Role of Inferior Parietal Lobule

Home , Arcuate fasciculus, Inferior parietal lobule, Parietal lobe, Primary sensory areas, Superior parietal lobule

Topic issue | psychiatry | neurology | neuroscience| neurosurgery

Language processing – role of inferior parietal lobule

nikola prpić university of zagreb school of medicine 0000-0002-0170-8858

DOI: http://dx.doi.org/10.17486/gyr.3.1037

Summary: It has long been known and demonstrated that activation of Wernicke’s area causes simultaneous activation of Broca’s area and the premotor frontal cortex via the arcuate fasciculus. Such simultaneous activation occurs in other areas, as well. One of those areas is the lower parietal cortex, which consists of the supramarginal and angular gyri. The supramarginal gyrus acts to analyze phonological properties of words and to merge syllables into words. It is consequently considered to be part of the Wernicke area by many authors. The angular gyrus is surrounded by secondary somatosensory, visual and auditory cortical areas and is essential in multimodal, highly complex synthesis of information. This is evident in both lesion studies, showing disturbed semantic processing in angular gyrus damage, and functional imaging studies showing angular activation in complex sentence structure interpretation.

Keywords: language processing, lower parietal lobule, supramarginal gyrus, angular gyrus

Language and speech have long been distinguishing characteris- when we realize that it is the lower precentral areas (primary mo- tics of the human species. In fact, many have proposed that it was tor areas) that send fibers to mouth, tongue, cheeks and vocal language that allowed Homo sapiens to conquer the world. But cords. These conclusions are supported by functional imaging3 how are words internalised? How do they connect to visual rep- and lesion studies4. So far areas involved in hearing (Wernicke) resentations of objects? These problems have puzzled many for and producing sounds (Broca) have been described – but for as long as there has been writen language. Norman Geschwind speech to be comprehensible, these areas must somehow be con- was one of the first scientists who confronted this problem and nected. Neuroanatomical studies have shown a thick fiber bundle, it was after him that the whole inferior parietal area was named labeled the arcuate fasciculus, connecting these areas. It should „Geschwind territory“1. be noted that the arcuate fasciculus occurs almost exclusively in humans. From an evolutionary standpoint, this helps explain the Language processing complexity of human speech.5

When a word is heard, it is first processed by the primary auditory Functional webs cortex for its elementary phonological aspects i.e. how different sounds and frequencies interact to give us the sound of the syl- Functional imaging studies have shown that even when we hear lable being analyzed. Sound registration activates specific neurons speech, there is extensive activation of Broca’s area. Similarly, assigned to specific sounds. Neurons assigned to similar sounds when we speak, Wernicke’s area activates. This mutual activation are located near each other. This pattern of neural allocation is is evident in lesion studies, which show that damage to Wernicke’s termed tonotopic organization. area causes speech impediments, even though it is primarily an These neurons, located in the primary auditory perisylvian cor- auditory area. Alongside these two areas, speaking and listening tex (Brodmann area 41), activate the secondary cortex, located to speech also activates the frontal, lower temporal and parietal lower in the temporal lobe, where sound information is further cortices. All these findings have put forward a theory that all processed. In this part of the cortex lies the Wernicke area, tradi- neurons involved in language comprehension are connected into tionally related to understanding spoken words. The role of the functional webs. Neurons coding auditory, articulatory, seman- Wernicke area is best demonstrated by well-localized lesions such tic and other aspects of words cause mutual excitation, creating as strokes or hemorrhages. Patients with such lesions in the area near instantaneous activation of all cortical fields significant to often display receptive aphasia. Receptive aphasia is a disorder language processing. This in turn means that, when thinking of a characterized by loss of ability to understand spoken or written word, the entire web of neurons specific to that word is activated, language and to produce meaningfull sentences, even though the including neurons coding how to say this word, what the word grammar and intonation used when speaking is correct.2 sounds like, what it means, what it looks like and more.6 To fully understand the mechanism of this occurence, we must This interconnection between systems provides the basis for look toward the frontal lobe, specifically, the lower frontal gyrus learning in infants. When babies first start creating sounds, they – the location of Broca’s area. Broca’s area has a function analo- activate their Broca’s areas (involved in articulation). Likewise, gous with the rest of the premotor area. Just as higher frontal upon hearing those sounds, their Wernicke’s areas activate. premotor areas plan and refine ideas for movement, so does the Through long-term potentiation (LTP) neuronal webs are cre- Broca area refine and plan articulation. This becomes obvious ated, connecting hearing and speaking. Later, new information is

Fig. 2 Angular gyrus (yellow) and other Fig 1 Broca and Wernicke areas cortical areas that send information to it. Secondary visual (blue), auditory (red) and somatosensory (green) areas. added to the webs - for instance, the meaning of a word, what the words in hopes of making it easier for them to understand writ- object defined by the word looks like, what the object can do etc. ten language. This method serves as a very good model to explain the damaging effects observed in cases of receptive aphasia. For Parietal lobe a word to be registered, its functional web needs to be activated. This may happen in any part of the web and be triggered by any The parietal lobe is bordered anteriorly by the central sulcus (Ro- aspect of the word – seeing the object, imagining it, hearing the landic fissure), laterally by the lateral sulcus (Sylvian fissure) and word or seeing it written. When a written word is presented, it is posteriorly by the parieto-occipital fissure. Relevant parts of the automatically processed through both the visual system and the parietal lobe include: the primary somatosensory area, comprised temporal regions, is internally read and activates the functional of Brodmann areas 3, 1 and 2, all located in the postcentral gyrus; web.8 Consequently, aphasic patients with damage to the lower the superior parietal lobule, comprised of Brodmann areas 5 and parietal lobe lose the ability to differentiate a word when hearing 7, involved in spatial orientation and stereognosia (object recogni- it, when producing it but also when reading it because their ability tion by touch) and speech recognition; and the inferior parietal to „sound out“ words has been compromised. lobule, comprised of Brodmann areas 39 and 40, responsible for multisensory synthesis. The inferior parietal lobule is composed Semantic properties of the supramarginal and angular gyri. Their position gives a very important clue regarding their function. The supramarginal gyrus The angular gyrus is anatomically connected almost exclusively lies directly above the Sylvian fissure and is often described as a with other association regions and receives little to no direct input part of Wernicke’s area. This location suggests that supramarginal from primary sensory areas. Cytoarchitectonicaly, it corresponds neurons decode rough auditory information and send them for to PGa and PGp, regions almost nonexistent in lower primates. further processing. This processing is now highly associative and The previous section focused on how how our brains react to combines different word elements such as the sound of the word, hearing words, but the same process of neural web/network the look of the object, the feel of the object etc. activation happens when viewing objects represented by those Logic indicates that the area that performes these complex op- words. There are even studies describing different categories of erations should be strategically placed between the secondary words activating different areas as seen in fig 3. The connection auditory cortex (superior temporal and supramarginal gyrus), between language and internalisation of surroundings should secondary somatosensory cortex (Brodmann area 5 and 7 in the not be overlooked. Let us take, for instance, the Piraha people of superior parietal lobe) and secondary visual cortex (rostral oc- Amazon who only have words for the numbers 1 and 2. When cipital lobe). This is precisely the location of the angular gyrus a Piraha subject is shown 4 or 5 objects and later tested on the (as seen in Fig. 2), where the semantic processing of words is number of these objects, they cannot remember exactly how considered to take place. many objects they were presented with. This supports the idea that memory and language are unseparable and that language Phonological properties influences the perception of one’s surroundings.9. Unfortunately, functional imaging studies weren’t performed on these particular As a part of Wernicke’s area (according to some authors), the subjects, but studies proving lower parietal activation in semantic supramarginal gyrus is involved in analyzing phonological and analysis do exist.10 temporal aspects of words. This was best demonstrated in an Additional evidence comes from lesion studies. Lesions to the left fMRI study examining cerebral activaty in subjects who were angular gyrus caused alexia (inability to recognize or read written asked to single words based on differences in pitch or spectral words or letters), agraphia (inability to write), anomia (inability content of tones and in the initial stop consonant for syllables. to name objects), transcortical sensory aphasia (characterized by When singling out the words, there was activation in the superior poor auditory comprehension, relatively intact repetition, and temporal gyrus (especially when identifying acoustic differences) fluent speech with semantic paraphasias present) and sentence and supramarginal gyrus (especially when identifying phonologi- comprehension impairment11. An interesting paper written in cal units).7 From this, it can be gathered that the Wernicke area 2003 speculated that the angular gyrus was at least partially re- complex allows for analysis of speech and its segmentation into sponsible for understanding metaphors.12 There is some evidence sound-based components. to support this claim, but no in-depth research. For instance, pa- It is often suggested to children learning to read to „sound out“ tients with angular gyrus lesions had difficulties with interpreting

Fig 3 Action vs object word cortical activation showing different functional webs relating Fig 4. Bouba/kiki effect. The object on the left was named kiki and the object on the to their processing. right was named bouba by 95% of subjects.

proverbs – when asked what they thought a proverb meant, they tention an interesting role of the parietal lobe in mathematical would devise complicated, but wrong answers. One such example processes. Neuroimaging has confirmed and greatly elaborated was a patient presented with the proverb “Not all that glitters the findings from neurological patients. It suggests that the an- is gold”. When asked to explain it meant, the patient replied it gular gyrus is the locus of core numerical processing. Functional was advice not to trust salesmen when buying gold because they imaging has shown activation of this area whenever numbers might try to cheat you. In addition to these disturbances, patients and numerical magnitudes are implicated, even if subjects are with angular gyrus lesions were observed to have a disturbed not aware of it or it is irrelevant for the task13. bouba/kiki effect. Bouba/kiki effect manifests itself when a subject is presented with two objects as in fig 4 and asked to name Conclusion them arbitrarily. Most will name pointy objects kiki and round objects bouba. This effect is speculated to stem from cortical Understanding language processing requires that we first un- connections uniting visual perception of objects (spiked versus derstand functional webs and mental representations of words. round), appearance of the speaker’s lips (open and round versus These webs explain how different aspects of words are connected narrow and wide), and feelings related to hearing those sounds. and why lesions in different parts of the brain cause dysfunction These connections are all centered around the angular gyrus. in producing or understanding words. Among other activated areas in these webs are lower parietal gyri. The anterior part of the Gerstmann’s syndrome lower parietal lobule is the supramarginal gyrus, which constitutes the Wernicke area along with the superior temporal areas. In the When mentioning lesions in this area of the brain, Gerstmann’s supramarginal gyrus, phonological and temporal analysis takes syndrome, caused by damage to the angular and supramarginal place. It is the part of the brain that allows formation of words gyri, shouldn’t be overlooked. The symptoms include dysgraphia, out of syllables. The posterior part of the lower parietal lobule dyscalculia, finger agnosia (inability to distinguish fingers on the consists of the angular gyrus, a highly complex area concerned hand) and left-right disorientation. This syndrome brings to at- with semantic analysis and mulitisensory synthesis.

Literature: 1. Geschwind Norman. Disconnexion syndromes in animals and man, Brain 1965, 88 (3) 237-294 2. Caspari, Isabelle LaPointe, Leonard L. , (2005).Aphasia and related neurogenic language disorders (3rd ed.). New York, NY, US: Thieme New York 3. Jean-Fraoncois Demonet et al. THE ANATOMY OF PHONOLOGICAL AND SEMANTIC PROCESSING IN NORMAL SUBJECTS Brain Dec 1992, 115 (6) 1753-1768; DOI: 10.1093/ brain/115.6.1753 4. Binder J.R. et al. Human Brain Language Areas Identified by Functional Magnetic Resonance Imaging The Journal of Neuroscience, 1997, 17(1): 353-362; 5. James K Rilling et al. The evolution of the arcuate fasciculus revealed with comparative DTI Nature Neuroscience (2008) 11, 426 - 428 6. Pulvermuller Friedemann, The Neuroscience of Language: On Brain Circuits of Words and Serial Order, Cambridge University Press 2002 7. P. Celsisa Differential fMRI Responses in the Left Posterior Superior Temporal Gyrus and Left Supramarginal Gyrus to Habituation and Change Detection in Syllables and Tones NeuroImage 1999, 9(1) 135–144 doi:10.1006/nimg.1998.0389 8. C. Stoeckel et al. Supramarginal gyrus involvement in visual word recognition, Cortex (2009) 45 (9) 1091–1096 doi:10.1016/j.cortex.2008.12.004 9. http://www.newrepublic.com/article/117485/multilinguals-have-multiple-personalities 10. Noppeney U, Price CJ. Functional imaging of the semantic system: retrieval of sensory-experienced and verbally learned knowledge. Brain Lang. 2003 Jan;84(1):120-33 11. Sakurai Y1, Asami M, Mannen T. Alexia and agraphia with lesions of the angular and supramarginal gyri: evidence for the disruption of sequential processing. J Neurol Sci. 2010 Jan 15;288(1-2):25-33. doi: 10.1016/j.jns.2009.10.015. 12. E Hubbard, VS Ramachandran, Phenomenology of synaesthesia Journal of Consciousness Studies 2003, 10 (8), 49-57 13. Zago L. et al. Neural Correlates of Simple and Complex Mental Calculation NeuroImage (2001) 13, 314–327 doi:10.1006/nimg.2000.0697 Procesuiranje govora - uloga donjeg parijetalnog režnjića Sažetak: Odavno je poznato i dokazano da aktivacija Wernickeove areje uzrokuje da se istovremeno aktiviraju Brocina areja i premotorni frontalni korteks putem fasciculus arcuatusa. Takva istovremena aktivacija zahvaća i druge dijelove mozga. Jedan od tih dijelova je donji dio parijetalnog režnja, koji se sastoji od gyrus supramarginalis i gyrus angularis. Gyrus supramarginalis analizira fonološke osobine riječi i tvori riječi od slogova. Posljedično ga neki autori smatraju dijelom Wernickeove areje. Gyrus angularis okružen je sekundarnim somatosenzornim, vizualnim i auditornim kortikalnim poljima i važno je središte multimodalne, vi- soko složene sinteze informacija. To je vidljivo pri proučavanju lezija, koje pokazuju poremećaje semantičkog procesuiranja kod oštećenja gyrus angularisa, kao i pri funkcionalnim pretragama, koje pokazuju angularnu aktivaciju pri interpretiranju složenih rečenica.

Ključne riječi: procesuiranje govora, donji parijetalni režnjić, gyrus supramarginalis, gyrus angularis

September 2015 | Gyrus Vol III no. 3 | 175