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W H Zaagman

Publications and source records attributed to W H Zaagman.

7 recordsLinked to original sources
PubMed1989

Microprocessor-controlled vector scan display system for generation of real-time visual stimuli.

The paper describes a low-cost, versatile, widely applicable digital display system in which a high-resolution, high-intensity display unit is driven by a real-time vector scan controller. The actual online display control of this device is preceded by an offline software stimulus-frame editor, designed to create and support a library of stimulus patterns (standard frames without any timing information and without any referencing to a special display co-ordinate system). The display times can be set individually for each single frame (containing at maximum 1023 pixels) with relative ease. Display frequency is set to 1 kHz. Writing time per pixel is less than 1 microsecond. A graphic display system like the one presented in this paper can be applied to generate a variety of real-time optical stimuli, such as intensity modulation of single spots or distinct display areas, abrupt displacements of a certain pattern, continuous movement or onset and offset of steady motion of a pattern.

Animals↗
PubMed1988

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.

Attention↗
PubMed1984

Saturation in a wide-field, directionally selective movement detection system in fly vision.

In the third optic lobe of the fly large-field spiking neurons are found which detect movement in a directionally selective way. For a wide variety of stimuli the responses of one of these, the H1 neuron, can very well be described and predicted by Reichardt's correlation model which is based on behavioural optomoter responses. However, when the spike rate is driven to large values with intense stimuli, the correlation model needs to be extended in order to account for the saturation phenomena that occur. Saturation can take place in all the elements of the large and extensive movement detecting system which contains interneurons in the optic lobes which process and guide the signals from the peripheral receptors to the central H1 neuron. To separate saturation at the peripheral site from that at the central level of the H1 neuron, a special stimulation technique was chosen. In measuring the saturation at the central level the stimulus parameter was the size of the stimulus field, while the modulation depth of the moving grating in this field stayed constant. Saturation at the peripheral site of the system was studied with a stimulus in which the modulation depth of the moving grating was the parameter and the size of the stimulus field was small and constant. When a simple feedback loop is incorporated in the final stage of the correlation model the saturation phenomena at the central level for steady-state stimulus conditions can very well be described. Saturation phenomena at the peripheral site of the system can also be explained by the same kind of feedback mechanisms in the input channels.

Action Potentials↗
PubMed1978

On the correlation model: performance of a movement detecting neural element in the fly visual system.

The applicability of the basic principles of the correlation model to the description of the activity of a movement detecting neuron in the third optic ganglion of the fly's visual system has been investigated. This wide field neuron is supposed to sum the outputs of a large number of correlators (i.e. multiplying units followed by time averagers) that are distributed over almost the entire eye. The model describes and predicts the experimental results in a satisfactory way if a uniformly distributed system or correlators is assumed. The sampling base of the correlators in this system equals the interommatidial angles deltaphi. The half width of the spatial sensitivity distribution of the visual inputs of the correlators, deltarho, is equal to the half width of the retinula cells of the 1--6 system.

Action Potentials↗
PubMed1976

Huntington's chorea. A random process.

The purpose of the present study was to investigate statistically the irregular nature of the choreatic jerks in Huntington's Chorea. EMG-bursts of certain muscles in a patient with Huntington's Chorea were taken as a measure of the jerks. Statistical analysis of the measurements, i.e. durations of bursts and intervals between bursts, revealed that the irregular nature of the choreatic jerks originate from a random process. Each choreatic jerk appears to be an entirely independent and unpredictable event.

Buttocks↗