PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Contrastive learning”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,171 records · Page 65Linked to original sources
PubMed1992

Conservation acquisition, maintenance, and generalization by mentally retarded children using equality-rule training.

To teach conservation to mildly retarded, preoperational preadolescents (9 to 12 years old), a number-representational system was used to mediate the invariance of quantity as part of "equality-rule" training. Before viewing a perceptually misleading configuration of either two equal quantities or two unequal quantities entailing the dimensions of number or length, participants learned that equality involved the assignment of two identical numbers and inequality the assignment of two different numbers. These numbers were then applied to the actual quantities and became the basis for rule statements that were rehearsed, memorized, and subject-generated each time the quantities were transformed into a new configuration. Following equality training, correct judgments and verbal justifications of conservation were high (M greater than or equal to 85%) during both immediate and delayed post-tests and for near-generalization (number and length) and far-generalization (weight, liquid substance, and solid substance) tasks. In contrast, the post-test levels of comparable participants given learning-set (LS) training, LS plus verbal-rule training (a combination of statements about identity, negation, and compensation), or no training at all were substantially and significantly lower. Equality training is of theoretical and practical interest because of its potential to teach a symbolic representational rule that directly and meaningfully articulates the quantitative properties of objects and counteracts the overreliance on task-specific and faulty perceptual cues inherent in the conservation task.

Child↗
PubMed1995

[Cognitive dysfunction following anterior communicating artery aneurysm rupture. Comparison with alcoholic Korsakoff syndrome on neuropsychological performance].

The present study aims to compare neuropsychological performance of patients following anterior communicating artery aneurysm rupture (ACoA) with that of patients with alcoholic Korsakoff syndrome (AKS). Fifteen ACoA patients and ten age-and education-matched AKS patients were included in the study. All the patients were tested at least one year post onset of their illness at a stable condition. The WAIS and forward digit span scores of AKS were also matched to ACoA, and simple attention and general intelligence were well preserved both in ACoA and AKS. Frontal function as measured by the Wisconsin card sorting test (Keio version) (KWCST) was equivalently impaired in the two groups. Anterograde memory as measured by Wechsler memory scale subtests, serial seven word learning test, Rey auditory verbal learning test, and logical memorizing test (Luria's paired word-picture association), was more severely impaired in AKS than ACoA in contrast to the comparable attention, intelligence, and frontal function: (1) memory tasks with low correlations to KWCST (serial word learning tasks and paired verbal associates), reflecting primary simple serial memorizing, and (2) memory tasks with high correlations to KWCST (logical memory and logical memorizing), reflecting higher and complicated strategic mnemonic activities. However, the correlations between these anterograde memory subtests and KWCST were substantially equivalent in ACoA and AKS. This suggests that the differences in anterograde amnesia demonstrated in ACoA and AKS may be of quantitative, not of qualitative property. The extent of deficits in semantic encoding as measured by Wickens' release from proactive interference paradigm (PI release) was also milder in ACoA than AKS. Both AKS and ACoA failed to show PI release in contrast to normal PI release demonstrated in age-matched ten healthy subjects. PI release in ACoA, however, was in between AKS and healthy subjects. The results were interpreted in the light of a recently postulated hypothesis that a combination of frontal lobe damage and memory impairment is crucial for causing a deficit in semantic encoding. The extent of damage in the memory circuit in ACoA may be variable, which may result in milder degree of anterograde amnesia and semantic encoding than AKS in the present study.

Alcohol Amnestic Disorder↗
PubMed2006

Feature detection and letter identification.

Seeking to understand how people recognize objects, we have examined how they identify letters. We expected this 26-way classification of familiar forms to challenge the popular notion of independent feature detection ("probability summation"), but find instead that this theory parsimoniously accounts for our results. We measured the contrast required for identification of a letter briefly presented in visual noise. We tested a wide range of alphabets and scripts (English, Arabic, Armenian, Chinese, Devanagari, Hebrew, and several artificial ones), three- and five-letter words, and various type styles, sizes, contrasts, durations, and eccentricities, with observers ranging widely in age (3 to 68) and experience (none to fluent). Foreign alphabets are learned quickly. In just three thousand trials, new observers attain the same proficiency in letter identification as fluent readers. Surprisingly, despite this training, the observers-like clinical letter-by-letter readers-have the same meager memory span for random strings of these characters as observers seeing them for the first time. We compare performance across tasks and stimuli that vary in difficulty by pitting the human against the ideal observer, and expressing the results as efficiency. We find that efficiency for letter identification is independent of duration, overall contrast, and eccentricity, and only weakly dependent on size, suggesting that letters are identified by a similar computation across this wide range of viewing conditions. Efficiency is also independent of age and years of reading. However, efficiency does vary across alphabets and type styles, with more complex forms yielding lower efficiencies, as one might expect from Gestalt theories of perception. In fact, we find that efficiency is inversely proportional to perimetric complexity (perimeter squared over "ink" area) and nearly independent of everything else. This, and the surprisingly fixed ratio of detection and identification thresholds, indicate that identifying a letter is mediated by detection of about 7 visual features.

Adolescent↗
PubMed2001

Ontogeny of locomotion in rhesus macaques (Macaca mulatta): II. Postural and locomotor behavior and habitat use in a free-ranging colony.

This study quantifies changes in postural and locomotor behavior as well as habitat use across the life span of free-ranging rhesus macaques (Macaca mulatta) in the Cayo Santiago colony in Puerto Rico. It focuses on developmentally related changes from birth to adulthood, and complements an earlier study by Turnquist and Wells ([1994] J Hum Evol 26:487-499) on the early postnatal ontogeny of the musculoskeletal system of the same colony. A total of 6,551 locomotor and postural events was analyzed. Selection and use of substrate correlated well with age. The more sedentary adult and dependent infant select safe, wide, horizontal arboreal settings in contrast to the older Infant IIs and Juveniles, who are learning locomotor and postural skills through independent chase and play. Infant macaques, when independent, often employ a low center of gravity and widely abducted limbs in order to broaden their contact with the base of support. This study shows that the previously reported ontogenetic changes in morphology are closely paralleled by changes in postural and locomotor behavior, and these in turn are correlated to changes in habitat use, particularly during the formative years.

Adaptation, Physiological↗
PubMed2003

Comparison of computational models of familiarity discrimination in the perirhinal cortex.

This study compares the efficiency and plausibility of published computational models of familiarity discrimination in the perirhinal cortex. Substantial evidence indicates that the perirhinal cortex is involved in both the familiarity discrimination aspect of recognition memory and in perceptual functions involved with representations of complete stimuli (i.e., object identification). Published models of how the perirhinal cortex may perform familiarity discrimination can be divided into two groups. The first group assumes that a proportion of perirhinal neurons form a network specialised just for familiarity discrimination (these models may be based on Hebbian or anti-Hebbian synaptic plasticity). In contrast, the second group assumes that both familiarity discrimination and learning representations of complete stimuli are performed within a single combined network. This study establishes that when the responses of neurons that provide input to the familiarity discrimination network are correlated (as indicated by experimental data), specialised networks based on anti-Hebbian learning may recognise the previous occurrence of many more stimuli (i.e., have a capacity up to thousands of times larger) than specialised networks based on Hebbian learning. The currently published combined models do not learn an optimal stimulus representation (they do not fully extract statistically independent features), and hence their capacities are even lower than those of the specialised models based on Hebbian learning. Hence, the combined models published thus far are critically less efficient than the specialised models based on anti-Hebbian learning. This study also compares the consistency of the models with experimental observations concerning what is known of synaptic plasticity in the perirhinal cortex and the responses of its neurons. Many theoretically important parameters remain undetermined, and experiments are suggested to provide information critical for refining and distinguishing between the various models. However, the above theoretical arguments and currently published data favour the existence of a separate network specialised for familiarity discrimination.

Animals↗
PubMed2003

Perception of depth from shading in infant chimpanzees ( Pan troglodytes).

We investigated the ability to perceive depth from shading, one of the pictorial depth cues, in three chimpanzee infants aged 4-10 months old, using a preferential reaching task commonly used to study pictorial depth perception in human infants. The chimpanzee infants reached significantly more to three-dimensional toys than to pictures thereof and more to the three-dimensional convex than to the concave. Furthermore, two of the three infants reached significantly more to the photographic convex than to the photographic concave. These infants also looked longer at the photographic convex than the concave. Our results suggest that chimpanzees perceive, at least as early as the latter half of the first year of life, pictorial depth defined by shading information. Photographic convexes contain richer information about pictorial depth (e.g., attached shadow, cast shadow, highlighted area, and global difference in brightness) than simple computer-graphic graded patterns. These cues together might facilitate the infants' perception of depth from shading.

Age Factors↗
PubMed1996

Left and right visual field advantages are a function of scotopic and photopic retinal adaptation, respectively, in simple reaction time to near-threshold targets.

Modulation of stimulus luminance in a tachistoscopic face discrimination task has been found to significantly invert visual hemifield advantage in reaction time (RT) (Sergent, 1982a, Sergent, 1982b). However, there is no more physiological rationale for that than for a similar effect, say, of retinal adaptation, and it is even conceivable that the latter may have confounded the former in past experiments. The experiments reported here were therefore designed to tease out the relative contributions of stimulus luminance and of background illumination (i.e., retinal adaptation) in a simple RT task. Two equally difficult conditions of dim targets were set up, one with light-adapted subjects and one with dark-adapted subjects. Similarly, two equally difficult conditions of bright targets were set up with light and dark-adapted subjects. It was found that dim targets (near detection threshold) yielded a significant right visual field RT advantage in light-adapted subjects and that dim targets (equally near detection threshold) yielded a significant left visual field RT advantage in dark-adapted subjects. Future experiments will determine whether cone-mediated RT to detection is left hemisphere dominant and whether rod-mediated RT to detection is right hemisphere dominant.

Adaptation, Ocular↗
PubMed1984

The effect of familial sinistrality on perceptual learning.

Perceptual learning as measured by adaptation to prismatically displaced vision was contrasted in groups of familial sinistral (FS) and familial dextral (FD), strongly right-handed adult females. The FS group exhibited significantly larger target pointing and eye positioning adjustments than the FD group. Further, only the FS group showed transfer of training from the hand used during prism viewing to the unobserved one.

Adult↗
PubMed1991

Visual recognition memory in squirrel monkeys: effects of serotonin antagonists on baseline and hypoxia-induced performance deficits.

Cognitive deficits resulting from neuropathological brain changes such as Alzheimer's Disease or normal aging are most likely due to alterations in multiple neurotransmitter systems. While the majority of preclinical studies have focused on the effects of acetylcholine (ACh), it has been shown that activation of the serotonergic (5-HT) pathways in the central nervous system interferes with passive avoidance retention in rats. In contrast, decreased 5-HT activity has been shown to improve learning and memory in rats using similar procedures. In the present experiment, 5-HT antagonists were evaluated for their effects on performance in a delayed match to sample task (DMTS) in two groups of squirrel monkeys: one in which the baseline level of performance was low (less than 65% correct, N = 5; group 1) and another in which DMTS performance was high (greater than 80% correct, N = 3; group 2) but impaired by exposure to hypoxia. Initial parametric tests exposing group 2 to various levels of oxygen deprivation were conducted to determine optimal conditions for performance deficits. Each monkey in both normoxia (group 1) and hypoxia (group 2) served as his own control and received an individualized range of doses for each test compound. For both groups, ketanserin and mianserin, the 5-HT2-selective antagonists, produced dose-dependent increases in DMTS performance at 0.3-1.5 mg/kg PO and 0.05-1.5 mg/kg PO, respectively. Pirenperone, another 5-HT2-selective antagonist, was active in improving performance in group 1 at 0.001 to 0.2 mg/kg PO but was not effective against hypoxia-induced performance deficits.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2004

Effects of prenatal undernutrition on emotional reactivity and cognitive flexibility in adult sheep.

The experiment reported was designed to test two hypotheses: that prenatal undernutrition (a) increases emotional reactivity and (b) impairs cognitive flexibility in sheep. The mothers of one group were fed live weight maintenance requirements throughout pregnancy (control, C) while those of another group were fed 50% of that amount from days 1 to 95 of pregnancy and 100% from then onwards (prenatal undernutrition, PU). At 18 months of age, PU sheep were more active during restraint (P < 0.05) and approached a novel stimulus more slowly (P = 0.02). In response to a sudden stimulus, PU males and C females showed a higher initial level of locomotion compared to C males, which only gradually declined, while PU females started at a high initial rate, changing rapidly to immobility. In a T-maze, PU resulted in a shift of side preferences (laterality) from a general right-bias to neutrality in males and to a left-bias in females (P < 0.05). In the two reversal tasks, C males and PU females had a preference for one side over the other, while PU males showed no preference. In contrast to C males, PU males failed to improve their learning speed from the first to the second reversal (P < 0.05). It is concluded that PU can lead to increased emotional reactivity and changes in side preference in both sexes and impaired cognitive flexibility in males. Undernutrition during pregnancy, therefore, not only affects the welfare of the dam, but also the personality of her offspring.

Animals↗
PubMed2006

Visual training improves underwater vision in children.

Children in a tribe of sea-gypsies from South-East Asia have been found to have superior underwater vision compared to European children. In this study, we show that the improved underwater vision of these Moken children is not due to better contrast sensitivity in general. We also show that European children can achieve the same underwater acuity as the Moken children. After 1 month of underwater training (11 sessions) followed by 4 months with no underwater activities, European children showed improved underwater vision and distinct bursts of pupil constriction. When tested 8 months after the last training session in an outdoor pool in bright sunlight-comparable to light environments in South-East Asia-the children had attained the same underwater acuity as the sea-gypsy children. The achieved performance can be explained by the combined effect of pupil constriction and strong accommodation.

Accommodation, Ocular↗
PubMed2001

When more means less: neural activity related to unsuccessful memory encoding.

The neural correlates of memory encoding have been studied by contrasting neural activity elicited by items at the time of learning according to whether they were later remembered or forgotten [1]. Previous studies have focused on regions where neural activity is greater for subsequently remembered items [2-8]. Here, we describe regions where activity is greater for subsequently forgotten items. In two experiments that employed the same incidental learning task, activity in an overlapping set of cortical regions (posterior cingulate, inferior and medial parietal, and dorsolateral prefrontal) was associated with failure on a subsequent memory test.

Adult↗
PubMed2002

Programs of gene expression during the laying down of memory formation as revealed by DNA microarrays.

Many experiments in the past have demonstrated the requirement of de novo gene expression during memory formation. In contrast to the initial reductionistic view that genes relevant to learning and memory would be easily found and would provide a simple key to understand this brain function, it is becoming apparent that the genetic contribution to memory is complex. Previous approaches have been focused on individual genes or genetic pathways and failed to address the massively parallel nature of genome activities and collective behavior of the genes that ultimately control the molecular mechanisms underlying brain function. In view of the broad variety of genes and the cross talk of genetic pathways involved in this regulation, only gene expression profiles may reflect the complete behavior of regulatory pathways. In this review we illustrate how DNA microarray-based gene expression profiling may help to dissect and analyze the complex mechanisms involved in gene regulation during the acquisition and storage of memory in the mammalian brain.

Animals↗
PubMed1996

Surface range and attitude probing in stereoscopically presented dynamic scenes.

The authors report on different methods to probe the structure of visually perceived surfaces in 3 dimensions. The surfaces are specified by patterns of shading with Lambertian and specular components, which deform over time and over stereoscopic views. Five observers performed 2 probe tasks, 1 involving the adjustment of a punctate probe so as to be on the apparent surface and the other involving the adjustment of a small gauge figure that indicates surface attitude. The authors found that these rather different methods yielded essentially identical depth maps up to a linear transformations and that the observers all deviate slightly from veridicality in basically identical ways. The nature of this deviation appears to be correlated with the rough topography of the specularities.

Attention↗
PubMed1996

Effects of lighting on the perception of facial surfaces.

A series of experiments is reported that investigated the effects of variations in lighting and viewpoint on the recognition and matching of facial surfaces. In matching tasks, changing lighting reduced performance, as did changing view, but changing both did not further reduce performance. There were also differences between top and bottom lighting. Recognizing familiar surfaces and matching across changes in viewpoint were more accurate when lighting was from above than when it was from below the heads, and matching between different directions of top lighting was more accurate than between different directions of bottom lighting. Top lighting also benefited matching between views of unfamiliar objects (amoebae), though this benefit was not found for inverted faces. The results are difficult to explain if edge- or image-based representations mediate face processing and seem more consistent with an account in which lighting from above helps the derivation of 3-dimensional shape.

Adult↗
PubMed1999

2-D center-surround effects on 3-D structure-from-motion.

This study investigates how mechanisms for amplifying 2-D motion contrast influence the assignment of 3-D depth values. The authors found that the direction of movement of a random-dot conveyor belt strongly inclined observers to report that the front surface of a superimposed, transparent, rotating, random-dot sphere moved in a direction opposite to the belt. This motion-contrast effect was direction selective and demonstrated substantial spatial integration. Varying the stereo depth of the belt did not compromise the main effect, precluding a mechanical interpretation (sphere rolling on belt). Varying the speed of the surfaces of the sphere also did not greatly affect the interpretation of rotation direction. These results suggest that 2-D center-surround interactions influence 3-D depth assignment by differentially modulating the strength of response to the moving surfaces of an object (their prominence) without affecting featural specificity.

Adult↗
PubMed1999

Representational momentum for rotations in depth: effects of shadings and axis.

Representational momentum is a positive memory distortion for an object's final position following the presentation of an implied event (J.J. Freyd, 1987). Positive memory distortions occur when observers accept test positions beyond the final presented position, or forward along the implied trajectory, as the same more readily than positions behind the final position. Four experiments explored implied events depicting rotations about various depth axes in shaded and silhouette conditions. Positive memory distortions were observed for all depth rotations under certain shading conditions, with some differences in the size of the distortion between axes. No directional effects (e.g., right vs. left) were observed. The overall positive memory distortions observed for depth rotations contrasted with the negative distortions previously observed for translation motion in depth (T.L. Hubbard, 1995 ).

Adult↗