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PubMed2001

Visual perception of action categories and the "bowl-throw" decision in cricket.

Cricket umpires, cricket bowlers, and physical education students (who were knowledgeable about the rules of cricket), were shown 72 videotaped point-light displays of cricket deliveries with varying extents of elbow flexion such that they ranged from highly "bowl-like" to highly "throw-like". The observers made a bowl-throw decision about each display, and the umpires and bowlers reported their confidence on a 5-point scale. The percentage of displays reported as a "bowl" was 59, 40, and 44 for the umpires, bowlers, and students respectively. Umpires made significantly more bowl decisions than both the bowlers and students, but there was no difference between the latter groups. Umpires were significantly more confident than the bowlers in both their bowl and throw decisions. Thus, in an experimental setting, with no apparent costs or benefits associated with their decision-making, umpires "called" a bowler significantly less frequently for throwing than other knowledgeable observers. The procedures devised for this experiment demonstrate that psychophysical methods can be applied to the problem of discrete action-category nominations in sport (e.g., bowl or throw, walk or run).

Analysis of Variance↗
PubMed1989

Single-channel vibrotactile supplements to visual perception of intonation and stress.

Two experiments were conducted to explore the effectiveness of a single vibrotactile stimulator to convey intonation (question versus statement) and contrastive stress (on one of the first three words of four 4- or 5-word sentences). In experiment I, artificially deafened normal-hearing subjects judged stress and intonation in counterbalanced visual-alone and visual-tactile conditions. Six voice fundamental frequency-to-tactile transformations were tested. Two sentence types were voiced throughout, and two contained unvoiced consonants. Benefits to speechreading were significant, but small. No differences among transformations were observed. In experiment II, only the tactile stimuli were presented. Significant differences emerged among the transformations, with larger differences for intonation than for stress judgments. Surprisingly, tactile-alone intonation identification was more accurate than visual-tactile for several transformations.

Communication Devices for People with Disabilities↗
PubMed1999

Visual perception of motion, luminance and colour in a human hemianope.

Human patients rendered cortically blind by lesions to V1 can nevertheless discriminate between visual stimuli presented to their blind fields. Experimental evidence suggests that two response modes are involved. Patients are either unaware or aware of the visual stimuli, which they are able to discriminate. However, under both conditions patients insist that they do not see. We investigate the fundamental difference between percepts derived for the normal and affected hemifield in a human hemianope with visual stimuli of which he was aware. The psychophysical experiments we employed required the patient, GY, to make comparisons between stimuli presented in his affected and normal hemifields. The subject discriminated between, and was allowed to match, the stimuli. Our study reveals that the stimulus parameters of colour and motion can be discriminated and matched between the normal and blind hemifields, whereas brightness cannot. We provide evidence for associations between the percepts of colour and motion, but a dissociation between the percepts of brightness, derived from the normal and hemianopic fields. Our results are consistent with the proposal that the perception of different stimulus attributes is expressed in activity of functionally segregated visual areas of the brain. We also believe our results explain the patient's insistence that he does not see stimuli, but can discriminate between them with awareness.

Adult↗
PubMed1980

Psychophysical measurements concerning the range of visual perception.

Two adjacent semicircular testfields of luminances l and L + delta L, were presented on an adapting field of luminance LU. In earlier experiments, it was found that the just noticeable difference of brightness (jnd) delta L, can be described by delta L = delta Ls + bLc (Lingelbach and haberich 1977). For foveal observations and LU values within the photopic range, delta Ls and b are power functions of LU. In this study, these investigations area continued for scotopic background luminances and extrafoveal conditions (6 degrees and 12 degrees temporal). The following results area obtained: 1. Under scotopic conditions of background luminance, LU, the jnd's for foveal observations are nearly independent of LU. delta L is dependent on the luminance L and can be expressed by the same curve for different levels of LU. 2. For extrafoveal conditions, the results can be described in the same way as for the fovea. The jnd is found to be delta L = delta Ls + bLc. In the total range of LU investigated (0.001 less than or equal to LU less than or equal to 100 cd/m2), the parameters delta Ls and b are power functions of LU. c increases with log L from about 1.1--1.5. 3. The sum of jnd-steps as separate "units of sensation" form a "dynamic characteristic" for each adaptational level. It is different for fovea and periphery. 4. The Fechner scaling of brightness, which corresponds to the sum of jnd-steps up to each adaptational luminance LU ("static characteristic"), shows an unexpected similarity to the Stevens power function.

Adult↗
PubMed2002

Visual perception of movement kinematics and the acquisition of "action prototypes".

Recognizing a class of movements as belonging to a "nominal" action category, such as walking, running, or throwing, is a fundamental human ability. Three experiments were undertaken to test the hypothesis that common ("prototypical") features of moving displays could be learned by observation. Participants viewed moving stick-figure displays resembling forearm flexion movements in the sagittal plane. Four displays (presentation displays) were first presented in which one or more movement dimensions were combined with 2 respective cues: direction (up, down), speed (fast, slow), and extent (long, short). Eight test displays were then shown, and the observer indicated whether each test display was like or unlike those previously seen. The results showed that without corrective feedback, a single cue (e.g., up or down) could be correctly recognized, on average, with the proportion correct between .66 and .87. When two cues were manipulated (e.g., up and slow), recognition accuracy remained high, ranging between .72 and .89. Three-cue displays were also easily identified. These results provide the first empirical demonstration of action-prototype learning for categories of human action and show how apparently complex kinematic patterns can be categorized in terms of common features or cues. It was also shown that probability of correct recognition of kinematic properties was reduced when the set of 4 presentation displays were more variable with respect to their shared kinematic property, such as speed or amplitude. Finally, while not conclusive, the results (from 2 of the 3 experiments) did suggest that similarity (or "likeness") with respect to a common kinematic property (or properties) is more easily recognized than dissimilarity.

Biomechanical Phenomena↗
PubMed1988

[Psychophysiological characteristics of visual perception in patients with schizophrenia during polarization of the occipital and frontal cortices].

Purposeful change of the functional status of the occipital and frontal cortex in relation to distinguishing visual stimuli shows that one of the signs indicative of schizophrenia is impairment of the active component of the regulatory mechanism of perception mediating the psychological orientation toward the recognition of meaningful signals and related to functions of the frontal cortex. Analysis of the alterations in the index of importance (serving as a quantitative parameter of changes in the psychological orientation in cases of shifts of the frontal cortex status) have demonstrated that in schizophrenics, unlike in healthy subjects, there is no relationship between changes in the frontal cortex potential and regulation of the critical sphere of perception.

Electrodes↗
PubMedDate not supplied

On the neural correlates of visual perception.

Neurological findings suggest that the human striate cortex (V1) is an indispensable component of a neural substratum subserving static achromatic form perception in its own right and not simply as a central distributor of retinally derived information to extrastriate visual areas. This view is further supported by physiological evidence in primates that the finest-grained conjoined representation of spatial detail and retinotopic localization that underlies phenomenal visual experience for local brightness discriminations is selectively represented at cortical levels by the activity of certain neurons in V1. However, at first glance, support for these ideas would appear to be undermined by incontrovertible neurological evidence (visual hemineglect and the simultanagnosias) and recent psychophysical results on 'crowding' that confirm that activation of neurons in V1 may, at times, be insufficient to generate a percept. Moreover, a recent proposal suggests that neural correlates of visual awareness must project directly to those in executive space, thus automatically excluding V1 from a related perceptual space because V1 lacks such direct projections. Both sets of concerns are, however, resolved within the context of adaptive resonance theories. Recursive loops, linking the dorsal lateral geniculate nucleus (LGN) through successive cortical visual areas to the temporal lobe by means of a series of ascending and descending pathways, provide a neuronal substratum at each level within a modular framework for mutually consistent descriptions of sensory data. At steady state, such networks obviate the necessity that neural correlates of visual experience project directly to those in executive space because a neural phenomenal perceptual space subserving form vision is continuously updated by information from an object recognition space equivalent to that destined to reach executive space. Within this framework, activity in V1 may engender percepts that accompany figure-ground segregations only when dynamic incongruities are resolved both within and between ascending and descending streams. Synchronous neuronal activity on a short timescale within and across cortical areas, proposed and sometimes observed as perceptual correlates, may also serve as a marker that a steady state has been achieved, which, in turn, may be a requirement for the longer time constants that accompany the emergence and stability of perceptual states compared to the faster dynamics of adapting networks and the still faster dynamics of individual action potentials. Finally, the same consensus of neuronal activity across ascending and descending pathways linking multiple cortical areas that in anatomic sequence subserve phenomenal visual experiences and object recognition may underlie the normal unity of conscious experience.

Adaptation, Physiological↗
PubMed1980

The physics of visual perception.

By measuring the contrast threshold for gratings of different waveform and spatial frequency, Campbell & Robson suggested in 1968 that there may be 'channels' tuned to different spatial frequencies. By using the technique of adapting to a high contrast grating, it was possible to measure the band-pass characteristics of these channels. Similar techniques were used to establish the orientational tuning of the channels. Reasons are put forward why it is advantageous to organize the visual system in this manner.

Form Perception↗
PubMed1989

Lateralization in the visual perception of Chinese characters and words.

It has been reported that tachistoscopic perception of single Chinese characters is better with a left-visual-field (LVF) than with a right-visual-field (RVF) presentation and that of Chinese words consisting of characters is better with a RVF presentation (O. J. L. Tzeng, D. L. Hung, B. Cotton, & S.-Y. Wang, 1979, Nature (London), 382, 499-501). In this study, the nature of this character-word difference in lateralization was explored in a task in which stimuli were presented unilaterally to a visual field for recognition test. Four types of stimuli were used: Single character, single pseudo- or noncharacter, two-character word, and two-character pseudoword. Results show (a) no visual-field advantage for illegal characters and words, (b) a LVF-advantage effect for characters associated with a more prominent LVF than RVF character-superiority effect, (c) a RVF-advantage effect for words associated with a more prominent RVF than LVF word-superiority effect, and (d) these two visual-field effects for characters and words being not absolute, they occur only with a low rather than with a high recognition for their respective illegal counterparts. These results suggest that the character-word difference is due to a more efficient lexical interpretation of character stimuli in the right than in the left hemisphere and a more efficient lexical interpretation of word stimuli in the left than in the right hemisphere.

Adult↗
PubMed2006

Ongoing eye movements constrain visual perception.

Eye movements markedly change the pattern of retinal stimulation. To maintain stable vision, the brain possesses a variety of mechanisms that compensate for the retinal consequences of eye movements. However, eye movements may also be important for resolving the ambiguities often posed by visual inputs, because motor commands contain additional spatial information that is necessarily absent from retinal signals. To test this possibility, we used a perceptually ambiguous stimulus composed of four line segments, consistent with a shape whose vertices were occluded. In a passive condition, subjects fixated a spot while the shape translated along a certain trajectory. In several active conditions, the spot, occluder and shape translated such that when subjects tracked the spot, they experienced the same retinal stimulus as during fixation. We found that eye movements significantly promoted perceptual coherence compared to fixation. These results indicate that eye movement information constrains the perceptual interpretation of visual inputs.

Algorithms↗
PubMed1989

Binocular depth perception, visual acuity and visual fields in cats following neonatal section of the optic chiasm.

We studied the role of the transcallosal pathway in stereopsis by measuring binocular and monocular depth perception in two cats that had undergone section of the optic chiasm at the age of 21 d. To ensure that the surgery did not impair vision to the extent that depth perception could not be evaluated, visual acuity and visual fields were also measured. In both of the chiasm-sectioned animals the visual fields were reduced and the visual acuity was substantially lower than in normal cats, with a maximum of about 2 cyc deg-1. Binocular depth thresholds of the chiasm-sectioned cats were worse than those of the normal cat but were better than their own monocular thresholds. These results suggest that the chiasm-sectioned animals were still able to use binocular cues to judge depth and indicate that the indirect pathway through the corpus callosum is sufficient to mediate binocular depth perception.

Depth Perception↗
PubMed1989

[Asymmetry of visual perceptive activity of faces and emotional facial expressions].

Eye movements were studied in 40 right-handed subjects during perception of symmetrical chimerical faces. These movements were recorded using an original system based on a differential optical method for the detection of corneal reflection and of the pupil made shiny. Under basal conditions, the first fixation was usually located in the left visual hemifield. The subjects spent more time gazing at the right hemiface (P less than 0.04). This visuo-spatial asymmetry in favour of the left hemispace was increased when the subject was requested to determine the emotional expressions of a new series of faces (P less than 0.002). Compared with basal conditions, the increase in the percentage of time spent in the left area was significant (P less than 0.035). An analysis performed on the first 3 seconds confirmed and amplified the differences observed. These results can partly be explained by reading habits and exploratory activity for symmetrical shapes. However, activation of the right hemisphere, specialized in the perception of faces and facial expressions, probably influenced visual exploration by drawing attention to the left area and favouring the left visual hemifield.

Adult↗
PubMed1981

[Slight variations of vitamin A in blood levels and their effect on visual perception].

The purpose of this study was to find out a simple perceptual visual test that gives the possibility to detect the visual incidence of slight variations of the vitamin A blood rate. The visual function was tested in 30 young subjects without refraction problems at the same time as blood concentration of vitamin A was measured. Visual tests comprised visual acuity at high and low luminance level, contrast sensitivity, glare sensitivity as well as attenuation characteristics of the eye toward flickering stimuli. Among all these tests, only the flicker test showed some relation with the plasma vitamin A levels, which were situated between 35 and 75 mcg/100 ml.

Adolescent↗
PubMed1982

Half-field asymmetry in visual perception of distance.

Distances were presented to the right and left halves of the visual field during steady fixation of a central point. Two methods of evaluation were used: direct scaling and the method of limits. A small but statistically significant difference was found between the estimates of distances presented to the left and to the right of the fixation point by direct scaling. No difference was found when the method of limits was used. The results are interpreted as evidence for the existence of functional asymmetry connected with the computation-and-speech ability of the hemispheres. No evidence for perception asymmetry could be found.

Adult↗
PubMed2001

Synaesthetic photisms influence visual perception.

When C, a digit-color synaesthete, views black digits, she reports that each digit elicits a highly specific color (a photism), which is experienced as though the color was externally projected onto the digit. We evaluated this claim by assessing whether C's photisms influenced her ability to perceive visually presented digits. C identified and localized target digits presented against backgrounds that were either congruent or incongruent with the color of her photism for the digits. The results showed that C was poorer at identifying and localizing digits on congruent than incongruent trials. Such differences in performance between congruent and incongruent trials were not found with nonsynaesthete control participants. These results suggest that C's colored photisms influence her perception of black digits. We propose a model in which color information influences the perception of digits through reentrant pathways in the visual system.

Adult↗