PubMed Health⌕ Search

Biomedical subjects

Stuart Anstis

Publications and source records attributed to Stuart Anstis.

12 recordsLinked to original sources
PubMed2006

Selectivity for the configural cues that identify the gender, ethnicity, and identity of faces in human cortex.

We used psychophysical and functional MRI (fMRI) adaptation to examine how and where the visual configural cues underlying identification of facial ethnicity, gender, and identity are processed. We found that the cortical regions showing selectivity to these cues are distributed widely across the inferior occipital cortex, fusiform areas, and the cingulate gyrus. These regions were not colocalized with areas activated by traditional face area localizer scans. Traditional face area localizer scans isolate regions defined by stronger fMRI responses to a random series of face images than to a series of non-face images. Because these scans present a random assortment of face images, they presumably produce the strongest responses within regions containing neurons that are face-sensitive but not highly tuned for face type. These areas might be expected to show only weak selective adaptation effects. In contrast, the largest responses to our selective adaptation paradigm would be expected within areas containing more selectively tuned neurons that might be expected to show only a sparse collective response to a series of random faces. Many aspects of face processing (e.g., prosopagnosia, recognition, and configural vs. featural processing) are likely to rely heavily on regions containing high proportions of neurons that show selective tuning for faces.

Cerebral Cortex↗
PubMed2006

Induced movement: the flying bluebottle illusion.

Two small objects (flies) followed identical circular orbits. However, a large background that circled around behind them in different phases made one orbit look twice as large as the other (size illusion) or made the circles look like very thin horizontal or vertical ellipses with aspect ratios of 7.5:1 or more (shape illusion). The nature of the perceptual distortion depended upon the relative phase between the movements of the background and those of the flies. Brief snatches of the moving background that added up to a circular motion were also effective.

Humans↗
PubMed2006

In honour of Lothar Spillmann - filling-in, wiggly lines, adaptation, and aftereffects.

I have studied a number of visual phenomena that Lothar Spillmann has already elucidated. These include: Neon spreading: when a small red cross is superimposed on intersecting black lines, the red cross seems to spread out into an illusory disk. Unlike the Hermann grid, neon spreading is relatively unaffected when the black lines are curved or wiggly. This suggests that the Hermann grid, but not neon spreading, involves long-range interactions. Neon spreading can be shown in random-dot patterns, even without intersections. It is strongest when the red crosses are equiluminous with the gray background. Adaptation, aftereffects, and filling-in: direct and induced aftereffects of color, motion, and dimming. Artificial scotomata and filling-in: the "dam" theory is false. Staring at wiggly lines or irregularly scattered dots makes them gradually appear straighter, or more regularly spaced. I present evidence that irregularity is actually a visual dimension to which the visual system can adapt. Conjectures on the nature of peripheral fading and of motion-induced blindness. Some failed experiments on correlated visual inputs and cortical plasticity.

Adaptation, Physiological↗
PubMed2006

Background stripes affect apparent speed of rotation.

A gray line that rotated about its own center against a stationary background of vertical stripes appeared to double in perceptual speed as it rotated through the vertical position and thus momentarily aligned with the background. Four factors may contribute to this speed-up: (i) landmarks, in which the tip of the moving vertical line moves horizontally across the maximum number of stationary stripes; (ii) orientation repulsion of the moving line by the vertical stripes, which may distort the line's perceived position and hence its perceived speed; (iii) the orientation of an induced brightness pattern along the line; and (iv) the motion of the induced brightness pattern, which moves physically most rapidly along the line when the line is near vertical. To test these possibilities, an annulus display provided landmarks but no intersections, and this almost abolished the effect. A rotating-slit display provided an oriented, moving pattern that mimicked the induced brightness but had no landmarks, and this increased the effect. We conclude that the motion, but not the orientation, of the intersections [option (iv)] was responsible for the illusion. The fact that this motion along the length of the line affected the perceived speed of the line orthogonal to its own length indicates a failure on the part of the visual system to fully decouple tangential from radial motion.

Contrast Sensitivity↗
PubMed2005

Local and global segmentation of rotating shapes viewed through multiple slits.

Rotating outline squares and circles were viewed through a sunburst pattern of stationary radial slits. At slow rotation rates the (dotted) square was perceived globally as a single rotating shape, and at higher rates, as a set of independent local dots moving in and out radially. An eccentrically rotating circle was seen as a dotted circle; the dots comprising the circle actually moved in and out along straight radial paths, but observers could never see this. Instead, they saw the dots as running around the rim of the circle. The common motions were rejected, perhaps by subtracting the mean motion of all points from each point. Only relative motion could be seen, and absolute dot motions were not available to consciousness. Thus the visual motion system parsed patterns of absolute motion vectors into patterns of relative motion vectors.

Humans↗
PubMed2005

Last but not least.

When two three-letter words are flashed up in sequence, observers cannot tell whether the top halves of the words are the same or different. It follows that words, like faces, are processed holistically, not as a set of separate features.

Brain↗
PubMed2004

Factors affecting footsteps: contrast can change the apparent speed, amplitude and direction of motion.

Contrast can affect the apparent speed of a moving stimulus. Specifically, when a grey square drifts steadily across stationary black and white stripes, it appears to stop and start as its contrast changes--the so-called 'footsteps illusion'. We now show that what matters is the contrast of the leading and trailing edges, not of the lateral edges. The stripes act by altering the stimulus contrast, and are not merely stationary landmarks. Back and forth apparent motion appears smaller in amplitude at low contrasts, even on a spatially uniform (non-striped) surround, and this is a specific motion phenomenon, not a result of misjudging static position. Contrast also affects the perceived direction of a moving stimulus. A vertically jumping grey diamond on a surround of black and white quadrants appears to change its direction of movement depending on the relative contrast of its left-oblique versus right-oblique edges against the surround. Thus, the perceived direction, amplitude and speed of moving objects depend greatly on their luminance contrast against the surround. A model of motion coding is proposed to explain these results.

Adult↗
PubMed2004

Metacontrast masking is specific to luminance polarity.

UNLABELLED: A 1 degrees -spot was flashed up on a screen, followed by a snugly fitting annular mask. We measured the amount of masking as a function of stimulus luminance. The surround was always mid-gray, the masking ring was either black or white, and the luminance of the spot target ranged from 0% to 100% of white in 4% steps. Observers reported the apparent lightness of the masked spot by adjusting a matching spot. RESULTS: A black annular mask made all spots that were darker than the gray surround appear to be transparent, that is, of the same luminance as the surround (complete masking). The black ring had virtually no masking effect on spots that were lighter than the surround. Conversely, a white ring made all spots that were lighter than the gray surround look apparently the same luminance as the surround (complete masking), but had virtually no masking effect on spots that were darker than the surround. In summary, a black ring masked spatial decrements but not increments, whilst a white ring masked spatial increments but not decrements. Thus masking occurred only when the spot and the ring had the same luminance polarity. This same-polarity masking still occurred when the target spot was larger than the 'donut hole' of the masking ring, so that the target and ring partly overlapped. This ruled out simple edge-cancellation theories. Instead, masking disrupts the filling-in process that normally propagates inward from the edges of a spot [Vision Res. 31 (7-8) (1991) 1221]. We conclude that metacontrast masking occurs within, but not between, separate visual ON and OFF pathways.

Contrast Sensitivity↗
PubMed2002

The Purkinje rod-cone shift as a function of luminance and retinal eccentricity.

UNLABELLED: In the Purkinje shift, the dark adapted eye becomes more sensitive to blue than to red as the retinal rods take over from the cones. A striking demonstration of the Purkinje shift, suitable for classroom use, is described in which a small change in viewing distance can reverse the perceived direction of a rotating annulus. We measured this shift with a minimum-motion stimulus (Anstis & Cavanagh, Color Vision: Physiology & Psychophysics, Academic Press, London, 1983) that converts apparent lightness of blue versus red into apparent motion. We filled an iso-eccentric annulus with radial red/blue sectors, and arranged that if the blue sectors looked darker (lighter) than the red sectors, the annulus would appear to rotate to the left (right). At equiluminance the motion appeared to vanish. Our observers established these motion null points while viewing the pattern at various retinal eccentricities through various neutral density filters. RESULTS: The luminous efficiency of blue (relative to red) increased linearly with eccentricity at all adaptation levels, and the more the dark-adaptation, the steeper the slope of the eccentricity function. Thus blue sensitivity was a linear function of eccentricity and an exponential function of filter factor. Blue sensitivity increased linearly with eccentricity, and each additional log(10) unit of dark adaptation changed the slope threefold.

Adult↗
PubMed2002

The boogie-woogie illusion.

A grid of vertical and horizontal lines, each composed of light and dark squares, is moved rigidly at 45 degrees to the vertical on a gray surround. When the luminance of the background is set midway between the luminances of the light and dark squares, the squares appear to race along the lines even though they are actually 'painted' on the lines. The effect arises from the unequal apparent speeds of the lines and their textures. The light and dark squares along the lines define a first-order pattern whose apparent speed, parallel or along the line, is close to veridical. The lines themselves have no overall luminance difference from the background so that they are defined by a second-order difference. As reported elsewhere, apparent speed is reduced for second-order motion so that the motion perpendicular to the line is perceived as slower than the motion along the line even though they are physically equal. The imbalance creates the impression that the small squares are moving along the lines rather than moving rigidly with them.

Form Perception↗
PubMedDate not supplied

Moving objects appear to slow down at low contrasts.

Moving cars give the illusion of slowing down in foggy conditions, because low contrast reduces perceived speed. A grey square that drifts horizontally across a surround of black and white vertical stripes appears to stop and start as it crosses each stripe, because its contrast keeps changing. A moving square whose vertical and horizontal edges have different contrasts will show illusory distortions in perceived direction. Contrast also affects the apparent amplitude and salience of back-and-forth apparent motion. Finally, a line of black and white dots on a grey surround moves in illusory directions, because of a mismatch in the contrasts along and across the dotted line. Thus, motion signals in the early parts of the visual system are profoundly altered by stimulus luminance and contrast. This suggests that motion is coded by the relative firing rates of neural channels tuned to fast and slow motion, with contrast-dependence being a motion analog of the Bezold-Brucke hue shift.

Accidents, Traffic↗