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PubMed2005

Disparity-defined objects moving in depth do not elicit three-dimensional shape constancy.

Observers generally fail to recover three-dimensional shape accurately from binocular disparity. Typically, depth is overestimated at near distances and underestimated at far distances [Johnston, E. B. (1991). Systematic distortions of shape from stereopsis. Vision Research, 31, 1351-1360]. A simple prediction from this is that disparity-defined objects should appear to expand in depth when moving towards the observer, and compress in depth when moving away. However, additional information is provided when an object moves from which 3D Euclidean shape can be recovered, be this through the addition of structure from motion information [Richards, W. (1985). Structure from stereo and motion. Journal of the Optical Society of America A, 2, 343-349], or the use of non-generic strategies [Todd, J. T., & Norman, J. F. (2003). The visual perception of 3-D shape from multiple cues: Are observers capable of perceiving metric structure? Perception and Psychophysics, 65, 31-47]. Here, we investigated shape constancy for objects moving in depth. We found that to be perceived as constant in shape, objects needed to contract in depth when moving toward the observer, and expand in depth when moving away, countering the effects of incorrect distance scaling (Johnston, 1991). This is a striking example of the failure of shape constancy, but one that is predicted if observers neither accurately estimate object distance in order to recover Euclidean shape, nor are able to base their responses on a simpler processing strategy.

Cues↗
PubMed2001

Prior knowledge on the illumination position.

Visual perception is fundamentally ambiguous because an infinite number of three-dimensional scenes are consistent with our retinal images. To circumvent these ambiguities, the visual system uses prior knowledge such as the assumption that light is coming from above our head. The use of such assumptions is rational when these assumptions are related to statistical regularities of our environment. In confirmation of previous visual search experiments, we demonstrate here that the assumption on the illumination position is in fact biased to the above-left rather than directly above. This bias to the left reaches 26 degrees on average in a more direct shape discrimination task. Both right-handed and left-handed observers have a similar leftward bias. We discuss the possible origins of this singular bias on the illumination position.

Adolescent↗
PubMed1998

The more you know the less you can tell: inhibitory effects of visuo-semantic activation on modality specific visual misnaming.

WH, a 77-years old right-handed psychoanalyst, displayed modality specific visual misnaming as a sequel of an embolic stroke in the left posterior cerebral artery. WH's errors in visual object naming consisted mainly of semantic paraphasias and perseverations. His verbalizations during testing sometimes manifested a conflict between correct responses and perseverations. Analysis of the stream of information from visual perception via semantics to phonology suggested incomplete access from vision to semantics as the source of errors. The disconnection did not affect verbo-visual matching, though he was unable to reject names that did not correspond to visual stimuli. Action naming was relatively spared, but naming of pictures of actions with objects was worse than naming of pictures of intransitive actions. Tactile naming worsened with simultaneous vision of objects. In visual object naming the error rate increased with increasing familiarity of objects. We propose that an interaction of excitation and inhibition within a single semantic system can explain the clinical phenomena of modality specific visual misnaming.

Aged↗
PubMed2003

Binding personal and extrapersonal space through body shadows.

Shadows in visual scenes can have profound effects on visual perception. Here we have found that visual distracters distant from the body interfere with human spatial discrimination of tactile targets at a hand, particularly when the shadow of the stimulated hand stretches toward them in extrapersonal space. These findings suggest that shadows cast by a person's own body parts can bridge the gap between personal and extrapersonal space.

Adult↗
PubMed1982

[Psychosomatic correlations in pediatric ophthalmology].

Somatopsychic disturbances encompassing both ophthalmologic as well as child-psychiatric phenomena mainly occur in ADD: Central disorders of eye motoricity (strabismus) and disorders of visual perception are encountered. Dyslexia or dysorthography often occur as secondary phenomena. The psychosomatic or psychogenic disturbances affect not only the lid mechanism but also visualization. In the lid region, nictation tics and trichotillomania of the eyebrows and eyelashes occur. Psychic impairment of vision may range from a slight amblyopia, often associated with limitation of the field of vision, to complete amaurosis. The symptomatology is described and illustrated with examples.

Adolescent↗
PubMed1976

Away from a unisex psychology: individual differences in visual sensory and perceptual processes.

Four tests of visual perception were given to twenty-five men and twenty-five women. These were a test of acuity, threshold for four field positions, visual persistence, and a measure of comfortable brightness. Subjects also completed five personality questionnaires. In most measures, differences were found to be related to sex rather than to personality factors. In fact, the analyses performed suggest that persenality tests do not measure equivalent processes in men and women. Correlational anaylsis showed all visual functions to be independent of one another with the exception of photopic acuity and scotopic threshold, which were highly correlated. Two new findings on the visual system emerged which have not been reported elsewhere: (i) Four distinct dark adaptation curves were produced, and have been labeled as exponential, flat-exponential, linear, and plateau. All subjects fell into one of these categories and showed a consistent trend to exhibit these curves for all field positions. (ii) Highly significant differences were found in sensitivity for the four visual fields, the upper field was superior, followed by the right, then left, with the lower visual field considerably poorer.

Adolescent↗
PubMed2002

Large-scale neural model for visual attention: integration of experimental single-cell and fMRI data.

A computational neuroscience framework is proposed to better understand the role and the neuronal correlate of spatial attention modulation in visual perception. The model consists of several interconnected modules that can be related to the different areas of the dorsal and ventral paths of the visual cortex. Competitive neural interactions are implemented at both microscopic and interareal levels, according to the biased competition hypothesis. This hypothesis has been experimentally confirmed in studies in humans using functional magnetic resonance imaging (fMRI) techniques and also in single-cell recording studies in monkeys. Within this neuro-dynamical approach, numerical simulations are carried out that describe both the fMRI and the electrophysiological data. The proposed model draws together data of different spatial and temporal resolution, as are the above-mentioned imaging and single-cell results.

Algorithms↗
PubMed1982

Functional organization of neurons in cat striate cortex: variations in ocular dominance and receptive-field type with cortical laminae and location in visual field.

1. Binocularity and receptive-field type of cortical neurons were assessed relative to the cortical layer in which the neurons were recorded and to receptive-field position in the visual field. 2. Receptive fields were observed up to 2 degrees into the ipsilateral half of the visual field. In the region up to 2 degrees on either side of the vertical meridian, the relative contribution of the ipsilateral eye was reduced. This progression in ocular dominance from ipsilateral to contralateral visual field agrees well with the distribution of X-cells about the nasotemporal division. 3. The region of maximum binocularity in each hemifield was found to be a 12 degree wide vertical strip extending from the vertical meridian to 12 degrees contralateral. In the representation of the central 12 degree strip, most units in all cortical layers were binocular. 4. Low levels of binocularity were observed at a considerable distance before the monocular portion of the visual field was reached. 5. The decrease in binocularity for simple cells occurred closer to the vertical meridian than for complex cells. 6. The proportions of cells classified as simple or complex did not change with position in the visual field. 7. At all locations in the visual field, complex cells showed a higher percentage of binocularity than simple cells. 8. The proportions of two types of simple cells, I and II, and complex cells were variable between cortical layers. Layer IV contained predominantly simple II cells, whereas layer V contained predominantly complex cells. 9. The results are discussed in terms of visual perception and the dynamic pattern of visual stimulation around a moving animal, the optic flow field.

Animals↗
PubMed1976

Field dependence in visually and nonvisually involved learning disabled children.

39 visually and nonvisually perceptually impaired 8- to 11-yr.-old boys with learning disabilities were compared with a control group of 35 "normal" learners on the Rod-and-frame and Children's Embedded-figures Tests. Previous findings of greater field dependence of learning disabled children are confounded because the experimental tasks involved visual perception. In our study the 27 "visuals" were more field-dependent than either the 12 "nonvisuals" or the controls. The latter groups did not differ significantly from one another, which may in part be a function of the small sample of nonvisual children identified. Alternative explanations, e.g., the visual nature of the field-dependence measures and the lack of reading difficulty of the nonvisual group, are considered. For the visually disabled Ss only Vocabulary scores, suggesting that among such children those with higher verbal intelligence may be more field-independent.

Child↗
PubMedDate not supplied

[Posterior cortical atrophy].

INTRODUCTION: Posterior cortical atrophy (PCA) is a dementing syndrome characterized by an early alteration of higher visual functions. Patients usually develop a perceptive visual agnosia related to Balint syndrome. DEVELOPMENT: Verification showed Alzheimer s disease (AD) in most PCA observations (13/14 cases), and it can be concluded that there is a posterior variant of AD with prominent visual symptomatology. However, most PCA cases have not been verified and the nature of the disorder remains unknown in these observations. An early and severe unilateral occipital horn dilatation was found in one out of every six cases with non verified PCA. To the best of our knowledge, this image has not been described in AD and these observations could be of different nature. On the other hand, some non verified PCA cases have occasionally manifested as an associative visual agnosia. The nature of this type of associative disorder is unknown, but it could be similar to the nature of semantic dementia, a non Alzheimer s syndrome related to unspecific lesions of temporal lobes.

Agnosia↗
PubMed2002

Visuoperceptual and visuomotor deficits in developmental stutterers: an exploratory study.

Although the cause of stuttering is unknown, there is strong evidence for it being a neuromotor disorder characterised by an abnormality of higher control encompassing not only speech but other motor systems. The aim of this exploratory study was to look for the presence of non-speech/language deficits--in particular, visuomotor and visuoperceptual deficits--in persons who stutter. Twelve moderate to severe developmental stutterers were compared with a group of fluent speakers, matched for age and sex, on a range of computerized sensory-motor tasks. These tasks covered various aspects of visuomotor function--ballistic movement, dynamic steadiness, and several types of tracking--and visuoperceptual function--acuity, static perception, and dynamic perception. A novel technique was used to remove the visuospatial component from tracking performance. Stutterers had slower reaction times, less accurate random tracking, and impaired dynamic visual perception. Severity of stuttering correlated with reaction time and dynamic perception. Removal of the visuoperceptual component from tracking performance indicated that the impaired tracking in the stutterers was predominantly due to reduced dynamic perception. This is the first study to provide preliminary evidence for the presence of non-linguistic visuoperceptual and upper-limb visuomotor tracking deficits in people with moderate to severe stuttering. These findings support a neurogenic aetiology for stuttering and are compatible with evidence of an overactive dopamine system in stutterers.

Adult↗
PubMed2002

[Pareidolia in Beato de Liébana's illuminated Visigothic codes].

The visual perception alterations constitute a characteristic etymological field in some neurological and psychiatric processes (such as hallucinations, hallucinosis, etc.) that can also appear in healthy people (metamorphosias, pareidolias). The pareidolia is a phenomenon characterized by the non auto-provoked visualization of a perception in which reality and daydream are combined. It constitutes a source of inspiration for different artistic manifestations, is the basis of some common psychological explorations (such as Rorcharch test) and it even can be the explication to some situations considered as paranormals, for example the vision of faces in the Moon or in Mars or the apparition of sacred figures either in spots or shades... Beato of Liébana was a cantabrian presbyter, author in the eight century of the Comments of the Apocalypse exegesis, copied and illuminated repeated times all through the following centuries. Given the marked revealing and eschatological component of the Apocalypse, starting from the Leon's School (known also as Mozarabic since the Ninth Century), the original was enriched adding another texts with their own illustrations. From them, the Comments to Daniel's Book, from Saint Jeronimus, stands out. The whole of the illuminated Hispanic manuscripts constitutes a "corpus" of amazing codices international known today as Beatos. We have demonstrated that the first History's graphic representation of a pareidolia phenomenon is found in the miniature Visigoth Hispanic manuscripts known as Beatos; we have arrived to this conclusion studying two facsimiles from the x Century (Beato of Valcavado and Beato of Gerona) specially the miniature called Baltasar's Feast; in both codices, is illustrated how this Babylon king perceives the way a phantasmagoric hand spurts up from a candlestick flame and writes in the adjacent wall three mysterious words: MANE, TECEL, FARES, which were only interpreted by Prophet Daniel, requested ad hoc. Baltasar mixed up unintentionally the perceived and the daydreamed starting from a visual field barely structured, we conclude that he suffered from a pareilolia phenomenon, graphically represented by the first time in History in the Beatos.

Fantasy↗
PubMed1990

Source of the eye-movement signal reaching real-motion cells.

The stability of visual perception despite eye movements suggests the existence in the visual system of neurons able to recognize whether the movement of a retinal image is due to the actual movement of an object or is self-induced by the ocular movement. We found neurons of this type in several areas of the monkey visual cortex and named them "real-motion" cells. Extracellular recordings were carried out from single neurons of the cortical prestriate area V3A of two awake macaque monkeys. Eighty-seven neurons were studied by comparing their responses during stimulus movement across the stationary receptive field, and receptive-field movement across the stationary stimulus. This visual stimulation was presented against a uniform visual background, in darkness or against a textured background. Neurons which were not real-motion in light (45/87) maintained their behaviour in darkness, while about 40% of real-motion cells lost this behaviour in darkness. Both real-motion and non real-motion cells maintained the same behaviour when tested against a uniform or textured visual background but often, texture increased the difference in the response that real-motion cells showed between stimulus and eye movement. These data suggest that the eye-movement signal which reaches real-motion cells and is responsible for their behaviour may be either retinal or extraretinal in nature. This double innervation is in good agreement with perceptual phenomena related to the detection of movement in the visual field.

Animals↗
PubMed1987

The effects of optical vergence, contrast, and luminance on the accommodative response to spatially bandpass filtered targets.

The steady-state monocular accommodative response was measured using a new type of stimulus, a spatially bandpass filtered luminance distribution known as a difference of Gaussian or DOG. The independent variables were: spatial frequency, optical vergence, contrast, and mean luminance. High-contrast DOGs of varying peak spatial frequencies were presented monocularly over a range of target optical vergences. In addition, DOGs were presented at a fixed dioptric vergence (1) over a range of contrasts with a constant mean luminance, and (2) over a range of mean luminances with a constant contrast. The magnitude of the accommodative error was found to depend on the total stimulus condition, i.e. the particular level of each independent variable. Changes in retinal-image contrast from the background (zero-defocus) level, which were brought about by the errors of accommodation under the various stimulus conditions, were mathematically determined and were used to calculate the "accommodative (contrast discrimination) Weber fractions". Comparisons between accommodative and psychophysical contrast discrimination data revealed that there are limits to the degree of shared processing of contrast discrimination information between accommodation and visual perception.

Accommodation, Ocular↗