Electrophysiological Methods for Mapping Brain Motor and ...ric.uthscsa.edu/.../RADI6017-chapters-9-2010.pdf · Methods for Mapping Brain Motor and Sensory Circuits RADI 6017 Oct

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