Biomedical subjects

O J Braddick

Publications and source records attributed to O J Braddick.

11 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↗

PubMed1979

Binocular single vision and perceptual processing.

Stimuli with small binocular disparities are seen as single, despite their differing visual directions for the two eyes. Such stimuli also yield stereopsis, but stereopsis and single vision can be dissociated. The occurrence of binocular single vision depends not only on the disparities of individual stimulus elements, but also on the geometrical relation of different parts of the pattern presented to each eye. A pair of vertical bars with opposite binocular disparities is seen as single if the pair is moderately widely spaced but not if it is narrow. Vertical alignment and identity in length of such bars also increase the occurrence of double vision. It is argued that these effects reflect the extraction of features of the monocular patterns, with these detected monocular features determining the binocular percept. Single and double vision of bars differing in orientation can be similarly analysed. The occurrence of relatively elaborate processing of monocular signals does not exclude the possibility that binocular interaction can occur between signals that have not been so processed. Multiple sites or types of binocular interaction are likely.

Functional Laterality↗