Electrophysiological Methods for Mapping Brain Motor and Sensory

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
Methods of Brain Stimulation
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
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
Microelectrodes
• Sharpened rods of tungsten, platinum/iridium
• Insulated with glass, epoxy
• Uninsulated tip of 5-12µA
• Low impedance: 0.7 – 1.5 MΩ
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)
500 pulses
14,000 triggers
40x10 pulses