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Electrophysiological Methods for Mapping Brain Motor and Sensory Circuits
RADI 6017 Oct 27th, 2010
Historical Perspective
• Periphery mapped to central brain regions
• Motor and Sensory maps:
• Localization of function
• Way the brain is organized
• How does the brain work?
Maps of Motor Cortex by Electrical Stimulation
Fritsch and Hitzig (1870) :
Dog cortex
orderly and specific peripheral movements
Ferrier (1876, 1886): Rhesus monkey and dogs
Motor and sensory humunculus
Direct Electrical Stimulation • surface electrodes • Deep needle electrodes • Single-unit microelectrode
Transcranial • Electrical stimulation (TES): Merton and Morton, 1980 • Magnetic Stimulation (TMS): Barker et al, 1987
Methods of Brain Stimulation
Structural vs. Functional Brain maps
Structural maps • Neural hardware/anatomical connectivity • By electrical stimulation of neurons and pathways • Stimulate at one site and record at a remote site • Output variables:
• Intracellular potentials • Field potentials • Extracellular unit potentials • Electromyography (EMG)
Functional maps: • Spatial representation of a natural parameter • Localization of natural stimuli (sensory) and movements (motor)
Strengths of Electrophysiological mapping
Advantages • Spatial resolution: at the level of single neuron • Construct more global mapping • Temporal resolution: milliseconds
Disadvantages • invasive • Time and labor intensive • Limited to primary motor and sensory (somatosensory, auditory and visual) areas
• Sharpened rods of tungsten, platinum/iridium
• Insulated with glass, epoxy
• Uninsulated tip of 5-12µA
• Low impedance: 0.7 – 1.5 MΩ
Microelectrodes
Sensory vs. Motor Mapping
Sensory Maps • Mapping a specific sensory parameter to a brain region • One input variable: Stimulus • One output measure: unit recording from region of interest • One anatomical map and one functional map • Receptive fields: naturally occurring stimulus modality to which the neuron is most responsive
Sensory vs. Motor Mapping
Motor Maps • Several input and output measures • Harder than sensory mapping • Activation of muscles in isolation is difficult • Motor fields: all movements that engage a neuron
• Functional (type of movement) • Structural (target muscles)
• Neuroantomic labeling • TMS
Topographical organization of pathways from somatosensory cortex, pontine nuclei, and cerebellar hemispheres
Leergaard et al., European Journal of Neuroscience, Vol. 24, pp. 2801–2812, 2006
★
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(active and passive)
Input Measures: Electrical stimulation
Common method in motor mapping Magnitude of response depends on:
• Stimulus current • Stimulus duration • Stimulus polarity • Electrode dimensions
Extent of effective current spread from a stimulating electrode I = a + kd2
Input: Electrical Stimulation
Transcranial Stimulation
TES
TMS
Mapping Motor Output with Electrical Stimulation of the cortical surface�
Mapping Motor Output with Intracortical Microstimulation (ICMS)�
Mapping Motor Output with High-Density Microelectrode Arrays�
Mapping Motor Output: Other Methods
Spike triggered averaging of EMG activity from single neurons
Stimulus triggered averaging of EMG activity (single pulse ICMS)
40x10 pulses 500 pulses 14,000 triggers