Spatial processing of visual information in the movement-detecting pathway of the fly

1. Spatial processing of visual signals in the fly's movement-detecting pathway was studied by recording the responses of directionally-selective movement-detecting (DSMD) neurons in the lobula plate. The summarized results pertain to a type of neuron which preferentially responds to horizontal movement directed toward the animal's midline. Three kinds of visual stimuli were used: moving gratings, reversing-contrast gratings and reversing-contrast bars.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47087/1/359_2004_Article_BF00613743.pd

Apparent Movements Induced by Luminance Modulations:A Model Study

When the receptive-field profiles of the different units in the primary visual cortex are described by a series of different functions which are given by a Gaussian distribution and its first, second, and so on, spatial derivatives, a full analysis of the input-output processing of these units (under the assumption of linearity for small signals) can be achieved for a wide variety of optical stimuli consisting of closely adjacent fields modulated independently in intensity. Once the input-output relationship for one particular unit has been obtained, it is possible to calculate in a straightforward manner the spatial representation of the stimulus pattern in a two-dimensional distribution of such units. Investigations are reported into how a stimulus pattern (a dark or bright bar between two fields modulated in illuminance) is represented in a hierarchical structure of such layers of units, each layer containing just one type of receptive-field profile from the Gaussian family of derivatives. It is shown that if a visual percept is associated with the behaviour of the extrema or zero-crossings of the representations in the first few layers of such an architecture, a complete description can be given of the experimental results obtained by Gregory and Heard in their psychophysical experiments on illusory movement perception induced by luminance intensity modulations

Adaptation of transient responses of a movement-sensitive neuron in the visual system of the blowfly Calliphora erythrocephala

We studied temporal response properties of the H1 neuron by extracellular recording. This neuron is a wide-field movement-sensitive element in the visual system of the blowfly (Calliphora erythrocephala). If the neuron is stimulated with a stepwise pattern displacement in its preferred direction, it responds with a burst of action potentials. By repeating the stimulus step one obtains the average of the step response: a 20ms latency time followed by a sharp increase in average firing rate and a slower decay to the resting activity. We report that the characteristic decay time of the step response depends on the stimulus history. If the stimulus moved prior to the step, the higher the pattern velocity, the faster was the decay of the step response to the resting level. In quantitative terms, for velocities in the range 0.4–100°/s, the decay time-constant varies from 300–10ms and is smaller for higher velocities. The time-constant is only weakly affected by other stimulus parameters such as modulation depth or spatial wavelength, and is set independently in different areas of the visual field where it is tuned to the local velocity. We discuss a possible advantage of this form of adaptation for the processing of visual signals: The performance of the nolinear operations that extract information from the visual input can be optimized by prefiltering signals in the individual visual columns with a time-constant that decreases with stimulus velocity. It will be shown that both the test step response and the response to continuous movement can be described reasonably well by a correlation model with input filters that adapt their time-constant

'Exit' in deeply divided societies: regimes of discrimination in Estonia and Latvia and the potential for Russophone migration

Hirschman’s model of ‘exit’ envisages that mass migration can communicate feedback to a state and elicit modifying policy behaviour. The regimes of discrimination against Russophones in Estonia and Latvia are examined to demonstrate that in certain conditions of inter-ethnic conflict the model does not work as predicted. In deeply divided societies the mass migration of a minority can be intentionally promoted by a majority regime of discrimination and thus does not perform a feedback function. Equally, in such conditions migration may operate as a safety valve to release the build up of minority antagonisms against the discriminatory regimes. The cases of Estonia and Latvia also illustrate the limitations of international conditionality from the EU and OSCE to prevent anti-minority policies, when there is a lack of international commitment and when governing elites resist. Given the lack of political will in Estonia and Latvia to modify the regimes of discrimination, and the poor prospects for integration or assimilation, the article predicts a significant out-migration by Russophones to other EU member states when restrictions on freedom of movement are eased after the transitional period