97th DOG Annual Meeting 1999
K701
MOTION-ERG/VEP: A "TRUE" MOTION RESPONSE?
M. Bach, A. Unsöld, M. Hoffmann
Perception of visual motion has turned out to be an important channel of our visual system; ERGs and VEPs evoked by motion onset allow to objectively asses our neural motion mechanisms. But are retinal and cortical potentials elicited by motion onset really specific for motion?
Methods: Simultaneously, retinal and cortical (Oz/Ot vs. Ohr derivation) potentials were recorded in 10 subjects to a contracting/expanding dartboard stimulus (speed 22º/s). Specificity for motion was tested via a motion-adaptation paradigm. In a second experiment we studied in 8 subjects the direction specificity of the motion VEP over 8 directions, spaced in 45º-increments.
Results: Expansion and contraction evoked similar responses. The retinal response was dominated by a pattern-ERG-like positivity (P70), the cortical response was dominated by N2 (160 ms). Adaptation reduced the responses as follows: In the adapted direction the ERG was reduced by 36%, the VEP by 90%; in the opposite direction adaptation reduced the ERG by 28% and the VEP by 54%. In experiment 2 we found the same VEP results for the 0º and 180º direction relative to the adapting direction; a weak direction-specific adaptation effect was seen at ±45º whereas ±90º and ±135º did not differ from the 180º condition.
Conclusions: Motion stimuli imply local luminance modulation, which adapts retinal and cortical mechanisms. The low direction-specificity of retinal adaptation indicates that there are few (if any) specific motion detectors in the human retina. Cortical effects of adaptation are very strong, while only about one third of this effect is specific for direction. This suggests that motion stimuli also markedly activate phasic mechanisms that are not specific for motion.
Univ.-Augenklinik Freiburg, Killianstr. 5, D-79106 Freiburg
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