Synaesthesia and autism: Different developmental outcomes from overlapping mechanisms?

Figures & data

Figure 1. Illustrations of synaesthesia. Examples of grapheme-colour and week-day colour synaesthesia (left) and sequence-space synaesthesia (right). In sequence-space synaesthesia, sequences (in this case months) are experienced in a certain spatial configuration, sometimes situated around the body of the synaesthete. Sequence-space synaesthesia may involve colours but that is not always the case [To view this figure in colour, please see the online version of this journal].

Figure 2. Predictive coding account of autism and synaesthesia. When input arrives at low-level brain areas, excitatory activity travels up the brain’s hierarchy through feedforward connectivity (black upward arrows). Signals from higher order areas (downward arrows) based on higher or mid-level predictions (priors) are integrated with the feedforward excitatory signal from low-level areas, leaving some prediction error. In the case of autism and synaesthesia, however, inhibitory feedback from mid-level areas may be limited due to overly specific priors (narrow gray downward arrow), leaving excess excitatory activity (thick black upward arrow) to linger in the brain (excess prediction error). In synaesthesia, high-level priors are not proposed to be affected while overly specific mid-level priors could be the mechanism underlying synesthetic perception; in schizophrenia, high-level priors may be modified to fit with the altered sensory evidence, leading to hallucinations. For autism, the specific role of low vs high-level priors remains unclear to date [To view this figure in colour, please see the online version of this journal].