Angular gyrus

Anatomy

Connections

Left and right angular gyri are connected by the dorsal splenium and isthmus of the corpus callosum.

Boundaries

• Anteriorly by the supramarginal gyrus.
• Superiorly by the intraparietal sulcus.
• Posteriorly by the parieto-occipital sulcus.
• Inferiorly the angular gyrus of the parietal lobe is continuous as the superior and middle temporal gyri. Also, the angular sulcus, which is capped by the angular gyrus, is continuous as the superior temporal sulcus inferiorly.

Function

The angular gyrus is the part of the brain associated with complex language functions (i.e. reading, writing and interpretation of what is written). Lesion to this part of the brain shows symptoms of the Gerstmann syndrome: effects include finger agnosia, alexia (inability to read), acalculia (inability to use arithmetic operations), agraphia (inability to copy), and left-right confusion.

Language

Norman Geschwind proposed that written word is translated to internal monologue via the angular gyrus.

V. S. Ramachandran, and Edward Hubbard published a paper in 2003 in which they hypothesized the angular gyrus to play a role in understanding metaphors. They stated:There may be neurological disorders that disturb metaphor and synaesthesia. This has not been studied in detail but we have seen disturbances in the Bouba/Kiki effect (Ramachandran & Hubbard, 2001a) as well as with proverbs in patients with angular gyrus lesions. It would be interesting to see whether they have deficits in other types of synaesthetic metaphor, e.g. 'sharp cheese' or 'loud shirt'. There are also hints that patients with right hemisphere lesions show problems with metaphor. It is possible that their deficits are mainly with spatial metaphors, such as 'He stepped down as director'.

The fact that the angular gyrus is proportionately much larger in hominids than other primates, and its strategic location at the crossroads of areas specialized for processing touch, hearing and vision, leads Ramachandran to believe that it is critical both to conceptual metaphors and to cross-modal abstractions more generally. However, recent research challenges this theory.

Research by Krish Sathian (Emory University) using functional magnetic resonance imaging (fMRI) suggests that the angular gyrus does not play a role in creating conceptual metaphors. Sathian theorizes that conceptual metaphors activate the texture-selective somatosensory cortex in the parietal operculum.

Brownsett and Wise highlight the role of the left angular gyrus in both speaking and writing.

Arithmetic and spatial cognition

Since 1919, brain injuries to the angular gyrus have been known to often cause arithmetic deficits. Functional imaging has shown that while other parts of the parietal lobe bilaterally are involved in approximate calculations due to its link with spatiovisual abilities, the left angular gyrus together with left Inferior frontal gyrus are involved in exact calculation due to verbal arithmetic fact retrieval. When activation in the left angular gyrus is greater, a person's arithmetic skills are also more competent.

Attention

The right angular gyrus has been associated with spatiovisual attention toward salient features. It may allocate attention by employing a bottom-up strategy which draws on the area's ability to attend to retrieved memories. Furthermore, the angular gyrus has been associated with orienting in three dimensional space, not because it interprets space, but because it may control attention shifts in space.

Other functions

Default mode network

The angular gyrus is part of the default mode network, a network of brain regions activated during multi-modal activities that are independent of external stimuli.

Awareness

The angular gyrus reacts differently to intended and consequential movement. This suggests that the angular gyrus monitors the self's intended movements and uses the added information to compute differently, as it does for consequential movements. By recording the discrepancy, the angular gyrus maintains an awareness of the self.

Memory retrieval

Activation of the angular gyrus shows that not only does it mediate memory retrieval, but it also notes contradictions between what is expected from the retrieval, and what is unusual. The angular gyrus can access both content and episodic memories and is useful in inferring from these the intentions of human characters. Furthermore, the angular gyrus may use a feedback strategy to ascertain whether a retrieval is expected or unusual.

Out-of-body experiences

Experiments have demonstrated the ability of stimulation of the right angular gyrus to induce out-of-body experiences. Stimulation of the left angular gyrus in one experiment caused a woman to perceive a shadowy person lurking behind her. The shadowy figure is actually a perceived double of the self. Another such experiment gave the test subject the sensation of being on the ceiling. This is attributed to a discrepancy in the actual position of the body, and the mind's perceived location of the body.

Clinical significance

Damage to the angular gyrus manifests as Gerstmann syndrome. Damage may impair one or more of the below functions.

1. Dysgraphia/agraphia: deficiency in the ability to write
2. Dyscalculia/acalculia: difficulty in learning or comprehending mathematics
3. Finger agnosia: inability to distinguish the fingers on the hand
4. Left-right disorientation

Additional images

File:Angular gyrus animation small.gif|Position of angular gyrus (shown in red). File:Gray725 angular gyrus.png|Lateral surface of left cerebral hemisphere, viewed from above. Angular gyrus is shown in orange. File:Gray726 angular gyrus.png|Lateral surface of left cerebral hemisphere, viewed from the side. Angular gyrus is shown in orange. Image:Gehirn, lateral - Hauptgyri beschriftet.svg|Lateral view of a human brain, main gyri labeled.

File:Slide3HAN.JPG|Cerebrum. Lateral view.Deep dissection. File:Slide4HAN.JPG|Cerebrum. Lateral view.Deep dissection. File:Angular gyrus coronal sections.gif|Angular gyrus highlighted in green on coronal T1 MRI image File:Angular gyrus sagittal sections.gif|Angular gyrus highlighted in green on sagittal T1 MRI image File:Angular gyrus transversal sections.gif|Angular gyrus highlighted in green on transversal T1 MRI image

References

References

1. Seghier, Mohamed L.. (February 2013). "The Angular Gyrus". The Neuroscientist.
2. John, Hall. (2010). "Guyton and Hall Textbook of Medical Physiology". Saunders.
3. (2008). "Corpus callosal connection mapping using cortical gray matter parcellation and DT-MRI". Human Brain Mapping.
4. (2005). "Segmentation of Subcomponents within the Superior Longitudinal Fascicle in Humans: A Quantitative, In Vivo, DT-MRI Study". Cereb. Cortex.
5. (2010). "Dissociable Connectivity within Human Angular Gyrus and Intraparietal Sulcus: Evidence from Functional and Structural Connectivity". Cereb. Cortex.
6. (2006). "Connection patterns distinguish 3 regions of human parietal cortex". Cereb Cortex.
7. (2007). "The occipitofrontal fascicle in humans: A quantitative, in vivo, DT-MRI study". NeuroImage.
8. (2007). "Anatomical traces of vocabulary acquisition in the adolescent brain". J Neurosci.
9. Gai, Dayu. "Angular gyrus {{!}} Radiology Reference Article {{!}} Radiopaedia.org".
10. (1965). "Disconnexion Syndromes in Animals and Man". Brain.
11. (2003). "The Phenomenology of Synaesthesia". Journal of Consciousness Studies.
12. (2011). "Metaphorically feeling:Comprehending textual metaphors actives somatosensory cortex". Brain and Language.
13. (2009-06-16). "The Contribution of the Parietal Lobes to Speaking and Writing". Cerebral Cortex.
14. (1919). "On language, music and calculation mechanisms and their localisation in the cerebrum". Zeitschrift für die Gesamte Neurologie und Psychiatrie.
15. (1940). "Syndrome of finger agnosia, disorientation for right and left, agraphia and acalculia—Local diagnostic value". Arch Neurol Psychiatry.
16. (1999). "Sources of mathematical thinking: behavioral and brain-imaging evidence". Science.
17. (2007). "Individual differences in mathematical competence predict parietal brain activation during mental calculation". NeuroImage.
18. (2012). "The angular gyrus: multiple function ad multiple subdivisions". Neuroscientist.
19. (2011). "Is 2+2=4? Meta-analyses of brain areas needed for numbers and calculations". NeuroImage.
20. (2011). "TMS over the left angular gyrus impairs the ability to discriminate left from right". Neuropsychologia.
21. (Sep 2012). "Neural mechanisms of attentional reorienting in three-dimensional space". J Neurosci.
22. (2018-05-01). "Distant from input: Evidence of regions within the default mode network supporting perceptually-decoupled and conceptually-guided cognition". NeuroImage.
23. (January 2017). "Angular default mode network connectivity across working memory load". Human Brain Mapping.
24. Farrer C, Frey SH, Van Horn JD, Tunik E, Turk D, Inati S, Grafton ST. The angular gyrus computes action awareness representations. Centre de Neuroscience Cognitive.
25. [https://www.nytimes.com/2006/10/03/health/psychology/03shad.html Out-of-Body Experience? Your Brain Is to Blame - New York Times]
26. (2006). "Induction of an illusory shadow person: Stimulation of a site on the brain's left hemisphere prompts the creepy feeling that somebody is close by.". Nature.
27. Vallar G. (July 2007). "Spatial neglect, Balint-Homes' and Gerstmann's syndrome, and other spatial disorders". CNS Spectr.
28. (2004). "Defective spatial imagery with pure Gerstmann's syndrome". Eur. Neurol..