Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Perceptual Distortion”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

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

At least 901 records · Page 50Linked to original sources
PubMed2005

Theta and alpha oscillations reflect differences in memory strategy and visual discrimination performance in patients with Parkinson's disease.

The aim of the present study was to investigate alterations in brain oscillatory activity during transient intentional encoding of abstract novel shapes in patients with Parkinson's disease (PD). Combined time-frequency analyses of alpha and theta power revealed a clear dissociation between PD patients (N=10) and age-matched healthy controls (N=10). PD patients used sub-optimal stimulus encoding strategies for subsequent maintenance and recall, whereas the controls mainly used categorization processes. In contrast to controls, PD patients showed significantly less theta increase and upper alpha suppression at frontal locations and significant laterality of early posterior theta and lower-2 alpha oscillations at right posterior locations. These findings suggest that the fronto-striatal circuits participate in visual categorization processes. Furthermore, theta and alpha oscillations appear to be involved in mediating the integration processes in mnemonic networks underlying visual categorization.

Aged↗
PubMed2005

Self-face recognition activates a frontoparietal "mirror" network in the right hemisphere: an event-related fMRI study.

Self-recognition has been demonstrated by a select number of primate species and is often used as an index of self-awareness. Whether a specialized neural mechanism for self-face recognition in humans exists remains unclear. We used event-related fMRI to investigate brain regions selectively activated by images of one's own face. Ten right-handed normal subjects viewed digital morphs between their own face and a gender-matched familiar other presented in a random sequence. Subjects were instructed to press a button with the right hand if the image looked like their own face, and another button if it looked like a familiar or scrambled face. Contrasting the trials in which images contain more "self" with those containing more familiar "other" revealed signal changes in the right hemisphere (RH) including the inferior parietal lobule, inferior frontal gyrus, and inferior occipital gyrus. The opposite contrast revealed voxels with higher signal intensity for images of "other" than for "self" in the medial prefrontal cortex and precuneus. Additional contrasts against baseline revealed that activity in the "self" minus "other" contrasts represent signal increases compared to baseline (null events) in "self" trials, while activity in the "other" minus "self" contrasts represent deactivations relative to baseline during "self" trials. Thus, a unique network involving frontoparietal structures described as part of the "mirror neuron system" in the RH underlies self-face recognition, while regions comprising the "default/resting state" network deactivate less for familiar others. We provide a model that reconciles these findings and previously published work to account for the modulations in these two networks previously implicated in social cognition.

Adult↗
PubMed2005

Gamma-band activity over early sensory areas predicts detection of changes in audiovisual speech stimuli.

Oscillatory activity in the gamma-band range in human magneto- and electroencephalogram is thought to reflect the oscillatory synchronization of cortical networks. Findings of enhanced gamma-band activity (GBA) during cognitive processes like gestalt perception, attention and memory have led to the notion that GBA may reflect the activation of internal object representations. However, there is little direct evidence suggesting that GBA is related to subjective perceptual experience. In the present study, magnetoencephalogram was recorded during an audiovisual oddball paradigm with infrequent visual (auditory /ta/ + visual /pa/) or acoustic deviants (auditory /pa/ + visual /ta/) interspersed in a sequence of frequent audiovisual standard stimuli (auditory /ta/ + visual /ta/). Sixteen human subjects had to respond to perceived acoustic changes which could be produced either by real acoustic or illusory (visual) deviants. Statistical probability mapping served to identify correlations between oscillatory activity in response to visual and acoustic deviants, respectively, and the detection rates for either type of deviant. The perception of illusory acoustic changes induced by visual deviants was closely associated with gamma-band amplitude at approximately 80 Hz between 250 and 350 ms over midline occipital cortex. In contrast, the detection of real acoustic deviants correlated positively with induced GBA at approximately 42 Hz between 200 and 300 ms over left superior temporal cortex and negatively with evoked gamma responses at approximately 41 Hz between 220 and 240 ms over occipital areas. These findings support the relevance of high-frequency oscillatory activity over early sensory areas for perceptual experience.

Adult↗
PubMed2004

Sensorimotor effects on central space representation: prism adaptation influences haptic and visual representations in normal subjects.

Prism adaptation improves visual and haptic manifestations of left neglect, and can induce a small but reliable simulation of left visual neglect in normal individuals. Here, we present two experiments in which the effects of prism adaptation on the representation of space were explored. In Experiment 1, normal subjects were required to locate the centre of a haptically explored circle, before and after adaptation to leftward displacing prisms. In Experiment 2, a visual circle centring task was used. In both tasks, prism adaptation induced a significant rightward shift of performance. In addition, in both experiments, three classical measures of visuo-manual adaptation were taken: the visual shift, the proprioceptive shift and the total shift. The effects found on the haptic and visual tasks did not correlate with any of these measures. This suggests that the effects of prism adaptation on the circle centring tasks did not depend directly on the sensorimotor consequences of the adaptation. These results imply that prism adaptation can affect noetic levels of space representation in normal subjects, supporting the hypothesis that this low-level sensorimotor intervention can exert a bottom-up structuring influence on higher levels of cognitive integration.

Adaptation, Psychological↗
PubMed2004

Neural correlates of action attribution in schizophrenia.

Patients with first-rank symptoms (FRS) of schizophrenia do not experience all of their actions and personal states as their own. FRS may be associated with an impaired ability to correctly attribute an action to its origin. In the present study, we examined regional cerebral blood flow (rCBF) with positron emission tomography during an action-attribution task in a group of patients with FRS. We used a device previously used with healthy subjects that allows the experimenter to modulate the subject's degree of movement control (and thus action attribution) of a virtual hand presented on a screen. In healthy subjects, the activity of the right angular gyrus and the insula cortex appeared to be modulated by the subject's degree of movement control of the virtual hand. In the present study, the schizophrenic patients did not show this pattern. We found an aberrant relationship between the subject's degree of control of the movements and rCBF in the right angular gyrus and no modulation in the insular cortex. The implications of these results for understanding pathological conditions such as schizophrenia are discussed.

Adult↗
PubMed2004

Differential impairments of facial affect recognition in schizophrenia subtypes and major depression.

The goal of this study was to assess facial affect recognition abilities in subjects with various schizophrenia subtypes and subjects with major depression. A total of six disorganized, 21 paranoid and 18 residual subjects with schizophrenia (DSM-IV criteria) were compared with 21 subjects with major depression (DSM-IV criteria) and 30 matched healthy control subjects. Two experimental tasks requiring the sorting and rating of emotional facial expressions were applied. Disorganized and paranoid subjects showed strong impairments in the sorting of emotional facial expressions. Depressive subjects displayed only minor deficits, and residual subjects were unimpaired. Subjects with disorganized schizophrenia rated emotional facial expressions as more aroused, and depressive subjects rated them as less aroused, than the other study groups. Our study demonstrates strong deficits in facial affect recognition in subjects with schizophrenia and pronounced disorganized or psychotic symptoms. Deficits in facial affect recognition are specific to schizophrenia. They may be considered as a state marker of schizophrenia.

Adult↗
PubMed2004

Depression biases the recognition of emotionally neutral faces.

Functional abnormalities in emotion-related brain systems have been implicated in depression, and depressed patients may therefore attribute emotional valence to stimuli that are normally interpreted as emotionally neutral. The present study examined this hypothesis by comparing recognition of different facial expressions in patients with moderate to severe depression. Eighteen depressed patients and 18 matched healthy controls made a forced-choice response to briefly presented neutral, happy, and sad faces. Recognition accuracy and response time were measured. Twelve patients were retested after showing signs of symptom remission. Depressed patients and controls were equally accurate at recognizing happy and sad faces. Controls also recognized neutral faces as accurately as happy and sad faces, but depressed patients recognized neutral faces less accurately than either happy or sad faces. Depressed patients were also particularly slow to recognize neutral faces. The impairment in processing of neutral faces was still evident after symptom remission. Error analyses showed that depressed patients attributed not only sadness, but also happiness (in remission), to neutral faces. These results suggest that, unlike healthy subjects, depression-prone individuals do not seem to perceive neutral faces as unambiguous signals of emotional neutrality.

Adult↗
PubMed2004

Brief emotion training improves recognition of facial emotions in chronic schizophrenia. A pilot study.

Impaired emotional communication may be an important contributing factor to poor social function in schizophrenia. This pilot study examined the effect of emotion training exercises on the perception of facial emotional expression. Twenty male chronic schizophrenia patients underwent three training sessions using a computerized Emotion Training program, developed for teaching autistic children, which was adapted to the clinical setting. Patients were assessed before and after training with validated tests of identification of facial emotions (PEAT, ER40), differentiation of facial emotions (EmDiff) and working memory. In comparison to baseline, patients performed significantly better on the PEAT and ER40 tests after training. No change was observed in EmDiff or in cognitive test performance. Brief Emotion Training can improve recognition of facial emotional expressions in chronic schizophrenia patients. This may be due to increased patient awareness of emotional aspects of stimuli and/or improvement in specific emotional perceptual skills. Further studies of Emotion Training as a potential treatment modality are warranted.

Adult↗
PubMed2006

A generalized bias against disconfirmatory evidence in schizophrenia.

Fixation onto false/unrealistic beliefs is a core feature of schizophrenic delusions. A recent study conducted by our research group has provided evidence for the presence of a bias against disconfirmatory evidence (BADE) in patients with schizophrenia. Importantly, this bias was found with delusion-neutral material. To further validate a BADE as an underlying component of schizophrenic delusions, we recruited 34 presently deluded and non-deluded patients with schizophrenia, along with 26 healthy and 46 mixed psychiatric control participants. Participants were administered a closure task. On each trial, a common object (e.g., elephant) was increasingly disambiguated (i.e., shown in decreasing degrees of fragmentation). The participants were required to assess the plausibility of different interpretations at each of the up to eight stages in each trial. In line with the main hypothesis, patients with schizophrenia downgraded the ratings for incorrect interpretations significantly less over the course of task completion than did healthy and psychiatric controls. In contrast, the gradual upgrading of correct interpretations was similar across all groups, suggesting that the pattern of results obtained for incorrect interpretations reflects a BADE and not a mere repetition of prior responses or a lack of attention to the task at hand. The present study suggests that a BADE is a core feature of schizophrenia, and that this style of thinking is not confined to delusion-congruent scenarios.

Adult↗
PubMed2005

The cognitive neuroscience of auditory distraction.

We are often aware of the content of distracting sound, although typically remain unaware of the processes by which that sound is disruptive. Disruption can occur even when the sound is ignored and unrelated to the task being performed. In a recent major development, Gisselgård et al. have used positron emission tomography to reveal how distracting sounds recruit the involvement of dorsolateral prefrontal cortex.

Auditory Perception↗
PubMed2006

What's so special about the average face?

The past few years have seen a marked increase in our understanding of how faces are represented in the brain, with the discovery of new anatomical structures and new algorithms for representing faces. Still, the basic computational mechanism used by the primate visual system to identify faces remains a topic of intense debate. Are faces represented by matching to a set of stored exemplars, or by measuring the distance from a standard prototype along a set of different axes? A recent article by Rhodes and Jeffery provides compelling psychophysical evidence in favor of the latter 'axis' model.

Association Learning↗
PubMed2004

Depth-dependent blur adaptation.

Variations in blur are present in retinal images of scenes containing objects at multiple depth planes. Here we examine whether neural representations of image blur can be recalibrated as a function of depth. Participants were exposed to textured images whose blur changed with depth in a novel manner. For one group of participants, image blur increased as the images moved closer; for the other group, blur increased as the images moved away. A comparison of post-test versus pre-test performances on a blur-matching task at near and far test positions revealed that both groups of participants showed significant experience-dependent recalibration of the relationship between depth and blur. These results demonstrate that blur adaptation is conditioned by 3D viewing contexts.

Adaptation, Ocular↗
PubMed2004

Illusory reversal of temporal order: the bias to report a dimmer stimulus as the first.

When two objects are presented in rapid succession, observers find it difficult to discriminate their temporal order. Below certain limit (e.g., 20-70 ms), the rate of correct temporal order judgement is reported to be about 50% (i.e., close to chance level). However we have found stimulus conditions where order discrimination drops significantly below chance level: the stimulus that is presented as the second is reported as the first. It is necessary that the stimuli are very brief, spatially overlapping, clear-cut backward and forward masking is absent, stimulus onset asynchronies are very short, and luminance contrast of the following stimulus is considerably lower than luminance contrast of the first stimulus. The higher the contrast ratio, the stronger the order reversal effect. However, because also in the conditions where the two stimuli are presented synchronously, the dimmer target is perceived as the first, the effect should be attributed to some implicit bias which enforces subjects to regard a more contrasted stimulus as the one that appears subsequent to the less contrasted stimulus.

Humans↗
PubMed2004

Contrast dependency of sacadic compression and suppression.

In the occurrence of a saccadic eye movement vision becomes suppressed. Supra-threshold visual stimuli that are briefly presented at that time become perceptually compressed towards the saccade target (saccadic compression) and shifted in saccade direction (saccadic shift). We show that the strength of saccadic compression, like the strength of saccadic suppression, varies with stimulus contrast. Low contrast stimuli lead to stronger compression than high contrast stimuli. The similarity of contrast dependence and time course suggests that saccadic compression is related to saccadic suppression. Because the saccadic shift did not depend on contrast we suggest that shift and compression are different effects.

Adult↗
PubMed2005

Center-surround inhibition deepens binocular rivalry suppression.

When dissimilar stimuli are presented to each eye, perception alternates between both images--a phenomenon known as binocular rivalry. It has been shown that stimuli presented in proximity of rival targets modulate the time each target is perceptually dominant. For example, presenting motion to the region surrounding the rival targets decreases the predominance of the same-direction target. Here, using a stationary concentric grating rivaling with a drifting grating, we show that a drifting surround grating also increases the depth of binocular rivalry suppression, as measured by sensitivity to a speed discrimination probe on the rival grating. This was especially so when the surround moved in the same direction as the grating, and was slightly weaker for opposed directions. Suppression in both cases was deeper than a no-surround control condition. We hypothesize that surround suppression often observed in area MT (V5)-a visual area implicated in visual motion perception-is responsible for this increase in suppression. In support of this hypothesis, monocular and binocular surrounds were both effective in increasing suppression depth, as were surrounds contralateral to the probed eye. Static and orthogonal motion surrounds failed to add to the depth of rivalry suppression. These results implicate a higher-level, fully binocular area whose surround inhibition provides an additional source of suppression which sums with rivalry suppression to effectively deepen suppression of an unseen rival target.

Humans↗
PubMed2005

Phenomenal regression to the frontal and natural picture.

The retinal image of a figure on a slanted picture is narrower than that of a figure on a frontal picture. In this study, the perceived width of various figures (horizontal line segments, ellipses, faces, symbolic faces, and artistic pictures) on a slanted picture plane was measured. The width of the figures was magnified or reduced in order to vary the naturalness of the original figures. The perceived width was found to be much closer to the width of the original figures than to the retinal images of the slanted figures. The width of the original figures was also found to affect the perceived width of the slanted figures; the perceived width was observed to be more biased toward a more natural width. On the other hand, the naturalness of the figures did not affect the perceived slant. These results suggest that the visual system corrected the width of the figures on a slanted plane, taking into the account naturalness or prägnanz as well as the slant.

Discrimination Learning↗
PubMed2005

Holistic processing unites face parts across time.

When complementary halves of different familiar faces are combined into a new face, there is interference in the identification of either half. This "composite face effect" has been taken as strong evidence that faces are processed holistically. Here, we demonstrate that this effect can persist when the two parts of a face are separated by up to 80 ms of visual noise, showing that the parts of a face interact not only spatially but also temporally. We suggest that the processing underlying robust identification accepts an accumulation of evidence over time.

Adolescent↗
PubMed2005

Orientation tuning of human face processing estimated by contrast matching in transparency displays.

Upright images of faces appear more salient than faces of other orientations. We exploited this effect in a titration experiment where faces were superimposed in transparency. By manipulating the physical contrast of the component images, we measured the degree of perceptual dominance as function of the orientation of the face in the image plane. From these measurements, we obtain the orientation tuning of face processing, which is well approximated by a Gaussian function with a SD of about 45 deg and mean centered on upright. Faces predominantly lit from above and from below produced very similar results. However, when presented with scrambled faces observers showed no orientation preference. We argue that these results can be explained by the existence of specialized face processing mechanisms with an orientation tuning with a bandwidth of approximately 90 deg, predominantly centered on the upright orientation and easily disrupted by alterations of the normal facial configuration.

Contrast Sensitivity↗