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At least 1,207 records · Page 67Linked to original sources
PubMed2000

Competition for consciousness among visual events: the psychophysics of reentrant visual processes.

Advances in neuroscience implicate reentrant signaling as the predominant form of communication between brain areas. This principle was used in a series of masking experiments that defy explanation by feed-forward theories. The masking occurs when a brief display of target plus mask is continued with the mask alone. Two masking processes were found: an early process affected by physical factors such as adapting luminance and a later process affected by attentional factors such as set size. This later process is called masking by object substitution, because it occurs whenever there is a mismatch between the reentrant visual representation and the ongoing lower level activity. Iterative reentrant processing was formalized in a computational model that provides an excellent fit to the data. The model provides a more comprehensive account of all forms of visual masking than do the long-held feed-forward views based on inhibitory contour interactions.

Adult↗
PubMed2000

Race as a visual feature: using visual search and perceptual discrimination tasks to understand face categories and the cross-race recognition deficit.

One of the most familiar empirical phenomena associated with face recognition is the cross-race (CR) recognition deficit whereby people have difficulty recognizing members of a race different from their own. Most researchers assume that the CR deficit is caused by failure to generalize perceptual encoding expertise from same-race (SR) faces to CR faces. However, this explanation ignores critical differences in the social cognitions and feature coding priorities associated with SR and CR faces. On the basis of data from visual search and perceptual discrimination tasks, it appears that the deficit occurs because people emphasize visual information specifying race at the expense of individuating information when recognizing CR faces. In particular, it is possible to observe a paradoxical improvement in both detection and perceptual discrimination accuracy for CR faces that is limited to those who recognize them poorly. These findings support a new explanation for the CR recognition deficit based on feature coding differences between CR and SR faces, and appear incompatible with similarity-based models of face categories.

Adult↗
PubMed2001

Is position "special" in visual attention? Evidence that top-down processes guide visual selection.

The role of spatial position in selective visual processing has been the source of recent debate. Experiments by Tsal and Lavie (1988) and van der Heijden et al. (1996) have been designed to establish the status of position in visual selection. Tsal and Lavie found that observers tend to select letters from a briefly presented array according to position. Using the same paradigm, van der Heijden et al. found that observers tend to select according to colour, except under conditions of low contrast and unrestricted eye movements. The present study attempted to reconcile these findings by exploring the influence of top-down processes (task instructions) while explicitly controlling for eye movements. Experiment 1 demonstrated there was no inherent selection bias for stimuli similar to those used by van der Heijden et al., suggesting that the tendency to select according to colour found by van der Heijden et al. was due to task demands. Experiment 2 further established the role of top-down factors by replicating the results of van der Heijden et al. with our stimuli. Experiment 3 demonstrated that selection can be switched from colour to position by changing the demands of the task. These results suggest that selection may be accounted for by task demands (e.g., instructions) with no priority access for position information.

Analysis of Variance↗
PubMedDate not supplied

Pattern-reversal visual evoked potentials in migraineurs with or without visual aura.

To determine the possible electrophysiologic changes in migraineurs with or without visual aura, we investigated pattern-reversal visual evoked potentials in 39 patients. We compared the mean P100 latency and amplitude of 16 patients with aura, 23 patients without aura, and 17 age- and sex-matched normal subjects. There were no significant differences between groups. There was no correlation between age and the parameters in any group. However, in 7 of 23 patients without aura, the P100 latency was longer than the mean control value +2 SD. The mean disease duration in this subgroup was significantly longer than the means of the remaining 16 patients without aura or the patients with aura (P < 0.05 for each). This suggests the possibility that P100 latency prolongation is a consequence, but not an entity caused by the pathogenetic mechanism of the disease from the beginning.

Adult↗
PubMed2006

[Value of different measurements of the preoperative visual acuity and perception of Purkinje's vessel shadows for prediction of the postoperative visual acuity after vitrectomy in diabetic vitreous hemorrhage].

BACKGROUND: In cases of strong vitreous hemorrhage due to diabetic vitreo-retinopathy there is an uncertainty for the prediction of postoperative visual acuity after vitrectomy. This study compares the value of different preoperative measurements for the prognosis of postoperative visual acuity. METHOD AND PATIENTS: We evaluated retrospectively the reports of all patients who have been operated by vitrectomy due to diabetic vitreous hemorrhage at our hospital in 2004 (first vitreoretinal surgery). The following measurements were evaluated: preoperative distance and reading VA, last known VA before hemorrhage, laser interference VA, Purkinje's vessel shadow and postoperative distance VA. RESULTS: 42 patients had entire documentation of all evaluated parameters. The preoperative distance VA was 20/1000 (light perception to 20/60), only 3 patients had a reading VA. Laser interference VA was better than red light perception in 13 patients (7 without red light perception). Purkinje's vessel shadows were positive in 22 patients, negative in 17 patients, but 3 patients did not understand the procedure. Prehemorrhage VA was known in 19 patients. Laser interference VA and last known VA prior to hemorrhage (in mean 20 months old) had the best correlation to postoperative VA (p > 0.05). Purkinje's vessel shadow predicts -- if positive -- a postoperative VA of 20/300 or better. CONCLUSIONS: Postoperative VA after vitrectomy due to diabetic vitreous hemorrhage is best predicted by prehemorrhage VA or laser interference VA, especially if no prehemorrhage VA is known. Positive Purkinje's vessel shadow predicts a postoperative VA of 20/300 or better.

Comorbidity↗
PubMed2004

Projections from primary visual cortex to cytochrome oxidase thin stripes and interstripes of macaque visual area 2.

It has been controversial whether the cytochrome oxidase (CO)-dense blobs in primate primary visual cortex (V1) and CO-dense thin stripes in visual area 2 (V2) are parts of a cortical color-processing stream that is segregated from other functional streams. One of the key pieces of evidence for the segregated color stream is the previous report of specific connections between blobs and thin stripes, which is parallel to the connections between interblobs and interstripes. To study the degree of the segregation between the proposed different streams, in the current study, anatomical tracers were injected into different V2 compartments with the functional guidance of optical imaging. The spatial relationship between each labeled cell and the CO blobs in V1 were analyzed quantitatively. After tracer injections in the color-preferring modules in CO thin stripes, equal amounts of labeled cells were found in the blobs and interblobs. However, the density of the labeled cells was more than twice as high in the blobs as that in the interblobs, and most of the clusters of labeled cells partially overlapped with the blobs. Tracer injections in the interstripes labeled cells predominantly in the interblobs. Our results suggest that both the blobs and interblobs project to the thin stripes and call into question the proposition that different CO compartments in V1 and V2 are connected in parallel to form highly segregated functional streams.

Animals↗
PubMed1998

Physiological and psychological evaluation for visual display colour readability: a visual evoked potential study and a subjective evaluation study.

This study was conducted, by subjective and objective evaluation, to clarify the complex relationships among VDT (visual display terminal) display colour readability and the related physical and physiological factors. Readability was defined as participants being able to read sentences easily on a VDT screen irrespective of their meanings. In the subjective evaluation, paired comparison tests and a colour impression test using the semantic differential (SD) method with factor analysis were carried out for 30 colour stimuli with different dominant wavelengths and stimulus purity values. The former test yielded identification of the most readable stimulus purity, which was in the range of 0.2 to 0.55 for any dominant wavelength. From the latter test, a 'conspicuousness factor' and a 'comfort factor' were extracted for evaluating readability. In the objective evaluation, three kinds of dominant wavelengths and three levels of stimulus purity were used. This evaluation showed that P100 peak latencies of colour-VEPs (visual evoked potentials) evoked by the subjectively optimal stimulus purity colours fully displayed on the screen were significantly shorter than those evoked by any other stimulus purity colours. Moreover, VEP P100 peak latencies evoked by the text colours that had the most readable stimulus purities were also shorter than those evoked by any other stimulus purity colours. Consequently, it was concluded that the optimum VDT display colour was that which had the most readable stimulus purity for each dominant wavelength, and that colour-VEP peak latency could serve as an indicator for quantification of VDT display colour readability.

Adult↗
PubMed1993

Sociocultural aspects of blindness in an Egyptian delta hamlet: visual impairment vs. visual disability.

Through ophthalmological exams, structured interviews and participant observation, this study examines the experience of blindness in rural Egypt, and finds that villagers' subjective assessments of their vision differ substantially from ophthalmic measurements of their vision. Individuals with profound visual loss remain independent in their daily activities and contribute to their families' subsistence. While they may agree that they have "weak eyesight," they do not perceive themselves to be disabled. Stigmatizing attitudes that the blind are completely dependent and unable to fulfill their social roles further encourage those with decreased vision to deny the extent of their visual loss.

Adolescent↗
PubMed1997

Screening for visual-perceptual deficits following closed head injury: a short form of the Visual Form Discrimination Test.

The purpose of this study was to evaluate the concurrent validity and clinical utility of a short form of the Visual Form Discrimination Test in persons with closed head injuries (n = 62). Given the homogeneity of the items, and patients' apparent consistency in responding, we hypothesized that scores from an eight-item short form would be highly similar to the scores from the full version of the test (i.e. 16 items). The mean difference between the short form and full version was less than one point, and the correlation between the two forms was 0.86. Applying a clinical decision rule resulted in a 'normal/impaired' overall correct classification rate of 98.4%. The short form shows considerable promise as a brief test of visual discrimination in persons with closed head injuries.

Adult↗
PubMed1998

The Lighthouse Strategy: use of a visual imagery technique to treat visual inattention in stroke patients.

Eight male and eight female stroke patients, aged 45-77, were asked to imagine their eyes as horizon-sweeping beams of a lighthouse and were then cued to use this image in functional and therapy training tasks as part of their treatment for visual inattention within a comprehensive day rehabilitation programme. Treatment Group patients' performance on the Mesulam Verbal Cancellation Test given at admission and discharge were significantly improved (p = 0.002). In comparison with controls matched for diagnoses, race and age, significant improvement (p = 0.007, f = 8.389) also occurred by discharge in the overall attention of patients taught to use the 'Lighthouse Strategy' as measured by a facility rating scale and reports by family and caregivers. Further potential applications of visual imagery to the retraining of compensatory cognitive and other skills are discussed.

Aged↗
PubMed1991

Effects of long-term visual display terminal usage on visual functions.

Since their rapid introduction into the workplace visual display terminals (VDT's) have been the source of a number of studies aimed at detecting effects on office workers. The published studies have been either short-term or cross-sectional, with one exception. The results have shown varying degrees of relation but by not having a comparable control group all have been limited in their value. We set out to monitor at regular intervals over a 2-year period specific visual functions that have been reported to be affected by VDT usage, and to compare VDT and non-VDT users in the same office environment. By assessing the VDT operators from the time the systems were introduced we have an accurate baseline to work from when assessing change due to the introduction of the VDT.

Accommodation, Ocular↗
PubMed1980

Visual information processing: the structure and creation of visual representations.

For human vision to be explained by a computational theory, the first question is plain: What are the problems that the brain solves when we see? It is argued that vision is the construction of efficient symbolic descriptions from images of the world. An important aspect of vision is therefore the choice of representations for the different kinds of information in a visual scene. An overall framework is suggested for extracting shape information from images, in which the analysis proceeds through three representations: (1) the primal sketch, which makes explicit the intensity changes and local two-dimensional geometry of an image; (2) 2 1/2-D sketch, which is a viewer-centred representation of the deplth, orientation and discontinuities of the visible surfaces; and (3) the 3-D model representation, which allows an object-centred description of the three-dimensional structure and organization of a viewed shape. The critical act in formulating computational theories for process capable of constructing these representations is the discovery of valid constraints on the way the world behaves, that provide sufficient additional information to allow recovery of the desired characteristic. Finally, once a computational theory for a process has been formulated, algorithms for implementing it may be designed, and their performance compared with that of the human visual processor.

Depth Perception↗
PubMed1989

Functional development of the visual system in normal and protein-deprived rats. VII. Lamination of oxidative enzyme activity in the visual cortex during post-natal development.

Lamination of activity of two mitochondrial enzymes-succinate dehydrogenase (SDH) and menadione-dependent alpha-glycerophosphate dehydrogenase (M-GPDH) - was examined in the visual cortex of control (C) and protein-deprived (PD) rats from 10 days old to adult. In C rats, lamination of enzyme activity was evident from 10 days by a band of higher activity in lamina IV. A band of higher SDH activity was seen in superficial lamina VI from 15 days. Lamination of M-GPDH activity became less pronounced with increasing age, whereas distinct bands of high SDH activity in lamina IV and VI remained also in adult rats. PD rats showed a developmental delay of approximately 5 days with regard to the general increase in neuropile activity. A band of higher enzyme activity in lamina IV was present from 10 days, but it was markedly less distinct in the PD rats compared to the C rats between 15 and 25 days. A band of higher SDH activity in superficial lamina VI was not seen until 25 days in PD rats. No apparent differences were seen between adult C and PD rats. The observed developmental alterations are discussed in relation to other delays, distortions and deficits found in the visual system of PD rats.

Animals↗
PubMed1991

An intracellular analysis of the visual responses of neurones in cat visual cortex.

1. Extracellular and intracellular recordings were made from neurones in the visual cortex of the cat in order to compare the subthreshold membrane potentials, reflecting the input to the neurone, with the output from the neurone seen as action potentials. 2. Moving bars and edges, generated under computer control, were used to stimulate the neurones. The membrane potential was digitized and averaged for a number of trials after stripping the action potentials. Comparison of extracellular and intracellular discharge patterns indicated that the intracellular impalement did not alter the neurones' properties. Input resistance of the neurone altered little during stable intracellular recordings (30 min-2 h 50 min). 3. Intracellular recordings showed two distinct patterns of membrane potential changes during optimal visual stimulation. The patterns corresponded closely to the division of S-type (simple) and C-type (complex) receptive fields. Simple cells had a complex pattern of membrane potential fluctuations, involving depolarizations alternating with hyperpolarizations. Complex cells had a simple single sustained plateau of depolarization that was often followed but not preceded by a hyperpolarization. In both simple and complex cells the depolarizations led to action potential discharges. The hyperpolarizations were associated with inhibition of action potential discharge. 4. Stimulating simple cells with non-optimal directions of motion produced little or no hyperpolarization of the membrane in most cases, despite a lack of action potential output. Directional complex cells always produced a single plateau of depolarization leading to action potential discharge in both the optimal and non-optimal directions of motion. The directionality could not be predicted on the basis of the position of the hyperpolarizing inhibitory potentials found in the optimal direction. 5. Stimulation of simple cells with non-optimal orientations occasionally produced slight hyperpolarizations and inhibition of action potential discharge. Complex cells, which had broader orientation tuning than simple cells, could show marked hyperpolarization for non-optimal orientations, but this was not generally the case. 6. The data do not support models of directionality and orientation that rely solely on strong inhibitory mechanisms to produce stimulus selectivity.

Action Potentials↗
PubMed1982

Visual evoked response in transient monocular visual loss.

The pattern-reversal visual evoked response (VER) was recorded in 2 patients during transient monocular visual reduction. In both cases the VER was initially abolished. With recovery of vision there was gradual return of amplitude over a 3- to 8-minute period, while latencies remained unchanged from preattack values. These findings are discussed with regard to current understanding of the origins of the VER and relevant aspects of retinal electrophysiology.

Adult↗
PubMed1992

Role of inferior temporal neurons in visual memory. I. Temporal encoding of information about visual images, recalled images, and behavioral context.

1. Lesions of the inferior temporal (IT) cortex selectively hamper monkeys in tasks requiring visual memory. A system that recognizes images must be able to encode a current stimulus, recall the code of a previous stimulus, compare the codes of the two stimuli, and make a decision on the basis of the outcome of the comparison. Therefore, IT neurons must be involved in at least one of these processes. To determine the specific role of IT neurons in visual memory, we measured the information conveyed in the neuronal responses about current patterns, recalled patterns, and behavioral context. 2. Two monkeys were trained to perform a sequential matching task using a set of 32 black and white Walsh patterns. In the course of an experiment, each pattern was presented repeatedly in sample, match, and nonmatch behavioral contexts. While the monkeys were performing the task, we recorded the activity of 76 neurons from area TE of IT. The neuronal responses to the stimuli were converted to spike density functions, and the resultant waveforms were quantified using their principal components. The relationships between the responses and the stimuli were studied using analysis of variance and information theory. 3. The analysis of variance was applied to the neuronal response waveforms using the context (sample, match, or nonmatch) and the patterns of the stimuli as independent variables and the spike count or the coefficients of the principal components as the dependent variables. We found that the waveforms of most neurons were significantly modulated by both the pattern and the context of the stimulus presentation. 4. We also analyzed the stimulus-response relationships using information theory. The input codes were based on the pattern and context of the stimuli, and the output codes were based on the spike count or the principal components of the responses. The neuronal response waveforms were found to convey significant amounts of information about both the pattern and context of the stimuli. Transmitted information was greatest when the response of a neuron was interpreted as a message about the combination of pattern and context. Nevertheless, there was information about context independent of pattern and vice versa. 5. We also used information theory to determine whether the neuronal responses to the second, or test, stimulus conveyed information about the pattern of the first, or sample, stimulus. The input codes were based on the patterns of the sample stimuli, and the output codes were based on the responses to the nonmatch test stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of Variance↗
PubMed1996

A theory of the visual motion coding in the primary visual cortex.

This paper demonstrates that much of visual motion coding in the primary visual cortex can be understood from a theory of efficient motion coding in a multiscale representation. The theory predicts that cortical cells can have a spectrum of directional indices, be tuned to different directions of motion, and have spatiotemporally separable or inseparable receptive fields (RF). The predictions also include the following correlations between motion coding and spatial, chromatic, and stereo codings: the preferred speed is greater when the cell receptive field size is larger, the color channel prefers lower speed than the luminance channel, and both the optimal speeds and the preferred directions of motion can be different for inputs from different eyes to the same neuron. These predictions agree with experimental observations. In addition, this theory makes predictions that have not been experimentally investigated systematically and provides a testing ground for an efficient multiscale coding framework. These predictions are as follows: (1) if nearby cortical cells of a given preferred orientation and scale prefer opposite directions of motion and have a quadrature RF phase relationship with each other, then they will have the same directional index, (2) a single neuron can have different optimal motion speeds for opposite motion directions of monocular stimuli, and (3) a neuron's ocular dominance may change with motion direction if the neuron prefers opposite directions for inputs from different eyes.

Color Perception↗