Neuroimaging includes various techniques that either directly or indirectly image the structure and the function of the human brain. Thus, neuroimaging can be divided into two categories: structural imaging and functional imaging.
Structural imaging examines the structure of the brain (like gray and white matter) and the possible changes that occur in these structures with factors such as learning and aging. Popular methods to investigate changes in brain structure include voxel-based morphometry (VBM), which enables investigation of changes in the brain’s anatomy, and diffusion tensor imaging (DTI), which enables examination of image neural tracts by measuring the restricted diffusion of water in the brain.
In contrast, functional imaging is used to observe the working brain. Functional brain imaging offers new insights into topics that lie at the heart of sport psychology. For example, research on motor imagery has reached a new level demonstrating that motor imagery is based on neural activation of core motor areas in the brain. This widely accepted finding has dramatically influenced approaches to motor rehabilitation.
Imaging of the living brain has to deal with the fundamental problem of the scale of observation. Research on the mirror neuron system (MNS), for example, is based on single-cell recordings in the monkey brain. However, cognitive neuroscience is typically interested in examining the relevance and interconnectivity of defined whole brain areas during specific tasks. In the case of the MNS, the role of the parietofrontal circuit for action recognition has been uncovered.
Functional imaging enables researchers to identify brain regions whose activation is associated with specific action-linked processes, such as action observation or action imitation processes. Possible methods for this are positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). In PET studies, radioactive-marked molecules (e.g., radioactive-markered glucose) are administered into the participant’s blood right before the study. The tomograph detects the radiation and therefore shows exactly where the molecules are being used in the brain. On the other hand, fMRI does not need an injection and is based on the different magnetic properties of the human blood. Both methods examine metabolic brain activity.
In sports and motor neuroscience, most published work uses fMRI because it has good temporal and spatial resolution properties. PET is used less often for research because radioactive-marked molecules have to be administered. Therefore, the fMRI will be described in more detail.
In humans, fMRI has proven to be an efficient method to study task-relevant brain activation. The resting brain is not silent and shows neural activity even during sleep. For this reason, fMRI studies attempt to understand brain activation by examining differences of brain activities between two or more tasks, such as action observation and motor imagery. Research on the functions of brain areas for specific tasks relies heavily on cytoarchitectonic results (structural information about anatomical regions of interest in the brain, based on the cellular composition) and on research with patients with defined cortical lesions.
To map neural activity, fMRI uses the change of blood oxygen flow within the brain. More precisely, the measurements rely on the different magnetic properties of oxygen-rich and oxygenpoor blood. Oxygen-rich blood is diamagnetic and therefore has less impact on the magnetic field, whereas oxygen-poor blood is paramagnetic, which leads to stronger interferences in the magnetic field. Thus, the strength of the measured signal depends on the degree of the oxygenation of the blood. The dependency between the image quality and the oxygen saturation of the blood is called blood oxygenation level dependency (BOLD). Changing blood flow and the related BOLD response is directly associated with neural activation in a certain brain region.
During fMRI scanning, it is necessary for participants to lie in a strong, permanent magnetic field with high homogeneity. Certain nuclei in the human body, the hydrogen nuclei, provide magnetic properties. Being in a strong magnetic field, hydrogen nuclei behave like a compass needle; they all align with the magnetic field. During fMRI scanning, radiofrequency impulses are applied to the aligned magnetic system. This results in a change of the orientation of the hydrogen nuclei. After the radio pulse ceases, the hydrogen nuclei
return to their original orientation by emitting energy, which is detected by an antenna of the system. The source of this signal is specified by magnetic field gradients that vary the strength of the magnetic field and hence allow determination of the specific signal source and position. The position of the brain in the magnetic field is defined at the very beginning of the experiment. Therefore, it is crucial for the later analysis of the data that the participants do not move their head during the experiment. Otherwise a mislocalization of a detected increased activation may be possible.
Generally, science starts with a research question that in turn generates (neuroanatomical) hypotheses, which can then be tested by performing an experiment. For fMRI, the experimental strategy is to observe the brain’s response (the BOLD response) to certain kinds of stimulation: for example, an observation task with different body movements. Over the last decade, three design types have dominated fMRI studies: the blocked design, the event-related design, and the mixed design. These designs vary in terms of stimulus presentation and timing. The blocked design is characterized by presenting a time interval with stimuli of only one condition, alternating this with intervals representing stimuli of other conditions. The main advantage of this type of paradigm is increased statistical power and robustness. In contrast, the event-related design presents random short-duration events drawn from the different conditions within the experiment, providing superior temporal resolution characteristics. This approach permits the temporal characterization of BOLD signal changes. A mixed design contains features of both these design types.
After completed data collection, the critical question is whether there are differences or commonalities between the different experimental conditions. To test for this, several types of comparison are possible. One central comparison strategy is the subtraction method, in which the BOLD response for the experimental condition has subtracted from it the BOLD response acquired from the control condition. The factorial strategy is an alternative to the subtraction strategy in which all experimental conditions are processed as experimental factors. This strategy also allows testing for interactions between the conditions. Some experimental tasks show different levels of difficulty. Given this, a parametric design can be used to test whether there is an increase of the BOLD effect that systematically varies with an increase of task difficulty. Each of the comparison strategies aims to detect differences between experimental conditions. In contrast, a conjunction analysis offers the possibility to detect the commonalities between the BOLD patterns of two conditions by calculating the intersection between the two conditions.
Functional magnetic resonance imaging (fMRI) has already had a strong impact on research in fields, such as action observation, motor imagery, and attention, and has great potential to impact other key topics in sport psychology and motor control as interactive actions, emotion, and empathy. Recently, imaging genetics has started to reveal new directions for brain imaging. Genes have an effect on neural activity on the molecular level. Different concentrations of neurotransmitters moderate neural activity in different cognitive tasks. Brain imaging may help to elucidate this complex interaction between genes and neural activity.
The striking development of functional brain imaging has been driven by the technical advances of the last 20 years; fMRI has become a standard tool in cognitive neuroscience. It is complemented by magnetoencephalograpy (MEG), which records magnetic fields produced by electrical currents in the working human brain; near infrared spectroscopy (NIRS), which measures changes in cerebral blood flow, similar to fMRI but vulnerable to movement, only useful on the cortex, and does not reach deeper regions; and electroencephalography (EEG), which measures electrical activity along the scalp. EEG also offers tools for functional brain imaging with low-resolution brain electromagnetic tomography (LORETA). These methods differ with respect to the fundamental limitations concerning the range of active movements feasible during data recording, with EEG and NIRS offering an advantage in this regard.
There has been no doubt that the advent of new methods of brain imaging, data recording, and data analysis has facilitated progress in understanding cognitive processes. Neuroimaging must build on, rather than replace, the importance of-well-designed research with strong theory-driven hypotheses.
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