Is there such a thing as a unisensory brain region?

From decoding rapid-fire phonemes that occur in milliseconds to tracking syntax and semantics across entire conversations, the brain engages an intricate network of regions just to make sense of what’s being said. For instance some regions are involved in syntactic structure and production, while others in comprehension and meaning inference. We even have areas like the superior temporal gyrus firing within milliseconds to help distinguish between similar sounds like “ba” and “pa.” But interestingly, we’ll see, the brain doesn’t demand that this information arrive through the ears or the eyes or the hands. It only asks that it arrives in a form that’s structured, interpretable, and worth responding to.

Why does the brain care so much about language? And more interestingly, what even is language to the brain? Most commonly we associate language with speech, but once we look at the brain, it doesn't always associate language with speech. It seems the brain is not interested in the modality of language but rather in anything with some sort of communicative potential. That is, it’s not the sound waves, hand gestures, or inked letters that matter but the structured, meaningful message beneath them.

From an evolutionary standpoint this is quite a genius move. A brain wired for just one kind of sensory input like sound may work well in a world where we only need sound to communicate, but this is seldom the case. We are receptive to important information more than once from the way someone looks at you, their facial expressions, or even just a gesture. We live in a dynamic world where sensory input varies and not everyone receives the same input. So if you were designing a system to extract meaning from the world, would you build something rigid and fixed? Or something flexible, adaptive, and responsive to different ‘modalities’ of communication?

Language as a Function, not a Format

Traditionally, parts of the brain have been labelled by what we think they do: this area processes sounds, this area processes images, this one is for touch. But findings from neuroimaging and sensory deprivation research indicates that brain’s language network for example, does not seem to be bound to specific senses but tuned to meaning, in whichever format meaning is presented. 

Bernstein and Liebenthal (2014) looked at how sign language is processed in deaf and hearing individuals. Participants included:

• DS: Deaf native signers of British Sign Language (BSL)

• DO: Deaf oral users—highly proficient in spoken English, but no knowledge of BSL

• HN: Hearing non-signers

  
  

All participants were shown videos of sign language while their brain activity was recorded using fMRI. Cleverly, researchers included three groups: deaf individuals who were native users of British Sign Language (BSL), deaf individuals who didn’t know BSL but used spoken language (oral deaf), and hearing people with no knowledge of sign language. This setup allowed the researchers to tease apart the effects of language from the effects of deafness. If a particular brain region was active in both deaf groups (regardless of whether they knew sign language) it likely reflected a general adaptation to deafness. But if that region was active only in the deaf signers, then it was much more likely to be involved in processing the linguistic content of the signs themselves, not just visual motion.

They found that the left superior temporal cortex lit up in deaf signers when watching BSL, which is a region usually involved in processing spoken language in hearing people. Crucially, this region did not light up in oral deaf participants who couldn’t sign and not in hearing non-signers either.

It seems this region is not tuned to spoken input like we usually frequent, but rather any input with communicative meaning, regardless of spoken or signed. We see that the brain may be tracking content, not format.

What we also begin to see here is a striking example of brain plasticity and the presence of multimodal processing units in the brain. In hearing individuals who are proficient in sign language, the superior temporal cortex exhibits significantly less activation compared to that of deaf signers, likely because this region is already predominantly engaged in processing auditory input. But in deaf individuals, where auditory input isn’t available, that same neural real estate is simply repurposed for something equally useful: processing visual language. It becomes a key player in decoding the structure and meaning of signs. As we are beginning to see, brain regions are not strictly "auditory" or "visual" or "tactile" but flexible systems that can be tuned to handle whatever mode of communication is most relevant.

Here not only flexibility is highlighted but moreso how experience may shape the functional architecture of the brain. The modalities that present themselves as useful as you grow up may shape which brain regions process what. 

An interesting interlude here is the idea that the more the brain uses a particular sense, the better that sense becomes, especially when others are no longer available. To most of us, a notion of ‘super hearing’ for example has strong comical undertone, but only because we’ve never had to rely on a single modality to navigate the world. Our brains are constantly taking in multisensory information like sight, sound and touch together to help us build a coherent percept of something. But when one of those senses is missing, as we see the brain seems to reallocate resources to maximise information extraction from other sensory inputs. So, do they become better or more sensitive in certain senses? Well, deaf individuals have been shown to detect touch and subtle vibrations with greater precision than hearing people, a phenomenon linked to increased activity in auditory brain regions being used by the somatosensory system (Levänen et al., 1998). Similarly, blind individuals often perform better on tasks requiring auditory localization or pitch discrimination, with their visual cortex playing an unexpected supporting role in auditory processing. Once again, brain region functionality seems to alter to maximise the utility of the senses that are still available. Just like we become good at language because we use it constantly, those who rely on specific sensory channels every day become finely tuned to them too.

This theme of neural flexibility continues across sensory domains. The primary visual cortex (V1), which is responsible for the earliest stages of visual processing in sighted people, shows something fascinating in those who are blind. In blind individuals (who don’t engage in visual processing at all) V1 activates in response to spoken language, exclusively responding to structured, meaningful language. Once again, we see the brain repurposing an unused region to process relevant and significant information.

Echo-locators who are blind, for instance, show substantial activation in their visual cortices when judging sound-based spatial cues, something their sighted but blindfolded counterparts do not. Logically, sound-based spatial cues are especially important for individuals with visual impairments, providing critical environmental information. Just as observing someone walking toward you conveys meaning visually, auditory cues likewise inform perception. This underscores the pragmatic nature of the brain in utilising available sensory inputs where region unused for its original function is reassigned to something else that’s useful. 

Marina Bedny’s research group expanded on this idea through their work with Braille readers. In blind participants, the ventral occipitotemporal cortex (VOTC) (a region typically responsible for visual word recognition in the sighted) showed strong activation when participants read Braille words. Critically, this activation wasn’t triggered by random textures; it was the structure and meaning in the language (conceptualized through braille) that drove the response. A visual reading area had become a tactile reading area.

Even more strikingly, in a 2015 study, Bedny’s team found that V1 itself responded more strongly to grammatically structured spoken sentences than to backward or nonword speech in congenitally blind individuals. The brain region that, in sighted people, is used to detect light and edge contrast was now tuned to analyze linguistic structure.

Taken together, these findings show that brain plasticity is not only a passive response to deprivation but rather an active reallocating of brain areas based on what’s useful for who. Whether through vision, hearing, or touch, the brain flexibly recruits available networks to support meaningful, structured communication despite the sensory modality.

This kind of active neural flexibility has been demonstrated even in people with completely typical sensory experiences. You might assume that recruiting the visual cortex for non-visual tasks only happens in those who’ve been blind since birth, where early sensory deprivation gradually rewires the brain over a lifetime but seemingly ,the brain is much more responsive

Taking Braille reading as an example, in blind individuals, disrupting activity in the visual cortex using transcranial magnetic stimulation (TMS) significantly impairs their ability to read by touch. For sighted individuals, the same task is disrupted not by targeting visual areas, but the somatosensory cortex, the brain’s usual hub for tactile processing. So far, this aligns with expectation: if you’ve never had sight, your brain adapts, however such repurposing doesn’t require decades of sensory deprivation.

In a study by Merabet and colleagues, sighted participants were blindfolded and trained to read Braille for five days. By Day 5, their visual cortices were lighting up during Braille reading, and disrupting that region with TMS impaired their performance. Then, as soon as the blindfolds came off for a while , the effect vanished. Cross-modal plasticity does not appear to depend solely on slow structural rewiring, rather the brain may possess inherently flexible circuits or pathways that remain latent under typical conditions but become engaged when required.

Conclusion

This research may be seen to call into question long-held assumptions about how the brain is organised. We don’t seem to have a “hearing part” and a “sight part” and a “language part” each sealed off from one another and compartemptalised. We seem to have systems that are adaptable, overlapping, and shaped by their utility, no matter the communicative medium.

Second, we can see how our experiments mostly study one kind of person. If your theories about language are based on the speech of hearing, sighted, English-speaking undergrads from elite universities, you might be missing… a lot.

Finally, these findings tell us something about what the brain values here is meaning and communication - it just so happens that a lot of ours is auditory.  But language isn’t locked to a single sense, or a single format, or even a single brain map.

The brain is not modality-specific because the world is not modality-specific. If the goal is to extract meaning and interact with others, then it makes far more sense to wire a brain that is adaptive. Spoken language happens to be what our society uses the most, but it does not mean this is all our brain can use. It seems that our brain may not always be finely functionally compartmentalised but multisensory by nature.