Biomedical subjects

Daniel Saumier

Publications and source records attributed to Daniel Saumier.

9 recordsLinked to original sources

PubMed2007

Atypical categorical perception in autism: autonomy of discrimination?

A diminished top-down influence has been proposed in autism, to account for enhanced performance in low-level perceptual tasks. Applied to perceptual categorization, this hypothesis predicts a diminished influence of category on discrimination. In order to test this hypothesis, we compared categorical perception in 16 individuals with and 16 individuals without high-functioning autism. While participants with and without autism displayed a typical classification curve, there was no facilitation of discrimination near the category boundary in the autism group. The absence of influence of categorical knowledge on discrimination suggests an increased autonomy of low-level perceptual processes in autism, in the form of a reduced top-down influence from categories toward discrimination.

Adolescent↗

PubMed2005

Establishing visual category boundaries between objects: a PET study.

Individuals with Alzheimer's disease (AD) often have problems in recognizing common objects. This visual agnosia may stem from difficulties in establishing appropriate visual boundaries between visually similar objects. In support of this hypothesis, showed that AD subjects have difficulties in establishing visual category boundaries between continuously graded shapes. In an attempt to investigate the neural basis of these impairments, the current study required a group of neurologically healthy elderly participants to categorically classify a series of ellipses varying in width while regional blood flow changes were measured using positron emission tomography (PET). Two categorization conditions were compared in order to isolate changes in cortical activity that dissociated the categorization of shapes situated either near or far from a category boundary that divided the width continuum. The participants produced a discontinuity in the probability and speed of categorizing the shapes at some point along the continuum, suggesting that the objects were classified into distinct categories. Moreover, a comparison of the PET scans obtained while the subjects were categorizing the shapes situated near vs. far form the category boundary revealed significant differences in cortical activity in the parietal and frontal brain areas. These findings suggest that both visuo-spatial and decision making mechanisms may be involved in establishing categorical distinctions between continuously graded stimuli. It is proposed that the functional role of the parietal and frontal cortical regions in establishing visual boundaries between categories of objects may be relevant for understanding object recognition impairments in AD.

Aged↗

PubMed2005

The effect of Alzheimer's disease and aging on conceptual combination.

Alzheimer's disease (AD) subjects, healthy elderly, and young adults interpreted a series of novel noun-noun expressions composed of familiar object words. Subjects interpreted each item by selecting one of three possible definitions: a definition in which the referents of each noun were associated together in a particular context (e.g., rabbit cat: a cat that is raised by rabbits); a definition based on integrating the semantic attributes from each noun into a single referent (e.g., rabbit cat: a cat that has long ears and hops); and a semantic foil. The results show that the AD subjects selected significantly fewer integrational definitions and significantly more foils than healthy elderly subjects. Healthy elderly participants were also found to select significantly more foil definitions than the young adult subjects. These findings suggest that AD individuals have difficulty in integrating semantic features when interpreting novel noun-noun expressions and that both AD and healthy elderly individuals have morphosyntactic impairments related to the identification of the modifying and head nouns of noun-noun expressions.

Adult↗

PubMed2005

Semantic and syntactic aspects of the mass/count distinction: a case study of semantic dementia.

Semantic and syntactic contributions to processing of mass and count nouns were assessed by examining the performance of a patient suffering from a pure semantic deficit. Semantic and syntactic processing was evaluated on grammaticality judgement and sentence-picture matching tasks, respectively, where each task involved mass and count readings of metonymic nouns. While the patient did not show impaired performance on the grammaticality judgment task, he manifested difficulties in making mass/count distinctions in the sentence-picture matching task. It is thus argued that while distributionally the mass/count distinction may be established on a purely syntactic basis, cognitive processing of mass/count information requires both intact syntactic and semantic knowledge.

Aged↗

PubMed2005

Focused attention deficits in patients with Alzheimer's disease and mild cognitive impairment.

Reaction time (RT) tasks take various forms, and can assess psychomotor speed, (i.e., simple reaction time task), and focused attention (i.e., choice reaction time (CRT) task). If cues are provided before stimulus presentation (i.e., cued choice reaction time (CCRT) task), then a cueing effect can also be assessed. A limited number of studies have addressed the nature of focused attention impairments in Alzheimer's disease (AD). Additionally, it is unknown whether similar impairments occur in Mild Cognitive Impairment (MCI). The current study used three RT tasks to address the nature of focused attention impairments in AD and MCI subjects. The results suggest that there were significant CRT and CCRT differences in AD subjects when compared to NECs. Furthermore, slowed RTs were also present in the MCI group, which provides evidence for impaired focussed attention and the inability to benefit from a cue in both the MCI and AD groups. The implications of the impairments related to the MCI group could potentially prove useful in early diagnosis of cognitive impairments in the elderly.

Aged↗

PubMed2004

Independent processing of parts and of their spatial organization in complex visual objects.

A visual search experiment using synthetic three-dimensional objects is reported. The target shared its constituent parts, the spatial organization of its parts, or both with the distractors displayed with it. Sharing of parts and sharing of spatial organization both negatively affected visual search performance, and these effects were strictly additive. These findings support theories of complex visual object perception that assume a parsing of the stimulus into its higher-order constituents (volumetric parts or visible surfaces). The additivity of the effects demonstrates that information on parts and information on spatial organization are processed independently in visual search.

Humans↗

PubMed2003

Distinct mechanisms account for the linear non-separability and conjunction effects in visual shape encoding.

In a series of visual search experiments involving simple 2D shapes, Arguin and Saumier (2000) showed that targets that were made of conjunctions of distractor features or that were a linear combination of distractor features were searched at significantly slower rates than single-feature linearly separable targets. The present study assessed whether these conjunction and linear nonseparability effects can be attributed to distinct mechanisms. Specifically, we studied the impact of target-distractor similarity on the search rates for single-feature, conjunction, and linearly nonseparable targets. The results replicate the conjunction and linear nonseparability effects obtained by Arguin and Saumier. They also show that the conjunction and linear separability effects are differently modulated by variations in target-distractor similarity. This dissociation demonstrates that both effects are based on distinct mechanisms. The possible nature of these mechanisms is discussed.

Adult↗

PubMed2002

Semantic memory.

Our concepts about objects, states, and events are stored in a cognitive structure termed semantic memory. There are several types of neurologic disorders that may cause impairments of semantic memory. Clinical evaluations of these impairments are complex, because semantic memory is linked to other cognitive systems that, when damaged, may produce related syndromes or difficulties. In an attempt to gain further understanding of these breakdown patterns, we review data from both neuropsychologic and brain activity research that have been concerned with how object concepts are represented and localized in the brain. Although these data have spawned varying and controversial views regarding the content and organization of semantic knowledge, converging evidence suggests that semantic memory is mainly localized in the posterior region of the left temporal lobe, and that particular categories of knowledge may be represented in different but overlapping regions within this area.

Functional Laterality↗

PubMedDate not supplied

Visual object agnosia as a problem in integrating parts and part relations.

Current models of vision generally assume that the recognition of visual objects is achieved by encoding their component parts, as well as the spatial relations among parts. The current study examined how the processing of parts and their configurations may be affected in visual agnosia due to brain damage. Both a visual agnosic patient (AR) and healthy control subjects performed a visual search task in which they had to discriminate between targets and distractors that varied according to whether they shared their parts and/or their configuration. The results show that AR's visual search rates are disproportionally slow when targets and distractors share the same configuration than when they have different configurations. AR is also found to be disproportionately slow in discriminating targets and distractors that share identical parts when the targets and distractors share the same configuration. With differently configured targets and distractors, AR shows no part sharing effect. For controls, in contrast, the part and configuration sharing effects occur independently of one another. It is concluded that AR's object recognition deficit arises from difficulties in discriminating objects that share their configuration, and from an abnormal dependency of part information processing upon object configuration.

Adult↗