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K H Ruddock

Publications and source records attributed to K H Ruddock.

At least 37 records · Page 2Linked to original sources
PubMed1990

Effects of stimulus complexity on simple spatial discriminations.

We have extended previous measurements on the detection of a target, differentiated in orientation or magnification from a number of identical reference elements. In these new experiments, we have used more than one class of reference element from which the target must be discriminated. For example, in the case of orientation discrimination, the reference elements are of two classes, which differ from each other in orientation, but are otherwise identical. Representatives of the two classes are present in equal numbers, and the target to be detected is presented at an orientation angle different from that of either class of reference element. Data for discrimination of target magnification or orientation are given for simple geometric elements, namely lines, squares and triangles. In nearly all cases, the characteristic time required for 50% probability of target detection, T1/2, is greater with two classes of reference elements than with a single class. In nearly all cases, T1/2 values for the mixed reference elements increase with the number of reference elements, N, corresponding to discrimination by serial processing. This remains the case even when detection is parallel (T1/2 independent of N) for either class of reference element used separately. We discuss the properties of the spatial discrimination mechanisms which give rise to these responses.

Depth Perception↗
PubMed1990

Spatial, colour and contrast response characteristics of mechanisms which mediate discrimination of pattern orientation and magnification.

We have measured response times for the detection of a single target presented against a set of reference elements which are characterised by combinations of four different stimulus parameters; colour, contrast polarity, magnification and orientation. The aim of the experiments was to determine the response characteristics of visual mechanisms which mediate target detection through the discrimination of orientation and magnification. In the first experiments, we determined sensitivity to differences in colour and contrast polarity, and show that the mechanisms responsible for the discrimination of orientation and of magnification are both selective in their responses to colour and to contrast polarity. There are, nonetheless, residual interactions between patterns of different contrast polarities and between those of different colour, and in the latter case, weak interactions persist under equiluminance conditions. In a second set of experiments, we examined the interactions between orientation and magnification. We conclude that the responses of visual mechanisms which mediate target detection through discrimination of orientation are markedly dependent on stimulus magnification whereas those which mediate detection through discrimination of magnification are, in contrast, relatively insensitive to stimulus orientation.

Adult↗
PubMed1988

The limits of parallel processing in the visual discrimination of orientation and magnification.

We present the results of an experimental study on visual pattern discrimination by human subjects. The stimuli consisted of a single target element embedded in a multi-element reference field and response times, T1/2, for 50% probability of detection of the target element were measured as a function of the number of reference elements, N. Two response patterns were observed, one in which T1/2 is effectively independent of N, and the other in which it is approximately proportional to N. We argue that these two response classes reflect two different modes of discrimination, the former mediated by parallel processing, which does not require scanning of the elements in order to discriminate the target, whereas the other involves sequential scanning of the elements, in order that the target can be discriminated under foveal examination. We show that a target element differentiated from the reference elements by virtue of orientation or magnification can be discriminated under parallel processing, except when the value of the distinguishing parameter is too small, or when the individual elements are insufficiently well defined. We also establish that targets differentiated from the reference elements by (partial) charges in symmetry can also be discriminated by parallel processing.

Humans↗
PubMed1988

Simultaneous processing of spatial and chromatic components of patterned stimuli by the human visual system.

We report measurements on discrimination of orientation and magnification made for elements differentiated in colour and/or luminance from their background. By performing measurements at a series of background luminances and for fixed luminance of the elements, we show that with colour contrast, discrimination for both spatial parameters is unimpaired when the background is at isoluminance with the elements. Under simple luminance contrast, however, these discriminations become poorer when the background luminance is within some +/- 5% of that of the elements, and are completely absent when the two values are the same. A deuteranomalous subject is unable to make the spatial discrimination around the isoluminance point for colour contrasts which are too small for him to distinguish, but for which subjects with normal colour vision maintain spatial discriminations at isoluminance. This observation establishes that the physiological mechanisms of normal colour vision, rather than stimulus artefacts, mediate the observed spatial discriminations. We conclude that the visual processing of colour and spatial parameters such as orientation and magnification are intrinsically related to each other.

Adult↗
PubMed1987

Residual vision in patients with retrogeniculate lesions of the visual pathways.

Twenty-five subjects have been studied who, as a result of damage to the striate cortex, were 'blind' in extensive areas (scotomata) of the visual fields. Of these 25 subjects, 5 exhibited residual vision in response to transient lights presented within the scotoma, which enabled them to locate the stimuli by hand-reaching or by eye movements; the latter have been measured by electro-oculography. The residual vision underlying their responses was characterized by low flicker-fusion and by sensitivity in detection of movement which increased as target speed was raised. Discrimination for the direction of target movement was poor, but spatial resolution in the discrimination of target displacement was essentially normal. The subjects were unable to recognize or discriminate the spatial structure of targets located within the 'blind' field, and the observed dissociation between spatial discrimination of displacement and pattern is examined in relation to the 'two systems' hypothesis of visual function. There is no obvious correlation between the extent of neuronal damage as revealed by CT scans and the existence of residual vision.

Adult↗
PubMed1987

Visual discrimination of simple geometrical patterns: I. Measurements for multiple element stimuli.

We describe a method for the measurement of visual discrimination between simple patterns. The target to be discriminated is embedded in a background consisting of multiple, randomly positioned but identical elements, and is distinguished by a single parameter such as magnification or relative rotation. The positions of the target and background elements are varied randomly between presentations and discrimination for different values of the target parameter is measured in terms of the time taken for detection of the target. Using this method, we have studied discrimination of rotation and of magnification for simple pattern elements such as lines, triangles and squares. The results for rotation discrimination are interpreted as evidence for the activity of two discrimination mechanisms, one sensitive to the orientation of the lines from which the pattern elements are constructed and the other to the orientation of the pattern element relative to the visual field.

Adolescent↗
PubMed1987

Visual discrimination of simple geometrical patterns: II. Atypical responses in a subject suffering difficulties with pattern recognition.

We have applied the experimental methods described by Ike et al. (Spatial Vision, 1987, 2, 13-29) in the study of pattern discrimination by subjects who, during childhood, experienced severe difficulties with reading and spelling. Of the six subjects studied, one still suffers from such difficulties and we show that she performs atypically in certain of the discrimination tasks. In particular, her discrimination responses for elements such as delta are markedly different from those of all other subjects. We interpret these abnormal responses on the basis of the discrimination mechanisms for line orientation and rotation of the elements which were discussed by Ike et al. (1987).

Adult↗
PubMed1986

Neuropharmacological actions of kynurenic and quinolinic acids on horizontal cells of the isolated fish retina.

We have studied the effects of naturally occurring metabolites of tryptophan, kynurenic and quinolinic acids, on the electrophysiological responses of retinal horizontal cells in the fish (Rutilus rutilus, the roach). Quinolinic acid usually hyperpolarizes the cells and reduces their light evoked responses (S-potentials) but on occasion, it causes a slight depolarization of the membrane potential. These actions are similar to those found with N-methyl-D-aspartate (NMDA) and our results are consistent with the proposal that it acts at NMDA binding sites. Kynurenic acid (greater than or equal to mM) invariably hyperpolarizes horizontal cells to their potassium Nernst equilibrium potential and, more significantly, blocks the depolarizing actions exerted on them by excitatory amino acids, such as kainic and quisqualic acids. We show that this action persists in the presence of the synaptic blocker, cobalt chloride, and thus is not mediated by chemical synaptic activity. Kynurenic acid does not reverse depolarization of horizontal cells by dopamine or gamma-aminobutyric acid, thus its inhibitory effects are selective to the actions of excitatory amino acids. Neither xanthurenic acid, a close structural analogue of kynurenic acid, nor quinolinic acid are effective in blocking depolarizations by excitatory amino acids.

Animals↗
PubMed1986

A study of systematic visual perseveration involving central mechanisms.

The subject of this investigation experiences vivid after-images which persist for tens of seconds following stimulation by light insufficiently intense to produce after-images in normal subjects. The after-images occur at all points in the visual field, and have spatial structure closely similar to that of the stimuli which elicit them, the only distortion being the absence of high (greater than or equal to 10 cycles X deg-1) spatial frequencies. The duration of the after image is dependent on the spatial structure and spectral content of the eliciting stimulus, being maximum for low, nonzero spatial frequencies (1-2 cycles X deg-1) and yellow or green wavelengths. The after-images appear 'khaki' or 'mustard-green' in colour, regardless of the spectral composition of the light stimulus, and those produced by flickering or moving targets do not themselves exhibit temporal fluctuations, although movement produces a streak along the path described by the moving object. After six weeks of treatment with the anticonvulsant drug, carbamazepine, the duration of the after-images fell by about 30%, but no further reduction occurred over the following three months. In spite of the prolonged after-images, the subject has normal sensitivity for detection of flicker and movement, but that for detection of a single flash is raised abnormally at low background illumination. Neither CT nor MRI brain scans revealed any abnormality. These responses are discussed in relation to those found in other subjects suffering visual perseveration or palinopsia, and we examine their possible causes.

Adult↗
PubMed1986

The contribution of blue-sensitive cones to spatial responses of post-receptoral visual channels in man.

Psychophysical methods developed for the investigation of spatial and temporal pathways in human vision have been applied in combination with the two-colour increment threshold technique of W. S. Stiles to study the way in which signals from blue-sensitive cones are transmitted along the visual pathways. A flicker sensitive spatio-temporal filter, designated 'ST2', has been examined by background modulation methods, and spatial filters sensitive to bars of a specific width by grating adaptation methods employing dichoptic presentation of stimuli. It is shown that the blue-sensitive (pi 3) spectral mechanism contributes to both classes of filter response, in a manner similar to that observed for the red-sensitive spectral mechanism. The binocularly driven, bar-sensitive filters have broad-band spectral response characteristics, thus the data demonstrate that signals arising in blue-sensitive cones converge onto a luminance channel. The results of this investigation, together with those previously published for a second (ST1) spatio-temporal filter, describe a variety of post-receptoral responses involving the pi 3 spectral mechanism.

Color Perception↗
PubMed1985

Rapid fading of visual sensations in a subject with a parietal-occipital tumour.

We present psychophysical data measured for a single subject, who had a tumour in the left parieto-occipital region, located by CT scan and post-mortem study. Her vision was characterised by rapid fading during voluntary fixation of images located in the right hemifield. We describe a number of simple tests which we developed in order to study this phenomenon. The results of these tests established that significant loss of function occurs within 1 s of fixation and that the effects associated with fixation are spatially localised. We compare our subject's response characteristics with those observed in other cases of visual disturbance and we examine the possible underlying mechanisms which give rise to the rapid fading of her visual images.

Adult↗
PubMed1984

Electrophysiological effects of GABA on fish retinal horizontal cells are blocked by bicuculline but not by picrotoxin.

We have studied electrophysiologically the actions of gamma-aminobutyric acid (GABA) and related pharmacological agents on fish retinal horizontal cells by recording intracellularly from isolated retinae perfused with Ringer containing the various drugs. We show that although GABA usually hyperpolarizes the membrane potential relative to its dark level, it sometimes and particularly at higher (greater than or equal to 5 mM) concentration produces membrane depolarization, with reduction in the light evoked responses (S-potentials) in both cases. These effects are reversed by bicuculline but not by picrotoxin, although both agents antagonize GABA in many other preparations [5, 25]. The GABA uptake blocker nipecotic acid [15] hyperpolarizes horizontal cells and reduces their light evoked responses, and again these effects are reversed by bicuculline but not by picrotoxin. beta-Alanine, which blocks glial GABA transport [29], and diaminobutyric acid (DABA), which blocks neuronal GABA transport [14, 29, 31], have effects similar to those of nipecotic acid. We discuss these actions of GABA and of the other related drugs and their differential sensitivity to bicuculline and picrotoxin.

Aminobutyrates↗
PubMed1983

Two spatio-temporal filters in human vision. 1. Temporal and spatial frequency response characteristics.

We have studied visual detection of a circular target moving across a spatially and/or temporally modulated background. Illumination, It, for threshold detection of the target has been measured as a function of background modulation frequency and changes in It associated with background modulation provide a means of determining the frequency response characteristics of visual channels. Temporal frequency responses obtained with temporally modulated, spatially uniform backgrounds have pass-band characteristics and the temporal frequency for peak response increases with increase in mean background illumination. These temporal frequency responses resemble those of the de Lange (1954) filter, but the latter incorporates the incremental thresholds for steady backgrounds. The amplitude of this temporal response saturates at low (approximately 40%) background modulation, decreases to zero as the target velocity falls to zero, and is maximum for a circular target of diameter 2 degrees. The spatial characteristics of this temporal filter were measured with a background field consisting of alternate steady and flickering bars. The resulting spatial frequency curve peaks at 1 cycle deg-1 for all background illuminations and is independent of the background grating orientation. This spatial response differs significantly from the IMG spatial functions observed with a background grating (Barbur and Ruddock, 1980). The spatial and temporal responses reviewed above exhibit similar parametric variations and we therefore associate them with a single spatio-temporal filter, ST2. A second temporal response, with low-pass frequency characteristics, was observed with a background field consisting of two matched gratings, presented in spatial and temporal antiphase. This response has parametric properties similar to those of the IMG spatial response described previously by Barbur and Ruddock (1980), thus we associated the two sets of data with a single spatio-temporal filter, ST1. We show that the ST2 responses can be obtained by combining ST1 responses, and we present a network incorporating the two filters. We review other psychophysical studies which imply the activity of two spatio-temporal filters with properties of the kind revealed in our studies. We argue that filter ST1 has properties equivalent to those of X-type and filter ST2 has properties equivalent to those of Y-type electrophysiological mechanisms.

Adult↗
PubMed1983

Two spatio-temporal filters in human vision. 2. Selective modification in amblyopia, albinism, and hemianopia.

We have used the psychophysical methods described in the first paper of this series (Holliday and Ruddock, 1983) to determine selected spatial and temporal response characteristics of the ST1 and ST2 filters for subjects suffering visual defects. Data are given for 19 amblyopes, an albino and a hemianope, and comparison data are also given for a number of subjects with normal vision. The ST1 spatial responses for both the "normal" and "amblyopic" eyes of 12 convergent strabismic amblyopes are displaced to low spatial frequencies compared to the normal curve, which implies that there is a loss of fine spatial tuning. In all but one subject, the curve for the "amblyopic" eye peaks at a spatial frequency lower than that for the "normal" eye, thus the former deviates further from the normal pattern than the latter. The ST1 spatial responses of 6 refractive amblyopes are also displaced to the low frequency side of the normal curve, although on average the shift is smaller than in the case of the strabismic amblyopes. For each subject, the response curve of the "amblyopic" eye peaks at a lower spatial frequency than does that for the "normal" eye. ST1 spatial responses were measured for targets located up to 30 degrees off-axis along the horizontal meridian and sample data are given for one strabismic and one refractive amblyope and for two normal subjects. It is concluded from these data that the changes in the spatial responses associated with amblyopia do not simply reflect eccentric fixation of the target. The ST2 spatial response was measured for the "normal" and "amblyopic" eyes of 9 amblyopes (7 strabismic and 2 refractive). There is no significant difference between the average amblyopic response and that of normal subjects, and only in one case does the response for an "amblyopic" eye peak at a frequency lower than the peak frequency for normal vision. The ST2 temporal response for 9 amblyopes shows no systematic deviations from the normal response. For the albino, both the ST1 and ST2 spatial responses peak at around 0.3 cycles deg-1, and both curves are displaced considerably to the low spatial frequency side of the normal ST2 spatial response. The albino's ST2 temporal response is essentially normal. Measurements for the hemianope's "blind" hemifield under conditions appropriate to the isolation of the ST1 and ST2 spatial responses reveal no tuning curves. The ST2 temporal response for the "blind" hemifield, however, is of large amplitude, with a peak at 2 Hz, well below the normal frequency response peak. It is argued that the loss of fine spatial tuning which occurs in the ST1, but not the ST2, spatial responses of the amblyopes is consistent with the sequential organisation of these two filter classes proposed by Holliday and Ruddock (1983). Further, for the only two subjects whose ST2 spatial response curves are displaced to abnormally low frequencies (the albino and a strabismic amblyope) the ST1 spatial response is shifted to low spatial frequencies compared to the normal ST2 curve...

Albinism↗
PubMed1983

Visual mechanisms for the analysis of spatial pattern.

After a brief outline of the structure and electrophysiology of the normal visual pathways, the responses, as revealed by psychophysical studies, of the visual system to spatially and temporally varying stimuli are reviewed. An appropriate network model, involving two sequentially organized classes of visual channel, is presented. Examples are given in which the psychophysical methods developed to analyse the normal visual pathways are applied to cases in which these pathways are defective (amblyopia, albinism, hemianopia, parietal cortical lesion, inhibitory central colour vision defect). These cases not only illustrate the value of psychophysical techniques in analysing disturbances of visual function but are also suggestive of the mechanisms involved in higher processing of the retinal image.

Adolescent↗
PubMed1982

Hyperpolarization of retinal horizontal cells by excitatory amino acid neurotransmitter antagonists.

Membrane potentials of retinal horizontal cells have been recorded fro isolated fish retinae perfused with Ringer solutions containing various drugs. We show that glutamate diethyl ester (GDEE) and gamma-D-glutamylglycine (DGG), respectively antagonists of the excitatory neurotransmitter agonists quisqualate and kainate, hyperpolarize horizontal cells and, at mM concentrations, suppress the light evoked S-potentials. They were not, however, generally effective in blocking the depolarizing effects of exogenous kainate and quisqualate, but in the 10% of the experiments where they were effective GDEE blocked selectively quisqualate binding the DGG blocked selectively kainate binding. In contrast, folic acid consistently hyperpolarized the horizontal cells and at mM concentrations, blocked partially the effects of kainate and quisqualate. These results are consistent with the depolarizing actions of kainate and quisqualate on retinal horizontal cells [9, 10], but the inconsistent blocking of kainate and quisqualate effects by DGG and GDEE seriously restricts their value as a tool in retinal neurophysiology.

Animals↗
PubMed1982

Depolarization of retinal horizontal cells by excitatory amino acid neurotransmitter agonists.

We have recorded the intracellular membrane potential of horizontal cells, second-order interneurones of the vertebrate retina, from fish retinae perfused with Ringer solution containing agonist drugs of the excitatory neurotransmitters, L-glutamate and L-aspartate. We show that at concentrations greater than about 10 microM, kainate and quisqualate have a potent depolarizing effect on horizontal cells and suppress their light evoked electrical responses (S-potentials). In contrast, a third agonist, N-methyl-D-aspartate, hyperpolarizes horizontal cells. The depolarizing action of kainate and quisqualate persists in the presence of the synaptic blocker, cobalt chloride, which implies that they bind directly on the horizontal cell membranes. Two kainate-related drugs, which are active on invertebrate neurones, were also examined, one, alpha-ketokainate, simulates the action of kainate, but the other, dihydrokainate, is ineffective on horizontal cells. L-Glutamate binds with high affinity at quisqualate sites, whereas L-aspartate binds with high affinity at NMDA sites, thus we conclude that L-glutamate is the likely neurotransmitter at the photoreceptor-horizontal cell synapse.

Amino Acids↗