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F A Kingdom

Publications and source records attributed to F A Kingdom.

15 recordsLinked to original sources
PubMed1997

Integrating contours within and through depth.

To better understand the role of disparity in contour integration we compared detection performance of "paths" composed of elements confined either to a single depth plane, or spanning multiple depth planes. In both cases paths defined by alignment of elements were embedded in a noise background-field made up of similar, but randomly positioned, elements covering the same depth range as the path elements. We show that a systematic disparity cue can enhance the detectability of paths which traverse depth, but that this detectability is weak compared to paths made up of elements of the same disparity. These results suggest that the outputs of disparity detectors tuned to different disparities can be linked to define contours.

Depth Perception↗
PubMed1997

On the independence of chromatic and achromatic stereopsis mechanisms.

The extent to which the processing of stereoscopic depth information can take place separately in colour-contrast-sensitive and luminance-contrast-sensitive mechanisms has been investigated. Contrast thresholds for stereoscopic depth identification (front/back) were measured using 0.5 c/deg Gabor patches. The stimuli possessed different amounts of colour and luminance contrast ranging from isoluminance (red/green) to isochrominance (yellow/black) through intermediate values. Two models for combining chromatic and achromatic stereopsis information were tested. The first (single-pathway) model assumed colour and luminance contrast summation within a single luminance-contrast-sensitive mechanism before stereoscopic judgement. The second (dual-pathway) model assumed probability summation between independent chromatic and achromatic stereopsis mechanisms. The latter model provided the better fit to the data. In providing evidence in favour of an independent chromatic stereopsis mechanism, it was shown that luminance artifacts were unlikely to be the cause of maintained stereopsis at isoluminance. The possible neural substrates of chromatic stereopsis are discussed.

Color Perception↗
PubMed1996

A linear systems approach to the detection of both abrupt and smooth spatial variations in orientation-defined textures.

Two distinct paradigms have characterized most previous studies of texture perception: one has dealt with texture segregation, the other with the processing of texture gradients. Typically, studies of texture segregation have used stimuli with abrupt textural variations, whereas studies of texture gradient processing have used stimuli with smooth textural variations. In this study we have asked whether the mechanisms which process abrupt and smooth textural variations are the same, by considering whether a simple linear model can account for the detection of orientation modulation in micropattern-based textures with three types of modulation: sine-wave (SN), square-wave (SQ) and missing fundamental (MF). The MF waveform was constructed by removing the fundamental harmonic from a square-wave. We found a clear overall ordering of sensitivity: SQ > SN > MF. We found that sensitivity to the SQ and MF stimuli could be predicted very well from the SN data if one assumed that the r.m.s. output of a single linear channel underlay the detection of the orientation modulation. This suggests that the detection of both abrupt and smooth changes in orientation-defined textures is subserved by a common mechanism which mimics the operation of a single linear channel.

Humans↗
PubMed1996

Contrast discrimination at high contrasts reveals the influence of local light adaptation on contrast processing.

Previous measurements of contrast discrimination threshold, delta C, as a function of pedestal contrast, C, for sine-wave gratings have shown a power law relationship between delta C and C at suprathreshold levels of C. However, these studies have rarely used contrasts greater than 50%. Whittle (1986), using incremental and decremental patches, found that delta C increased with C only up to about 50%. At higher contrasts it decreased. Since a periodic stimulus can be considered to be composed of increments and decrements, we thought we might find such an inverse U-shaped function for gratings if we used contrasts up to 100%. We tested this for both sine-wave and square-wave stimuli at spatial frequencies from 0.0625 to 8.0 c/deg. We found that for frequencies up to 0.5 c/deg, delta C in nearly all cases 'dipped down' after about C = 50% contrast. At 4.0 and 8.0 c/deg, however, no dip-down occurred. Additional experiments showed that the dip-down was unlikely to be due to cortical long-term adaptation and most likely an effect of localized light adaptation to the dark bars. We argue that the absence of dip-down at high spatial frequencies was mainly due to the attenuation of contrast by the optics of the eye. As for the results of Whittle (1986), a Weber's Law in W = (Lmax-Lmin)/Lmin describes the inverse U-shaped contrast discrimination function well. Two other contrast expressions also linearize the data on log-log plots. We show how some familiar notions about the physiological operation of localized light adaptation can easily account for the form of the contrast discrimination function. Finally we estimate the number of discriminable steps in contrast from detection threshold to maximum contrast for the various spatial frequencies tested.

Adaptation, Ocular↗
PubMed1996

Stereoacuity and colour contrast.

We have measured the contrast dependence of stereoacuity using both horizontally and vertically oriented, isoluminant (red-green) and isochromatic (yellow-black), 0.5 c/deg Gabor patches. For comparison, contrasts were computed in multiples of detection threshold, where detection threshold was defined as the contrast required for the stimulus to be simultaneously detectable in each eye. Disparity thresholds (1/stereoacuity) for vertical chromatic Gabors were higher than those for vertical luminance Gabors by a factor of between 4 and 9 depending on contrast, and declined less steeply with contrast. Disparity thresholds for horizontal chromatic Gabors were very high (130-210 min arc) compared with horizontal luminance Gabors (by a factor of between 9 and 17) and were only measurable at contrasts above 10 times simultaneous monocular detection threshold. These results support the view that chromatic stereoscopic processing is less precise than luminance stereoscopic processing, and that there is a special deficit in the processing of disparity with horizontally oriented chromatic stimuli. The implications of these results for the role of colour vision in stereopsis are discussed.

Color Perception↗
PubMed1996

Facilitation of luminance grating detection by induced gratings.

Grating induction causes a homogeneous test field surrounded by sinewave gratings to possess an induced counterphase grating [McCourt M. E. (1982). Vision Research, 22, 119]. There is currently no consensus about the stage of visual processing at which illusory phenomena such as simultaneous brightness contrast are signaled. We measured the masking efficacy of induced gratings by measuring contrast detection thresholds for targets (sinewave luminance gratings) added in phase to both real and induced gratings which were matched in apparent contrast. At spatial frequencies below c. 0.5 c/deg, target detection and discrimination were comparably facilitated by both real and induced low-contrast pedestals (0.5-2%). At higher spatial frequencies (above 1.0 c/deg) facilitation continued to be observed for targets added in-phase to real grating pedestals, but occurred only for targets added out-of-phase with induced pedestal gratings. Higher inducing frequencies by themselves were not responsible for the observed phase shift of facilitation, however, since both real and induced pedestals produced similar target contrast discrimination functions when inducing frequency was varied by manipulating viewing distance (which holds the ratio of inducing grating period and test field height constant). The results imply the existence of at least two types of lateral interactive processes: one producing in-phase facilitation, and a second producing out-of-phase facilitation. The relative contribution of each process depends upon the ratio of inducing grating period and test field height.

Contrast Sensitivity↗
PubMed1996

Losses in peripheral colour sensitivity predicted from "hit and miss" post-receptoral cone connections.

On the basis of the early primate neurophysiological recordings, it was thought that the different cone types of the primate retina project selectively into the centre and surround of the receptive fields of cone opponent neurons, and more recently this view has been reasserted on the basis of physiological results. An alternative idea is that these projections are in fact unselective for cone type, and, therefore, cone opponency arises from chance variations in the proportions of different cone types in centre and surround. The issue is presently controversial with anatomical or physiological support for both hypotheses. Our results show that there is a selective loss of red-green colour sensitivity across the human visual field. Furthermore, this selective loss occurs under low temporal frequency conditions (0.5 Hz) which were selected to favour the mediation of both colour and luminance detection by a common P cell pathway and to exclude an M cell contribution to detection threshold. We show that "hit and miss" post-receptoral cone projections will produce a decline in cone opponency that is sufficient to account for this selective loss, thus providing psychophysical evidence consistent with this hypothesis.

Color Perception↗
PubMed1996

Pattern discrimination with increment and decrement Craik-Cornsweet-O'Brien stimuli.

Previous studies have reported that the magnitude of induced brightness in Craik-Cornsweet-O'Brien (CCOB) figures is greater for decrement compared with increment figures. Moulden and Kingdom (Spatial Vision, 1990, 5, 101-121) suggested this was due to larger Off-centre compared with On-centre receptive fields. Such an explanation would also predict an increment-decrement difference in the contrast threshold for discriminating a CCOB stimulus from its step-edge equivalent. To test this prediction contrast thresholds were measured for discriminating 'cusp' from 'square-bar' stimuli, for both increment and decrement forms. Contrary to prediction however, no difference was found between increment and decrement discrimination thresholds. These findings suggest that On- versus Off-centre receptive field size differences are unlikely to underlie the polarity-based asymmetry in induced brightness in CCOB figures. More generally they demonstrate that the magnitude of induced brightness in CCOB figures does not have a direct parallel in the ability of observers to discriminate those stimuli from their step-edge equivalents. The significance of these findings for models of brightness coding is discussed.

Contrast Sensitivity↗
PubMed1995

Detection of orientationally multimodal textures.

Oriented textures were produced with the use of probability density functions modulated sinusoidally over orientation. Orientational contrast sensitivity functions (OCSFs) for a task involving the discrimination of these patterns from orientationally-random textures were found for several human observers. An inverse Fourier transform of this OCSF yielded a weighting function, or filter, defined over orientation. The weighting function is broad, with a half-height full-width of 34 deg. This orientational filter was able to predict human performance in further discrimination tasks employing a variety of probability density functions over orientation.

Contrast Sensitivity↗
PubMed1995

Sensitivity to orientation modulation in micropattern-based textures.

We have measured the sensitivity of the human visual system to sinusoidal modulations of orientation in micropattern-based textured stimuli. The result is the orientation modulation function, or OMF, which describes this sensitivity as a function of the spatial frequency of orientation modulation. We found that the OMF was bandpass with peak sensitivity at spatial frequencies ranging between 0.06 and 0.2 c/deg, depending on the size of the micropatterns. The OMF was found to be scale invariant, that is its position on the spatial frequency axis did not change with viewing distance when spatial frequency was measured in object rather than retinal units. This scale invariance was shown to result from the visual system taking into account the scale rather than the density of the micropatterns as viewing distance was changed. It has been argued by Bergen [(1991) Vision and visual dysfunction (Vol. 10B) New York: Macmillan] that scale invariance in textures is a consequence of the coupling of mechanisms which detect textural features with those which detect local luminance contrasts. We reasoned that Gabor micropattern textures might therefore show narrower OMFs compared to line micropattern textures. However we found no difference in OMF bandwidth between the Gabor and line micropattern textures, suggesting that the line micropatterns were acting as selectively as the Gabor micropatterns for the spatial scale of the mechanisms which detected the orientation modulation. Evidence is presented which suggests that the mechanisms which detected the orientation modulation in our stimuli are non-linear. Finally we showed similar OMFs for sine-wave and square-wave modulations of micropattern orientation, and similar OMFs for modulations of micropattern with orientation about the horizontal and about the vertical, the direction of modulation in both cases being horizontal. The implications of these findings for the mechanisms involved in orientation-defined texture processing is discussed.

Contrast Sensitivity↗
PubMed1995

The perceived orientation of aliased lines.

The use of raster display devices for the display of graphics causes problems of aliasing when edges or lines are produced. This can be significant in those psychophysical experiments where the orientational properties of the stimulus are important. We have assessed the perceived orientation of a selection of aliased lines by comparing them with the orientation of pairs of dots. It is found that the perceptual orientation is modelled well by a least-squares metric on the pixels that compose the line. Small deviations from this metric were found, and were also found in a control experiment employing anti-aliased lines. They appear to be due to range effects. Averaged across subjects, orientational acuity was only slightly lower for aliased lines.

Data Display↗
PubMed1995

Separating colour and luminance information in the visual system.

In our visual world we can distinguish with ease between chromatic and luminance contrasts. However, in our retinae most neurones are responsive to both chromatic and luminance changes and therefore send ambiguous or 'multiplexed' information to the higher visual centres. Psychophysical evidence suggests that some cortical process must subsequently separate out this information into its chromatic and luminance components. The purpose of this communication is to review and critically evaluate the different existing schemes for doing this. To assist in this evaluation a linear systems analysis is employed in which model cortical neurones are imputed with the property of providing information about either colour or luminance. It is concluded that there is currently no unified scheme available to explain a separation of colour and luminance information in the visual system. Some theoretical considerations and most promising approaches to solving the problem are noted, but it is suggested that there may be definite limits to the ability of the visual system to achieve complete separation of colour and luminance from the retinal signal.

Color Perception↗
PubMed1995

Differences between stereopsis with isoluminant and isochromatic stimuli.

Contrast thresholds for stereoscopic depth identification (crossed or uncrossed) were measured as a function of disparity by use of isoluminant (red-green) and isochromatic (yellow-black) 0.5 cycles/deg Gabor patches. For the purposes of comparison, stimulus contrasts were scaled by their respective detection thresholds. The Gabor patches could be either vertically or horizontally oriented. It was found that the disparity dependence of the depth-identification contrast thresholds was similar for both chromatic and luminance patterns if the stimuli were vertically oriented, with the overall level of performance worse for the chromatic patterns by a factor of approximately 2 (6 dB). With horizontal patterns this difference was much larger, by a factor of approximately 7 (17 dB). These results suggest first that stereopsis in the absence of luminance cues is supported by a less-contrast-sensitive linear mechanism than that which supports stereopsis in the presence of luminance cues and second that the corresponding nonlinear chromatic stereo mechanism is either nonexistent or very weak. The implications of these data for previous studies of stereopsis at isoluminance is discussed.

Color Perception↗
PubMed1994

Contrast thresholds for stereoscopic depth identification with isoluminant and isochromatic stimuli.

Contrast thresholds for stereoscopic depth identification (crossed or uncrossed) were measured as a function of disparity using isoluminant (red-green) and isochromatic (yellow-black) 0.5 c/deg Gabor patches. For the purposes of comparison stimulus contrasts were scaled by their respective detection thresholds. The detection thresholds employed were computed from the monocular detection thresholds of the stereo half-images, based on the assumption that simultaneous detection of these half-images in each eye was a sufficient condition for stereopsis. It was found that the disparity tuning of both chromatic and luminance mechanisms was similar, with a performance peak for a binocular phase disparity of 50-120 deg. However, more contrast was required, relative to detection threshold, for the chromatic patterns to evoke a sensation of stereoscopic depth. These results suggest that stereopsis in the absence of luminance cues is supported by a less-contrast-sensitive analogue of the system that supports stereopsis in the presence of luminance cues. The results are also consistent with there being a lower density of disparity-selective mechanisms in the chromatic pathway. The implications of these data for previous studies of stereopsis at isoluminance is discussed.

Calibration↗