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PubMedDate not supplied

What does the eye see best?

Our eyes see so much in such varied conditions that one might consider the question posed in the title to be meaningless, but we show here that, within the range that we have been able to test, there is a particular spatiotemporal pattern of light that is detected better than any other. At least two plausible theories of visual detection predict that a stimulus will be seen best (will have greatest quantum efficiency) when it matches the weighting function of the most efficient detector. We have measured quantum efficiency for detecting a wide variety of spatiotemporal patterns using foveal vision in bright light. The best stimulus found so far is a small, briefly exposed circular patch of sinusoidal grating having a spatial frequency of approximately 7 c deg-1, drifting at approximately 4 Hz. We propose that this is the weighting function of the most efficient human contrast detector. We believe this answer to the question is unexpected and may have fundamental implications with regard to the mechanisms of visual perception.

Brain↗

PubMed1987

[Selectivity of the neuronal responses of the human brain to different angular orientations of visual stimuli].

In parkinsonic patients with long-term intracerebral electrodes implanted into different nuclei of the thalamus and striopallidar complex the neuronal impulse activity was recorded during visual testing for orientation sensitivity. Visual testing revealed significant responses in multiple unit activity of studied neuronal populations to the angular orientations of the presented stimuli differing from background stimulus orientation. This responses were spatially specific and were not the result of modulation of the activity of these units by the common afferent signal. Neuronal populations differently responding to stimuli which differ only in the angular orientation were observed. These findings might be interpreted as an orientation sensitivity. Such orientation sensitivity of various units differs both in optimal orientation angle and sharpness of responses, some unit groups responding to the definite orientation of the stimuli only. It is suggested that the neuronal populations with significant responses to the visual stimuli used are the elements of thalamostriopallidar system which plays an important role in the maintenance of motor functions. The convergence of motor and visual information within this system probably enables their comparison and interaction, related to the visual perception constancy.

Brain Mapping↗

PubMedDate not supplied

[The level of various intellectual abilities in patients with vertebrobasilar arterial syndromes].

Several methods such as: Wechsler-Bellevue intelligence scale, memory scale of M. Choynowski, tests of Couve, Bourdon, Piéron, Baley, Graham-Kendall and Benton were used for establishing the level of certain intellectual abilities in patients with vertebrobasilar arterial syndrome. The tested group included 50 patients and a control group was selected which was matched for sex, age, educational level and number. Statistical analysis showed that the level of intellectual abilities in the patients with vertebrobasilar syndrome was lower than in healthy controls and this change was observed in the levels of many parameters of intelligence, memory, attention and visual perception.

Adult↗

PubMed2006

Vertical-horizontal illusion present for sighted but not early blind humans using auditory substitution of vision.

This experiment was undertaken to investigate the effect of sensory modality (vision vs. audition) and of visual status (early blind vs. sighted) on susceptibility to the vertical-horizontal illusion. Early blind volunteers and blindfolded sighted subjects explored variants of the vertical-horizontal illusion using a device that substituted audition for vision, whereas sighted subjects from an independent group inspected the same stimuli visually. Sensitivity to the vertical-horizontal illusion, including an illusion of moderate strength when using the sensory substitution device, was observed only in the two sighted groups. The existence of an illusion effect when using such a device supports the idea of a visual perception provided by sensory substitution, whereas the attenuation of the vertical-horizontal illusion strength is consistent with the visual field shape theory (Künnapas, 1955a). The absence of the illusion effect in early blind subjects suggests that the sensory experience influences the nature of perception and that the visual experience plays a crucial role in the vertical-horizontal illusion, in accordance with the size-constancy scaling theory (Gregory, 1963).

Adolescent↗

PubMed1996

Disorders of visual and spatial perception in the early stage of Alzheimer's disease.

A battery of visual and spatial perception tests (VOSP) has been administered to a sample of 25 mild Alzheimer's disease (AD) patients and 25 age- and education-matched controls in order to assess visuospatial skills in the early phase of the disease. Among visual object perception tests, AD patients were impaired only in the task where they had to name the stimulus (Silhouettes task). The score in this task significantly correlated with the Boston Naming Test score (r = .73; p < 0.0001). The performance on the Object decision task (which does not require naming), figure-ground analysis, and identification of degraded stimuli was unimpaired. Similarly there were no significant differences for any of the spatial perception tests. The results showing that the performances on most measures of visual and spatial perception is unimpaired, indicate that the "early" perceptual processes, as well as the access to structural knowledge, are not defective in early AD. On the other hand, AD patients are impaired at the level of semantic access and name retrieval. The finding of an highly significant correlation among tests requiring visual access to structural and semantic knowledge suggests that a rigid separation between perceptual and semantic mechanisms may be unwarranted.

Aged↗

PubMed2002

Visual discrimination testing of ultraviolet transmitting and ultraviolet blocking acrylic thermoplastics.

BACKGROUND: Visual perception has hindered the development of blinded, placebo-controlled ultraviolet light exposure conditions in humans. New acrylic thermoplastics that block or transmit ultraviolet light have visual properties that may provide a solution. METHODS: In a series of triangle taste tests, 60 subjects were tested for the ability to visually perceive ultraviolet light transmission by two of four types of acrylic thermoplastics. RESULTS: Forty-six of sixty (67%) subjects were unable to visually detect ultraviolet light transmitting acrylics among ultraviolet blocking acrylics in both an ambient light background condition and an ambient light background condition with ultraviolet A light supplementation. CONCLUSION: Acrylic thermoplastics allow for the production of blinded, placebo-controlled ultraviolet light exposure conditions by eliminating visual discrimination differences between active and control groups.

Acrylic Resins↗

PubMed2006

Hemispheric specialization for the visual control of action is independent of handedness.

The idea that visually guided action is independent of visual perception has been supported by neurological, neuropsychological, and behavioral studies. In healthy subjects, evidence for this distinction has come from psychophysical studies of the effects of visual illusions on perceptual judgments and object-directed grasping. This evidence is limited, however, by the fact that virtually all studies have involved right-handed subjects using their dominant hand, which is presumably controlled by the left hemisphere. There is tentative evidence from earlier neurological studies that the left hemisphere may in fact play a special role in the integration of visual and motor information during grasping. We designed two experiments to test this idea. The first experiment involved pictorial illusions, which are known to have robust effects on perceptual judgments but little influence on grasping. Right- and left-handed subjects reached out and grasped objects embedded in two different visual illusions with either their dominant or their nondominant hand. For both right- and left-handed subjects, precision grasping with the left hand, but not with the right, was affected by the illusions. In a follow-up experiment, we examined precision grasping in a more natural setting and showed that left-handed subjects use their nondominant (right) hand significantly more as compared with right-handed subjects. We conclude that visuomotor mechanisms encapsulated in the left hemisphere play a crucial role in the visual control of action and that this hemispheric specialization evolved independently of handedness.

Brain↗

PubMed2000

Neural correlates of reafference: evoked brain activity during motion perception and saccadic eye movements.

The ability to perceive a stable visual environment despite eye movements and the resulting displacement of the retinal image is a striking feature of visual perception. In order to study the brain mechanism related to this phenomenon, an EEG was recorded from 30 electrodes spaced over the occipital, temporal and parietal brain areas while stationary or moving visual stimuli with velocities between 178 degrees/s and 533 degrees/s were presented. The visual stimuli were presented both during saccadic eye movements and with stationary eyes. Stimulus-related potentials were measured, and the effects of absolute and relative stimulus velocity were analyzed. Healthy adults participated in the experiments. In all 36 subjects and experimental conditions, four potential components were found with mean latencies of about 70, 140, 220 and 380 ms. The latency of the two largest components between 100 and 240 ms decreased while field strength increased with higher absolute stimulus velocity for both stationary and moving eyes, whereas relative stimulus velocity had no effect on amplitude, latency and topography of the visual evoked potential (VEP) components. If the visual system uses retinal motion information only, we would expect a dependence upon relative velocity. Since field strength and latency of the components were independent of eye movements but dependent upon absolute stimulus velocity, the visual cortex must use extraretinal information to extract stimulus velocity. This was confirmed by the fact that significant topographic changes were observed when brain activity evoked during saccades and with stationary eyes was compared. In agreement with the reafference principle, the findings indicate that the same absolute visual stimulus activates different neuronal elements during saccades than during fixation.

Adult↗

PubMedDate not supplied

Understanding diverse effects of visual attention with the VAP-filters metaphor.

The Variable and Permeable Filters (VAP-Filters) metaphor is presented with an explanation of its advantages over other popular metaphors in accounting for attention effects in many different research paradigms. Research from laboratories of the author and others are discussed briefly and shown to result in diverse facilitatory and inhibitory attention effects on visual perception. All of these effects are consistent with the VAP-Filters metaphor.

Attention↗

PubMed1998

Competition between past and present. Assessment and interpretation of verbal perseverations.

Perseveration consists of the inappropriate repetition of a preceding behaviour when a new adapted response is expected. We have developed statistical tools that make it possible to reveal such perseverations, assess their significance and study their finer characteristics, such as their temporal course and impaired processing level. This approach is illustrated and evaluated through analyses of naming errors produced by three patients with impairments affecting different stages of the processing chain leading from visual perception to speech production. These examples of perseverations include the intrusion not only of whole words (patient R.A.V.) but also of isolated phonemes (patient D.U.M.) or of visual features (patient Y.M.) from previous trials. In all cases, the probability that an error is a perseveration from a previous trial is an exponentially decreasing function of the lag between the two trials considered. This suggests that perseverations reflect a decaying internal variable, such as an internal level of activation of previous utterances. Based on these empirical results, we put forward a tentative mechanism for the generation of perseverations: whenever a given processing level is deprived of its normal input, persistent activity inherited from previous trials is no longer overcome by current input, and is revealed in the form of perseverations.

Adult↗

PubMed2000

Dynamics of striate cortical activity in the alert macaque: I. Incidence and stimulus-dependence of gamma-band neuronal oscillations.

Using single and multiunit recordings in the striate cortex of alert macaque monkeys, we find that gamma-band (20-70 Hz) oscillations in neuronal firing are a prominent feature of V1 neuronal activity. The properties of this rhythmic activity are very similar to those previously observed in the cat. Gamma-band activity is strongly dependent on visual stimulation, largely absent during spontaneous activity and, under the conditions of our experiment, not time-locked to the vertical refresh of the computer monitor (80 Hz) used to present the stimuli. In our sample, 61% of multiunit activity (MUA) and 46% of single-unit activity (SUA) was significantly oscillatory, with mean frequencies of 48+/-9 and 42+/-13 Hz, respectively. Gamma-band activity was most likely to occur when cells were activated by their optimal stimuli, but still occurred, although less often and with lower amplitude, in response to nonoptimal stimuli. The frequency of gamma-band activity also reflected stimulus properties, with drifting gratings evoking higher-frequency oscillations than stationary gratings. As in the cat, the spike trains of single cells showing gamma-band oscillations often displayed a pattern of repetitive burst firing, with intraburst firing rates of 300-800 Hz. The overall similarity of rhythmic neuronal activity in the primary visual cortex of cats and monkeys suggests that the phenomenon is not species-specific. The stimulus-dependence of the rhythmic activity is consistent with a functional role in visual perception.

Action Potentials↗

PubMed1999

A set of 400 pictures standardized for French: norms for name agreement, image agreement, familiarity, visual complexity, image variability, and age of acquisition.

The present article provides French normative measures for 400 line drawings taken from Cycowicz, Friedman, Rothstein, and Snodgrass (1997), including the 260 line drawings that were normed by Snodgrass and Vanderwart (1980). The pictures have been standardized on the following variables: name agreement, image agreement, familiarity, visual complexity, image variability, and age of acquisition. These normative data also include word frequency values and the first verbal associate (taken from Ferrand & Alario, 1998). The six variables obtained are important because of their potential effect in many fields of psychology, especially the study of cognitive processes such as visual perception, language, and memory.

Art↗

PubMedDate not supplied

[Conditioned reflex autoregulation of human respiration during tachystoscopic presentation of cues].

The changes in external respiration were studied in humans during visual perception of test signals, as well as the changes in relations between the moment of signal presentation and the phase of respiratory cycle in the process of complication of experimental tasks and training. The method of temporal stereotype of test signals sequence permitted to reveal the process of self-regulation of cerebral functional state through the changes in respiration. This process is envisaged as a component of the entire structure of conditioned reflex to time. Its principal meaning is to prepare the functional level of the central nervous system providing for the stability of the given level of perception and processing of information in time.

Adolescent↗

PubMed1993

Brain lateralization and severity of stuttering in children.

Cerebral lateralization in visual perception was investigated in 9 severe stuttering, 11 mild stuttering and 48 fluent speakers. The subjects were asked to identify words presented in the left or right visual field for 20 ms. Children responded by pointing to the exposed test word on a response card which contained four different words. Errors committed in the left and right visual fields were analyzed. The data showed a left hemisphere superiority in the processing of words in both the mild stutterers and the fluent speakers, but a right hemisphere advantage in the severe stutterers. The results suggest a close relationship between the severity of stuttering and functional brain organization.

Child↗

PubMed1987

A theory of wheelchair wheelie performance.

The results of this analytical study of wheelchair wheelie performance can be summarized into two wheelchair design equations, or rules of thumb, as developed in the paper. The equation containing the significant parameters involved in popping a wheelie for curb climbing is: fh = 0.8 mg theta c.g. [A] where fh is handrim force, m is the mass of the wheelchair + user less rear wheels, g is acceleration of gravity (9.807 m/s2), and theta c.g. is "c.g. angle," i.e., the angle between the vertical through the rear axle and a line connecting the rear axle and the system center-of-gravity. Equation [A] shows that reducing the mass and/or the c.g. angle will make it easier to pop a wheelie. The c.g. angle is reduced by moving the rear axle position forward on the wheelchair. Wheelie balance is the other aspect of performance considered; where the user balances the wheelchair on the rear wheels for going down curbs or just for fun. The ease with which a system can be controlled (balanced) is related to the static stability of the system. The static stability is defined as: omega 2 = mgl/J [B] where J is the mass moment of inertia at the center of gravity of the system about the direction perpendicular to the sideframe. For better wheelchair control during wheelchair balance the static stability should be reduced. Measurements of the value for the polar mass moment of inertia for a typical wheelchair + user of m = 90 kg was found to be J = 8.7 kg-m2. In order to decrease the value of the static stability, Equation [B], one can increase J or decrease m and/or l, where l is the distance from the rear axle to the c.g. of the system. It is also shown that balancing a rod in the palm of the hand (inverted pendulum) is a mathematical problem similar to the wheelie balance problem, and a rod of length 1.56 meters is similar to a wheelchair + user system mass of 90 kg. However, balancing a rod is done primarily by using visual perception, whereas wheelie balance involves human joint proprioceptors and visual plus vestibular (inner ear) perception. Thus, a simple test of determining the shortest length of rod one can balance in the palm of the hand (plus measuring handrim force capability and simple reaction time) may indicate if a wheelchair user will find it easy to do a wheelie balance.

Humans↗

PubMed1969

Visual transient phenomenon: its polarity and a paradox.

Luminance stimuli modulated at frequencies above flicker fusion create perceptible visual transient responses when the frequency is changed abruptly. The polarity of these transients, as directly perceived and objectively confirmed, is shown enough by itself to yield a powerful criterion for visual models. An apparent paradox when light flashes are superposed on the frequency discontinuity has further implications for models and suggests a possible non-conservation of polarity in the brightness perception process.

Flicker Fusion↗

PubMed1995

Perceptual abnormalities in Parkinson's disease: top-down or bottom-up processes?

A wide variety of perceptual impairments have been reported in patients with Parkinson's disease (PD) in recent years; the underlying causes of these impairments have been variously attributed to different levels of the visual-cognitive system, from the retina to frontal cortex. Parkinsonian perceptual abnormalities could thus be interpreted as indirectly caused either by 'bottom-up' effects, stemming from dopaminergic dysfunction in the retina, or by 'top-down' effects, stemming from deficits in attention due to disturbances in the striatal-frontal system. Alternatively, a direct visuospatial impairment, perhaps related to the motor symptoms, has been considered. Data on three basic aspects of visual perception (3-D stereo vision, figure-ground discrimination, and pattern perception) which might be expected to give difficulty to PD subjects suffering changes in early processing mechanisms are reported. Visual complexity and the degree of mental manipulation of the material required both varied in different parts of the tests. PD patients were on stable drug regimens. It was found that disease severity interacted with performance: patients with mild PD showed little perceptual abnormality, patients with moderate PD showed only top-down effects, and patients with severe PD showed evidence both of top-down and of bottom-up deficits. Thus it appears that any retinal effects on perception in PD occur only in the advanced stages of the disease; in earlier stages any visual dysfunction probably reflects top-down disturbances from higher levels of the cognitive-behavioural system.

Adult↗

PubMed2006

A visual model for object detection based on active contours and level-set method.

A visual model for object detection is proposed. In order to make the detection ability comparable with existing technical methods for object detection, an evolution equation of neurons in the model is derived from the computational principle of active contours. The hierarchical structure of the model emerges naturally from the evolution equation. One drawback involved with initial values of active contours is alleviated by introducing and formulating convexity, which is a visual property. Numerical experiments show that the proposed model detects objects with complex topologies and that it is tolerant of noise. A visual attention model is introduced into the proposed model. Other simulations show that the visual properties of the model are consistent with the results of psychological experiments that disclose the relation between figure-ground reversal and visual attention. We also demonstrate that the model tends to perceive smaller regions as figures, which is a characteristic observed in human visual perception.

Animals↗

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