Search PubMed⌕ Search

Biomedical subjects

O J Braddick

Publications and source records attributed to O J Braddick.

At least 19 recordsLinked to original sources
PubMed1991

Discrimination of spatial phase shows a qualitative difference between foveal and peripheral processing.

Detection and discrimination of compound grating stimuli were examined in foveal and peripheral vision. At the fovea, stimuli containing two components (spatial frequencies F and 3F) can be discriminated on the basis of their relative spatial phase when the 3F component is at a contrast below its independent detection threshold. This is no longer the case at increasing retinal eccentricity, where phase discrimination thresholds fall off much more steeply than simple detection thresholds. This relative fall-off in discrimination performance is still present for stimuli scaled for the cortical magnification factor, and is not attributable to fading of peripheral images due to the Troxler effect. The results therefore must imply a qualitative change in the processing of phase information between foveal and peripheral vision.

Contrast Sensitivity↗
PubMed1991

The temporal integration and resolution of velocity signals.

The temporal properties of human visual motion detection were explored. Experiment 1 measured thresholds for speed discrimination as a function of stimulus duration. Thresholds fell asymptotically to a Weber fraction around 0.06 over a period of approx. 100 msec, with faster speeds asymptoting at slightly shorter stimulus durations. A second experiment required subjects to discriminate a pattern that was modulated between two speeds from one which remained at a constant speed. The minimum depth of the modulation required to make this judgement was found to be equivalent to a Weber fraction of 0.3 at low modulation rates, around five times greater than when the velocities were presented in isolation (expt 1). At some higher modulation rate performance dramatically declined. The modulation rate at which this occurred decreased with stimulus speed, and increased with stimulus size. The results of expt 1 seem consistent with the known properties of primary motion sensors, while the results of the latter experiments may arise from a later stage integrating the output of these primary motion sensors.

Discrimination, Psychological↗
PubMed1991

Serial search for targets defined by divergence or deformation of optic flow.

The optic flow field can be described in terms of the local differential measures, divergence, deformation, and rotation, which are informative about observer motion and the 3-D structure of the environment. Does an explicit representation of these measures exist in human visual processing in the form of a feature map? Triesman's criteria were used to investigate this; ie is there 'pop-out' for a target defined as different in local divergence or deformation from surrounding elements, or is a serial search necessary? The stimulus arrays contained 3, 5, or 9 square or rectangular elements, which each underwent repeated cycles of expansion, contraction, or deformation. The time required to detect a target undergoing the opposite transformation increased steeply with the number of elements, implying very slow serial search. (The mean time was 210 ms per element for divergence targets and 542 ms per element for deformation). The process was clearly still serial when the density and number of elements was increased up to 48 in an array 2.16 deg x 2.16 deg. In contrast, a single line element undergoing the opposite direction of translation motion to the rest of the display did show pop-out. It is concluded that no parallel processes seem to exist which are sensitive to the spatial uniformity of divergence and of deformation of optic flow. These differential properties may be derived as conjunctions of signals from a primary process which extracts local velocity. This result contrasts with our findings for targets defined by stereo disparity gradient, which show parallel processing in analogous experiments.

Adult↗
PubMed1991

Pre-attentive detection of a target defined by stereoscopic slant.

Does the visual system represent stereoscopic depth purely as a map of local disparities, or does it explicitly represent local relationships of disparity, such as disparity gradients? Experiments are reported in which visual search for a target containing the same disparity range as other elements in the display, but differing in the relationship of the disparities (stereo slant), was used to determine whether the target showed 'pop-out' like a unitary feature, or the serial search characteristic of feature conjunctions. Each stereo pair of elements was selected randomly from a range of outline parallelograms leaning to the right or to the left, so that the target could not be identified using any monocular shape cue. Response times for detection of the target (present on 50% of the trials) were independent of the number of elements in the display. This result was confirmed by varying element size and spacing, and by using oblique crosses rather than parallelograms as stimuli. It is concluded that stereoscopically defined slant, or disparity gradient, can be processed and compared in parallel across the display, and acts in this respect as an explicit unitary visual property. This contrasts with findings in analogous experiments on movement, which show that targets defined by divergence or deformation of optic flow can only be identified by serial search.

Attention↗
PubMed1990

Direction discrimination for band-pass filtered random dot kinematograms.

When an array of random dots is displaced, the ability to report the direction of apparent motion is subject to an upper spatial limit (dmax). As the size of the displacement is increased, direction discrimination errors show a monotonic increase that becomes asymptotic at a chance level. We have measured direction discrimination using spatially band-pass filtered random dots. These stimuli do not yield a monotonic increase in errors. Rather, for displacements greater than around 1 cycle of the stimulus centre frequency (Fc), performance oscillates about chance, with displacements of 1 1/4 cycles of Fc yielding systematic errors in perceived direction. We analyse this pattern of performance in terms of the stimulus autocorrelation function and conclude that dmax can be taken as lying on the initial rising portion of the displacement versus error function. Using this definition we find, in line with the results of Chang and Julesz (1985), that dmax scales inversely with Fc. Contrary to the results of Chang and Julesz, we find that this scaling holds beyond 4 c/deg.

Discrimination, Psychological↗
PubMed1990

Masking of low frequency information in short-range apparent motion.

When an array of random dots is displaced, the ability to report the direction of apparent motion is subject to an upper spatial limit (dmax). Using spatially low-pass filtered random dot kinematograms we show that dmax is dependent on the upper cut-off frequency of the stimulus (Fh). The extent of this dependence is critically dependent on the size of the stimulus. Our results suggest a process whereby low spatial frequency motion information is masked by the presence of high spatial frequencies in the same region of the field, analogous to phenomena occurring in the perception of static form (e.g. the Abraham Lincoln effect). The effects of stimulus size on dmax, found for broad-band stimuli by ourselves and others, result from a loss of high frequency sensitivity at increased retinal eccentricities; this loss reduces the masking effect of high frequencies, as stimulus size increases.

Discrimination, Psychological↗
PubMed1990

Differences in the processing of short-range apparent motion at small and large displacements.

Using random dot patterns we have compared performance on direction discrimination tasks for single and multi-step sequences of apparent motion at a range of displacement sizes. Performance was measured by varying the correlation between the frames. For "small" displacements we found that no improvement in performance occurs with stimulus duration (number of frames) if the movement of individual elements within the pattern was restricted to one step, whereas if elements undergo multiple steps, performance improves with duration. For "large" displacements, on the contrary, performance improves with increasing stimulus duration irrespective of whether individual elements are restricted to single steps. These results suggest that small and large displacements are processed in different ways. We review possible psychophysical and physiological correlates of this suggestion.

Discrimination, Psychological↗
PubMed1989

The combination of motion signals over time.

The improvement in performance with increasing number of frames in a random-dot kinematogram (temporal recruitment) was assessed by measuring threshold signal-to-noise ratios of direction discrimination. At fast frame presentation rates (50 Hz) thresholds fell sharply as the number of frames in the sequence increased, whereas at slow frame presentation rates (20 and 10 Hz) there was a less dramatic fall in thresholds. The similarity between the results at 20 and 10 Hz suggests that the mechanism of this less dramatic rise is relatively independent of temporal factors. The recruitment effect also does not appear to be limited by a maximum spatial range. We propose that temporal recruitment may occur via two mechanisms. One involves stimulating motion detectors with greater spans and delays, whilst the other involves the co-operative interaction of signals from units tuned to similar directions and have similar spans and delays. This distinction is supported by a further experiment which eliminates the first of these recruitment mechanisms by destroying possible correlations between non-adjacent frames.

Humans↗
PubMed1989

Extension of displacement limits in multiple-exposure sequences of apparent motion.

In order to examine the processes by which motion signals are combined over time, we presented subjects with random dot kinematograms which could vary in terms of the number of frames in the sequence and the duration between the onset of each stationary frame. Performance (as measured by the greatest displacement at which subjects could discriminate opposite directions of movement) improved with increasing number of displacements up to around 5 displacements, whilst manipulations of the frame duration had no affect upon this figure. Thus the results cannot be described in terms of a limited integration time. By creating sequences in which no dots underwent more than a single displacement we show that the improvement is not specific to individual dot paths. We suggest that these results could be accounted for in terms of a co-operative network in which mutual facilitation can propagate between detectors tuned to a common direction of motion.

Humans↗
PubMed1989

'Where' and 'what' in visual search.

A line segment target can be detected among distractors of a different orientation by a fast 'preattentive' process. One view is that this depends on detection of a 'feature gradient', which enables subjects to locate where the target is without necessarily identifying what it is. An alternative view is that a target can be identified as distinctive in a particular 'feature map' without subjects knowing where it is in that map. Experiments are reported in which briefly exposed arrays of line segments were followed by a pattern mask, and the threshold stimulus-mask interval determined for three tasks: 'what'--subjects reported whether the target was vertical or horizontal among oblique distractors; 'coarse where'--subjects reported whether the target was in the upper or lower half of the array; 'fine where'--subjects reported whether or not the target was in a set of four particular array positions. The threshold interval was significantly lower for the 'coarse where' than for the 'what' task, indicating that, even though localization in this task depends on the target's orientation difference, this localization is possible without absolute identification of target orientation. However, for the 'fine where' task, intervals as long as or longer than those for the 'what' task were required. It appears either that different localization processes work at different levels of resolution, or that a single localization process, independent of identification, can increase its resolution at the expense of processing speed. These possibilities are discussed in terms of distinct neural representations of the visual field and fixed or variable localization processes acting upon them.

Attention↗
PubMed1986

Development of the discrimination of spatial phase in infancy.

The ability to discriminate grating patterns, containing the same spatial frequency components but in different phase relationships, has been studied in infants by comparing looking times following habituation to one pattern. The performance of 1-month-olds was compared with that of 2/3-month-old infants. Both age groups could discriminate a set of components in square-wave-phase (fundamental 0.18 c/deg) from components of the same amplitude combined in random phase. However, these compounds differ in peak-to-trough contrast, which infants of both ages could discriminate even for a constant waveform. When contrast was randomized from presentation to presentation, the older group still demonstrated discrimination, implying that they were sensitive to the pattern differences, but the younger group did not. The younger group also failed to demonstrate discrimination between the two waveforms when they were of fixed, matched, peak-to-trough contrast, indicating that the previous absence of discrimination was not simply due to distraction by the contrast variations. We conclude that 1-month-olds are insensitive to the configuration of these compound grating patterns even when they are capable of detecting their components. This loss of spatial information has some analogies with adult peripheral and amblyopic vision. Like other aspects of vision, it shows striking development between 1 and 3 months of age.

Aging↗
PubMed1985

Eccentricity-dependent scaling of the limits for short-range apparent motion perception.

The ability to report the direction of apparent motion when an array of random dots is displaced fails when the displacement exceeds a limiting value (dmax). We find that dmax increases rapidly with retinal eccentricity, in a manner different from spatial measures such as acuity which are believed to depend on the "magnification factor" of projection to area 17. The minimum displacement giving detectable motion (dmin) shows a shallower increase with eccentricity which is more compatible with the variation of cortical magnification. The dependence of apparent motion on the timing variables (exposure duration, inter-stimulus interval) changes negligibly with eccentricity. Consequently the dynamic range and the upper limit of detectable velocities increases greatly with eccentricity. The increase of dmax with eccentricity means that the perception of apparent motion will show an approximate invariance with display scale, even though dmax has a locally fixed value depending on receptive field structure.

Fixation, Ocular↗
PubMed1985

Temporal properties of the short-range process in apparent motion.

A study is reported of the perception of random-dot two-frame apparent motion in which the durations of each exposure and the interstimulus interval between them were varied. The results are largely consistent with the rule that, for optimal motion detection, a portion of each exposure must fall within the same time interval of about 40 ms. In addition, motion perception is separably dependent on the displacement from one exposure to the next and on the time interval between those exposures, rather than on the 'velocity' implied by their ratio.

Computers↗
PubMed1985

Eccentric photorefraction: optical analysis and empirical measures.

An optical analysis of a photographic technique, "eccentric photorefraction," designed to measure refraction and accommodative states along a single meridian of the eye, is presented. Empirical measures taken from a model eye support the theoretical derivation. The application of the technique for use with human infants is discussed with reference to measurements taken from human eyes.

Humans↗
PubMed1984

Screening for refractive errors in 6-9 month old infants by photorefraction.

The method of isotropic photorefraction has been used in a trial of refractive screening of 6-9 month old infants. Data are presented on the calibration of the method against retinoscopic measurements and its reliability. In photorefractive screening of 1096 infants under cyclopentolate cycloplegia 5% were found to be hypermetropic (over +3.5 D), 4.5% myopic, and 1.3% anisometropic (over 1 D). These refractive errors were confirmed on retinoscopic follow-up (with the exception of a few anisometropes). Follow-up of controls shows that one small refractive error was missed in 52 infants. We conclude that photorefraction is a valid and practical screening technique. Longitudinal study of infants with refractive errors will assess the value of early detection, in particular for prediction and prevention of strabismus.

Adult↗
PubMed1983

The effects of screen size and eccentricity on acuity estimates in infants using preferential looking.

The study examined whether screen size (10 degrees vs 19 degrees dia.) and separation (3 degrees vs 10 degrees eccentricity of inner edges) affect the estimates of acuity obtained with 1-3 month infants tested by forced-choice preferential looking. One and 2-month infants (but not 3-month olds) showed higher acuity estimates with the larger screens. Screen separation did not significantly affect acuity estimates for any of the age groups. Possible factors underlying these results are discussed.

Fixation, Ocular↗
PubMed1983

Some recent findings on the development of human binocularity: a review.

Evidence on the development of binocular function in infancy is reviewed. (1) Visual evoked potentials (VEP) may be recorded from infants in response to dynamic random dot stimuli which alternate between positive and negative binocular correlation. Such responses can only arise in neurones receiving binocular input. (2) Infants' looking behaviour may be shown to depend on the presence of binocular disparity in the stimulus (either random-dot or line stereograms). Results of these techniques agree that binocular function normally develops initially between 2 and 4 months of age. Our own data using VEP show a median age of first binocular response of 13 weeks but with marked individual variations. Binocular development involves the interplay of sensory interaction and oculomotor coordination, but it is unlikely that alignment of the two eyes is the dominant constraint determining the onset of binocular vision. It is possible, but not yet established, that the detection of binocular correlation may precede the ability to discriminate stereoscopic disparities. Infants in the first 3 months of life show an asymmetry of monocular optokinetic nystagmus (MOKN). The response to temporalwards field motion which they lack is driven in cat by a pathway via binocular cortex: thus the development of this response in human infants might depend on development of binocularity. However, the correlation across individual infants between the age of onset of binocularity and the age at which symmetrical MOKN is attained is relatively weak. It is possible that the neuroanatomical basis of MOKN control differs between cat and human.

Child Development↗