Selective attention and coding in visual perception.
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Nonveridical visual perception in human amblyopia.
PURPOSE: Amblyopia is a developmental disorder of spatial vision. There is evidence to suggest that some amblyopes misperceive spatial structure when viewing with the affected eye. However, there are few examples of these perceptual errors in the literature. This study was an investigation of the prevalence and nature of misperceptions in human amblyopia. METHODS: Thirty amblyopes with strabismus and/or anisometropia participated in the study. Subjects viewed sinusoidal gratings of various spatial frequencies, orientations, and contrasts. After interocular comparison, subjects sketched the subjective appearance of those stimuli that had nonveridical appearances. RESULTS: Nonveridical visual perception was revealed in 20 amblyopes ( approximately 67%). In some subjects, misperceptions were present despite the absence of a deficit in contrast sensitivity. The presence of distortions was not simply linked to the depth of amblyopia, and anisometropes were affected as well as those with strabismus. In most cases, these spatial distortions arose at spatial frequencies far below the contrast detection acuity cutoff. Errors in perception became more severe at higher spatial frequencies, with low spatial frequencies being mostly perceived veridically. The prevalence and severity of misperceptions were frequently found to depend on the orientation of the grating used in the test, with horizontal orientations typically less affected than other orientations. Contrast had a much smaller effect on misperceptions, although there were cases in which severity was greater at higher contrasts. CONCLUSIONS: Many types of misperceptions documented in the present study have appeared in previous investigations. This suggests that the wide range of distortions previously reported reflect genuine intersubject differences. It is proposed that nonveridical perception in human amblyopia has its origins in errors in the neural coding of orientation in primary visual cortex.
Adolescent↗
Experience, context, and the visual perception of human movement.
Why are human observers particularly sensitive to human movement? Seven experiments examined the roles of visual experience and motor processes in human movement perception by comparing visual sensitivities to point-light displays of familiar, unusual, and impossible gaits across gait-speed and identity discrimination tasks. In both tasks, visual sensitivity to physically possible gaits was superior to visual sensitivity to physically impossible gaits, supporting perception-action coupling theories of human movement perception. Visual experience influenced walker-identity perception but not gait-speed discrimination. Thus, both motor experience and visual experience define visual sensitivity to human movement. An ecological perspective can be used to define the conditions necessary for experience-dependent sensitivity to human movement.
Adolescent↗
Magnetic coil suppression of visual perception at an extracalcarine site.
Perception of extrafoveal visual targets can be suppressed by magnetic stimulation over the occipital lobes, but the site of interference for this and similar phenomena has not been well defined. We modified a previously used technique to determine the locus of magnetic activation. Using butterfly stimulus coils of different sizes and electric field profiles, we determined a scalp position of minimum threshold and a level of stimulator output that produced 50% error rates for each coil. Intersection of the corresponding electric field profiles in air and in a saline model head was similar and identified superficial occipital cortex rather than the primary visual area as the site of perceptual suppression. Less direct analyses involving the distribution of induced electric fields produced the same conclusion. These results suggest specific hypotheses about the effects of magnetic stimulation on visual physiology.
Attention↗
[Visual perception in normal children and adults I. Developmental changes in dominance of visual field in normal children].
Developmental changes of dominance of visual field was evaluated using a tachistoscope in 70 normal subjects from 4 years of age up to adulthood (35 males and 35 females). The time of exposure was measured under two experimental conditions; simultaneous stimulation of different types to both left and right fields, or a single stimulus only to one visual field. One of the following stimuli was exposed to the visual field; one, two or three letters of Japanese characters ("hirakana"), one of Chinese characters ("kanji"), figures, and Roman alphabets. Simultaneous bilateral stimulation revealed frequent dominance in the right visual field, which was established by 4 years of age. The average time of exposure become shorter with age up to 6-7 years for one "hirakana" letter and one "kanji", and up to 10 years for two and three "hirakana" letters. All four left-handed subjects in this study showed dominance of the right visual field. Dominance of the left visual field was observed in 3 of 52 subjects with dominance of the right eye (5.8%), and in 5 of 15 with dominance of the left eye (33.3%). This difference was statistically significant (chi 2 = 8.41, p less than 0.01). It was suggested that dominance of the visual field was related with dominance of the eye, and not with handedness.
Adolescent↗
[Deficit in the transmission of visual perceptions in patients with cerebral hemispherical lesions with no alterations of the visual field].
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Brain Damage, Chronic↗
Suppression of visual perception by transcranial magnetic stimulation--experimental findings in healthy subjects and patients with optic neuritis.
The influence of noninvasive magnetic brain stimulation by a magnetic coil (MC) placed over the occiput on perception and correct reporting of a briefly presented set of 3 letters of the alphabet was examined in 15 patients with prolonged VEP latencies due to neuritis of the optic nerve. The results derived from observing these patients were compared to the results obtained from an age-matched control group of 20 healthy voluntary subjects examined under the same experimental conditions. In both groups it was possible to demonstrate that transcranial magnetic stimulation is able to suppress recognition of the letters if applied with a certain delay time after a brief presentation of the visual stimulus. The groups were compared to each other with regard to the delay with which it was possible to demonstrate the most effective suppression. In the healthy subjects, this delay was found between 60 and 100 msec. In the patients, it was prolonged to 80-140 msec. This prolongation was closely related to the VEP latency (P100). Furthermore, visual suppression and the influence on it by different parameters were studied in detail in healthy subjects; the visual suppression depends on visual (e.g., brightness, duration) and magnetic (e.g., intensity) stimulus conditions. The method described seems to be of considerable value in the investigation of basic mechanisms of visual perception. This includes pathophysiological changes caused by optic neuritis and possibly other disorders affecting the visual system.
Adult↗
Effects of frequency and similarity neighborhoods on pharmacists' visual perception of drug names.
To minimize drug name confusion errors, regulators, drug companies, and clinicians need tools that help them predict which names are most likely to be involved in confusions. Two experiments, carried out in the United States, examined the effects of stimulus frequency (i.e., how frequently a target name is prescribed), neighborhood frequency (i.e., how frequently prescribed are the "neighbors" of the target name), and neighborhood density (how many names are within a fixed distance of the target name) on the probability of pharmacists making an error in a visual perceptual identification task. In both experiments, the task was to correctly identify a series of blurry drug names after a 3s presentation on a computer monitor. In the first experiment, 45 pharmacists viewed 160 typewritten names, incorrectly identifying 60.6% of them. Random effects regression revealed a significant beneficial effect of stimulus frequency and a detrimental effect of neighborhood density. Significant two-way interactions were observed between stimulus frequency and neighborhood density and neighborhood frequency and neighborhood density. In the second experiment, 37 pharmacists viewed 156 handwritten drug names, incorrectly identifying 45.7%. Random effects regression revealed significant main effects of stimulus frequency and neighborhood density. These were contained within a significant three-way interaction: The interaction between stimulus frequency and neighborhood density was present at high but not low neighborhood frequency. Objectively measurable frequency and neighborhood characteristics have predictable effects on errors in pharmacists' visual perception. Organizations that coin and evaluate drug names, as well as hospitals, pharmacies, and health systems, should consider these characteristics when assessing visually confusing names.
Decision Making↗
The imprint of action: motor cortex involvement in visual perception of handwritten letters.
Humans are able to recognize handwritten texts accurately despite the extreme variability of scripts from one writer to another. This skill has been suggested to rely on the observer's own knowledge about implicit motor rules involved in writing. To investigate the possible neural correlates of such an ability, we monitored with magnetoencephalography (MEG) the approximately 20-Hz oscillations originating from the motor cortex. The oscillations were more suppressed after visual presentation of handwritten than printed letters, indicating stronger excitation of the motor cortex to handwritten scripts. These results support the idea of embodied visual perception of handwritten scripts and the involvement of the motor cortex in the underlying action-perception link.
Adult↗
A mismatch between kinesthetic and visual perception in Parkinson's disease.
Kinesthesia may be defective in patients with Parkinson's disease (PD), and this defect conceivably has a role in parkinsonian hypokinetic symptoms. In the present study, PD patients used kinesthetic perception to estimate the amplitude of passive angular displacements of the index finger about the metacarpophalangeal joint and to scale them as a percentage of a reference stimulus. The reference stimulus was either a standard kinesthetic stimulus preceding each test stimulus (task K) or a visual representation of the standard kinesthetic stimulus (task V). In task V, the PD patients' underestimation of the amplitudes of finger perturbations was significantly greater than that of normal subjects, but not for task K. PD patients' underestimation was also greater in task V than in task K; the difference between the underestimations was significantly greater than for normal subjects. These results suggest that, when kinesthesia is used to match a visual target, distances are perceived to be shorter by the PD patients. Assuming that visual perception is normal, kinesthesia is "reduced" in PD patients. This reduced kinesthesia, when combined with the well-known reduced motor output and probably reduced corollary discharges, implies that the sensorimotor apparatus is "set" smaller in PD patients than in normal subjects.
Aged↗
[Results of visual perception tests in prematurely born children of pre-school age (author's transl)].
Taking into account the criterions "birth weight", "small for date" (birth weight in relation to gestational age), "perinatal complications", "normal delivery" and "oxygen post partum" 32 children of pre-school age, who had been prematurely born tested with the following variables of the FROSTIG developmental tests of visual perception: visuo-motor coordination, figure-ground discrimination, perceptual constancy, space perception and perception of spacial relations. There were not significant differences between the premature-groups. No relations could be found between the quantity of O2 post partum and the tested abilities. After comparing the test subjects and a group of full-term born controls significant differences could be established. The different abilities were not related to the birth weight. Extensive deficits were observed in the "small for date" born prematures. The results are discussed with regard to their therapeutic relevance.
Birth Weight↗
Visual perception of texture in aggressive behavior of Betta splendens.
In order to elucidate the role of texture in fish vision, the agonistic behavior of male Siamese fighting fish (Betta splendens) was tested in a response to models composed by means of image processing techniques. Using the models with the contour shape of a side view of Betta splendens in an aggressive state, the responses were vigorous when there was a fine distribution of brightness and naturalistic color, producing textures like a scale pattern. Reactions became weaker as the brightness and color distribution reverted to more homogeneous levels and the scale pattern disappeared. When the artificial models with the circular contour shape were used, models with the scale pattern evoked more aggressive behaviors than those without it, while the existence of spherical gradation affected the behavior slightly. These results suggest that texture plays an important role in fish visual perception.
Aggression↗
Visual perception of writing and pointing movements.
Studies of movement production have shown that the relationship between the amplitude of a movement and its duration varies according to the type of gesture. In the case of pointing movements the duration increases as a function of distance and width of the target (Fitts' law), whereas for writing movements the duration tends to remain constant across changes in trajectory length (isochrony principle). We compared the visual perception of these two categories of movement. The participants judged the speed of a light spot that portrayed the motion of the end-point of a hand-held pen (pointing or writing). For the two types of gesture we used 8 stimulus sizes (from 2.5 cm to 20 cm) and 32 durations (from 0.2 s to 1.75 s). Viewing each combination of size and duration, participants had to indicate whether the movement speed seemed "fast", "slow", or "correct". Results showed that the participants' perceptual preferences were in agreement with the rules of movement production. The stimulus size was more influential in the pointing condition than in the writing condition. We consider that this finding reflects the influence of common representational resources for perceptual judgment and movement production.
Adult↗
[Speed of visual perception, alpha frequency and electroencephalographic data].
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Electroencephalography↗
[Visual perception of movement. A normative study].
BACKGROUND: A battery of tests has been developed to better define the variety of defects which can occur in the process of visual perception of motion. METHODS: The tests assessed the perception of 1) coherent motion, in each of the four cardinal directions; (2) form from motion; (3) flow in depth; and (4) motion of a target moving in depth, within or without an optic flow surround. A normative study was conducted with 34 subjects, 21 to 69 years of age. RESULTS: Recognition of form from motion was better with upwards than with downwards motion. Perception of motion in depth was better with backward than forwards motion. The presence of an optic flow altered the perception of a motion in depth. CONCLUSION: The recognition of form from motion demonstrated an asymmetry in threshold between the upward and downward movements. This asymmetry did not occur with global coherent motion, thus reflecting the variety of mechanisms involved in the visual process of motion. Moreover, alteration of perception of motion in depth within an optic flow reflected background effect.
Adult↗
Pulse configuration-dependent effects of repetitive transcranial magnetic stimulation on visual perception.
Transcranial magnetic stimulation (TMS) is a noninvasive technique for direct stimulation of the neocortex. In the last two decades it is successfully applied in the study of motor and sensory physiology. TMS uses the indirect induction of electrical fields in the brain generated by intense changes of magnetic fields applied to the scalp. It encompasses two widely used waveform configurations: mono-phasic magnetic pulses induce a single current in the brain while biphasic pulses induce at least two currents of inverse direction. As has been shown for the motor cortex, efficacy of repetitive transcranial magnetic stimulation (rTMS) may depend on pulse configuration. In order to clarify this question with regard to visual perception, static contrast sensitivities (sCS) were evaluated before, during, immediately after and 10 minutes after monophasic and biphasic low frequency (1 Hz) rTMS applied to the occipital cortex of 15 healthy subjects. The intensity of stimulation was the phosphene threshold of each individual subject. Using 4 c/d spatial frequency, significant sCS loss was found during and immediately after 10 min of monophasic stimulation, while biphasic stimulation resulted in no significant effect. Ten minutes after the end of stimulation, the sCS values were at baseline level again. However, reversed current flow direction resulted in an increased efficacy of biphasic and decreased efficacy of monophasic stimulation. Our results are in agreement with previous findings showing that primary visual functions, such as contrast detection, can be transiently altered by low frequency transcranial magnetic stimulation. However the effect of modulation significantly depends on the current waveform and direction.
Adult↗
Effect of gradually increasing carboxyhaemoglobin saturation on visual perception and psychomotor performance of smoking and nonsmoking subjects.
The effect of gradually increasing COHb saturation on human visuoperceptual and psychomotor performance was studied in 22 nonsmokers and 22 smokers. Each subject performed two sessions in randomized order, one during air breathing and the other during CO breathing on two separate days. Testing and COHb saturation measurement were repeated six times during each session. Gas breathing was between the test periods. The increase of COHb saturation up to 12--13 per cent units had no effect (p greater than 0,05) on perceptual speed and accuracy as measured by the Bourdon--Wiersma test. Finger tapping speed was also unaffected. Visual perception measured with critical flicker frequency (CFF) was sensitive to CO. The gradual increase in COHb saturation caused a linear decrease in CFF in the both groups. An increase of one per cent unit in COHb saturation caused significant decrease in CFF (p less than 0.001), when intraindividual changes were taken into account. During acute exposure to CO there was no difference in any test performance between the groups. During air preathing there was no difference in performance although there was a significant difference (p less than 0,001) in the COHb saturation levels. This negative finding might be due to adaptation of smokers to chronic exposure of CO because of smoking.
Adult↗
The locus of spatial attention during the temporal integration of visual memories and visual percepts.
Temporal integration is a process by which two serially presented visual stimuli are mentally integrated to form a composite representation. In the present research, we explored how spatial selective attention is used during the delay separating stimuli, in order to determine the contents of spatial working memory in this task. A two-task situation was created. On the primary task, two dot arrays were serially presented within a grid, leaving one space empty, which subjects identified. On the secondary task, instead of the second array, a discrimination probe was presented. Integration accuracy increased through delays of 1,500 msec, revealing an estimate of the time required to form an optimal memory trace for integration. Once the memory trace was formed (but not before), response time to the probe was faster if it was presented in a location previously occupied by a dot from Array 1. This indicates that during the delay separating the arrays, the subjects assigned spatial attention to the locations occupied by the first array and actively maintained the leading array in working memory. Implications for theories of visual processing and memory are discussed.
Attention↗