Mobile phone related articles are
Please note that a publication can be assigned to several endpoints, i.e. the sum of publications from the individual thematic points and subpoints can be greater than the total sum of actual publications.
The human brain consists of neuronal networks, which can be in different states of activation (e.g. neural transmission via chemical or electrical signals or functional activity via metabolic processes). Brain activity can be measured by several indicators, e.g.
Each of these indicators can be assessed by specific measurement techniques, e.g.
The EEG reflects the summation of the attenuated electric activity of neurons of the cerebral cortex, recorded through the skull and the scalp. Several electrodes are placed onto the scalp and voltage differences (potential differences) between electrode pairs are measured, amplified and recorded as brain waves. Then the frequency spectrum of these brain waves is analysed. The spontaneous cortical activity of the waking and sleep EEG occurs in a frequency range between 0-50 Hz. For clinical applications this range is split into frequency bands that are associated with different states of consciousness:
The frequency band of 27-50 Hz (gamma waves) is less explored and may be associated with hyperactivity, anxiety states and tension.
Other wave forms in the EEG are event-related potentials (ERP) or evoked potentials. In contrast to the spontaneously measured EEG (which is independent of specific events), these wave forms can either be "evoked" through sensory perceptions (e.g. visual, acoustic, motor or somatosensory stimuli) or they are "related" to cognitive processes (e.g. attention, memory). Evoked potentials represent the processing of a physical stimulus, whereas event-related potentials are triggered by "higher" processes. To measure these potentials, short EEG recordings in response to the stimulus or event are analysed. To extract the relevant potentials many trials must be conducted to average out random brain activity. With a value of a few µV, the amplitude of the potentials is covered by the signals of the spontaneous EEG. To extract the potentials from the background of the EEG, selective analysing techniques are necessary.
There exists other structural elements in the waves of the EEG or the rhythmicity of the brain, which provide information about certain disturbances (e.g. seizures, tumors, trauma, poisoning). Such irregularities in the EEG can be identified by visual inspection of the EEG.
With the rapidly growing use and development of new mobile phone technologies, public concerns about potential health risks arise. Due to the close proximity of the mobile phone device to the head, the brain is exposed to relatively high specific absorption rates (SAR), compared with the rest of the body (see Hossmann et al. 2003). As a discrete amount of radiofrequency emitted by mobile phones is absorbed through the skull and reaches the brain, a (non-thermal) physiological interaction could be hypothesized between these radiofrequency electromagnetic fields and human cerebral activity (see Valentini et al. 2007). Because several functions of the brain (e.g. attention, reaction time, memory) are based on electrical and chemical processes, the EEG can be used to evaluate the performance as well as deficits in the brain-functions (see Gehlen et al. 1996).
Currently, it is difficult to evaluate whether exposure-related changes (e.g. alpha wave activity in the EEG) facilitate or impair cognitive functions, as it is unclear what these changes might represent (Croft et al. 2008). The measurement of event-related potentials in the EEG provides a better insight into the different stages of information processing, as they are specifically related to the presentation of stimuli. These stimuli are often elicited as part of tasks of neuropsychological tests (e.g. oddball paradigm) which in turn provide information about changes in cognitive performance (see Hamblin et al. 2006). Most studies investigate the effects of acute mobile phone exposure on brain activity in humans. Studies using animals, e.g. rats or mice, are performed less often. The advantages of animal studies are that large samples can be examined under controlled experimental conditions and interactions of psychotropic drugs with radiofrequency exposure on brain activity can be investigated (e.g. Lopez-Martin et al. 2009; Lopez-Martin et al. 2006).
The results from several studies on the effects of mobile phone related radiofrequency electromagnetic fields on brain activity, EEG and evoked potentials are difficult to compare, because the are partially inconsistent or contradicting. Possible reasons are for example different exposure durations and output powers of the applied mobile phones, testing of subjects during or after exposure (time of EEG recording), sample characteristics (size, demographic differences) as well as a diversity of tests to assess effects on brain activity. Moreover, different tools and techniques for analysing brain activity can lead to different results.
From 55 available studies (see table as of august 2009), 48 studies were conducted with humans and 7 studies were conducted with animals. Thirty studies investigated spontaneous EEG activity, 10 studies investigated event-related potentials and another 10 studies examined evoked potentials. In three studies seizures were assessed with the EEG. It has to be noted that more than one of these endpoints can be assessed in one article. The remaining studies investigated other indicators or brain activity, e.g. firing rate of neurons, binding properties of neurotransmitters, magnetic field changes (using MEG), brain stem activity, sympathetic nervous system activity or electric potentials of the cerebral cortex (using electrocorticogram). Overall, 16 studies did not show significant effects of radiofrequency exposure. In ten studies on spontaneous EEG, effects of radiofrequency exposure on alpha wave activity were observed, which was either increased (e.g. Bardasano et al. 2007; Regel et al. 2007; De Seze et al. 2001) or decreased (e.g. Perentos et al. 2008; Maby et al. 2006). Three EEG studies indicated specific gender differences in response to exposure (Nanou et al. 2009; Nanou et al. 2005; Papageorgiou et al. 2004). One study that compared adults and children (Kramarenko et al. 2003), observed in both groups slow delta wave activity during waking EEG, with is considered harmful. Because this activity was higher in children the authors hypothesized that children might be more susceptible for harmful exposure effects. In two studies on event-related potentials, significant effects from previous studies could not be replicated in later experiments (Hamblin et al. 2006 vs. Hamblin et al. 2004; Krause et al. 2004 vs. Krause et al. 2000). This might indicate potential instability of results in studies on event-related potentials.
Due to the aforementioned experimental variations between studies and inconsistency between results, a final evaluation of effects of mobile phone-related radio frequency electromagnetic fields on brain activity, EEG and evoked potentials is a complex and time-consuming process that has to be enforced by an expert body. In the course of such an evaluation, all publications of a certain frequency range type have to be collected and evaluated regarding their quality (e.g. dosimetry, study design, sample size, statistics). The results have to be reproducible and should be evaluated with regard to comparable research studies.
At the international level, the WHO is particularly responsible for statements on the effects of electromagnetic fields. For public health related issues, specific recommendations are available. At the national German level, the federal office for radiation protection (Bundesamt für Strahlenschutz, BfS) is the responsible authority. In 2009, the federal office for radiation protection (Bundesamt für Strahlenschutz, BfS) published a review on the literature concerning "Brain, cognition and sleep" (BfS 2009). The Swiss Federal Office of the Environment (FOEN) published a comprehensive evaluation on radiofrequency exposure and health and drew the following conclusions (FOEN 2006) for the subtopic brain physiology (brain waves and event-related potentials): "Overall, regarding the evidence from the studies, it is considered likely that mobile phone exposure influences spontaneous EEG activity during resting wakefulness or sleep as well as the response of the brain to stimuli. The nature and significance of effects cannot be estimated currently."
The following authors provided reviews on the effects of electromagnetic fields in the frequency band of mobile phones, especially with regard to influences on brain actvity, EEG and evoked potentials:
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