences that required different levels of arithmetic ability. The second question concerns the effects of modality of presentation on performance. The numbers in the original version of the PSAT were presented visually with a slide projector (Sampson, 1956). However, due to practical circumstances, virtually all of the clinical findings have been reported using the auditory version where the digits are presented via an audiotape. As a result of this switch in presentation modality, the degree to which the clinical efficacy of the PASAT is related to the aural presentation of the stimuli is unclear. If the clinical results reflect primarily the ability to rapidly process information, then similar effects should be produced regardless of mode of presentation. However, several studies have suggested that this is not the case (Bateman, 1997; Diamond, DeLuca, Kim, & Kelley, 1997; Feinstein, Brown, & Ron, 1994; Fos, Greve, South, Mathias, & Benefield, 2000; Hiscock et al., 1998). Consequently, the current research employed both a visual (Adjusting-PVSAT) and an auditory (Adjusting-PASAT) version.
2. Method 2.1. Research participants The 60 participants were volunteers from an Introductory Psychology course and received course credit. The following exclusionary criteria were used: (1) history of neurological, mood, or psychotic disorder, (2) substance abuse, (3) antidepressant, tranquilizer, or antipsychotic drugs, or (4) head injury. Participants were randomly assigned to one of four conditions based on 2 × 2 factorial combination of mode of presentation (visual vs. auditory) and type of number list (unrestricted sums = 2–18 vs. restricted sums = 2–10) forming four groups: visual-unrestricted; visual-restricted; auditory-unrestricted; auditory-restricted. The average age of the participants was 24.6 years (S.D. = 6.2) with the age of the participants in the visual-unrestricted group slightly older (27.1 years) than those in the other three groups (23.7 years). All groups had comparable levels of education (13.6 years, S.D. = 1.3). 2.2. Materials 2.2.1. Adjusting-PSAT A series of digits from 1 to 9 were presented either visually by computer screen or aurally by headphones. A visual mask followed each of the visually presented digits to prevent the formation of after images. Aural digits were presented by a male human voice through a Windows PCM WAV file format. The ISI was controlled by the 32-bit multimedia timer function of Windows 98. The duration of the aurally presented digits varied between 321 and 398 ms. All visual stimuli were presented for a 350 ms duration.
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On each trial, the participant was required to add the two preceding digits. The participant said the sum aloud and the experimenter typed the response into the computer using the numeric keypad. The computer program was written in such a manner to compensate for the duration of time required for the experimenter to enter the responses. Throughout the test trials the duration of the ISI was determined by the “correctness” of the response. That is, the ISI was adjusted using a titrated/stairstep procedure so that each correct response produced a 20 ms decrease in the next interval and each incorrect response produced a 20 ms increase. The test series consisted of 200 trials and began with the presentation of a single-digit followed by a 2.4-s interval. The test series was preceded by a practice session of 12 trials that began with a 3.0-s interval. The same procedure was used with both the visual and auditory versions. 2.2.2. Math test The addition test from the French Kit (French, Ekstrom, & Price, 1963) was used to examine the effects of math ability on Adjusting-PSAT performance. The participants were instructed to provide the sums for a series of addition problems. Each problem consisted of three numbers (single and double digits) arranged in a column on a sheet of paper. Part one consisted of nine practice problems that were to be solved as quickly as possible. Parts two and three required the participants to complete as many addition problems as they could in 2 min. The total number of correct answers on parts two and three was used as the measure of math ability. 2.2.3. Neuropsychological tests The following three neuropsychological tests that are frequently used to measure attention and executive functioning were administered. It should be noted that none of these tests can be viewed as a “pure” measure of attention. At best, they can be conceived as possessing a large attentional component. (1) Trail Making Test Parts A and B (TMT; Reitan & Wolfson, 1985). On part A, the participant consecutively joined 25 numbers as rapidly as possible. On part B, the participant alternated between numbers and letters. (2) Symbol Digit Modality Test (SDMT; Smith, 1982). The participant was required to match numbers with the appropriate symbol. (3) Digit Span (Schmidt & Tombaugh, 1995). Participants repeated a series of progressively longer series of digits in either the same order (forward span) or in the reverse order (backward span). 2.3. Procedure The testing session took approximately 1 h to complete. Participants were introduced to experimental procedures with the informed consent form and medical health questionnaire. Following the administration of the Adjusting-PSAT, the math test and the neuropsychological tests were given in the order listed above. The order was held constant across all participants.
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3. Results 3.1. Effects modality and number list 3.1.1. Threshold Since the Adjusting-PSAT is primarily a test of speed of information processing and working memory, a threshold was chosen as the primary measure for analysis as it reflects the fastest speed at which information can be processed. The threshold was defined as the shortest ISI where a correct response occurred. A 2 (visual vs. auditory) by 2 (restricted vs. unrestricted) ANOVA revealed only a significant main effect for modality: modality, F(1, 56) = 29.6, P < .001; number list, F(1, 56) = .40, P > .05; modality × number list, F(1, 56) = .00, P > .05. Inspection of Figure 1 demonstrates that the modality effect was due to lower visual thresholds. 3.1.2. Number of correct responses A 2 by 2 ANOVA was performed on the number of correct responses to provide further evidence for the effects of modality on Adjusting-PSAT performance. Similar to the results for threshold, Figure 2 indicates that visual presentation resulted in more correct responses than the auditory presentation. No differences were observed for number list nor was there a significant interaction between modality and number list: modality, F(1, 56) = 27.5, P < .001; number list, F(1, 56) = .32, P > .05; modality × number list, F(1, 56) = .11, P > .05.
Fig. 1. The effects modality (auditory and visual) and type of number list (restricted and unrestricted) on the Adjusting-PSAT threshold.
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Fig. 2. The effects modality (auditory and visual) and type of number list (restricted and unrestricted) on the number of correct responses on the Adjusting-PSAT.
3.1.3. Correct latency Given the effects of modality on threshold and on the number of correct responses, it was expected that visual response latencies for correct responses would be shorter than the auditory latencies. As Figure 3 reveals, the anticipated relationship occurred: modality, F(1, 56) = 39.7, P < .001; number list, F(1, 56) = 3.6, P > .05; modality × complexity, F(1, 56) = .39, P = .08. 3.2. Simple versus complex sums The above analyses revealed that no significant differences existed between the two different types of number lists. This finding was rather unexpected since the restricted number list (i.e., sum = 2–10) was constructed to represent a much less challenging list than the traditional unrestricted list (i.e., sum = 2–18). However, a consistent non-significant trend of better performance for the restricted number list existed for several measures. The following analyses were undertaken to determine if failure to achieve a significant effect was due to the fact that both simple sums (2–10) and complex sums (11–18) occurred within the unrestricted number list. For these analyses, performance on the simple sums and the complex sums within the unrestricted number list was analyzed separately. This produced a “pure” simple and “pure” complex condition where a complexity effect was more likely to be observed. An ANOVA performed on baseline adjusted scores (i.e., proportions of correct responses) revealed significant main effects for modality and complexity (simple vs. complex sums):
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Fig. 3. The effects modality (auditory and visual) and type of number list (restricted and unrestricted) on correct response times on the Adjusting-PSAT.
modality, F(1, 27) = 11.44, P < .001; complexity, F(1, 27) = 60.4, P < .001; modality × complexity, F(1, 27) = 3.6, P > .05. There were proportionately more correct responses on the visual than on the auditory version, and there were proportionately more correct responses for the unrestricted-simple sums than for the unrestricted-complex sums. A second analysis was undertaken where response latencies for correct responses were used. Visual latencies were shorter than auditory latencies and latencies for unrestricted-simple sums were shorter than for unrestricted-complex sums: modality, F(1, 27) = 13.0, P < .01; complexity, F(1, 27) = 53.9, P < .001; modality × complexity, F(1, 27) = .00, P > .05. 3.3. Correlations and multiple regressions Correlational analyses were performed to determine the relationship among threshold values, math, and neuropsychological tests of attention. In Table 1, threshold values are shown separately for each modality as well as when combined. There was a strong and significant negative correlation between math scores and threshold. Furthermore, a significantly higher correlation was found between math and the auditory modality than between math and the visual modality. Of all the measures of attention, threshold was most highly correlated with TMT-B. The SDMT test was also highly correlated with threshold with the higher value occurring for the visual threshold. In order to account for threshold variance, multiple regressions were performed with modality and number list. Modality accounted for a substantial amount of variance across all groups
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Table 1 Correlations among Adaptive-PSAT thresholds, math, and neuropsychological measures of attention Threshold Measure
Combined
Auditory
Visual
Math
TMT-A
TMT-B
SDMT
Forward score
Math TMT-A TMT-B SDMT Forward score Backward score
−.63∗∗∗ .33∗∗ .60∗∗∗ −.47∗∗∗ −.13 −.31∗∗
−.74∗∗∗ .35 .53∗∗ −.36∗ −.32 −.37∗
−.53∗∗ .20 .56∗∗∗ −.54∗∗ −.12 −.41∗
−.14 −.41∗∗∗ .36∗∗ .06 .29∗
.31∗ −.11 −.09 .01
−.39∗∗ −.21 −.25
−.09 .06
.67∗∗∗
∗
Significant at P < .05. Significant at P < .01. ∗∗∗ Significant at P < .001. ∗∗
while number list accounted for very little of the variance: modality (34%): F(1, 58) = 30.5, P < .001; number list: (1%); F(1, 58) < 1; modality + number list (35%): F(1, 58) = 15.3, P < .001. Since math was highly and significantly correlated with threshold, math was entered after modality and number list to determine how much of the residual variability in the threshold could be accounted for by math ability. The results showed that math ability accounted for 26% of the threshold residual variance: F(1, 55) = 34.8, P < .001. Of particular interest was the finding that math accounted for a greater amount of variance with the unrestricted list (33%) in comparison to the restricted list (18%). The regression analysis with TMT-B scores showed that it accounted for only 18% after modality and number list: F(1, 55) = 21.5, P < .001. Finally, when TMT-B was entered after math it only accounted for 6% of the variance: F(1, 54) = 9.6, P < .05.
4. Discussion The present study clearly demonstrated that lower thresholds occurred for the visual version of the Adjusting-PSAT. The effects of modality on threshold measures were supplemented by other analyses showing that the number of correct responses was significantly higher, and the latency for correct responses was significantly shorter. Moreover, when the Adjusting-PSAT was further analyzed by comparing performance on the unrestricted-simple and unrestricted-complex numbers, significant modality and complexity effects were found for proportion of correct responses and latencies for correct responses. These findings extend previous research (Bateman, 1997; Feinstein et al., 1994; Fos et al., 2000; Hiscock et al., 1998), and suggest that the visual and auditory threshold measures may assess different cognitive processes. In the present situation, it is likely that a conflict between the stimulus input and response output produced an ‘interference effect’. That is, the aural articulation of the response may have masked the aural presentation of the digits. This explanation is similar to that posited by several other authors who have suggested that
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some type of interference effect is responsible for the lower performance on the auditory version (Bateman, 1997; Fos et al., 2000; Hiscock et al., 1998; Madigan, DeLuca, Diamond, Tramontano, & Averill, 2000). All of these researchers postulated that in the auditory version the person has access to a single information processing channel, which processes the spoken word. If only one channel is being accessed, then conflict between articulating a response and listening to the next digit could occur, particularly when ISI length becomes shorter. In fact, participants in the auditory group of the current study often commented that they had a hard time “listening to the voice” while saying their answers aloud. The effects of this interference could be reduced either by changing the output from auditory to non-auditory (e.g., pressing different response keys) or by changing the input modality from auditory to visual. The second alternative occurred in the present experiment. While it may not be easy to articulate a response and watch for the next digit, it is much easier than attempting to articulate and listen for the next digit at the same time. In this sense, because the articulated response and incoming visual stimuli are processed along separate pathways, there is less interference on the visual Adjusting-PSAT than on the auditory Adjusting-PSAT. This interference effect has important implications for interpreting the effects that TBI has had on the traditional PASAT. Although the PASAT is acknowledged to be one of the more sensitive measures of how TBI affects speed of information processing, its sensitivity is based on results obtained with the auditory presentation of the digits. The current results cast considerable doubt as to whether the PASAT’s sensitivity can be attributed solely to a decline in information processing speed. It is more likely that lower performance on the PASAT reflects the joint effects that TBI has on decreasing speed of information processing and increasing susceptibility to interference effects. That is, it is well known that one of the effects of TBI is to make a patient more vulnerable to the effects of distracting stimuli (Kewman, Yanus, & Kirsch, 1988; Van Zomeran & Brouwer, 1987, 1994). In view of this, it appears that in addition to indexing reduced speed of information processing, the auditory versions of the PSAT may also measure the effects that TBI has on other cognitive processes that are manifested as interference effects. Thus, it is the combined effect of these two processes that makes the auditory version of the PSAT so sensitive to the effects of TBI. Since the visual Adjusting-PSAT is relatively free of interference effects, it is reasonable to assume that, in fact, the visual version may provide a purer measure of how information processing speed is affected by TBI than does the auditory version. However, this does not necessarily mean that it will have greater clinical utility than the auditory version. In fact, the reduction of the interference effects may actually end up reducing the clinical utility of the visual Adjusting-PSAT in predicting readiness for return to work in patients with TBI and perhaps other neurological disease groups as well. Unlike the results found for modality, those found for the number list were not as obvious. The initial assumption was that the inclusion of complex sums on the unrestricted Adjusting-PSAT would make the task more difficult and would thus have higher thresholds than the restricted version that only contained simple sums. However, the type of number list did not affect the threshold. This lack of effect may have been due to the unrestricted Adjusting-PSAT containing both simple and complex sums. That is, the simple sums within the unrestricted version may have been easier than the complex sums but the combination of the two may have averaged or counteracted the effects of the complex sums. To test this hypothesis, the
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performance on unrestricted-simple and unrestricted-complex sums were analyzed separately. As predicted, performance was better for unrestricted-simple than for unrestricted-complex sums. The importance of math as a modulator variable was even more apparent when it was regressed after modality and number list and accounted for 25% of threshold. These results add to the growing literature indicating that performance on the PSAT is affected by math ability (Bateman, 1997; Chronicle & MacGregor, 1998; Deary et al., 1991; Johnson et al., 1988; Sherman et al., 1997; Ward, 1997). The fact that math accounted for a greater amount of variance with the unrestricted list (33%) in comparison to the restricted list (18%) suggests that the effects of math ability can be decreased by using a number list where the answers vary between 2 and 10. In closing, the current results suggest that the Adjusting-PSAT offers many advantages that recommend its use in clinical situations. Although speculative in nature, but certainly capable of empirical verification, some of these advantages are listed below. First, the procedure used in the Adjusting-PSAT allows calculation of a threshold measure, as well as the number of correct responses. This offers a potentially more sensitive measure of cognitive impairment than does the traditional PSAT procedure where the interval between digits is not response contingent and one must rely solely on the number of correct responses. Second, having the option of using either an auditory or visual version of the Adjusting-PSAT increases the potential clinical utility of the test. That is, the auditory version may be more sensitive to the effects of TBI on both interference and information processing speed, while the visual version may be more sensitive to the effects on information processing speed alone. Third, the current results suggest that a restricted number list (sums = 2–10) should be used rather than the more traditional unrestricted number list (sums = 2–18). The net effect of using a restricted number list is that it reduces the effects of moderators variables such as math ability, potentially decreases the level of frustration by making the test easier, and reduces the time required to emit a response by making most answers a single digit number. Finally, it is possible that the Adjusting-PSAT may minimize some of the frustration and stress that is often associated with the traditional PASAT (Hugenholtz, Stuss, Stethem, & Richard, 1988; Lezak, 1995; Spreen & Strauss, 1998). This reduction may occur because with the Adjusting-PSAT a person does not have to endure a large number of trials at an interval that is clearly beyond his/her ability level, as typically occurs at the shorter intervals (e.g., 1.2 s) with the traditional PASAT.
Acknowledgments The research presented in this paper was supported by a grant from National Academy of Neuropsychology awarded to Drs. T. N. Tombaugh and L. Rees.
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Tombaugh, T. N. (1999). Administrative manual for The Adjusting-Paced Serial Addition Test (Adjusting-PSAT). Ottawa, Ont.: Ottawa Concussion Clinic, Carleton University. Van Zomeran, A. H., & Brouwer, W. H. (1987). Head Injury and concepts of attention. In H. S. Levin, J. Grafman, & H. M. Eisenberg (Eds.), Neurobehavioral recovery from head injury. New York: Oxford University Press. Van Zomeran, A. H., & Brouwer, W. H. (1994). Clinical neuropsychology of attention. New York, NY: Oxford University Press. Ward, T. (1997). A note of caution for clinicians using the Paced Auditory Serial Addition Task. British Journal of Clinical Psychology, 36, 303–307.
Brief report
The Adjusting-Paced Serial Addition Test (Adjusting-PSAT): thresholds for speed of information processing as a function of stimulus modality and problem complexity夽 J. Royan a , T.N. Tombaugh b,c,∗ , L. Rees c,d , M. Francis e a
University of Victoria, Victoria, BC, Canada Carleton University, Ottawa, Ont., Canada ON K1S 5B6 c Ottawa Concussion Clinic, Carleton University, Rm B550 Loeb Bldg., 1125 Colonel By Drive, Ottawa, Ont., Canada ON K1S 5B6 d The Rehabilitation Centre, Ottawa, Ont., Canada e University of Auckland, Auckland, New Zealand b
Accepted 3 September 2002
Abstract A modified computer version of the PASAT (Adjusting-PSAT; Tombaugh, 1999) is described that measures speed of information processing and working memory by means of a temporal threshold rather than number of correct responses. This is accomplished by making the duration of the interval between numbers depend on the correctness of responding—a correct response decreases the interval between digits and an incorrect response increases the interval. Modality of presentation (visual and auditory) was factorially combined with problem difficulty (answers between 2–10 or 2–18). Performance of 60 healthy student volunteers on the Adjusting-PSAT was compared to that obtained on several traditional neuropsychological measures (Digit Span, Trail Making Test, and Symbol Digit Modality Test) and on a test of basic addition skills. The visual version of the test produced a lower threshold than did the auditory version, but problem difficulty did not produce a significant effect. Of the neuropsychological
夽 This article was presented in part at the 21 annual meeting of the National Academy of Neuropsychology, San Francisco, CA, November, 2001. This article was based in part on a thesis by Jodie Royan submitted to Carleton University in partial fulfillment of the requirements for a Master’s degree in Psychology. ∗ Corresponding author. Tel.: +1-613-520-2659; fax: +1-613-520-3667. E-mail address: [email protected] (T.N. Tombaugh).
0887-6177/$ – see front matter © 2002 National Academy of Neuropsychology. PII: S 0 8 8 7 - 6 1 7 7 ( 0 2 ) 0 0 2 1 6 - 0
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tests, Trails-B (TMT-B) was most highly correlated with thresholds. However, regression analyses revealed that math ability accounted for more variance than did TMT-B. The clinical implications of these finding are discussed. © 2002 National Academy of Neuropsychology. Published by Elsevier Science Ltd. All rights reserved. Keywords: Information processing; Stimulus intervals; PASAT
1. Introduction The Paced Serial Addition Test (PSAT) was originally developed by Sampson (1956) to examine temporal integration. The basic procedure involved presenting a series of single digit numbers and requiring the person to add the two most recent digits. Speed of information processing was manipulated by presenting the numbers at different rates on each of four trials. The interstimulus intervals (ISIs) were 2.4, 2.0, 1.6 and 1.2 s on Trials 1, 2, 3, and 4, respectively. The number of correct responses was calculated for each trial (maximum = 60) and then combined over all four trials for a total score. Originally, Sampson (1956) developed both a visual (PVSAT) and an auditory (PASAT) version of the test. However, as a result of Gronwall and Sampson’s (1974) pioneering research using the PASAT to assess the effects of traumatic brain injury (TBI), the auditory version is now used almost exclusively in neuropsychology. Currently, the PASAT is one of the more frequently used neuropsychological tests to measure attention and concentration (Gordon & Zillmer, 1997). Evidence of its popularity is witnessed by the fact that it was recommended as the primary test to evaluate neuropsychological functions in clinical trials for multiple sclerosis (Rudick et al., 1997). In spite of its popularity, the PASAT has several weaknesses. Perhaps the most important of these is that it does not determine precisely at what speed information processing actually breaks down. Stated slightly differently, the PASAT does not provide any temporal threshold measure as to when an individual is no longer able to consistently process information. This lack of precision seriously curtails its ability to assess the initial effects that various neurological insults have on speed of information processing and to track recovery from the injury. The lack of a threshold measure can be attributed, at least in part, to the technology that was available when the Gronwall and Sampson (1974) first used the PASAT clinically. At that time, the necessity of using an audiotape to present the digits required the presentation of a fixed series of 4 ISIs and reliance on the number of correct responses as the measure of performance. The present experiment describes a computerized test that combines the basic PSAT methodology with the psychophysical stair-step/titration procedure for determining thresholds. This procedure, called the Adjusting-Paced Serial Addition Test (Adjusting-PSAT; Tombaugh, 1999), makes the ISI contingent on the “correctness” of the response. That is, the interval between numbers increases by 20 ms whenever an incorrect response occurs and decreases by 20 ms whenever a correct response is made. The continuous adjustment of the ISI allows a temporal threshold to be determined where the speed of digit presentation outstrips the
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person’s ability to process information. The shortest interval where a correct response occurs represents this threshold. Making the duration of the interval depend on the correctness of responding eliminates another methodological criticism of the original PASAT. Snyder, Aniskiewicz, and Snyder (1993) observed that individuals frequently employ a “chunking” strategy to cope with the increased difficulty that occurs when presentation speed exceeds a person’s processing capabilities. Thus, at the shorter intervals an individual may abandon any attempt to add consecutive digits and adopt the “alternate answer” strategy of adding two numbers, skipping one, adding two numbers, skipping one etc. Often this type of strategy will produce a spuriously superior performance on the original PASAT. However, it will not substantially affect the threshold score on the Adjusting-PSAT. This is because the ISI associated with the correct responses will be offset by the increased ISI produced when no response is made on the next trial. In effect, the “alternate answer” strategy will produce a non-changing, oscillating threshold when calculated over a series of trials. A third advantage of the using the Adjusting-PSAT is that it greatly reduces or eliminates the problem of making an a priori decision about what ISI intervals should be employed. Currently, a variety of different ISIs are used in research applications and clinical practice. For example, Gronwall and Sampson (1974) originally used 4 ISIs (2.4, 2.0, 1.6, and 1.2 s) with TBI patients, Deary, Langan, Hepburn, and Frier (1991) selected 4 and 2 s ISIs to assess cognitive functioning in diabetic patients, and Rudick et al. (1997) recommend that a 3-s ISI be used for clinical trials with multiple sclerosis patients. The selection of the most appropriate ISI is dictated primarily by the specific level of cognitive processing that occurs with different patients, and is based on the expected performance of the “average” patient. In many cases, however, the interval that is selected is insensitive to the level of cognitive functioning of individual patients because it is either too long or too short. This problem is largely eliminated in the Adjusting-PSAT because the threshold is individually determined regardless of the initial value. That is, if the initial ISI is too “easy” subsequent responding will progressively decrease it and if it is too “hard” subsequent responding will progressively increase it. Although the procedure used in the adjusting procedure may overcome some of the shortcomings of the traditional PASAT, it does not directly address two other fundamental questions. The first question concerns the degree that mathematical ability affects performance. There is considerable evidence to suggest that the traditional PASAT is affected by mathematical ability (Chronicle & MacGregor, 1998; Crawford, Obonsawin, & Allan, 1998; Egan, 1988; Hiscock, Caroselli, & Kimball, 1998; Sherman, Strauss, & Spellacy, 1997). Most of the evidence to support this relationship has been derived from experiments that have correlated different measures of mathematical ability with performance. However, the construct of mathematical ability is broad and encompasses many dimensions and skills that may not be directly relevant for Serial Addition Tests. The skill that appears to be most directly related to Serial Addition Tests is the ability to solve basic arithmetic problems. It is this ability that is measured in the present experiment. The basic measurement of arithmetic skills does not, however, provide any practical solution as to how to counteract the effects of arithmetic ability if, in fact, it is shown to be biasing factor. Johnson, Roethig-Johnston, and Middleton (1988) proposed one solution to this problem when they created a children’s version
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of the PASAT. They reduced the complexity of the answers by restricting them to a sum that ranged between 2 and 10 rather than between 2 and 18 as occurs in the adult version. A similar approach is taken in the present study, where the difficulty of the Adjusting-PSAT was manipulated by using two number sequences that required different levels of arithmetic ability. The second question concerns the effects of modality of presentation on performance. The numbers in the original version of the PSAT were presented visually with a slide projector (Sampson, 1956). However, due to practical circumstances, virtually all of the clinical findings have been reported using the auditory version where the digits are presented via an audiotape. As a result of this switch in presentation modality, the degree to which the clinical efficacy of the PASAT is related to the aural presentation of the stimuli is unclear. If the clinical results reflect primarily the ability to rapidly process information, then similar effects should be produced regardless of mode of presentation. However, several studies have suggested that this is not the case (Bateman, 1997; Diamond, DeLuca, Kim, & Kelley, 1997; Feinstein, Brown, & Ron, 1994; Fos, Greve, South, Mathias, & Benefield, 2000; Hiscock et al., 1998). Consequently, the current research employed both a visual (Adjusting-PVSAT) and an auditory (Adjusting-PASAT) version.
2. Method 2.1. Research participants The 60 participants were volunteers from an Introductory Psychology course and received course credit. The following exclusionary criteria were used: (1) history of neurological, mood, or psychotic disorder, (2) substance abuse, (3) antidepressant, tranquilizer, or antipsychotic drugs, or (4) head injury. Participants were randomly assigned to one of four conditions based on 2 × 2 factorial combination of mode of presentation (visual vs. auditory) and type of number list (unrestricted sums = 2–18 vs. restricted sums = 2–10) forming four groups: visual-unrestricted; visual-restricted; auditory-unrestricted; auditory-restricted. The average age of the participants was 24.6 years (S.D. = 6.2) with the age of the participants in the visual-unrestricted group slightly older (27.1 years) than those in the other three groups (23.7 years). All groups had comparable levels of education (13.6 years, S.D. = 1.3). 2.2. Materials 2.2.1. Adjusting-PSAT A series of digits from 1 to 9 were presented either visually by computer screen or aurally by headphones. A visual mask followed each of the visually presented digits to prevent the formation of after images. Aural digits were presented by a male human voice through a Windows PCM WAV file format. The ISI was controlled by the 32-bit multimedia timer function of Windows 98. The duration of the aurally presented digits varied between 321 and 398 ms. All visual stimuli were presented for a 350 ms duration.
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On each trial, the participant was required to add the two preceding digits. The participant said the sum aloud and the experimenter typed the response into the computer using the numeric keypad. The computer program was written in such a manner to compensate for the duration of time required for the experimenter to enter the responses. Throughout the test trials the duration of the ISI was determined by the “correctness” of the response. That is, the ISI was adjusted using a titrated/stairstep procedure so that each correct response produced a 20 ms decrease in the next interval and each incorrect response produced a 20 ms increase. The test series consisted of 200 trials and began with the presentation of a single-digit followed by a 2.4-s interval. The test series was preceded by a practice session of 12 trials that began with a 3.0-s interval. The same procedure was used with both the visual and auditory versions. 2.2.2. Math test The addition test from the French Kit (French, Ekstrom, & Price, 1963) was used to examine the effects of math ability on Adjusting-PSAT performance. The participants were instructed to provide the sums for a series of addition problems. Each problem consisted of three numbers (single and double digits) arranged in a column on a sheet of paper. Part one consisted of nine practice problems that were to be solved as quickly as possible. Parts two and three required the participants to complete as many addition problems as they could in 2 min. The total number of correct answers on parts two and three was used as the measure of math ability. 2.2.3. Neuropsychological tests The following three neuropsychological tests that are frequently used to measure attention and executive functioning were administered. It should be noted that none of these tests can be viewed as a “pure” measure of attention. At best, they can be conceived as possessing a large attentional component. (1) Trail Making Test Parts A and B (TMT; Reitan & Wolfson, 1985). On part A, the participant consecutively joined 25 numbers as rapidly as possible. On part B, the participant alternated between numbers and letters. (2) Symbol Digit Modality Test (SDMT; Smith, 1982). The participant was required to match numbers with the appropriate symbol. (3) Digit Span (Schmidt & Tombaugh, 1995). Participants repeated a series of progressively longer series of digits in either the same order (forward span) or in the reverse order (backward span). 2.3. Procedure The testing session took approximately 1 h to complete. Participants were introduced to experimental procedures with the informed consent form and medical health questionnaire. Following the administration of the Adjusting-PSAT, the math test and the neuropsychological tests were given in the order listed above. The order was held constant across all participants.
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3. Results 3.1. Effects modality and number list 3.1.1. Threshold Since the Adjusting-PSAT is primarily a test of speed of information processing and working memory, a threshold was chosen as the primary measure for analysis as it reflects the fastest speed at which information can be processed. The threshold was defined as the shortest ISI where a correct response occurred. A 2 (visual vs. auditory) by 2 (restricted vs. unrestricted) ANOVA revealed only a significant main effect for modality: modality, F(1, 56) = 29.6, P < .001; number list, F(1, 56) = .40, P > .05; modality × number list, F(1, 56) = .00, P > .05. Inspection of Figure 1 demonstrates that the modality effect was due to lower visual thresholds. 3.1.2. Number of correct responses A 2 by 2 ANOVA was performed on the number of correct responses to provide further evidence for the effects of modality on Adjusting-PSAT performance. Similar to the results for threshold, Figure 2 indicates that visual presentation resulted in more correct responses than the auditory presentation. No differences were observed for number list nor was there a significant interaction between modality and number list: modality, F(1, 56) = 27.5, P < .001; number list, F(1, 56) = .32, P > .05; modality × number list, F(1, 56) = .11, P > .05.
Fig. 1. The effects modality (auditory and visual) and type of number list (restricted and unrestricted) on the Adjusting-PSAT threshold.
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Fig. 2. The effects modality (auditory and visual) and type of number list (restricted and unrestricted) on the number of correct responses on the Adjusting-PSAT.
3.1.3. Correct latency Given the effects of modality on threshold and on the number of correct responses, it was expected that visual response latencies for correct responses would be shorter than the auditory latencies. As Figure 3 reveals, the anticipated relationship occurred: modality, F(1, 56) = 39.7, P < .001; number list, F(1, 56) = 3.6, P > .05; modality × complexity, F(1, 56) = .39, P = .08. 3.2. Simple versus complex sums The above analyses revealed that no significant differences existed between the two different types of number lists. This finding was rather unexpected since the restricted number list (i.e., sum = 2–10) was constructed to represent a much less challenging list than the traditional unrestricted list (i.e., sum = 2–18). However, a consistent non-significant trend of better performance for the restricted number list existed for several measures. The following analyses were undertaken to determine if failure to achieve a significant effect was due to the fact that both simple sums (2–10) and complex sums (11–18) occurred within the unrestricted number list. For these analyses, performance on the simple sums and the complex sums within the unrestricted number list was analyzed separately. This produced a “pure” simple and “pure” complex condition where a complexity effect was more likely to be observed. An ANOVA performed on baseline adjusted scores (i.e., proportions of correct responses) revealed significant main effects for modality and complexity (simple vs. complex sums):
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Fig. 3. The effects modality (auditory and visual) and type of number list (restricted and unrestricted) on correct response times on the Adjusting-PSAT.
modality, F(1, 27) = 11.44, P < .001; complexity, F(1, 27) = 60.4, P < .001; modality × complexity, F(1, 27) = 3.6, P > .05. There were proportionately more correct responses on the visual than on the auditory version, and there were proportionately more correct responses for the unrestricted-simple sums than for the unrestricted-complex sums. A second analysis was undertaken where response latencies for correct responses were used. Visual latencies were shorter than auditory latencies and latencies for unrestricted-simple sums were shorter than for unrestricted-complex sums: modality, F(1, 27) = 13.0, P < .01; complexity, F(1, 27) = 53.9, P < .001; modality × complexity, F(1, 27) = .00, P > .05. 3.3. Correlations and multiple regressions Correlational analyses were performed to determine the relationship among threshold values, math, and neuropsychological tests of attention. In Table 1, threshold values are shown separately for each modality as well as when combined. There was a strong and significant negative correlation between math scores and threshold. Furthermore, a significantly higher correlation was found between math and the auditory modality than between math and the visual modality. Of all the measures of attention, threshold was most highly correlated with TMT-B. The SDMT test was also highly correlated with threshold with the higher value occurring for the visual threshold. In order to account for threshold variance, multiple regressions were performed with modality and number list. Modality accounted for a substantial amount of variance across all groups
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Table 1 Correlations among Adaptive-PSAT thresholds, math, and neuropsychological measures of attention Threshold Measure
Combined
Auditory
Visual
Math
TMT-A
TMT-B
SDMT
Forward score
Math TMT-A TMT-B SDMT Forward score Backward score
−.63∗∗∗ .33∗∗ .60∗∗∗ −.47∗∗∗ −.13 −.31∗∗
−.74∗∗∗ .35 .53∗∗ −.36∗ −.32 −.37∗
−.53∗∗ .20 .56∗∗∗ −.54∗∗ −.12 −.41∗
−.14 −.41∗∗∗ .36∗∗ .06 .29∗
.31∗ −.11 −.09 .01
−.39∗∗ −.21 −.25
−.09 .06
.67∗∗∗
∗
Significant at P < .05. Significant at P < .01. ∗∗∗ Significant at P < .001. ∗∗
while number list accounted for very little of the variance: modality (34%): F(1, 58) = 30.5, P < .001; number list: (1%); F(1, 58) < 1; modality + number list (35%): F(1, 58) = 15.3, P < .001. Since math was highly and significantly correlated with threshold, math was entered after modality and number list to determine how much of the residual variability in the threshold could be accounted for by math ability. The results showed that math ability accounted for 26% of the threshold residual variance: F(1, 55) = 34.8, P < .001. Of particular interest was the finding that math accounted for a greater amount of variance with the unrestricted list (33%) in comparison to the restricted list (18%). The regression analysis with TMT-B scores showed that it accounted for only 18% after modality and number list: F(1, 55) = 21.5, P < .001. Finally, when TMT-B was entered after math it only accounted for 6% of the variance: F(1, 54) = 9.6, P < .05.
4. Discussion The present study clearly demonstrated that lower thresholds occurred for the visual version of the Adjusting-PSAT. The effects of modality on threshold measures were supplemented by other analyses showing that the number of correct responses was significantly higher, and the latency for correct responses was significantly shorter. Moreover, when the Adjusting-PSAT was further analyzed by comparing performance on the unrestricted-simple and unrestricted-complex numbers, significant modality and complexity effects were found for proportion of correct responses and latencies for correct responses. These findings extend previous research (Bateman, 1997; Feinstein et al., 1994; Fos et al., 2000; Hiscock et al., 1998), and suggest that the visual and auditory threshold measures may assess different cognitive processes. In the present situation, it is likely that a conflict between the stimulus input and response output produced an ‘interference effect’. That is, the aural articulation of the response may have masked the aural presentation of the digits. This explanation is similar to that posited by several other authors who have suggested that
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some type of interference effect is responsible for the lower performance on the auditory version (Bateman, 1997; Fos et al., 2000; Hiscock et al., 1998; Madigan, DeLuca, Diamond, Tramontano, & Averill, 2000). All of these researchers postulated that in the auditory version the person has access to a single information processing channel, which processes the spoken word. If only one channel is being accessed, then conflict between articulating a response and listening to the next digit could occur, particularly when ISI length becomes shorter. In fact, participants in the auditory group of the current study often commented that they had a hard time “listening to the voice” while saying their answers aloud. The effects of this interference could be reduced either by changing the output from auditory to non-auditory (e.g., pressing different response keys) or by changing the input modality from auditory to visual. The second alternative occurred in the present experiment. While it may not be easy to articulate a response and watch for the next digit, it is much easier than attempting to articulate and listen for the next digit at the same time. In this sense, because the articulated response and incoming visual stimuli are processed along separate pathways, there is less interference on the visual Adjusting-PSAT than on the auditory Adjusting-PSAT. This interference effect has important implications for interpreting the effects that TBI has had on the traditional PASAT. Although the PASAT is acknowledged to be one of the more sensitive measures of how TBI affects speed of information processing, its sensitivity is based on results obtained with the auditory presentation of the digits. The current results cast considerable doubt as to whether the PASAT’s sensitivity can be attributed solely to a decline in information processing speed. It is more likely that lower performance on the PASAT reflects the joint effects that TBI has on decreasing speed of information processing and increasing susceptibility to interference effects. That is, it is well known that one of the effects of TBI is to make a patient more vulnerable to the effects of distracting stimuli (Kewman, Yanus, & Kirsch, 1988; Van Zomeran & Brouwer, 1987, 1994). In view of this, it appears that in addition to indexing reduced speed of information processing, the auditory versions of the PSAT may also measure the effects that TBI has on other cognitive processes that are manifested as interference effects. Thus, it is the combined effect of these two processes that makes the auditory version of the PSAT so sensitive to the effects of TBI. Since the visual Adjusting-PSAT is relatively free of interference effects, it is reasonable to assume that, in fact, the visual version may provide a purer measure of how information processing speed is affected by TBI than does the auditory version. However, this does not necessarily mean that it will have greater clinical utility than the auditory version. In fact, the reduction of the interference effects may actually end up reducing the clinical utility of the visual Adjusting-PSAT in predicting readiness for return to work in patients with TBI and perhaps other neurological disease groups as well. Unlike the results found for modality, those found for the number list were not as obvious. The initial assumption was that the inclusion of complex sums on the unrestricted Adjusting-PSAT would make the task more difficult and would thus have higher thresholds than the restricted version that only contained simple sums. However, the type of number list did not affect the threshold. This lack of effect may have been due to the unrestricted Adjusting-PSAT containing both simple and complex sums. That is, the simple sums within the unrestricted version may have been easier than the complex sums but the combination of the two may have averaged or counteracted the effects of the complex sums. To test this hypothesis, the
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performance on unrestricted-simple and unrestricted-complex sums were analyzed separately. As predicted, performance was better for unrestricted-simple than for unrestricted-complex sums. The importance of math as a modulator variable was even more apparent when it was regressed after modality and number list and accounted for 25% of threshold. These results add to the growing literature indicating that performance on the PSAT is affected by math ability (Bateman, 1997; Chronicle & MacGregor, 1998; Deary et al., 1991; Johnson et al., 1988; Sherman et al., 1997; Ward, 1997). The fact that math accounted for a greater amount of variance with the unrestricted list (33%) in comparison to the restricted list (18%) suggests that the effects of math ability can be decreased by using a number list where the answers vary between 2 and 10. In closing, the current results suggest that the Adjusting-PSAT offers many advantages that recommend its use in clinical situations. Although speculative in nature, but certainly capable of empirical verification, some of these advantages are listed below. First, the procedure used in the Adjusting-PSAT allows calculation of a threshold measure, as well as the number of correct responses. This offers a potentially more sensitive measure of cognitive impairment than does the traditional PSAT procedure where the interval between digits is not response contingent and one must rely solely on the number of correct responses. Second, having the option of using either an auditory or visual version of the Adjusting-PSAT increases the potential clinical utility of the test. That is, the auditory version may be more sensitive to the effects of TBI on both interference and information processing speed, while the visual version may be more sensitive to the effects on information processing speed alone. Third, the current results suggest that a restricted number list (sums = 2–10) should be used rather than the more traditional unrestricted number list (sums = 2–18). The net effect of using a restricted number list is that it reduces the effects of moderators variables such as math ability, potentially decreases the level of frustration by making the test easier, and reduces the time required to emit a response by making most answers a single digit number. Finally, it is possible that the Adjusting-PSAT may minimize some of the frustration and stress that is often associated with the traditional PASAT (Hugenholtz, Stuss, Stethem, & Richard, 1988; Lezak, 1995; Spreen & Strauss, 1998). This reduction may occur because with the Adjusting-PSAT a person does not have to endure a large number of trials at an interval that is clearly beyond his/her ability level, as typically occurs at the shorter intervals (e.g., 1.2 s) with the traditional PASAT.
Acknowledgments The research presented in this paper was supported by a grant from National Academy of Neuropsychology awarded to Drs. T. N. Tombaugh and L. Rees.
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