Dokument

Summary:
This thesis comprises fife studies. In the first study we investigated the localization of brief visual targets during reflexive eye movements (optokinetic nystagmus). Localization during OKN, corrected for the bias observed during fixation, was shifted in the direction of the slow eye movement. This bias decreased shortly before a fast-phase and temporarily increased afterwards. Control experiments showed that localization errors were mainly due to the performance of eye movements rather than due to the background motion. Hence, our data show for the first time an influence of reflexive eye movements on the localization of briefly presented visual targets. In the second study, we determined mislocalization of flashed visual targets during optokinetic afternystagmus (OKAN). These eye movements are quite unique in that they occur in complete darkness, and are generated by subcortical control mechanisms. We found that during OKAN slow-phases subjects mislocalize targets away from the fovea. This corresponds to a perceived expansion of visual. Around the OKAN fast-phases, we found a bias in the direction of the fast-phase prior to its onset and opposite to the fast-phase direction thereafter. Such a biphasic modulation has also been reported in the temporal vicinity of saccades, and during optokinetic nystagmus (OKN). A direct comparison, however, showed that the modulation during OKAN was much larger and occurred earlier relative to fast-phase onset than during OKN. In the third study we analyzed the fast-phases of OKAN and OKN as well as visually guided and spontaneous saccades under identical background conditions. Our data clearly show that fast-phases of OKAN and OKN differ with respect to their main sequence. OKAN fast-phases were characterized by their lower peak-velocities and longer durations as compared to OKN fast-phases. Furthermore we found that the main sequence of spontaneous saccades depends heavily on background characteristics, with saccades in darkness being slower and lasting longer. On the contrary, the main sequence of visually guided saccades depended on background characteristics only very slightly. This implies that the existence of a visual saccade target largely cancels out the effect of background luminance. Our data underline the critical role of environmental conditions (light vs. darkness), behavioral tasks (e.g. spontaneous vs. visually guided) and the underlying neural networks for the exact spatio-temporal characteristics of fast eye movements. It is widely debated whether fast-phases of the reflexive optokinetic nystagmus (OKN) share properties with another class of fast eye movements, visually guided saccades. Conclusions drawn from previous studies were complicated by the fact that a subject’s task influences the exact type of OKN: stare- vs. look nystagmus. Therefore in the fourth study we set out to determine in the same subjects the exact dynamic properties (main sequence) of fast-phases of look- and stare-nystagmus as well as visually guided saccades. Our data clearly show that fast-phases of look- and stare-nystagmus differ with respect to their main sequence. Fast-phases of stare-nystagmus were characterized by their lower peak-velocities and longer durations as compared to fast-phases of look-nystagmus. Visually guided saccades were on the same main-sequence as fast-phases of look-nystagmus, while they had higher peak-velocities and shorter durations than fast-phases of stare-nystagmus. Our data underline the critical role of behavioral tasks (e.g. reflexive vs. intentional) for the exact spatio-temporal characteristics of fast eye movements. Primates perform saccades to stationary and moving targets. Yet, saccades towards moving targets are computationally more demanding since the oculomotor system must use speed and direction information to program an adequate saccade. In non-human primates different brain regions have been implicated in the control of voluntary saccades. One of these regions is the lateral intraparietal area (LIP). It is not known whether LIP neurons show differential activation related to the control of saccades towards stationary as compared to moving targets. To this end in the fifth study we recorded single unit activity in area LIP of two monkeys (Macaca mulatta). Monkeys performed visually guided saccades to either stationary targets or moving targets in pseudo-randomized order. Stationary saccade targets were located such that the amplitudes of saccades towards moving and stationary targets were identical. Confirming previous results, many LIP neurons showed saccade related discharges that were tuned for amplitude and direction. Yet, given identical saccade metrics, discharge varied depending on whether saccades were followed by stable fixation or smooth pursuit in about one third of these neurons. We conclude that area LIP is involved in the control of saccades towards stationary and moving targets.

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