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PubMed2005

An fMRI study of imitation: action representation and body schema.

Recent neuropsychological investigations of apraxia have led to new hypotheses about the representational defects associated with imitation impairments in neurological patients. This fMRI experiment investigated the relation between imitation and the body schema in healthy subjects. Experimental conditions were derived from a factorial plan, and participants were asked to watch a human model performing bodily gestures and then to execute either an identical or a different action, with the same or different limbs. Brain areas activated when subjects imitated the model were traced to the representation of the action (main effect of performing an identical action regardless of limb), to the body schema (using the same limb regardless of action), or to both. The first set of analyses yielded a network associated with visual perception, indicating that action representation is primarily visuospatial not motor, while the second analysis highlighted regions involved in body schema including the inferior parietal cortex and the insula. It is suggested that imitation of simple body gestures requires both a visuospatial description of the observed model, sustained by visual perception areas in the right occipitotemporal and superior parietal cortices and a visuospatial description of one's own body, supported by the left inferior parietal lobule. These results favor a model of praxis proposing that imitation deficits in left inferior parietal lobe patients with apraxia reflect primarily an impairment of the body schema, while deficits of praxis in right parietal patients are limited to gestures demanding in terms of visuospatial analysis.

Adult↗
PubMed2001

Psychophysical aspects in medical illumination techniques.

The relation between image and visual perception of the human eye is an important point in digital imaging systems. Research aims should therefore pay attention to psychophysical aspects. Optimising of digital imaging systems can only be reached if the important final steps in the diagnostic process--visual perception and signal detection--are taken into account.

Computer Terminals↗
PubMed2006

Shape recognition alters sensitivity in stereoscopic depth discrimination.

A fundamental question in visual perception is to characterize how information from sensory input is integrated with prior probabilities. The role of prior probabilities is controversial for elementary visual processes, which are often believed to be immune from higher-level influences. In this paper, we demonstrate such influences. We tested human observers' abilities to discriminate stereoscopic depth defined by points embedded in a biological pattern--a human figure. Our results indicate that the internal representation of a walking human figure imposed constraints on depth discrimination of a static stimulus. This constraint was manifested when the stimulus was recognized as a human figure. When the expected human figure configuration (forearms having equal length) was inconsistent with the sensory input information, discrimination of forearm lengths was impaired. In contrast, when there was no inconsistency (the hand-hip distance on the left was not expected to be equal to that on the right), discrimination between the two distances was improved, presumably because the human figure configuration provided a more accurate frame of reference for stereoscopic depth. Both the impairment and improvement were due to changes of discrimination sensitivity rather than decision bias. Our findings support the view that visual perception is an inference process constrained by Bayesian prior probabilities.

Bayes Theorem↗
PubMed2004

Detection of contour continuity and closure in three-month-olds.

The present study used an operant conditioning procedure and contour integration stimuli to test three-month-olds' sensitivity to both contour continuity and contour closure. The data demonstrate an immaturity of continuity detection and a lack of closure detection at that age, relative to a previous finding of a heightened sensitivity to closed contours in adult observers. This finding modifies the general view of infant visual perception that has been more focused on the quantitative development of various aspects of visual perception, including contrast sensitivity, binocular disparity processing, perceptual completion, and other perceptual skills. These results suggest qualitative change in terms of the organization of visual information during development, and implications of this finding for visual maturation of mechanisms suggested to underlie these detection abilities are discussed.

Conditioning, Operant↗
PubMed2005

The local mean decomposition and its application to EEG perception data.

This paper describes the local mean decomposition (LMD), a new iterative approach to demodulating amplitude and frequency modulated signals. The new method decomposes such signals into a set of functions, each of which is the product of an envelope signal and a frequency modulated signal from which a time-varying instantaneous frequency can be derived. The LMD method can be used to analyse a wide variety of natural signals such as electrocardiograms, functional magnetic resonance imaging data, and earthquake data. The paper presents the results of applying LMD to a set of scalp electroencephalogram (EEG) visual perception data. The LMD instantaneous frequency and energy structure of the EEG is examined, and compared with results obtained using the spectrogram. The nature of visual perception is investigated by measuring the degree of EEG instantaneous phase concentration that occurs following stimulus onset over multiple trials. The analysis suggests that there is a statistically significant difference between the theta phase concentrations of the perception and no perception EEG data.

Action Potentials↗
PubMed2006

A somatotopic map of vibrissa motion direction within a barrel column.

Most mammals possess high-resolution visual perception, with primary visual cortices containing fine-scale, inter-related feature representations (for example, orientation and ocular dominance). Rats lack precise vision, but their vibrissa sensory system provides a precise tactile modality, including vibrissa-related 'barrel' columns in primary somatosensory cortex. Here, we examined the subcolumnar organization of direction preference and somatotopy using a new omni-directional, multi-vibrissa stimulator. We discovered a direction map that was systematically linked to somatotopy, such that neurons were tuned for motion toward their preferred surround vibrissa. This sub-barrel column direction map demonstrated an emergent refinement from layer IV to layer II/III. These data suggest that joint processing of multiple sensory features is a common property of high-resolution sensory systems.

Animals↗
PubMed1996

Identification of the nonlinear state-space dynamics of the action-perception cycle for visually induced postural sway.

Human subjects standing in a sinusoidally moving visual environment display postural sway with characteristic dynamical properties. We analyzed the spatiotemporal properties of this sway in an experiment in which the frequency of the visual motion was varied. We found a constant gain near 1, which implies that the sway motion matches the spatial parameters of the visual motion for a large range of frequencies. A linear dynamical model with constant parameters was compared quantitatively with the data. Its failure to describe correctly the spatiotemporal properties of the system led us to consider adaptive and nonlinear models. To differentiate between possible alternative structures we directly fitted nonlinear differential equations to the sway and visual motion trajectories on a trial-by-trial basis. We found that the eigenfrequency of the fitted model adapts strongly to the visual motion frequency. The damping coefficient decreases with increasing frequency. This indicates that the system destabilizes its postural state in the inertial frame. This leads to a faster internal dynamics which is capable of synchronizing posture with fast-moving visual environments. Using an algorithm which allows the identification of essentially nonlinear terms of the dynamics we found small nonlinear contributions. These nonlinearities are not consistent with a limit-cycle dynamics, accounting for the robustness of the amplitude of postural sway against frequency variations. We interpret out results in terms of active generation of postural sway specified by sensory information. We derive also a number of conclusions for a behavior-oriented analysis of the postural system.

Humans↗
PubMed1990

A longitudinal study of the cognitive function of children with renal failure.

Fifty-six children with chronic renal failure who either received a kidney transplant, were hemodialyzed or peritoneally dialyzed, or who were being medically managed were given a series of neuropsychological tests every 6 months for a total of four testing sessions. Each child was matched by age, sex, and race to healthy children who received the same sequence of tests. The performance of each treatment group was compared with their controls longitudinally, using a repeated measure analysis of variance. In general, renal subjects performed at lower levels than their controls on tasks of verbal ability, visual perception, memory and visual motor skills. Visual motor performance was the most clinically affected. When compared with controls, the renal subjects did not improve as much or actually deteriorated on some measures of memory and learning skills.

Adolescent↗
PubMed2005

[Diagnosis and differentiation of children with language development disorders. What role can be attributed to intelligence?].

BACKGROUND: Specific language impairment (SLI) is defined as a developmental disorder in which language comprehension and the child's ability to use expressive spoken language is markedly below the appropriate level for his or her mental age. The intelligence of SLI children is in the normal range, while their language abilities are impaired. "Normal intelligence", the defining feature of SLI is questioned in this study. PATIENTS AND METHODS: Using IDIS (an inventory of diagnostic information in language impairment), we examined 138 children aged 5 and 6 years with severe language impairment; 108 SLI and 30 LI children. Various indicators of speech and language such as articulation, the ability to discriminate sounds, lexicon, grammar and pragmatic abilities but also auditory and visual perception, auditory and visual memory, fine and gross motor function were assessed. RESULTS: The performance of the SLI children was significantly higher in most of the tests than that of the LI children. Factor analysis showed that the two groups also differed in the structure of performance. Auditory short-term memory was reduced in most children irrespective of intelligence. CONCLUSIONS: We propose the retention of the differentiation of subgroups of developmental speech and language disorders depending on the level of intelligence.

Child↗
PubMed2006

[The clinical syndrome of posterior cortical atrophy].

Posterior cortical atrophy (PCA) is a syndrome that involves distinct neuropsychological deficits. This paper presents the clinical and neuropsychological findings recorded in four patients with PCA and reviews the characteristics of the syndrome and other conditions that need to be considered in the differential diagnosis. The cardinal symptoms of PCA are deficits of higher visual and spatial functions (mostly taking the form of Balint's syndrome), variably associated with disorders of visual perception, topographical disorientation, visual object agnosia and prosopagnosia, and deficits affecting reading, copying, drawing, and calculation. PCA is mostly associated with histopathological changes similar to those found in dementia of Alzheimer type (DAT), which are located predominantly in posterior brain regions. Memory and language functions tend to be preserved better and for a longer time in PCA than in the normal variant of DAT. SPECT and PET show deficits of perfusion and metabolism in both parietal and occipital lobes. The diagnosis of PCA is based on neuropsychological and imaging findings.

Cerebral Cortex↗
PubMedDate not supplied

The neuropsychology of trichotillomania.

The pathophysiology of trichotillomania (TM) is not well understood. Overlap with obsessive compulsive disorder (OCD) has been proposed, although extant data are inconsistent in this regard. In the neuropsychological domain, some data have supported the proposed TM-OCD overlap. However, the available studies are limited in number, and they typically have sampled a restricted range of performance domains. To examine further neuropsychological functioning in TM, the present study compared performance of 21 patients with TM and 17 normal control (NC) participants on a broad battery of tests assessing intellectual functioning, auditory perception and language, visual perception, somatosensory function, motor ability, memory, concept formation, attention and information processing speed, impulsivity, and cerebral dominance. The TM group demonstrated poorer performance on all measures of divided, but not focused, attention. Correlational data suggested the potentially important role of negative affect in TM. Implications of the data for the conceptualization of TM are discussed.

Adolescent↗
PubMed1997

Intact perceptual memory in the absence of conscious memory.

Declarative memory enables conscious recollection of the past and has been proposed to be distinct from priming, a perceptual form of memory that operates nonconsciously and improves the ability to detect or identify recently presented stimuli. Yet, it has been difficult to obtain unambiguous evidence for the independence of declarative memory and priming. The authors report the first demonstration, using matched tests, of fully intact perceptual memory (priming) in a profoundly amnesic patient (E.P.), despite at-chance recognition memory. The priming and recognition tests included tests that were matched with respect to test materials, length of the study and test lists, and the kind of cues available at test. Priming appears to reflect neural changes within perceptual processing systems that occur before information reaches the brain systems that transform visual perception into conscious visual memory.

Aged↗
PubMed2001

Spatial scale and cellular substrate of contrast adaptation by retinal ganglion cells.

Human visual perception and many visual system neurons adapt to the luminance and contrast of the stimulus. Here we describe a form of contrast adaptation that occurs in the retina. This adaptation had a local scale smaller than the dendritic or receptive fields of single ganglion cells and was insensitive to pharmacological manipulation of amacrine cell function. These results implicate the bipolar cell pathway as a site of contrast adaptation. The time required for contrast adaptation varied with stimulus size, ranging from approximately 100 ms for the smallest stimuli, to seconds for stimuli the size of the receptive field. The differing scales and time courses of these effects suggest that multiple types of contrast adaptation are used in viewing natural scenes.

Adaptation, Ocular↗