Cognitive deficits due to thermal stress: An exploratory study on soldiers in deserts

Abstract

Background

It is well known that environmental factors play an important role in human performance. High temperature and humidity particularly impair mental performance by altering brain neurochemistry and electrolyte disturbance which in turn affect one's overall efficiency. While the physiological responses to environmental heat have been well established, it is less clear about its impact on cognition. Study aims to investigate the impact of thermal strain on cognition.

Methods

One hundred (100) healthy soldiers aged between 20 and 30 years who had spent minimum of one year in desert conditions prior to their induction in the study formed sample of the study. The subjects were evaluated on memory and on cognitive functions twice i.e. in the month of February and June. The data so generated was analyzed by appropriate statistical methods.

Results

The mean age of the subjects were 25.8 yrs. There was a significant decline in cognitive performance in hot climate as compared to normal weather on Post graduate Institute (PGI) memory scale (P < 0.05). The effect was more marked for tests requiring sustained attention, concentration, psychomotor performance, verbal memory and tests of executive function.

Conclusion

The present study is the first study to be conducted in actual desert conditions. Findings revealed a detrimental impact of thermal stress on the cognitive performance of soldiers in deserts.

Introduction

Military service and climatic hardship share a unique relationship. Many key operational zones across the world are located in places where not only the enemy but weather is equally hostile. However, in spite of these testing conditions, the soldier is expected to perform at optimum levels at all times. Since any compromise of his mental or physical efficiency can lead to dire consequences, it is imperative that his body and mind remain fit and function in complete harmony.

Effect of high temperature on human behavior in terms of more irritability and decreased efficiency is well known. Studies have also confirmed that thermal stress leads to general physiological hyper-arousal and decline in cognitive performance.^{1, 2, 3} Studies on the effect of exercise under hot and dry conditions suggest that there are significant cognitive decrements under such conditions.⁴ The decline in cognitive performance is more with increase in complexity of the task.⁵ In brief, cognitive decline under thermal strain is positively correlated with severity of heat strain and the complexity of the cognitive task at hand. Plethora of research indicates that high heat and humidity pose significant stress on the cardiovascular system in maintaining homeostasis⁶ though very little research has been conducted to explore the effect of heat on the cognitive performance of soldiers in deserts. The issue gains further importance as so many strategically important areas lie in deserts across the world. Present study aims to investigate the effect of thermal strain on various aspects of cognitive performance of soldiers in deserts.

Material and methods

Sample

Male troops are deployed all along the western Indian border where temperature touches more than 50 °C during summer months. The previous year meteorological data was obtained from the Rajasthan State Meteorological Department (Met Dept) and Corps Headquarters (Corps HQ) Medical Branch located somewhere in the western border region. Sample of the study comprised of the troops deployed along the border in deserts at extremes of temperature. Based on the met data, zones of high temperatures were identified along the border in desert areas. One of the infantry battalion company deployed in that dry arid high temp zone was selected for study. Troops deployed on western border were selected because of its strategic location and higher cognitive demands to maintain vigil and state of readiness. The inclusion criteria used for sample selection was:

• (a)Healthy soldiers aged between 20 and 30 years who had spent minimum of one year in deserts.
• (b)Soldiers who are not likely to be posted out in next 6 months.
• (c)Soldiers with normal or corrected vision and hearing and absence of any psychological morbidity or physical disease.

Same sample was assessed in the month of February and June. Since the study was to be conducted before and after extreme temp conditions, the Commanding Officer was requested to make maximum soldiers available for the study. At the outset of the study, the authors did not have any baseline data available as such kind of study had not been undertaken in Armed Forces context in the past. Adequate hydration was ensured and no amount of physical exertion was permitted while the sample was being tested on psychological measures. They all had adequate sleep the night prior to cognitive testing. The company had total 136 soldiers and based on our inclusion criteria 107 were selected. However, 7 soldiers were not made available during collection of data in June hence, removed from analysis.

Tools

(a) Sociodemographic and clinical data sheet for recording of age, service, trade, date of induction into deserts, duration of actual time spent in desert conditions (excluding the period of leave/temporary duty), educational qualification, any disability/current medications were recorded.

(b) Psychological tests administered were Post graduate Institute memory scale (PGIMS) also referred here as PGI memory scale, Trail Making test, Reaction timer and Computerized version of Wisconsin Card Sorting Test (WCST). In the PGI memory test, out of 10 subtests, tests of recent memory, remote memory and mental balance test were excluded because they demonstrated a ceiling effect. Trail making test comprises of two parts i.e. A & B. The time taken by the subject for the two sub parts was measured using a stop watch. Three scores were obtained. Time (seconds) taken to complete task A, Time (seconds) taken to complete task B, Trail Shift (time difference i.e. Time B minus Time A). Reaction time is a measure of time elapsed between exposure to a sensory stimulus and the subject's response to this stimulus. Reaction timer comprised of equipment wherein the subject was expected to respond to auditory or visual stimuli in the shortest possible time. Ten such readings were taken for each subject and the mean was counted as mean visual reaction time. Responses were measured for both hand and foot for all the subjects. Reaction timer tests audio-visual psychomotor response and is a good measure of its speed and integrity.

Wisconsin card sorting test (WCST) is used to evaluate executive functions especially those involving the frontal lobes. All the psychological instruments are standardized and Indian norms for them are available. These tasks measure a range of cognitive processes including attention, concentration, memory, verbal learning, information processing and executive function.^{7, 8, 9, 10}

Methods

The study design and the protocol were approved by the institutional ethical committee. The study was conducted during daylight hours in tents in actual desert conditions because they represented the actual conditions in which the soldiers stay and work and no artificial cooling means were allowed. The months of February and June were chosen for the study. The temperature is comfortable 25–28 °C in February and in June it usually crosses 42 °C which was supposed to represent hot weather. 25–28 °C has been found to be most conducive as far as human cognitive functioning is concerned (1). The measure of temperature was obtained using a standardized electronic hygrothermometer and later compared with State Met Dept and Med Br of Corps HQ who maintain temp chart. Subjects were allowed to hydrate themselves with plain water depending on their choice.

Participants were told that they are a test group for a new type of lecturing method, via videotape of approximately 30 min. Informed consent was taken from all the participants of the study and they were asked not to speak to each other or discuss among themselves about the test. They were assured that the tests would have no implications toward their service and this was purely an academic exercise. The assistants used a table and a chair to mark the result on a blank sheet of paper for the memory test and trail making test whereas results of computerized Wisconsin Card Sorting test (WCST) and the reaction timer was directly recorded on the hard disk of computer. Sufficient illumination was ensured in the examination tent.

Statistical analysis

Mean scores for each of the four instruments was calculated along with standard deviation. The performance on cognitive tests being conducted in hot and cold weather was compared i.e. cool weather during February and hot weather during the month of June. Two tailed paired T test was performed to test for significance of difference between the two readings for all the tests and a P < 0.05 was considered statistically significant.

Results

Table 1 shows the environmental conditions during the month of February and Jun in the same location. The temperature rose from cool 25–27 °C in February to hot 42–43 °C during June. The mean age of the subjects was 25.8 years. The sample comprised of male infantry soldiers. It was found that the average duration of exposure to the desert conditions was 21 months. Most of the subjects were having education till high school. A small minority had higher education.

Table 1.

Environmental conditions during psychological testing.

	Temperature	Humidity
March	24–27 °C	26–30%
Jun	42–43 °C	28–31%

Neurocognitive function: A look at Table 2, Table 3, Table 4, Table 5 reveals that subjects had a significant decline in cognitive performance in hot climate as compared to normal temperature. PGI score shows that there is significant decline in performance on the sub-tests for attention and concentration, delayed recall, immediate recall, verbal retention for dissimilar pairs, visual retention and visual recognition (Table 2). Reaction time. There is no significant difference between the normal temperature and hot temperature readings for audio reaction time (both hand and foot), and visual reaction time for foot. However, the difference is just significant for visual reaction time for the hand (Table 3). Trail making test. There was significant decline on all the three parameters of this test i.e. Trail A, Trail B and Trail shift during hot weather (Table 4). Wisconsin card sorting test recording shows a significant decline in executive task performance with rise in ambient temperature. Total errors and perseverative errors were more than non-perseverative errors (Table 5).

Table 2.

PGI memory scale readings.

S. No	Test	Recording at 24–27 °C Humidity 23% Mean score (S.D.)	Recording at 42–43 °C Humidity 27% Mean score (S.D.)	T score	P value α = 0.05
1	Attention and concentration	9.6 (1.82)	8.96 (1.77)	2.52	0.013
2	Delayed recall	11.88 (0.44)	8.72 (0.84)	33.61	0.001
3	Immediate recall	9.48 (1.94)	6.52 (1.94)	10.77	<0.00001
4	Verbal retention for similar pairs	5.00 (0.00)	4.28 (0.46)	1.5721	0.119
5	Verbal retention for dissimilar pairs	14.84 (0.37)	10.44 (2.48)	17.52	<0.00001
6	Visual retention	12.88 (0.33)	11.24 (2.31)	7.015	<0.00001
7	Recognition	9.96 (0.20)	9.48 (0.65)	2.28	0.0247

Table 3.

Reaction time.

S. No.	Test	Recording at 24–27 °C Humidity 23% (mean time in milliseconds) (S.D.)	Recording at 42–43 °C Humidity 27% (mean time in milliseconds) (S.D.)	T score	P value α = 0.05
1	ARTH	1.34 (0.58)	1.31 (0.56)	0.4231	0.966
2	ARTF	1.13 (0.60)	1.26 (0.50)	1.7027	0.0917
3	VRTH	0.74 (0.24)	0.68 (0.22)	1.96	0.0528
4	VRTF	0.77 (0.36)	0.80 (0.27)	0.7273	0.4687

ARTH: audio reaction time of hand.

ARTF: audio reaction time of foot.

VRTH: visual reaction time of hand.

VRTF: visual reaction time of foot.

S.D.: std deviation.

Table 4.

Trail making test.

S No	Test	Recording at 24–27 °C Humidity 23% (mean time in seconds) (S.D.)	Recording at 42–43 °C Humidity 27% (mean time in seconds) (S.D.)	T score	P value α = 0.05
1	Trail making test A	25.78 (7.95)	29.33 (8.54)	2.62	<0.0101
2	Trail making test B	57.24 (18.09)	69.47 (14.39)	5.29	<0.00001
3	Trail shift	31.47 (6.89)	40.14 (7.48)	3.63	0.00045

Table 5.

Wisconsin card sorting test.

S No	Test	Recording at 24–27 °C Humidity 23% Mean score (SD)	Recording at 42–43 °C Humidity 27% Mean score (SD)	T score	P value α = 0.05
1	Raw score	33.76 (11.71)	28.02 (7.43)	4.14	0.000073
2	Total errors	30.24 (11.71)	34.02 (7.43)	2.72	0.0077
3	Perseverative errors	18.44 (11.85)	21.75 (10.69)	2.072	0.040
4	Non-perseverative errors	11.8 (7.49)	12.69 (7.35)	0.8521	0.396

Discussion

Present study was unique because it was first time that soldiers were tested psychologically in actual desert conditions. Other studies have been conducted in laboratories where conditions were simulated.^{1, 3} Another attribute of this study was the high temperature at which the cognitive performance was tested. Majority of the subjects were young male healthy soldiers who participated actively in the study. They had spent more than one year in their current location. Same subjects underwent psychological testing in the month of February and June in actual desert conditions.

A close look at Table 2, Table 3, Table 4, Table 5 reveals that there is significant decline in the performance of subjects on the various parameters of the neurocognitive functions due to rise in ambient temperature. This was the primary objective of the study wherein the decrements in cognitive performance due to rising temperature was postulated. The effect of humidity cannot be expected to be primarily responsible for this decline as there was no significant difference between the cool and hot weathers (at around 30%) and which was well within the comfort zone. However, the rise in temperature from cool 25–27 °C to hot 42–43 °C in June degrees is highly significant. PGI memory scale score (Table 2) shows that for attention and concentration, delayed recall, immediate recall, verbal retention for dissimilar pairs, visual retention and visual recognition the difference is significant. Similarly psychomotor dexterity, speed, and executive frontal lobe tasks as reflected on the Trail making test and Wisconsin Card Sorting Tests reflected declining performance with rise in ambient temperature (Table 4, Table 5). Reaction time readings in normal temperature and hot weather conditions (Table 3) reveal that there is no significant difference between the normal temperature and hot temperature readings for audio reaction time (both hand and foot referred to as ARTH, ARTF in Table 3), and visual reaction time for foot (VRTF). However, the difference is nearing significance for visual reaction time for the hand (VRTH). Taken together, it can be inferred that performance on simple cognitive task is not much affected during hot weather as compared to normal weather. The findings conform to findings of Hancock¹ who suggested that simple cognitive task is unlikely to be affected and may even improve in hot climate provided the homeostasis is well maintained. Razmjou⁵ had suggested that under thermal stress, cognitive fatigue and decline occurs when the complexity of the task is increased.

Effect of extreme climate on cognitive performance such as that of high altitude, extreme cold or heat, and low ocean depths is well known.^{11, 12} High temperature (i.e. beyond 35 °C) negatively affects mood and ability to concentrate.^{13, 14} Role of conscious effort and motivation are important but as the operations get prolonged, these factors lose their impact on overall performance.¹⁵ However, how much is the overall cognitive decline because of thermal stress and underlying neurological mechanisms have not been studied systematically.

In our study, no amount of dehydration was allowed and the physical exertion leading to mental exhaustion was also excluded. The humidity was same for both the cold climate and hot climate and well within the comfort zone. Therefore, our findings are more consistent for the effect of temperature solely as compared to earlier studies in this direction where effect of both dehydration and thermal stress were studied together.^{14, 15} Recent technological advances have allowed for measurement of brain temperature and blood flow, which were previously only available during animal studies.^{16, 17} All these studies indicate that hyperthermic brain is associated with reduced blood flow, reduced supply of oxygen and glucose and decreased voluntary activation of peripheral nervous pathways. It appears to be the most plausible explanation for cognitive decline at a higher environmental temperature.

Nutrition based studies have found that common vitamin B12 and folate deficiencies and high blood homocysteine levels are often associated with cognitive decline and dementia. Results came from animal studies where scientists artificially induced raised blood homocysteine levels by modifying diet with reduced folate or vitamin B12 deficiency or by high intake of methionine. Similarly magnesium and zinc deficiencies were also found to have negative impact on cognition. It is not clear if and how these conditions contribute to the observed cognitive impairments. It is plausible that increased sweating to maintain thermoregulation in hot weather may be leading to loss of these nutrients along with water with consequent decline in cognitive performance.⁶

Of all the factors discussed above it appears that a combined effect of electrolyte and nutrient imbalance, neurotransmitter disturbance and reduced conscious effort may be responsible for the cognitive decline with rising ambient temperature. It is noteworthy that effect of humidity alone may be entirely different if studied independently and that has not been one of the objectives of the current study.

These findings assume more importance in the present scenario in view of global warming. Incorporation of NBC (nuclear biological chemical) suit which becomes extremely warm from inside after sometime, operating inside an armored combat vehicle are few of the likely scenarios where soldiers perform under extremes of heat. Operating sophisticated computer based weaponry requiring analysis, precision and quick response in the face of enemy threat or combat under such circumstances will be a challenging option.

The study holds the distinction of being the first of its kind in actual desert conditions. The strength of the study is its unique model in which same sample of large size was studied twice in actual ground conditions. The healthy subjects were chosen for the study to exclude bias due to ill health. They were all educated and were able to comprehend the instructions. No instruments were used to ascertain the mood states or boredom levels of the soldiers which may be studied independently in future studies. Further it was not verified if the findings are transient or permanent. These are few of the limitations of the study.

Conclusion

To our knowledge, the present study was first of its kind wherein effect of rise in environmental temperature on the cognitive performance was studied on Indian soldiers in deserts. The study concluded that thermal stress leads to impairment in performance on complex cognitive tasks. It is recommended that future studies in this direction should be carried out to suggest strategies to overcome cognitive decrement due to thermal stress.

Conflicts of interest

The authors have none to declare.