Download Language processing – role of inferior parietal lobule

neuroscience| neurosurgery
Topic issue | psychiatry | neurology | 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 characteristics of the human species. In fact, many have proposed that it was
language that allowed Homo sapiens to conquer the world. But
how are words internalised? How do they connect to visual representations of objects? These problems have puzzled many for
as long as there has been writen language. Norman Geschwind
was one of the first scientists who confronted this problem and
it was after him that the whole inferior parietal area was named
„Geschwind territory“1.
Language processing
When a word is heard, it is first processed by the primary auditory
cortex for its elementary phonological aspects i.e. how different
sounds and frequencies interact to give us the sound of the syllable being analyzed. Sound registration activates specific neurons
assigned to specific sounds. Neurons assigned to similar sounds
are located near each other. This pattern of neural allocation is
termed tonotopic organization.
These neurons, located in the primary auditory perisylvian cortex (Brodmann area 41), activate the secondary cortex, located
lower in the temporal lobe, where sound information is further
processed. In this part of the cortex lies the Wernicke area, traditionally related to understanding spoken words. The role of the
Wernicke area is best demonstrated by well-localized lesions such
as strokes or hemorrhages. Patients with such lesions in the area
often display receptive aphasia. Receptive aphasia is a disorder
characterized by loss of ability to understand spoken or written
language and to produce meaningfull sentences, even though the
grammar and intonation used when speaking is correct.2
To fully understand the mechanism of this occurence, we must
look toward the frontal lobe, specifically, the lower frontal gyrus
– the location of Broca’s area. Broca’s area has a function analogous with the rest of the premotor area. Just as higher frontal
premotor areas plan and refine ideas for movement, so does the
Broca area refine and plan articulation. This becomes obvious
when we realize that it is the lower precentral areas (primary motor areas) that send fibers to mouth, tongue, cheeks and vocal
cords. These conclusions are supported by functional imaging3
and lesion studies4. So far areas involved in hearing (Wernicke)
and producing sounds (Broca) have been described – but for
speech to be comprehensible, these areas must somehow be connected. Neuroanatomical studies have shown a thick fiber bundle,
labeled the arcuate fasciculus, connecting these areas. It should
be noted that the arcuate fasciculus occurs almost exclusively in
humans. From an evolutionary standpoint, this helps explain the
complexity of human speech.5
Functional webs
Functional imaging studies have shown that even when we hear
speech, there is extensive activation of Broca’s area. Similarly,
when we speak, Wernicke’s area activates. This mutual activation
is evident in lesion studies, which show that damage to Wernicke’s
area causes speech impediments, even though it is primarily an
auditory area. Alongside these two areas, speaking and listening
to speech also activates the frontal, lower temporal and parietal
cortices. All these findings have put forward a theory that all
neurons involved in language comprehension are connected into
functional webs. Neurons coding auditory, articulatory, semantic and other aspects of words cause mutual excitation, creating
near instantaneous activation of all cortical fields significant to
language processing. This in turn means that, when thinking of a
word, the entire web of neurons specific to that word is activated,
including neurons coding how to say this word, what the word
sounds like, what it means, what it looks like and more.6
This interconnection between systems provides the basis for
learning in infants. When babies first start creating sounds, they
activate their Broca’s areas (involved in articulation). Likewise,
upon hearing those sounds, their Wernicke’s areas activate.
Through long-term potentiation (LTP) neuronal webs are created, connecting hearing and speaking. Later, new information is
September 2015 | Gyrus Vol III no. 3 | 173 neuroscience| neurosurgery
Topic issue | psychiatry | neurology | Fig 1 Broca and Wernicke areas
added to the webs - for instance, the meaning of a word, what the
object defined by the word looks like, what the object can do etc.
Parietal lobe
The parietal lobe is bordered anteriorly by the central sulcus (Rolandic fissure), laterally by the lateral sulcus (Sylvian fissure) and
posteriorly by the parieto-occipital fissure. Relevant parts of the
parietal lobe include: the primary somatosensory area, comprised
of Brodmann areas 3, 1 and 2, all located in the postcentral gyrus;
the superior parietal lobule, comprised of Brodmann areas 5 and
7, involved in spatial orientation and stereognosia (object recognition by touch) and speech recognition; and the inferior parietal
lobule, comprised of Brodmann areas 39 and 40, responsible for
multisensory synthesis. The inferior parietal lobule is composed
of the supramarginal and angular gyri. Their position gives a very
important clue regarding their function. The supramarginal gyrus
lies directly above the Sylvian fissure and is often described as a
part of Wernicke’s area. This location suggests that supramarginal
neurons decode rough auditory information and send them for
further processing. This processing is now highly associative and
combines different word elements such as the sound of the word,
the look of the object, the feel of the object etc.
Logic indicates that the area that performes these complex operations should be strategically placed between the secondary
auditory cortex (superior temporal and supramarginal gyrus),
secondary somatosensory cortex (Brodmann area 5 and 7 in the
superior parietal lobe) and secondary visual cortex (rostral occipital lobe). This is precisely the location of the angular gyrus
(as seen in Fig. 2), where the semantic processing of words is
considered to take place.
Phonological properties
As a part of Wernicke’s area (according to some authors), the
supramarginal gyrus is involved in analyzing phonological and
temporal aspects of words. This was best demonstrated in an
fMRI study examining cerebral activaty in subjects who were
asked to single words based on differences in pitch or spectral
content of tones and in the initial stop consonant for syllables.
When singling out the words, there was activation in the superior
temporal gyrus (especially when identifying acoustic differences)
and supramarginal gyrus (especially when identifying phonological units).7 From this, it can be gathered that the Wernicke area
complex allows for analysis of speech and its segmentation into
sound-based components.
It is often suggested to children learning to read to „sound out“
174 | Gyrus Vol III No 3 | September 2015
Fig. 2 Angular gyrus (yellow) and other
cortical areas that send information to it.
Secondary visual (blue), auditory (red) and somatosensory (green) areas.
words in hopes of making it easier for them to understand written language. This method serves as a very good model to explain
the damaging effects observed in cases of receptive aphasia. For
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
aspect of the word – seeing the object, imagining it, hearing the
word or seeing it written. When a written word is presented, it is
automatically processed through both the visual system and the
temporal regions, is internally read and activates the functional
web.8 Consequently, aphasic patients with damage to the lower
parietal lobe lose the ability to differentiate a word when hearing
it, when producing it but also when reading it because their ability
to „sound out“ words has been compromised.
Semantic properties
The angular gyrus is anatomically connected almost exclusively
with other association regions and receives little to no direct input
from primary sensory areas. Cytoarchitectonicaly, it corresponds
to PGa and PGp, regions almost nonexistent in lower primates.
The previous section focused on how how our brains react to
hearing words, but the same process of neural web/network
activation happens when viewing objects represented by those
words. There are even studies describing different categories of
words activating different areas as seen in fig 3. The connection
between language and internalisation of surroundings should
not be overlooked. Let us take, for instance, the Piraha people of
Amazon who only have words for the numbers 1 and 2. When
a Piraha subject is shown 4 or 5 objects and later tested on the
number of these objects, they cannot remember exactly how
many objects they were presented with. This supports the idea
that memory and language are unseparable and that language
influences the perception of one’s surroundings.9. Unfortunately,
functional imaging studies weren’t performed on these particular
subjects, but studies proving lower parietal activation in semantic
analysis do exist.10
Additional evidence comes from lesion studies. Lesions to the left
angular gyrus caused alexia (inability to recognize or read written
words or letters), agraphia (inability to write), anomia (inability
to name objects), transcortical sensory aphasia (characterized by
poor auditory comprehension, relatively intact repetition, and
fluent speech with semantic paraphasias present) and sentence
comprehension impairment11. An interesting paper written in
2003 speculated that the angular gyrus was at least partially responsible for understanding metaphors.12 There is some evidence
to support this claim, but no in-depth research. For instance, patients with angular gyrus lesions had difficulties with interpreting
neuroscience| neurosurgery
Topic issue | psychiatry | neurology | Fig 3 Action vs object word cortical activation showing different functional webs relating
to their processing.
proverbs – when asked what they thought a proverb meant, they
would devise complicated, but wrong answers. One such example
was a patient presented with the proverb “Not all that glitters
is gold”. When asked to explain it meant, the patient replied it
was advice not to trust salesmen when buying gold because they
might try to cheat you. In addition to these disturbances, patients
with angular gyrus lesions were observed to have a disturbed
bouba/kiki effect. Bouba/kiki effect manifests itself when a subject is presented with two objects as in fig 4 and asked to name
them arbitrarily. Most will name pointy objects kiki and round
objects bouba. This effect is speculated to stem from cortical
connections uniting visual perception of objects (spiked versus
round), appearance of the speaker’s lips (open and round versus
narrow and wide), and feelings related to hearing those sounds.
These connections are all centered around the angular gyrus.
Gerstmann’s syndrome
When mentioning lesions in this area of the brain, Gerstmann’s
syndrome, caused by damage to the angular and supramarginal
gyri, shouldn’t be overlooked. The symptoms include dysgraphia,
dyscalculia, finger agnosia (inability to distinguish fingers on the
hand) and left-right disorientation. This syndrome brings to at-
Fig 4. Bouba/kiki effect. The object on the left was named kiki and the object on the
right was named bouba by 95% of subjects.
tention an interesting role of the parietal lobe in mathematical
processes. Neuroimaging has confirmed and greatly elaborated
the findings from neurological patients. It suggests that the angular gyrus is the locus of core numerical processing. Functional
imaging has shown activation of this area whenever numbers
and numerical magnitudes are implicated, even if subjects are
not aware of it or it is irrelevant for the task13.
Conclusion
Understanding language processing requires that we first understand functional webs and mental representations of words.
These webs explain how different aspects of words are connected
and why lesions in different parts of the brain cause dysfunction
in producing or understanding words. Among other activated areas in these webs are lower parietal gyri. The anterior part of the
lower parietal lobule is the supramarginal gyrus, which constitutes
the Wernicke area along with the superior temporal areas. In the
supramarginal gyrus, phonological and temporal analysis takes
place. It is the part of the brain that allows formation of words
out of syllables. The posterior part of the lower parietal lobule
consists of the angular gyrus, a highly complex area concerned
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, visoko 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