# Abstract

 

## Background Heat stress potentially has detrimental effects on brain function. Hence, cognitive function may be impaired during physical activity in a hot environment. Skin cooling is often applied in a hot environment to counteract heat stress. However, it is unclear to what extent neck cooling is effective for cognitive impairment during exercise in a hot environment. The purpose of this study was to examine the effects of temporal neck cooling on cognitive function during strenuous exercise in a hot environment. 

## Methods Eight male young participants (mean ± SD, age = 26.1 ± 3.2 years; peak oxygen uptake = 45.6 ± 5.2 ml/kg/min) performed Spatial delayed response (DR) task (working memory) and Go/No-Go task (executive function) at rest and during exercise in the Hot and Hot + Cooling conditions. After the participants completed the cognitive tasks at rest, they cycled the ergometer until their heart rate (HR) reached 160 beats/min. Then, they cycled for 10 min while keeping their HR at 160 beats/min. The cognitive tasks were performed 3 min after their HR reached 160 beats/min. The air temperature was maintained at 35°C and the relative humidity was controlled at 70%. Neck cooling was applied to the backside of the neck by a wet towel and fanning. We used accuracy of the Spatial DR and Go/No-Go tasks and reaction time in the Go/No-Go task to assess cognitive function. 

## Results Neck cooling temporarily decreased the skin temperature during exercise. The accuracy of the cognitive tasks was lower during exercise than that at rest in the Hot and Hot + Cooling condition (p \< 0.05). There were no differences in the accuracy between the Hot and Hot + Cooling conditions (p = 0.98). Neither exercise (p = 0.40) nor cooling (p = 0.86) affected reaction time. These results indicate that temporal neck cooling did not alter cognitive function during strenuous exercise in a hot environment. 

## Conclusions The present study suggests that temporal neck cooling with a wet towel and fanning is not effective for attenuating impairment of working memory and executive function during strenuous exercise with a short duration in a warm and humid environment. # Results and discussion 

## Exercise intensity and HR Table [1](#) shows exercise intensity and HR after the participants reached a HR of 160 beats/min. We observed a significant main effect of Time Course (F [9,63] = 45.02, p \< 0.001, η^2^ = 0.87) on exercise intensity. There were no significant main effect of Condition (F [1,7] = 1.62, p = 0.24, η^2^ = 0.19) and interaction (F [9,63] = 1.01, p = 0.44, η^2^ = 0.13). These results indicate that exercise intensity decreased as time elapsed and the degree of decrease was not different between conditions. Similarly, we found a significant main effect of Time Course (F [9,63] = 6.39, p \< 0.001, η^2^ = 0.48) on HR. There were no significant main effect of Condition (F [1,7] = 2.91, p = 0.13, η^2^ = 0.29) and interaction (F [9,63] = 0.81, p = 0.61, η^2^ = 0.10). The significant main effect of Time Course may be ascribed to a slight higher HR around the first 3 min after the participants' HR reached 160 beats/min. Overall, HR appears to be well controlled around the target level in the present study. ::::table-wrap

::: caption
Changes in intensity and HR during exercise after the participants' HR
reached 160 beats/min
:::

```
                       Time after the participants' HR reached 160 beats/min (min)                                                                                                                           
```

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1 2 3 4 5 6 7 8 9 10\
Exercise intensity (W)\
Hot 164.5 ± 25.3 149.3 + 20.0 137.3 ± 29.9 132.5 ± 30.1 134.1 ± 27.7 130.4 ± 26.0 127.8 ± 27.4 127.9 ± 25.6 125.0 ± 29.8 123.9 ± 30.2
Hot + Cooling 163.3 ± 22.8 149.8 ± 29.8 136.6 ± 29.9 126.0 ± 34.0 125.6 ± 33.4 123.8 ± 33.8 121.9 ± 35.1 121.0 ± 36.1 117.1 ± 35.9 114.6 ± 35.6
Heart rate (beats/min)\
Hot 162.3 ± 1.7 163.5 ± 2.2 163.6 ± 2.3 161.3 ± 2.0 159.9 ± 1.3 161.4 ± 1.7 162.1 ± 2.1 160.3 ± 3.1 160.0 ± 1.3 160.3 ± 1.9
Hot + Cooling 162.6 ± 1.2 164.0 ± 3.0 165.1 ± 3.0 162.8 ± 1.7 160.8 ± 1.9 160.4 ± 1.6 161.1 ± 1.8 160.9 ± 1.4 161.0 ± 1.7 160.9 ± 1.3

Values are mean ± SD.:::: 

## RPE, blood lactate concentration, body weight, and body temperature Table [2](#) shows RPE, blood lactate concentration, body weight, and body temperature. We found significant main effects of Exercise on RPE (F [1,7] = 174.49, p \< 0.001, η^2^ = 0.96), blood lactate concentration (F [1,7] = 43.02, p \< 0.001, η^2^ = 0.86), body weight (F [1,7] = 55.30, p \< 0.001, η^2^ = 0.89), and body temperature (F [1,7] = 56.82, p \< 0.001, η^2^ = 0.89). In contrast, Condition did not affect these parameters. There were no significant interactions. Collectively, exercise increased RPE and blood lactate concentrations to the same extent in both conditions. The degree of dehydration and increase in body temperature were not different between conditions. Indeed, we observed no difference in weight reduction between conditions (0.41 ± 0.07% in the Hot condition and 0.37 ± 0.22% in the Hot + Cooling condition). ::::table-wrap
::: caption
RPE, blood lactate concentration, body weight, and body temperature
:::

Condition Variable Rest Exercise After

______________________________________________________________________

Hot RPE 7.3 ± 0.4 15.9 ± 2.0\*\
Blood lactate concentration, mmol/l 1.0 ± 0.2 5.3 ± 1.9\*\
Body weight, kg 69.1 ± 7.5 68.8 ± 7.5\*\
Body temperature, °C 36.1 ± 0.6 38.4 ± 0.4\*\
Hot + Cooling RPE 7.1 ± 0.9 15.8 ± 1.7\*\
Blood lactate concentration, mmol/l 1.0 ± 0.2 5.5 ± 1.7\*\
Body weight, kg 68.9 ± 7.7 68.6 ± 7.7\*\
Body temperature, °C 36.5 ± 0.6 38.3 ± 0.3\*

Values are mean ± SD.

*RPE* ratings of perceived exertion.

\* p \< 0.001, vs. rest.:::: 

## Effects of neck cooling on skin temperature Figure [2](#) illustrates the skin temperature during exercise. The main effects of Time Course (F [1,9] = 9.60, p \< 0.001, η^2^ = 0.58) and Condition (F [1,7] = 8.03, p \< 0.05, η^2^ = 0.53) were significant. There was a significant interaction between Condition and Time Course (F [9,63] = 8.88, p \< 0.001, η^2^ = 0.56). These results indicate that skin temperature changed in a different manner between conditions. We observed significant differences in skin temperature between conditions after neck cooling. However, the effects of neck cooling were temporal and did not last until the end of the exercise. <figure> <p><img src="" /></p> <figcaption>Neck skin temperature in the Hot and Hot + Cooling conditions during exercise. The <em>upper dashed arrow</em> indicates the time when neck cooling was started. <sup>‡‡</sup><em>p</em> \< 0.01 between the conditions.</figcaption> </figure> 

## Cognitive function Figure [3](#) illustrates the accuracy of the cognitive task (A) and reaction time in the Go-trial (B). We observed a significant main effect of Exercise on the accuracy of the cognitive task (F [1,7] = 8.78, p \< 0.05, η^2^ = 0.56). We found neither a significant main effect of Condition (F [1,7] = 0.001, p = 0.98, η^2^ \< 0.001) nor a significant interaction (F [1,7] = 0.27, p = 0.62, η^2^ = 0.04). These results indicate that the accuracy of the cognitive task significantly decreased during exercise, and that temporal neck cooling had no effects on the accuracy of the cognitive task. Reaction time in the Go-trial was not affected by Exercise (F [1,7] = 0.802, p = 0.40, η^2^ = 0.10) or Condition (F [1,7] = 0.03, p = 0.86, η^2^ = 0.005) in the present study. <figure> <p><img src="" /></p> <figcaption><strong>a</strong> Accuracy of the cognitive task at rest and during exercise. <sup>#</sup><em>p</em> \< 0.05 vs. rest. <strong>b</strong> Reaction time in the Go-trial at rest and during exercise.</figcaption> </figure> To our knowledge, no studies have examined the effects of temporal neck cooling on cognitive impairment during strenuous exercise in a hot environment. We tested if temporal neck cooling with a wet towel and fanning has beneficial effects on cognitive function during strenuous exercise with a short duration in a hot environment. The main finding of this study was that temporal neck cooling did not attenuate the detrimental effects on cognitive function during strenuous exercise in a hot environment. The present finding suggests that temporal neck cooling with a wet towel and fanning is not effective enough to attenuate cognitive impairment during strenuous exercise with a short duration in a warm and humid environment. We assessed cognitive function using the combination of spatial DR task and Go/No-Go task. The Spatial DR task requires working memory \[[25](#), [29](#)\]. The Go/No-Go task requires executive function, including selective attention, response inhibition and interference control \[[30](#), [31](#)\]. The prefrontal and parietal cortices play a key role in spatial working memory \[[32](#)--[35](#)\]. Activation in the prefrontal cortex is crucial for executive function \[[36](#), [37](#)\]. Furthermore, the frontopolar cortex, which is located at the most anterior part of the frontal cortex, is selectively activated in multi-tasking where execution of the primary task was maintained in a pending state while performing a secondary task \[[38](#)\]. Given that the present cognitive task was a modified version of multi-tasking, the present cognitive task required activation of the frontopolar cortex. Taken together, we can assume that the present cognitive task was appropriate to assess higher cognitive function in humans. In the present study, HR was maintained at 160 beats/min during exercise. Indeed, RPE was approximately 16 immediately after the cognitive tasks during exercise. Furthermore, blood lactate concentration increased above 5 mmol/l after exercise in both conditions. Hence, in the present study, we can assume that exercise intensity was classified as strenuous. Skin cooling may reduce thermal strain and/or discomfort at rest and during exercise \[[5](#), [9](#)--[13](#)\]. Thus, physiological as well as psychological changes induced by skin cooling may somehow attenuate the impairment in cognitive function during exercise in a hot environment. Previous studies indicated that head and neck cooling with frozen packs prevented the impairment of short-term memory \[[14](#), [39](#)\]. In the present study, the accuracy of the cognitive task decreased during exercise in the Hot and Hot + Cooling conditions. We observed no differences in the accuracy of the cognitive task during exercise between conditions. These results suggest that temporal neck cooling for a short period did not have beneficial effects on cognitive function during exercise in a hot environment under the present condition. In the present study, while neck cooling temporarily decreased skin temperature, neck cooling did not alter body temperature. These results mean that the neck cooling applied in this study was not effective enough to decrease body temperature. The present results are in line with the findings showing that skin cooling did not reduce body temperature \[[9](#)--[11](#), [40](#)\]. Moreover, we found no differences in RPE between conditions. Therefore, the lack of beneficial effects of temporal neck cooling on cognitive function would be ascribed to that physiological and/or psychological changes induced by brief neck cooling was not sufficient enough to attenuate the detrimental effects on cognitive function. Dehydration is one of the potential factors that may impair cognitive function in a hot environment. Hence, we used strenuous exercise with a short duration. We expected that severe dehydration is unlikely to occur even in a hot environment in the present study. Indeed, weight reduction was less than 0.5% in the Hot and Hot + Cooling conditions. Previous studies suggested that dehydration may impair complex cognitive tasks when weight reduction was greater than 2% \[[21](#), [22](#), [39](#)\]. Indeed, Gaoua et al. \[[39](#)\] suggested that, when weight reduction did not exceed 0.5%, fluid replacement is adequate and dehydration does not alter cognitive function. Hence, we can assume that the effects of dehydration on cognitive function were negligible in the present study. In contrast to the accuracy of the cognitive task, reaction time in the Go-trial was not altered by exercise. These results suggest that the effects of strenuous exercise in a hot environment are different between accuracy and speed of response. Previous studies indicated that nerve conduction velocity increases in the hot environment \[[14](#), [39](#)\]. Thus, exercise in a hot environment may increase in nerve conduction velocity, which leads to the decrease in reaction time. However, in the present study, it is unclear whether reaction time was not affected by strenuous exercise or whether increases in conduction velocity in a hot environment was not effective, possibly due to a short duration of exercise. Furthermore, we cannot rule out the possibility that increases in nerve conduction velocity compensated for the negative effects of strenuous exercise on speed of response. Provided that multiple factors are responsible for changes in response speed during exercise, further studies will be required to elucidate these points. It has been suggested that increase in arousal level induced by exercise affect cognitive function \[[41](#), [42](#)\]. From this viewpoint, the present results may suggest that arousal level went beyond the optimal level during strenuous exercise in a hot environment, and that temporal neck cooling was not effective to alter arousal level. However, physiological mechanisms underlying the alterations in arousal level during exercise remains to be elucidated. Thus, further studies are required to understand the association between arousal level and cognitive function.


Extracted trial result:
Outcome: accuracy of the cognitive tasks (continuous)
Intervention: Exercise - Hot condition
Comparator: At rest - Hot condition
Intervention group size: 8
Comparator group size: 8