Abstract
BackgroundWhen a moving stimulus and a briefly flashed static stimulus are physically aligned in space the static stimulus is perceived as lagging behind the moving stimulus. This vastly replicated phenomenon is known as the Flash-Lag Effect (FLE). For the first time we employed biological motion as the moving stimulus, which is important for two reasons. Firstly, biological motion is processed by visual as well as somatosensory brain areas, which makes it a prime candidate for elucidating the interplay between the two systems with respect to the FLE. Secondly, discussions about the mechanisms of the FLE tend to recur to evolutionary arguments, while most studies employ highly artificial stimuli with constant velocities.Methodology/Principal FindingSince biological motion is ecologically valid it follows complex patterns with changing velocity. We therefore compared biological to symbolic motion with the same acceleration profile. Our results with 16 observers revealed a qualitatively different pattern for biological compared to symbolic motion and this pattern was predicted by the characteristics of motor resonance: The amount of anticipatory processing of perceived actions based on the induced perspective and agency modulated the FLE.Conclusions/SignificanceOur study provides first evidence for an FLE with non-linear motion in general and with biological motion in particular. Our results suggest that predictive coding within the sensorimotor system alone cannot explain the FLE. Our findings are compatible with visual prediction (Nijhawan, 2008) which assumes that extrapolated motion representations within the visual system generate the FLE. These representations are modulated by sudden visual input (e.g. offset signals) or by input from other systems (e.g. sensorimotor) that can boost or attenuate overshooting representations in accordance with biased neural competition (Desimone & Duncan, 1995).
Highlights
The Flash-Lag Effect (FLE) When a moving stimulus and a briefly flashed static stimulus are shown spatially aligned the static stimulus is perceived as lagging behind the moving stimulus
The FLE has received a great deal of attention because it initiated a fundamental discussion about the properties of the visual system: Does the early visual system already compensate for neural delays in terms of visual prediction as proposed by Nijhawan [1,2] or is such a predictive mechanism only observed within the sensorimotor system? The predictive mechanisms of the motor system are widely accepted since without compensation for neural delays motor actions in response to dynamic events would always lag behind [2, for review]
We did predict an interaction between perspective and agency for biological but not for symbolic motion because the biological 1st person perspective should generate the strongest motor resonance with the observer’s mirror neuron system (MNS), potentially enhancing the FLE
Summary
The Flash-Lag Effect (FLE) When a moving stimulus and a briefly flashed static stimulus are shown spatially aligned the static stimulus is perceived as lagging behind the moving stimulus. When a flash was administered in perfect alignment with the (invisible) moving limb participants showed an FLE This reveals the tight connection between the visual and the sensorimotor system and further suggests that the two systems employ similar (predictive) mechanisms [2]. Resuming our predictions for the flash-terminated condition, we expected an increased FLE for the condition that generates the strongest overshooting representation, i.e. the strongest predictive motor resonance within the MNS This is clearly the case for biological motion in the 1st person perspective, yet, as pointed out, possibly in only one of the agency conditions–which exactly, depends on whether an illusion of control is induced and whether illusionary control of a biological motion attenuates or boosts the FLE
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