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 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 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 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 fMRI Voxels in Primary cortex tracked all stimuli

Experimental Design in fMRI Voxels in area MT tracked only the onset of motion

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

Limitations of fMRI All techniques have constraints and limitations A good scientist is careful to interpret data within those constraints

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

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 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 Single or multiple electrodes are inserted into the brain “chronic” implant may be left in place for long periods

Intracranial and “single” Unit Single electrodes may pick up action potentials from a single cell An electrode may pick up the combined activity from several nearby cells spike-sorting attempts to isolate individual cells

Intracranial and “single” Unit Simultaneous recording from many electrodes allows recording of multiple cells

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” 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” 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 Intracranial electrodes typically cannot be used in human studies

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

Electroencephalography and the Event-Related Potential 1929 – first measurement of brain electrical activity from scalp electrodes (Berger, 1929)

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 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 pyramidal cells span layers of cortex and have parallel cell bodies their combined extracellular field is small but measurable at the scalp!

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

Electroencephalography Electrical potential is usually measured at many sites on the head surface More is sometimes better

Electroencephalography EEG changes with various states and in response to stimuli