The visual word form area (VWFA) is a functional region of the left fusiform gyrus and surrounding cortex (the right-hand side is part of the fusiform face area) that is thought to be involved in identifying words and letters from lower-level shape images prior to association with phonology or semantics.
Because the alphabet is a relatively new development in human evolution, it is unlikely that this region evolved as a result of selection pressures related to word recognition per se; however, this region may be highly specialized for certain types of shapes that occur naturally in the environment and are thus likely to surface within written language.
Reading difficulties have been linked to abnormalities in the activation of this area. Electrical brain stimulation to the visual word form area disrupts reading and can result in letter misinterpretation.
If the region is surgically lesioned, the patient will experience a demonstrable impairment in reading ability but not in object, name, or face identification or general language abilities. Over the following six months, there will be some improvement, but reading will still take twice as long as it did before surgery.
VWFA in Hyperlexia and Dyslexia
A meta-analysis of studies on children and adults with dyslexia reveals that underactivation of the left occipitotemporal region, specifically the VWFA, may be involved in dyslexics’ trouble with fluent reading. These reading difficulties may possibly be attributed to an impaired connection between the VWFA and adjacent parietal cortical regions responsible for visual attention.
Research findings indicate that children diagnosed with autism spectrum disorders (ASD) may exhibit a greater dependence on visual perception domains, such as the VWFA, while placing a comparatively lesser emphasis on phonological domains when engaged in reading tasks, in contrast to youngsters without ASD.
Greater VWFA activation may be particularly significant in youngsters with hyperlexia (reading skills beyond one’s training). Hyperlexia is known to be related with ASD, with prevalence estimates ranging from 6 to 20.7% in autistic children.
Visual Word Form Area Criticisms
In 2003, functional imaging investigations cast doubt on the visual word form’s existence. This skepticism has virtually vanished.
Yet, the area’s magnitude appears to vary greatly. An area that falls within this mental organ in one person may fall outside of it in another.
Functional MRI scans have shown that the area specifically activates when people read, as opposed to recognizing other objects, such as faces or houses. And people with lesions in the region lose the ability to recognize whole words, reduced to letter-by-letter reading.
However, fMRI studies cannot demonstrate a causal role for the visual word form area, and lesions involving the VWFA invariably involve other regions as well.
Scholars, led by Laurent Cohen of the Hôpital de la Salpêtrière, were presented with an opportunity to establish that the VWFA does, in fact, contribute causally to the capacity for word recognition in 2006, when a patient’s epilepsy-relieving surgery specifically disrupted the VWFA.
The findings of reading, language, and object recognition tests before and after the surgery on the 46-year-old man were reported in the April 20, 2006, issue of Neuron. Prior to surgery, they discovered that his reading skills were normal. However, post-surgery tests revealed quite different findings.
“Although we studied reading more extensively than the perception of other types of visual stimuli, our patient presented a clear-cut reading impairment following surgery, while his performance remained flawless in object recognition and naming, face processing, and general language abilities,”
reported the researchers.
“Such selectivity may be difficult to observe in patients with more customary lesions resulting from strokes or tumors, which often affect a larger extent of cortex and white matter. The small size of the present lesion thus provides precious support to the idea of partial regional selectivity for word perception in the ventral cortex,”
they wrote.
Importantly, the researchers observed that the patient could detect lengthy words as rapidly as small ones before the operation; however, following the surgery, the recognition time increased linearly as a function of word length. Such findings revealed that the patient’s recognition of words had been restricted to letter by letter.
“How could there be a piece of neural tissue dedicated to a recently invented cognitive skill like word recognition?”
wondered Alex Martin in a preview of the paper in the same issue of Neuron. Nevertheless, Martin commented, Cohen and his colleagues “report a unique set of findings in favor of the existence of the VWFA that will surely add fuel to the debate.”
He concluded:
“The single case study…provides compelling evidence that the VWFA plays a causal role in the chain of neural events that underlie normal reading.”
Alternative Functions Proposed
According to a 2006 study from Joseph and colleagues, the left posterior fusiform gyrus is not a ‘word form area’ per se, but rather is dedicated to determining word meaning. That is, this part of the brain is where bottom-up information (visual shapes of words (form) and other visual qualities as needed) collides with top-down information (semantics and phonology of words).
As a result, the left fusiform gyrus is assumed to be the interface in word processing rather than a dictionary that computes a word solely based on its form, as the lexical word form hypothesis claims. The evidence presented in this research also refutes the lexical theory.
Another significant difference between this hypothesis and the preceding ones is that it is not restricted to words alone but to any “meaningful stimulus”; in fact, nonsensical items may activate the posterior fusiform cortex in order to derive their meaning from higher-level processes.
The finding that disruption of the visual word form area due to surgical lesions or electrical brain stimulation has little effect on a person’s ability to extract meaning from non-word stimuli, on the other hand, provides strong evidence that the VWFA’s function is primarily limited to processing words and not “any meaningful stimulus.”
There is evidence, however, that the VWFA may also be involved in other types of visual processing, despite the fact that it is not specifically designed for reading. Rather, it possesses a set of properties and functions that render it practical for reading and especially crucial for fluid reading.
VWFA involvement appears to be dependent on a stimulus’s visual complexity, and it appears to process recognized visual stimuli that are grouped together. This could explain why “letter by letter” reading is possible even when the VWFA has lesions that interfere with fluid reading abilities.
This may also explain why line drawings and Amharic characters activate the VWFA more strongly than written words familiar to research participants.
References:
- Devlin, Joseph T.; Jamison, Helen L.; Gonnerman, Laura M.; Matthews, Paul M. (2006). The role of the posterior fusiform gyrus in reading. Journal of Cognitive Neuroscience. 18 (6): 911–922. doi: 10.1162/jocn.2006.18.6.911
- Gaillard et al.: Direct Intracranial, fMRI, and Lesion Evidence for the Causal Role of Left Inferotemporal Cortex in Reading. Neuron, 50, 191-204, April 20, 2006. DOI 10.1016/j.neuron.2006.03.031
- Hirshorn, Elizabeth A.; Li, Yuanning; Ward, Michael J.; Richardson, R. Mark; Fiez, Julie A.; Ghuman, Avniel Singh (2016-07-19). Decoding and disrupting left midfusiform gyrus activity during word reading. Proceedings of the National Academy of Sciences. 113 (29): 8162–8167. doi: 10.1073/pnas.1604126113
- McCandliss, Bruce D.; Cohen, Laurent; Dehaene, Stanislas (2003). The visual word form area: Expertise for reading in the fusiform gyrus. Trends in Cognitive Sciences. 7 (7): 293–299. doi: 10.1016/S1364-6613(03)00134-7
- Ostrolenk, Alexia; Forgeot d’Arc, Baudouin; Jelenic, Patricia; Samson, Fabienne; Mottron, Laurent (2017). Hyperlexia: Systematic review, neurocognitive modelling, and outcome. Neuroscience & Biobehavioral Reviews. 79: 134–149. doi: 10.1016/j.neubiorev.2017.04.029
- Price, Cathy J; Devlin, Joseph T (2003). The myth of the visual word form area. NeuroImage. 19 (3): 473–81. doi:10.1016/S1053-8119(03)00084-3
- Richlan, Fabio; Kronbichler, Martin; Wimmer, Heinz (2011). Meta-analyzing brain dysfunctions in dyslexic children and adults. NeuroImage. 56 (3): 1735–1742. doi:10.1016/j.neuroimage.2011.02.040
- Vogel, Alecia C.; Petersen, Steven E.; Schlaggar, Bradley L. (2014). The VWFA: it’s not just for words anymore. Frontiers in Human Neuroscience. 8: 88. doi:10.3389/fnhum.2014.00088