Spatial Transfer in Prism Adaptation
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1
Institute for Theoretical Physics, Germany
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2
Institute for Human Neurobiology, Germany
Humans can adapt very quickly to changes in visual stimuli such as the ones induced by horizontally shifting prisms. When fixating a target, the effect of wearing prisms is mainly to change the orientation of the eyes relative to the head. We systematically studied the role of eye position in pointing movements during and after prism adaptation. In our experiment, subjects first adapted to one target while wearing prisms (PRISM-phase). Subsequently, prisms were removed and the adaptation aftereffect was tested for a different target position (POST-phase). We found large inter-individual differences in the timescale of adaptation, in both PRISM- and POST-phases, as well as in the direct effect (i.e., the initial deviation of the pointing movement from the target when putting on prisms). Normalising the movements of different subjects allows to compare them properly and to fit the resulting data adequately with a single exponential decay plus an offset. A Leave-One-Out statistical test reveals an offset greater than zero, distributed around 19.15% +/- 0.37% of the direct effect. We also found that the mean direct effect only amounts to 40% of the optical shift induced by the prisms. This phenomenon is caused by integration of cues about the unshifted scene which were not removed by wearing the prisms (e.g. perspective). Most interestingly, we found comparable, linear patterns of spatial transfer for a change in eye position between the PRISM and POST-phases: a change in one direction led to an increase of the aftereffect, and a change in the other direction resulted in a decrease of the aftereffect (confirming the results from Wischhusen and Fahle (2008)). To understand the observed dynamics of the adaptation process, we developed a mathematical model of the adaptation process based on the literature on human muscle receptors. Our main assumption is that adaptation on eye position proprioceptors aims to minimise an error signal in the system. In the PRISM-phase, the model adapts to one spatial position. In the POST-phase, this adaptation leads to an underestimation of the change in eye position, which results in an increase or else decrease of the aftereffect as observed in the experiments. Thus our model provides a consistent explanation of the linear pattern seen in the spatial transfer of adaptation by assuming that the gain of only one muscle receptor is changed (or, for example, that the gain of one receptor is changed on a different time scale than the gain of the antagonist receptor, depending on the different activation levels of the receptors caused by the prisms). Noise during the adaptation process in the model leads to a statistical bias which also predicts a remaining offset in the PRISM-phase. However, it turns out that this bias is too small to explain the observed effect. We currently explore extensions of the model which will provide a better quantitative prediction of this feature.
Conference:
Bernstein Symposium 2008, Munich, Germany, 8 Oct - 10 Oct, 2008.
Presentation Type:
Poster Presentation
Topic:
All Abstracts
Citation:
Arevalo
O,
Hochstein
L,
Bornschlegl
M,
Ernst
U,
Pawelzik
KR and
Fahle
M
(2008). Spatial Transfer in Prism Adaptation.
Front. Comput. Neurosci.
Conference Abstract:
Bernstein Symposium 2008.
doi: 10.3389/conf.neuro.10.2008.01.026
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Received:
13 Nov 2008;
Published Online:
13 Nov 2008.
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Correspondence:
Orlando Jose Arevalo, Institute for Theoretical Physics, Bremen, Germany, orlando@neuro.uni-bremen.de