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Electrophysiology Neurons are Electrical • Remember that Neurons have electrically charged membranes • they also rapidly discharge and recharge those membranes (graded potentials and action potentials) 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 • 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 signals from several nearby cells – spike-sorting attempts to isolate individual cells Intracranial and “single” Unit • Simultaneous recording from several 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 QuickTime™ and a decompressor are needed to see this pict ure. 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” QuickTime™ and a decompressor are needed to see this pict ure. 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 Intracranial and “single” Unit • Some complications: – Suppose we observe an increase in spike rate in two discrete regions of the brain in response to a sensory stimulus: What are the possible interpretations? Intracranial and “single” Unit • Some complications: – Suppose we observe an increase in spike rate in two discrete regions of the brain in response to a sensory stimulus: What are the possible interpretations? 1. Area A “drives” area B 2. Area B “drives” area A 3. Area A and B are controlled by a third area independently Intracranial and “single” Unit • Some complications: – Suppose we observe an increase in spike rate in two discrete regions of the brain in response to a sensory stimulus: What are the possible interpretations? 1. Area A “drives” area B 2. Area B “drives” area A 3. Area A and B are controlled by a third area independently and their activity is unrelated How might you differentiate these possibilities Intracranial and “single” Unit How might you differentiate these possibilities • Timing of spikes might help: – if A and B are synchronized they are probably functionally related – if A leads B then it is likely to be the first in the signal chain Subdural Grid • Intracranial electrodes cannot be used in human studies Subdural Grid • Intracranial electrodes cannot be used in human studies • It is possible to record from the cortical surface Subdural grid on surface of Human cortex Electroencephalography • It is also possible to record from outside the skull altogether! 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) Electroencephalography • Electrical potential is usually measured at many sites on the head surface QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this pict ure. QuickTime™ and a decompressor are needed to see this picture. 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 The Event-Related Potential (ERP) • Embedded in the EEG signal is the small electrical response due to specific events such as stimulus or task onsets, motor actions, etc. The Event-Related Potential (ERP) • Embedded in the EEG signal is the small electrical response due to specific events such as stimulus or task onsets, motor actions, etc. • Averaging all such events together isolates this event-related potential The Event-Related Potential (ERP) • We have an ERP waveform for every electrode The Event-Related Potential (ERP) • We have an ERP waveform for every electrode The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful • Sometimes we want to know the overall pattern of potentials across the head surface – isopotential map The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful • Sometimes we want to know the overall pattern of potentials across the head surface – isopotential map Sometimes that isn’t very useful - we want to know the generator source in 3D Brain Electrical Source Analysis • Given this pattern on the scalp, can you guess where the current generator was? Brain Electrical Source Analysis • Given this pattern on the scalp, can you guess where the current generator was? Brain Electrical Source Analysis • Source Analysis models neural activity as one or more equivalent current dipoles inside a headshaped volume with some set of electrical characteristics Brain Electrical Source Analysis Initiate the model Brain Electrical Source Analysis Initiate the model Project “Forward Solution” Brain Electrical Source Analysis Initiate the model Project “Forward Solution” Compare to actual data Brain Electrical Source Analysis Adjust the model Project “Forward Solution” Compare to actual data Brain Electrical Source Analysis This is most likely location of dipole Project “Forward Solution” Compare to actual data Brain Electrical Source Analysis • EEG data can now be coregistered with highresolution MRI image Anatomical MRI Brain Electrical Source Analysis • EEG data can now be coregistered with highresolution MRI image Anatomical MRI 3D volume is rendered and electrode locations are superimposed Brain Electrical Source Analysis • EEG data can now be coregistered with highresolution MRI image Magnetoencephalography • For any electric current, there is an associated magnetic field Electric Current Magnetic Field Magnetoencephalography • For any electric current, there is an associated magnetic field • magnetic sensors called “SQuID”s can measure very small fields associated with current flowing through extracellular space Electric Current Magnetic Field SquID Amplifier Magnetoencephalography • MEG systems use many sensors to accomplish source analysis • MEG and EEG are complementary because they are sensitive to orthogonal current flows • MEG is very expensive QuickTime™ and a decompressor are needed to see this picture.