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The Effect of ModerateLevels of Simulated Altitudeon Sustained CognitivePerformanceCathy J. Bartholomew , Warren Jensen , ThomasV. Petros , F. Richard Ferraro , Kevin M. Fire , DaveBiberdorf , Earl Fraley , Joe Schalk & Dave BlumkinPublished online: 13 Nov 2009.

To cite this article: Cathy J. Bartholomew , Warren Jensen , Thomas V. Petros , F.Richard Ferraro , Kevin M. Fire , Dave Biberdorf , Earl Fraley , Joe Schalk & DaveBlumkin (1999) The Effect of Moderate Levels of Simulated Altitude on SustainedCognitive Performance, The International Journal of Aviation Psychology, 9:4, 351-359,DOI: 10.1207/s15327108ijap0904_3

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THE INTERNATIONAL JOURNAL OF AVIATION PSYCHOLOGY, 9(4), 351-359 Copyright Q 1999, Lawrence Erlbaum Associates, Inc.

The Effect of Moderate Levels of Simulated Altitude on Sustained

Cognitive Performance

Cathy J. Bartholomew Deportment of Psychology University of North Dakota

Warren Jensen Department of Aviation

University of North Dakota

Thomas V. Petros and F. Richard Ferraro Department of Psychology University of North Dakota

Kevin M. Fire and Dave Biberdorf Department of Communication Sciences and Disorders

University o f North Dakota

Earl Fraley, Joe Schalk, and Dave Blumkin Department of'Aviation

University of North Dakota

Previous work has documented cognitive deficits at high altitudes (15,000-25,000 ft), but there is controversy for lower altitudes. This study looked at the effects of moderate altitudes-1 2,500 ft and 15,000 ft--on short-term memory in comparison to 2,000 ft. Seventy-two student pilots and instructors were first administered the Vo- cabulary, Digit Span, and Digit Symbol subtests from the Wechsler Adult Intelli-

Requests for reprints should be sent to Thomas V. Petros, Department of Psychology, University of North Dakota, Grand Forks, ND 58202-8380.

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gence Scale-Revised, the Vandenberg Mental Rotation Test, and the near-contrast sensitivity portion of the Vistech VCTS 6000 chart. Participants then spent 1 % hr at their designated altitude for cognitive testing. Participants performed a 30-min vigi- lance task while listening to an audiotape with instructions to recall radio calls pref- aced by their assigned call sign. Half of the radio calls were high memory loads (at least 4 pieces of information), and half were low memory loads (no more than 2 pieces of information). No effects of altitude were found in performance on the Vigilance task. However, for readbacks of high memory load, significant deficits in recall were observed at 12,500 ft and 15,000 ft, whereas no effect of altitude was observed on re- call of readbacks with low memory loads. These results indicate that, at altitude, short-term memory was exceeded for the readbacks requiring a larger amount of in- formation to be recalled, and that cognitive deficits are found at lower altitudes than previously observed.

The impact of altitude on human performance is a critical concern in the field of avi- ation. At higher altitudes, humans suffer from a variety of symptoms that result from oxygen deficiency in the blood. These symptoms of hypoxia can include headaches, dizziness, nausea, concentration difficulties, and, in more severe cases, impaired vision and impaired cognitive processes (Bonnon, Noel-Jorand, & Theme, 1995; Fowler, Paul, Porlier, Elcombe, & Taylor, 1985; Fowler, Prlic, & Brabant, 1994).

The impact of hypoxia on cognitive performance has been examined using three methods of inducing hypoxia. One method consists of testing cognitive perfor- mance at several altitudes during a mountain climb. For example, Kramer, Coyne, and Strayer (1993) tested mountain climbers (on Mount Denali in Alaska) at 3,028 ft, at 14,301 ft after their attempt at the summit (which is 20,3 16 ft), and again after theirreturn to 3,028 ft. A variety of cognitive tasks were administered on each testing session. The cognitive tasks also were administered to a control group at intervals comparable to those of the experimental group. The results indicated that experi- mental participants performed worse than controls on most cognitive tasks. Also, performance of the control group improved with repeated testing, whereas no such improvement in performance was observed for the experimental group. The results of Kramer et al. demonstrated the importance of a control group for the proper inter- pretation of research in this area. If a control group had not been used, this study would have erroneously concluded that altitude has no impact on performance.

One limitation of the generalizability of mountaineering studies to the aviation environment is that adverse environmental conditions encountered during a moun- tain climb also could contribute to the cognitive deficits observed. Therefore, other researchers have used breathing mixtures to induce hypoxia while at sea level. For example, Fowler et al. (1 994) had participants breathe mixtures of oxygen and nitro- gen to reduce SaOs (arterial oxyhemoglobin) saturation level in the blood to 64% to 66% (1 3,900-14,400 ft); in a separate testing condition,participants were allowedto breathe normal air. Participants were tested under both conditions in one session

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ALTITUDE AND COGNITNE PERFORMANCE 353

with a 20-min interval when switching from one condition to the other. Each condi- tion (hypoxic and normoxic) provided 20 min of breathing prior to the onset of cog- nitive testing. Short-term memory was tested in each condition using the Sternberg (1969) memory scanning task. In that task, participants were given memory sets of two, four, or six digits to memorize. Participants were then given a probe item and asked to respond as quickly as possible as to whether the probe was a member of the memory set. The slope ofthe line relating memory set size to response time is used as an estimate oftherate of scanning short-term memory. The results indicatedthat par- ticipantsresponded significantly slower in the hypoxic thannormoxic condition, yet the rate of scanning short-term memory was similar in both groups.

One limitation of the study by Fowler et al. (1994) was the brief time between hypoxic and normoxic conditions, which may have resulted in carryover effects. Although the authors reported no significant order effects, the analysis of order ef- fects involved only 6 participants in each between-subject condition. One limita- tion of previous examinations of the effect of hypoxia is that altitude was most often manipulated as a within-subjects factor. The potential for practice effects and carryover effects in this situation was clearly documented by Kramer et al. (1993) and Bonnon et al. (1 995). This has led to a complicated pattern of results and possi- bly an underestimation of the impact on aviators of hypoxia at lower altitudes. The purpose of this study was to examine the effect of moderate altitude on short-term memory using three groups of participants: one group tested at 15,000 fi, one group tested at 12,500 ft, and a control group tested at 2,000 ft.

METHOD

Participants

Participants were 72 volunteer students and flight instructors, 59 men and 13 women, from the Center for Aerospace Sciences at the University ofNorth Dakota. All participants were required to either have or be working on instrument ratings and to have completed a course in aerospace physiology. Demographic characteris- tics and flight experience measures are presented in Table 1.

Materials

When participants reported to the chamber, they were asked to complete a consent form and then a series of pretests. One measure was the vocabulary subtest of the Wechsler Adult Intelligence ScaleRevised (WAIS-R; Wechsler, 198 l), in which participants are read 35 vocabulary words of increasing difficulty and asked to pro- vide a definition orally. The Digit Span subtest from the Wechsler Memory

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354 BARTHOLOMEW ET AL.

TABLE 1 Means and Standard Deviations for Demographic Variables

- - - -- - -

2, 000fP 12,500 fP 15,000 fP

Measure M SD M SD M SD F

Age 21.8 1.73 23.7 4.16 22.1 1.91 3.17 Height 70.8 3.96 70.6 3.56 69.9 2.84 0.46 Weight 173.7 27.01 170.8 32.96 177.2 30.53 0.27 Flight hours 346.2 354.10 533.9 664.20 487.8 551.80 0.81 Instrument hours 52.1 42.70 53.4 46.00 71.6 66.40 1.03

Scale-Revised also was administered. In that test, participants are read a series of digits in groups of two to nine and must recall them, the first time in order and then, in the second half of the test, in reverse order (Wechsler, 1987). The Vandenberg Test of Mental Rotation (Vandenberg & Kuse, 1987) also was administered. In that test, participants look at one geometrical figure and are asked to pick, out of four choices, the two that are the same but rotated fiom the original figure. Finally, the Digit Symbol subtest of the WAIS-R was given, which presents participants with the digits one through nine, each paired with a symbol. Participants then have a list of 92 numbers under which they must put the appropriate symbol as quickly as pos- sible. Participants' scores on the vocabulary, mental rotation, digit symbol, and digit span measures are presented in Table 2.

Procedure

Participants were tested at either 2,000 A, 12,500 ft, or 15,000 ft, ascending to their target altitude at a rate of 1,700 ft per minute. The testing was performed in a hyperbaric chamber in the Center for Aerospace Sciences at the University ofNorth Dakota and was administered on Apple IIe computers. The participants were tested in groups of up to 4 participants simultaneously. Fifteen minutes after reaching their target altitude, participants began a 30-min vigilance task in which they were presented with a series of digits, one digit every 2 sec. Their task was to press the space bar on the computer whenever an "8" was presented that had been preceded by a "3." Errors of omission, errors of commission, and response latencies for cor- rect responses were recorded by the computer. During the vigilance task, partici- pants also listened to a series of 80 radio calls that had been prerecorded by an air traffic controller. Each radio call was preceded by one of four call signs, and partici- pants were asked to recall verbatim the radio call if it had been preceded by their call sign. Participants were asked to recall the radio call orally. The recalls were taped for later transcription and scoring. Half the radio calls placed a high memory load

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ALTITUDE AND COGNITIVE PERFORMANCE 355

on the participant (4-5 bits of information) and half the radio calls placed a low memory load on the participant (1-2 bits of information). An example of a high memory load readback was "Piper 3627, turn right heading 230, descend and main- tain at or above 3,600 until established on the localizer, cleared for the ILS [interna- tional landing system] runway 26." An example of a low memory load readback was "Piper 882, squawk 2427." Each participant was provided with a pencil and pa- per to write down any information they chose.

RESULTS

To examine whether our groups were different on the variety of individual differ- ence measures we obtained, a series of one-way analyses of variance (ANOVAs) were conducted on age, height, weight, number of flight hours, number of instru- ment hours, vocabulary scores, mental rotation scores, digit symbol scores, and digit span-forward and digit span-backward scores (see Tables 1 and 2). The only significant difference observed was for age, F(2,69) = 3 . 1 7 , ~ < .05. A subsequent Tukey HSD revealed that the average age of the participants tested at an altitude of 12,500 ft was significantly greater than the age of the participants tested at either 2,000 or 15,000 ft. When we removed one extreme age score of 40 from the group at 12,500 ft, group differences in age were no longer significant, F(2,68) = 2.19, p > .05. However, this person's data were included in all subsequent analyses.

The bivariate correlations between the number of flight hours, number of in- strument hours, vocabulary scores, mental rotation scores, digit symbol scores, digit span scores, and the recall scores for the radio calls are presented in Table 3. An examination of Table 3 indicates that digit span-forward and -backward were significantly correlated, recall of radio calls with a high memory load was signifi- cantly correlated with recall of radio calls with a low memory load, and mental ro- tation scores were significantly correlated with recall of radio calls with a low memory load.

TABLE 2 Means and Standard Deviations for Individual Difference Variables

Measure -- -- -- -- - - - - -

Vocabulary 49.0 6.16 51.3 6.82 53.3 7.29 2.51 Mental rotation 21.9 8.76 19.8 8.85 19.3 5.95 0.76 Digit symbol 73.0 11.44 72.7 8.92 73.8 10.10 0.07 Digit span (forward) 10.0 1.91 8.9 2.00 9.4 2.48 1.54 Digit span (backward) 8.6 2.41 8.1 1.50 7.8 2.89 0.82

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Recall of the readbacks was scored blind by two independent raters. The per- centage of agreement ranged from 8 1.3% to loo%, with the average agreement be- ing 96.89%. The proportion of readbacks correctly recalled (see Table 2) was then subjected to a 3 (altitude) x 2 (memory load) mixed ANOVA, with altitude as a be- tween-subject variable and memory load as a within-subjects variable. The ANOVA revealed a significant main effect of memory load, F(l ,55) = 2 5 5 . 4 5 , ~ < .Ol, along with an Altitude x Memory Load interaction, F(2, 55) =5.64, p c.0 1 . The basis of this interaction was examined using a Tukey-Kramer test of painvide contrasts (Kirk, 1995). The results of these comparisons revealed that altitude did not significantly affect recall of the low memory load readbacks, but for the high memory load, recall at both 12,500 ft and 15,000 ft was significantly lower than re- call at 2,000 ft (see Table 4).

The number of correct responses (inverse of errors of omission), number of er- rors of commission, and the response latencies for the number of correct responses

TABLE 3 Correlations Among Individual Difference Variables and Readback Measures

1. Vocabulary 2. Mental rotation 3 . Digit symbol 4. Digit span (fornard) 5 . Digit span (backward) 6. RB-high 7. RB-low 8. Flight hours 9. Instrument hours

Note. RB = readbacks of radio calls. *p < .05.

TABLE 4 Means and Standard Deviations for Percentage Correct

for Readback as a Function of Load and Altitude

Memory Load

High Low

Altitude M SD M SD

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ALTITUDE AND COGNITIVE PERFORMANCE 357

TABLE 5 Median Response Latencies (in milliseconds) and Mean Number Correct, Both as a Function of

Altitude and Blocks for the Vigilance Task

Block

I 2 3 4 5 6

A[titude M SD M SD M SD M SD M SD M SD

Response latencies for correct responses 2,000 f? 365 104 387 125 403 161 402 156 441 212 444 200 12,500 f? 377 114 427 138 391 110 387 118 382 88 382 122 15,000ft 382 105 405 133 444 189 464 164 447 181 452 175

Number correct 2,000 f? 7.778 1.478 8.455 0.93 8.182 1.46 7.864 1.490 7.955 1.588 9.455 0.800 12,500ft 8.444 1.247 7.833 1.08 8.056 1.43 8.000 1.815 8.667 1.328 9.167 0.924 15,OOOfi 7.333 1.857 7.458 2.05 7.917 1.50 7.542 1.382 7.625 1.663 8.792 1.250

(see Table 3) were computed separately for each 5-min block of the vigilance task. A 3 (altitude) x 6 (block) mixed ANOVA was conducted separately for each of these measures. Altitude was a between-subject variable, and block of testing was a within-subjects variable. The analyses revealed no significant effects for the number of correct responses and errors of commission. The ANOVA of the re- sponse latencies only revealed a significant main effect of block, F(5,305) = 2.98, p < .05, indicating that latencies were slower across blocks (see Table 5).

DISCUSSION

The main finding of this study was the significant effect of altitude on recall of readbacks during high memory load. Because there were no significant altitude ef- fects for low memory loads, it is unlikely that the difference for high memory loads was due to some physical factor such as diminished auditory sensitivity. It is inter- esting that, even at moderate altitudes, differences were observed only for readbacks of high memory load. This suggests that, at altitude, short-term memory was exceeded for the readbacks requiring a larger amount of information to be re- called, but short-term memory was not exceeded for the same amount of informa- tion at the control altitude. Information-processing theorists have argued that hu- mans have a limited pool of cognitive resources to process information. Many factors, such as alcohol, fatigue, and circadian variations (Petros, Beckwith, &An- derson, 1990; Petros, Kerbel, Beckwith, Sacks, & Sarafolean, 1985), can influence the amount of cognitive resources available at any given time to process informa- tion. The results of the readback task in this study indicate that altitude also may in- fluence the amount of cognitive resources available to process information. This

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could lead to dangerous situations, such as missed indications of engine problems, incorrect reading of instruments, and added difficulty in handling unusual situa- tions, such as extreme weather conditions or emergencies. This study suggests that civil aviators may be more susceptible to accidents while flying without supple- mental oxygen at, or even after flying at, altitudes the Federal Aviation Administra- tion (FAA) finds acceptable. If the effects last into descent and approach and are coupled with a high workload situation, the risk of an accident might increase.

Performance on the vigilance task was not affected by a lack of oxygen result- ing from the simulated altitude. This suggests that a lack of oxygen alone does not impair the basic skills of monitoring and attending to a single channel of informa- tion for a sustained period of time. Typically, when navigating aircraft, pilots are monitoring several channels of information simultaneously while monitoring ra- dio calls. Possibly, the readback deficits observed in this study would have been magnified if the vigilance task required the simultaneous allocation of attention to multiple channels of information. In the flying environment, pilots must monitor many things, including flight instruments, engine instruments, and one or more ra- dios. Future research may consider a more difficult monitoring task.

A possible limitation of this study relates to generalizability. Participants in this study had a relatively low number of flight hours-an average of fewer than 550 hr-so the data may not apply to those with more experience, such as airline pilots and military pilots. However, the results would generalize to the population of ci- vilian aviators without much experience.

Future research should examine factors that might exacerbate cognitive deficits at moderate altitudes, such as circadian rhythms, hangover effects, antihistamine usage, or fatigue. In addition, researchers should explore how long effects from ex- posure to moderate altitudes persist, allowing predictions about pilot performance in landing, one of the most critical and dangerous phases of flight. Replication of this study using lower altitudes, such as 10,000 ft or even 8,000 ft, could reveal in- teresting findings and would directly relate to FAA policy.

ACKNOWLEDGMENT

We acknowledge the funding support for this project provided by theNorth Dakota Experimental Program to Stimulate Competitive Research (EPSCoR) Project, NSF Grant No. OSR-9452892.

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