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At least 1,297 records · Page 72Linked to original sources

PubMed2004

Perceptual grouping of biological motion promotes binocular rivalry.

Investigation of perceptual rivalry between conflicting stimuli presented one to each eye can further understanding of the neural underpinnings of conscious visual perception. During rivalry, visual awareness fluctuates between perceptions of the two stimuli. Here, we demonstrate that high-level perceptual grouping can promote rivalry between stimulus pairs that would otherwise be perceived as nonrivalrous. Perceptual grouping was generated with point-light walker stimuli that simulate human motion, visible only as lights placed on the joints. Although such walking figures are unrecognizable when stationary, recognition judgments as complex as gender and identity can accurately be made from animated displays, demonstrating the efficiency with which our visual system can group dynamic local signals into a globally coherent walking figure. We find that point-light walker stimuli presented one to each eye and in different colors and configurations results in strong rivalry. However, rivalry is minimal when the two walkers are split between the eyes or both presented to one eye. This pattern of results suggests that processing animated walker figures promotes rivalry between signals from the two eyes rather than between higher-level representations of the walkers. This leads us to hypothesize that awareness during binocular rivalry involves the integrated activity of high-level perceptual mechanisms in conjunction with lower-level ocular suppression modulated via cortical feedback.

Dominance, Ocular↗

PubMed1999

Unconscious letter discrimination is enhanced by association with conscious color perception in visual form agnosia.

Adaptive behavior guided by unconscious visual cues occurs in patients with various kinds of brain damage as well as in normal observers, all of whom can process visual information of which they are fully unaware [1] [2] [3] [4] [5] [6] [7] [8]. Little is known on the possibility that unconscious vision is influenced by visual cues that have access to consciousness [9]. Here we report a 'blind' letter discrimination induced through a semantic interaction with conscious color processing in a patient who is agnosic for visual shapes, but has normal color vision and visual imagery. In seeing the initial letters of color names printed in different colors, it is normally easier to name the print color when it is congruent with the initial letter of the color name than when it is not [10]. The patient could discriminate the initial letters of the words 'red' and 'green' printed in the corresponding colors significantly above chance but without any conscious accompaniment, whereas he performed at chance with the reverse color-letter mapping as well as in standard tests of letter reading. We suggest that the consciously perceived colors activated a representation of the corresponding word names and their component letters, which in turn brought out a partially successful, unconscious processing of visual inputs corresponding to the activated letter representations.

Adult↗

PubMed2006

[Functional results after intraocular lens implantation with or without blue light filter: an intraindividual comparison].

BACKGROUND: The new generation of intraocular lenses (IOL) with an additional blue light filter has a slight yellowish colour (compared to the IOLs with only a UV filter) due to its different light transmitting properties. This could have an effect on contrast sensitivity and subjective visual perception. PATIENTS AND METHODS: In this intraindividual prospective comparative study including 14 cataract patients without further ocular pathology, a blue light filtering IOL (SN60AT, Alcon) was implanted in one eye and 1 month later a conventional single-piece IOL (SA60AT, Alcon) was fitted into the fellow eye. The visual acuity and the clinical findings were assessed at one day and four weeks post-operatively. Contrast sensitivity was tested under defined mesopic (6 cd/m(2)) and high mesopic (18.8 cd/m(2)) light conditions in the Ginsburg Box using the Functional Acuity Contrast Test (F.A.C.T.). Furthermore, subjective differences in the visual perception of both eyes were noted. RESULTS: The contrast sensitivity testing 1 day and 1 month postoperatively, revealed no statistically significant differences (P > 0.008) between both IOL types for all spatial frequencies (1.5/3/6/12/18 cpd) and light levels. 13 patients (85 %) reported no differences in the colour perception of both eyes, and none had visual disturbances. CONCLUSION: This intraindividual comparison revealed no relevant differences concerning mesopic contrast sensitivity and the subjective visual perception after implantation of one IOL with or without a blue light filter.

Aged↗

PubMed1991

Results of speech perception and speech production training for three prelingually deaf patients using a multiple-electrode cochlear implant.

Five studies were conducted to measure changes in the perception and production of selected speech targets, with training, in three prelingually deaf patients. The two adults and one adolescent were implanted with the Cochlear (Nucleus) multiple-electrode prosthesis. The studies were perception and production of nasal consonants; perception of syllable-final consonants; perception and production of alveolar consonants; auditory-visual perception of alveolar consonants; and perception and production of vowels. Perceptual data were collected in the audition (implant)-alone condition, except for the auditory-visual perception of alveolar consonants where the audition-alone, vision-alone, and auditory-visual conditions were used. Speech perception data in the audition-alone condition were also collected from four postlingually deaf adult implant patients, without training, to indicate differences between the two classes of patients. The three prelingually deaf patients generally showed some improvements in speech production. In perception, improvements were recorded only for individual patients in some studies. The performance of the adolescent was better than that of the two adults in all cases. The perceptual performance of the postlingually deaf patients was superior to that of the prelingually deaf patients in all cases.

Adolescent↗

PubMed2006

Auditory-visual speech perception and synchrony detection for speech and nonspeech signals.

Previous research has identified a "synchrony window" of several hundred milliseconds over which auditory-visual (AV) asynchronies are not reliably perceived. Individual variability in the size of this AV synchrony window has been linked with variability in AV speech perception measures, but it was not clear whether AV speech perception measures are related to synchrony detection for speech only or for both speech and nonspeech signals. An experiment was conducted to investigate the relationship between measures of AV speech perception and AV synchrony detection for speech and nonspeech signals. Variability in AV synchrony detection for both speech and nonspeech signals was found to be related to variability in measures of auditory-only (A-only) and AV speech perception, suggesting that temporal processing for both speech and nonspeech signals must be taken into account in explaining variability in A-only and multisensory speech perception.

Acoustic Stimulation↗

PubMed1999

Categorization of ambiguous sentences as a function of a changing prosodic parameter: a dynamical approach.

The present study investigates the dynamics of changes in interpretation of ambiguous sentences. The sentences used had alternative interpretations with different surface structures (bracketing). Continua were created by systematic manipulation of prosodic cues (relative foot duration), resulting in stimulus sentences that spanned the range between the two interpretations. Continua were presented to subjects who were requested to indicate as quickly as possible which meaning they perceived. The pattern of responses and response times revealed the presence of hysteresis. That is, when the values of prosodic parameters are congruent with both interpretations, the individual's recent history decides which meaning will be perceived. Thus, we can treat this categorization process as a transition from an initially stable meaning that loses stability with variations in prosody (our control parameter). The same underlying dynamics have been observed in studies of perception of syllables and aspects of visual perception. Apart from demonstrating the same characteristics of pattern formation at various levels of cognition, the study points to the usefulness of the dynamical approach in the investigation of language understanding.

Cognition↗

PubMed1992

[Ambulatory autonomy and visual motion perception in a case of almost total cortical blindness].

A 37-year-old man experienced cortical blindness following a bilateral stroke in the territory of the posterior cerebral arteries. Four years later, the measurement of visual field defects (Goldmann perimeter) showed persistence of bilateral blindness with a 2-degree preservation of macular vision and a perifoveal sparing between 10 to 30 degrees of eccentricity in the left inferior quadrant. Despite this visual impairment, the subject was able to perform visually-guided locomotion. Moreover he consciously perceived visual motion in the blind parts of his visual field. CT and MRI showed a lesion involving most of the striated cortex. The visual cortex located in the internal occipito-parital regions was relatively spared. The contribution of this structure to extra-striated vision of motion is discussed.

Adult↗

PubMed2004

Functional equivalence of spatial representations derived from vision and language: evidence from allocentric judgments.

Past research (e.g., J. M. Loomis, Y. Lippa, R. L. Klatzky, & R. G. Golledge, 2002) has indicated that spatial representations derived from spatial language can function equivalently to those derived from perception. The authors tested functional equivalence for reporting spatial relations that were not explicitly stated during learning. Participants learned a spatial layout by visual perception or spatial language and then made allocentric direction and distance judgments. Experiments 1 and 2 indicated allocentric relations could be accurately reported in all modalities, but visually perceived layouts, tested with or without vision, produced faster and less variable directional responses than language. In Experiment 3, when participants were forced to create a spatial image during learning (by spatially updating during a backward translation), functional equivalence of spatial language and visual perception was demonstrated by patterns of latency, systematic error, and variability.

Female↗

PubMed1992

The effect of unilateral brain lesions on perception of visual illusions.

The study tested the effect of the left and right hemisphere lesions on the susceptibility to visual illusions. Twenty-five patients with left hemisphere damage, 22 patients with right hemisphere damage and 23 control subjects inspected series of figures producing four different illusions: Ponzo, Ehrenstein-Orbison, Poggendorff and Zoellner. Series of stimuli configurations were constructed so that different degrees of distortion opposite to the illusory effect were introduced in each pattern. The amount of distortion was increased in steps so that at a certain point of the series the patterns produced percepts opposite to the illusory ones. That point indicated the strength of the illusion. Stimuli of a given series were presented to each subject 5 times in random order and subject's judgements of illusions were collected. The illusion strength was established using the Spearman distribution method for determining psychophysical thresholds. The results showed that the left and right hemisphere lesions either did not affect the subject's susceptibility to illusions (in Ponzo and Poggendorff) or their effects were similar, i.e. they reduced the strength of the perceived illusions (in Ehrenstein-Orbison and Zoellner). Our data contradict the hypothesis of hemisphere differences in the perception of visual illusions and support the view that those phenomena are multiple-caused.

Attention↗

PubMed1991

Visual motion perception from stimulation of the human medial parieto-occipital cortex.

Visual phenomena evoked by direct electrical stimulation of extrastriate cortex were observed in 30 epileptic patients as part of a presurgical investigation. An incremental sequence of low-level bipolar stimulation trains was delivered at medial and lateral pairs of contacts of stereotaxically-implanted multilead intracerebral electrodes in parietal, occipital and posterior temporal regions. Diffusion of stimulus afterdischarges was monitored by electrodes in temporal and frontal lobes and by the non-stimulated contacts of the stimulated electrode. Localized stimulations evoked few visual phenomena. The strongest anatomo-perceptual correlation was found for stimulation in the medial parieto-occipital fissure which evoked visual motion phenomena in all three patients stimulated in that region. The evoked motion perceptions were not associated with eye movements or any particular localization of the epileptic focus. These perceptions were only evoked once outside of the medial PO region at the 61 sites examined. The results suggest that the medial parieto-occipital region is closely linked to the human visual motion processing system.

Brain Mapping↗

PubMed1996

Perception of visual apparent movement and topographic analysis of related evoked potentials.

Most of the studies in long-range apparent movement have been concerned with the psychophysical properties in central vision. However, apparent movement in peripheral vision is not clearly understood. This study focuses on the properties of the perception of apparent movement between central and peripheral vision. The results show that judgements of apparent movement in central vision indicate high performance at less than 4 degrees of spatial separation, and that judgements in peripheral vision indicate high performance at greater than 4 degrees. Evoked potentials associated with long-range apparent movement were also measured, and topographic maps were constructed to evaluate the active areas corresponding to visual apparent movement. The topographic maps show that the central regions are activated above 300 msec after the onset of the second apparent-movement stimulation. The results suggest that visual apparent movement occurs in the visual association area, or at higher levels in the central nervous system.

Brain↗

PubMed1990

Towards a developmental cognitive science. The implications of cross-modal matching and imitation for the development of representation and memory in infancy.

This chapter began with a query about whether there was any content to an enterprise called "developmental cognitive science," and if so, whether the findings could inform work in adult cognition and neuropsychology. Both questions can now be answered in the affirmative. Evidence has been marshaled from infant studies concerning five topics of enduring interest in the cognitive and neuro-sciences: cross-modal integration, imitation, the coordination of perception and action, memory, and representation. The data show that young human infants can detect equivalences between information picked up by different sensory modalities. This was demonstrated both in tactual-visual perception of objects and auditory-visual perception of speech. Results also show that perception and production are intertwined literally from the earliest phases of infancy, with 4-month-olds demonstrating vocal imitation and newborns reproducing elementary gestures they saw an adult perform. There seems to be a transparency between the perceptual and motor systems, and it is conceivable that they may draw on the same internal code. Infants' proclivity to imitate was used to investigate early memory. It was found that young infants were not constrained to immediate mimicry, but could imitate after significant delays. The findings support the inference that infants, perhaps as early as birth, have a functioning memory system that cannot be reduced to "habit formation" or an exclusively "procedural memory." It was proposed instead that there is a kernel of some higher level memory system right from the earliest phases of human infancy. This does not imply that there is no development in the representational world of infants. Data were reviewed suggesting that there is a watershed transformation in childhood cognition at about 18 months of age. However, this is not a change from a stage in which there was a purely sensorimotor or habit-based system. Rather the development was characterized as a shift from using empirical or experience-based representations to using hypothetical representations, which concern possible realities. This developmental shift allows children to project into the future "what must be" and deduce from the past "what must have been," in advance of, and sometimes in the absence of, strictly perceptual evidence. This capacity provides the underpinnings for the conduct of science itself. Its origins are to be found in infancy.

Aging↗

PubMed1988

Visual event perception in alcoholics.

A series of microcomputer-based procedures were devised to investigate the ability of alcoholics to detect and locate rapid visual changes, and to assess deficits of visual processing in this patient population. Forty alcoholics were tested 7-13 days after cessation of drinking (mean period of abstinence = 9.5 days), and 24 were retested 2-3 weeks later (mean abstinence = 27.8 days). In comparison to a matched control group the alcoholics were impaired on initial testing at locating visual events. Alcoholics also had significantly slower movement times in a visual-choice reaction time task, and were slower on visual search. At retest the alcoholics were impaired on one of the event perception tasks. Alcoholics were not impaired initially on a measure of visual memory span but were impaired at retest, possibly due to a learning deficit. It is concluded that alcoholics show a persisting deficit in the ability to notice and locate rapid visual changes, together with a transitory impairment on timed tasks requiring visually guided movement.

Adult↗

PubMed2006

Auditory motion perception activates visual motion areas in early blind subjects.

We have previously shown that some visual motion areas can be specifically recruited by auditory motion processing in blindfolded sighted subjects [Poirier, C., Collignon, O., De Volder, A.G., Renier, L., Vanlierde, A., Tranduy, D., Scheiber, C., 2005. Specific activation of V5 brain area by auditory motion processing: an fMRI study. Brain Res. Cogn. Brain Res. 25, 650-658]. The present fMRI study investigated whether auditory motion processing may recruit the same brain areas in early blind subjects. The task consisted of simultaneously determining both the nature of a sound stimulus (pure tone or complex sound) and the presence or absence of its movement. When a movement was present, blind subjects had to identify its direction. Auditory motion processing, as compared to static sound processing, activated the brain network of auditory and visual motion processing classically observed in sighted subjects. Accordingly, brain areas previously considered as specific to visual motion processing could be specifically recruited in blind people by motion stimuli presented through the auditory modality. This indicates that the occipital cortex of blind people could be organized in a modular way, as in sighted people. The similarity of these results with those we previously observed in sighted subjects suggests that occipital recruitment in blind people could be mediated by the same anatomical connections as in sighted subjects.

Adolescent↗

PubMedDate not supplied

Neuromediators in the neuronal mechanisms for attention and the perception of visual information during food-seeking behavior.

Neurons of the visual cortex were investigated in experiments with cats during the expectation of and at the time of milk presentation in a feeder. The milk was delivered either as a result of the animal pressing a pedal or extraneously, without pressing. It was determined that the presence of the directed visual attention of the animal towards the site of reinforcement appearing after pedal pressing is accompanied by an increase in the level of the impulse activity of individual neurons. The activation of cortical neurons at the time of the perception of the appearance of milk in the feeder was also pronounced in cases of expected reinforcement. The neuromediators tended to alter the discharge activity of individual neurons in both experimental situations.

Acetylcholine↗

PubMed1990

Form perception and visual acuity in a chimpanzee.

A 6.5-year-old female chimpanzee learned to distinguish perfectly every letter of the alphabet in a matching-to-sample task with 26 letters as choice alternatives. Confusion of letters during the initial training was used to scale them in a multidimensional similarity space and to associate them in hierarchical clusters. The results resembled those obtained from similarity judgements by humans. Using letters of various sizes, a visual acuity test revealed that the chimpanzee's acuity was about 1.5, comparable to that in normal humans. The chimpanzee also readily learned to use letters as names of individual humans and chimpanzees. A species-typical feature was identified in the perceptual processes associated with complex forms, such as those involved in individual recognition.

Algorithms↗

PubMed2001

[Studies of vision by Leonardo da Vinci].

Leonardo was an advocate of the intromission theory of vision. Light rays from the object to the eye caused visual perceptions which were transported to the brain ventricles via a hollow optic nerve. Leonardo introduced wax injections to explore the ventricular system. Perceptions were assumed to go to the "senso comune" in the middle (3rd) ventricle, also the seat of the soul. The processing station "imprensiva" in the anterior lateral horns together with memory "memoria" in th posterior (4th) ventricle integrated the visual perceptions to visual experience. - Leonardo's sketches with circular lenses in the center of the eye reveal that his dependence on medieval optics prevailed over anatomical observations. Drawings of the anatomy of the sectioned eye are missing although Leonardo had invented a new embedding technique. In order to dissect the eye without spilling its contents, the eye was first boiled in egg white and then cut. The procedure was now repeated and showed that the ovoid lens after boiling had become spherical. - Leonardo described that light rays were refracted and reflected in the eye but his imperfect anatomy prevented a development of physiological optics. He was, however, the first to compare the eye with a pin-hole camera (camera obscura). Leonardo's drawings of the inverted pictures on the back wall of a camera obscura inspired to its use as an instrument for artistic practice. The camera obscura was for centuries a model for explaining human vision.

Eye↗

PubMed2006

Quality evaluation of ultrasound imaging in the carotid artery based on normalization and speckle reduction filtering.

Image quality is important when evaluating ultrasound images of the carotid for the assessment of the degree of atherosclerotic disease, or when transferring images through a telemedicine channel, and/or in other image processing tasks. The objective of this study was to investigate the usefulness of image quality evaluation based on image quality metrics and visual perception, in ultrasound imaging of the carotid artery after normalization and speckle reduction filtering. Image quality was evaluated based on statistical and texture features, image quality evaluation metrics, and visual perception evaluation made by two experts. These were computed on 80 longitudinal ultrasound images of the carotid bifurcation recorded from two different ultrasound scanners, the HDI ATL-3000 and the HDI ATL-5000 scanner, before (NF) and after (DS) speckle reduction filtering, after normalization (N), and after normalization and speckle reduction filtering (NDS). The results of this study showed that: (1) the normalized speckle reduction, NDS, images were rated visually better on both scanners; (2) the NDS images showed better statistical and texture analysis results on both scanners; (3) better image quality evaluation results were obtained between the original (NF) and normalized (N) images, i.e. NF-N, for both scanners, followed by the NF-DS images for the ATL HDI-5000 scanner and the NF-DS on the HDI ATL-3000 scanner; (4) the ATL HDI-5000 scanner images have considerable higher entropy than the ATL HDI-3000 scanner and thus more information content. However, based on the visual evaluation by the two experts, both scanners were rated similarly. The above findings are also in agreement with the visual perception evaluation, carried out by the two vascular experts. The results of this study showed that ultrasound image normalization and speckle reduction filtering are important preprocessing steps favoring image quality, and should be further investigated.

Carotid Arteries↗