Human multisensory fusion of vision and touch: detecting non-linearity with small changes in the sensory environment

Previous investigations using relatively large amplitude sensory stimuli or complete removal of sensory input have demonstrated non-linear processing of sensory information for postural control. In the present study, we asked whether a linear range of sensory fusion exists when smaller amplitude sti muli are used. The amplitudes of visual and somatosensory input were simult aneously co-varied within a trial. The postural responses were characterize d by analyzing how the Fourier transform of postural sway at the driving fr equency varied with sensory movement amplitudes. If the postural control sy stem is linear with constant weighting of sensory inputs, then the pattern of Fourier transforms should be a linear function of movement amplitude. Ho wever, in 28 of 58 trials we observed non-linearity in this function. The r esults clearly show that even at very small amplitudes of sensory change, t he nervous system processes multisensory information in a non-linear fashio n. (C) 2001 Elsevier Science Ltd. All rights reserved.