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Results for “Perceptual Distortion”

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PubMed2001

Deficit of visual contour integration in dyslexia.

PURPOSE: The visual processing of text occurs spontaneously in most readers. Dyslexic persons, however, often report both somatic symptoms and perceptual distortions when trying to read. It is possible that the perceptual distortions experienced by those with dyslexia reflect a disturbance in the basic mechanisms supporting perceptual organization at the early stages of visual processing. Integration of information over extended areas of visual space can be measured psychophysically in a task that requires the detection of a path defined by aligned, spatially narrow-band elements on a dense field of otherwise similar elements that are randomly oriented and positioned. In the present study a contour integration task was used to investigate such perceptual organization in dyslexia. METHODS: The detection of contours or paths composed of Gabor micropatterns was performed within a field of randomly oriented distracter elements in a 2-alternate forced choice (AFC) task. The stimuli were manipulated by randomly varying both the density of the background noise elements and the number of elements that defined a path of constant length. RESULTS: In all observers, sensitivity to the paths increased with the number of target elements comprising the path, and subjects in both groups exhibited similar trends in relative density of the stimuli. However, in all conditions, dyslexic observers were two to three times less sensitive to path stimuli than the control group. CONCLUSIONS: In the present study the authors have described a visual deficit in a global integration task in dyslexia. The pattern of deficits reported suggest that abnormal cooperative associations may be present in dyslexia that are indicative of poor perceptual integration.

Adult↗
PubMed1989

Psychodynamic features in borderline personality disorder identified by a subliminal test, the Defense Mechanism Test.

Twenty-seven psychiatric inpatients, diagnosed according to DSM-III and the Diagnostic Interview for Borderline (DIB), and 7 healthy controls were tested with the Defense Mechanism Test (DMT), a test of subliminal perception based on psychoanalytic theory. In the test a specific stimulus is presented subliminally in a tachistoscope and the patient's perceptual distortions are registered. Patients with borderline personality disorder (BPD) were compared with patients with other personality disorders, patients with schizophrenic disorders and healthy controls. Specific perceptual distortions were identified among patients with BPD. Some individual DMT signs correlated with some deviant behaviors as identified by the DIB. The findings supported the psychodynamic validity of the concept of borderline personality disorder. It is concluded that DMT is a promising research instrument that provides an empirical approach to crucial psychodynamic phenomena.

Adult↗
PubMed1997

Depersonalization and derealization during panic and hypnosis in low and highly hypnotizable agoraphobics.

The primary aim of the present study was to investigate the association between spontaneous experiences of depersonalization or derealization (D-D) during panic states and hypnosis in low and highly hypnotizable phobic individuals. Secondarily, the association among level of hypnotizability, capacity for imaginative involvement, and severity of phobic complaints was also assessed. Sixty-four patients with panic disorder with agoraphobia according to the DSM-III-R (American Psychiatric Association, 1987) criteria participated in the study. Proneness to experience D-D during hypnosis was positively related to hypnotizability, but only for agoraphobic patients who had already experienced these perceptual distortions during panic episodes. Correlations of level of hypnotizability and capacity for imaginative involvement with severity of agoraphobic complaints were not significant. These findings suggest that hypnotizability may be a mediating variable between two different, although phenotypically similar, perceptual distortions experienced during panic states and hypnosis. Implications for both theory and clinical practice are discussed.

Adolescent↗
PubMed2005

How perception impacts on drawings.

In 3 experiments the authors investigate how errors in perception produce errors in drawings. In Experiment 1, when Shepard stimuli were shown as a pair of tables, participants made severe errors in trying to adjust 1 part of the stimulus to match the other. When the table legs were removed, revealing a pair of parallelograms with minimal perspective cues, the illusion was weaker. The authors predicted that participants would err when drawing the table but not the parallelogram stimuli. The results of Experiment 2 support the prediction and establish a direct link between degree of perceptual distortion of the table stimuli and the severity of error in drawing. When drawing only the right-hand part of the figure, participants also erred to a greater degree in drawing the table than the parallelogram (Experiment 3). Collectively, the results suggest that perceptual distortion is linked with errors in drawing the table stimuli.

Adult↗
PubMed1986

Out-of-body experiences in schizophrenia. A questionnaire survey.

Questionnaires on perceptual distortions, symptoms of schizophrenia, and out-of-body experiences (OBEs) were completed by 71 volunteers with a history of schizophrenia and 40 control subjects (patients in a hospital accident ward). Significantly more of the schizophrenics (42%) than of the control group (13%) answered "yes" to a question about OBEs. However, a follow-up questionnaire showed that only 14% of schizophrenics (i.e., the same as the control group) had had "typical" OBEs, in which a change of viewpoint was reported. Those reporting typical OBEs did not report more perceptual distortions or symptoms of schizophrenia than did those reporting no OBEs, although those reporting other atypical experiences did. On this basis there is no evidence to consider the typical OBE as pathological or as symptomatic of schizophrenia.

Adult↗
PubMed2006

Screening for Meares-Irlen sensitivity in adults: can assessment methods predict changes in reading speed?

Two methods of assessing candidates for coloured overlays were compared with the aim of determining which method had the most practical utility. A total of 58 adults were assessed as potential candidates for coloured overlays, using two methods; a questionnaire, which identified self-reported previous symptoms, and a measure of perceptual distortions immediately prior to testing. Participants were classified as normal, Meares-Irlen sensitive, and borderline sensitive. Reading speed was measured with and without coloured overlays, using the Wilkins Rate of Reading Test and the change in speed was calculated. Participants classified as normal did not show any significant benefit from reading with an overlay. In contrast, a significant reading advantage was found for the borderline and Meares-Irlen participants. Current symptom rating was found to be a significant predictor of the change in reading speed, however the previous symptom rating was not found to be a reliable predictor. These data indicate that the assessment of perceptual distortions immediately prior to measuring colour preference and reading speed is the most meaningful method of assessing pattern glare and determining the utility of coloured overlays.

Adolescent↗
PubMed2002

The effect of coloured filters on the rate of reading in an adult student population.

Meares-Irlen Syndrome is characterised by visual stress (visual discomfort) and visual perceptual distortions that can be alleviated by individually prescribed coloured filters. The benefit from coloured filters can be demonstrated with the Wilkins Rate of Reading Test (WRRT). Previous research using individually prescribed coloured overlays (sheets of plastic placed on a page) found that between one-fifth and one-third of unselected school-children show a significant (> 5%) improvement in their rate of reading with their chosen overlay. This 5% cut-off has good sensitivity and specificity for predicting those children who will continue to voluntarily use their overlay for a sustained period. Previous research has concentrated on children, and we sought to investigate the immediate effect of overlays on rate of reading in an adult population. Subjects were 113 unselected university students who answered a symptom questionnaire and were tested with the Wilkins Intuitive Overlays and WRRT. Some symptoms were common: 73% reported sore or tired eyes when reading and 40% reported four to 12 headaches a year. One hundred of the subjects chose an overlay as improving their immediate perception of text. These subjects were significantly more likely to report perceptual distortions and visual discomfort on viewing text than subjects who did not choose an overlay. The 100 subjects read 3.8% faster with the overlay than without any overlay (p < 0.00001), whereas the 13 subjects who did not choose an overlay read 1.7% slower with a placebo overlay than without (p = 0.37). Of the subjects who chose an overlay, 38% read more than 5% faster with the overlay and 2% read more than 25% faster. These results are comparable with those obtained for children. We conclude that Meares-Irlen Syndrome is likely to be as common in adults as it is in children.

Adolescent↗
PubMedDate not supplied

Fixity of belief, perceptual aberration, and magical ideation in obsessive-compulsive disorder.

Clinicians and researchers have pondered the intersection of obsessive-compulsive disorder (OCD) and psychosis. We examined the records of 395 individuals seeking treatment for OCD and classified participants according to their most frequent or distressing obsession and compulsion. All participants completed measures of fixity of belief, perceptual distortions, magical ideation, and psychotic symptoms. Results indicated that individuals who reported fear of harming self or others via overwhelming impulse or by mistake, and those with religious obsessions, had poorer insight and more perceptual distortions and magical ideation than did individuals with other types of obsessions. These results did not appear to reflect mere differences in OCD severity. Results are discussed in light of previous findings showing that psychotic-like symptoms are associated with attenuated treatment outcome in OCD. More research is needed to assess the absolute magnitude of psychotic-like features in OCD patients with impulse/mistake and religious obsessions and to examine whether these features interfere with standard cognitive-behavioral therapy.

Adult↗
PubMed1999

[Effects of rotation and distortion on perceptual priming].

We conducted two experiments to specify the properties of the representation underlying perceptual priming. We transformed a particular property of stimuli between study and test. We used novel stimuli to eliminate contamination by semantic memory. They were rotated by 0 degree, 90 degrees, or 180 degrees in Experiment 1 and were distorted horizontally in Experiment 2. These transformations were chosen for topological structures of stimuli to be kept unchanged. In Experiment 1 the priming effect of the rotated stimuli was smaller than that of the identical ones. The rotational angles had no effect. More surprisingly, the horizontally distorted stimuli showed the same amount of priming as the identical ones in Experiment 2. In contrast, recognition performance of the rotated or horizontally distorted stimuli was lower than that of the identical ones. Statistical independence between priming and recognition was found in both experiments. Thus, recognition was statistically and functionally independent of priming. These results suggest that the priming representation encodes both orientation-independent and orientation-dependent information, and that the global topological structure is critical for priming.

Adult↗
PubMed1993

Autoerotic fatalities with power hydraulics.

We report two cases in which men used the hydraulic shovels on tractors to suspend themselves for masochistic sexual stimulation. One man developed a romantic attachment to a tractor, even giving it a name and writing poetry in its honor. He died accidentally while intentionally asphyxiating himself through suspension by the neck, leaving clues that he enjoyed perceptual distortions during asphyxiation. The other man engaged in sexual bondage and transvestic fetishism, but did not purposely asphyxiate himself. He died when accidentally pinned to the ground under a shovel after intentionally suspending himself by the ankles. We compare these cases with other autoerotic fatalities involving perceptual distortion, cross-dressing, machinery, and postural asphyxiation by chest compression.

Adult↗
PubMed2004

Beyond the K-hole: a 3-year longitudinal investigation of the cognitive and subjective effects of ketamine in recreational users who have substantially reduced their use of the drug.

RATIONALE: Ketamine is a dissociative anaesthetic that is also a drug of abuse. Previous studies have demonstrated persisting episodic and semantic memory impairments in recreational ketamine users 3 days after taking ketamine. However, the degree to which these deficits might be reversible upon reduction or cessation of ketamine use was not known. OBJECTIVE: To follow-up a population of ketamine users tested 3 years previously and examine whether impairments observed 3 days after drug use are enduring or reversible. METHODS: Eighteen ketamine users and 10 polydrug controls from studies conducted between 3 and 4 years earlier were re-tested on the same battery of cognitive tasks and subjective measures. These tapped episodic, semantic and working memory and executive and attentional functioning. Subjective schizotypal, dissociative, mood and bodily symptoms were also examined and a drug use history recorded. RESULTS: The ketamine users had reduced their frequency of use of ketamine by an average of 88.3%. Performance of ketamine users on tasks tapping semantic memory had improved and this improvement was correlated with their reduction in ketamine use. On tasks tapping episodic memory and attentional functioning, ketamine users still showed deficits compared to polydrug controls. Higher levels of schizotypal symptoms and perceptual distortions were exhibited by the ketamine group, although dissociative symptoms were similar to controls. CONCLUSIONS: These findings indicate that semantic memory impairments associated with recreational ketamine are reversible upon marked reduction of use; however, impairments to episodic memory and possibly attentional functioning appear long-lasting. In addition, schizotypal symptoms and perceptual distortions may persist after cessation of ketamine use. Ketamine users, or potential users, should be aware of the enduring effects of this drug on aspects of memory and subjective experience.

Adult↗
PubMed2000

Haptic interaction with virtual objects. Spatial perception and motor control.

This paper considers interaction of the human arm with "virtual" objects simulated mechanically by a planar robot. Haptic perception of spatial properties of objects is distorted. It is reasonable to expect that it may be distorted in a geometrically consistent way. Three experiments were performed to quantify perceptual distortion of length, angle and orientation. We found that spatial perception is geometrically inconsistent across these perceptual tasks. Given that spatial perception is distorted, it is plausible that motor behavior may be distorted in a way consistent with perceptual distortion. In a fourth experiment, subjects were asked to draw circles. The results were geometrically inconsistent with those of the length perception experiment. Interestingly, although the results were inconsistent (statistically different), this difference was not strong (the relative distortion between the observed distributions was small). Some computational implications of this research for haptic perception and motor planning are discussed.

Distance Perception↗
PubMed1997

The influence of societal factors on female body image.

This study was designed to identify factors associated with the perceptual and attitudinal components of female body image. The influence of society and factors thought to mediate the relationship between body image and society (field dependence, locus of control, and self-esteem) were investigated. Age and body mass index (BMI) were also included as independent variables. A total of 101 female university students in Australia ranging in age from 18 to 55 years (M = 24.11) participated in the study. A video camera apparatus (VCA) was used to assess perceptual distortion of body size. The VCA, the Body Esteem Scale, and the Appearance Evaluation subscale of the Multidimensional Body Self Relations Questionnaire were used to assess body satisfaction. On average, women underestimated their body sizes by 4%, and they typically wanted to be smaller than their actual body sizes. About two fifths of the women expressed moderate to strong negative feelings about both individual body parts and their bodies as a whole. Multiple regression analyses revealed that perceptual distortion of body size could not be predicted from the independent variables. Body satisfaction was best explained by societal factors, self-esteem, and BMI.

Adolescent↗
PubMed2001

Systematic distortions of auditory space perception following prolonged exposure to broadband noise.

Perceptual distortions referred to as aftereffects may arise following exposure to an adapting sensory stimulus. The study of aftereffects has a long and distinguished history [Kohler and Wallach, Proc. Am. Philos. Soc. 88, 269-359 (1944)] and a range of aftereffects have been well described in sensory modalities such as the visual system [Barlow, in Vision: Coding and Efficiency (Cambridge University Press, Cambridge, 1990)]. In the visual system these effects have been interpreted as evidence for a population of cells or channels specific for certain features of a stimulus. However there has been relatively little work examining auditory aftereffects, particularly in respect of spatial location. In this study we have examined the effects of a stationary adapting noise stimulus on the subsequent auditory localization in the vicinity of the adapting stimulus. All human subjects in this study were trained to localize short bursts of noise in a darkened anechoic environment. Adaptation was achieved by presenting 4 min of continuous noise at the start of each block of trials and was maintained by a further 15-s noise burst between each trial. The adapting stimulus was located either directly in front of the subject or 30 degrees to the right of the midline. Subjects were required to determine the location of noise burst stimuli (150 ms) in the proximity of the adapting stimulus following each interstimulus period of adaptation. Results demonstrated that following adaptation there was a general radial displacement of perceived sound sources away from the location of the adapting stimulus. These data are more consistent with a channel-based or place-based process of sound localization rather than a simple level-based adaptation model. A simple "distribution shift" model that assumes an array of overlapping spatial channels is advanced to explain the psychophysical data.

Adult↗
PubMed2001

Size perception in hemianopia and neglect.

Some debate remains as to whether underestimation of the horizontal size of objects in the left part of visual space is a general disturbance in spatial neglect. The issue is unclear because size perception may be influenced by factors other than neglect, e.g. visual field defects such as hemianopia. To disentangle these effects, we compared the performance of patients with pure neglect, pure hemianopia or both on the same size-comparison test. Whereas pure neglect was accompanied by misperception of horizontal object size, patients with pure hemianopia showed an even greater impairment of size perception. Accordingly, the area of maximum lesion overlap of these patients with impaired size perception was not centred around the parietotemporal cortex, which is typically associated with spatial neglect, but rather was found in the occipital lobe (Brodmann areas 17 and 18). The results suggest that spatial neglect is not an inevitable consequence of distorted size perception. The perceptual distortion of objects in the leftward parts of visual space is not sufficient to account for the occurrence of visuospatial neglect.

Aged↗
PubMed2005

Systematic distortions of perceptual stability investigated using immersive virtual reality.

Using an immersive virtual reality system, we measured the ability of observers to detect the rotation of an object when its movement was yoked to the observer's own translation. Most subjects had a large bias such that a static object appeared to rotate away from them as they moved. Thresholds for detecting target rotation were similar to those for an equivalent speed discrimination task carried out by static observers, suggesting that visual discrimination is the predominant limiting factor in detecting target rotation. Adding a stable visual reference frame almost eliminated the bias. Varying the viewing distance of the target had little effect, consistent with observers underestimating distance walked. However, accuracy of walking to a briefly presented visual target was high and not consistent with an underestimation of distance walked. We discuss implications for theories of a task-independent representation of visual space.

Computer Simulation↗
PubMed2003

The phenomenology of body image distortions induced by regional anaesthesia.

Patients with peripheral nerve or spinal cord lesions frequently report perceptual distortions related to position, shape, texture or temperature of the affected areas. This study aimed to describe the phenomenology of such body image alterations during the course of upper limb, lower limb or spinal anaesthetic blocks in patients (n = 36) undergoing orthopaedic surgery. Multimodal sensory testing and assessment of motor function were performed at regular intervals, and the relationship between the reported body image distortions and the progression of sensory and motor impairment was analysed. We found that perceptual changes concerning the shape and size of the deafferented limb occurred in the great majority of patients. In all of them, illusions of swelling, elongation or shortening of the limb coincided with the impairment of warm, cold and/or pinprick sensations, suggesting that thin myelinated Adelta- and/or unmyelinated C-fibres may provide a source of tonic modulation to the limb's cortical representation. Such perceptual alterations of shape and size of body parts differed clearly from postural illusions in terms of frequency, time course and influence of vision. In addition to perceptual changes in the deafferented area, almost half of the patients felt their unanaesthetized lips and/or mouth swelling during the course of upper limb block, suggesting the unmasking of dynamic interactions between somatotopically adjacent cortical representations. Conflicting sensations could co-exist in the patient's body image, such as the illusion of swelling of a limb, which, at the same time, was felt to be missing. The sense of ownership of the deafferented limb was impaired in some cases. These observations show that the perception of body shape and the awareness of its postural variations are built from different plastic models. They also underline the contribution of peripheral afferent activity to the maintenance of a unified body image.

Adolescent↗
PubMed2006

Macular hole: perceptual filling-in across central scotomas.

The present study examined perceptual distortions of a vertical line before and after macular hole (MH) surgery in 25 eyes of 24 patients. Participants' perceptual reports of distortions were classified as solid, bent right/left, thinned at the center, or broken. The majority of patients (72%) reported symmetrical distortions of the line pre-operatively. After surgery, participants with larger MHs were more likely to retain residual distortions. Of particular interest is the group reporting thinning of the line preoperatively, as the center should be perceptually missing. Examination of MH diameters in relation to the line perceptions indicated that the shape of the perceived line can be explained at the retinal level, while its continuity must be perceptually created at the cortical level.

Adult↗