There are two paths for visual information, one to the motor areas (dorsal ‘where’ stream) and one to the areas concerned with consciousness, memory and cognition (ventral ‘what’ stream). The visual ventral stream has areas for the recognition of various categories of object: faces, body parts, letters for example. But are these areas really ‘visual’ areas or can they deal with input from other senses? There is recent research into an area concerned with numerals. (see citation below) There are some reasons to doubt a ‘vision only’ processing in these areas. “…cortical preference in the ‘visual’ cortex might not be exclusively visual and in fact might develop independently of visual experience. Specifically, An area showing preference for reading, at the precise location of the VWFA (visual word-form area), was shown to be active in congenitally blind subjects during Braille reading… large-scale segregation of the ventral stream into animate and inanimate semantic categories have also been shown to be independent of visual experience. More generally, an overlap in the neural correlates of equivalent tasks has been repeatedly shown between the blind and sighted using different sensory modalities.” Is an area specialized in one domain because of cultural learning through visual experience or is the specialization the result of the specific connectivity of an area?
Abboud and others used congenitally blind subjects to see if the numeral area could process numerals arriving from auditory signals. Congenitally blind subjects cannot have categorical area that are based on visual learning. The letter area and numeral area are separate even though the letter symbols and numeral symbols are very similar – in fact can be identical. The researchers predicted that the word area had connections to language areas and the numeral area connected to quantitative areas.
The subjects were trained in eye-music, a sight substitute based on time, pitch, timbre and volume. While being scanned, the subjects heard the same musical description of an object and were asked to identify the object as part of a word, part of a number, or a colour. Roman numerals were used to give a large number of identical musical descriptions of numbers and letters. What they found was that the numeric task gave activation in the same area as it does in a sighted person and that blind and sighted subjects had the same connections, word area to language network and numeral area to quantity network. It is the connectivity patterns, independent of visual experience, that create the visual numeral-form area. “…neither the sensory-input modality and visual experience, nor the physical sensory stimulation itself, play a critical role in the specialization observed in this area. ” It is which network is active (language or quantity) that is critical.
“…these results are in agreement with the theory of cultural recycling, which suggests that the acquisition of novel cultural inventions is only feasible inasmuch as it capitalizes on prior anatomical and connectional constraints and invades pre- existing brain networks capable of performing a function sufficiently similar to what is needed by the novel invention. In addition, other factors such as the specifics of how literacy and numeracy are learned, as well as the distinctive functions of numerals and letters in our education and culture, could also account for the segregation of their preferences.”
Here is the abstract: “Distinct preference for visual number symbols was recently discovered in the human right inferior temporal gyrus (rITG). It remains unclear how this preference emerges, what is the contribution of shape biases to its formation and whether visual processing underlies it. Here we use congenital blindness as a model for brain development without visual experience. During fMRI, we present blind subjects with shapes encoded using a novel visual-to-music sensory-substitution device (The EyeMusic). Greater activation is observed in the rITG when subjects process symbols as numbers compared with control tasks on the same symbols. Using resting-state fMRI in the blind and sighted, we further show that the areas with preference for numerals and letters exhibit distinct patterns of functional connectivity with quantity and language-processing areas, respectively. Our findings suggest that specificity in the ventral ‘visual’ stream can emerge independently of sensory modality and visual experience, under the influence of distinct connectivity patterns. ”
Abboud, S., Maidenbaum, S., Dehaene, S., & Amedi, A. (2015). A number-form area in the blind Nature Communications, 6 DOI: 10.1038/ncomms7026