Brain Imaging

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.

Experimental Designs

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.

Implications

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.

References:

  1. Amaro, E., Jr., & Barker, G. J. (2006). Study design in fMRI: Basic principles. Brain and Cognition, 60, 220–232.
  2. Baars, B. J., & Romsøy, T. (2007). The tools: Imaging the living brain. In B. J. Baars & N. M. Gage (Eds.), Cognition, brain, and consciousness: Introduction to cognitive neuroscience (pp. 87–120). Amsterdam: Elsevier.
  3. Eickhoff, S. B., Lotze, M., Wietek, B., Amunts, K., Enck, P., & Zilles, K. (2006). Segregation of visceral and somatosensory afferents: An fMRI and cytoarchitectonic mapping study. NeuroImage, 31, 1004–1014.
  4. Huettel, S. A., Song, A. W., & McCarthy, G. (2008). Functional magnetic resonance imaging (2nd ed.). Sunderlan d, MA: Sinauer Associates.
  5. Logothetis, N. K., Pauls, J., Augath, M., Trinath, T., & Oeltermann, A. (2001). Neurophysiological investigation of the basis of the fMRI signal. Nature, 412, 150–157.

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