At a very basic level, we recognize each other's speech by tuning to sounds that are particularly suited for the human supralaryngeal vocal tract (SVT) to produce 1.9. Unlike other species our capacity to discriminate particular phonemic streams is very elaborate. Our speech detectors seem to be genetically transmitted. Infants respond to basic vowels and stop consonants shortly after birth [41] 1.10.
However, sounds do not occur in the absence of other perceptual cues. Consequently, we assume that phonemic streams are entrenched in the experience of other sensory experiences. In most adults, sensory maps are synaptically segregated; an oral stimulus will not activate, say, the color area of the visual cortex. This being said, the angular-gyrus is a crucial area in polymodal convergence of sensory stimuli which provide coherence between the senses in apprehending situations.
Studies in people with a peculiar disorder called synaesthesia provide more detailed insight into how, synaptically, we come to associate sounds and shapes, colors and graphemes or even sounds and activities. Synaesthesia is a condition in which people report being stimulated in a second or third modality though only receiving stimulus in one. The most common form of synaesthesia is the association of graphemes and color, that is, synesthetes will report seeing specific colors when presented with specific letters or numbers. So, for example, one sees the color purple when perceiving the number five. There are also cases of synaesthesia that have been reported in which synesthetes see shapes along with some sounds or music.
In Synaesthesia - A Window Into Perception, Thought and Language, V.S. Ramachandran and E.M. Hubbard suggest that synaesthesia is a hyperconnectivity in the fusiform and angular gyrus; they say it is the result of a mutation that causes defective pruning of connections between sensory maps [55].
We identify different subtypes of number/colour synaesthesia and propose that they are caused by hyperconnectivity between colour and number areas at different stages in processing; lower synesthetes may have cross-wiring (or cross-activation) within the fusiform gyrus, whereas higher synesthetes may have cross-activation in the angular gyrus. This hyperconnectivity might be caused by a genetic mutation that causes defective pruning of connections between brain maps. The mutation may further be expressed selectively (due to transcription factors) in the fusiform or angular gyri, and this may explain the existence of different forms of synaesthesia. If expressed very diffusely, there may be extensive cross-wiring between brain regions that represent abstract concepts, which would explain the link between creativity, metaphor and synaesthesia (and the higher incidence of synaesthesia among artists and poets). -Ramachandran and Hubbard, 2001, p.3-
Ramachandran and Hubbard suggest this hypothesis because the fusiform gyrus hosts both the areas of color (V4 and V8) and the visual grapheme area, most specifically in the left hemisphere adjacent to V4. They also suggest that immature brains are significantly more connected between and within these areas and that a process of pruning eliminates many of these connections. Some suggest that infants are born with synaesthesia and that segregation is achieved by the age of four months[5].
Maybe more interestingly, Ramachandran and Hubbard propose a theory of evolution of language and the emergence of proto-language based on their discoveries on synaesthesia.
We suggest, also, that the study of synaesthesia can help us understand the neural basis of metaphor and creativity. Perhaps the same mutation that causes cross-wiring in the fusiform, if expressed very diffusely, can lead to more extensive cross-wiring in their brains. If concepts are represented in brain maps just as percepts are, then cross-activation of brain maps may be the basis for metaphor and this would explain the higher incidence of synaesthesia in artists, poets and novelists (whose brains may be more cross-wired, giving them greater opportunity for metaphors). Our speculations on the neural basis of metaphor also lead us to propose a novel synesthetic theory of the origin of language. We postulate that at least four earlier brain mechanisms were already in place before language evolved; a non-arbitrary synesthetic link between object shapes and sound contours (e.g., bouba and kiki), a synesthetic mapping between sound contour and motor lip and tongue movements (mediated, perhaps, by the recently discovered mirror neurons system in the ventral premotor area that must represent the movements of others, including vocal movements), a synesthetic correspondence between visual appearance and vocalizations (e.g., petite, teeny and little for diminutive objects mimed synaesthetically by a small /I/ formed by the lips and a small vocal tract), and cross-activation between motor maps concerned with gesticulation and vocalizations. This would have allowed an autocatalytic bootstrapping culminating in the emergence of a vocal proto-language. Once this was in place other selection pressures could kick in to refine it (through the combined effects of symbol manipulation/semantics and of the exaptation provided by the syllabic structure for syntactic deep structure). -Ramachandran and Hubbard, 2001, p.28-[55]
Unfortunately, a theory of the emergence of linguisticity in individuals does not entirely describe how language is shared and how we come to have all this vocabulary. It also does not explain how we recognize grammatical structures from non-grammatical ones or how language changes over time.
If we believe Ramachandran and Hubbard, we may associate sounds and objects or activities similarly across the species but we must assume that there are some differences. Even synesthetes do not necessarily agree on the color for particular graphemes; for example, two synesthetes may not see the same color when exposed to the same grapheme.