Electronmicroscopical image of the synaptic complex of cone photoreceptors in the zebrafish retina. The structures with the black labeling are dendrites of horizontal cells


to impaired vision. Working in conjunction with the foundation for the blind, Bartiméus, Kamermans’


contribution of blindness


team began to discern the these proteins


in humans. Patients


to night showing


these symptoms were studied, alongside mice and zebrafish specimens that shared similar mutations. This allowed for rigorous analysis – leading to some unexpected conclusions.


“According thinking, eye movement


to orthodox problems


are


actually caused by the brain rather than the eye,” says Kamermans. “But we believe that mutations in the retina actually lead to brain issues, which then cause eye problems. This


is a significant conventional theory.”


“We’ve begun to consider how information is selected and compressed before it is sent to the brain via the optic nerve” Another critical goal for Kamermans and


his body of ten researchers is to examine how the large amount of redundant data is filtered out of


the visual information


Schematic representation of the ephaptic feedback synapse between horizontal cells and cones. For details see: Kamermans et al. (2001) Science 292:1178-1180


Kamermans. The properties of the synapses (a structure which permits a neuron to pass an electrical signal to another cell) between those cells are critical to the function this network performs, he says. Building on earlier work, the researchers found that horizontal cells in fact feed back signals to cones using a unique, ultrafast and essentially noiseless inhibitory mechanism hitherto undiscovered. major


impact in neuroscience if


It would have a similar


synaptic mechanisms occur elsewhere in the central nervous system, making this one of the key questions they are addressing. To substantiate


their observations, the


team experimented on strains of mutated zebrafish. Using molecular biological and behavioural experiments, were able for


the first demonstrate that the feedback www.projectsmagazine.eu.com the scientists


time to directly from


horizontal contributes


cells to cones significantly to


contrast enhancement.


Having documented the basic mechanisms at work, the group will next examine how this particular synapse can be modulated. Since the eye has to perform optimally under vastly


different light conditions


(think star light versus a sunny day on the beach), it has to adjust the properties of its synapses when going from daylight vision to night vision. Another promising breakthrough,


potentially of interest to medicine, has been achieved through analysing the components of a further synapse, which links photoreceptors


and ON-bipolar cells,


neurons that form the pathway through the retina. During their investigations, the researchers uncovered new proteins, which when mutated yield to eye diseases leading


stream and how unnecessary unpredicted events are processed with high fidelity. “We’ve begun to consider how information is selected and compressed before it is sent to the brain via the optic nerve,” he says. “It’s critical that we understand what the system is doing when it’s healthy, before looking at illness and therapy. We have assessed the retinal information flow, and can now begin to look at what computations are performed – and attempt to obtain a quantitative description of how the eye codes its visual environment.” Analogue and digital coding schemes are


both used economically by the retina in order to use its neuronal capacity optimally. For instance, the retina mostly differential


sends images images to the brain. Rather


than operating like a camera – which captures entire still images – it senses and shares


in dimensions. “There several different are significant


differences between the way such devices and the human eye actually operate,” says Kamermans. “Non stabilised cameras create blurred imagery. But, strangely, in human


31 challenge to


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