Abstract
BackgroundIn the continuum between a stroke and a circle including all possible ellipses, some eccentricities seem more “biologically preferred” than others by the motor system, probably because they imply less demanding coordination patterns. Based on the idea that biological motion perception relies on knowledge of the laws that govern the motor system, we investigated whether motorically preferential and non-preferential eccentricities are visually discriminated differently. In contrast with previous studies that were interested in the effect of kinematic/time features of movements on their visual perception, we focused on geometric/spatial features, and therefore used a static visual display.Methodology/Principal FindingsIn a dual-task paradigm, participants visually discriminated 13 static ellipses of various eccentricities while performing a finger-thumb opposition sequence with either the dominant or the non-dominant hand. Our assumption was that because the movements used to trace ellipses are strongly lateralized, a motor task performed with the dominant hand should affect the simultaneous visual discrimination more strongly. We found that visual discrimination was not affected when the motor task was performed by the non-dominant hand. Conversely, it was impaired when the motor task was performed with the dominant hand, but only for the ellipses that we defined as preferred by the motor system, based on an assessment of individual preferences during an independent graphomotor task.Conclusions/SignificanceVisual discrimination of ellipses depends on the state of the motor neural networks controlling the dominant hand, but only when their eccentricity is “biologically preferred”. Importantly, this effect emerges on the basis of a static display, suggesting that what we call “biological geometry”, i.e., geometric features resulting from preferential movements is relevant information for the visual processing of bidimensional shapes.
Highlights
As established by many psychophysical studies, biological motion is a special stimulus for the visual system [1,2,3]
Because the graphomotor task was used to analyze the discrimination task according to a distinction between motor preferential and non-preferential ellipses, we describe it before the dual tasks
In order to investigate whether motor-perceptual interactions occur during visual discrimination of static ellipses, we used a finger-thumb opposition sequence which is well known to selectively and strongly activate the motor neural networks [44,45,46,47]
Summary
As established by many psychophysical studies, biological motion is a special stimulus for the visual system [1,2,3]. RP manipulation generates ellipses varying between right slanted stroke (ellipse with the maximal eccentricity) and circle (ellipse with no eccentricity), including ovals (ellipse with intermediary eccentricity) (see Fig. 1 for the correspondence between ellipses, eccentricity and RP) Thanks to this dual-task paradigm, we computed as a variable of interest the cost of the motor task on the discrimination sensitivity (d’) relative to a control condition where the discrimination task was performed in isolation. When the motor task was performed with the non-dominant hand, and when the discriminated ellipses were non-preferential, the motor task had no significant cost on the discrimination sensitivity This finding has two major implications: first, because we used a static display, it suggests that movement geometry is coded within the visual system without being mediated by kinematic variables, bringing the question of the neural mechanisms allowing its emergence; second it indicates that for a given individual, motor knowledge is relevant only for the visual processing of the shapes that would be the most spontaneously produced
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