For many years, the effect of a temperature drop on EEG has been studied, mainly because of its clinical use, as it has protective effects on neurons in different conditions, such as acute ischemic stroke, traumatic brain injury, or inflammation of brain tissue . Conversely, hyperthermia is a more common condition in nature, but it has received less attention, at least taking into consideration its relationship with brain activity in healthy subjects. With this investigation, we aim to study the impact of hyperthermia on neural activity, a condition in which core temperature rises above the physiological threshold necessary to maintain normal body functions. Our results demonstrate that hyperthermia has an important impact on neural activity measured by EEG. Hyperthermia decreased not only the amplitude of the recorded signal but also induced a reduction in the theta, alpha, and beta frequency bands when the temperature reached 42 ºC.
EEG signals represent a reliable tool for evaluating cortical function, a dynamic process that changes continuously and is affected by pathological conditions (e.g., infections) and physiological homeostasis. Regarding physiology, it is known that body temperature is an important factor driving neuronal activity, as the metabolism and integrity of brain cells depend on it .
EEG during febrile status has been intensively studied and analyzed and is usually related to other pathological processes, such as seizures, infections, cancer treatment [18,8] or syphilis . However, the effect of hyperthermia on EEG in healthy subjects has not been deeply studied, even when fever is a common process that all of us suffer from at some point in life.
In the situations mentioned above, when EEG alterations are detected, it is often very difficult to disentangle which part can be attributed to the underlying pathological process and which to the temperature itself. Furthermore, the available results are not clear. Lifshitz et al.  concluded after studying the EEG of patients without neurological pathologies during episodic fever that “fever in adults seems not to provoke rapidly reversible EEG changes.” This conclusion is completely different from the profound depression of recorded activity reported by Cabral et al. . In both cases, the increase in temperature was a consequence of a previous condition that could affect the results. On the other hand, Reilly et al. , studying cancer patients on hyperthermia treatment, found reversible changes in EEG signals related to temperatures above 41.5 ºC, mainly a decrease in the voltage signal and a shift to lower EEG frequencies.
The present study demonstrates a gradual reduction in signal intensity as the temperature increases, but in no case was there a suppression  or an increase  of EEG signal as previously reported, indicating that those extreme results could be the consequence of the specific pathology. On the other hand, a gradual increase from 39 to 41 ºC followed by an abrupt decrease in EEG amplitude has also been reported in curarized rats . Although it is a different animal model, it fits well with our data.
Our results also showed that in control conditions, the EEG was dominated by low frequencies; as a consequence of anesthesia, delta frequencies rule the spectrum , and this predominance was maintained without changes even during the high temperature peaks. However, significant changes were detected at theta, alpha, and beta frequencies, suggesting that these frequency bands are more sensitive to high temperatures.
We must keep in mind that our control situation is anesthetized mice; hence, anesthesia influences both the initial EEG and the EEG obtained during hyperthermia. There are a number of studies describing the effect of different anesthetics on EEG signals [24,25,26] and showing that anesthesia tends to produce a common pattern on EEG, reducing the variability that characterizes the awake resting state. This pattern is characterized by a prominent presence of delta frequencies, usually with no differences between anesthetic agents. Delta frequencies have been proposed to be the default activity pattern of cortical networks since they have been detected in disconnected preparations [27,28]. Theta, alpha and low beta frequencies are also present in anaesthetized animals, but the intensity and particular characteristics can vary with the anesthesia level and the anesthetic agent . We are aware that our control situation cannot be defined as a normal physiological state, but on the other hand, it allows us to obtain a more stable standard initial point, decreasing the variability between animals. Additionally, anesthesia guarantees that changes are generalized, and two electrodes at distant places are sufficient to obtain a representative measurement of brain activity . We observed a reduction in alpha, theta and beta frequencies, compatible with the described disruption of functional connectivity in the brain network under hyperthermia . Only delta frequencies, which can be present even in slice preparations [27,28], maintained stable power.
These results show that hyperthermia is able to change cortical circuitry activity, affecting brain oscillations in anesthetized mice. Since alpha, theta and beta frequencies have been associated with different levels of awareness and different cognitive tasks [31,32,33] and hyperthermia has been negatively correlated with cognitive functions [34,35,36], it is tentative to speculate that those changes in cortico-cortical synchronization could be behind some of the cognitive effects. Additionally, such effects open the door to the possibility that small temperature changes in awake animals may have a strong effect on cortical oscillations, affecting gamma rhythm (not detectable in our anesthetized animals), which is related to sensory stimulation, attention and cognitive tasks [37,38,39]. Another open question worth considering in the future is whether longer periods of hyperthermia could produce stronger changes lasting longer or perhaps trigger some compensatory plastic phenomena.
In summary, acute elevation of body temperature is an important factor that is able to modify the electrical activity of the brain, probably impacting its metabolic activity. Our results showing the effect of hyperthermia on the amplitude and the synchronization of EEG signals pave the way for future research and represent a reference point for experiments involving a specific pathological condition in which temperature is a trigger (e.g., a mouse model of seizures where temperature is the trigger parameter for seizures).