Parietal+Lobe

= Parietal Lobes = Senior Executive of the Body in Physical & Visual Space Body Image, Visual Space, Neglect, Denial, the "Lobe of the Hand" Apraxia and Gerstmann's Syndrome

The parietal lobes are the senior executive of the physical-body-in-space and maintains one's personal image of the body, both physical and visual. The parietal lobes receives distinct sensory impressions from the entire body and can feel "pain" or a bug crawling on one's arm, leg, or face. It is the parietal lobe which guides the movement of the body in space, coordinating body movement while running, walking, skipping, or climbing over obstacles. The parietal lobe are also considered a "lobe of the hand" and receives sensory sensations from the bones, tendons, muscles, and skin of the hand, and guides the movement of the hand in visual-space. Therefore, the ability to reach for and manipulate a tool, open and remove the cap from a bottle and pour the contents into a glass, are made possible by the parietal lobe in association with the frontal motor areas and the visual cortex. The parietal lobes are not a homologous tissue but consists of cells which are responsive to a variety of divergent stimuli, including movement, hand position, objects within grasping distance, audition, eye movement, pain, heat, cold, as well as complex and motivationally significant visual stimuli

Example: Area 7 of the superior parietal lobe: receives considerable visual input, particularly from the lower visual fields and periphery; also complex somesthetic stimuli regarding the hand and objects beyond and within reaching distance. Moreover, different neurons in different regions of area 7 perform somewhat different functions. Even the subareas within the primary somesthetic neocortex, areas 3ab,1, 2, respond to different stimuli and receive projections from different subregions within the thalamus, which in turn receive different forms of input from skin, joints and muscles.

Damage/Injury
Because of diverse yet related functions, damage can result in a variety of disturbances depending on which area involved, Including abnormalities involving: -Somesthetic and pain sensation -The body image -Visual-spatial relations -Temporal-sequentual motor activity -Language -Grammar -Numerical calculation -Emotion -Attention

Associated with inferior parietal lobe / Multimodal assimilation area Anomia Agraphia Conduction Aphasia Lateralized Temporal-Sequential Functions Apraxia Acalculia (Including Number Agnosia, Alexia, Agraphia) Right-Left Disorientation Attention and Neglect (particularly left sided neglect), including secondary delusional denial, disconnection, confabulation, gap filling, delusional playmates, egocentric speech
 * GERSTMANN'S SYNDROME:** **Finger Agnosia**

**PRE-PARIETAL PROCESSING**
There is no general consensus as to peripheral receptors or the specific roles they play in somesthetic perception, some argue that specific somatic receptors transmit modality-specific signals which provide distinct information along discrete neural pathways, others argue that there are few "organized receptors" and that most "receptors are not essential for the recognition of..." for example, "cold and warmth", and that "skin receptors have specialized properties" only "for the transduction of particular ranges of stimuli... " and do not transmit "modality-specific information". Adams and Victor (1993) do admit, however, that "the quality of sensation depends on the type of fiber that is stimulated."

Different putative receptors for sensation: mechanoreceptors -
 * Pacini's and Meissner's corpuscles (superficial):** rapidly adapting, signal the onset and offset of stimulation
 * Ruffini's and Meissner's corpuscles (subcutaneous):** slowly adapting, transmit information over long time periods. However, both rapid and slowly adapting receptors are found in the superficial skin (Pacini's and Ruffini's) and subcutaneous tissue (Meissner's and Meissner's).

Hence, both slow and fast adapting receptors may be stimulated simultaneously and provide a wealth of information regarding external stimulation--information.
 * Information transmission:** to sensory neurons located in the dorsal root ganglia -> dorsal columns of the spinal cord where they terminate in the gracile and cuneate nucleus of the medulla -> cerebellum and upper brainstem via the dorsal column pathways, projecting through through the medial lemnsical pathway where axons carry information terminates in the ventral posterior nucleus of the thalamus (VPL and VPM), wherea again, considerable processing takes place. Thalamic nuclei subdivisions (subnuclei) : each projects to specific regions within the primary and association somesthetic receiving areas, as well as to the insular region of the parietal lobe.

Specifically VPL and VPM act to relay impulses received from the ascending trigeminothalamic, spinothalamic, and medial lemniscus pathways to cortical layer IV. This input is somatotopically organized and the entire body surface comes to be spatially represented in the parietal neocortex.



PARIETAL TOPOGRAPHY
Nine major somesthetic areas within the parietal lobe: PRIMARY SOMESTHETIC RECEIVING AREAS (Brodmann's areas 3ab,1,2) Somesthetic (Supplementary) Association Area Polymodal Receiving Area (visual, motor, somesthetic) (Area 7ab) Granular insular Area

Pain: Posterior Parietal Areas 5, 7, & Supramarginal Gyrus The Inferior Parietal Lobe - Multimodal-assimilation area (Areas 7, 39, 40)

The primary somesthetic as well as portions of the association area contribute almost one third of the fibers which make up the cortical-spinal (pyramidal) tract. Hence, this region is very involved in motor functioning; e.g., the sensory and postural guidance of movement, including hand movements and the direction of gaze.

Moreover, the primary motor and somesthetic association regions are richly interconnected with the primary areas transmitting to the association areas which in turn project to the motor cortex.In order to make motoric responses with some precision, there must be tremendous sensory feedback concerning proprioception, including data regarding the positions of the various joints and tendons, etc. --information which is provided by the somesthetic cortices. Together, the motor and somesthetic areas comprise a single functional unit which some have referred to as the sensorimotor cortex.



THE RIGHT & LEFT PARIETAL LOBE: LESIONS & LATERALITY
Lesions of the superior and inferior parietal lobule (of which area 7 is part) and the parietal-occipital junction can greatly disturb: -Ability to make eye movements -Maintain or shift visual attention -Visually follow moving objects -In the extreme result in oculomotor paralysis
 * ATTENTION & VISUAL SPACE**

Right parietal lesions are associated with deficiencies in depth perception and stereopsis -AbilIty to determine location, distance, spatial orientation and object size -Visual constructional abilities may also be compromised -Visual-spatial disorientation and appear clumsy. -Defective performance on line orientation tasks, maze learning, -Ability to discriminate between unfamiliar faces -Ability to select from the visual environment stimuli which are of importance -Can result in a complete neglect of the left half of visual space VS. right parietal-occipital damage = also -Deficient on tasks requiring detection of imbedded figures -May also have severe problems with dressing (e.g. dressing apraxia) -May become easily lost or disoriented even in their own homes.

VS. Left parietal injuries result in only minimal right sided neglect. This is because the right parietal lobe not only attends to both halves of the body, but both halves of visual space. LOCALIZATION OF OBJECTS IN SPACE Right hemisphere lesion: perform defectively on general visual localization tasks. Left hemisphere lesions: seems to impact/control localization of only objects withing grasping/manipulation distance

EMOTION Some of the effects of lesions in this region also include altered emotional-motivational functioning, body and visual-spacial neglect, as well as clumsiness and visual-spatial disorganization. With massive right parietal lesions involving area 7, many patients are often initially hypokinetic and seem very passive, inattentive, unresponsive and take very little interest in their environment. Moreover, when their disabilities are pointed out (e.g. paresis, paralysis), they may seem indifferent or conversely euphoric. Area 7, as well as 5 (and the inferior parietal lobule) receive auditory information and are capable of discerning the emotional-motivational significance of this input as well as differentiating between different vocal-emotional characteristics --especially right parietal neurons. Hence, when the right parietal region is damaged, patients not only seem unconcerned about their disability, but they may have difficulty perceiving and differentiating between different forms of emotional speech.

Inferior? Agraphia Lateralized Temporal-Sequential Functions Apraxia Acalculia (Including Number Agnosia, Alexia, Agraphia) Right-Left Disorientation Attention and Neglect (particularly left sided neglect), including secondary delusional denial, disconnection, confabulation, gap filling, delusional playmates, egocentric speech
 * GERSTMANN'S SYNDROME:** **Finger Agnosia**

SUMMARY & OVERVIEW The parietal lobes, although commonly associated with the mediation of somesthetic stimuli, are also concerned with motor and attentional functioning, the perception of spatial relations, including depth, orientation, location, and the identification of motivationally significant auditory, somesthetic, and visual stimuli. It is via the integrative interaction of the parietal lobe that an individual can feel an object and not only determine its physical qualities, (e.g. shape, size, weight, texture), but can visualize, verbally lable, and even write out it's name. It is also via the interaction of these various cell assemblies that an individual can attend to specific objects in space as well as reach out and manipulate them. Indeed, the parietal lobe is very important for mediating movement in visual space. That is, in order for a movement to be correctly planned and carried out signals must be directed to the right muscle groups as based on efferent visual, somesthetic, as well as auditory input. Moroever, one requires sensory information as to the position of the body and limbs in space, otherwise movements will be clumsy. This is accomplished through the dense interconnections linking the primary somesthetic with the motor areas in the frontal lobe and via impulses transmitted down the cortocal-spinal tract. Moreover, parietal neurons appear to guide and monitor movements as they occur in visual space. Although the entire parietal lobule makes important contributions to movement, the inferior parietal lobule of the left hemisphere appears to be the central region of concern in regard to the performance of skilled temporal-sequential motor acts. These engrams assist in the programing of the motor frontal cortex where the actions are actually executed. If the inferior parietal region is destroyed the patient loses the ability to perform actions in an appropriate temporal-sequence or to even appreciate when they have performed an action incorrectly. This condition is referred to as apraxia. In contrast, right parietal injuries are associated with severe disturbances of emotion, constructional deficiencies, as well as a host of visual-spatial perceptual abnormalities including left sided inattention and neglect. For example, with severe lesions in this area patients may demonstrate a profound inattention to all forms of stimuli falling to their left. When drawing pictures they may fail to draw the left half of an object, when writing or reading they may ignore the left half of words or the left half of the page, and they may fail to perceive and respond to individuals standing to their left, or even the left half of their own body. Largely this condition is secondary to a destruction of neurons which are sensitive to various forms of visual and somesthetic input. Nevertheless, it is important to emphasize that lesions to the parietal lobe (or anywhere within the brain) are seldom localized to one particular quadrant (e.g. inferior, superior), or even restricted to the parietal lobe. That is, damage may be parietal-occipital, parietal-temporal, frontal-parietal, or even bilateral (such as due to cerebrovascular disease or compression from a unilateral tumor). Therefore, an afflicted individual may display agraphia but normal reading, stereognosis in the absence of apraxia, or conversely a wide mixture of seemingly unrelated symptoms.