Showing papers by "Hermann Wagner published in 1996"

A neuronal learning rule for sub-millisecond temporal coding

Abstract: A paradox that exists in auditory and electrosensory neural systems is that they encode behaviorally relevant signals in the range of a few microseconds with neurons that are at least one order of magnitude slower. The importance of temporal coding in neural information processing is not clear yet. A central question is whether neuronal firing can be more precise than the time constants of the neuronal processes involved. Here we address this problem using the auditory system of the barn owl as an example. We present a modelling study based on computer simulations of a neuron in the laminar nucleus. Three observations explain the paradox. First, spiking of an 'integrate-and-fire' neuron driven by excitatory postsynaptic potentials with a width at half-maximum height of 250 micros, has an accuracy of 25 micros if the presynaptic signals arrive coherently. Second, the necessary degree of coherence in the signal arrival times can be attained during ontogenetic development by virtue of an unsupervised hebbian learning rule. Learning selects connections with matching delays from a broad distribution of axons with random delays. Third, the learning rule also selects the correct delays from two independent groups of inputs, for example, from the left and right ear.

Emmetropization and optical development of the eye of the barn owl (Tyto alba).

Abstract: 1. We have studied the development of the refractive state in young barn owls (Tyto alba pratincola). Strikingly, the eyes had severe refractive errors shortly after lid opening (which occurred around day 14 after hatching; average from 6 owls: 13.83 ± 1.47 days). Refractive errors vanished in the subsequent one or two weeks (Fig. 1, Fig. 2). 2. Refractive errors did not differ by more than 1 diopter (D) in both eyes of an individual (Fig. 2). Thus, non-visual control of eye growth was sufficient to produce non-random refractions. However, visual input was finally required to adjust the optical system to emmetropia. 3. Using in-vivo A-scan ultrasonography of ocular dimensions (Fig. 4A), photokeratometric measurements of corneal radius of curvature (Fig. 4B), and frozen sections of excised eyes (Fig. 3), we developed paraxial schematic eye models which described age-dependent changes in ocular parameters and were applicable through the ages from lid opening to fledging (Table 1). A schematic eye for the adult barn owl (European subspecies: Tyto alba alba) is also provided. Eye sizes in an adult owl of the American (Tyto alba pratincola) and the European subspecies (T. alba alba) were similar despite of different body weights (500 g and 350 g, respectively). 4. The schematic eyes were used to test which ocular parameters might have caused the recovery from refractive errors. However, none of the ocular dimensions measured underwent obvious changes in their growth curves as visual input became available. Apparently, coordinated growth of several ocular components produced emmetropia. 5. From the schematic eye model, the developmental changes in image brightness and image magnification were calculated (Fig. 5). In barn owl eyes, image size was not quite as extreme as in the tawny owl or the great horned owl. However, the image was larger and the f/number was lower than in diurnal birds of comparable weight (pigeon, chicken). The observation supports a conclusion that image size is maximised in owls to permit a higher degree of photoreceptor convergence for higher light sensitivity at dusk while spatial acuity remains comparable to diurnal birds with smaller eyes.

Coordination in a six-legged walking system. Simple solutions to complex problems by exploitation of physical properties.

Abstract: A network for controlling a six-legged, insect-like walking system is proposed. The network contains internal recurrent connections, but important recurrent connections utilize the loop through the environment. This approach leads to a subnet for controlling the three joints of a leg during its swing which is arguably the simplest possible solution. The task for the stance subnet appears more difficult because the movements of a larger and varying number of joints (9-18: three for each leg in stance) have to be controlled such that each leg contributes efficiently to support and propulsion and legs do not work at cross purposes. Already inherently non-linear, four factors further complicate this task: 1) the combination of legs in stance varies continuously, 2) during curve walking, legs must move at different speeds, 3) on compliant substrates, the speed of the individual leg may vary unpredicatably, and 4) the geometry of the system may vary through growth and injury or due to non-rigid suspension of the joints. We show that an extremely decentralized, simple controller, based on a combination of negative and positive feedback at the joint level, copes with all these problems by exploiting the physical properties of the system.