What happens in the brain when we grow up without hearing?

When the brain develops deprived of one of the senses, it reorganizes itself in ways that continue to surprise science. In the case of congenital deafness, it was already known that vision can recruit areas normally dedicated to hearing. However, one central question remained precisely unanswered: do these regions actually begin to encode visual information (with spatial organisation), or do they merely react in a non‑specific way?

To address this question, Alessio Fracasso and colleagues studied young congenitally deaf individuals and young hearing individuals while they viewed simple visual patterns inside an fMRI scanner. Using classical stimuli that systematically traverse different regions of the visual field, the researchers analysed how various brain areas responded to this stimulation.

In hearing participants, the results followed the expected pattern: the visual cortex was activated according to the location of the stimuli, while the auditory cortex showed no relevant modulation. In deaf participants, however, an unexpected phenomenon emerged. Instead of increasing activity, the auditory cortex exhibited a systematic deactivation of the brain signal when visual stimuli appeared. The authors demonstrated that these deactivations were stimulus‑dependent rather than mere noise, suggesting organised visual representation in auditory regions deprived of sound input.

More detailed exploratory analyses revealed that these negative responses predominantly represented the opposite side of the visual field, favored regions near the center of vision, and displayed large receptive fields, features typically associated with areas specialized in spatial coding.

This study opens a new perspective on brain plasticity, showing that the brain does not simply replace one sense with another through increased activation: it can also do so through selective deactivation, perhaps as a way to filter irrelevant information or optimise visual attention. This study was published in the scientific journal Human Brain Mapping, in the article The neural organization of visual information in the auditory cortex of the congenitally deaf, as a part of research project 203/20 - Dynamic eye-movement encoding in human cortex using ultra-high field fMRI (7Tesla), supported by the Bial Foundation.

ABSTRACT

Neuroplasticity is the brain's ability to reorganise its structural and functional architecture throughout life. In congenital deafness, the sensory-deprived auditory cortex can be recruited to represent sensory information belonging to other modalities, a process known as cross-modal plasticity. Previous studies have indicated that the auditory cortex of congenitally deaf, but not of hearing individuals, is recruited during visual tasks. However, it remains unclear whether and to what extent these cross-modal responses represent low-level visual spatial information or map the visual field. Here, we addressed this question using two complementary fMRI experiments focusing on cross-modal processing in the auditory cortex of both deaf and hearing individuals during passive viewing of conventional visual stimuli. The first experiment, at the group level, revealed that, unlike in hearing individuals, the auditory cortex of deaf individuals predominantly exhibited negative BOLD signals in early and associative auditory areas — a surprising finding given the prevailing focus on activations in prior work. These negative BOLD signals — commonly interpreted as deactivation responses — suggest that visual information may be represented via cross-modal deactivation mechanisms. We complement the investigation with an exploratory follow-up analysis using pRF modeling in a subset of participants. Together, our findings indicated that, in congenitally deaf individuals, cross-modal visual processing in the auditory cortex may be mediated by deactivation signals, offering new insights into the neural basis of sensory reorganization.