In our experiments we typically present visual, auditory and/or tactile stimuli in combination with a specific task (e.g. detection of a specific stimulus).
Often, more complex stimuli are presented as well (e.g. speech stimuli, three-dimensional virtual reality). In response to the task, reaction times, error rates and/or interpretations of stimuli are measured. Most of our studies are controlled via computers.
Electroencephalography (EEG) and Event-Related Potentials (ERP)
The electroencephalogram measures the electrical activity of the brain. These electrical potentials, which can be recorded from the scalp, are very small and fall into the range of millivolt. To measure these small voltages, special instruments are necessary. A typical EEG study looks like the following:
The participant puts on a cap, where the electrodes are mounted. Fitting the cap and mounting the electrodes takes around 1 - 2 hours. Afterwards, the participant takes a seat in an electrically and acoustically shielded room. This is important to get a clear signal of the brain activity and to avoid acoustic distractions from outside the room. In the room, visual, auditory and/or tactile stimuli are presented to the participant.
Stimuli are subject to a task (e.g. pay attention to words spoken by the female speaker and ignore words spoken by the male speaker; push a button everytime, the female speaker utters two different syllables).
During the task performance, the electroencephalogram is recorded with 64 to 128 electrodes, amplified and stored. For every stimulus and every response, the stimulus computer gives a signal to the data acquisition computer. This is essential for the following, post-hoc data analysis, as it allows the coupling of the stimulus and the EEG.
Data analysis requires different steps. First, the electroencephalographic response to the stimlui have to be extracted from the spontaneous EEG; the so called event-related postentials (ERPs). These are the systematic responses of the brain, preceding, accompanying and following specific events.
The resulting voltage-time-diagram is characterized by ups and downs, considered to represent different processing stages (sensory processing, metal processes, motor planning, movement execution).
The amplitude of a potential reflects the strenght of activation of the neural processors in the brain (and correlated for example with subjective task difficulty).
The exact time point of a potential shift reflects the beginning of the underlying processing stage and therefore contains information about the sequence and pace of information processing (e.g. we showed that potential shifts following tactile stimuli happen earlier in blind individuals).
Besides event-related potentials, the EEG allows the analysis of frequency bands, which provide information about the integration (binding) of different brain regions. For analyzing the EEG, special computer programs (e.g. Brain Vision Analyzer) are needed.
In addition to each single ERP one can study the distribution of potentials accross the whole scalp. This allows an approximation of the source of the potential and the underlying cognitive process in the brain. Such a localization can be achieved only with data recording from 64 or, even better, from 128 electrodes. Very elaborate calculations can be performed to calculate a three-dimensional map of the brain out of two-dimensional data to approximate the location of the underlying generators.
Imaging data of the same participant, such as structural and/or functional magnetic resonance imaging, can help to further identify the location of the perceptual and cognitive processes active in a specific task.
Structural images of the brain allow the calculation of the source of the potential for the individual brain (as all heads and brains are slightly different in size, anatomy, structure). Instead, functional images can show the location corresponding to a task.
Functional Magnetic Resonance Imaging (fMRI)
Functional magnetic resonance imaging (fMRI) exploits the fact, that our brain requires oxygen for cognitive processes. Brain areas receive and consume oxygen relative to the amount of their activation; the most active regions will receive the most oxygen and vice versa. As oxygen is transported in the blood, regional blood flow increases as a function of activation. This can be measured with magnetic resonance imaging.
Like in the EEG studies, stimuli and task of different complexity are presented to participants. Resulting data has to be analyzed with special analyzing software (BrainVoyager, SPM). As a result of data analysis, the fMRI provides pictures of the brain with a resolution in the range of millimeters. While the structural magnetic resonance imaging (MRI) provides an anatomical picture of the brain, the fMRI shows differential activation of brain areas for the specific tasks and stimuli.
As an example, with the help of fMRI we showed that brain regions, normally involved in processing visual information, are active in auditory and tactile tasks in blind individuals. Other studies show, that the right (instead of the normally involved left) hemisphere engaged in speech perception after left hemisphere damage after a stroke.
While ERPs provide a very fine grained picture of temporal processing and time point of perceptual and cognitive processes, the fMRI method can localize these processes with a high spatial precision in the brain - which is only rudimentary possible with the EEG. Because of these different and distinct characteristics, a combination of both techniques, requiring a participant to take part in both an EEG and an equivalent fMRI study is very useful and commonly done in neuroscience.
Currently there are attempts to combine both methods in a single experimental design.
Transcranial Magnetic Stimulation (TMS)
For transcranial magnetic stimulation (TMS), a magnetic field is created via a headcoil positioned on the scalp. According to the stimulation parameters, underlying brain regions can temporally be inhibited or activated. The placement of the head coil is depends on the prior knowledge about possible source locations, which were identified with the ERP or fMRI technique.
The TMS is useful to study, if a given brain region is necessary for a specific task (e.g. speech comprehension). In Hamburg there is great expertise in TMS, therefore we are planning to implement this method in our future studies as well. Research questions that can be answered with TMS for example are: Is the observed activation of the right hemisphere in patients with a left sided lesion in brain regions associated with speech perception a pathologic or an adaptive mechanism for the recuperation of verbal functions?
Studies in Individuals with peripheral and central alterations of the central nervous system
Studies in persons with peripheral damage of the nervous system (e.g. blind and deaf individuals) or or damage to the central nervous system (e.g. patients with brain lesions) give inside into diffeent important issues of neuroscience.
On the one hand studies in these populations help identifying essential and sufficient brain areas for the performance of different tasks.
On the other hand plastic capacities of the brain can be identified. Related research questions, for example, are: can brain regions reorganize in a way that they can process information they "normally" do not process (e.g. Can the visual cortex in blind individuals perform different tasks, like process auditory information? Can other brain regions compensate the damage caused by a brain lesion?)
In case of reafferentiation after a period of sensory deprivation (e.g. cataract surgery after period of congenital blindness; cochlea implantation after period of auditory deprivation) the timecourse of these plastic capacities can be studied as well, with related research questions such as: Are ther critical periods for the reorganization of the brain? When, how and under what constrains can sensory functions be recuperated?
Studies including Children and Elderly (> 65 years)
Studying the trajectory of a developing system from a simple to a highly complex system and how it changes throughout development may allow conclusions about general functional principles and capacities of the brain.
For example, knowing the different stages of speech acquisition and the typical errors children have to take during development, helps infering how the speech system works in general. Additionally measuring neural changes in and between different stages of such development, information about the adult functioning of the brain can be derived.
In turn, knowledge about different stages and underlying brain organization in children and adults is essential for (i) developing early interventions in childhood to prevent, compensate for and/or recuperate from specific developmental disorders and/or retardation.; (ii) rehabilitation training after brain lesion or (iii) prevention of cognitive decay in the elderly.