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Cerebral Localization in Neurosurgery- Pre and Intraoperative techniques

Cerebral Localization: Definition

1. "Mapping of the cerebral cortex into areas, and the correlation of these areas with cerebral function"

2. "Diagnosis of the location of a brain lesion in the cerebrum, done either by
       Signs and symptoms manifested
       Using any investigation modality"

History of Cerebral Localization

History of Cerebral Localization

History of Cerebral Localization

History of Cerebral Localization

Cerebral Localization Techniques

  • Clinical Localization
  • EEG
  • fMRI
  • MRI Tractography
  • Intracarotid Amytal or Wada Test 
  • Magnetic Encephalography
  • Transcranial Magnetic Stimulation

  • Electrocortical Stimulation 
  • Neuronavigation
  • Intraoperative Ultrasound
  • Intraoperative CT Scan
  • Intraoperative MRI
  • Optical Imaging 

Clinical Localization: Frontal Lobe

Localizing Symptoms/ Signs
Precentral gyrus
       Monoplegia or hemiplegia depending
on extent of damage
Broca’s area
       Motor or Expressive dysphasia
Supplementary motor area
       Paralysis of head and eye movement to
opposite side
       Head turns ‘towards’ diseased hemisphere and eyes look in the same direction.

Prefrontal areas
Three pre-frontal syndromes

1. Orbitofrontal syndrome
       Poor judgement
       Emotional lability
2.Frontal convexity syndrome
       Poor abstract thought
3. Medial frontal syndrome
       Sparse verbal output

Clinical Localization: Parietal Lobe

       Confusion of right and left limbs.
       ANOSOGNOSIA (lack of insight)

Postcentral gyrus
Contralateral disturbance of cortical sensations:
       Postural sensation
       Sensation of passive movements
       Tactile localization
       2 Point Discrimination (normally 4 mm on finger tips)
       Appreciation of size, shape, texture and weight (astereognosis).
       Perceptual rivalry (sensory inattention)- two stimuli applied to each side simultaneously, the patient is only aware of that one contralateral to the normal parietal lobe. As the time gap between application of stimuli is increased (approaching 2–4 seconds) the patient
becomes aware of both.
Supramarginal and angular gyri
       Wernicke’s dysphasia
Optic Radiation
       Lower homonymous quadrantanopia

Clinical Localization: Temporal Lobes 

Auditory cortex
Upper surface of the superior temporal gyrus, buried in the lateral sulcus (Heschl’s gyrus)
Cortical deafness
Bilateral lesions – Rare (eg: Bilateral Heschl's Gyrus Infarct), may result in complete deafness

Dominant - difficulty in hearing spoken words
Non Dominant - difficulty in appreciating rhythm/music (AMUSIA)
Middle and inferior temporal gyri
       complex partial seizures with déjà vu, jamais vu.
       Memory/ learning disturbances
Limbic lobe
       Olfactory hallucination with complex partial seizures
       Aggressive or antisocial behaviour
       Inability to establish new memories
Optic radiation
       upper homonymous quadrantanopia

Clinical Localization: Occipital Lobes

STRIATE cortex and PARASTRIATE cortex
Homonymous hemianopia with or without involvement of the macula, depending on the posterior extent of the lesion
Only occipital pole : Central hemianopia field defect involving the macula
Anton’s syndrome
       Involvement of both the striate and the parastriate cortices
       affects the interpretation of vision.
       Patient is unaware of his visual loss and denies its presence
Balint’s syndrome
       Inability to direct voluntary gaze
       Visual agnosia
       Due to bilateral parieto-occipital lesions

Functional MRI

Mechanism of Functional MRI

Uses of functional MRI

  1. as a screening tool to establish candidate regions of interest for a particular function 
  2. as an alternative to Wada testing to establish hemispheric dominance for language function or 
  3. following spatial and temporal patterns of neural plasticity to aid in timing of reoperation 
  4. to provide some level of functional data in patients unable to undergo awake craniotomy 

 Anisotropic Diffusion Tensor Imaging

  • demonstrate white matter tracts by using MRI to measure the direction of diffusion of water molecules as a marker for the axis of these tracts 
  • based on the restriction of diffusion of water by axonal membranes and myelin 
  • Water diffusion in axon tracts is direction dependent, or anisotropic. 
  • Demonstrates anatomic relationships of major white matter tracts relevant for tumor surgery 
  • Yields information regarding structure, not function 
  • Intrinsic brain tumors are associated with significant edema, which can distort DTI tractography locally 

Patterns of DTI

Four patterns of anisotropy have been observed:

  1. Normal signal with altered position or direction, corresponding to tract displacement; 
  2. Decreased but present signal with normal direction and location, thought to correspond to vasogenic edema
  3. Decreased signal with disrupted direction maps, thought to correspond to infiltration
  4. Loss of anisotropic signal corresponding to fiber tract obliteration or destruction 

Intracarotid Amytal or Wada Test  

  • Inhibition method in which the territory perfused by the internal carotid artery (ICA) on one side of the brain is temporarily anesthetized by injection of sodium amytal into the ICA
  • We observe onset of contralateral hemiparesis and EEG changes, a battery of behavioral tests is applied 
  • Examiner has only a few minutes to test each hemisphere 
  • Test is invasive, carrying a 0.6 to 1% risk of stroke 
  • Cross flow between hemispheres when it exists, can result in anesthesia of both hemispheres from a unilateral injection – resulting in false localisation

Magnetic Encephalography

Advantages of Magnetic Encephalography

  • Non invasive
  • MEG signal is not attenuated by the cranium and scalp 
  • Primarily use in the presurgical evaluation of epilepsy patients to localize epileptogenic foci
  • Has better spatial resolution of approximately 2 mm and better signal-to-noise ratio 
  • Primarily use in the presurgical evaluation of epilepsy patients to localize epileptogenic foci 
  • identifying function or absence of function near the tumor preoperatively 
  • 100% negative predictive value and a 64% positive predictive value for correlation of intraoperative stimulation of functional sites with MEG (Martino J, et al. 2011)

Disadvantages of Magnetic Encephalography

  • Currently MEG scanners are very expensive and have limited availability 
  • Not used routinely yet, as outcome data is not available

Transcranial Magnetic Stimulation

  • TMS involves the stimulation or inhibition of neuronal electrical activity via a magnetic field delivered at the scalp 
  • Activation - single pulse (used to map the motor cortex)
  • Inhibition - repetitive TMS (used to disrupt language processing )
  • poor spatial resolution and the risk of repetitive TMS causing seizures 

Intraoperative localization techniques


  • Electrode leads are placed directly over surgically-exposed cortical surface 
  • Needle-shaped depth electrodes are also used to monitor deep mesial temporal structures, such as the hippocampus
  • synchronized postsynaptic potentials  -> primarily in cortical pyramidal cells 
  • identification epileptogenic zones
  • ECoG monitoring can detect higher frequency EEG signals than those captured by non-invasive EEG (greater than 60 Hz) (Crone et al.,2006). 
  • Recent studies have shown potential use of pathological high-frequency oscillations (pHFOs) as markers for underlying epileptogenic foci (Engel et al., 2009) 


  • Accurately project computed tomography (CT) or magnetic resonance imaging (MRI) data into the operative field for defining anatomical landmarks, pathological structures and tumor margins.
  • frameless, interactive, computer-aided technique 
  • Shows real time position of the tip of an instrument in the corresponding images, without requiring a stereotactic frame for calculation
  • Main Reasons for limitations in accuracy:
  1. Intra-operative brain deformation (brain shift) 
  2. Local Tissue Deformation 
  3. Device/ Technique Related errors

Neuronavigation Workflow and Possible Origins of Deviation

Placement of skin fiducials
Improper placement/positioning
Preoperative neuroimaging
       Limited resolution of MRI/CT
       Patient position in scanner not identical with position in OR (soft- tissue displacement by gravity, head rest)
       Motion artifacts in MRI/CT
Delay until begin of surgery
       Displacement of skin fiducials
       Use of too few skin fiducials
       Deviations by Improper selection of skin fiducial center
       Image fusion mismatch
       Software-dependent deviation

Patient positioning
       Soft-tissue displacement by head holder frame
       Soft-tissue displacement by tracheal tube
       Improper fixation of reference frame
       Deviations by Physical error of optical localization method
       Improper threshold of 3-D skin reconstruction for surface matching
       Displacement of skin fiducials during Paired-point matching or improper targeting of fiducial center
       Software-dependent deviation
Preparations for surgery
       Movement of head in head holder frame by weight or traction from attachment of surgical drapes
       Improper attachment of reference marker balls to instruments
       Deviation by accidental movement

Minimising Brain shift in Neuronavigation

  • Displacement is greatest in the direction of gravity
  • Minimize diuretic usage
  • CSF shunting or drainage should be avoided before the primary approach has been made
  • Debulking of tumors “inside to outside” approach also yields significant tissue shifts
  • Defining outer margins before proceeding with debulking may minimize brain shift

Techniques of Intraoperative Brain Mapping

After dural opening -> Cortical Stimulator is applied with following parameters

  • biphasic square wave
  • 60 Hz, 1-ms duration
  • current range of 1 to 8 mA peak to peak
  • Stimulation using a bipolar electrode to the cortical surface for 2 seconds 
  • Positive cortical sites are labelled with sterile numbered paper squares

Motor Mapping
  • current increased in intervals of 0.5 mA until either an overt motor response or reproducible EMG activity 
  • EMG-based method is better -> decreased risk for intraoperative seizure 

Sensory mapping 
  • done similarly within the postcentral gyrus  
  • patients typically report dysesthesias
Subcortical mapping 

  • same stimulation parameters as for cortical mapping 
  • Done when the resection nears the CST system or Somatosensory pathways
  1. descending corticospinal fibers - e.g., deep, posterior limit of precentral gliomas
  2. Internal capsule  - deep border of insular gliomas
  3. Cerebral peduncle - deep border of temporal lobe gliomas
  4. Thalamocortical somatosensory pathways- retrocentral gliomas (elicits dysesthesias in awake patient )

Techniques of Intraoperative Mapping: Language

  • all sedation is discontinued before dural opening 
  • orientation and counting are checked to ensure a baseline level of patient function and cooperation 
  • begins with a simple counting 
  • sites of speech arrest on stimulation are identified
  • picture-naming task given and then stimulation is given
  • stimulation induced errors for at least two out of three trials
  • nature of the error is recorded
  1. Semantic paraphasia
  2. Phonemic paraphasia
  3. Anomia
  4. Perseveration

  • For multilingual patients -> map each language separately, starting with the patient’s primary language.
  • Throughout the period of tumor resection, the patient is asked to perform a “double task,” which includes regular movement of the arm (hand grasp and flexion at the elbow) in parallel with the picture-naming

Techniques of Intraoperative Mapping:Visual Pathways

Tumors at the junction of the parietal, temporal, and occipital lobes
On Cortical Stimulation-> primary visual cortex -> phosphenes seen in the contralateral hemifields
Subcortical Stimulation -> sequential slides that have a central fixation point as well as two pictures placed diagonally on the screen


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