Kinematic studies of transport across an island wake, with application to the Canary islands

ArXiv pre-print: http://arxiv.org/abs/nlin/0605051.-- Final full-text version of the paper available at: http://dx.doi.org/10.1111/j.1600-0870.2006.00199.x.Transport from nutrient-rich coastal upwellings is a key factor influencing biological activity in surrounding waters and even in the open ocean. The rich upwelling in the North-Western African coast is known to interact strongly with the wake of the Canary islands, giving rise to filaments and other mesoscale structures of increased productivity. Motivated by this scenario, we introduce a simplified two-dimensional kinematic flow describing the wake of an island in a stream, and study the conditions under which there is a net transport of substances across the wake. For small vorticity values in the wake, it acts as a barrier, but there is a transition when increasing vorticity so that for values appropriate to the Canary area, it entrains fluid and enhances cross-wake transport.M.S. and U.F. would like to acknowledge the financial support by the DFG grant FE 359/7-1(2003). E.H-G. and C.L. acknowledge financial support from MEC (Spain) and FEDER through project CONOCE2 (FIS2004-00953). Both groups have benefited from a MEC-DAAD joint program. C.L. is a Ramón y Cajal fellow of the Spanish MEC

Deficits of smooth pursuit initiation in patients with degenerative cerebellar lesions.

It is well known that cerebellar dysfunction can lead to an impairment of eye velocity during sustained pursuit tracking of continuously moving visual target. We have now studied the initiation of smooth pursuit eye movements towards predictable and randomized visual step-ramp stimuli in six patients with degenerative cerebellar lesions and six age-matched healthy controls using the magnetic scleral search-coil technique. In comparison with the control subjects, the cerebellar patients showed a significant delay of pursuit onset, and their initial eye acceleration was significantly decreased. These cerebellar deficits of pursuit initiation were similarly found in response to both randomized and predictable step-ramps, suggesting that predictive input does not compensate for cerebellar deficits in the initiation period of smooth pursuit. When we compared initial saccades during smooth tracking of foveofugal and foveopetal step-ramps, the absolute position error of these saccades did not significantly differ between patients and controls. In fact, none of the patients showed any bias of the saccadic position error that was related to the direction or velocity of the ongoing target motion. This work presents further evidence that the effect of cerebellar degeneration is not limited to the impaired velocity gain of steady-state smooth pursuit. Instead, it prolongs the processing time required to initiate smooth pursuit and impairs the initial eye acceleration. These two deficits were not associated with an abnormal assessment of target velocity and they were not modified by predictive control mechanisms, suggesting that cerebellar deficits of smooth initiation are not primarily caused by abnormal information on target motion being relayed to the cerebellum

Lateralized EEG components with direction information for the preparation of saccades versus finger movements

During preparation of horizontal saccades in humans, several lateralized (relative to saccade direction), event-related EEG components occur that have been interpreted as reflecting activity of frontal and parietal eye fields. We investigated to what degree these components are specific to saccade preparation. EEG lateralization was examined within the interval (1 s) between a first (S1) and a second (S2) stimulus, after which a response had to be made (look left or right, or press a button with the left or right index finger). The visual S1 indicated either the direction (left vs right) and/or the effector (eye vs finger), and S2 (visual/auditory in different blocks) added the information not given by S1. An occipital component (220 ms after S1) was effector-independent, probably reflecting processing of the direction code. The following parietotemporal component (320 ms after S1) was specific for direction information. This component seems more relevant for finger movements than for saccades and may reflect a link between visual perception to action. A later frontal component (480 ms after S1) was specific for direction information and may be related to the planning of a lateral movement. One component was entirely specific for the preparation of a finger movement (the lateralized readiness potential before S2). Thus, several different lateralized processes in the S1-S2 interval could be delineated, reflecting hand-specific preparation, processing of the direction code, and the coordination of perception and action, but no components were observed as being specific for saccade preparation