Visual word form area

The Visual Word Form Area (VWFA) is an area of the brain that is hypothesized to be involved in the process of reading textual material.

What/Where is the Visual Word Form Area (VWFA)?

The visual word form area (VWFA) is a functional region of the left fusiform gyrus and surrounding cortex (right-hand side being part of the fusiform face area) that is hypothesized to be involved in identifying words and letters from lower-level shape images, prior to association with phonology or semantics. Because the alphabet is relatively new in human evolution, it is unlikely that this region developed 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 therefore likely to surface within written language.

In addition to word recognition, the VWFA may participate in higher-level processing of word meaning.

In 2003, functional imaging experiments have also raised doubts about whether the VWFA is an actual region. This skepticism has largely disappeared, however there seems to be much variability in its size. An area that may fall within this mental organ in one person may fall outside it in someone else

Anomalies in the activation of this region have been linked to reading disorders. If the area is subjected to a surgical lesion, the patient will suffer a clear impairment to reading ability but not to recognition of objects, names, or faces or to general language abilities. There will be some improvement over the next six months, but reading will still take twice as long as it had before surgery.

Visual Word Form Hypotheses

Pre-lexical Visual Word Form Hypothesis

Put forward by Cohen and colleagues (2000). The basics of this theory state that the neurons in the ventral occipital-temporal cortex (vOT) - of which the posterior fusiform gyrus is a part of - have receptive fields that are sensitive to bigrams, or two letter combinations that commonly occur in words. The neurons sense and process the bigrams, to detect their legality. Here the posterior left fusiform gyrus (part of the vOT), is thought to be one station in a long line of processing areas. The processing starts with visual feature detectors in extrastriate cortex, proceeding through letter detectors and letter-cluster detectors in the posterior fusiform, and then activating lexical representations stored in more anterior multimodal fusiform area. The theory states the function of the VWFA is pre-lexical as it occurs before the word is understood to have meaning.

Lexical Visual Word Form Hypothesis

Put forward by Kronbilcher et al. (2004), was based on functional imaging data that showed, in a parametric fMRI study, that a decrease in activation in the left fusiform gyrus was seen in response to an increase in the frequency of the word - where the frequency is how common the word is. These data refute the previous pre-lexical theory as if the VWFA was pre-lexical one would expect equal activation throughout all frequencies. Instead a lexical theory was proposed where the left fusiform gyrus neurons are thought not to detect words by attempting to match them to stored representations of known words. This would explain the data as more common words would take less time to detect than the less common words, reducing the energy needed for computation and therefore potentially reducing the magnitude of the haemodynamic response that is detected by BOLD fMRI.

Alternative Functions for the cortical area ascribed to the VWFA

Devlin et al. (2006) states that the left posterior fusiform gyrus is not a 'word form area' as such, but instead hypothesizes that the area is dedicated to determining word meaning. That is to say, that this area of the brain is where bottom-up information (visual shapes of words (form), and other visual attributes if necessary) comes into contact with top-down information (semantics and phonology of words). Therefore the left fusiform gyrus is thought to be the interface in the processing of the words not a dictionary that computes a word based on its form alone, as the lexical word form hypothesis states. This paper also presents evidence that refutes the lexical hypothesis.

Another major difference between this hypothesis and the prior ones mentioned is that it is not limited to words alone but to any "meaningful stimulus", in fact non-sensical objects may activate the posterior fusiform cortex in order to extract their meaning from higher-level processes.

Criticism

Some researchers are critical of the notion of the Visual Word Form Area and suggest that the area(s) so far identified have multiple functions and that "there is no evidence that visual word form representations are subtended by a single patch of neuronal cortex and it is misleading to label the left midfusiform region as the visual word form area."

See also

• Neuronal recycling hypothesis