The Left Hemisphere
"The Left Hemisphere" From: Neuropsychiatry, Neuropsychology, Clinical Neuroscience by Rhawn Joseph, Ph.D.
Handedness and Hemispheric Functional Laterality
The left half of the brain controls the right hand. The left hemisphere also perceives visual and physical sensations from the right half of the body. Handedness, that is, if one is right or left handed, is directly related to language representation in the brain--the exceptions being those who were forced to become right handed, or who became left-handed following a brain injury
Among 80-90% of right handers, and over 50-80% of those who are left handed, the left cerebral hemisphere provides the neural foundation and mediates most aspects of expressive and receptive linguistic functioning (Frost et al., 1999; Pujol, et al., 1999). The left half of the brain is dominant for the perception and production of almost all aspects of non-emotional language (Albert, 1993; Barr et al. 1990; Benson, 1993; DeRenzi et al. 1987; Evers et al., 1999; Frost, et al., 1999; Goodglass & Kaplan,1999; Heiss, et al., 1999; Kapura et al. 1992; Kertesz, 1983ab; Kimura, 1993; Levin, 1993; Pujol, et al., 1999), including reading, writing, speaking, spelling, naming, and the comprehension of the grammatical, syntactical, and descriptive components of language, including time sense, rhythm, musical scales, verbal concept formation, analytical reasoning, and verbal memory (Albert et al.1972; Albert, 1993; Barr et al. 1990; Benson, 1993; Carmazza & Zurif, 1976; DeRenzi et al. 1987; Efron,1963; Frisk & Milner 1990; Goodglass & Kaplan, 2000; Hecaen & Albert, 1978; Heilman & Scholes, 1976; Kapura et al. 1992; Kertesz, 1983ab; Kimura, 1993; Levin, 1993; Levine & Sweet, 1983; Luria, 1980; Milner, 1970; Parsons & Fox, 1997; Price, 1997; Vignolo, 1983; Zurif & Carson, 1970).
As demonstrated with functional imaging and blood flow studies, when reading, speaking, and naming, the left hemisphere becomes highly active (Buchel et al., 1998; Evers et al., 1999; Frost et al., 1999; Peterson et al., 1988, 1990; Price, 1997; Pujol, et al., 1999). Likewise, when playing musical scales, or listening to the rhythmical aspects of music, the left temporal lobe increases in activity (Evers et al., 1999; Parsons & Fox, 1997).
As demonstrated through lesion studies, functional imaging, and dichotic listening, the human left hemisphere adominates in the perception and processing of real words, word lists, rhymes, numbers, backwards speech, morse code, consonants, consonant vowell syllables, nonsense syllables, the transitional elements of speech, and single phonemes (Blumstein & Cooper, 1974; Cutting, 1974; Frost et al., 1999; Haglund et al. 1993; Kimura, 1961, 1993; Kimura & Folb, 1968; Levy, 1974; Mills & Rollman, 1979; Papcunn et al. 1974; Price, 1997; Pujol et al., 1999; Shankweiler & Studert-Kennedy, 1966, 1967; Studdert-Kennedy & Shankweiler, 1970). It is also dominant for recognizing phonetic, conceptual, and verbal (but not physical) similarities; for example, determining if two letters (g & p vs g & q) have the same vowel ending (Cohen, 1972; Levy, 1974; Moscovitch, 1973).
In this regard, it could be argued that because the lexical, syntactic, grammatical, and denotative aspects of language are localized to the left hemisphere, those aspects of consciousness which are dependent on language are also associated with the left hemisphere; a position advocated by a number of independent neuroscientists (e.g. Albert et al. 1976; Bogen 1969; Dixon 1981; Galin 1974; Hoppe 1977; Ornstein 1972; Miller 1991; Popper & Eccles 1977.
Language and consciousness are tightly related, such that the verbal-dependent, linguistic-aspects of consciousness are clearly associated with the left hemisphere in the majority of the population.
The Language Axis The IPL (which includes the angular and supramarginal gyrus) assimilates associations received from yet other areas of the left and right hemisphere (including the amygdala and cingulate gyrus), fills any gaps with relevant associations, and then injects the resulting verbal associations into the stream of language and thought via the arcuate and longitudinal fasciculus which interlinks the language areas. Hence, the concept of a "language axis" (Joseph, 1982, 1999e,f).
That is, when engaged in language and other cognitive tasks, the brain functions in both a parallel and localistic mode and thus engages in parallel/distributed as well as localized representation and processing. This involves the parallel and localized activation of Wernicke's and Broca's areas, and the inferior parietal lobule (IPL), with the IPL serving as the ultimate convergence zone. Language areas of the left hemisphere becoming activated during language tasksAs detailed by Joseph, (Joseph, 1982, 1986a, 1988a; Joseph et al., 1984), and subsequently confirmed in numerous studies (see below), language processing is sequential, serial, and parallel, involving the activation of widespread areas of both the right and left hemisphere and specific cortices in the left half of the cerebrum. In a monograph which is now considered a classic, Geschwind (1965) reviewed and synthesized these theories and reintroduced Freud's concept of disconnection. Specifically, Geschwind (1965) showed how lesions to various regions of the brain can prevent the language areas from gaining access to necessary information, thus resulting in naming, reading, and related language disorders due to disconnection; i.e. disconnection syndromes. In 1861 and again in 1964, Paul Broca localized the loss of the "speech faculty" to the left frontal operculum; an area that is now referred to as Broca's expressive speech area.
Ten years later, Wernicke (1874) demonstrated that a lesion to the posterior portion of the superior temporal lobe resulted in memory loss for the "auditory images" of words; a region now referred to as Wernicke's area. Around 4,500 years ago, An Egyptian physician wrote in the Ebers Papyrus, of a man who suffered a head injury and "lost his ability for speech without paralysis of tongue." Seven hundred years later, another Egyptian physician described in the "Edwin Smith Surgical Papyrus" the symptoms of two patients with injuries to the brain, including loss and recovery of speech.
THE INFERIOR PARIETAL LOBE AND LANGUAGE
Moreover, simultaneous activation of the right hand or Broca's area can result in spreading excitation such that both neocortical areas become excited simultaneously (see below); which in turn can result in mutual interference (Kimura & Archibald, 1974; Kinsbourne & Cook, 1971). That is, speaking and engaging in a right handed motor activity simultaneously disrupts both. In fact, immediately adjacent to Broca's expressive speech is a huge expanse of neocortical tissue representing the hand (see chapter 19), and the hand and mouth areas are also richly interconnected. The association between handedness, language and motor functioning has in fact, been noted by numerous investigators, such that there is now a convergence of opinion that the neural substrate and the evolution of language and linguistic thought are related to and are outgrowth of right hand and left hemisphere temporal-sequential motor activity (Bradshaw and Nettleton, 1982; Corballis, 1991; Corballis & Morgan, 1978; Faaborg-Anderson, 1957; Hewes, 1973; Jacobson, 1932; Joseph, 1982, 1993, 1999e; Kimura, 1973, 1976. 1979, 1980, 1993; MacNeilage, 1993; MacNeilage et al. 1987; McGuigan, 1978; Morgan & Corballis, 1978). This linkage accounts for why individuals often gesture with the right hand when they speak and why Broca's aphasia is almost invariably accompanied by paralysis of the right upper extremity.
In fact, as demonstrated through lesion studies, and tachistiscopic and dichtotic listening studies, the perception, organization and categorization of information into discrete temporal units or within a linear and sequential time frame are all left hemisphere mediated activities, including the sequential control of finger, hand, arm and articulatory movements (Beaumont, 1974; Christman, 1994; Corina et al. 1992; Efron, 1963; Eisele & Aram, 1994; Heilman et al. 1983; Kimura, 1977, 1993; Mateer, 1983; Haaland & Harrington, 1994; Haglund, et al. 1993; Lenneberg, 1967; Wang & Goodglass, 1992). The left half of the brain is sensitive to rapidly changing acoustics be they verbal or non-linguistic and is specialized for sorting, separating and extracting in a segmented fashion, the phonetic and temporal-sequential or articulatory features of incoming auditory information so as to identify speech units (Eisele & Aram, 1994; Joseph, 1993; Kimura, 1993; Luria, 1980; Shankweiler & Studdert-Kennedy, 1967; Studdert-Kennedy & Shankweiler, 1970). The left hemisphere, therefore is provided a competitive advantage over the right in motor control, such that in response to tool making and gathering activities by men and women over the course of human evolution, this half of the brain became specialized for imposing temporal sequences on both external and internal stimuli (Chapters 6,7). Among almost 90% of the population the left hemisphere is also dominant for fine motor functioning including handedness (reviewed in Corballis, 1991).
EVOLUTION OF HANDEDNESS, LANGUAGE & LEFT HEMISPHERE SPECIALIZATION
However, the inferior parietal lobule (the angular and supramarginal gyrus) is not just a "hand" area but a multi-modal language area which acts to sequence language as well as inject words and categories into the stream of language and thought (Joseph, 1982). The role of the IPL in language is not only evident as based on lesion studies (as will be discussed) but functional imaging. For example, as based on functional imaging, it appears that the supramarginal gyrus may act as a phonological storehouse that becomes activated during short-term memory and word retrieval (Demonet, et al., 1994; Paulesu, et al., 1993; Price, 1997); whereas conversely, deficits in phonological processing are the most common correlate of reading disability (Brady & Shankweiler, 1991). Simply looking at words will activate the left supramarginal gyrus (Bookheimer, et al., 1995; Vandenberghe, et al., 1996; Menard, et al., 1996; Price, 1997) which also becomes active when performing syllable judgements (Price, 1997), and when reading (Bookheimer, et al., 1995; Menard, et al., 1996; Price, et al., 1996). Likewise, the IPL become highly active when retrieving the meaning of words during semantic processing and semantic decision tasks and activation increases as word length increases (Price, 1997). The reason apes do not understand grammar or use spoken language is because they are almost completely lacking a recent neurological evolutionary acquisition, the angular gyrus of the IPL. It is the inferior parietal lobule which makes not only fully formed ASL possible, but the evolution of grammatically correct language, be it spoken, or gestural in the form of drawing, painting, or writing.
LIMBIC LANGUAGE & THE INFERIOR PARIETAL LOBE
Brain of a Human vs Ape A Neanderthal (top) vs a CroMagnon H. Erectus Fine motor control and handedness as well as the grammatical and temporal sequential, denotative aspects of language, appear to have been a acquired rather gradually. For example, there is some evidence which suggests that by two or three million years ago up to 60-70% of Australopithecines were right handed (Dart 1953) whereas by 1.5 million years ago, about 80% of H. habilis were similarly inclined (Toth, 1985). However, it was not until about 150,000 years ago, that up to 90% of archaic humans had become right handed (Cornford, 1986), which is similar to modern day estimates of handedness. THE
ORGANIZATION OF LINGUISTIC THOUGHT
As noted, a large area of parietal lobe neocortex is devoted to hand and finger representation and the guidance of the hand and arm during reaching, throwing, temporal sequential and related movements (chapter 20). Indeed, motor functioning is dependent on sensory feedback which in turn is provided by the parietal lobe (chapter 20). That is, if not for the sensory feedback provided by the muscles and joints (information which is transmitted to the parietal lobes) movement would become clumsy and uncoordinated since a person would not know where their limbs were in space and in relation to one another. Since the parietal lobes and the motor areas in the frontal lobes are richly interconnected, they serve in many ways as a single neurocortical unit, i.e. sensorimotor cortex (Luria, 1980).
CONFABULATION & GAP FILLING ASSIMILATION AND ASSOCIATION WITHIN THE INFERIOR PARIETAL LOBE PART II
Assimilation of input from diverse sources is a major feature of the Language Axis in general. However, if due to an injury the language axis is functionally intact but isolated from a particular source of information about which the patient is questioned, he or she may then suffer word finding difficulty, anomia, or angosia (Davidoff & De Bleser, 1994; Geschwind, 1965; Shelton et al. 1994).
BROCA'S APHASIA PSYCHOSIS & BLUNTED (NEGATIVE) SCHIZOPHRENIA
As noted, within the left frontal convexity there is a large region which is responsible for the expression of speech; i.e. Broca's area, which is named after Paul Broca who over 100 years ago delineated the symptoms associated with damage to this area (Broca, 1878). Immediately adjacent to Broca's area is the portion of the primary motor area which subserves control over the oral-facial musculature and the right hand.
SOUND PERECEPTION, LANGUAGE & APHASIA
Different forms of schizophrenia are also associated with destruction or dysfunction of specific cerebral nuclei; e.g. the temporal lobe, caudate-putamen, medial frontal lobe, and the dopamine system (e.g. McGuire et al. 1998; Bruder et al., 1999; Salisbury et al. 1998; Jacobsen et al. 1998; Shidhabuddin et al. 1998; Kwon et al., 1999). However, the medial and left (and right) frontal and caudate-putamen and dopamine system interact in regard to a number of functions as a single unit. As such, when this unit becomes dysfunctional, obsessive compulsive behaviors or a schizophrenic psychosis may ensue (see chapter 16, 19; and below). However, different regions of the brain contribute to different symptoms. . "SCHIZOPHRENIA" Located within the superior temporal lobe of both hemispheres are the primary auditory receiving areas. If these areas are destroyed bilaterally the individual becomes cortically deaf. Although able to hear they cannot perceive or comprehend non-verbal sounds or understand spoken lanugage.
GLOBAL APHASIA
Because individuals with receptive aphasia display unusual speech, have lost comprehension, and failure to comprehend that they no longer comprehend or "make sense" when speaking, they are at risk for being misdiagnosed as psychotic or suffering from a formal thought disorder, i.e. "schizophenia". Indeed, individuals with abnormal left temporal lobe functioning sometime behave and speak in a "schizophrenic-like" manner (see Chapter 21). Given that they may also behave in a euphoric and/or paranoid manner only increases the likelihood of a misdiagnosis. Global aphasia is essentially a total aphasia due to massive left hemisphere damage involving the entire language axis, i.e. the frontal, parietal and temporal convexity. Comprehension is severely reduced as is the ability to speak, read, write, or repeat. Patients are usually but not always (Legatt, Rubin, Kaplan, Healton, & Brust, 1987), paralyzed on the right side due to damage extending into the motor areas of the frontal lobe.Frequently this disturbance is secondary to cerebrovascular disease involving the middle cerebral artery. However, tumors and head injuries can also create this condition.
ISOLATION OF THE SPEECH AREA (Or Transcortical Aphasia) LANGUAGE & TEMPORAL-SEQUENTIAL MOTOR CONTROL Isolation of the speech area is a condition where the cortical border zones surrounding the language axis have been destroyed due to occlusion of the tiny terrtiary blood vessels which supplies these regions (Geschwind et al. 1968). That is, the Language Axis of the left hemisphere becomes completely disconnected from surrounding cortical tissue but remains (presumably) an intact functional unit. This is in contrast, to global aphasia in which the three major zones of language have been destroyed.
NAMING, KNOWING, COUNTING, FINGER RECOGNITION & HAND CONTROL
As discussed in detail above and in chapters 6, 7, there is considerable evidence that the evolution of language and linguistic thought are related to and in part are outgrowth of right hand temporal-sequential motor activity. The right hand appears to serve as a kind of motoric extension of language and thought in that it acts at the behest of lingustic impulses (Joseph, 1982). In fact, the hand and oral-facial musculature are neuronally represented in adjacent cortical space and are intimately interconnected with Broca's area which lies immediately posterior-lateral and anterior to these motor areas.
AGNOSIA, APRAXIA, ACALCULI, & ORIENTATION IN SPACE
Ontogenetically, it is first via the hand that one comes to know the world so that it may be named and identified. For example, the infant first uses the hand to grasp various objects so they may be placed in the mouth and orally explored. As the child develops, rather than mouthing, more reliance is placed solely on the hand (as well as the visual system) so that information may be gathered through touch and manipulation.As the child and it's brain matures, instead of predominantly touching, grasping, and holding, the fingers of the hand are used for pointing and then naming the object indicated. It is these same fingers which are later used for counting and the development of temporal-sequential reasoning; i.e. the child learns to count on his or her fingers, then to count (or name) by pointing at objects in space.
APRAXIA
Abormalities involving spatial-perceptual motor functioning can occur with lesions to either hemisphere. However, the nature of the disturbance (as well as the severity) differ depending on which half of the brain has been compromised. For example, right cerebral injuries usually have a more pronounced effect on visual-spatial and related perceptual abilities, and more greatly disrupt the overall perception and expression of configurational relationships (e.g. disruption of the overall gestalt) as demonstrated on drawing or constructional tasks. Individuals with left sided damage tend to preserve spatial relations but show a reduction in the number of parts represented. Moreover, left hemisphere lesions tend to more severely effect motoric aspects of spatial-perceptual functioning. As such, left hemisphere injured patients tend to recognize their errors.
ALEXIA & AGRAPHIA
The left cerebral hemisphere has been shown to be superior to the right in the control of certain types of complex, sequenced motor acts (Haaland & Harrington, 1994; Kimura, 1980, 1993; McDonald et al. 1994). If the left hemisphere is damaged, patients may be impaired in their ability to acquire or perform tasks involving sequential changes in the hand or upper musculature or those requiring skilled movement (Barrett et al., 1998; Buxbaum et al., 1998; Haaland & Harrington, 1994; Heilman et al. 1982; Kimura, 1979. 1980, 1993; McDonald et al. 1994). Indeed, the deficit may extend to well learned and even stereotyped motor tasks such as lighting a cigarette or using a key. This condition is referred to as apraxia.
EVOLUTION OF READING & WRITING
There are a variety of theories which purport to explain the mechanisms involved in reading and the comprehension of written language. Reading, of course, requires activation of the visual areas in the occipital and occipital and temporal lobes, so that form is perceived, thus revealing that the form is a word. In addition, letters or groups of letters must be recognized and their temporal order ascertained ("orthographic" processing); there must be semantic processing so that the meaning of the word can be derived, and there may be phonological processing so that the sound of the word may be heard within the privacy of one's head. These latter stages of linguistic analysis involve activation of Wernicke's and Broca's area, and the IPL--as also demonstrated by functional imaging (Demonet, et al., 1994; Paulesu, et al., 1993; Price, 1997).
ALEXIA
Hands of an ApePresumably the evolution of the capacity to read and write is directly associated with the evolution of the hand and the inferior parietal lobule and is related to tool making, gathering, gestulating and gossiping for over 100,000 years. The Left Hemisphere and Consciousness Exner's Area (EX) is above Broca's Area and adjacent to the motor areaLesions localized to this vincinity have been reported to result in disturbances in the elementary motoric aspects of writing, i.e. Frontal Agraphia (Penfield & Roberts, 1959), sometimes referred to as Pure Agraphia. However, Pure Agraphia has also been attributed to left parietal lesions (Basso et al. 1978; Strub & Geschwind, 1983). In general, with frontal agraphia, grapheme formation becomes labored, incoordinated, and takes on a very sloppy appearance. Cursive handwriting is usually more disturbed than printing. The ability to spell, per se, may or may not be effected, whereas with parietal lesions spelling is often abnormal. Rather, there are disturances in grapheme selection and the patient may seem to have "forgotten" how to form certain letters, and/or they may abnormally sequence or even add unnecessary letters when writing (see Hecaen & Albert, 1978).
AGRAPHIA
Just as one theory of reading proposes that visual graphemes are converted into phonological units (visual images into sound), it has been proposed that in writing one transcodes speech sounds into grapheme clusters, i.e. a phoneme to grapheme conversion (Barron, 1980; Ellis, 1982; Friederici et al., 1981). In the lexical route, there is no phonological step. Instead the entire word is merely retrieved. This condition has been described as Global Alexia by some authors. However, global alexia is a more pervasive disorder in which the ability to write (agraphia) and name objects is also compromised. Thus there are several subtypes of reading disturbances which may occur with left cerebral damage or congenital disturbances involving these tissues (Coltheart 1998; Miceli et al., 1999; Miozzo & Caramazza, 1998). These include literal, verbal, and global alexia, and alexia for sentences, as well as developmental dyslexia. In addition, alexia can sometimes result from right hemisphere lesions, a condition referred to as Spatial Alexia. All these disorders, however, are acquired and should be distinguished from developmental dyslexia which is present since childhood (see Njiokiktjien, 1988).
