vannuscorps, g. | bioRxiv Science

bioRxiv2020

Typical lipreading and audiovisual speech perception without motor simulation

All it takes is a face to face conversation in a noisy environment to realize that viewing a speakers lip movements contributes to speech comprehension. Following the finding that brain areas that control speech production are also recruited during lip reading, the received explanation is that lipreading operates through a covert unconscious imitation of the observed speech movements in the observers own speech motor system - a motor simulation. However, motor effects during lipreading do not necessarily imply simulation or a causal role in perception. In line with this alternative, we report here that some individuals born with lip paralysis, who are therefore unable to covertly imitate observed lip movements, have typical lipreading abilities and audiovisual speech perception. This constitutes existence proof that typically efficient lipreading abilities can be achieved without motor simulation. Although it remains an open question whether this conclusion generalizes to typically developed participants, these findings demonstrate that alternatives to motor simulation theories are plausible and invite the conclusion that lip-reading does not involve motor simulation. Beyond its theoretical significance in the field of speech perception, this finding also calls for a re-examination of the more general hypothesis that motor simulation underlies action perception and interpretation developed in the frameworks of the motor simulation and mirror neuron hypotheses.

neuroscience↗

bioRxiv2020

Shape-centered representations of bounded regions of space mediate the transformation of retinotopic representations into conscious perception of objects

The primary visual cortex represents the retinotopic orientation of visual primitives (edges, blobs, bars), but our conscious perception is of orientated objects (e.g., dogs, forks) in the environment. How this transformation operates remains unknown. We report here the study of a young woman presenting with an extraordinarily clear and informative visual disorder that affects highly specific aspects of object perception allowing precise inferences about the type and properties of visual representations that mediate this transformation. Davida perceives sharp-edged 2D bounded regions of space of medium to high contrast as if they were plane-rotated by 90, 180 or 270 degrees around their center, mirrored across their own axes, or both. In contrast, her perception of strongly blurred or very low contrast shapes, and of compound shapes emerging from a collection of bounded elements, is intact. The nature of her errors implies that visual perception is mediated by a representation of each bounded region of space in a shape-centered coordinate system aligned on either the shapes most elongated part or on the shapes axis of symmetry and centered either at the midpoint of the shapes most elongated part or at the shapes centroid. The selectivity of her disorder to sharp-edged medium to high-contrast stimuli additionally suggests that duplicate shape-centered representations are computed in parallel from information derived from the parvocellular and magnocellular subcortical channels and integrated precisely at the level at which shape representations must be mapped onto a behaviorally relevant frame of reference.

neuroscience↗