Biomedical subjects

D R Badcock

Publications and source records attributed to D R Badcock.

At least 19 recordsLinked to original sources

PubMed1992

Two-stage analysis of the motion of 2-dimensional patterns, what is the first stage?

Humans↗

PubMed1992

Analysis of the motion of 2-dimensional patterns: evidence for a second-order process.

The sum of two differently orientated moving sinusoidal gratings of similar spatial frequency, contrast, and velocity appears as a single coherent "plaid" pattern. The visual system is thought to analyse the motion of plaids in two stages, first analysing the motion of the (1-D) components, and then calculating a speed and direction which is consistent with those 1-D motions. We studied the apparent direction of motion of plaids made by adding two components that had the same spatial frequency and contrast, and were symmetrically oriented about the vertical axis. The gratings moved in jumps, and we studied the effect of varying the size of the jump, the angle between the component gratings, and the temporal interval between the jumps, on the perceived direction of motion. When the size of the jumps was increased to 3/8 of their spatial period, the perceived direction of motion of the plaid pattern reversed, although if one component were presented alone, its direction of movement did not reverse. Reversed motion of this type was consistently obtained if the angle between the components was greater than about 140 degrees, if the interval between jumps was at least 25 msec, and if the spatial frequency of the component gratings was less than about 4 c/deg. When the angle between the components was smaller, or the time between jumps was greater, most observers saw normal motion in the direction predicted by the two-stage hypothesis. When the spatial frequency was raised, observers saw no consistent motion.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗

PubMed1991

The shift effect can be elicited with both foveal and peripheral masks.

Foveal target detection thresholds are elevated by presenting a counterphasing, vertical squarewave grating in the peripheral retina. This psychophysical "shift effect" has been considered to be an analogue of the neurophysiological "periphery effect" first described by McIlwain (1964; Journal of Neurophysiology, 27, 1154-1173). Physiological response properties of cells from the retina and lateral geniculate nucleus of cat and primate visual systems predict that sensitivity thresholds should also be elevated for peripheral targets in the presence of a foveal counterphasing mask. In these experiments, contrast sensitivities for human observers were obtained using a two-interval forced-choice procedure for peripheral target sinusoids in the presence of a foveal counterphasing mask. A suppressive shift effect was elicited by the foveal counterphasing squarewave mask, but only for counterphasing peripheral sinusoids. Masking was only obtained at the lowest spatial frequencies for both the peripheral and foveal shift effects.

Contrast Sensitivity↗

PubMed1991

The impact of jitter on separation discrimination: combination of monocular inputs.

The eyes are in continuous motion. A robust system required to make spatial separation judgments should be resistant to the positional noise produced by such movements. Two parallel lines have been jittered horizontally in order to assess the impact of stimulus movement on the retina. Jitter that maintains the separation between the two lines has minimal effect on separation discrimination thresholds, regardless of whether the targets in the two eyes jitter in a positively correlated, negatively correlated or uncorrelated manner. Presenting both eyes with line pairs but only altering the separation in one eye in the second interval yields poorer performance. However, if one eye receives a change in separation while the other views a dark screen then that monocular threshold is very similar to the binocular threshold. These results are most simply explained by procedures which average the monocular separation estimates.

Convergence, Ocular↗

PubMed1990

Resistance to positional noise in human vision.

Human eyes are in constant and rapid motion even when observers try to maintain steady fixation. Also, the visual system has a sluggish temporal response. In combination, these two factors would be expected to blur stimuli and reduce spatial sensitivity. But observers are able to detect a difference in separation of a few seconds of arc between two closely spaced parallel lines. Here we report that even very large amounts of positional jitter of the line pair has minimal impact on this ability. This result is in marked contrast to the deterioration observed when targets are swept linearly across the retina, but is consistent with a system that must ignore oculomotor jitter. To explain these results will require a re-evaluation of current models of position coding in human vision.

Eye Movements↗

PubMed1990

The sensitivity of separation discrimination to spatiotemporal jitter.

Differences of less than 20 sec of visual angle in the separation of a pair of closely spaced parallel lines can be reliably detected. This ability is known as a hyperacuity because the thresholds are smaller than the diameter of one foveal cone. It is shown that this ability does not require a stationary pattern. Indeed, correlated horizontal jitter of the line pair has little detrimental effect on performance for jitter that ranges up to 8 min arc for two lines with a separation of only 6 min arc. Uncorrelated jitter of the two lines, which allows the actual separation to vary from moment to moment, causes performance to deteriorate at a rate similar to the rise of signal uncertainty. The results reflect the operation of a system which is not only extremely robust to oculomotor instability but is also robust to positional variation that could not be produced by eye movements.

Discrimination, Psychological↗

PubMed1990

Phase- or energy-based face discrimination: some problems.

Fourier analysis may be used to obtain the amplitude and phase spectra of images. Spectra from two images can then be recombined in an attempt to produce images with identical phase spectra but different amplitude spectra or with identical amplitude but different phase spectra. Kleiner and Banks (1987) and Kleiner (1987) used this technique to determine if the amplitude (energy) or phase characteristics of images form the basis for infant face preferences. This article describes how some of the procedures used by these researchers, for example, luminance normalization and photographic production of transparencies of images, probably prevented the images from having the desired characteristics, thus making interpretation of their studies very difficult. Additional potentially confounding issues to be addressed when repeating these or similar experiments are also discussed. These difficulties arise from the distinction between the numerical phase values obtained through the mathematical process and those that are visually effective.

Attention↗

PubMed1990

Low-frequency filtering and the processing of local-global stimuli.

The role of low-spatial-frequency information in the processing of global stimuli made up of local elements was examined. After selective removal of low spatial frequencies two major changes occurred in the pattern of results. First, response times to global stimuli were significantly slower and the usual speed advantage of global over local processing was lost. Second, when processing local features the usual decrease in response speed when the local and global letters are not the same (consistency effect) was not obtained. These effects could not be explained by changes in error rate, by contrast variation resulting from the process of filtering, or by loss of visual sensitivity due to greater eccentricity of global images.

Adult↗

PubMed1989

Detecting the displacements of spatial beats: no role for distortion products.

When two sinusoidal gratings of the same orientation and similar spatial frequency are summed, the resulting pattern has a periodic spatial variation or beat in contrast. Although the pattern contains no luminance modulation component at the beat frequency, it behaves in some respects as if it did: human observers for example are very good at detecting spatial displacements of the beat. We wished to test the possibility that a non-linearity in the visual system generates a component (a "distortion product") at the beat frequency, and that it is displacement of the distortion product that observers detect. Attempting to "null" the distortion product by adding to the beat pattern a sinusoidal component of the same spatial frequency as the distortion product but 180 deg out of phase with it does not impair performance in detecting motion of the beat; there is no nulling at any amplitude of the added component. Reducing the phase shift of the hypothetical distortion product by adding a static sinusoid to the moving beat pattern fails to produce the predicted fall in performance. These results suggest that distortion products do not contribute to our sensitivity to the displacement of beat patterns. Reversing the contrast of a beat pattern when it is displaced, slightly increases sensitivity to displacement, the same manipulation impairs performance with luminance patterns. This is consistent with the notion that the beat is detected as an unsigned local contrast signal.

Contrast Sensitivity↗

PubMed1989

Uniform field flicker: masking and facilitation.

The influence of uniform field flicker (UFF) on the contrast threshold for stationary sinusoidal gratings was measured as a function of flicker depth. Changing from no flicker to a flicker depth of 10% produces reliable differences in threshold. Further increases in flicker depth to 60% had little additional influence. The presence of UFF produces masking with low spatial frequency stimuli as expected but at higher spatial frequencies facilitation is obtained. The results are discussed in the context of the distinction between primate Magnocellular and Parvocellular pathways.

Adult↗

PubMed1988

Discrimination of spatial phase changes: contrast and position codes.

Changing the relative phase of the frequency components of a stimulus usually also produces local contrast variations. Using stimuli composed of the product of a sinusoid (carrier) and a spatial envelope, an attempt was made to distinguish between the visual system's ability to code spatial phase on the one hand and local contrast and position cues on the other. The experiments assess the ability of observers to detect which of two stimuli is farther to the left. In the main experiments a large, easily detectable, envelope shift is presented on every trial and performance is measured as a function of the size of a carrier shift in the same direction. Increasing the size of the carrier shift gradually increases the size of the phase difference between the two stimuli in a trial but simultaneously reduces the contrast change in the bars of the stimulus. If the visual system can code phase directly the ability of observers to detect a change in location should improve as the size of the carrier shift increases but if local contrast is coded performance should be poorer over a small range of carrier shifts than that obtained without a carrier shift. It is shown that a region of poorer performance is obtained and therefore it is concluded that the visual system does not code spatial phase explicitly.

Contrast Sensitivity↗

PubMed1987

Detecting the displacements of spatial beats: a monocular capability.

Sensitivity to the sudden displacement (phase shift) of a single monocularly presented sinusoidal grating is increased when a static grating of similar spatial frequency is presented to the same eye. If the static grating is presented to the other eye instead sensitivity is, at best, halved. This demonstration implies that monocular and binocular visual pathways differ in their sensitivity to spatial variations of contrast. In addition it provides another example in which the monocular visual pathways are more sensitive to spatial displacements than the binocular pathways.

Depth Perception↗

PubMed1987

Masking by uniform field flicker: some practical problems.

The technique of uniform field flicker (UFF) masking has frequently been used to address issues concerning the relative performance of sustained and transient neural channels in the human visual system. Unfortunately there has been an artifact in the implementation of this method in most published experiments which has meant that the contrast of the target has been flickered in synchrony with the mean luminance. A study is reported in which the artifact was corrected and the effects of UFF masking on the contrast sensitivity function then examined. With this correction, masking was still restricted to low spatial frequencies but it was much weaker than reported originally. It is argued that the original evidence suggesting that UFF masking can be used to examine the functioning of transient and sustained channels has not been interpreted correctly and that the basis for such a claim is weak.

Adult↗

PubMed1986

Detection of spatial beats: non-linearity or contrast increment detection?

We measured sensitivity of human observers to the 1 c/deg beat between sinusoidal gratings of 9 and 10 c/deg, at different contrasts of the 2 components. Raising the contrast of one component increases the contrast required in the other component to detect the beat. This is consistent with the hypothesis that the beat is detected because of the local increments in contrast which it produces, but not with the hypothesis that it is detected when a difference-frequency distortion product generated by non-linear transduction of luminance reaches threshold.

Discrimination, Psychological↗

PubMed1985

Separate detectors for simple and complex grating patterns?

Grating having two sinusoidal components show a periodic variation in contrast which is visible as a "beat" pattern. The spatial frequency of the beat is the difference between the frequencies of the two components. Thresholds for a number of detection and discrimination tasks were measured using beat patterns of 1 c/deg (with components of 9 and 10 c/deg), and gratings of 1 and 10 c/deg. Temporal modulation at 6 Hz lowered detection thresholds for 1 c/deg gratings, but not for beats or 10 c/deg gratings. The effect of contrast on the range of temporal frequencies over which direction of movement can be discriminated differs for the three types of pattern: beats resemble neither low nor high spatial frequency gratings. Low and (for 2 of 3 observers) high spatial frequency gratings, but not beat patterns, are susceptible to a movement after effect induced by a low spatial-frequency grating. Beat patterns induce little or no movement after effect. We conclude that beat patterns are not detected by the same mechanisms that detect simple gratings.

Figural Aftereffect↗

PubMed1985

The low level motion system has both chromatic and luminance inputs.

Adaptation to moving isoluminant gratings induces a motion after-effect (MAE). Isoluminant gratings are less effective at inducing and at nulling MAEs than are luminance gratings. These results are consistent with a low-level motion detection system which operates on signals from mechanisms which show both spatial and chromatic opponency.

Adaptation, Ocular↗

PubMed1985

Detecting the displacement of periodic patterns.

Observers were asked to detect the direction of displacement of a 30 c/deg grating. They were virtually unable to perform this task when the component was presented alone but when either a 28 or a 32 c/deg component (neither of which moved) was added to the 30 c/deg component observers were extremely sensitive to displacements of the 30 c/deg component. These results suggest that the detection of displacements cannot take place within narrow-band spatial channels, but relies on a mechanism which compares the output of channels in different spatial positions.

Form Perception↗

PubMed1985

Spatial location and hyperacuity: the centre/surround localization contribution function has two substrates.

Vernier acuity and jump detection were investigated using a perturbation technique, in which a flanking line is placed to one side of the target line. The size and direction of vernier displacement, or jump, required for no apparent change of location is strongly influenced by the separation between the flanking line and the test line and by its polarity. For flanks within a zone extending approximately 3'-4' to either side of the target line, the target's location is assigned to a weighted centroid of the complete luminance distribution: The target is pulled towards the flank, when the flank has a positive contrast polarity, and repelled when the polarity is negative. The effects of a dark flank on one side and a bright flank on the other are additive. Outside this central zone repulsion effects are obtained independent of the contrast polarity of the flank and flanks on opposite sides of the target line can cancel each other's influence. Varying the duration of the flank produces maximal effects in the surround with shorter duration than that required for maximal effects in the centre. Thus, while the localization contribution function resembles the popular difference of gaussians receptive field profile, it has two components reflecting differing mechanisms. In the centre the earlier centroid hypothesis can be applied with the addition of distance dependent weights. The surround has characteristics resembling the feature interaction seen in figural after-effects.

Humans↗