THE INFERIOR PARIETAL LOBE
by Rhawn Joseph, Ph.D.
FUNCTIONAL NEUROANATOMY OF THE INFERIOR PARIETAL LOBE
INFERIOR PARIETAL LOBE
There are nine major somesthetic areas within the parietal lobe, such that the primary, association, and assimilation areas actually consist of numerous subareas. Broadly, and most generally, however, the parietal lobe may be subdivided into a primary receiving area (involving Brodmann's areas 3ab,1,2) within the post central gyrus, an immediately adjacent somesthetic association area (Brodmann's area 5ab), a polymodal (visual, motor, somesthetic) receiving area located in the superior-posterior parietal lobule (area 7ab), a granular insular area which is located in the inferior convexity and encompasses part of the marginal gyrus, and a multimodal-assimilation area within the inferior parietal lobule (areas 7, 39, 40) which encompasses the angular and supramarginal gyrus.
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 (Cohen et al. 1994; Dong et al. 1994; Snyder et al., 1998). Moreover, the primary motor and somesthetic association regions are richly interconnected (Jones & Powell, 1970) with the primary areas transmitting to the association areas which in turn project to the motor cortex. Indeed, 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 (Cohen et al. 1994; Dong et al. 1994; Lebedev et al. 1994; Pred'Homme & Kalaska 1994; Snyder, et al., 1998). Together, the motor and somesthetic areas comprise a single functional unit which some have referred to as the sensorimotor cortex (Luria, 1980).
THE INFERIOR PARIETAL LOBULE
Developmentally, of all cortical regions, the inferior parietal lobule is one of the last to functionally and anatomically mature (Blinkov & Glezer, 1968; Flechsig, 1901; Conel, 1937-1943; Joseph & Gallagher, 1985, Joseph et al., 1984). Hence, many capacities mediated by this area (e.g. reading, calculation, the performance of reversible operations in space) are late to develop appearing between the ages of 5-8.
Sitting at the junction of the temporal, parietal, and occipital lobes, the inferior region (which includes the angular and supramarginal gyri) has no strict anatomical boundaries, is partly coextensive with the posterior-superior temporal gyrus, and includes part of area 7 as well as area 37. It maintains rich interconnections with the visual, auditory, and somesthetic associations areas including the middle (basal) temporal lobe, the superior colliculus via the pulvinar, the lateral geniculate nucleus of the thalamus, and massive interconnections with the frontal lobes, inferior temporal region, and other higher order assimilation areas throughout the neocortex (Bruce, Desimone & Gross, 1986;Burton & Jones, 1976; Geschwind, 1965; Jones & Powell, 1970; Seltzer & Pandya, 1978; Zeki, 1974).
THE MULTI-MODAL ASSIMILATION AREA
Over the course of evolution the amygdala, hippocampus, and medial temporal lobe began to balloon outward and upward, giving rise to superior temporal lobe, and then continuing to expand in a posterior direction, forming part of the angular and marginal gyrus. Hence, this portion of the inferior parietal lobule has auditory and thus (in the left hemisphere) language capabilities. However, with the evolution of the thumb and the capability of utilizing a precision grasp coupled with tool making and related temporal-sequential tasks, the superior parietal lobule also expanded, thereby also giving rise to inferior parietal neocortical tissue.
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Given its location at the border regions of the somesthetic, auditory, and visual neocortices, and containing neurons and receiving input from these modalities, as the inferior parietal lobule evolved it became increasingly multimodally responsive; a single neuron simultaneously receiving highly processed somesthetic, visual, auditory and movement related input from the various association areas. Hence, many of the neurons in this area are multi-specialized for simultaneously analyzing auditory, somesthetic, and spatial-visual associations, and have visual receptive properties which encompass almost the entire visual field, with some cells responding to visual stimuli of almost any size, shape, or form (Bruce et al. 1982, 1986; Hyvaerinene & Shelepin, 1979).
Inferior parietal neurons are involved in the assimilation and creation of cross modal associations and act to increase the capacity for the organization, labeling and multiple categorization of sensory-motor and conceptual events (Geschwind, 1965; Joseph 1982). One can thus create visual, somesthetic, or auditory equalivalents of objects, actions, feelings, and ideas, simultaneously. For example, conceptualizing a "chair" as a word, visual object, or in regard to sensation, usage, and even price. That is, the IPL is directly involved in naming--as demonstrated by functional imaging (Price, 1997). The left IPL becomes activated when reading ( Bookheimer, et al., 1995; Menard, et al., 1996; Price, 1997 Price, et al., 1996; Vandenberghe, et al., 1996) during semantic processing (Price, 1997), and when generating words (Shaywitz, et al., 1995; Warburton, et al., 1996) or when making syllable judgements (Price, 1997). Indeed, the IPL appears to act as a pholological storehouse that becomes activated during short-term memory and word retrieval (Demonet, et al., 1994; Paulesu, et al., 1993; Price, 1997) and becomes highly active when retrieving the meaning of words during semantic processing and semnatic decision tasks (Price, 1997).
Because of it's involvement in functions such as those described above, one side-effect of damage to the left angular gyrus, is a condition called anomia, i.e. severe word finding and confrontive naming difficulty. These individuals have difficulty naming objects, describing, pictures, etc. Moroever, lesions involving the angular gyrus, or when damage occurs between the fiber pathways linking the left inferior parietal lobule with the visual cortex, there can also result Pure Word Blindness. This is due to an inability to receive visual input from the left and right visual cortex and to transmit this information to Wernicke's area so that auditory equivalents may be called up. Such patients are thus unable to read and suffer from alexia.
Because the inferior parietal lobule also acts as a relay center where information from Wernickes region can be transmitted, via the arcuate fasciculus, to Broca's area (for expression) destructive lesions, particulary to the supramarginal gyrus of the left cerebral hemisphere can result in conduction aphasia (see chapter 11). Although comprehension would be intact and a patient would know what she wanted to say, she would be unable to say it. Nor would she be able to repeat simple statements, read out loud, or write to dictation. This is because Broca's area is diconnected from the posterior language zones.
It has been argued that the sensory motor engrams necessary for the production and perception of written language are stored within the parietal lobule of the left hemisphere (Strub & Geschwind, 1983). In fact, given that the parietal lobes are concerned with the hands and lower visual fields, they not only guide and observe hand movements, but learn and memorize these actions, including those involved in writing.
Hence, when lesioned, patients sometimes have difficulty writing and forming letters due to an inability to access these engrams (Strub & Geschwind, 1983; Vignolo, 1983); i.e. they suffer from agraphia, an inability to write (see chapter 11). Writing samples may be characterized by mispellings, letter ommissions, distortions, temporal-sequential misplacements, and inversions (Kinsbourn & Warrington, 1964). Sometimes agraphia is accompanied by alexia; inability to read (Benson & Geschwind, 1969; Hecaen & Kremin, 1977).
LATERALIZED TEMPORAL-SEQUENTIAL FUNCTIONS
Because it is a recepient of so much information and aids the rest of the brain in various forms of analysis, one function of the inferior parietal lobe is to maintain track of input/output so that information may be organized appropriately in either a sequential (i.e. first, middle, last), or spatial framework. Hence, another side effect of lesions localized to the inferior parietal lobule is a disruption of visual-spatial functioning, temporal-sequencing ability (e.g. apraxia), as well as logic, grammar, and the capacity to perform calculations; depending on which hemisphere is compromised.
Individuals with lesions involving the inferior-parietal-occipital border of either hemisphere may have difficulty carrying out spatial-sequential tasks. For example, drawing "a square beneath a circle and a triangle beneath a square" (Luria, 1980). Often they may draw the objects in the order described (i.e. square, circle, triangle, square). That is, they have difficulty in conceptualizing how to place the objects in relation to each other.
Those with left inferior parietal lesions have trouble with more obvious sequential-grammatical relationships (Luria, 1980). For example, they may be unable to understand the question: "John is taller than Jim but shorter that Pete. Who is taller?" In part, this is not only a function of left parietal dysfunction but the right hemispheres difficulty in dealing with temporal-sequential and grammatical relations.
Because the right brain does not understand grammatical relationships, a sentence that starts which the name "John" is interpreted by the right parietal area as all about "John". i.e. the first word of the sentence is undertood by the right brain as the "agent" regardless of semantics or grammar (Chernigovaskaya & Deglin, 1986). In this manner, if presented with the sentence, "give me the book after you give me the pencil", the right brain would respond to the order of presentation rather than their grammatical relationship and would thus present the book then the pencil. When left parietal input is abolished, proper temporal-sequential and grammatical programming/comprehension thus suffers.
THE SENSORY GUIDANCE OF MOVEMENT
The parietal lobe is highly concerned with the mediation of movement. As noted in chapter 19, the primary motor cortex extends well beyond area 4 and includes portions of the somatosensory regions which in turn contribute almost one third of the fibers which make up the pyramidal tract. These areas are in fact richly interconnected (Jones & Powell, 1970). Together, the motor and somesthetic regions comprise a single functional unit, i.e. the sensorimotor cortex.
Nevertheless, it is important to emphasize, as pointed out by Luria (1980) that every voluntary movement is in fact comprised of a series of movements which are spatially organized in accordance with successively changing input from other modalities (Barrett, et al., 1998; Buxbaum et al., 1998; Kimura 1993). 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 streams of visual, somesthetic, as well as auditory input. This includes information regarding the position of the body and limbs in space. Indeed, movement becomes extremely difficulty without sensory feedback and guidence. Because of this, parieal lesions can result in unilateral paresis and even wasting (i.e. parietal wasting). Hence, the somesthetic cortex is very important in the guidence of movement, and in fact some neurons fire prior to making a movement (Lebedev et al. 1994).
Although the entire parietal lobule makes important contributions , the superior and 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. This is because the motor engrams for performing these acts appear to be stored in the left angular and supramarginal gyri (Geschwind, 1965; Heilman, 1993) -a consequence, in part of its unique ability to guide, visually observe, and thus selectively learn hand movements, gestures, and complex temporal sequential actions such as involving tool construction (Joseph 1993, 1999e). Related memories are therefore stored in this cortex.
Conversely, these hand movement related memories assist in the programing of the motor frontal cortex where the actions are actually executed (e.g. Deibert et al., 1999). However, the inferior parietal lobule in turn is dependent on input from the primary and association somesthetic areas.
There is some evidence of laterality in regard to all of the above, the right half of the brain may be more concerned with movement of the trunk and the lower extremities. This would include navigational movement through space, running, certain types of dancing, and actions requiring analysis of depth and balance. The left cerebral hemisphere exerts specialized influences on the upper extremities including the control of certain types of complex, sequenced motor acts such as those requiring alterations in the orientation and position of the upper limbs (Haaland & Harrington 1994; Kimura, 1982, 1993).
If the left 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 been performed incorrectly. They may also be impaired in their ability to acquire or perform tasks involving sequential changes in the hand or upper musculature (Kimura, 1979, 1982, 1993), including well learned, skilled, and even stereotyped motor tasks such as lighting a cigarette or using a key.
Apraxia is a disorder of skilled movement in the absence of impaired motor functioning or paralysis (Barrett et al., 1999; Buxbaum et al., 1999). Usually apraxic patients show the correct intent but perform the movements in a clumsy fashion. Like many other types of disturbances, patients and their families may not notice or complain of apraxic abnormalities. This is particularly true if they're aphasic or paralyzed on the right side. That is, clumsiness with either extremity may not seem significant. Hence, this is something that requires direct evaluation.
Performance deteriorates the most when required to imitate or pantomime certain actions including the correct usage of some object (McDonald et al. 1994). For example, the patient may be asked to show the examiner, "how you would use a key to open a door", or "hammar a nail into a piece of wood". In many cases the patient may use the body, i.e. a finger, as an object (e.g. a key), rather than the finger and thumb holding the key. Although performance usually improves when they use the real objects (Geschwind, 1965; Goodglass & Kaplan, 1972), a rare few may show the disturbance when using the real object as well (Heilman, 1993; Kimura 1993).
In addition, patient's with apraxia may demonstrate difficulty properly sequencing their actions (Barrett et al., 1998). For example, they may pretend to stir a cup of coffee, then pretend to pour the coffee into the cup, and then take a sip. However, the individual acts may be performed accurately.
Broadly speaking, there are several forms of apraxia, which like many of the disturbances already discussed may be due to a number of causes or anatomical lesions. These include, ideational apraxia, ideomotor apraxia, bucal facial apraxia, constructional apraxia and dressing apraxia. With the exception of dressing and constructional apraxia, apraxic abnormalities are usually secondary to left hemisphere damage, in particular, injuries involving the the left frontal and inferior parietal lobes.
For example, Kimura (1982, 1993) found that the ability to perform meaningless oral or hand movements was related to the frontal or posterior nature of the lesion, such that those with frontal lesions were impaired on oral whereas those with parietal lesions had the most difficulty making hand postures or complex movements of the extremities (Kolb & Milner, 1981). Thus, apraxic abnormalities secondary to left cerebral lesions tend to either involve destruction of the inferior parietal lobule (IFP) or lesions resulting in disconnection of the frontal motor areas (or the right cerebral hemisphere) from this more posterior region of the brain.
If the inferior parietal region is destroyed the patient loses the ability to appreciate when they have performed an action incorrectly. If the motor region is destroyed, although the act is still performed inaccurately (due to disconnection from the IFP), the patient is able to recognize the difference (Heilman, 1993; Kimura 1993).
Ideomotor apraxia is usually associated with lesions within the inferior parietal lobe of the left hemisphere. Rather than problems with temporal-sequencing of motor acts per se, these individuals tend to be very clumsy when performing an act, and/or they may perseverate and erroneously perform a previous movement. These individuals also tend to be very deficient when attempting to perform an action via pantomime or when engaged in meaningful imitation, meaningless imitation, and the meaingful use or meaningless use of actual objects (Goodlass & Kaplan, 1972; Heilman, 1973; Kimura & Archibald, 1974; McDonald et al. 1994). This is presumably due to destruction of the engrams important in motor performance. Patients will demonstrate apraxic abnormalities in both the right and left hand.
In addition, patients with ideomotor apraxia tend to have difficulty with simple versus complex movements, although various elements within a complex action may be performed somewhat abnormal (Hecaen & Albert, 1978; McDonald et al. 1994). Hence, actions such as waving goodbye, throwing a kiss, making the "sign of the cross", may be performed deficiently. Moreover, many patients tend to uncontrollably comment on their actions; i.e. "verbal overflow". That is, when asked to "wave goodbye", they may say "goodbye" while waving even when instructed to say nothing.
It has been suggested that ideomotor apraxia can occur in the absence of ideational apraxia, but that the converse is not true (Hecaen & Albert, 1978). In this regard, ideomotor apraxia may be a less severe form of ideational apraxia (Kimura 1993).
This form of apraxia is usually due to severe disturbances in the temporal sequencing of motor acts (Buxbaum et al., 1998). That is, the separate chain of links which constitute an entire movement become dissociated, such that the overriding idea of the movement in it's entirety is lost. Hence, these individuals commit a number of temporal and spatial errors when making skilled movements, although the individual elements, in isolation, may be preserved and performed accurately (Hecaen & Albert, 1978; Luria, 1980). For example a patient (via pantomime) may rotate their hand before inserting the key, drink from a cup before filling it from a pitcher of water, or puff from a cigarette and then lighting it. Thus they incorrectly sequence a series of acts. Both hands are effected.
Because of conceptual, ideational abnormalities, they may also have difficulty using actual objects correctly. During pantomime they may use a body part as object such as an index finger for a key. Even so, their actions are out of temporal sequence. Hence, these patients seem to be unable to access the motor engrams (or "memories") which would allow them to perform appropriately (Luria, 1980). In this regard, patients are sometimes hesistant to perform a task as they have difficulty understanding what has been asked of them. However, they can often describe verbally what they are unable to perform (Heilman, 1993).
LEFT SIDED OR UNILATERAL APRAXIA (also called Callosal & frontal apraxia)
Patients with unilateral (callosal/frontal) apraxia are unable to imitate or perform certain movements with their left (but not right) hand and are clumsy in their use of objects. Left sided apraxia is sometimes due to a lesion of the anterior corpus callosum or left frontal motor cortex. This is because lesions of the corpus callosum or premotor and motor region of the left hemisphere can result in a disconnection syndrome; i.e. the motor areas of the right hemisphere cannot gain access to the motor engrams stored within the left inferior parietal lobe. Thus, with a left frontal lesion there results an apraxia of the left hand and paralysis of the right. Often this is secondary to strokes within the distribution of the anterior cerebral artery such that the anterior portion of the corpus callsom is destroyed (Geschwind, 1965).
It is noteworthy that patients also may show deficient finger tapping performance in the left hand due to apraxic abnormalities secondary to left hemisphere injury (Heilman, 1975). In these instances, reduced finger tapping is bilateral.
Dressing apraxia is usually secondary to right hemisphere lesions involving the inferior parietal region, and as the name implies, the patient has difficulty putting on their clothes. For example, a patient may attempt to put a shirt on upside down, then inside-out, and then backwards. Severe spatial-perceptual abnormalities as well as body image disturbances are usually contributing factors.
Aphasia & Apraxia
Many patients who are aphasic also appear apraxic because they have severe difficulty comprehending language and understanding motor commands. That is, a patient may fail to peform a particular action because he doesn't comprehend what is being asked.
To distinguish between receptive aphasic abnormalities and apraxia one must ask "yes" and "no" questions ("are you in a hospital?"); require them to perform certain actions via pantomime ("show me how you would throw a ball" or "show me how a soldier salutes"); as well require pointing response ("point to the lamp"). If they can answer appropriately "yes" or "no" or point to objects named but cannot execute commands they have apraxia. It is important to note that in severe cases apraxic patients may have difficulty even with pointing.
Individuals with damage involving the left parietal lobule not only make errors when performing motor acts but comprehending, recognizing and discriminating between different types of motor acts such as demonstrated via pantomime (Heilman et al. 1982; McDonald et al. 1994). Moreover, individuals with lesions in the left inferior occipital lobe have also been shown to have difficulty verbally understanding, describing or differentiating between pantomimes (Rothi et al. 1986). That is, in the extreme, if one were to pantomime the pouring of water into a glass vs. lighting and smoking a cigarette, these indviduals have problems describing what they have viewed or in choosing which was which.
Deficits in pantomime recognition occur frequently among indviduals with aphasia (Varney, 1978). Moreover, this disturbance is also significantly correlated with reading comprehension (Gainotti & Lemmo, 1976; Varney, 1978, 1982). In this regard, indviduals with alexia frequently suffer from pantomime recognition deficits as well. Because of this relationship it has been suggested that the ability to read may be based on or derived from the abiity to understand gestural communciation, i.e. the reading of signs (Joseph 1993; Varney, 1982).
Wang and Goodglass (1992) and Kimura (1993) argue that pantomime imitation and production are related to both apraxia and the ability to interpret purposeful movements; the engrams for which are located in the inferior parietal lobule (Heilman et al. 1982; Joseph 1993; Wang & Goodglass, 1992). By contrast to left anterior lesions which may impair motor functioning and the capacity to imitate the ability to comprehend pantomime is retained (Heilman et al. 1982).
As noted, the inferior and superior parietal lobule receives considerable visual input, particularly from the periphery and lower visual field -the area in which the hands are most likely to be viewed. Hence, this area of the brain views, manipulates, guides and mediates hand-object coordination and reaching movements, including the comprehension of hand movements; i.e. gestures (Joseph, 1993). Hence, when the left superior and inferior parietal lobule is destroyed gestural comprehension, including the understanding of (as well as the capacity to execute) complex gestures, including ASL and the capacity to engage in complex temporal sequential acts is significantly impacted. If the right parietal area is destroyed, these deficits may also include constructional apraxia (chapter 10).
Constructional apraxia is by no means a unitary disorder (Benton, 1969; Benson & Barton, 1970) and can may be expressed in a number of ways. On a drawing or copying task this may include the addition of unncessary/non-existant details or parts, misalignment or inattention to details, disruptions of the horizontal and verticle axis with reversals or slight rotations in reproduction, and scattering of parts. For example, in performing the Block Design subtest from the WAIS-R, the patient may correctly reproduce the model but angle it incorrectly. In drawing or copying figures, the patient may neglect the left half, draw over the model, and misalign details.
Moreover, although constructional deficits are more severe after right hemisphere damage (Arrigoni & DeRenzi, 1964; Black & Strub, 1976; Benson & Barton, 1970; Critchley, 1953; Hier et al., 1983; Joseph, 1988a; Kimura 1993; Piercy et al. 1960), disturbances involving constructional and manipulo-spatial functioning can occur with lesions to either half of the brain (Arrigoni & DeRenzi, 1964; Mehta et al., 1987; Piercy et al., 1960). Hence, depending on the laterality, as well as the extent and site of the lesion, the deficit may also take different forms. For example, following posterior reight cerebral lesions, rather than apraxic, the patient is spatially-agnosic, i.e. suffering from constructional agnosia and a failure to perceive and recognize visual-spatial and object interrelationships. In other cases, such as following left cerebral injury, the disturbance may be secondary to a loss of control over motor programming (Kimura 1993; Warrington et al., 1966; Warrington, 1969).
Although visual motor deficits can result from lesions in either hemisphere (Arrigoni & DeRenzi, 1964; Piercy et al., 1960; Kimura 1993), visual-perceptual disturbances are more likely to result from right hemisphere damage. In contrast, lesions to the left half of the brain may leave the perceptual aspects undisturbed whereas visual motor functioning and selective organization may be compromised (Kim et al. 1984; Mehta et al., 1987; Poeck et al. 1973). As such the patient is likely to recognize that errors have been made.
In general, the size and sometimes the location of the lesion within the right hemisphere has little or no correlation with the extent of the visual-spatial or constructional deficits demonstrated, although right parietal lesions tend to be worst of all. With right parietal involvement patients tend to have trouble with the general shape and overall organization, the correct alignment and closure of details, and there may be a variable tendency to ignore the left half of the figure or to not fully attend to all details. Moreover the ability to perceive (or care) that errors have been made is usually compromised.
Conversely, constructional disturbances associated with left hemisphere damage are positively correlated with lesion size, and left anterior lesions are worse than left posterior (Benson & Barton, 1970; Black & Bernard, 1984; Black & Strub, 1976; Kimura 1993; Lansdell, 1970). This is because the capacity to control and program the motor system has been compromised. The larger the lesion, the more extensive the deficit.
Moreover, because the left hemisphere is concerned with the analysis of parts or details and engages in temporal- sequential motor manipulations, lesions result in oversimplification and a lack of detail although the general outline or shape may be retained (Gardner, 1975, Levy, 1974). However, in some cases, when drawing, there may be a tendency to more greatly distort the right half of the figure with some preservation of left sided details.
RIGHT & LEFT FRONTAL