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At least 829 records · Page 46Linked to original sources
PubMed2004

Perceptual dominance time distributions in multistable visual perception.

Perceptual multistability, alternative perceptions of an unchanging stimulus, gives important clues to neural dynamics. The present study examined 56 perceptual dominance time series for a Necker cube stimulus, for ambiguous motion, and for binocular rivalry. We made histograms of the perceptual dominance times, based on from 307 to 2478 responses per time series (median=612), and compared these histograms to gamma, lognormal and Weibull fitted distributions using the Kolmogorov-Smirnov goodness-of-fit test. In 40 of the 56 tested cases a lognormal distribution provided an acceptable fit to the histogram (in 24 cases it was the only fit). In 16 cases a gamma distribution, and in 11 cases a Weibull distribution, were acceptable but never as the only fit in either case. Any of the three distributions were acceptable in three cases and none provided acceptable fits in 12 cases. Considering only the 16 cases in which a lognormal distribution was rejected ( p<0.05) revealed that minor adjustments to the fourth-moment term of the lognormal characteristic function restored good fits. These findings suggest that random fractal theory might provide insight into the underlying mechanisms of multistable perceptions.

Algorithms↗
PubMedDate not supplied

The right hemisphere as an anomaly detector: evidence from visual perception.

V. S. Ramachandran (1998) has suggested that the right hemisphere, which tends to be specialized for the analysis of global-level information, serves as an anomaly detector. Its role is to judge whether a given percept is possible and whether there are elements of that percept that seem incongruent with the other elements. In contrast, the left hemisphere tends to create a "story" to make sense of the incongruities. In the current study, pictures of possible or impossible objects were displayed for brief exposure durations to either the left visual field/right hemisphere or to the right visual field/left hemisphere). The results provide tentative support for the work of Ramachandran. In male participants, the right hemisphere was superior to the left in detecting impossible objects.

Brain↗
PubMedDate not supplied

[Amplitude-temporal parameters of the evoked potentials of the visual and motor areas in the visual perception and mental representation of an image].

Amplitude-temporal analysis was carried out of the EP components of the visual and motor areas elicited by neutral (diffuse light) and structural (checker board pattern) stimuli in different situations, defined by instruction. Interserial comparisons showed that at any instruction, the latency of the first EP component of the motor areas is reduced; as a result it can appear here simultaneously with the EP of the visual areas. At the instruction involving the subject in the process of active change of perception, activation of the right hemisphere, including the motor area, is manifest by EP parameters, while the right occipital area is activated in response to the structural stimulus, and the left one--in response to the neutral stimulus. At complication of the stimulus or instruction, the period is prolonged when the latency of EP components of the motor area is shorter than the latency of the isopolar components of the visual area--from 120 to 150 ms in response to the neutral stimuli and the neutral with their counting; from 90 to 150 ms in response to the structural stimuli; from 80 to 210 ms in response to the neutral stimuli with mental representation of the structural one.

Adolescent↗
PubMedDate not supplied

Influence of internal structure of hair fiber on hair appearance. II. Consideration of the visual perception mechanism of hair appearance.

The optical properties of hair fibers were studied, focusing on the reflections (highlights) from both the front and the back surfaces of the fiber in consideration and on the effect these have on the perceptions of hair appearance. The two reflections are distinguished from each other by sight, because only the back surface reflection is colored by melanin granules and/or dyestuffs inside the fiber. When we observe a flat plate as a model for hair without a cuticle structure, the visual angle between the two light loci correlates with the thickness of the model plate and gives an impression of depth. In the case of hair with a cuticle angle, the visual angle is maintained even when the fiber thickness is reduced. This visual angle causes an overestimation of the thickness and enhances the impression of depth. The visual angle changes dramatically with a curl curvature change of the hair tress, meaning that the impression of depth is also dynamically changed by a small change in hairstyle. The dynamic change in the impression of depth probably causes a vibrant impression. The following are required for beautiful hair appearance along with an impression of depth and vibrancy: (a). Internal structure without light scattering origins is essential to observe vivid colored highlights from the back surface. (b). Well-ordered cuticles are essential to get intensive double highlights from the front and back surfaces. (c). A properly curved hairstyle is essential to obtaining a more vibrant impression.

Female↗
PubMed2001

Modulation of visual perception by eye gaze direction in patients with spatial neglect and extinction.

Parietal neurons have retinotopic receptive fields whose response is modulated by eye gaze signals. We examined the role of eye position and eye movement direction on visual extinction in three patients with parietal damage and left neglect, characterized by impaired perception of contralesional field stimuli despite intact visual cortex. Patients tracked a horizontally moving fixation point, while discriminating fine changes at the fixated point and detecting more salient targets flashed on either side of fixation. Eye position modulated extinction, with worse detection of left field targets during eccentric fixation towards the left. Direction of eye movement also modulated extinction, with worse detection of left targets during rightward scanning. These results demonstrate that extinction in contralesional field is influenced by extra-retinal factors related to eye position and eye movements, consistent with a convergence of these signals in parietal cortex.

Aged↗
PubMed2005

Vagaries of visual perception in autism.

Three classes of perceptual phenomena have repeatedly been associated with autism spectrum disorder (ASD): superior processing of fine detail (local structure), either inferior processing of overall/global structure or an ability to ignore disruptive global/contextual information, and impaired motion perception. This review evaluates the quality of the evidence bearing on these three phenomena. We argue that while superior local processing has been robustly demonstrated, conclusions about global processing cannot be definitively drawn from the experiments to date, which have generally not precluded observers using more local cues. Perception of moving stimuli is impaired in ASD, but explanations in terms of magnocellular/dorsal deficits do not appear to be sufficient. We suggest that abnormalities in the superior temporal sulcus (STS) may provide a neural basis for the range of motion-processing deficits observed in ASD, including biological motion perception. Such an explanation may also provide a link between perceptual abnormalities and specific deficits in social cognition associated with autism.

Autistic Disorder↗
PubMed1975

[Prosopagnosia. Role of the right hemisphere in visual perception. (Apropos of a case after right occipital lobectomy)].

The authors describe a case of prosopagnosia which appeared a month after right occipital lobectomy. Results obtained by means of various tests devised to analyse this perception disorder have produced the following conclusions: 1) results of pairing tests which require a high level of perceptive differentiation are successful but after abnormally long delays; 2) in spite of this success, faces--even the patient's own--are not recognized; 3) this disorder extends to affect every task requiring recognition of the individual nature of a stimulus; 4) this individuality has the peculiarity of being based on visual patterns which it is impossible, or almost impossible, to express verbally; 5) the part played by defective visual representation in the brain is discussed; 6) a posterior lesion of the minor hemisphere is a prerequisite of prosopagnosia; 7) these facts, in conjunction with the findings in cases of inter-hemispheric disconnection lead one to think that the minor hemisphere, in right-handed subjects, plays a major part in the development and treatment of highly differentiated visual patterns which it is impossible, or almost impossible, to express verbally.

Adult↗
PubMed1986

Visual perception and image display terminals.

It should be clear that there is no prescription for an ideal picture terminal or for an ideal image work station. Image informativeness, the ability to transmit the relevant diagnostic information to the viewer, is the most important medical consideration that has an impact on the individual patient, but short of doing extensive observer performance studies, there is no way of being absolutely sure that any given display method is transmitting visual information optimally. Certain principles are clear, however. Pixel clutter should be eliminated, and stochastic noise should be minimized. Contrast should be maximized, but there must be a rational relationship between the intensity of the stored image and the luminance of the displayed image. In my opinion, there should be a monotonically increasing relationship. Cross-sectional images and projection images, especially those of large body parts such as the chest, may not be analogous with regard to the method for contrast enhancement. Windowing, which works well for cross-sectional images, may produce bizarre appearances in projection images because of the nonlinearities in image intensity that are caused by overlapping structures. There is little objective information about the best format for displaying single images of large body parts or multiple images of any type. Although the development of picture terminals will undoubtedly be driven by economics and by the available technology, it might prove very fruitful to make the effort required to define the specifications for a picture terminal on the basis of the optimal transfer of visual information. Fortunately, people are highly adaptable and can learn to use almost anything to their advantage, but it would be a pity if digital image readers of the future have to decide which terminal or station is best on the basis of image aesthetics rather than diagnostic performance.

Computers↗
PubMed2000

Visual perception: Learning to see through noise.

New studies show that perceptual learning does not reduce the noise inherent to the neuronal mechanisms of perception. Rather, learning boosts the brain's capability to extract and make use of the relevant outside signal - but where and how the neuronal changes occur is still unknown.

Humans↗
PubMed1985

Hemispheric asymmetry in visual perception of temporal order.

The perception of temporal order is investigated. Suitable localization of the signals is also used to study the functional asymmetry of the cerebral hemispheres during perception of temporal order. The experimental data obtained correlate well with earlier assumptions of ours, namely: (a) separate mechanism for perception of temporal order; (b) the right hemisphere is dominant for perception of the order of events in time; (e) dependence of the subjective threshold for ordering of events in time on different physical characteristics of the stimuli.

Dominance, Cerebral↗
PubMed2005

Hallucinations and pathological visual perceptions in Maupassant's fantastical short stories--a neurological approach.

Maupassant excelled as a realist writer of the nineteenth century, with fantastical short stories being an outstanding example of his literary genius. We have analysed four of his fantastical stories from a neurological point of view. In "Le Horla," his masterpiece, we have found nightmares, sleep paralysis, a hemianopic pattern of loss and recovery of vision, and palinopsia. In "Qui sait" and in "La main" there is also an illusory movement of the objects in the visual field, although in a dreamlike complex pattern. In "Lui," autoscopy and hypnagogic hallucinations emerge as fantastical key elements. The writer suffered from severe migraine and neurosyphilis involving the optic nerve, which led to his death by general paralysis of the insane (GPI). Visual loss and visual hallucinations affected the author in his last years, before a delirant state confined him to a nursing home. Our original hypothesis, which stated that he could have translated his sensorial experiences coming from this source to his works, had to be revised by analyzing some of his earliest works, notably "Le Docteur Héraclius Gloss" and "La main d'écorché" (1875). We found hallucinatory symptoms, adopting the form of autoscopy and other elaborated visual misperceptions, in stories written at age 25, when Maupassant was allegedly healthy. Therefore, we hypothesize that they may be related to his hypersensitive disposition, assuming that no pathology is necessary to experience such vivid experiences. In addition, Maupassant's abuse of drugs, as illustrated in "Rêves," could have provided an additional element to outline his painstaking visual depictions. All these factors, in addition to his up-to-date neurological knowledge and attendance at Charcot's lectures at "La Salpêtrière," armed the author for repetitive and enriched hallucinatory experiences, which were transferred relentlessly into his works from the beginning of his career.

Fantasy↗