Effects of Neck Muscle Vibration and Caloric Vestibular Stimulation on the Perception of Subjective ‘Straight Ahead’ in Man
For accurate motor behavior, like grasping or fixating a target, the correct perception of the target’s spatial location relative to the body is essential. That is, the spatial location of a target has to be transformed into an egocentric, body-centered coordinate system. In recent years, strong evidence has been found that for this purpose the brain uses abstract, neural representations of space interposed between sensory input and motor output (Andersen et al., 1993). These representations seem to be organized in non-retinal, egocentric coordinates. Several authors (Ventre et al., 1984; Biguer et al., 1988; Karnath et al., 1991; Karnath et al., 1993) have shown that the perception of ‘straight ahead’body orientation appears to be very closely connected with the neural generation of the reference frames that underlie the subject’s mental representation of space. The processes behind this generation of a neural representation of egocentric spatial information is relatively complex. Several coordinate transformations are necessary. For example, when reaching or grasping for a stationary object, the positions of the eyes and head may vary from moment to moment although the relevant spatial location of the target with respect to the body may not change. If the visual target location was coded in retinal coordinates only, then each time the eyes moved, the coded location would change as well. For a quick and accurate response, the retinotopic coordinates of the target must be transformed into a coordinate system based on a non-retinal, body-centered frame of reference. To locate the direction of gaze in space and to relate this information to the orientation of the body, the input from the retina has to be combined with eye-position signals as well as head-position information. Therefore, it can be expected that the perception of ‘straight ahead’ is influenced by different external information sources.
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