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PubMed2001

The Fourier theory of vision.

The historical roots of the Fourier theory of spatial visual perception are traced. The development of the underlying concepts and the psychophysical experiments that led to them, and that they in turn spawned, are examined, as well as their relation to the current knowledge of neural substrates in the retina and primary visual cortex. Allowing nonlinearities or even substituting other types of basis functions does not eliminate the difficulties faced by any theory of visual perception that is built on the notion of fixed spatial filters.

Fourier Analysis↗
PubMed2004

What do lateralized displays tell us about visual word perception? A cautionary indication from the word-letter effect.

A common assumption underlying laterality research is that visual field asymmetries in lateralized word perception indicate the hemispheric specialisation of processes generally available for the perception of words, including words viewed in a more typical setting (i.e. in the central visual field). We tested the validity of this assumption using a phenomenon (the word-letter effect) frequently reported for displays viewed in the central visual field, where letters in words are perceived more accurately than the same letters in isolation. Words and isolated letters were presented in the left visual field (LVF), right visual field (RVF) and central visual field (CVF), the Reicher-Wheeler task was used to suppress influences of guesswork, and an eye-tracker ensured central fixation. In line with previous findings, lateralized displays revealed a RVF-LVF advantage for words (but not isolated letters) and CVF displays revealed an advantage for words over isolated letters (the word-letter effect). However, RVF and LVF displays both produced an advantage for isolated letters over words (a letter-word effect), indicating that processing subserving the advantage for words when participants viewed stimuli in the central visual field was unavailable for lateralized displays. Implications of these findings for studies of lateralized word perception are discussed.

Attention↗
PubMed2003

Neuropsychological developmental change in a case with Noonan syndrome: longitudinal assessment.

The authors describe the neuropsychological development of a 10-year-old boy with Noonan syndrome. The subject's IQ showed normal intelligence, although there was a discrepancy between verbal and performance IQs. Visual perception was delayed, with clumsiness and inattention affecting his performance. Both visual perception and hyperactivity improved with the subject's general development, but his inattention seemed to increase after the age of 9. The behavioral features and cognitive profile of our case resembled those of attention-deficit/hyperactivity disorder. We recommend that clinicians should evaluate cautiously the specific profile of cognitive development in Noonan syndrome with particular focus on controlling the patient's inattention.

Attention↗
PubMedDate not supplied

[Neuropsychological implications of Crouzon syndrome: a case report].

INTRODUCTION: Crouzon syndrome (CS) has traditionally been associated with moderated mental retardation. However, very few studies have quantified in these patients the degree of this mental retardation, and even less research has been carried out to evidenciate the typical neuropsychological profile, including affective, cognitive and executive functions. CLINICAL CASE: We present a single case-study of a 16 year-old, right-handed female affected of CS. Neuropsychological functions assessed included attention, concentration, memory, visual perception, language, arithmetic processes, praxis, executive functions, reasoning and general intelligence. Deficient results have been obtained in learning and memory, attention, language, graphic praxis, comprehension of social situations and executive functions (frontal lobes). Nevertheless, the patient kept within normal limits her spatial and temporal orientation, visual perception, visual-spatial and manipulative abilities and arithmetic processes. CONCLUSIONS: Results suggest that the intellectual defect in CS is not general and uniform and that some intellectual abilities are preserved, while others are affected in different degrees. The importance of carrying out a thorough neuropsychological assessment in these cases is emphasized.

Adolescent↗
PubMed2005

Use of visual information in speech perception: evidence for a visual rate effect both with and without a McGurk effect.

The McGurk effect, where an incongruent visual syllable influences identification of an auditory syllable, does not always occur, suggesting that perceivers sometimes fail to use relevant visual phonetic information. We tested whether another visual phonetic effect, which involves the influence of visual speaking rate on perceived voicing (Green & Miller, 1985), would occur in instances when the McGurk effect does not. In Experiment 1, we established this visual rate effect using auditory and visual stimuli matching in place of articulation, finding a shift in the voicing boundary along an auditory voice-onset-time continuum with fast versus slow visual speech tokens. In Experiment 2, we used auditory and visual stimuli differing in place of articulation and found a shift in the voicing boundary due to visual rate when the McGurk effect occurred and, more critically, when it did not. The latter finding indicates that phonetically relevant visual information is used in speech perception even when the McGurk effect does not occur, suggesting that the incidence of the McGurk effect underestimates the extent of audio-visual integration.

Adolescent↗
PubMed1990

Visual skill. Correlation with detection of solitary pulmonary nodules.

A series of visual tests was administered to 65 individuals: 9 Board-certified radiologists, 41 radiology residents, and 15 fourth-year medical students on the senior radiology elective. Results of these visual tests were compared with performance in detection of solitary pulmonary nodules. Among radiology residents or board-certified radiologists, no correlation was found between performance on tests of visual perception and the ability to correctly locate pulmonary nodules. A correlation between visual test scores and nodule detection was evident among medical students. These findings suggest that factors other than skill in visual perception determine a radiologist's ability to correctly identify solitary pulmonary nodules.

Internship and Residency↗
PubMed1997

The possible meaning of the upper and lower alpha frequency ranges for cognitive and creative tasks.

This study is aimed at verifying the functional independence of two frequency bands within the alpha range. It is based on experiments that examined the role of these two hands with regard to the amount of local electrogenesis (amplitude) and the cooperation of brain areas (coherence) in mental tasks concerning: (1) visual perception and imagery; (2) listening to and composing music; (3) verbal and visual creativity; and (4) aspects of mood. In all experiments EEG were recorded for at least 1 min during each task, separated one from another by at rest periods of at least equal lengths. EEG electrodes were pasted according to the 10/20 system (averaged ear lobes as reference). After FFT power was calculated for all 19 electrodes, coherence was estimated for all possible electrode pairs (i.e. 171). This was done for six frequency ranges between 1.5 and 31.5 Hz, the alpha range having been divided into two (7.5-9 Hz and 9.5-12.5 Hz). The spectral parameters obtained during each task were compared with those of the merged EEG at rest, significant changes (P < or = 0.01-P < or = 0.05) were entered into schematic maps of the brain. Generally, fewer differences were found for amplitude than for coherence. In all four tasks concerning visual perception the clearest differences were found in single person studies. But also in group studies more or less distinct differences were found between alpha 1 and 2. Also in the series with music the two alpha bands did not behave uniformly, nor were uniform features found in the two series of musically trained and untrained subjects. Distinct discrepancies were also found in a verbal and visual imagery task. With respect to mood, only elevated mood was correlated with a decrease of coherence in alpha 2 and an increase of amplitude in alpha 1. This study though hinting at a different functional significance of these two alpha bands, however, does not allow to draw any conclusions as to their distinct functional meanings. Generally, the long-term coherence changes observed under these different mental tasks support the idea that part of information processing in the brain is reflected by the EEG. Structural peculiarities and microelectrode recordings of the cortex support this conclusion.

Adult↗
PubMed2005

Placing a tool in the spotlight: spatial attention modulates visuomotor responses in cortex.

Visual spatial attention has long been associated with facilitatory effects on visual perception. Here, we report that spatial attention can also modulate implicit visuomotor processing in dorsal regions of human cortex. Participants underwent fMRI scanning while performing a voluntary attentional orienting task that varied the category of a task-irrelevant object in the attended location (tool vs. non-tool). Data were then analyzed as a function of the attended location (left vs. right visual field) and the object category in that location. We found that the fMRI BOLD response in two visuomotor-related regions--the supplementary motor area (SMA) and the left inferior parietal lobule (IPL)--showed an interaction between the location of attention and the location of the tool in the bilateral display. Further, these responses were statistically distinct from those regions in dorsal cortex showing activity modulated only by the tool location or only by the attended location. While the effects of attending non-foveally within the visual field have been well documented in relation to visual perception, our findings support the proposal that voluntary visuospatial attention may also have consequences for the implicit planning of object-directed actions.

Adult↗
PubMed1995

Observer performance in the assessment of contract enhanced CT: effects of contrast medium and grey scale window on detection of defects in a simulated peripheral rim border.

The image contrast of X-ray CT systems depends on the properties of the imaging system, the number of grey scales used in the digital sampling process, and the addition or otherwise of a contrast medium. The effect of contrast medium is often emphasized in the peripheral rim of pathologic masses. In this study, a special hexagonal test object made of acrylic phosphate and a cylindrical container were employed. When the contrast medium IOPAMIRON 300 was used to fill the space between the test object and its cylindrical container, a contrast-enhanced peripheral rim with or without signal of defect was observed. Visual perceptibility was examined from the signals resulting from varying contrast by use of contrast medium and adjusting the grey scale by image processing. Threshold contrast influenced by window width was recorded in each image contrast, different ranges of window width was recorded in each instance using statistical analyses. While window width was inversely proportional to image contrast, different ranges of window width and image contrast provided better visual perceptibility for each concentration of the contrast medium. The range of window width for obtaining the best visual perceptibility was determined for a contrast-enhanced image.

Absorptiometry, Photon↗
PubMed2004

Inferential constraint sets in the organization of visual expectation.

Many lines of evidence indicate that considering visual perception as a passive, stimulus-driven, feedforward decoding process is no longer tenable. Visual perception naturally occurs within the context of an integrated array of ongoing cognitive processes involving memory, perception in other modalities, and motor control. In many situations, these processes allow expectations to be formed for likely visual events. This article explores the idea that the formation of visual expectations involves the active organization of visual cortical areas, providing a framework of contextual information within which expected events are interpreted. Retinal inputs are treated as constraints that feed into a complex system of interacting visual cortical areas and thalamic nuclei, which are concurrently imposing constraints on one another. Although the nature of expectational organization in the visual cortex is not well-understood, a reasonable hypothesis is that expectation involves the mutual constraint of spatiotemporal activity patterns in multiple visual cortical areas. In this scenario, expectation is instantiated by a set of activity patterns in high-level visual cortical areas that impose constraints on one another as well as on low-level areas according to the partial information that is available about expected retinal inputs. One approach to testing this proposal is through the analysis of simultaneously recorded local field potentials (LFPs) from local neuronal assemblies in multiple visual cortical areas. Analysis of LFPs by multivariate autoregressive modeling is showing promise in revealing the organization of expectation in visual cortex.

Animals↗
PubMed1997

Consciousness of perception after brain damage.

Disturbances of visual perception after brain damage provide clues to understanding consciousness and the brain. In this article we review six visual disorders in which perception and consciousness are dissociated as a result of brain damage: blindsight, implicit shape perception in apperceptive visual agnosia, covert recognition of faces in prosopagnosia, unconscious perception in neglect and extinction, implicit reading in pure alexia, and implicit object recognition in associative visual agnosia. We consider these six disorders from the standpoint of three main schools of thought concerning consciousness and the brain, namely a localized system for consciousness, consciousness as a state of integration, and consciousness as a property of graded representation. The findings suggest that these syndromes do not share a single mechanism and that it is conceivable that more than one explanation will be necessary both within and across syndromes. We conclude on the basis of the current evidence that it is unlikely that any single brain system is necessary for conscious awareness of perception that does not play a role in perception as well.

Agnosia↗
PubMed2006

Adaptation aftereffects in the perception of gender from biological motion.

Human visual perception is highly adaptive. While this has been known and studied for a long time in domains such as color vision, motion perception, or the processing of spatial frequency, a number of more recent studies have shown that adaptation and adaptation aftereffects also occur in high-level visual domains like shape perception and face recognition. Here, we present data that demonstrate a pronounced aftereffect in response to adaptation to the perceived gender of biological motion point-light walkers. A walker that is perceived to be ambiguous in gender under neutral adaptation appears to be male after adaptation with an exaggerated female walker and female after adaptation with an exaggerated male walker. We discuss this adaptation aftereffect as a tool to characterize and probe the mechanisms underlying biological motion perception.

Adaptation, Physiological↗
PubMed1976

Relationships between perceptual responses and accident patterns in children: a pilot study.

Many studies have been done to define factors related to accident tendencies in children, but few related these tendencies to developmental lags in visual perception. This pilot study explored the possibility that such a relationship exists. Twelve Caucasian females from similar socioeconomic backgrounds and age ranges were administered the Frostig Developmental Test of Visual Perception. Performance on the perceptual test was compared to accident frequency levels for each child to determine whether or not a correlation between visual perceptual performance and accident frequency existed. Although statistically significant results were not obtained, the need for more definite research related to spatial relations, position in space, and figure ground perception was demonstrated.

Accidents↗
PubMed2001

Visual categorical perception by rats with temporal, striate, or sham ablations.

Rats were trained to discriminate a 0 degrees stripe from a 90 degrees stripe in a two choice water maze. They were prepared with either Te2/3, partial striate (PS), or sham lesions and retrained on the preoperative discrimination. In two separate experiments, excellent savings was observed for all groups. Next, trials were administered with novel stripe orientations defined as either between- or within-category problems. Performance accuracy eroded rapidly for all groups in the first experiment, and no between-group differences were observed. In the second experiment, each session with categorical stimuli was preceded by four reminder trials with the original stimuli. This improved accuracy for all groups, but it was found that animals with PS lesions, not animals with T2/3 lesions, were impaired on between-category judgements. The impairment was not secondary to a disruption of basic visual sensory processing or significantly larger lesions relative to the Te2/3 group. As is the case for monkeys, accuracy with within-category stimuli was inferior to between-category stimuli for all groups. Possible reasons for this inter-species difference are discussed.

Animals↗
PubMed1981

Visual form perception and the spatial phase transfer function.

The quality of many imaging devices can be characterized, within certain constraints, by means of the modulation transfer function (MTF) and the phase transfer function (PTF). In many cases, it is possible to estimate, qualitatively, the effect of the MTF on the appearance of objects, and much progress has been made in making quantitative predictions of the detectability of objects and features within objects. This is not the case, however, for the PTF, and its influence is often neglected, even though nonideal PTF's obviously may degrade image quality. Experiments are described that attempt to assess the significance of the PTF for human visual performance. The effects of various PTF's were simulated by means of a technique that maintained the modulation of the spatial-frequency components closely constant. The visual detectability of such phase changes was measured by a discrimination method, and phase threshold levels were evaluated. Simple linear patterns provided the targets for all measurements, although some images are shown that illustrate qualitatively the phase-shift results on real two-dimensional targets. The work enables an assessment to be made of the effects of phase changes produced by various imaging systems.

Form Perception↗