Somatosensory information that will reach conscious perception travels in several pathways in the central nervous system. All of this decussates (crosses the midline), at different places, to eventually reach an area on the contralateral cerebrum called the primary somatosensory cortex, which is on the anterior part of the parietal lobe. Neurons from the primary somatosensory cortex then project to areas called association somatosensory cortex, which are also on the parietal lobe, posterior to the primary somatosensory cortex.
A complete lesion of the primary somatosensory cortex usually causes the complete loss of all the primary modalities of somatosensation of the contralateral face and body.
A complete lesion of association somatosensory cortex often spares perception of the primary somatosensory modalities, but usually causes loss of more complex perceptions (agnosias) from the contralateral side, such as recognizing the shape of objects by touch alone, called stereognosis, or recognizing symbols drawn on the skin by touch alone, called graphesthesia.
There is a map for how the body is represented on the primary somatosensory cortex called the somatosensory homunculus: the face and arm are lateral, the torso is superior, and the leg is medial.
Because of this, lesions of parts of the primary somatosensory cortex usually cause somatosensory abnormalities of the face and arm sparing the leg (lateral lesions), or of the leg sparing the face and arm (medial lesions). Deep in the cerebrum or the upper brainstem, and particularly in the thalamus, axons carrying somatosensory information from the face, arm, and leg travel close together, so that even small lesions usually cause somatosensory abnormalities of the face, arm, and leg together.
These patterns of somatosensory loss are often called cortical and subcortical.
Level 3 Unit 2 Part 09: Position, vibration, and fine touch perception of the body