Journal of Clinical and Experimental Neuropsychology

ISSN: 0168-8634 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/ncen19

Effects of severe closed-head injury on three stages of information processing Maureen E. Schmitter-edgecombe , William Marks , John F. Fahy & Charles J. Long To cite this article: Maureen E. Schmitter-edgecombe , William Marks , John F. Fahy & Charles J. Long (1992) Effects of severe closed-head injury on three stages of information processing, Journal of Clinical and Experimental Neuropsychology, 14:5, 717-737, DOI: 10.1080/01688639208402858 To link to this article: http://dx.doi.org/10.1080/01688639208402858

Published online: 04 Jan 2008.

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Journal of Clinical and Experimental Neuropsychology 1992, Vol. 14, NO.5, p ~717-737 .

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Effects of Severe Closed-Head Injury on Three Stages of Information Processing* Maureen E. Schmitter-Edgecombe, William Marks, John F. Fahy, and Charles J. Long

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Memphis State University

ABSTRACT The present study investigated the loci of the information-processing delay that characteristically follows severe closed-head injury (CHI). Stemberg’s additivefactors logic was used to determine the effects of severe CHI on the central information-processing stages of stimulus erlcoding, memory comparison, and decisionmaking/response-selection. The task variables used to define the stages operationally were stimulus quality, memory set size, and stimulus-responsecompatibility.Twenty subjects who had sustained a severe CHI more than 18 months earlier and 20 matched control subjects completed a stimulus encoding by response selection task in Experiment 1. and a Stemberg high-speed memory scanning task in Experiment 2. The CHI group performed the stimulus encoding and decision-makinghesponseselection stages of processing significantly slower than did the control group. However, no significant group differences were found on the memory comparison stage, suggesting that memory comparison processes may be relatively intact in long-term patients with severe head trauma. The results are discussed in relation to a global and a late-specificity hypothesis of central processing deficits following severe CHI. The possibility that cognitive processes demanding less attention may be more resilient to injury is also considered.

Slowing of information processing has been characterized as one of the most pronounced and persistent cognitive dysfunctions associated with severe closedhead trauma (VanZomeren, Brouwer, & Deelman, 1984). Although this slowing has been quantified by neuropsychologica1 tests that assess processing speed

* This research was supported by the Vidulich Research Fellowship awarded to the first author. Funding for the fellowship was provided by the Department of Psychology at Memphis State University through the Center of Excellence Program of the State of Tennessee. We gratefully acknowledge the assistance of Peter Endycke, Cathy McMahon, Amanda Turgeon, and Theresa Wismiller for their help in collecting and scoring the data. We also acknowledge the technical support of Jim Greer and Dennis Edgecombe. We want to thank Naftali Raz for his comments on earlier drafts of this manuscript. Correspondence concerning this article should be addressed to Maureen E. SchmitterEdgecombe, Department of Psychology, Memphis State University, Memphis, Tennessee 38152,USA. Accepted for publication: December 16, 1991.

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(e.g., MacFlynn, Montgomery, Fenton, & Rutherford, 1984; Van Zomeren & Deelman, 1976), the nature of the processing delay is unknown. A model-based cognitive science approach is used in the present study to investigate the information-processingdelay that follows severe closed-head injury (CHI). The present approach differsfrom traditional neuropsychologicalapproaches in that it goes beyond assessment of the presence or absence of a processing speed delay to explore cognitive processes that might underlie the delay. This theoretically driven approach rests on the assumption that certain experimental procedures assess specific cognitive processes. Figure 1 is a model of the major stages that have been identified empirically in a serial information-processingtask (Stemberg, 1969a; 1969b). Several studies employing choice reaction time (RT) tasks have attempted to locate the stage(s) in this serial information-processing model that are affected by severe closedhead trauma (e.g., Haut, Petros, Frank, & Lamberty, 1990; Rugg et al., 1988; 1989; Shum, McFarland, Bain, & Humphreys, 1990; Stokx & Gaillard, 1986; Van Zomeren, 1981). Although these studies have led to varying results regarding the locus of the processing delay in severely head-injured patients, they have yielded several general conclusions. First, while there is some evidence that peripheral processes associated with sensory and motor mechanisms contribute to the slower responding of patients with severe head trauma (Hannay, Levin, & Kay, 1982; Ruesch, 1944a; 1944b), empirical evidence suggests that central cognitive processes (e.g., stimulus encoding, memory comparison, and decision-makindresponse-selection) are the primary loci of the slowing (Shum et al., 1990; Van Zomeren, 1981). Second, studies employing diverse RT paradigms have consistently demonstrated inefficiencies in decision-making/response-selectionprocesses following severe head

Central Information Processes

"Late"

"Earlv"

Sensory Test Stimulus

Stimulus

Memory

S"-- Memory Set D egra~a~iw~~ Degradation Size

/ResponseSelection

P

Motor Response

S-R Compatibility Complexity

Fig. 1. The above stages appear to be the most important of the central cognitive processing stages which have been identified through systematic research manipulating combinations of task variables in a memory scanning task (Stemberg, 1969% 1969b). Below each central information processing stage is the corresponding task variable assumed to influence that stage.

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trauma (e.g., Gronwall & Sampson, 1974; Miller, 1970; Shum et al., 1990; Van Zomeren, 1981). In fact, the robustness of those findings, and the failure of early studies to find slower encoding processes, has led some to conclude that the slowness exhibited on RT tasks by CHI patients results almost entirely from an inefficiency in the late serial stage of decision-making/response-selection (e.g., Gronwall & Sampson, 1974; Miller, 1970). Current evidence, however, suggests that the stimulus encoding and memory comparison stages may also be affected by severe CHI (e.g., Curry, 1984; Haut et al., 1990; Rugg et al., 1988), indicating that a more global model of central information-processing slowing could be hypothesized. With regard to the stimulus encoding stage depicted in Figure 1, Shum et al. (1990) found that acute patients with severe CHI (less than 1 year postinjury) were impaired in encoding processes, whereas severely injured long-term patients (more than l year postinjury) were not. They interpreted this finding as indicating that difficulty with encoding processes may diminish over time following severe CHI. Stokx and Gaillard (1986) also found no impairment in the stimulus encoding stage of long-term patients with severe CHI. However, they did note a nonsignificant trend toward more prolonged RTs to degraded stimuli. In contrast to the above studies, research examining the latency of the P300 component of the event-related potentials in severe CHI has shown that P300 latency is slowed in long-term CHI patients (Curry, 1984; Deacon & Campbell, 1991). Converging evidence has suggested that the latency of the P300 component correlates well with the time taken to categorize a stimulus, but is relatively uninfluenced by response-related processes (Magliero, Bashore, Coles, & Donchin, 1984; McCarthy & Donchin, 1981). Therefore, the slowing in the latency of the P300 component following severe CHI in long-term patients is suggestive of a stimulus encoding deficit in this population. Turning to the memory comparison stage depicted in Figure 1, past studies investigating this stage have employed the Sternberg memory scanning task (e.g., Haut et al., 1990; Stokx & Gaillard, 1986). In this task, subjects are given a set of numbers or letters to remember. A test or probe item is presented following presentation of the memory set. The task is to make a manual response indicating whether the probe item belongs to the memory set. Sternberg (1969b, 1975) and other investigators (e.g., Anders & Fozard, 1973; Schmitt & Scheirer, 1977) have shown that RT to the probe item is a linear function of the number of items in the memory set. The slope of this function is assumed to reflect the time needed to scan each item held in memory and is termed memory comparison time. The intercept of the regression function is assumed to reflect processes that do not change with variations in the number of items in the memory set. These processes include encoding the probe, making a response decision, and responding. Using a varied set procedure of the Sternberg task in which a new memory set was given before each probe, Haut et al. (1990) found that memory scanning speed was slower in 12 long-term patients with severe CHI than in normal matched controls. They also found that the CHI group had larger intercept values, indicating

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a slowing in stimulus encoding processes, or response-related processes, or both. In contrast, utilizing a fixed set procedure of the Sternberg task in which each memory set was followed by the presentation of a number of single probe items, Stokx and Gaillard (1986) found that the memory comparison stage was not affected by severe head trauma. These investigators also found no difference between the groups in intercept values. These discrepant findings may reflect differences in the task procedures used to examine the memory comparison stage, i.e., a fixed set vs. varied set procedure. Alternatively, Stokx and Gaillard’s (1986) failure to find a slowing in the memory comparison stage may have resulted from the use of a small sample size (9 CHI and 9 controls). The fact that they found no significant difference in overall RT between the severe CHIpatients and controls supportsthis conjecture. Additionally, both fixed set and varied set procedures of the Sternberg task have been employed in other populations of brain-injured patients, such as mental retardates (Harris & Fleer, 1974). language-disordered children (Sininger, Klatzky, & Kirchner, 1989), and Parkinsonismpatients (Wilson, Kaszuiak, Klawans, & Garron, 1980). These studies have demonstrated a slowing in the speed of the memory comparison processes in these brain-injured populations. It would seem that the effects of severe CHI in long-term patients may not be limited to disruption of decision-making/response-selectionprocesses, but may also affect the stimulus encoding and memory comparison stages. Before such a conclusion can be accepted, however, it needs to be demonstrated that CHI and control subjects differ when manipulations of the stages are carried out with the same participants in the same experimental setting. The purpose of the present study was to examine more closely the possibility that severe head trauma results in more global central processing deficits. According to a global hypothesis, each of the central processing stages of stimulus encoding, memory comparison, and decision-making/response-selectionwould be affected by severe head trauma. Alternatively, a late-specificity hypothesis would be supported if the central processing deficit of CHI patients is more specific to the decision-makinghsponseselection stage. For clarity, the following investigation is reported as two separateexperiments. However, Experiments 1 and 2 were actually administered as part of the same research protocol. For half of the subjects in each group, Experiment 1 preceded Experiment 2, and for the other half of the subjects, Experiment 2 preceded Experiment 1. Experiment 1 was conducted to test the possibility that the stimulus encoding and decision-making/response-selectionstages are slowed following severe CHI. Experiment 2 was conducted to test the possibility that memory comparison processes are slowed following severe CHI. Sternberg’s (1969a) additive factors method was used to investigate these central stages of information processing. Sternberg’s additive-factors method assumes that stages in a serial information processing model can be isolated by manipulation of task variables. RT is considered to be the s u m of the component stage durations. According to this method, it is possible to determine whether

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variables have additive or interactive effects on RT by conducting factorial experiments. If two experimental variables affect the same processing stage, both variables will be contributors to the processing time at that stage, and their expected effect on mean RT will be an interaction. On the other hand, if two experimental variables affect two different stages, they will produce independent (additive) effects on total RT and will not interact in a statistical sense. Although some important methodological issues remain concerning the feasibility of this method (see McClelland, 1979; Pieters, 1983), the majority of results obtained with it are rather consistent (Callaway, 1983; Sanders, 1980). Those past studies that utilized Sternberg’s additive factors logic and found no impairment in the stimulus encoding and memory comparison processes of long-term patients with severe CHI had less than 10 subjects (Miller, 1970; Shum et al., 1990; Stokx & Gaillard, 1986). It is possible that there was not sufficient experimental power to detect a disproportionate slowing in stimulus encoding and memory comparison processes by the CHI group in these studies. The present study provides further investigation of these stages with a larger sample size. Additionally, the manipulation of all three central stages (stimulus encoding, memory comparison, and decision-making/response-selection)was carried out with the same subjects in the same experimental setting. We hypothesized that each of these stages would be affected by severe CHI in long-term patients, thus supporting a global slowing hypothesis.

EXPERIMENT 1. STIMULUS ENCODING AND DECISION-MAKING/ RESPONSE-SELECTION In the first experiment, we sought to clarify the effects of severe CHI on stimulus encoding and decision-makinghesponse-selection. The variables manipulated were the discriminability or quality of visual stimuli and stimulus-response (S-R) compatibility. Pilot testing of 20 neurologically normal undergraduates revealed that these manipulations had additive effects on mean RT. Therefore, the encoding and response-related processes could be investigated as separate stages. In line with a global hypothesis of the effects of severe CHI, we predicted that, within the severely injured CHI sample: (1) the cognitive operations used to complete the tasks would be qualitatively similar to the controls (i.e., that there would be no interaction between the stimulus encoding and decision-making/responseselection stages), (2) the ability to form a representation of stimuli would be slowed relative to controls, and (3) the ability to choose between response alternatives would be slowed relative to controls.

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METHOD Subjects Twenty subjects who had sustained severe CHI were recruited from the Memphis, Tennessee area. The primary means of recruitment involved contacting former clients from a neuropsychological laboratory, and advertising for head-injured individuals to either volunteer their time and/or participate in a memory training study. Coma was variously defied and inconsistently documented because the subjects were initially admitted to a variety of hospitals in the Memphis area as well as in other localities. Nevertheless, all subjects were reportedly unconscious for at least 48 hours and were, therefore, considered to have suffered severe to very severe closed-head injuries (Jennett & Teasdale, 1981). Additionally, consistent with Bond’s (1983) classification of severe CHI, duration of posttraumatic amnesia was reportedly greater than 7 days for each participant (range 8360 days, M = 125, SD = 121). All CHI subjects were more than 18 months postinjury (range 19-159 months, M = 65, SD = 38) at the time of testing and at least 15 years of age at the time of injury. The following exclusion criteria were applied: a history of multiple head injury or other neurologic disorder, history of treatment for substance abuse, less than 20/40 vision, a visual field deficit that would disrupt viewing of a computer screen. and/or severe motor deficits (such as a aemor in the upper limbs). Twenty neurologically normal undergraduate psychology students, participating as pan of a course requirement, served as control subjects. Control subjects were matched individually to the head-injured participants in terms of sex, age, level of education, and household income. Characteristics of both the patient and control group are summarized in Table 1. As can be seen from Table 1. the CHI subjects showed a typical sequelae of closed-head trauma (seeMandleberg & Brooks, 1975). That is, the CHI patients performed poorly on the Digit Symbol subtest of the Wechsler Adult Intelligence Scale-Revised (WAIS-R; Wechsler, 1981), while they were relatively unimpaired on the other WAIS-R subtests that were administered. Apparatus The experimental RT task was implemented in a set of programs written in Turbo C (Borland Corporation) and administered on an IBM compatible laptop personal computer. Subjects were seated approximately 30 cm from the computer screen. Each character of the visual stimuli was approximately 3 cm X 1.5 cm. All visual stimuli were presented in the center of the LCD computer screen. Responses to the target stimuli were recorded to the nearest millisecond. To reduce RT latencies, subjects rested their index fingers on the response keys (i.e., right index finger on the “/“ key and left index finger on the “Z”key) throughout the duration of the task. Response accuracy was also recorded.

Procedure Six different conditions were created orthogonally by varying the compatibility of the required response (compatible and incompatible) and the quality of the target stimuli (high, moderate, and low). In the compatible response condition, subjects were required to respond to the target word RITE with the right hand and the target word LEFT with the left hand. The compatible response condition was signaled by the cue word SAME.In the incompatible response condition, subjects were required to respond to the target word RITE with the left hand and the target word LEFT with the right hand. The incompatible response condition was signaled by the cue word OPPOSITE. The sequence of events on each trial (see Figure 2) was as follows: (1) A 0.5 s computer-generated auditory signal (60dB, lo00 Hz);(2) display of the cue word (SAME or OPPOSITE) for 3.0 s; (3) a 0.5 s computer-generated auditory warning tone (60dB, 1000 Hz);and, (4) presentation of the target word (LElT or RITE) 0.5 s after the warning tone ended. The target word was presented either intact (high quality condition) or degraded

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Table 1. Demographic Data and Mean Summary Data for Severe CHI and Control Groups (Standard Deviations in Parentheses). Severe CHI

Sex Male Female

Control

14 6

14 6

29.30 (8.23) 20-5 1

29.45 (9.48) 19-52

14.05 (2.26) 9-19

14.45 (1.10) 12-16

Age in Years

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M Range

Years of Education

M Range

Tapping. Dominant Hand Nondominant Hand

WAIS-R Estimated Full Scale IQb

47.95 (10.1 8>

56.40** (4.83)

45.20 (8.57)

52.35** (5.60)

95.60 ( 10.01)

102.80* (8.62)

8.40 (2.01)

10.25 (2.27)

Similarities

9.35 (2.13)

10.30 (1.49)

Arithmetic

9.10 (2.27)

9.90 (2.69)

Block Design

10.20 (2.59)

11.40 (2.11)

Digit Symbol

7.80 (2.39)

12.05** (2.11)

Digit Span

9.40 (2.39)

11.35 (2.76)

Subtest Scaled-Scores Vocabulary

~~

~~

~~

* p < -05. **p < .01. a Number of taps in a 10-s period. Vocabulary, Block Design, Arithmetic, and Similarities (Sattler, 1988).

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(moderate and low quality conditions). In the degraded conditions, stimulus quality was varied by placing a randomly generated patterned mask over the target word (Marks& Greer, 1989). This patterned mask randomly covered approximately 50% of the target word in the moderate quality condition and 75% of the target word in the low quality condition. The target word (RITE or LEFT) remained on the computer screen until the subject depressed one of the two response keys. The subjects received auditory feedback to inform them of the accuracy of their response. The subjects heard a high frequency tone for a correct response and a low frequency tone for an incorrect response. The intertrial interval was 3.0 s. All subjects were instructed to respond as rapidly as possible while maintaining a low error rate. The subjects received 120 trials preceded by 24 practice trials. The practice

A. High Quality CompatiMs Reeponee 1) 0.59

8. ModerateQuality Compatible Response

Tone

3.0S Presentationof C w Word 0.55 Wamlng TOIIO

SAME

2) 0.55 Interval V

3) Presentation of Targe3Word

LEFT

4) Response /

5) Auditory Feedbnck

'Beep'

ci

'Beep'

h '0UZZ'

6) 3.0s intenrial Interval

Fig. 2. The stimulus encoding by response selection RT task. A, B, and C represent three different experimental trials.

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trials were repeated if the subject had not mastered the demands of the task after the initial practice set. A subject-paced rest break followed every 40 trials. Both S-Rcompatibility and signal quality were selected randomly across trials, with a restriction that each target quality condition and each response compatibility condition be equally frequent. Furthermore, half of the responses in each of the six conditions were made with the right hand and the other half with the left hand.

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RESULTS AND DISCUSSION The means of the median RTs for correct responses as a function of stimulus quality (high, moderate, and low), S-R compatibility (compatible and incompatible), and group (CHI and controls) are displayed in Figure 3. For the present study, the following are the most important aspects of the data: First, in contrast to the predictions of a late-specificity hypothesis, the RTs of the head-injured subjects were slowed more than were those of the control subjects as stimulus quality decreased. This finding indicates that the CHI subjects were impaired in stimulus encoding processes. Second, the difference between the compatible and incompatible responses was larger for the head-injured participants than for the controls, suggesting that the CHI patients were impaired in decision-making/ response-selection processes. Other points to be noted from Figure 3 are that RTs increased as the quality of the stimuli was degraded with a patterned mask (see Table 2). Reaction times were slower for incompatible responses (1 107 ms) than for compatible ones (951 ms). Finally, overall RTs were considerably slower for CHI subjects (1279 ms) as compared to normal controls (780 ms).

Table 2. Group Means in Milliseconds for Compatible and Incompatible RT at each Level of Signal Quality (Standard Deviations in Parentheses). S-R Compatibility

Stimulus Quality

High

Moderate

Low

Severe CHI Compatible Incompatible

Compatible Incompatible

88 1 (244)

1146 (466)

1526 (63 1)

MAUREEN E. S C m - E D G E C O M B E ET AL. 2000

1600

CHICampatible

E

CHI Incompatible

R

ControlCompatble

I

Control Incompatible

.

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1400

13%

0

-

5%

3.5% 1.75%

5.25%

'-1

2.75%

2.25%

31

200

0

High

Moderate

Low

Stlmulus Quallty

Fig. 3. Reaction time as a function of stimulus quality and S-R compatibility for the CHI

and control groups. The numbers above the bars represent the percentage of incorrect responses in that condition. These conclusions were supported by a 2 (group) by 3 (stimulus quality) by 2

(S-R compatibility) repeated measures multivariate analysis of variance (MANOVA). This analysis yielded highly significant main effects of group, F(l, 38) = 29.03, p < .001; stimulus quality, F(2, 76) = 52.90, p < .001; and S-R compatibility, F(1, 38) = 95.56, p < .001, indicating that the control group performed better than the CHI group and that RTs were slowed by degraded stimuli and incompatible responses. When finger tapping speed and estimated Full Scale IQ were used as covariates to control for the motor and intellectual components of this processing task, the significantgroup difference remained, both Fs > 20.00.

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In addition, the analysis yielded significant interactions between group and stimulus quality, F(2, 76) = 14.14, p = .001, and group and S-R compatibility, F(1, 38) = 4.23, p < .05. These interactions indicate that the CHI subjects were impaired in both the stimulus encoding and decision-makinghesponse-selection stages of processing. There was no significant stimulus quality and S-Rcompatibility interaction, F(2, 76) = 1.27, indicating that the S-R compatibility manipulation and the stimulus quality manipulation affected separate processing stages. However, there was an unexpected higher order interaction between group, stimulus quality, and S-R compatibility,F(2,76) = 4.67, p < .05. Subsequent analysis revealed that this interaction reflects the anomalous finding that the incompatible responses of the CHI group produced only an 8% slowing at the moderate stimulus quality condition as compared to a 19% and 17% slowing at the nondegraded and low stimulus quality conditions, respectively. Furthermore, the incompatible responses of the control subjects produced a slowing of greater than 10%at each of the stimulus quality conditions. Because the previous analysis assumes a monotonic scale of measurement, a logarithmic transformation was performed on the median RT data to rule out the possibility that interactions were scale-dependent (Loftus, 1978). The three-way MANOVA was then repeated. This analysis revealed that the group and signal quality interaction, F(2,76) = 9.60, p < .001, and the three-way interaction, F(2, 76) = 4.60, p < .05,remained significant; however, the group and S-R compatibility interaction was no longer significant (F < 1). This analysis suggests that the increased encoding time of severe CHI subjects reflects an overall slowing in their ability to identify or encode a stimulus and not merely the effect of the measurement scale. This analysis also suggests that the previous finding of a significant group by S-R compatibility interaction could have resulted from a subset of subjects who had much larger median RTs in the incompatible conditions than in the compatible conditions. To examine this possibility, outliers were identified by computing simple difference scores for each subject. Individual difference scores were calculated by averaging the untransformed medians across the three levels of degradation and then subtracting the compatible average from the incompatible average. Two outliers were found in the CHI group. Both of these subjects showed the reverse pattern of larger RTs in the compatible condition (by 171 ms and 5 ms). No subject in the control group showed this pattern. This suggests that these two CHI subjects may have responded to the stimulus-response demands of the task differently than the other subjects. Additionally, over half of the CHI subjects’ simple difference scores (range from -171 to 399 ms) were higher than that of the control subject with the largest difference score (range from 21 to 219 ms). There were also no correlations between simple difference scores and the following variables in the CHI group: Coma, posttraumatic amnesia, time since injury, age, education, socioeconomic status, and IQ. Thus, there was no evidence to suggest that a specific subgroup of the CHI subjects had difficulty in the decision-making/ response-selection stage.

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The error rates were also analyzed by a repeated measures MANOVA. This analysis yielded significant main effects of stimulus quality, F(2,76) = 28.06,p < .001, and group, F(1,38)= 6.52, p < .05, indicating that error rates increased as stimulus quality decreased and were higher for the CHI participants. However, the overall error rate for both the CHI (5.4% ) and control (2.8%) group was small. A group by stimulus quality interaction was also found, F(1, 38) = 11.68, p < .001, indicating that the error rate of the CHI group increased at a greater rate than the control group as a function of stimulus quality. There was no evidence of a speedaccuracy trade-off, as both groups exhibited an increased number of errors as a function of stimulus quality and response incompatibility (see Figure 3). Both groups appear to have performed the RT task in a qualitatively similar manner, as there was no interaction between the stimulus encoding and decisionmaking/response-selectionstages for either group. The most important result of the present experiment, however, is that long-term CHI subjects are relatively inefficient as compared to controls in their ability to identify or form a representation of stimuli. Consistent with previous findings (e.g., Miller, 1970; Shum at al., 1990; Van Zomeren, 198l), the results are also suggestive of a slowing in decision-making/response-selectionprocesses following severe CHI. The present findings of a slowing at both the stimulus encoding and decision-making/responseselection stages following severe CHI are consistent with the predictions of the global hypothesis. However, the results do not support the late-specificityhypothesis that predicted a slowing specific to the decision-making/response-selectionstage.

EXPERIMENT 2. STERNBERG MEMORY SCANNING TASK The results of Experiment 1 supported the global slowing hypothesis by demonstrating that long-term severely injured patients are impaired in stimulus encoding and decision-making/response-selectionprocesses. In Experiment 2,we sought to further explore the global hypothesis by clarifying the effects of severe CHI on the memory comparison stage of processing. Memory set size was the variable manipulated in this experiment. Also, we chose to use the vaned set procedure of the Sternberg memory scanning task (SMS) since the fixed set procedure may put control subjects at an advantage. This is because control subjects could become well practiced with each memory set more quickly than head-injured subjects. In the present SMS task, digits were used as target stimuli and memory demands were kept within the short-term capacity (digit span) of the head-injured subjects. In line with a global slowing hypothesis, we expected that, within the severely injured CHI sample: (1) the speed of memory scanning would be slowed relative to controls, (2) the accuracy of memory scanning would be normal, and (3) the cognitive operations used to complete the scanning task would be qualitatively similar to the controls (i.e., an exhaustive search process in which subjects serially scan through the entire memory set before responding, irrespective of the

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location of the “yes” probe in the memory set). Such a serial search process would be reflected by linear response functions and near equality of the slopes of the “yes” and “no” functions for both the CHI and control group.

METHOD

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Subjects and Apparatus The subjects and apparatus were the same as in the previous experiment. Procedure The varied-set procedure of the Stemberg memory scanning task was utilized, with digits as the target stimuli. Six different conditions were created by varying the memory set size (1, 2, or 4 digits) and response type (“yes” or “no”). Each trial began with a 0.5 s computer-generated auditory warning signal (60 dB, lo00 Hz);this was followed by a sequential presentation of the digits in the memory set. Each digit of the memory set remained in the center of the computer screen for 1.2 s. For each trial, a different memory set of digits was presented. Stimuli were selected from the set 1 through 9 (the number 7 was excluded since it has two syllables). The end of each memory set was signaled by a 0.5 s computer-generated auditory warning tone (60 dB, lo00 Hz).The probe digit appeared 0.5 s after the warning tone ended, and remained on the screen until the subject depressed either the “yes” or “no” response key. The participants depressed the “yes” response key if the probe digit was in the memory set, and the “no” response key if it was not. All subjects were instructed to respond as rapidly as possible while maintaining a low error rate. The subjects received auditory feedback to inform them of the accuracy of their response (i.e., a high frequency tone for a correct response and a low frequency tone for an incorrect response). The intertrial interval was 3 s. The subjects received 120 trials preceded by 24 practice trials. The practice trials were repeated if the subject had not mastered the demands of the task after the initial practice set. A subject-paced rest break followed every 40 trials. Selection of the probe digit and memory set digits was random, with a restriction that each memory set size and each response type was equally frequent. For half of the subjects in each group, the key associated with their preferred hand was designated as the “yes” response; for the other half, the key associated with their preferred hand was designated as the “no” response.

RESULTS AND DISCUSSION Figure 4 displays the means of the median RTs for correct responses as a function of memory set size (1,2, and 4), response type (“yes” and “no”), and group (CHI and controls); the mean percentage of errors is given in parentheses. Overall, the mean percent error rate was 2.3 and 1.3 for the CHI and control subjects, respectively. Therefore, the task was performed with nearly perfect accuracy by both groups. The most important aspect of the present data is that the RTs of the head-injured subjects were not slowed more than were those of the control subjects as memory set size increased. This finding is inconsistent with the predictions of the global slowing hypothesis, since the results suggest that the CHI group was not significantly impaired in memory comparison processes. However, caution

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1100 1

-

1 m

-

900

-

800

r

(1 .E%)

-

--C

CHlYes

--O-

ControlYes

-

CHI&

(1.W -t-

(1 23%)

7m

-

600

-

500

-

400

! 0

1

I

1

2

3

4

ControlNo

5

Memory Set Slze

Fig. 4. Reaction time as a function of memory set size and response type for the CHI and control groups. The numbers in parentheses represent the percentage of incorrect responses in that condition.

is warranted, since there may not have been enough subjects to allow detection of a difference between the two groups at this processing stage. There are several other points to note in Figure 4. First, the overall pattern for the CHI and control groups suggests that both groups performed a serial exhaustive search of the memory set. Second, the CHI subjects had larger intercept values than did control subjects for both “yes” (718 vs. 471 ms) and “no” (849 vs. 525 ms) responses, indicating that stimulus encoding processes and/or response-related processes were slower in the CHI group than in the control group. Third, the difference between “yes” and “no” responses was greater for the CHI group (86 ms) than for the control group (29 ms), suggestingthat CHI subjects experienced

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Table 3. Group Means in Milliseconds for “Yes” and “No” RT at each Set Size (Standard Deviations in Parentheses). Response Type

Memory Set Size 1

2

4

Severe CHI

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Yes No

878 (202)

997 (233)

941

1061 (246)

( 184)

Yes No

Control 605 (125) 61 2 (124)

difficulty in the later serial processes of decision-makingjresponse-selection. Fourth, RTs increased as memory set size increased and were longer for “no” responses than for “yes” responses (see Table 3). Finally, overall RTs were considerably slower for CHI subjects (957 ms) as compared to normal controls (61 2 ms). These conclusions were supported by a 2 (group) by 3 (memory set size) by 2 (response type) repeated measures multivariate analysis of variance (MANOVA). This analysis yielded highly significant main effects of group, F(1, 38) = 38.01, p < .001; memory set size, F(2,76) = 102.51, p < .001; and response type, F(1,38) = 30.77, p < .001, indicating that the control group performed better than the CHI group, and that RTs were slowed by increases in memory set size and “no” responses. When finger tapping speed and estimated Full Scale IQ were used as covariates to control for the motor and intellectual components of this processing task, the significant group difference remained, both F s > 25.00. In addition, the analysis yielded a significant interaction between group and response type, F(1, 38) = 7.63, p < .01, suggesting that the CHI group was impaired in decision-makinglresponse-selectionprocesses. A significant response type and memory set size interaction was also found, F(2,76) = 8.74, p < .001. A subsequent analysis of this interaction revealed that the list length of 1 in the “no” condition produced abnormally long RTs in some subjects in both groups. It is likely that these subjects had difficulty in forming the concept of a list with only one element, and thus were inadequately prepared when the probe digit appeared. A similar result was reported by Marsh (1975) when testing an elderly population. None of the remaining interactions between group and any of the other variables approached significance, all F s < 1.70.

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MAUREEN E. SCHMIlTlB-EDGECOMBEET AL.

Table4. T-test Analysis on Mean Slope and Mean Y-Intercept Data of the CHI and

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Control Groups.

Response Type

GroupP

M

SD

Yes

CHI

70

41

Control

55

30

CHI

38

42

Control

34

42

CHI

745

185

Control

489

123

CHI

939

290

Control

584

160

No

T

df

P

1.32

38

.20

.32

38

.75

5.15

38

.001

4.80

38

.001

Y-intercept Yes

No

~~~~~

‘n = 20 for each

group.

As an alternative method of analysis, a least squares regression analysis was computed separately for each subject at each response type using memory set size as the predictor variable and response type as the dependent variable. For each individual subject, response times larger or smaller than two and one-half standard deviations from the mean RT in each treatment condition were removed. The purpose of this analysis was to compute a slope and intercept value for both the “yes” and “no” response functions for each subject. Comparisons of the mean slopes and intercepts of the two groups using t tests are displayed in Table 4. The data reveal that the CHI group had significantlylarger mean zero-intercepts for both the “yes” and “no” functions, whereas no significant group differences were found for the mean slopes of either response function. These results replicate the results of the MANOVA of group RT data presented above. Correlations between Stemberg measures (slope and intercept) and injury variables (coma, posttraumatic amnesia, and time since injury) were also computed for the CHI group. No significant correlations were found. The present data show an essential similarity in the type of processing of the two groups, since the shape of the RT functions and the error patterns were similar across the groups. Consistent with the findings of Experiment 1, the present group differences in intercept values indicate impairment of stimulus

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encoding and/or response-selection processes in the CHI group. The larger differences between “yes” and “no” responses for the CHI group indicates impairment in the late serial processes of decision-makinglresponse-selection. The most important result of the present experiment, however, is that the RT of the CHI group was not slowed significantly as compared to that of the control group as memory set size increased. This finding is inconsistent with the predictions of the global slowing hypothesis, and suggests that memory comparison processes may be relatively intact in long-term patients with severe CHI.

GENERAL DISCUSSION The results of this study do not support the late-specificity hypothesis which suggests that the effects of severe CHI in long-term patients are limited to impairment at the stage of decision-makingjesponse-selection. Also, the results do not appear to support the global hypothesis, which suggests that the effects of CHI result in impairment at all central processing stages. Consistent with both hypotheses, the CHI group exhibited significant impairment in decision-making/ response-selection processes relative to controls in both Experiments 1 and 2. However, in contrast to the predictions of the late-specificity hypothesis, the CHI group exhibited a deficiency in stimulus encoding processes in Experiment 1. And, in contrast to the predictions of the global hypothesis, there was no significant difference between the groups in memory comparison processes in Experiment 2. It is possible, however, that there were not enough subjects in the present study to detect a group difference in memory comparison processes. (To detect a significant group difference in the “yes” slopes at a power of .80 with the sample size used in the present study, the difference in “yes” slopes between the CHI and control groups needed to be approximately 35 ms instead of the present finding of 15 ms.) Alternately, the memory comparison stage may exhibit some resiliency to the effects of severe head trauma. Finally, in both experiments, the CHI group completed the RT tasks by employing processes qualitatively similar to the control group. While RT studies have established that long-term patients with severe CHI are impaired in decision-makinglresponse-selectionprocesses (Gronwall & Sampson, 1974; Miller, 1970 Shum et al., 1990; Van Zomeren & Deelman, 1976), the impairment in the stimulus encoding processes observed in the present study is a new finding. Although Shum et al. (1990) found an impairment in the encoding stage of CHI subjects who were less than one year post injury, they did not find a stimulus encoding deficit in long-term patients with severe CHI. The results of the present study suggest that a deficit in the efficiency of the encoding stage may exist even during the residual stages of severe CHI. These contrasting findings may reflect differences in the RT tasks used. The present experiment required subjects to identify degraded stimuli whereas Shum et al.’s experiment required visuo-spatial discrimination. Alternatively, it is possible that Shum et

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al. did not have significant experimentalpower to detect a disproportionate slowing by the CHI group at the stimulus encoding stage. The results of this study also appear to suggest that memory comparison processes may be relatively intact in long-term patients with severe CHI. The linear shapes of the response functions and the near equality of the slopes of the “yes” and “no” functions for both the CHI and control group, suggest that both groups used a similar serial exhaustive search process to scan items in short-term memory (STM) in Experiment 2. Previous studies have demonstrated that severe CHI subjects perform normally on measures of STM span (e.g., forward digit span; Baddeley, 1976; Mandleberg & Brooks, 1975) and have normal rates of forgetting from STM (Baddeley, Hams, Sunderland, Watts, & Wilson, 1987). The results of the present study suggest that long-term severely head-injured subjects may also have an intact rate of accessing information from STM. The present finding of no significant difference between the CHI and control groups in RTs as memory set size increased is in contrast to the results of Haut et al. (1990). Haut et al. found that the Stemberg slopes of a control goup were 70 ms faster than the slopes of a severe CHI group. With the present sample size, the power was high (approximately .88) for detectionof a group difference in memory comparison processes equal in effect size to that which Haut et al. obtained. Although Haut et al. employed memory set sizes of 2, 4, and 6, they found the greatest increase in group difference between set sizes 2 and 4. This result rules out memory set size as the reason for the contrasting conclusions between their study and the present one. However, the contrasting results may be attributable to methodological differences in stimulus presentation. Haut et al. used a blocked presentation of set sizes, whereas a random presentation of set sizes was used in the current study. Alternatively, the CHI sample of Haut et al. may have been more severely impaired. Their subjects exhibited an approximate 9% error rate with set size of 4, whereas the error rate in the current study was only 3.1% at set size of 4. Similar findings of intact memory comparisonprocesses and impaired encoding and response-related processes have been found to occur with aging (see Bashore, 1990 for a review). Based on a multiple resource model of attention (Navon & Gopher, 1979), one explanation for the relative intacmess of memory comparison processes is that these processes may be less attention-demanding(more automatic) than are stimulus encoding and response-related processes, the latter processes may stress the reduced attentional resources of the CHI population. Currently, more research is needed to discover whether the relative intacmess of the memory comparison stage can be replicated in this population and found in other groups of head-injured patients, such as acute patients with severe CHI (less than 1 year postinjury). Limitations to the relative intactness of the memory comparison stage in long-term patients with severe CHI should also be explored. For example, perhaps constraints may be found in memory comparison processes if the task requires larger memory set sizes, or words, rather than single digits as stimulus items.

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The present study’s results are inconsistent with the predictions of the global slowing hypothesis and appear to suggest that some processes, such as the ability to scan items in STM, may be more resilient to the effects of severe CHI. The findings of a slowing in both the stimulus encoding and decision-makinghesponseselection processes following severe CHI are also inconsistent with the predictions of the late-specificity hypothesis. Perhaps, the central processes that remain relatively intact in long-term patients with severe CHI are those that place the least demands on attentional resources for their execution. However, a word of caution concerning the present conclusions is warranted. The present results are based on a relatively small sample of 20 severe CHI subjects and, therefore, may need to be replicated with a larger sample size as well as with a different control poup. Although the CHI subjects performed both RT tasks in a manner that was qualitatively similar to that of the control subjects, college students may differ from the general population in both interest in intellectual tasks and desire to perform well on them. Understanding the information processing deficits of CHI patients at this foundational level is important, since more complex problem-solvingand heuristic reasoning tasks depend on the intactness of these lower level processes. The chronic nature of this processing delay suggests that rehabilitation efforts need to be aimed at compensation. For example, because the CHI subjects’ ability to encode and respond to stimuli is slower and less efficient, restructuring the CHI patient’s environment so information is presented clearly and choices (responses) are made easy, might prevent information overload to the processing system of the CHI patient. Finally, the possibility of utilizing the relatively more intact memory comparison processes in rehabilitation strategies provides a challenge that should be explored.

REFERENCES Anders. T.R.. & Fozard, J.L. (1973). Effects of age upon retrieval from primary and secondary memory. Developmental Psychology, 9,411-415. Baddeley, A.D. (1976). The psychology of memory. New York Basic Books. Baddeley, A., Harris, J., Sunderland, A., Watts, K.P., & Wilson, B.A. (1987). Closed head injury and memory. In H.S. Levin, J. Gratman, & H. M. Eisenberg (Eds.), Neurobehavioral recovery from head injury (pp. 295-3 17). New York Oxford University Press. Bashore, T.R. (1990). Age-related changes in mental processing revealed by analysis of event-related brain potentials. In J.W. Rohrbaugh, R. Parasuraman, & R. Johnson (Eds.), Event-related brainpotentials: Basic issues and application (pp. 242-275). New York Oxford University Press. Bond, M.R. (1983). Standardized methods of assessing and predicting outcome. In M. Rosenthal, E. Griffith, M. Bond, & J. Miller (Eds.), Rehabilitationof head injured adults (pp. 185-196). Philadelphia: Davis. Callaway, E. (1983). The pharmacology of human information processing. Psychophysiology, 20,359-370. Curry, S.H. (1984). Contingent negative variation and slow waves in a short interstimulus

Downloaded by [York University Libraries] at 20:36 05 November 2015

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interval Go/No-GOtask situation. Annals of New York Academy of Sciences, 425,171176. Deacon, D.. & Campbell, K.B. (1991). Effects of performance feedback on P300 and reaction time in closed head-injured outpatients. Electroencephalography and Clinical Neurophysiology, 78, 133-141. Gronwall, D.M.A., & Sampson. H. (1974). The psychological effects of concussion. Auckland: Auckland University Press. Hannay, H.J., Levin, H.S., & Kay, M. (1982). Tachistoscope visual perception after closed head injury. Journal of Clinical Neuropsychology, 4,117-129. Harris, G.J., & Fleer, R.E. (1974). High speed memory scanning in mental retardates: Evidence for a central processing deficit. Journal of Experimental Child Psychology, 17, 452-459. Haut, M.C., Petros, T.V., Frank, R.G., & Lamberry, G. (1990). Short-term memory processes following closed head injury. Archives of Clinical Neuropsychology, 5,299-309. Jennett, B., & Teasdale, G. (1981). Management of head injuries. Philadelphia: F.A. Davis. Loftus, G.R. (1978). On interpretation of interactions. Memory and Cognition, 6.3 12-319. Marks, W., & Greer, J.B. (1989). A set of color graphics routines for studying perceptual learning on the IBM PC. Behavior Research Methods, Instruments, and Computers, 21, 574-578. MacFlynn, G.. Montgomery, E.A., Fenton, G.W., & Rutherford, W. (1984). Measurement of reaction time following minor head injury. Journal of Neurology, Neurosurgery, and Psychiatry, 47, 1326-1331. Magliero, A., Bashore, T.R., Coles, M.G.H., & Donchin, E. (1984). On the dependence of P300 latency on stimulus evaluation processes. Psychophysiology, 21, 171-186. Mandleberg, I.A., & Brooks, D.N. (1975). Cognitive recovery after severe head injury. 1. Serial testing on the Wechsler Adult Intelligence Scale. Journal of Neurology, Neurosurgery, and Psychiatry. 38, 1121-1126. Marsh, G.R. (1975). Age differences in evoked potential correlates of a memory scanning process. Experimental Aging Research, I, 3-16. McCarthy, G., & Donchin, E. (1981). A metric for thought: A comparison of P300 latency and reaction time. Science, 311.77-80. McClelland, J.L. (1979). On the time relations of mental processes: An examination of processes in cascade. Psychological Review, 86,287-330. Miller, E. (1970). Simple and choice reaction time following severe head injury. Cortex,6, 121-127. , Navon. D., & Gopher, D. (1979). On the economy of the human processing system. Psychological Review, 86,214-255. Pieters. J.P.M. (1983). Stemberg’s additive factor method and underlying psychological processes: Some theoretical considerations. Psychological Bulletin. 93.41 1-426. Ruesch, J. (1944a). Intellectual impairment in head injuries. American Journal of Psychiatry, 100,480-496. Ruesch, J. (1 944b). Dark adaptation, negative after images, tachistoscopic examination, and reaction time in head injuries. Journal of Neurosurgery, I , 243-251. Rugg, M.D., Cowan, C.P., Nagy, M.E., Milner, A.D., Jacobson, I., & Brooks, D.N. (1988). Event related potentials from closed head injury patients in an auditory “oddball” task: Evidence of dysfunction in stimulus categorization. Journal of Neurology, Neurosurgery, and Psychiatry, 51,691-698. Rugg. M.D., Cowan, C.P., Nagy, M.E., Milner. A.D., Jacobson, I., & Brooks, D.N. (1989). CNV abnormalities following closed head injury. Brain, 112,489-506. Sanders, A.F. (1980). Stage analysis of reaction time. In G.E. Stelmach & J. Requin (Eds.), Tutorials in motor behavior (pp. 33 1-354). Amsterdam: Elsevier North-Holland Publishing Co.

Downloaded by [York University Libraries] at 20:36 05 November 2015

SEVERE HEAD TRAUMA AND INFORMATIONPROCESSING

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Sattler, J.M. (1988). Assessment of children (3rd ed.). San Diego: Jerome M. Sattler, Publisher. Schmitt, J.C., & Scheirer, C.J. (1977). Task difficulty as a mediator of subject strategy in memory scanning. American Journal of Psychology, 90, 565-575. Shum, D.H.K., McFarland, K., Bain, J.D., & Humphreys, M.S. (1990). Effects of closedhead injury on attentional processes: An information-processing stage analysis. Journal of Clinical and Experimental Neuropsychology, 12,247-264. Sininger, Y.S., Klatzky, R.L., & Kirchner, D.M. (1989). Memory scanning speed in language-disordered children, Journal of Speech and Hearing Research, 32,289-297. Stemberg, S . (1969a). The discovery of processing stages: Extensions of Donders’ method. Acta Psychologica 30, 276-3 15. Sternberg, S. (1969b). Memory scanning: Mental processes revealed by reaction-time experiments. American Scientist, 57,421 -457. Stemberg, S. (1975). Memory scanning: New findings and current controversies. Quarterly Journal of Experimental Psychology, 27, 1-32. Stokx, L.C., & Gaillard, W.K. (1986). Task performance of patients with a severe concussion of the brain. Journal of Clinical and Experimental Neuropsychology, 8,421-436. Van Zomeren, A.H. (1981). Reaction time and attention after closed head injury. Lisse: Swets & Zeitlinger. Van Zomeren. A.H., Brouwer, W.H., & Deelman, B.G. (1984). Attentional deficits: The riddles of selectivity, speed, and alertness. In N. Brooks (Ed.), Closed head injury: Psychological, social, and family consequences (pp. 74-107). Oxford: Oxford University Press. Van Zomeren, A.H., & Deelman, B.G.(1976). Differential effects of simple and choice reaction time after closed head injury. Clinical Neurology and Neurosurgery, 79,8190. Wechsler, D. (1981). Manual of the Wechsler Adultlnrelligence Scale-Revised. San Antonio: The Psychological Corporation. Wilson, R.S.. Kaszniak, A.W., Klawans, H.L., & Garron, D.C. (1980). High speed memory scanning in parkinsonism. Cortex, 16,73-82.