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Biomedical subjects

P Cavanagh

Publications and source records attributed to P Cavanagh.

At least 73 records · Page 4Linked to original sources
PubMed1992

Vision with equiluminant colour contrast: 2. A large-scale technique and observations.

A simple technique is described for producing large-scale, tritanopic displays. The technique reproduces the various phenomena of vision with equiluminous-colour contrast that have previously been reported with red/green stimuli. It is, however, much less demanding technically, robust against artifacts, and can be used on large-scale scenes. One advantage of the technique is that a piece of blue filter can be used individually by each observer to compare quickly tritanopic and luminance conditions.

Attention↗
PubMed1992

Achromatic form perception is based on luminance, not brightness.

Two figures were examined, one a subjective disk and the other a cup whose shape was revealed by shadows. The figures were presented in a single color on a background of a different color, and the observers adjusted the radiance of one color until, in the first case, the vividness of the subjective contour reached a minimum (minimum subjective contour) or, in the second case, the impression of depth that is due to shadows disappeared (shadow disappearance). The results for these two tasks followed the data for minimum flicker matches (made with the same stimuli) much more closely than those for direct brightness matching. We therefore claim that achromatic form perception in general and subjective contour and shadow perception in particular are based on the intensity dimension measured by flicker photometry, not on that measured by brightness matching. Finally, in agreement with these findings, bleaching of short-wavelength sensitive cones did not affect settings for subjective contours, shadows, or flicker photometry but did affect brightness matching.

Color Perception↗
PubMed1992

Orientation-contingent color aftereffects mediated by subjective transparent structures.

We examined whether the orientation-contingent color aftereffect (the McCollough effect) could be mediated by subjective horizontal and vertical structure induced by the perception of transparency. In our experiments, red vertical bars and green horizontal bars were alternated as an adapting stimulus. After adaptation, subjects (n = 6) were asked to adjust the green and red saturation of a test pattern until they obtained a neutral gray. Horizontal and vertical stripes were combined in the test pattern in three different ways: (1) overlapping with a luminance combination that gave rise to a perception of transparent overlays of horizontal and vertical stripes (valid transparency condition), (2) overlapping with luminance combinations that did not induce a perception of transparency (invalid transparency condition) and that appeared more as a patchwork of checks, and (3) presented in adjacent, nonoverlapping areas. Our results showed that the McCollough effect was significantly greater in the valid transparency condition than in the invalid transparency conditions. The effect in the valid transparency condition was nevertheless less strong than was the effect seen with the standard test stimulus made up of nonoverlapping vertical and horizontal stripes. Our results suggest that the McCollough effect can be mediated by the subjective spatial organization (inner representation of vertical and horizontal stripes) that accompanies the perception of transparency in our stimulus.

Adult↗
PubMed1991

The contribution of color to motion in normal and color-deficient observers.

By opposing drifting luminance and color gratings, we have measured the "equivalent luminance contrast" of color, the contribution that color makes to motion. We found that this equivalent contrast was highest (greater than 10%) for low spatial and temporal frequencies and was higher for red/green than for blue/yellow stimuli. Equivalent luminance contrast was about 4% for a green/purple stimulus that fell along the tritan confusion line, indicating a modest input to the motion pathway from the short wavelength-sensitive cones (B-cones). Contrast thresholds for the discrimination of the direction of motion showed that the contribution of color to motion was about the same (within a factor of 2) as that for luminance in terms of multiples of threshold contrast. These responses to moving, chromatic gratings could be mediated by any of several factors that can create a residual response in a luminance pathway: temporal phase lag between the responses to the colors of the stimuli, second harmonic distortion in the response and variability in equiluminance points across units. Each of these factors was evaluated experimentally and their combined effect could account for only a small portion of the contribution of color to motion. As a result, we attribute the perception of the motion of equiluminous stimuli to an opponent-color input to directionally selective cortical units. Chromatic stimuli had little or no equivalent contrast for color-deficient observers, whether the stimulus was red/green, which they discriminate less well than normals, or blue/yellow, which they discriminate almost as well as normals. The equivalent contrast measure provided an excellent basis for classifying normal, protan and deutan observers.

Color↗
PubMed1991

Texture and motion spreading, the aperture problem, and transparency.

When a cross of either static or moving texture is inserted in the intersection of an Ehrenstein figure, we find that the texture and its motion appear to spread outside of the cross. We suggest that these phenomena are analogous to neon color spreading, wherein an interpretation of a transparent surface makes the properties of that surface, which are physically visible only inside the cross, appear to spread over its apparent extent. In our experiments, the texture and the motion of the cross are attributed to the transparent surface. In addition, the spreading changes the apparent aperture within which the texture and its motion are seen, leading to a corresponding change in the apparent direction of motion.

Adult↗
PubMed1991

Fortysomething: recognizing faces at one's 25th reunion.

Twenty-four to 26 years after their high school graduation, subjects matched 40 photographs of former classmates taken from the high school yearbook with 40 current photographs of these same former classmates. Compared with age-matched control subjects who had no prior familiarity with the persons depicted in the photographs, the former classmates were superior at matching the old and new photographs. Generally, same-orientation photographs (i.e., old and new photographs taken from the same side) were matched more accurately than different-orientation photographs. Prior familiarity, however, rendered the orientation factor unimportant. These findings are discussed in terms of the processes that underlie facial recognition over very long intervals and in terms of their psycholegal implications.

Adult↗
PubMed1991

Purely chromatic perception of motion in depth: two eyes as sensitive as one.

Motion hyperacuity (phase) thresholds were measured for both lateral and stereoscopic oscillatory motion in both luminance and equiluminant red/green gratings of 2 cycles per degree. Thresholds for lateral chromatic motion did not exhibit the inhibitory fall-off at low temporal frequencies that was found for luminance motion. Phase thresholds for purely chromatic motion were substantially higher than those for luminance gratings, in proportion to the ratio of cone signal modulation, but they could be predicted from the corresponding contrast sensitivities for both types of stimulus. Stereomovement thresholds in luminance gratings showed the stereomovement suppression effect relative to monocular motion sensitivity previously reported for line stimuli, but purely chromatic gratings did not. Together with the lack of an inhibitory fall-off, these results imply that chromatic and luminance motion are processed by different neural pathways, and that the chrominance pathway is capable of supporting a strong percept of stereoscopic motion from purely chromatic gratings.

Attention↗
PubMed1990

Comparing the cerebral hemispheres on the speed of spatial shifts of visual attention: evidence from serial search.

We compared the speed at which visuo-spatial attention may be shifted from one stimulus to another as a function of the visual hemifield in which the items were displayed in a visual search task requiring serial processing. The increase in response time with the number of items displayed was similar for left- and right-hemifield presentations. This suggests that the rate at which visuo-spatial attention can be shifted from one stimulus to another during visual search does not differ between the cerebral hemispheres.

Adult↗
PubMed1990

ISI produces reverse apparent motion.

A moving random-dot stimulus was presented in two sequential frames separated by an interstimulus interval (ISI) during which the field was spatially uniform with luminance equal to either the average luminance of the stimulus field (grey) or that of the black dots (black). In Experiment 1, black ISIs did not affect perception of motion direction but grey ISIs produced motion in the direction opposite to the physical displacement (reverse motion). In Experiment 2, the contrast of the stimulus was reversed simultaneously with the displacement of the random-dot fields so that reverse motion would be seen with no ISI [Anstis & Rogers, Vision Research, 15, 957, 1975]. In this condition, grey ISIs reversed the reverse motion to produce a veridical perception. Finally, in Experiment 3, we examined whether the negative image that follows the stimulus offset was the source of the reversal in motion direction. A gradual offset of the stimulus necessarily reduces the amplitude of the negative response at stimulus offset and also reduced the frequency of seeing reverse motion, suggesting that the apparent reversal of motion direction with ISI can be attributed to the negative phase of a biphasic impulse response function. A simulation of the temporal response to the displacements of random-dot fields demonstrated that the negative phase of a biphasic impulse response function is sufficient to produce the reverse motion. We therefore claim that there is a significant biphasic temporal response function that precedes the analysis of motion in the visual system. This indicates that the overall temporal response function of the visual system is the result of a cascade of functions from early through late stages and that only a portion of the overall temporal response function can be attributed to stages involved in motion analysis.

Contrast Sensitivity↗
PubMed1990

Independent orientation-selective mechanisms for the cardinal directions of colour space.

Simultaneous, oppositely tilted, aftereffects were obtained for pairs of equiluminant gratings (e.g. magenta/cyan vs purple/greenish-yellow) and pairs of equiluminant and luminant gratings (e.g. magenta/cyan vs dark-grey/light-grey, or purple/greenish-yellow vs dark-grey/light-grey). Maximum aftereffects occurred for gratings whose colours were modulated along the principle axes of the colour space of Krauskopf, Williams and Heeley (1982): an axis of short-wavelength cone activation (B-cone axis), the difference between long- and medium-wavelength cone activation (R-G cone axis), and an axis of a contrast ratio of long-, medium- and short-wavelength cone activation (R + G + B cone axis). The results indicate that selectivity for orientation occurs independently in at least two chromatic channels and one achromatic channel.

Adaptation, Ocular↗
PubMed1990

Effect of surface medium on visual search for orientation and size features.

By using a visual search task, this study examined the encoding of orientation and size for stimuli defined in five different surface media: luminance, color, texture, relative motion, and binocular disparity. Results indicated a spatially parallel analysis of size and orientation features for all surface media, with the possible exception of binocular disparity. The data also revealed a search rate asymmetry in the orientation task for all media: Parallel or shallow search functions were obtained for oblique targets in vertical distractors, whereas steeper serial search functions were obtained for vertical targets in oblique distractors. No consistent asymmetry was found for the large and small targets in the size task. There seemed to be common principles of coding in all these different media, suggesting either a single analysis of shape features applied to a common representation or multiple analyses, one for each surface medium, with each extracting a similar set of features. The shared coding principles may facilitate the use of redundancy across media to reduce ambiguities in the locations and shapes of contours in the visual scene.

Adult↗
PubMed1989

Interattribute apparent motion.

Apparent motion can be seen between two alternating stimuli even if they are defined with respect to their background by attributes other than luminance (such as color, or texture). We measured motion strength as the maximum separation between two alternating stimuli which produced an impression of motion, for conditions in which the two stimuli were defined by the same attribute (intra-attribute) as well as conditions in which they were defined by different attributes (interattribute). The attributes used to define the stimuli were luminance, color, texture, relative motion, or stereopsis. The results indicate that motion was seen for all the intra-attribute conditions about equally well. The results also show that interattribute motion could be seen for all combinations studied. The motion strength in these cases was about 80% of that for the intra-attribute conditions. The process responsible for this motion perception must therefore be able to combine information from different attributes.

Adult↗
PubMed1989

Saccadic suppression of low-level motion.

We measured the detection of motion before, during and after a saccade to explore the effects of a saccade on motion perception. To isolate the low-level motion mechanism, the stimulus was a random-dot field displaced by small distance (0.3 deg) within a stationary frame. The displacement signaled motion clearly if eyes were fixated, but for the displacement during a saccade, motion was not detected whether the displacement was defined in spatial coordinates (expt 1) or in retinal coordinates (expt 2). Since motion could be seen with ISIs longer than the duration of a saccade (expt 3), the suppression cannot be attributed to visual loss during the saccade. Experiment 3 also showed that motion was never seen for a displacement that occurred during a saccade, even though the random dots were replaced by a uniform field during the eye movement thereby eliminating any masking effect of the sweep of the image across the retina. The purpose of the saccadic suppression of motion may be to block out unreliable motion signals that would be produced by a saccade. Since saccade distances are very often greater than the maximum distance over which the low-level motion mechanism can produce accurate direction discrimination for fine textures, motion signals would generally indicate false directions if they were not suppressed.

Eye Movements↗
PubMed1989

Shape from shadows.

The colors, textures, and shapes of shadows are physically constrained in several ways in natural scenes. The visual system appears to ignore these constraints, however, and to accept many patterns as shadows even though they could not occur naturally. In the stimuli that we have studied, the only requirements for the perception of depth due to shadows were that shadow regions be darker than the surrounding, nonshadow regions and that there be consistent contrast polarity along the shadow border. Three-dimensional shape due to shadows was perceived when shadow areas were filled with colors or textures that could not occur in natural scenes, when shadow and nonshadow regions had textures that moved in different directions, or when they were presented on different depth planes. The results suggest that the interpretation of shadows begins with the identification of acceptable shadow borders by a cooperative process that requires consistent contrast polarity across a range of scales at each point along the border. Finally, we discuss how the identification of a shadow region can help the visual system to patch together areas that are separated by shadow boundaries, to identify directions of surface curvature, and to select a preferred three-dimensional interpretation while rejecting others.

Attention↗
PubMed1989

Motion: the long and short of it.

Several authors have proposed that motion is analyzed by two separate processes: short-range and long-range. We claim that the differences between short-range and long-range motion phenomena are a direct consequence of the stimuli used in the two paradigms and are not evidence for the existence of two qualitatively different motion processes. We propose that a single style of motion analysis, similar to the well known Reichardt and Marr-Ullman motion detectors, underlies all motion phenomena. Although there are different detectors of this type specialized for different visual attributes (namely first-order and second-order stimuli), they all share the same mode of operation. We review the studies of second-order motion stimuli to show that they share the basic phenomena observed for first-order stimuli. The similarity across stimulus types suggests, not parallel streams of motion extraction, one short-range and passive and the other long-range and intelligent, but a concatenation of a common mode of initial motion extraction followed by a general inference process.

Adaptation, Ocular↗
PubMed1989

A new test of luminous efficiency for babies.

We used the minimum motion method devised by Anstis and Cavanagh (1983) to measure the luminous efficiency of red and green and of yellow and blue for "normal" 1- 3-month-old babies and for one 3-month-old boy destined to be color-deficient because of a deutan mother. Subjects watched a display which created apparent motion, the direction of which depended on the relative luminance of the colors. To determine the equiluminant points, we observed the optokinetic nystagmus elicited by the display as the relative luminance of the colors was varied. The equiluminant points of the normal mothers and their infants were similar to each other but different from those of the deutan mother and her son. Our new method demonstrates the early maturation of input from red and green cones into achromatic pathways. It can also be used to identify some color-deficient infants.

Color Perception↗
PubMed1988

Deep tectal cells in pigeons respond to kinematograms.

Deep tectal neurons in pigeons respond selectively to moving visual stimuli, and are inhibited by large background patterns moved in-phase with these stimuli. In this investigation we demonstrate that these same deep tectal neurons respond equally well to kinematograms as they do to traditional luminance contrast stimuli typically employed in visual experiments. Computer generated kinematograms, the motion domain equivalents of random dot stereograms, were used as stimuli in these experiments. These kinematograms, where a small centrally located set of random dots is moved coherently in one direction while the remaining dots are moved in a different direction, thus constitute a pure motion stimulus where the stimulus form is only visible in the dynamic pattern, but does not exist on any single frame. Both 'object' configured and 'hole' configured kinematograms were employed; the former appearing as regions of texture moving over, or in front of, the background texture, while the latter appear as windows through which a more distant textured surface is revealed. Extracellular recordings from isolated deep tectal cells showed that all units responded in a very similar manner whether the stimulus was an 'object' configured kinematogram or the more traditional luminance contrast variety. This similarity included directional selectivity, the in-phase inhibition anti-phase facilitation effect, and sensitivity to opposed motion independent of direction. However, when the kinematograms were configured as 'holes' none of the units tested responded to these stimuli. The significance of these observations for tectal functioning, image segmentation through motion and animal camouflage is discussed.

Animals↗