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Biomedical subjects

P Cavanagh

Publications and source records attributed to P Cavanagh.

At least 55 records · Page 3Linked to original sources
PubMed1995

The contribution of color to depth perceived from motion parallax.

Perceived depth was measured in a colored stimulus while stimulus movement yoked to head displacement simulated a depth of 1 cm. Velocity judgments were also made for similar stimuli moving at the same average speed but without head movement. Both measures decreased to a minimum of about 30-40% of the veridical values when the stimuli were equiluminous. Perceived depth and speed also decreased for a monochromatic stimulus as a function of luminance contrast but much more abruptly than for the chromatic stimuli. The results indicate that equiluminous color stimuli contribute to the perception of depth from motion parallax and that the contribution is not mediated by residual luminance.

Color Perception↗
PubMed1995

Enhanced displays of medical images: evaluation of the effectiveness of color, motion, and contour for detecting and localizing liver lesions.

RATIONALE AND OBJECTIVES: Many perceptual studies have shown that the detection of large, low-contrast targets is better either in color or in contrast-reversing presentations than in standard gray scale. We determined the value of several new display techniques for viewing liver computed tomography (CT) scans. METHODS: Eight observers (four radiologists and four nonradiologists) viewed sets of 100 liver CT images (50 with lesions and 50 without) under five display conditions on a Macintosh computer: (1) color (equiluminant color contrast); (2) color-luminance (combined luminance and chromatic contrast); (3) flicker (luminance contrast that reversed polarity at 2 Hz); (4) contour (shaded intensity mapping); and (5) control (conventional gray scale). Receiver operating characteristics (ROC) techniques were used for analysis. RESULTS: The measured ROC curve areas for the different viewing conditions were as follows: control = 0.77 +/- 0.01 (mean +/- standard error of the mean); color = 0.78 +/- 0.01; color-luminance = 0.82 +/- 0.01; flicker = 0.78 +/- 0.01; and contour = 0.76 +/- 0.01. The percentage of lesions correctly located ranged from 0.82 (color-luminance) to 0.75 (flicker). Performance under the color-luminance condition was significantly better than in the control condition (p = .01), whereas the other experimental conditions were not significantly different from the control condition (p > .21). CONCLUSION: The use of mixed color and luminance displays may have perceptual advantages for radiologists and can improve performance over that of gray-scale viewing.

Color↗
PubMed1995

Pictorial depth cues: a new slant.

Pictorial depth cues such as perspective projection, aspect ratio, and texture gradients can specify mathematically the slant of a planar surface. We performed experiments to measure the accuracy of human perception of surface slant from these cues. We calculated the perceived slant from judgments of the relative lengths of a pair of orthogonal lines embedded in the surface. Our results indicate that slant judgments are accurate to within 3 deg. This level of accuracy was achieved whether the cues were luminance differences or equiluminous color differences. We found no evidence of the recession to the frontal plane that has been reported by Gibson [J.J. Gibson, The Perception of the Visual World (Houghton Mifflin, Boston, Mass., 1950]) and others. We did find evidence suggesting that subjects do not make accurate depth estimates of disconnected surfaces. This may be the source of the discrepancy between our measures and those of Gibson and others. This research, combined with previous findings, supports a model of perception that involves at least two and possibly more representations of space: one local veridical representation of surface orientation derived primarily from pictorial cues and another global representation of observer-centered distance derived primarily from binocular disparity and motion parallax.

Depth Perception↗
PubMed1994

Visual feature integration with an attention deficit.

Treisman's feature integration theory proposes that the perception of illusory conjunctions of correctly encoded visual features is due to the failure of an attentional process. This hypothesis was examined by studying brain-damaged subjects who had previously been shown to have difficulty in attending to contralesional stimulation. These subjects exhibited a massive feature integration deficit for contralesional stimulation relative to ipsilesional displays. In contrast, both normal age-matched controls and brain-damaged subjects who did not exhibit any evidence of an attention deficit showed comparable feature integration performance with left- and right-hemifield stimulation. These observations indicate the crucial function of attention for visual feature integration in normal perception.

Aged↗
PubMed1994

Interhemispheric depth judgement.

Interhemispheric depth comparisons were studied by requiring subjects to align in depth two textured plates, one presented to the left hemifield and the other to the right. Callosal agenesis subjects and neurologically-normal control subjects adjusted the plates so that they appeared to be at the same distance. Subjects viewed the plates monocularly or binocularly while keeping their head still, moving it side-to-side or moving it up and down. Subjects fixated a target located between the two plates while performing the task. For all subjects, the results showed that the deviations from veridical settings were significantly smaller for the binocular than for the monocular viewing conditions. Moreover, there were no significant differences among the three binocular viewing conditions (horizontal, vertical or no head movement), indicating that neither vertical nor horizontal motion parallax improves the precision of depth judgement when binocular disparity is available. These results further suggest that the precision of interhemispheric comparison for binocular depth is not affected by the absence of the corpus callosum. Looking at the plates monocularly, the control subjects judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that motion parallax is a useful depth cue when relative motion is extracted from different hemifields. Unlike the control subjects, the callosal agenesis subjects did not judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that interhemispheric comparison of depth using relative motion is not possible without the corpus callosum.

Adult↗
PubMed1994

Motion capture of luminance stimuli by equiluminous color gratings and by attentive tracking.

Two experiments demonstrated motion capture of luminance-defined dots by gratings with no net luminance-based motion. In a series of two-frame experimental trials, we superimposed bright dots and a color grating rotating in opposite directions. Capture was observed at equiluminance and was facilitated by the presence of color in gratings over a range of luminance contrasts. In a second experiment, observers noted that when a counterphase grating was tracked in either direction with attention, the superimposed dots were captured in that direction. These results suggest motion capture is supported not only by luminance-based motion, but also by color- and attention-based motion. Indeed, we suggest that the most parsimonious explanation is that all capture is mediated by attention.

Attention↗
PubMed1994

Familiarity and pop-out in visual search.

In this paper, we report that when the low-level features of targets and distractors are held constant, visual search performance can be strongly influenced by familiarity. In the first condition, a [symbol: see text] was the target amid [symbol: see text]s as distractors, and vice versa. The response time increased steeply as a function of number of distractors (82 msec/item). When the same stimuli were rotated by 90 degrees (the second condition), however, they became familiar patterns--2 and 5--and gave rise to much shallower search functions (31 msec/item). In the third condition, when the search was for a familiar target, N (or Z), among unfamiliar distractors, [symbol: see text]s (or [symbol: see text]s), the slope was about 46 msec/item. In the last condition, when the search was for an unfamiliar target, [symbol: see text] (or [symbol: see text]), among familiar distractors, Ns (or Zs), parallel search functions were found with a slope of about 1.5 msec/item. These results show that familiarity speeds visual search and that it does so principally when the distractors, not the targets, are familiar.

Adolescent↗
PubMed1993

Influence of magnetic resonance imaging on a knee arthroscopy waiting list.

Magnetic resonance imaging (MRI) scans were performed on 155 patients who had been placed on a diagnostic knee arthroscopy waiting list 4 to 18 months previously. In all, 32 per cent were removed from the waiting list following this investigation. A total of 24 patients on the waiting list improved spontaneously, including 13 per cent of those with MRI-demonstrable meniscal tears. MRI had an accuracy of 93 per cent, a sensitivity of 100 per cent and a specificity of 67 per cent using arthroscopy as a standard; the false-positive rate was 7 per cent and the false-negative rate 1 per cent. It is concluded that the use of MRI can reduce the requirements for diagnostic arthroscopy, but that the ideal time for MRI is not clear.

Adult↗
PubMed1993

Transparent surfaces defined by implicit X junctions.

An X junction is known to be a strong cue for transparency whereas a T junction typically indicates occlusion by an opaque surface. In this article, however, we will demonstrate two cases in which special T junctions (implicit X) can support the perception of a transparent surface. The T junction is perceived as having an additional illusory contour rendering it as an implicit X junction. There is physically realizable condition in which transparency can produce a T junction and the existence of this special case may explain why T junctions are not necessarily taken as cues for opaque surface. The similar processing for X junctions and implicit Xs to form transparent surfaces is suggested.

Adult↗
PubMed1993

The perception of form and motion.

Although form and motion are two distinct aspects of visual processing, they do not start as separate entities in the visual system. Early analyses extract discontinuities in various image attributes and these can trace the outline of a form. When displaced, the same image features can give rise to impressions of motion. Recent work has overturned many of the assumptions about the contributions of different stimulus attributes to motion processing, and reorganized the classification of motion systems. The results have revealed unexpected interactions between attention and motion. Paralleling this research is work on the early stages of form learning and on the nature of the stored representations.

Attention↗
PubMed1993

Surface decomposition accompanying the perception of transparency.

At the retina, each location can have only one value of luminance or color. When transparency is perceived, however, different surface qualities can be redistributed to two or more apparently superimposed layers. The experiments described here explored the characteristics of this surface decomposition. It is shown that the surface decomposition occurs rapidly, it affects even early stages of visual processing, and it involves attributes such as texture and motion as well as color and brightness. In the first experiment, the recognition advantage for transparent overlapping digits demonstrated that the surface decomposition accompanying transparency occurs within 60 ms. In the second, the separation of overlying orthogonal grids due to surface decomposition was found to influence the strength of the McCollough effect, an effect attributed to early cortical processing. Finally, when a transparent surface appears to extend over areas that are physically identical to the background, qualities of the transparent overlay such as texture and motion, as well as color or brightness (e.g., the neon color effect) appear to spread to the illusory overlay.

Color Perception↗
PubMed1992

Attention-based motion perception.

Two "attentive" tracking tasks reveal the existence of an attention-based motion process. In the first task, oppositely rotating luminance and color gratings were superimposed. Because of masking from the color grating, the bars of the luminance grating were not visible; nevertheless, their motion was visible and it determined the perceived direction of the stimulus rotation. On the other hand, the bars of the color grating were visible but they could only be seen to move (in the opposite direction to the overall stimulus rotation) when they were tracked with attention. In a second task, the perceived velocity of a color grating, typically slow at equiluminance, speeded up when individual bars were attentively tracked. These findings demonstrate two independent motion processes: one that is "low-level" or automatic in that it signals motion even in the absence of attention to the stimulus, and one that is mediated by attention to visible features and provides accurate velocity judgments independently of the features being tracked.

Attention↗
PubMed1992

Visual persistence of figures defined by relative motion.

In order to measure visual persistence of figures that were solely defined by relative motion (motion-defined figures or motion figures), random-dot kinematograms were used to form stimulus figures in the two-frame, missing element task introduced by Di Lollo, V. (1977 Nature, 257, 241-243). Experiment 1 showed that motion-defined figures persisted for about 130 msec after the termination of the stimulus presentation (i.e. after the dots stopped moving). This was similar to but several tens of milliseconds longer than the visual persistence of figures which were defined by a luminance difference (luminance-defined figures or luminance figures) in the same random-dot pattern. Since motion detectors are not found in the retina or lateral geniculate in primates, our results strongly suggest that visual persistence is not only a retinal phenomenon but also a cortical one. Experiment 2 investigated the possible influence of motion aftereffects on the visual persistence of motion figures. The results showed that coherent movement of the dots over the whole display after the stimulus offset did not reduce the visual persistence of motion figures, suggesting that the source of this persistence is not a motion aftereffect. In Experiment 3, visual persistence for the motion-defined figures was shown to be longer than that for luminance-defined figures independently of the contrast of the stimulus figure as long as the stimuli could be seen clearly enough. This suggests that different mechanisms are involved in the visual persistence of motion-defined and luminance-defined figures.

Afterimage↗
PubMed1992

Depth capture and transparency of regions bounded by illusory and chromatic contours.

Spillmann and Redies noted that when a transparent textured pattern is held above the Ehrenstein figure, the subjective surfaces appear to lie not in the plane of the figures but in the plane of the overlying texture. In Experiment 1, we tested this phenomenon with chromatic squares and found that the perceived depth of regions bounded by the chromatic contours was captured by overlying texture planes when the square was equiluminous with the background. We then tested this phenomenon with a variety of illusory contour stimuli and found that it only occurs with figures involving fine line terminators, and not, for example, with the solid Kanizsa triangle. These results suggest that chromatic contours and the illusory contours induced by line terminators provide only weak binocular disparity signals and that these signals are easily overwhelmed by the disparity signals from the overlying luminance texture.

Color Perception↗
PubMed1992

Mental rotation, physical rotation, and surface media.

Subjects made mirror-normal discriminations on alphanumeric characters shown in different orientations in the picture plane. Either the characters or the background rotated during stimulus presentation in Experiments 1-3. Character rotation in the direction of mental rotation facilitated mental rotation, whereas rotation in the opposite direction inhibited it. In Experiment 4, characters were presented in different surface media so as to stimulate only one representation at a time. Mental rotation performance was similar whether the stimuli were defined by luminance, color, texture, relative motion, or binocular disparity, suggesting that mental rotation occurs at a level beyond that of the independent analyses of these different media. These results support those of Experiments 1-3 in excluding the participation of low-level motion analysis centers in the mental rotation processes.

Adult↗
PubMed1992

The role of transparency in perceptual grouping and pattern recognition.

The shortest stimulus exposure time for which transparency can be seen was examined. In the first experiment, overlapping digits were presented for 120 ms and the luminance in the overlapping regions was varied. Subjects reported, in separate blocks of trials, either the apparent transparency of the digits or the identity of the digits. When the luminance was set so that one set of digits appeared to be seen through the other, recognition of the digits was high. When the luminance in the overlapping regions did not produce impressions of transparency, digit recognition was low. In the second experiment, digit identification at several stimulus durations was compared between stimuli that had luminance that was valid for transparency and stimuli that had invalid luminance. Performance was found to be higher in the valid luminance condition than in the invalid condition after as little as 60 ms exposure duration. This result suggests that the impression of transparency requires only relatively short exposure durations.

Adult↗