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Biomedical subjects

K H Ruddock

Publications and source records attributed to K H Ruddock.

At least 19 recordsLinked to original sources
PubMed2000

Asymmetries of saccadic eye movements in oriented-line-target search.

Visual search for a line-element target differing sufficiently in orientation from a background of line elements can be performed rapidly, effortlessly, and without eye movements. There is, however, a response asymmetry: detection is better with an oblique target element in vertical or horizontal background elements than when these orientation are interchanged. If the underlying visual mechanisms also provide an input to the oculomotor system, then a similar asymmetry should be observed in eye-movement behaviour. To test this hypothesis, an experiment was undertaken in which eye movements were recorded while subjects searched for a line-element target in background of line elements; orientations were chosen from the range 0 degree, 30 degrees, 60 degrees, and 90 degrees to the vertical. Data from three subjects showed that (1) latencies for the initial saccade, (2) angular errors in initial-saccade direction, and (3) manual response times depended similarly on the combination of target- and background-element orientations, performance being better for 30 degrees or 60 degrees targets in 0 degree or 90 degrees backgrounds than vice-versa. The early orientation-selective mechanisms responsible for the rapid detection of oriented-line targets are probably the same as those providing signals for saccadic eye movements.

Adult↗
PubMed1998

Oscillopsia: visual function during motion in the absence of vestibulo-ocular reflex.

OBJECTIVES: To investigate (1) the effects of loss of vestibular function on spatiotemporal vision and (2) the mechanisms which enable labyrinthine defective (LD) patients to adapt to oscillopsia. METHODS: Visual function and eye movements were assessed in seven normal subjects and four LD patients with oscillopsia due to absent vestibulo-ocular reflex. Temporal vision was assessed by measurement of threshold sensitivity for detection of a target which moved across a flickering, spatially uniform background field. Spatial vision was investigated by measurements of threshold sensitivity for the detection of a target moving across a spatially modulated background in the form of square wave gratings. Velocity discrimination was assessed with drifting gratings. All measurements were made under static conditions and during oscillatory movement of either the visual stimulus or the subject (1 Hz, peak velocity 50 degrees/s). RESULTS: Temporal responses--Normal subjects and LD patients exhibited similar responses while static and under body oscillation. Spatial responses--The two groups achieved similar results under static conditions but body oscillation reduced threshold sensitivities and shifted the spatial response function towards lower spatial frequencies in the LD patients only. Similar changes in the spatial responses were seen during oscillation of the visual stimulus but these occurred in both normal subjects and LD patients. Velocity discrimination--Two LD patients achieved normal velocity discrimination but the other two showed abnormal responses to visual stimulus movement; one displayed a loss of velocity discrimination during whole body oscillation, and the other mismatched the velocity of two moving grating stimuli. CONCLUSIONS: The changes in the spatial responses are attributed to the presence of retinal slip during visual stimulus motion in all subjects or body oscillation in the LD patients. It is concluded that any visual adaptation to oscillopsia achieved by the LD patients does not influence the measured spatial response functions, which arise at an early stage of visual processing. The abnormal velocity discrimination may relate to the progressive improvement in oscillopsia reported by LD patients.

Adult↗
PubMed1997

Movement in the normal visual hemifield induces a percept in the 'blind' hemifield of a human hemianope.

We have investigated visual responses to moving stimuli presented to the normal hemifield of a hemianope, GY, who exhibits residual visual function in his right, 'blind' hemifield. Preliminary experiments established that his perception of moving stimuli localized in his 'blind' hemifield is retained when a similar stimulus is presented simultaneously in the normal hemifield. In response to a grating stimulus moving horizontally towards fixation in the non-foveal region of the normal, left hemifield, he perceives in addition to a normal motion percept in the left hemifield, a sensation of movement localized in the right hemifield. Qualitatively, this latter is indistinguishable from responses elicited by direct stimulation localized within his 'blind' hemifield by moving stimuli. We have investigated the characteristics of the mechanisms which induce the 'blind' field component of GY's responses to stimulation of the normal hemifield. We show that GY's sensitivity for detection of movement localized within his 'blind' hemifield is dependent on the direction of movement, the contrast and the velocity of a grating presented to the normal hemifield. No induced effects were recorded in response to colour or to non-moving, flickering stimuli. We examine the possible contribution of scattered light to our observations, and eliminate this factor by consideration of our experimental results. We discuss the neural mechanisms which may be involved in this response.

Humans↗
PubMed1997

Retinotopic organisation of cortical mechanisms responsive to colour: evidence from patient studies.

This paper deals with the visual responses of three patients who have impaired colour vision consequent on cortical dysfunction which, in two of them, is associated with demonstrable neuronal damage. The studies to be described are concerned particularly with the spatial attributes of their chromatic response mechanisms. Data are presented which establish that a hemianope GY has coarse chromatic discrimination for large stimuli located within his 'blind' hemifield. GY responds to stimuli containing differently coloured equiluminant components as if the coloured components were averaged over the whole field and it is speculated that such spatial averaging may correspond to the process which, in normal vision, provides compensation for change of illuminant in order to achieve colour constancy. Colour constancy is impaired in a second patient, BL, who has cortical lesions involving the lingual and fusiform gyri, areas which are partially spared in GY. It is shown that movement, but not colour, presented to GY's normal hemifield generates a response localised in his blind hemifield and disinhibitory interaction between movement and colour is illustrated for a patient MW, in whom colour chromatic stimuli generate spreading inhibition of visual responses. This inhibitory interaction is propagated between widely separated stimuli, including those which are located on opposite sides of the vertical meridian. We discuss these experimental results in relation to anatomical and physiological mechanisms of the primate visual cortex.

Adult↗
PubMed1997

Fixation patterns made during brief examination of two-dimensional images.

Measurements were carried out of saccadic eye movements made during brief (3 s) examination of images which the observer was asked to identify. Each image was identified in three forms: low-pass filtered, high-pass filtered, and unfiltered. The analysis of the eye-movement patterns was based on the locations of fixations made during examination of the images, for which purpose a least-squares measures of similarity between two sets of locations was introduced. It is shown that there is a high degree of similarity between fixations made by the same observer to the different versions of a given image and that for a given image there is a high degree of similarity between fixations made by the eighteen observers who participated in the experiments. The similarities are greater for the initial 1.5 s than for the full viewing period of 3 s. The similarity between the locations of fixations and those of selected image features such as local contrast, high-spatial-frequency content, and edge density was also examined. It is shown that there is only weak similarity between the locations of fixations and those of any given local image feature, and the tendency of observers to fixate centrally on the image is identified as the principal reason for the low similarity values. It is shown that if the nonuniform distribution of eye movements is taken into account, significant similarities are found between the locations of fixations and those of certain image features, such as edge density.

Adult↗
PubMed1997

Fixation sequences made during visual examination of briefly presented 2D images.

Eye movements made by eighteen observers in response to brief (3 s) presentations of eleven different images, each in three forms (unfiltered, high-pass filtered and low-pass filtered), have been analysed in order to identify both repeated sequences of fixations and image locations which attract re-fixations. It is shown that eye-movement traces made by different observers in response to the same image have few common temporal sequences involving the same fixation locations, even for sequences of only two fixations. There is a greater incidence of such sequences in eye-movement traces made by the same observer in response to two presentations of the same image, but average numbers are still low. Conserved sequences involving more than two identical locations occur at a much lower frequency, and the incidence of repeated sequences is not increased if consideration is restricted to regions of the image which attract large numbers of fixations. It is concluded that the temporal sequence in which fixations are made is not a significant factor in the analysis of the eye-movement data considered in this report. Calculations based on a least squares index of similarity are consistent with this conclusion. The analysis shows a relatively high incidence of re-fixation on certain locations in the images and there is evidence that such re-fixations are a significant factor in the high similarity between fixation locations established by different observers when viewing the same image.

Eye Movements↗
PubMed1996

A new abnormality of human vision provides evidence of interactions between cortical mechanisms sensitive to movement and those sensitive to colour.

We present the results of a psychophysical study on a human observer, MW, which describe his abnormal visual responses to moving stimuli. It has been shown previously that this subject has normal vision for stationary, achromatic patterns, but responds highly abnormally to saturated chromatic, and especially red stimuli. We now report that like chromatic stimuli, moving black and white patterns elicit an inhibitory response which extends beyond the visual field area covered by the moving stimulus itself, and suppresses detection of stationary, achromatic patterns. Although both chromatic and moving stimuli generate similar percepts in association with their inhibitory activities, these latter differ in several respects, and we conclude that they have different neural origins. We show that in addition to their separate inhibitory actions on detection of achromatic patterns, movement and colour exert mutually inhibitory effects. Movement is markedly effective in limiting the inhibitory spread associated with colour, regardless of the positions in the visual field of the moving and coloured stimuli. Such spatially diffuse activity is characteristic of higher visual processing in pre-occipital cortical visual areas.

Color Perception↗
PubMed1996

Orientation discrimination is impaired in the absence of the striate cortical contribution to human vision.

We have examined visual discrimination of orientation for bars and edges presented to the 'blind' hemifield of a patient, GY, who has an extensive lesion involving the left striate and prestriate cortex. The patient's residual vision in response to transient light stimulation of his 'blind' hemifield has been well characterized, and it provides discrimination on the basis of differences in target velocity, flicker frequency or colour. We now show that using his residual vision, GY can identify accurately the orientation of a flickering bar, although his performance is sub-normal for bars shorter than 10 deg. He is, however, unable to identify the orientation of a one-dimensional grating presented within a circular aperture, and is unable to detect a grating structure formed by equiluminant coloured bars or by alternating, equiluminant flickering and steady bars. We also show that he has poor orientation discrimination for edges formed by colour contrast, by contrast between moving stimuli or by flicker contrast. We conclude that in the absence of the striate cortical input, the residual mechanisms which provide discriminations for colour, flicker or movement are not well organized for the detection and discrimination of stimulus orientation. We discuss the physiological mechanisms responsible for the various aspects of GY's residual vision.

Adult↗
PubMed1996

The relationship between the locations of spatial features and those of fixations made during visual examination of briefly presented images.

The locations of features such as extremes of contrast or luminance, high spatial frequency content and edge density in a set of images have been determined, and the locations of fixations made by a group of eighteen human observers who examined the images during brief (3 s) presentations were also measured. The similarity between the locations of the eye movements and those of each stimulus feature was determined by means of a least squares index IS. For averages taken over data for all observers, the similarity determined in this way is much lower than values for pairs of fixation locations made by different observers. It is concluded that the pattern of fixations made to a given image which is highly conserved between different observers, cannot be associated with any one of the local features examined by us. It is shown further that the distribution of fixation locations over the images is non-uniform, with a marked bias to central areas, whereas the image features are more uniformly distributed. Weighting the distributions of feature locations to take account of the non-uniform distribution of fixations produces much higher IS values, but the dominant contribution to these high values is the weighting function itself. Only in the case of edge density is there significant similarity between the locations of eye movements and those of the image features.

Contrast Sensitivity↗
PubMed1995

Colour identification and colour constancy are impaired in a patient with incomplete achromatopsia associated with prestriate cortical lesions.

We have examined visual functions, including colour vision, in a patient with bilateral cortical lesions involving mainly the fusiform and lingual gyri, areas known to be involved in the central processing of chromatic stimuli. The patient has near normal (6/9) acuity, and his responses to tests of binocular function and spatial vision are normal, as are his discrimination of changes in target speed and surface lightness. He does, however, exhibit minor losses in the upper visual field, mild prosopagnosia and topographical agnosia, all conditions commonly associated with cerebral achromatopsia. Colour matches and spectral response data establish that his cone photoreceptors have normal spectral characteristics and his spectral sensitivity measured against a white background reveals normal postreceptoral chromatic function. The patient's colour discrimination for differences in wavelength, hue or saturation is, however, impaired and his colour naming is significantly disturbed, particularly for blues and greens. We have determined the areas of the chromaticity chart that correspond to his naming categories for surface colours, and show that changes in illuminant cause him to alter the names of surface colours in a manner consistent with the changes in their chromaticities. Other subjects with normal or congenital red-green deficient colour vision make many fewer name changes under changes in illuminant. We conclude that the patient's colour constancy is impaired as a consequence of abnormal central processing of colour vision.

Cerebral Infarction↗
PubMed1995

The characteristics of eye movements made during visual search with multi-element stimuli.

Saccadic eye movements during visual search made to detect a target embedded in a set of randomly distributed, identical elements have been measured. Both the time T1/2 taken to press a button to denote target detection (the manual response time) and the time Tf taken to fixate the target were determined. It is established that these two response times are closely related to each other, and have similar dependence on the number of reference elements N, both in search tasks for which T1/2 is independent of N (parallel search) and in those for which T1/2 is proportional to N (serial search). The properties of the eye movements made by two subjects in response to a variety of stimuli have been subjected to detailed analysis. It is shown that the number of fixations made during a given search is proportional to the manual response time T1/2 and that the probability of locating the target with a single fixation falls steeply as T1/2 increases. The latency of the first saccade is approximately linearly dependent on T1/2, and latencies made in performing a given task are distributed in a Gaussian manner the spread of the latency values increasing as the mean latency increases. It is conclude that the measurement of eye movements provides a more detailed description of visual search than that given by measurement of manual response times, and that in terms of eye movements, only a subset of those tasks for which T1/2 is independent of N can be classified as parallel.

Adult↗
PubMed1995

Automatic control of saccadic eye movements made in visual inspection of briefly presented 2-D images.

This paper describes an investigation of eye movements made by eighteen observers with normal spatial vision in response to eleven images of natural scenes each of which was presented in three versions, unfiltered filtered by low-pass and filtered by high-pass spatial frequency filters. The ability of observers to identify the different images was determined after each set of measurements. An index of similarity, calculated in terms of the sum of the squares of the distances between each fixation point and its nearest neighbour in the other set, was developed in order to provide a basis for the comparison of two sets of eye movements, each made by a different observer or both made on different occasions by the same observer. Application of this index shows that brief (1.5-s) presentation, there exists a high degree of similarity between fixations made by different observers in response to the same image. For longer (3-s) periods, however, the similarity for inter-observer comparisons is reduced. The data demonstrate that filtering of the image has little effect on the pattern of fixations made to a given image, but the duration of fixations is greatest for low-pass filtered images and least for unfiltered images, whereas the amplitude of saccades is greatest for unfiltered images and least for low-pass filtered images. It is proposed that for brief presentations, eye movements made during examination of an unfamiliar image are performed automatically in response to the spatial features of the image.

Adolescent↗
PubMed1995

Vision during motion in patients with absent vestibular function.

We have measured a spatial visual response and visual velocity discrimination in 4 patients with long standing vestibular loss and 6 controls. The spatial response was measured during; i) body and visual display stationary conditions, ii) whole-body oscillation (1 Hz +/- 50 degrees/s) and iii) visual stimulus oscillation (1 Hz +/- 50 degrees/s). Velocity discrimination was assessed during conditions i) and ii). The visual tests applied were selected on the basis that the spatial response is known to reflect peripheral processes of the retina, whereas velocity processing is more central in origin. Patients had normal spatial responses under static conditions and they suffered a degradation in their spatial responses during whole-body oscillation, whereas, normals' responses remained unaltered. During oscillation of the visual display both patients and normals suffered a degradation in their spatial responses, and for patients the change was very similar to that observed during whole-body oscillation. The changes in the spatial responses were dependent on the gain of the eye movements which compensated for the whole-body or visual display oscillation. In 3 patients and all controls whole-body oscillation did not alter the discrimination of velocity of a vertically moving horizontally orientated grating compared with when the subjects were stationary. One patient suffered a severe reduction in the ability to discriminate velocity under whole-body oscillation, which suggests that central suppression of motion perception reduces oscillopsia.

Acceleration↗
PubMed1995

Recovery from bilateral vestibular failure: implications for visual and cervico-ocular function.

We report a patient who sustained severe bilateral labyrinthine lesions during Streptococcus suis meningitis but considerably recovered vestibular function over a 7 month period. This unique case allowed us to examine the cervico-ocular reflex (COR) and visual function at various levels of activity of his vestibular system. The slow phase COR, elicited by trunk oscillation (0.2 Hz) with the head earth-stationary, was negligible immediately after the acute vestibular loss but rose to a gain of 0.51 one month after. Seven months later, when vestibular function was improved, COR gain dropped to a gain of 0.15. Measurements of spatial visual function during whole body oscillation in the acute stage and after 6 months showed marked improvement which correlated entirely with VOR measurements in the dark and during optic fixation. This patient also showed the unique feature that, in the acute stage, eye movement gain and visual function were poorer during whole body motion than during identical visual target motion. These findings suggest that: i) the COR may be inhibited by the presence of vestibular signals, ii) spatial vision measurements provide accurate assessment of the patient's visual blur during head motion, and iii) the severe oscillopsia experienced by patients in the acute stage of vestibular loss may not only be due to the absence of the VOR; additional degradation in eye movements during head motion, perhaps arising from acutely distorted labyrinthine signals, may also play a part.

Dark Adaptation↗
PubMed1994

Residual colour vision in a human hemianope: spectral responses and colour discrimination.

We present data for a patient, GY, with a right hemianopia caused by traumatic damage to the left occipital cortex. Previous studies have established that this patient has residual vision which enables him to detect and localize transient stimuli presented to his 'blind' hemifield. We have now examined spectral responses associated with this residual vision by using two-colour incremental threshold methods to measure II-spectral functions, and a white light background to examine spectral data for 'colour-opponent' characteristics. We report that both methods yield normal spectral response characteristics for GY's 'blind' hemifield. We have also investigated the patient's ability to identify, verbally, coloured stimuli presented to his 'blind' hemifield, and found that, in 'forced choice' experiments, he achieves a high proportion of correct responses to large stimuli. The patient reported that in threshold detection measurements his responses were based on the presence or absence of a percept associated with transient light stimulation of the 'blind' hemifield (residual vision), whereas colour naming was achieved without conscious perception of colour ('blindsight').

Accidents, Traffic↗
PubMed1994

Precortical dysfunction of spatial and temporal visual processing in migraine.

This paper examines spatial and temporal processing in migraineurs (diagnosed according to International Headache Society criteria, 1988), using psychophysical tests that measure spatial and temporal responses. These tests are considered to specifically assess precortical mechanisms. Results suggest precortical dysfunction for processing of spatial and temporal visual stimuli in 11 migraineurs with visual aura and 13 migraineurs without aura; the two groups could not be distinguished. As precortical dysfunction seems to be common to both groups of patients, it is suggested that symptoms that are experienced by both groups, such as blurring of vision and photophobia, may have their basis at a precortical level.

Adult↗
PubMed1990

Terbium simulates the action of calcium on electrophysiological activity in the isolated fish retina.

It has been suggested that terbium (Tb) could be substituted for calcium (Ca) in biological tissue and, because of its useful spectroscopic properties, provide an assessment of calcium distribution in the tissue. In order to assess their physiological effects, we have examined the action of free terbium ions on the membrane potential and light-evoked responses of horizontal cells, and on the massed light evoked response (ERG) of the neural retina. Measurements were performed by intracellular and extracellular recording from the isolated fish retina, superfused with Ringers of various ionic compositions, and certain technical difficulties which occurred in the use of terbium are discussed. We show that Tb ions mimic Ca ions in suppression of the ERG and in hyperpolarization of the horizontal cell membrane potential. The effects of Tb persist after its removal from the superfusate for a longer period than do those of calcium, which indicates that terbium binds more firmly at its site(s) activity. Tb and Ca both suppress the light-evoked S-potentials from the horizontal cells, although some differences were observed in the waveform of the S-potentials during the onset of suppression. We conclude that the physiological effects of Tb on the retina are essentially similar to those of Ca, and that Tb could, therefore, prove a useful marker of Ca distribution in neural tissue.

Animals↗