Somesthetic+(Supplementary)+Association+Area

Connections From
-Primary Somesthetic Cortices -Contraleral Primary zone of opposite hemisphere (area 5ab only) -Motor Association Areas (Area 6) in frontal Lobes -Posterior thalamus (i.e. ventral posterior complex)

Laterality
Due in large part to the callosal interconnections linking the secondary (supplementary) sensory areas in the right and left half of the brain, both halves of the body, the trunkal area in particular are represented in this region. Indeed, the two halves of the body appear to be superimposed such that the body is bilaterally represented in the secondary sensory areas.

However, bilateral representation is predominantly maintained within the right half of the human brain. It is this greater and bilateral degree of representation and the more unilateral nature of left parietal representation, which explains the greater incidence of neglect and the greater degree of hemiplegia and hemianesthesia which is seen after right vs left parietal lesions.

Hand Manipulation Cells
Small percentage of cells in area 5 also appear to be concerned with more complex activities such as the movement of the hand and arm and the manipulation of objects.A detailed representation of the cutaneous surface of the body, and in particular, the hand (and face), is maintained in here. These are referred to as "hand-manipulation cells".
 * Electrical stimulation** of area 5 can result in limb movements whereas experimentally induced tactile extinction of either the right or left hand can induce bilateral reductions in activity in this region.

In consequence, injury to this part of the brain can interfere with hand movements. Others neurons in area 5 are especially responsive to particular temporal-sequential patterns of sensation and can determine direction and rhythm of movement. It is presumably through the activity of these cells that one can "hear" via the detection of vibrations (such as reported by the deaf). In fact, auditory as well as visual simulation can induce activation of this region.

The Body In Space
Signals from joint and cutaneous receptors are transmitted to association neurons. Many association cells also receive converging input from primary neurons concerned with different body parts, and can combine these signals with visual information and are thus able to determine positional interrelationships.

Example: single association neuron may receive information regarding the elbow and the shoulder, and become activated only when these two body parts are simultaneously stimulated or in motion. A considerable number of cells are especially sensitive to the posture and position of the trunk and extremities during movement. By associating this convergent input these cells are thus able to monitor, coordinate and guide limb movement as well as determine the position of the body and objects in space.

Through the integrative and associative activities of the cell assemblies within area five, an interactional image of the body is maintained. In this manner, an individual is able to ascertain the positon of the body and the limbs at rest and in motion.

In part, this may be accomplished through comparisons with a more stable image of the body which is possibly maintained via the combined interactions of neurons in areas 3,1,2. That is a stable body image (or body image memories) are stored in these tissues. Hence, when the body has moved, this new information (received and processed in area 5) can be compared to the more stable trace (or memory maintained in the primary regions) so that the new position of the limbs and body can be ascertained. In this regard it could be argued that body-related memories are stored in the parietal lobe.

In the absence of movement (and in the absence of visual cues, such as when one wakes up in the middle of the night) one usually cannot tell where or in what position their arms or legs may be in. However, with a slight movement we can immediately determine position.

Tactile Discrimination Deficits & Stereognosis
-Abnormalities involving two-point discrimination -Position sense -Pressure sensitivity. -May be able to recognize that he is holding something in his hand he may be unable to determine what it might be (astereognosis). However, detection threshold is not altered.
 * massive destruction**: many of the same disturbances which occur following lesions of the primary region.

Example: May recognize touch, but be unable to localize what part of the body has been stimulated. i.e. naming the elbow when the leg is touched.


 * Small Lesions**: deficit will be only manifested when the part of the body represented is examined.

Example: may be able to localize touch and determine the direction of a moving stimulus when it is applied to the hand, face, or leg, but be unable to do so along the shoulder or arm if this area of the cortex has been compromised.

Lesions to the right parietal lobe are likely to give rise to bilateral abnormalities, whereas left parietal injuries generally effect only the right hand.
 * Laterality**

Astereognostic deficits require that the somesthetic area representing the hand be comprised.

Larger lesions extending into the posterior parietal lobe, area 7, also decrease the ability to perform size, roughness, weight and shape discriminations.