1. EE141 1 Imaging the Living Brain Janusz A. Starzyk Based on book Cognition, Brain and Consciousness ed. Bernard J. Baars Cognitive Architectures

2. EE141 2 Introduction  The brain imaging has been a breakthrough technology for cognitive neuroscience and cognitive psychology.  Before these techniques were developed brain study was based on experiments on animals, and injured human beings.  But brain injuries are imprecise, damaged areas are hard to locate, and often observed post-mortem (as in case of Broca’s and Wernicke’s patients).  Brain also compensates for the damage, lesions change over time, adaptation occurs, so that post mortem examination is very imprecise.  Animal studies depend on presumed homologies – not very convincing.  No other animals can speak to communicate clearly what they experience.

3. EE141 3 Introduction  The brain study was enhanced by imaging techniques like electroencephalography (EEG) based on X-rays computer tomography, positron emission tomography (PET), magnetic resonance imaging (MRI) etc.  We can observe functional activity of the brain  Magnetic imaging technique known as diffusion tractography allows to view white (myelinated) fiber tracts from cortex to the spinal cord.

4. EE141 4 Brain recording  Individual neuron’s activities can be recorded.  Picture shows spike counts for a single neuron in response to various images.  This particular neuron responds selectively to images of Jennifer Aniston.

5. EE141 5  Electroencephalography, (EEG)  Magnetoencephalography, (MEG)  Arteriography or Angiography  Computerized tomography, (CAT)  Single Photon Emission Computer Tomography, (SPECT)  Positron Emission Tomography, (PET)  Magnetic Resonance Imaging, (MRI)  Functional MRI, (fMRI)  Magnetic Resonance Spectroscopy, (MRS) Brain imaging techniques

6. EE141 6 Time-space tradeoff  Most popular imaging methods are compared for their time vs space resolution.  They do not have yet resolution to track a single neuron or a cluster of neurons.  fMRI has good spatial resolution and poor temporal resolution.  Magnetoencephalography (MEG) has a good temporal resolution but cannot locate precisely the source of firing.  Some studies combine EEG and fMRI

7. EE141 7 Single-neuron recording  Hubel and Wiesel (1962) received Nobel price for single-neuron activities recording in the cortex of a cat.  Depth electrodes used in humans only in very special cases – eg. before surgery in epileptic patients.

8. EE141 8 Single-neuron recording  Single neuron recording gives us only a partial information about the brain function.  Many scientists believe that brain processes can only be observed on the population of neurons. Conscious and unconscious observations

9. EE141 9 Animal and human studies  Until recently, studies of macaque monkeys were dominant source of information about vision, memory, attention and executive function of brain  Their brains have similar functional regions with minor anatomical differences

10. EE141 10 Electroencephalography EEG frequencies  Delta is the lowest frequency < 4 Hz and occur in a deep sleep or vegetative state of brain characterizing an unconscious person.  Theta has frequency 3.5-7.5 Hz, observed during some sleep states and during quiet focus (meditation). They are observed during memory retrieval.  Alpha waves are between 7.5 and 13 Hz. They originate from occipital lobe during relaxation with eyes closed but still awake.  Beta activity is fast irregular at low voltage 12-30 Hz. Associated with waking consciousness, busy or anxious thinking, and active concentration.  Gamma generally ranges between 26 and 70 Hz. Characterizes active exchange of information between cortical and subcortical regions.

11. EE141 11 EEG observations  EEG reveals patters during sleep, waking abnormalities, even response to music.  High density array of EEG electrodes placed on scalp at precise locations pick up signals from dendrites of the outside layers of cortex.  Fourier analysis of EEG signal helps to classify observed responses.

12. EE141 12 Magnetoencephalography (MEG)  Measures magnetic field produced by brain activities.  Is has spatial resolution of few millimeters and temporal resolution of few milliseconds.  MEG uses Magnetic Source Imaging (MSI) to superimpose magnetic activities onto brain anatomical pictures provided by MRI.  MSI is used before brain surgery to locate vital parts of the brain that must be protected during surgery.

13. EE141 13 Magnetoencephalography (MEG)  Due to magnetic field properties, MEG is sensitive to dendritic flow at the right angles to the walls of cortical folds (sulci). Magnetic field produced by a neuron Not sensitive to top or bottom neuron activities

14. EE141 14 Transcranial Magnetic Stimulation  TMS works at the milliseconds scale so it is a useful technique to study contribution of specific brain regions to cognitive process.  In this example TMS is applied to Brocka’s and Wernicke’s regions in the left hemisphere.  TMS is safe at mild levels of intensity and frequency.

15. EE141 15 fMRI  EEG and MEG measure brain activity directly.  Currently the most popular techniques fMRI (functional magnetic resonance imaging).  fMRI measures the oxygen level in local blood circulation technique called BOLD (blood-oxygen level dependent activity).  When neurons become active, local blood flow to those brain regions increases, and oxygen-rich blood occurs 2-6 sec later

16. EE141 16 fMRI principle of operation  Magnetic field aligns spins of oxygen atoms.  When the field is turned off spins return to their random orientations.  This relaxation of nuclear spin is picked up by sensitive coils and localized in 3D.

17. EE141 17 Positron emission tomography (PET) vs fMRI  PET was developed much earlier that MRI.  Provides a measure of metabolic brain activity.  It is very expensive and requires a cyclotron.  Subject must be injected with a radioactive tracer. PET scans showing speaking, seeing, hearing and producing words

18. EE141 18 Visual experiment with fMRI  fMRI images were obtained comparing face objects to nonface objects.  Subjects were supposed to match faces and their location.  Figure shows fMRI of brain activity in two different tasks.  Notice that location matching activates different brain area than face matching.

19. EE141 19  Brain imaging techniques can illustrate activities of a single neuron, large cortical structures, dynamic brain activity, and neurons connectivity.  We learned about a number of most important methods for brain imaging and discussed their properties.  Brain imaging transformed study of human cognition.  Combination of methods is used to enhance observation accuracy in time and space.  New methods are constantly being produced.Summary