Seeing through the static: Reduced imagery vividness in aphantasia is associated with impaired temporal lobe signal complexity

Aphantasia is the inability to experience mental imagery during full wakefulness without any prominent perceptual deficits. Visual aphantasia is associated with differences in distributed brain networks, but its neurobiological underpinnings remain a mystery. In particular, aphantasia may arise due to impairments in the top-down control over visual imagination. We predicted that this in turn would prevent the brains of aphantasic participants from differentiating neural activity encoding the contents of imagination from the background noise of resting activity, particularly within the ventral temporal lobes. To test this hypothesis, we re-analysed functional magnetic resonance imaging (fMRI) data collected from aphantasics (n = 21), hyperphantasics (those with "photographic imagery"; n = 20), and controls (n = 17) during a simple perception and imagery task. We used two measures of informational complexity to quantify the complexity of the spatial pattern of thresholded BOLD signals in the participants' temporal lobes during visual perception and imagery. Both measures of spatial complexity showed significant correlations with imagery vividness. We then performed dynamic functional connectivity analyses on the same data revealing that the higher-order networks of aphantasics were abnormally coupled with the temporal lobes during imagery (p < 0.05). These results provide a novel perspective, reframing aphantasia as an inability of the visual system to selectively activate regions encoding object-specific visual categories above background levels of noise.