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K H Ruddock

Publications and source records attributed to K H Ruddock.

At least 55 records · Page 3Linked to original sources
PubMed1982

Spectral sensitivity functions of post-receptoral responses in human vision.

1. Increment threshold illumination levels, It, for visual detection of a one-dimensional, spatially periodic test pattern (i.e. a grating) presented to the right eye have been measured following a period of adaptation to high contrast patterns. The right (test) eye was adapted to a grating, matched in bar-width and orientation to the test grating, and the left eye was adapted to a two-dimensional matrix array of circular spots. It is shown that the value of It is dependent on the spatial and spectral parameters of the spot matrix. 2. The variation of It with change in matrix wavelength, lambda, is greatest for matrix spots of some 10-15 min of arc diameter. 3. Values of It were determined as a function of the matrix wavelength, lambda. Three kinds of spectral response have been observed, each associated with a different stimulus geometry, and each characterized by the wavelength, lambda, for which It is minimum. With an adaptation matrix consisting of coloured spots on a dark surround, It is minimum for values of lambda around 600 nm, whereas with a matrix of dark spots on a coloured surround, the minimum occurs for lambda around 520 nm. The third spectral response is found when a blue test grating is superimposed on a yellow background, conditions under which detection is mediated by the blue-sensitive, pi 1-increment threshold mechanism. In this case, It is minimum for lambda equal to about 450 nm. 4. Three subjects with normal colour vision each give similar spectral responses, but the data for two deuteranopic red-green dichromats deviate significantly from the normal response pattern. 5. The binocular interaction mechanism through which the spot matrix influences threshold detection of the test grating is discussed in relation to electrophysiological and other psychophysical data.

Adaptation, Ocular↗
PubMed1981

The spatial and temporal organisation of motion perception units in human vision.

Measurements of threshold illumination levels for detection of retinally non-localised moving targets show that detection of a moving target is influenced by both the spatial and the temporal modulation of the background field. The temporal response characteristics obtained from these measurements are similar to those obtained from experiments on detection of temporal flicker. Experiments with spatially modulated background fields reveal visual mechanisms with spatial properties which are essentially independent of many stimulus parameters. The response amplitude of the spatial filter which characterises these mechanisms increases linearly as a function of background contrast and is independent of the relative orientation between the background structure and the direction of target movement. These properties are used to compute the two-dimensional spatial characteristics of mechanisms involved in the detection of moving targets.

Adult↗
PubMed1981

A new class of visual defect. Spreading inhibition elicited by chromatic light stimuli.

This paper deals with a single male subject, M., who has a well-defined visual defect of a kind not previously described. The defect is associated with a potent inhibitory response to long-wavelength stimuli, which spreads for up to 12 deg from the area of stimulation, suppressing M.'s detection of other high contrast stimuli. The effect can also be observed, weakly, with certain other coloured stimuli, but not with yellows or whites. Detailed psychophysical data are given for threshold sensitivity, colour matching, two-colour increment thresholds and visual acuity, all of which are more or less abnormal, depending on the stimulus colour. No abnormal effects were, however, observed with white light stimuli. On the basis of M.'s ability to fuse red and green random dot stereogram pairs, we argue that the inhibition arises centrally in the visual pathways. and we examine the implications of M.'s response characteristics for the analysis of central visual processing.

Adult↗
PubMed1980

Spatial characteristics of movement detection mechanisms in human vision. I. Achromatic vision.

Threshold illumination levels, It, for visual detection of white light targets, moving across spatially structured background fields, have been measured and it is shown that, for a given background illumination level, It depends upon the spatial characteristics of the background field structure. Thus, with the background composed of a square-waveform grating of fundamental spatial frequency f cycles/deg, It is maximum for an intermediate value of f, and falls as f increases or decreases from this value. The relationship between It and f characterizes the interaction between movement detection and the background grating, and is designated the IMG function. The parametric properties of the IMG functions are described and it is established that the mechanisms which give rise to these functions are sensitive to the movement, but not the spatial structure of the target. They correspond, therefore, to the movement-sensitive Y-type mechanisms, observed in electrophysiological studies of cat and primate visual pathways. The spatial distribution of sensitivity associated with the IMG functions has been computed by 2-D transform methods, the computation yielding circularly symmetric, centre-surround antagonistic "receptive field" distributions.

Adult↗
PubMed1980

Spatial characteristics of movement detection mechanisms in human vision. II. Chromatic stimuli.

Visual detection of targets moving against structured background fields has been studied with near-monochromatic stimuli, selected so as to isolate the different increment threshold spectral response mechanisms. It is shown that for foveal vision, the red- and green-sensitive mechanisms (pi 5 and pi 4 respectively) yield IMG functions (Barbur and Ruddock, 1980), similar to those found with white light. In contrast, the blue-sensitive (pi 1) mechanism yields a low-frequency IMG response quite unlike that found for the other mechanisms. There is also considerable variation between subjects in this case. Measurements taken 30 degrees off-axis with low (1.4 log trolands) background illumination level, yield a low frequency response IMG function for both rod and cone spectral mechanisms, similar to those found with white light stimuli. At high illumination levels (> 2.2 log trolands) the IMG function for the pi 5-mechanism is shifted to higher spatial frequencies, as is also observed with white light stimuli. A wavelength-selective binocular interaction effect, manifested in the detection of moving targets, is also described, and it is suggested that this may be of value in the study of defective colour vision.

Adult↗
PubMed1980

Human visual responses in the absence of the geniculo-calcarine projection.

This paper describes an investigation of responses elicited by flashed and moving light stimuli presented to the 'blind'half-field of a hemianopic subject. The detection mechanism is significantly less sensitive than that for vision in the normal half-field, but the increment threshold varies with the background illumination level in a manner similar to that for normal vision. The responses of the 'blind' hemifield are characterized by low critical fusion frequency (less than or equal to 8 Hz), large spatial summation and high sensitivity to fast moving targets. Velocity discrimination for targets located in the 'blind' hemifield is normal, but neither the shape nor the size of the target is discriminated. It is shown that the experimental data cannot be attributed to light scattered from the 'blind' into the normal hemifield, and other mechanisms which may be responsible for the observed responses are examined.

Adult↗
PubMed1979

Effects of picrotoxin and strychnine on fish retinal S-potentials: evidence for inhibitory control of depolarizing responses.

Simultaneous recording of the light evoked electrical signals from different classes of slow(S)-potential unit in the isolated fish retina established that picrotoxin, an antagonist of GABA, selectively suppresses the depolarizing component of C-type S-potentials. In contrast, equivalent concentrations of strychnine have no effect on S-potentials, but suppress a component of transient amacrine cell responses. On the basis of these and other experimental data [3,4] we propose a network diagram for the C-type S-potential units in the cyprinid fish retina. This network includes a GABA-ergic feed-back loop and is essentially similar to that proposed by Fuortes and Simon [5].

Animals↗
PubMed1979

A colour-dependent abnormality in human visual detection of stimulus motion and spatial structure.

We have studied the wavelength dependence of visual responses in a single human subject, M.W., who has an unusual visual defect. It is shown that for red stimuli, both spatial resolution for grating patterns and detection of target movement are grossly abnormal. On the basis of these and other [8] experimental data, we argue that M.W.'s response pattern implies interdependent central visual processing of colour, movement and spatial structure. We examine this conclusion in relation to recent anatomical and electrophysiological findings.

Color Perception↗
PubMed1979

The response characteristics of an inhibitory binocular interaction in human vision.

1. The contrast threshold level for visual detection of a linear grating consisting of parallel light and dark bars is increased by adaptation to a high contrast grating of spatial characteristics similar to those of the test grating (e.g. Blakemore & Campbell, 1969). This so-called contrast threshold elevation effect is significantly reduced if two adaptation gratings, with appropriate different spatial characteristics, are presented one to either eye (Ruddock & Wigley, 1976). We have studied the contrast threshold elevation effect obtained with a test and an adaptation grating presented to one eye and a second adaptation grating, referred to as the conditioning grating, presented to the other. 2. Preliminary data are presented for three subjects with normal stereoscopic vision. In each case, the contrast threshold elevation effect for a pair of spatially identical test and adaptation gratings is significantly reduced by a conditioning grating of spatial frequency in the range 1-5 c/deg. 3. Reduction in the contrast elevation effect is observed whether the conditioning and adaptation gratings are of the same or of different wave-lengths and the effect of the conditioning grating increases to a steady-state level over a period of some 30 sec following onset of its presentation. 4. Measurements were made with a 5 deg diameter circular test grating presented at retinal locations up to 8 deg in the horizontal and vertical meridians from a foveal fixation point. It was found that in the horizontal meridian, the amplitude of the suppression effect associated with the conditioning grating falls as the displacement angle of the test field from the fixation point increases. For displacements in the vertical meridian, however, the amplitude of the suppression effect is virtually independent of the retinal location of the test field. 5. Under experimental conditions which yield suppression of the contrast threshold elevation effect for subjects with normal stereoscopic vision, no suppression was found for three subjects who possessed neither global nor fine stereoscopic vision. Results for a subject who possessed fine, but not global stereoscopy, did, however, show the suppression effect.

Adaptation, Ocular↗
PubMed1978

Properties of length-selective and non-length-selective adaptation mechanisms in human vision.

The contrast threshold elevation effect has been measured for one dimensional (grating) and for two dimensional (spot) stimulus patterns. It has been shown previously (Burton and Ruddock, 1978) that such stimuli elicit, respectively, non-length-selective and length-selective adaptation effects. It is established that, unlike the frequency shift effect, the contrast threshold elevation effect is sensitive to the width of the light but not to that of the dark elements of the stimulus patterns. Adaptation to spot patterns elicits a significant threshold elevation for detection of both spot and grating test stimuli, but only under monoptic viewing conditions. The experimental findings are summarized in a block-diagram and it is shown that adaptation to grating patterns is successfully described by the spatial frequency response data given by Maudarbocus and Ruddock (1973).

Adaptation, Physiological↗