Melbourne School of Psychological Sciences - Research Publications

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    How many motoric body representations can we grasp?
    Kammers, MPM ; Kootker, JA ; Hogendoorn, H ; Dijkerman, HC (SPRINGER, 2010-04)
    At present there is a debate on the number of body representations in the brain. The most commonly used dichotomy is based on the body image, thought to underlie perception and proven to be susceptible to bodily illusions, versus the body schema, hypothesized to guide actions and so far proven to be robust against bodily illusions. In this rubber hand illusion study we investigated the susceptibility of the body schema by manipulating the amount of stimulation on the rubber hand and the participant's hand, adjusting the postural configuration of the hand, and investigating a grasping rather than a pointing response. Observed results showed for the first time altered grasping responses as a consequence of the grip aperture of the rubber hand. This illusion-sensitive motor response challenges one of the foundations on which the dichotomy is based, and addresses the importance of illusion induction versus type of response when investigating body representations.
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    Spatially localized time shifts of the perceptual stream
    Hogendoorn, H ; Verstraten, FAJ ; Johnston, A (FRONTIERS MEDIA SA, 2010)
    Visual events trigger representations in different locations and times in the brain. In experience, however, these various neural responses refer to a single unified cause. To investigate how representations might be brought into temporal alignment, we attempted to locally manipulate neural processing in such a way that identical, simultaneous sequences would appear temporally misaligned. After adaptation to a 20 Hz sequentially expanding and contracting concentric grating, a running clock presented in the adapted region of the visual field appeared advanced relative to an identical clock presented simultaneously in an unadapted region. No such effect was observed following 5-Hz adaptation. Clock time reports following an exogenous cue showed the same effect of adaptation on perceived time, demonstrating that the apparent temporal misalignment was not mediated by differences in target selection or allocation of attention. This effect was not mediated by the apparent speed of the adapted clock: a clock in a 20-Hz-adapted spatial location appeared slower than a clock in a 5-Hz-adapted location, rather than faster. Furthermore, reaction times for a clock-hand orientation discrimination task were the same following 5- and 20-Hz adaptation, indicating that neural processing latencies were not differentially affected. Altogether, these findings suggest that the fragmented perceptual stream might be actively brought into temporal alignment through adaptive local mechanisms operating in spatially segregated regions of the visual field.