A brief lecture on cortical sensation: components, when and how to examine and involved brain areas and tracts.

1. Cortical
Sensations
21/10/2018
Yasser Alzainy

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PosteriorColumn Pathway
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○ Proprioception, vibration, pressure, fine and
discriminative touch.
○ Three-order-neuron pathway
○ Project to:
• Somatosensory cortex.

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Anterolateral Pathway
○ Pain, temperature and crude touch
○ Three-order-neuron pathway
○ Project to:
• Somatosensory cortex: (spinothalamic tract) {localization}
• Pontine reticular formation: (spinoreticular tract) {emotion
and arousal}
• Mesencephalic pain modulation centers:
(spinomesencephalic tract) {pain modulation}
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Classificationof Neocortex
Neurology Department – Al-Azhar University
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○ Primary sensory and motor cortex
• Primary sensory cortex
• Primary motor cortex
○ Association cortex
• Unimodal (modality-specific)
 Somatosensory, visual or auditory association cortex, premotor
cortex, supplementary motor area
• Heteromodal (higher-order)
 Prefrontal cortex, parietal and temporal heteromodal
association cortex

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○ Primary sensory areas: perceive
○ Sensory association cortex: interpret
• Integrates information from more than one primary
modalities
• Correlates the perceived stimulus with memory of past
stimuli (discrimination, recognition)
• Crude sensations such as recognition of temperature and
pain are sub-served by the thalamus and has nothing to do
with the cortex which is concerned with finer aspects of
sensation such as recognition of intensity, location and
spatial relationships.
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○ Impairment = agnosia (loss of the power to recognize
the meaning of sensory stimuli.
○ It the term denotes lesion involving the association
cortex responsible for processing the primary sensory
modality.
○ The primary modalities must be relatively preserved
before concluding that a deficit in combined sensation
is due to a parietal lobe lesion.
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Neurology Department – Al-Azhar University
Testing for cortical sensation(e.g. stereognosis)
Primary Sensory Modalities
Intact
can be assessed
Impaired
Mild
depends on the judgment of the neurologist
Severe
can’t be assessed

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Cortical modalities
○ Stereognosis
○ Graphesthesia
○ Two-point discrimination
○ Sensory attention
○ Other gnostic / recognition functions
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Stereognosis
○ Cutaneous and proprioceptive sensations
○ Steps of object recognition
• Size
• Shape (2-D)
• Form (3-D)
• Identification of the object (memory).
○ Each step can be assessed individually
○ Tested only in the hands
○ Varieties: texture recognition (cotton, wool, glass,
metal)
○ Loss = Astereognosis
○ The earliest to be affected in parietal cortex lesion
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Graphesthesia
○ Cutaneous sensation
○ Letters or numbers drawn with blunt pin on finger pads,
palms and dorsum of feet.
• 1 cm on finger pads
• Larger elsewhere
○ Loss = Agraphesthesia / Grapheanaesthesia
○ Varieties: tactile movement sense / directional
cutaneous kinesthesia
• the ability to tell the direction of movement of a light
scratch stimulus drawn for 2 to 3 cm across the skin), which
may be a sensitive indicator of function of the posterior
columns and primary somatosensory cortex.
○ Even minimal impairment of primary sensory
modalities may cause agraphesthesia.
○ Loss of graphesthesia or the sense of tactile movement
with intact peripheral sensation implies a cortical
lesion, particularly when the loss is unilateral.
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Two point/ spatial discrimination
○ Tactile sensation.
○ Patient education (eyes closed)
• 2 points
• 1 point
• 2 points so close it feels like 1.
○ Then varied randomly and decreasing the distance till
the patient starts to give errors.
○ Result recorded as the minimum distance before the
patient starts to give errors.
○ Normal values:
• 1 mm at the tip of the tongue
• 2-3 mm on the lips
• 2-4 mm on the fingertips
• 8-12 mm on the palms
• 20-30 on the back of the hand
• 30-40 on the dorsum of the feet
○ The most sensitive test for cortical sensory impairment.
Neurology Department – Al-Azhar University
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SensoryAttention
○ The ability to perceive 2 simultaneous sensory stimuli
○ Usually comparing 2 sides of the body (can be done on
the same side); usually tested by light touch (can be
done by pin prick).
○ Loss of this function (one of the stimuli is not felt)=
sensory extinction, inattention or neglect.
• Severity of extinction can assessed by increasing the
intensity of the stimulus.
• Most likely due to parietal lobe lesion (can also occur with
thalamic and sensory radiation lesions)
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Stimulus Localization
○ Testing by asking the patient to point with his index
finger to the point touched by the examiner.
○ Accuracy varies as occurs with two-point
discrimination.
○ Loss of function: denotes lesion involving the
contralateral parietal lobe.
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Other SomatosensoryAgnosias
○ Autotopagnosia = inability to identify body parts, orient
the body, or understand the relation of individual parts
-a defect in the body scheme.
○ Hemineglect = lack of awareness of one-half of the
body.
○ Finger agnosia = inability to name or recognize fingers.
{usually part of Gerstmann’s syndrome}
○ Anosognosia = an absence of awareness, or denial of
the existence, of disease. {usually found in lesions of
the right parietal lobe}.
Neurology Department – Al-Azhar University
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‫فني‬‫ر‬‫عا‬ ‫مش‬‫أنا‬

15. Thank You
Yasser Alzainy
010 166 202 30
yasser.alzainy@gmail.com
www.facebook.com/yasser.alzainy