Raumkodierung während glatter Augenfolgebewegungen

During interaction with our environment, the perception of moving objects is a challenging task for our senses. Due to the fact that we can perceive objects with the highest resolution only in a small area of the retina, i.e. the fovea, moving objects are especially demanding for visual perception. Using so called smooth pursuit eye movements the oculomotor system keeps moving objects in the fovea. The position of stationary objects is constant in world coordinates, while it changes in retinal coordinates due to such eye movements. It is well known from the literature that smooth pursuit eye movements induce systematic errors in the spatial perception of briefly flashed targets. Under laboratory conditions, briefly presented visual targets are misperceived in the direction of the eye movement. Furthermore, the spatial position of these targets also has a strong impact on the localization performance. Subjects show larger localization errors for targets that are presented in the hemifield in which the eyes are moving, compared to targets presented in the opposite hemifield. Earlier studies also showed that smooth pursuit eye movements have little effect on the localization of auditory targets. In this thesis I have performed three experiments on the perception and coding of space during smooth pursuit eye movements. In the first study I examined the localization and integration of auditory and visual targets during smooth pursuit eye movements. During periodic smooth pursuit eye movements I presented brief visual and auditory targets. These targets were presented as unimodal, visual or auditory, stimuli as well as congruent bimodal audiovisual stimuli. I could show in this experiment that the localization of bimodal audiovisual targets could very well be predicted by a maximum-likelihood model of the unimodal auditory and visual localization responses. This result confirms that there is no supramodal representation of space during smooth pursuit eye movements. Spatial information processed by different sensory modalities leads to distinct localization patterns. The information provided by different modalities is however integrated in an optimal manner by a maximum-likelihood-model. Every day we observe moving targets in our environment, some of them move at constant speed, others are accelerated or decelerated. It is well known that the visual system is far less able to discriminate between different accelerations than it is for different velocities. So far the question if and how acceleration affects the localization during smooth pursuit was unanswered. In the second study of this thesis I analyzed the impact of acceleration on the localization of briefly flashed visual targets. The absolute value of the localization error vectors were significantly smaller while the eyes were accelerating as compared to conditions in which the eyes were decelerating or in which the velocity of the eyes was constant. This effect could be due to the fact that the neuronal coding of acceleration is improved compared to the coding of deceleration. From the observers point of view, approaching visual targets seem to be accelerated. Targets that are moving towards an observer are more likely to provoke a behavioral response than targets that are moving away (fight or flight). Thus, an observer might benefit from a better judgment of an approaching visual target. Recent psychophysical studies showed that detection thresholds for isoluminant, chromatic stimuli were reduced and the discrimination rate was improved during smooth pursuit eye movements in comparison to a condition in which these stimuli were presented during fixation. In the third study of my thesis I analyzed spatial and color coding in area V4 of the rhesus macaque monkey brain during smooth pursuit eye movements. This experiment had two main goals. First, I intended to investigate if the neuronal responses in area V4 would provide a neuronal correlate of the psychophysical data of improved processing of chromatic stimuli during smooth pursuit eye movements. The second aim was to compare the position of the receptive fields during fixation and smooth pursuit eye movements. I presented isoluminant, chromatic, vertical bars to the monkey while the monkey was maintaining a fixation or performed smooth pursuit eye movements. I found that the neuronal responses during smooth pursuit eye movements to isoluminant, chromatic stimuli were clearly stronger in the population analysis as compared to neuronal responses during fixation. Accordingly, these data represent a potential neuronal correlate for the improved sensitivity for chromatic stimuli that has been described psychophysically. The position of the receptive fields didn’t change significantly during smooth pursuit eye movements as compared to fixation. It can be deduced from this result that the mislocalization effects observed in psychophysical experiments during smooth pursuit eye movements are not due to a change or shift in the position of the receptive fields in area V4. The neuronal origin of this mislocalization effects remains unclear. Based on previous studies on mislocalization during saccades as well as spatial coding relative to the focus of attention it seems to become increasingly probable that the localization results during smooth pursuit eye movements might be based on erroneous decoding of eye position signals combined with an inhomogeneous distribution of attention across the visual field. Taken together the results of my thesis show that many external factors like acceleration of a visual target or auditory inputs can change the way we perceive space. In addition, internal signals responsible for controlling and maintaining smooth pursuit eye movements can change the way we perceive color or the position of briefly shown objects during smooth pursuit eye movements.