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PubMed1994

Anomalous and luminance contours produce similar angular induction effects.

One can shear a pattern of lines to produce an anomalous contour which has a perceptual influence similar to that of a straight line segment. Illusion effects have been found with configurations which contain these anomalous contours, as well as cross adaptation with respect to luminance contours. We have found that sheared-line contours will bias judgment of collinearity, ie perceived alignment, of a luminance contour. The angular induction effects are similar to those reported for interactions between luminance contours, and the same equation can be used to model both kinds of data. The results of this experiment support the neuroreductionist view that anomalous and luminance contours are processed at the same level of the nervous system. Additionally, we suggest that with both types of contour the perceptual system registers and responds to the alignment of local brightness differentials.

Adult↗

PubMed1995

Edge computation in human vision: anisotropy in the combining of oriented filters.

Above threshold, two superimposed sinusoidal gratings of the same spatial frequency (eg 1 cycle deg-1) and equal contrasts, and with orientations balanced around vertical, usually look like a compound structure containing vertical and horizontal edges. However, at large plaid angles (ie large differences between component orientations) and low plaid contrasts there is a tendency for the stimulus to appear as two overlapping gratings (component structure) with obliquely oriented edges. These dependencies of perceived spatial structure in plaids are incompatible with an edge-coding scheme that uses only circular filters to compute zero-crossings, but instead support the idea that different oriented filters can (compound percept) or cannot (component percept) be combined before edges are represented. Here, further evidence is presented in support of this hypothesis. Two-component plaid stimuli had plaid angles of 45 degrees or 90 degrees, and a range of plaid orientations (ie a range of orientations around which the plaid components were balanced). Observers indicated whether each stimulus was perceived as a compound or component structure for a range of plaid contrasts. In addition to angle and contrast effects, perceived spatial structure was also found to depend on plaid orientation: compound structures were perceived more often when the plaid components were balanced around the cardinal axes of the retina. It is suggested that the principles governing the combination of oriented-filter outputs might be learnt during the development of the visual system by using a Hebb-type rule: coactivated filters are more likely to combine their outputs when activated on future occasions. Given the prominence of vertical and horizontal orientations in a carpentered environment, this simple rule promotes a network that combines filters balanced around cardinal axes more readily than oblique axes, in agreement with the results.

Anisotropy↗

PubMed1995

Respective contribution of orientation contrast and illusion of self-tilt to the rod-and-frame effect.

The visual angle subtended by the frame seems to be an important determinant of the contribution of orientation contrast and illusion of self-tilt (ie vection) to the rod-and-frame effect. Indeed, the visuovestibular factor (which produces vection) seems to be predominant in large displays and the contrast effect in small displays. To determine how these two phenomena are combined to account for the rod-and-frame effect, independent estimates of the magnitude of each component in relation to the angular size subtended by the display were examined. Thirty-five observers were exposed to three sets of experimental situations: body-adjustment test (illusion of self-tilt only), the tilt illusion (contrast only) and the rod-and-frame test, each display subtending 7, 12, 28, and 45 deg of visual angle. Results showed that errors recorded in the three situations increased linearly with the angular size. Whatever the size of the frame, both mechanisms, contrast effect (tilt illusion) and illusory effect on self-orientation (body-adjustment test), are always present. However, rod-and-frame errors became greater at a faster rate than the other two effects as the size of teh stimuli became larger. Neither one nor the other independent phenomenen, nor the combined effect could fully account for the rod-and-frame effect whatever the angular size of the apparatus.

Adult↗

PubMed1996

A canonical microfunction for learning perceptual invariances.

An unsupervised method is presented which permits a set of model neurons, or a microcircuit, to learn low-level vision tasks, such as the extraction of surface depth. Each micro-circuit implements a simple, generic strategy which is based on a key assumption: perceptually salient visual invariances, such as surface depth, vary smoothly over time. In the process of learning to extract smoothly varying invariances, each microcircuit maximises a microfunction. This is achieved by means of a learning rule which maximises the long-term variance of the state of a model neuron and simultaneously minimises its short-term variance. The learning rule involves a linear combination of anti-Hebbian and Hebbian weight changes, over short and long time scales, respectively. The method is demonstrated on a hyperacuity task: estimating subpixel stereo disparity from a temporal sequence of random-dot stereograms. After learning, the micro-circuit generalises, without additional learning, to previously unseen image sequences. It is proposed that the approach adopted here may be used to define a canonical microfunction, which can be used to learn many perceptually salient invariances.

Computer Simulation↗

PubMed1996

Mach-band attenuation by adjacent stimuli: experiment and filling-in simulations.

Mach bands are illusory bright and dark bands seen where a luminance plateau meets a ramp, as in half shadows or penumbras. It has previously been shown that Mach bands are attenuated by placing stimuli, such as bars, nearby. It was shown in an experiment in which Mach-band attenuation for bar and Craik-O'Brien stimuli was compared that they are equally effective in attenuating Mach bands. The results suggest that the abrupt luminance transition of a stimulus adjacent to a ramp is responsible for the attenuation. The findings are interpreted in terms of a recent filling-in model of brightness perception and the results of computer simulations of stimuli are shown.

Attention↗

PubMed1996

Temporal-contrast discrimination and its neural correlates.

Reported differences in neuronal contrast processing between the parallel magnocellular (M) and parvocellular (P) visual pathways invite the hypothesis that contrast discrimination in the human visual system is more sensitive at low contrasts and less sensitive at high contrasts, for stimuli modulated at high compared with low temporal frequencies. In the present study, an edgeless temporally modulated uniform field was selected as the stimulus for psychophysical contrast discrimination, and contrast-increment thresholds for pedestal contrasts ranging from 5.5% to 78.2% were determined with a temporal two-alternative forced-choice staircase procedure. The increment thresholds for five normal subjects were adequately fit by power functions with exponents that shifted continuously from about 0.5 (square-root-law behavior) to about 1.0 (Weber's-law behavior) as stimulus temporal frequency increased from 1 to 30 Hz. A neural simulation, with the use of published contrast-response functions of magnocellular and parvocellular neurons, adjusted with an estimate of response variance, produced two distinct 'neural increment-threshold functions' that were similar to the psychophysical results obtained at the highest and the lowest temporal frequencies, respectively. A shift from a relatively more noise-limited neural mechanism to one whose response is predominantly determined by gain is suggested to account for the change of the contrast-increment-threshold function with increasing temporal frequency.

Adult↗

PubMed1998

Memory for language in deaf adults and children.

Cognitive psychologists have known for a long time that language and memory are intimately related in people who can hear. Why should the situation be any different for deaf children and deaf adults? This article considers the results of previous studies and some new findings in examining the possible impact of spoken language and sign language fluencies/preferences on the structure and process of memory in deaf individuals. Current evidence suggests that there are some differences in the organization of long-term memory in deaf as compared to hearing people, but no one has yet demonstrated such differences to be so large that they qualitatively or quantitatively affect learning in any real sense. In contrast, there is now abundant evidence to suggest that variation in spoken language abilities have a direct impact on memory span and perhaps on working memory more generally. These findings are discussed in terms of their implications for the education of students who are deaf or hard of hearing.

Adolescent↗

PubMedDate not supplied

Contrast constancy in natural scenes in shadow or direct light: A proposed role for contrast-normalisation (non-specific suppression) in visual cortex.

The range of contrasts in natural scenes is generally thought to far exceed the limited dynamic ranges of individual contrast-encoding neurons in the primary visual cortex. The visual system may employ gain-control mechanisms (Ohzawa et al. 1985) to compensate for the mismatch between the range of natural contrast energies and the limited dynamic range of visual neurons; one proposed mechanism is contrast normalisation or non-specific suppression (Heeger 1992a). This paper aims to evaluate the role of contrast normalisation in human contrast perception, using a computer model of primary visual cortex. The model uses orthogonal pairs of Gabor patches to simulate simple-cell receptive-fields to calculate local, band-limited contrast in a series of 50 digitised photographs of natural scenes. The average range of contrast energies in each image was 2.29 log units, while the "lifetime range" each model simple cell would see across all images was 2.98 log units. These ranges are greater than the dynamic range of real mammalian simple cells. Contrast normalisation (dividing contrast responses by the summed responses of all nearby neurons) reduces contrast ranges, perhaps sufficiently to match them to neurons' limited dynamic ranges. Comparison of images taken under diffuse and direct lighting conditions showed that contrast normalisation can sometimes match these conditions effectively. This may lead to perceptual contrast constancy in the face of spurious changes in contrast caused by natural environmental conditions.

Computer Simulation↗

PubMedDate not supplied

Critical band masking in optic flow.

Visual processing has been widely investigated with narrow band stimuli at low contrasts. We used a masking paradigm to examine how visual sensitivity under these conditions compares with the perception of the direction of heading in real scenes (i.e., with dynamic natural images at high contrasts). We first confirmed and extended previous studies showing biases in the amplitude distribution for spatial frequency, temporal frequency, speed and direction in dynamic natural movies. We then measured contrast thresholds for identification of the direction of motion for an observer traveling at various speeds. In spite of differences in contrast sensitivity and large non-uniformities in the amplitude content of the stimuli, contrast thresholds were relatively invariant of spatial frequency and completely invariant of temporal frequency, speed and direction. Our results suggest that visual processing normalises responses to supra-threshold structure at different spatial and temporal frequencies within natural stimuli and so equates their effective visibility.

Contrast Sensitivity↗

PubMedDate not supplied

Contrasting contributions of phonological short-term memory and long-term knowledge to vocabulary learning in a foreign language.

The contributions of phonological short-term memory and existing foreign vocabulary knowledge to the learning of new words in a second language were compared in a sample of 40 Greek children studying English at school. The children's speed of learning new English words in a paired-associate learning task was strongly influenced by their current English vocabulary, but was independent of phonological memory skill, indexed by nonword repetition ability. However, phonological memory performance was closely linked to English vocabulary scores. The findings suggest that in learners with considerable familiarity with a second language, foreign vocabulary acquisition is mediated largely by use of existing knowledge representations.

Adolescent↗

PubMedDate not supplied

Children's health and their mothers' risk of divorce or separation.

The purpose of this study is to determine how children's health conditions are related to their mothers' risk of divorce or separation. The study is based on data from over 7,000 children born to once-married mothers identified in the 1988 Child Health Supplement to the National Health Interview Survey. The effects of 15 childhood health conditions on the mothers' risk of divorce are estimated with Cox's proportional hazard models. Controlling for demographic, marital, and reproductive measures, we find that mothers' prospects for divorce are affected both positively or negatively by their children's health status, depending on the type of childhood condition and, in the case of low birth weight children, timing within the marriage. Women whose children have congenital heart disease, cerebral palsy, are blind, or had low birth weight appear to have higher risks of marital disruption than mothers of healthy children. In contrast, mothers whose children have migraines, learning disabilities, respiratory allergies, missing/deformed digits or limbs, or asthma have somewhat lower rates of divorce.

Adolescent↗

PubMed2004

Training-induced recovery of visual motion perception after extrastriate cortical damage in the adult cat.

Unilateral ibotenic acid lesions of the lateral suprasylvian (LS) cortex severely impair the ability of cats to integrate local motion signals (measured as direction range thresholds) and to extract motion signals from noise (measured as motion signal thresholds) in their contra-lesional visual hemifields. These deficits were found up to several months after the lesions and were limited to thresholds measured with random-dot stimuli, while contrast sensitivity for discriminating the direction of motion of sine-wave gratings remained unaffected. Our goal was to determine whether deficits of complex motion perception could recover and whether the recovery was spontaneous or required retraining. In each cat, a single location in the impaired visual hemifield was selected for visual retraining, which required the animals to discriminate motion direction using random-dot stimuli in which the range of dot directions was varied. Fifteen to 40 days of intensive retraining led to a gradual, complete recovery of motion integration. The recovery was stimulus specific since it did not transfer from direction range to motion signal thresholds, and it was largely restricted to the visual field locations retrained. Delaying the onset of retraining by several days to several months had no significant impact on the extent or rate of recovery. Once recovery was achieved, performance remained stable over a period of several months. These results suggest that recovery of complex visual motion perception after lesions of extrastriate visual cortex is an active process that requires extensive, stimulus- and retinotopically-specific visual retraining.

Animals↗

PubMed2000

Aging and the development of automaticity in conjunction search.

In two experiments, younger and older observers carried out feature searches for targets defined by their luminance contrast and orientation. Additionally, they received consistent-mapping (CM) training in luminance contrast by orientation conjunction search, followed by a brief exposure to conjunction search under reversal conditions. In Experiment 1, display size effects on reaction time suggested that both younger and older observers were conducting a parallel search in all conditions and showed equivalent disruption at reversal. Experiment 2 was a substantive replication of the first using more difficult conjunction search displays. In addition to latency, we measured the number, duration, and feature-based selectivity of fixations made during conjunction search. Display size effects were larger than in Experiment 2 and were of equivalent magnitude in younger and older people. There were no age differences in improvement in conjunction search and minimal age differences in disruption following reversal. Both age groups demonstrated early in training that they could select items possessing target features (i.e., the color white), and both age groups demonstrated that they could not completely reverse this selectivity when these features no longer defined the target. These experiments have several implications for models of visual attention and age differences therein.

Adult↗

PubMed1996

Neglect for low luminance contrast stimuli but not for high colour contrast stimuli: a behavioural and electrophysiological case study.

We describe a patient with a right hemisphere lesion involving the frontal lobe, the post-central gyrus and the superior parietal lobule. Behavioural testing demonstrated severe left unilateral neglect to low luminance contrast stimuli, but not to high colour contrast stimuli. Evoked potentials to low contrast luminance gratings presented in the left hemifield were not reliable. However, equiluminant coloured gratings presented in the same hemifield evoked reliable electrophysiological responses, although longer in latency than those evoked in the right hemifield. These findings suggest that the patient has severe damage of the high contrast sensitivity magnocellular pathway in the right hemisphere, with minor involvement of the parvocellular pathway.

Brain Damage, Chronic↗

PubMedDate not supplied

Word-reading thresholds in Alzheimer disease and mild memory loss: a pilot study.

PURPOSE: To determine whether subjects with early Alzheimer Disease (AD) or Mild Cognitive Impairment (MCI) would demonstrate significant abnormalities on tests of word reading threshold (WRT), spatial contrast sensitivity (SCS), and color discrimination (CD). METHODS: Prospective cohort study of 13 AD subjects, 13 subjects with Mild Cognitive Impairment (MCI), and 12 healthy elderly normal controls (ENC). CD was determined using the Farnsworth D-15 method. SCS was tested using a Vistech photographic chart. For WRT determination, pattern-masked words were presented successively at increasing target durations. The threshold was determined as the target duration at which 50% of the words were read. RESULTS: The mean number of errors in Color Discrimination and Spatial Contrast Sensitivity did not differ significantly among the three groups. WRT was significantly longer in the AD group (122.6 +/- 70.8 ms) in comparison with ENC group (53.8 +/- 15.9 ms) (p < 0.001). A WRT threshold limit of 85 msec correctly classified 11/12 ENC subjects and 10/13 AD subjects. Four of the 13 MCI subjects also exceeded this threshold and all four progressed to AD within 2 year follow-up. CONCLUSION: Word Reading Threshold determination discriminates mild AD subjects from ENC, and correlates with severity of cognitive impairment. It may have diagnostic and prognostic utility in early AD and MCI subjects.

Aged↗

PubMed2000

Adrenomedullary function during cognitive testing in attention-deficit/hyperactivity disorder.

OBJECTIVE: Reported correlations between epinephrine (EPI) excretion and classroom performance, the cognition-enhancing effects of EPI infusion, increased EPI excretion with stimulants, and reports of decreased EPI excretion in attention-deficit/hyperactivity disorder (ADHD) suggest that sympathoadrenomedullary function might be altered in ADHD. This hypothesis was tested by examining sympathetic and adrenomedullary functioning during cognitive testing in boys with diagnosed ADHD. METHOD: Urinary excretion of EPI and norepinephrine during a 3-hour cognitive test battery was assessed in 7- to 13-year-old boys. Excretion rates (nanograms per hour per square meter of body surface area) were determined in 200 individuals with ADHD (diagnosed according to DSM-IV criteria), with or without co-occurring oppositional defiant/conduct disorder or learning disorder. A non-ADHD contrast group (n = 51) with similar comorbidity was also studied. RESULTS: Substantially lower (mean +/- SE) urinary EPI excretion was observed in the ADHD-inattentive subtype (n = 71) compared with the control group (200 +/- 22 versus 278 +/- 24 ng/hr/m2; F = 5.99, p = .015, critical alpha = .017). No diagnostic group differences were seen for norepinephrine excretion. Correlational analysis of both parent- and teacher-rated behaviors revealed that inattention factors consistently negatively predicted urinary EPI excretion. CONCLUSIONS: The data extend findings of lower adrenomedullary activity during cognitive challenge in individuals with ADHD and suggest that the alteration is associated with inattentive behavior.

Adolescent↗

PubMed2001

Generalized common fate: grouping by common luminance changes.

A critical step toward object recognition is the segmentation of a scene into relevant regions. One of the most important cues for segmentation is that of common fate: Elements that move together are grouped together Here we describe a new instantiation of common fate, in which elements move together not through physical space, but through luminance space. Experiment I shows that when elements of a scene become brighter together or darker together, observers group those elements together Experiments 2 and 3 show that this effect is not due to the availability of fixed luminance differences between target and background regions, but requires common changes within each region in the direction of luminance over time. The effect is differentiated from the recently discovered grouping cue of temporal synchrony, and is considered instead to be an extension of Wertheimer's original grouping factor of common fate. Common fate for luminance, or generalized common fate, is an extremely strong cue for the segmentation of a scene, yielding a tremendous advantage over grouping by fixed luminance cues.

Contrast Sensitivity↗

PubMed2002

Lightness constancy: a direct test of the illumination-estimation hypothesis.

Many models of color constancy assume that the visual system estimates the scene illuminant and uses this estimate to determine an object's color appearance. A version of this illumination-estimation hypothesis, in which the illuminant estimate is associated with the explicitly perceived illuminant, was tested. Observers made appearance matches between two experimental chambers. Observers adjusted the illumination in one chamber to match that in the other and then adjusted a test patch in one chamber to match the surface lightness of a patch in the other. The illumination-estimation hypothesis, as formulated here, predicted that after both matches the luminances of the light reflected from the test patches would be identical. The data contradict this prediction. A second experiment showed that manipulating the immediate surround of a test patch can affect perceived lightness without affecting perceived illumination. This finding also falsifies the illumination-estimation hypothesis.

Adult↗