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Opportunity to Participate. EEG studies of vision/hearing/decision making – takes about 2 hours Sign up at www.tatalab.ca Keep checking back there for more time slots Two extra points added to your final grade!. Functional Imaging.

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Opportunity to participate

Opportunity to Participate

  • EEG studies of vision/hearing/decision making – takes about 2 hours

  • Sign up at www.tatalab.ca

    • Keep checking back there for more time slots

  • Two extra points added to your final grade!


Functional imaging

Functional Imaging

  • blood flow overshoots baseline after a brain region is activated

  • More oxygenated blood in that region increases MR signal from that region


Experimental design in fmri

Experimental Design in fMRI

  • A voxel in tissue that responds to the task shows signal change that matches the timecourse of the stimulus

Signal

Active

Rest

Active

Rest

60 sec

60 sec

60 sec

60 sec


Experimental design in fmri1

Experimental Design in fMRI

  • A real example of fMRI block design done well:

    • alternate moving, blank and stationary visual input

Moving

Blank

Stationary

Blank

40 sec

40 sec

40 sec

40 sec


Experimental design in fmri2

Experimental Design in fMRI

  • Voxels in Primary cortex tracked all stimuli


Experimental design in fmri3

Experimental Design in fMRI

  • Voxels in area MT tracked only the onset of motion


Experimental design in fmri4

Experimental Design in fMRI

  • Voxels in area MT tracked only the onset of motion

  • How did they know to look in area MT?


Pet another way to measure blood oxygenation

PET: another way to measure blood Oxygenation

  • Positron Emission Tomography (PET)

  • Injects a radioisotope of oxygen

  • PET scanner detects the concentration of this isotope as it decays


Advantages of fmri

Advantages of fMRI

  • Advantages of MRI:

    • Most hospitals have MRI scanners that can be used for fMRI (PET is rare)

    • Better spatial resolution in fMRI than PET

    • Structural MRI is usually needed anyway

    • No radioactivity in MRI

    • Better temporal resolution in MRI


Advantages of pet

Advantages of PET

  • Advantages of PET:

    • Quiet

    • A number of different molecules can be labeled and imaged in the body


Limitations of fmri

Limitations of fMRI

  • All techniques have constraints and limitations

  • A good scientist is careful to interpret data within those constraints


Limitations of fmri1

Limitations of fMRI

  • Limitations of MRI and PET:

    • BOLD signal change does not necessarily mean a region was specifically engaged in a cognitive operation

    • Poor temporal resolution - depends on slow changes in blood flow

    • expensive


Electrophysiology

Electrophysiology


Neurons are electrical

Neurons are Electrical

  • Remember that Neurons have electrically charged membranes

  • they also rapidly discharge and recharge those membranes (graded potentials and action potentials)

  • Review relevant textbook sections if this isn’t familiar to you


Neurons are electrical1

Neurons are Electrical

  • Importantly, we think the electrical signals are fundamental to brain function, so it makes sense that we should try to directly measure these signals

    • but how?


Intracranial and single unit

Intracranial and “single” Unit

  • Single or multiple electrodes are inserted into the brain

  • “chronic” implant may be left in place for long periods


Intracranial and single unit1

Intracranial and “single” Unit

  • Single electrodes may pick up action potentials from a single cell

  • An electrode may pick up thecombined activity from several nearby cells

    • spike-sorting attempts to isolate individual cells


Intracranial and single unit2

Intracranial and “single” Unit

  • Simultaneous recording from many electrodes allows recording of multiple cells


Intracranial and single unit3

Intracranial and “single” Unit

  • Output of unit recordings is often depicted as a “spike train” and measured in spikes/second

Stimulus on

Spikes


Intracranial and single unit4

Intracranial and “single” Unit

  • Output of unit recordings is often depicted as a “spike train” and measured in spikes/second

  • Spike rate is almost never zero, even without sensory input

    • in visual cortex this gives rise to “cortical grey”

Stimulus on

Spikes


Intracranial and single unit5

Intracranial and “single” Unit

  • By carefully associating changes in spike rate with sensory stimuli or cognitive task, one can map the functional circuitry of one or more brain regions


Subdural grid

Subdural Grid

  • Intracranial electrodes typically cannot be used in human studies


Subdural grid1

Subdural Grid

  • Intracranial electrodes typically cannot be used in human studies

  • It is possible to record from the cortical surface

Subdural grid on surface of Human cortex


Electroencephalography and the event related potential

Electroencephalography and the Event-Related Potential

  • Could you measure these electric fields without inserting electrodes through the skull?


Electroencephalography and the event related potential1

Electroencephalography and the Event-Related Potential

  • 1929 – first measurement of brain electrical activity from scalp electrodes (Berger, 1929)


Electroencephalography and the event related potential2

Electroencephalography and the Event-Related Potential

Voltage

Time

  • Place an electrode on the scalp and another one somewhere else on the body

  • Amplify the signal to record the voltage difference across these electrodes

  • Keep a running measurement of how that voltage changes over time

  • This is the human EEG


Electroencephalography and the event related potential3

Electroencephalography and the Event-Related Potential

  • 1929 – first measurement of brain electrical activity from scalp electrodes (Berger, 1929)

    • Believed to be artifactual and/or of no significance

    • Currently Google Scholar search for “human EEG” returns “about 383,000” hits

      • That’s about 13 papers per day


Electroencephalography

Electroencephalography

  • pyramidal cells span layers of cortex and have parallel cell bodies

  • their combined extracellular field is small but measurable at the scalp!


Electroencephalography1

Electroencephalography

  • The field generated by a patch of cortex can be modeled as a single equivalent dipolar current source with some orientation (assumed to be perpendicular to cortical surface)

Duracell


Electroencephalography2

Electroencephalography

  • Electrical potential is usually measured at many sites on the head surface

  • More is sometimes better


Electroencephalography3

Electroencephalography

  • EEG changes with various states and in response to stimuli


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