Traditionally, perceptual research has been compartmentalized into distinct and isolated categories according to individual modalities (i.e., visual, auditory, haptic, proprioceptive, vestibular, etc.).  This modular approach has treated individual sensory and motor processing as involving largely independent systems.  However, recently investigators are recognizing the importance of understanding cross-modal interactions and how they relate to perception and behaviour.

Much of the multisensory research up until this point has focused upon tasks involving discrete stimulus presentations in near body space. Such cue interactions of interest have included: visual-auditory integration, visual-proprioceptive integration, or visual-haptic integration. It is important, however, to investigate multisensory integration from the perspective of large-scale self-motion through action space. Unlike traditional approaches to examining the integration of two specific cues at a particular instance in time, navigating through one’s environment requires the dynamic integration of several cues across space and over time (i.e., visual flow, lower-limb proprioception, and vestibular information).  Understanding the principles underlying multimodal integration in this context of unfolding cue dynamics is very important as it provides insight into an important category of multisensory processing.

◘ Multi-sensory integration in the estimation of distance traveled
◘ Contribution of inertial information to the perception of walking speed
◘ Perception of visual speed while walking
◘ Adaptive treadmill control
◘ Vestibular perception is slow
◘ Perceived object stability
◘ Shape from shading

Contribution of inertial information to the perception of walking speed

People are capable of matching the speed of an optic flow field with their actual walking speed, although they tend to set the visual speed too high relative to veridicality. In any case, being able to perform this task at all implies that people have an estimate of walking speed based on other than visual information.

But, how is this non-visual estimate established? Our main interest in this project is to determine whether inertial cues (i.e., vestibular and somatosensory information) enter into this estimation, and if so, how and when.

To this end we use the circular Treadmill of the MPI for Biological Cybernetics (see Figure), a unique setup consisting of a large disc and a handlebar, which can be actuated independently from each other. It also features a computer display, placed in front of the participant, effectively a window to a virtual visual world, giving us full control of visual information, and the setup has a head tracking facility. The setup enables us to selectively isolate the different sensory systems that are involved in walking, such as vision and equilibrium.

Our main paradigm is a 2AFC discrimination task. Typical manipulations involve taking out part of the inertial information by having the participant walk in place and compare it to actual walking through space. Alternative schemes involve creating conflicts between, for instance, the inertial and proprioceptive inputs.

Another paradigm is to have participants adapt to such conflict scenarios. By doing so we attempt to systematically change the sensitivity of the perception of, for instance, inertial factors and examine if this subsequently influences the perceptual estimation of walking speed.

Results show that when inertial information tells participants that they are not moving through space (i.e., because they are walking in place on the treadmill) then they tend to underestimate their perceived walking speed relative to actually walking through space. One possible interpretation is that there is a mandatory integration of the ‘null’ signal coming from the inertial senses with those from the other senses telling the participants they are moving, thus necessarily understating perceived walking speed.

REFERENCE
◘ Frissen, I. , J.L, Souman & M. O. Ernst (2006). Aftereffects of prolonged locomotion on a circular treadmill. 7th annual meeting of the IMRF, Dublin, Ireland

FUNDED BY THE EU PROJECT
◘ Cyberwalk, Sixthe Framework Program of the European Commission

Circular Treadmill