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PubMed1997

Is working memory intact in alcoholics? An ERP study.

Few investigators have applied the working memory theory to studies on abstinent chronic alcoholics, though it has been reported that the deficits in short-term memory appear to be specific to visuo-spatial and problem-solving tasks. In the present study, we recorded ERPs from 40 male control subjects and 78 alcoholics performing a modified delayed matching to sample task. To minimize the possible confound of retinotopic projections for the matching stimuli, in contrast to the non-matching stimuli, we employed a unique set of stimuli in our delayed matching to sample task. Our results indicate that an ERP component, occurring at approximately 250 ms post-stimulus, may be a reflection of the ERP mnemonic effect for working memory. This component distinguishes the two groups at the right occipitotemporal region, providing evidence of right hemisphere dysfunction in alcoholics. Thus, the current experiment may show electrophysiological evidence of working memory deficits in alcoholics.

Adult↗

PubMed1988

Font regularity constraints on the process of letter recognition.

Strings of four unrelated letters were presented for subjects to identify, followed by a patterned mask and then a forced choice test of each letter position. In Experiment 1, the type style in the regular conditions was consistent--all of the letters were of a single type font--whereas in the mixed condition, each string contained letters from two type fonts. Compared with the mixed condition, accuracy in the regular conditions was higher overall and increased at a faster rate as a function of processing time. This held across four sessions. In Experiment 2, the font in the mixed condition was varied either between or within letter strings; sizeable advantages for the regular conditions were found with both mix-methods. The results are consistent with the idea of a schemalike perceptual system that becomes tuned to the regularities of a particular font in order to process visual information efficiently.

Adult↗

PubMed1983

Recovery at short intervals between masking flashes.

Recovery of a masked target (T + M) by a homogeneous flash (B) appears to follow a different rule at T-M intervals (t1) greater than 60 msec (Tenkink and Werner, 1981) than at shorter intervals (Purcell et al., 1982). The first study is now extended to shorter t1 and confirms the other study. Recovery peaks at a constant T-B interval (t2) of 60 msec (20 msec less than or equal to t1 less than or equal to 60 msec). Also a U-shaped curve relating correct performance to t1 was found (t2 greater than 120 msec). Two alternative explanations for the recovery phenomenon are presented. In both the important assumption is that the main effect of B is to suppress T and M activity at an early stage in the visual system.

Adult↗

PubMed1984

The contrast dependence of spatial frequency channel interactions.

Interactions between spatial frequency channels were tested in two ways: we measured the discriminability by the visual system between two compound spatial-frequency gratings, of components with spatial frequencies in the ratio 1:3, when the difference between the two gratings was an increase (or decrease) in contrast of both components of the compound grating (contrast discrimination), or when the difference between the two gratings was an increase in contrast of one component and a decrease in contrast of the other component (pattern discrimination). We found that the contrast: pattern discriminability ratio differs significantly from unity in most conditions. Furthermore, this ratio is generally greater when the components of the grating are in peaks-add relative phase than when in peaks-subtract phase. On the other hand, the ratio was close to unity for grating components of spatial frequencies 1 and 9 cycles/deg. These results suggest that the human visual system contains spatial frequency channels with bandwidths of between 1.6 and 3.2 octaves and that these relatively broad channels have peaks-add spatial profiles. The channels appear linear at intermediate contrasts and spatial frequencies, but super-linear at high spatial frequencies and contrasts. Contrast and spatial frequency may be interchangeable for the determination of the linearity of the visual system.

Discrimination, Psychological↗

PubMed1992

Visual acuity in the horse.

We assessed the ease with which horses could learn visual discriminations and measured their resolution acuity. We trained three horses to press their noses against one of two large wooden panels to receive a small food reward. Following training on a series of two-choice discrimination tasks, resolution acuity was measured. Although there was some variability between animals, the best acuity obtained was 23.3 c deg-1. Within the margin of error imposed by limited anatomical data, the obtained values are consistent with predictions based on retinal ganglion cell density estimates and posterior nodal distance/axial length ratios. They suggest that the resolution acuity of the horse is limited by ganglion cell density in the temporal portion of the narrow visual streak.

Animals↗

PubMed2004

The visual perception of 3D shape.

A fundamental problem for the visual perception of 3D shape is that patterns of optical stimulation are inherently ambiguous. Recent mathematical analyses have shown, however, that these ambiguities can be highly constrained, so that many aspects of 3D structure are uniquely specified even though others might be underdetermined. Empirical results with human observers reveal a similar pattern of performance. Judgments about 3D shape are often systematically distorted relative to the actual structure of an observed scene, but these distortions are typically constrained to a limited class of transformations. These findings suggest that the perceptual representation of 3D shape involves a relatively abstract data structure that is based primarily on qualitative properties that can be reliably determined from visual information.

Depth Perception↗

PubMed2005

The motion-induced position shift depends on the visual awareness of motion.

Visual motion signals distort the perceived positions of briefly presented stimuli; a briefly-flashed, stationary stimulus appears spatially displaced in the direction of a nearby motion. The present study examined the role of the visual awareness of motion in the motion-induced position shift by using exclusive dominance and suppression of binocular rivalry. Observers dichoptically viewed a flickering radial checkerboard and two sinusoidal gratings that drifted vertically in opposite directions. When observers viewed exclusively either the checkerboard or motion stimulus, two horizontal lines were flashed, one for each side of the rivalry stimulus. During the exclusive dominance of the grating motion, the lines appeared to shift in the directions of the nearby motions. The position shift was identical to that during non-rivalry, monocular viewing of the motion stimulus. However, when the grating motions were completely suppressed, no position shift was observed. These results demonstrate that the motion-induced position shift depends on the visual awareness of motion.

Awareness↗

PubMed1991

Effects of ectostriatal lesions on natural concept, pseudoconcept, and artificial pattern discrimination in pigeons.

Pigeons were trained on four different visual discrimination tasks: (1) concept of natural stimuli (food vs. non-food object discrimination); (2) arbitrary classification of natural stimuli (pseudoconcept); (3) concept of artificial stimuli (triangles generated by computer graphics); and (4) discrimination of one pair of artificial stimuli. Then, lesions of the ectostriatum were carried out. The ectostriatal lesions impaired the arbitrary classification of natural stimuli and the concept of artificial pattern but did not impair the natural concept or the simple discrimination of fixed two stimuli. Lesions in the neostriatum did not cause deficits in any discrimination task. The birds had to learn individual stimuli for the arbitrary classification of stimuli and the stimulus generalization test after the artificial pattern concept discrimination indicated that the pigeons formed a concept more complicated than "triangle" in human language. These results suggest that the ectostriatum plays a role in task discrimination that requires much visual processing to classify stimuli.

Animals↗

PubMed1996

Target localisation and identification in rapid visual search.

In recent models of visual search in has been proposed that, in rapid parallel search, information about the location of a target pattern among distractors and information about its identity would not be available simultaneously, but that target location is represented at earlier stages of visual processing than target identity. In the present study, the priority of location information over identity information in parallel visual search was investigated by means of one identification and two localisation tasks of different levels of difficulty. In all three tasks, the stimulus display was identical An oblique line segment, randomly 45 degrees or 135 degrees in orientation, was presented randomly at one quadrant of the display. In the identification task, the observer reported the orientation of the oblique line irrespective of its location. In the easy localisation task, the observer indicated whether the oblique line was at the left or right side of the display, and in the difficult localisation task whether it was in the upper or lower part of the display. In both localisation tasks, the observer ignored the orientation of the target line. The oblique line was accompanied by one, five, seventeen, or thirty-nine vertical distractor line segments in all tasks. The results showed that the response speed in the left vs right localisation was faster than in the identification task, whereas performance in the up vs down localisation was inferior. When the response factors (stimulus-response compatibility) in the left vs right localisation were taken into account, there were no performance differences between localisation and identification. Thus, these results demonstrated that direct performance comparisons between localisation and identification may be somewhat arbitrary, and they do not solve the issue of priority of location or identity information in rapid visual search.

Adult↗

PubMed1998

Selective attention to specific location cues: the peak and center of a patch are equally accessible as location cues.

Asymmetric patterns have several spatially distinct cues for spatial localization. These cues include the peak of the luminance distribution, the centroid of the contrast distribution, zero-crossings in the second derivative of the luminance profile, and the midpoint of the visible area. If these cues are represented as primitives in the visual system, the observer should be able to access them at will. To examine whether observers can selectively attend to particular cues, we measured perceived alignment for an asymmetric pattern with two distinct instructions: "align the peak", and "align the center". We found that observers could align the patterns in accord with the instructions with identical precision, suggesting that the peak and the center cues were equally accessible by the observer. We conclude that multiple localization cues are represented in and can be selectively accessed by the visual system.

Computer Graphics↗

PubMed2000

In search of remembrance: evidence for memory in visual search.

Observers searched for a target among distractors while the display items traded places every 110 ms. Search was slower when the target was always relocated to a position previously occupied by a distractor than when the items remained in place, showing the importance of memory for locations in a visual search task. Experiment 2 repeated a previous study in which items could move to any location within the display, but used a larger range of set sizes than tested in the earlier study. A cost in search times to relocating items was found at the larger set sizes, most likely reflecting that the probability that the target would replace a distractor increased with the set size. The findings provide strong evidence for the role of memory for locations within trials in a visual search task.

Adult↗

PubMed1997

A display controller for very brief image presentations.

The device described is a digital interface between a laboratory computer and a point-plotting oscilloscopic display. The combination of computer, display, and interface gives high spatial resolution and exceptionally low frame durations. This system is a powerful tool for studying the time-domain properties of visual perception.

Attention↗

PubMed1978

Influence of imposed metabolic fatigue on visual capacity components.

The present study was designed to investigate the effects of exercise to exhaustion on different components of visual capacity: visual field, and coincidence/anticipation capacity. 16 male subjects were first familiarized with the three visual tests, one day before being submitted to a prefatigue visual test. Metabolic fatigue was induced by working to exhaustion on a bicycle ergometer. Once fatigued, all subjects undertook the visual tests. Pre- and post-fatigue visual measurements were not significantly different. The results demonstrated the independence between metabolic fatigue and visual capacity.

Fatigue↗

PubMed1994

The KoWa model: metric relations in geometric optical illusions.

The KoWa model described here gives a parametric account of the geometric conditions for 6 optical illusions from which prediction of illusory qualities of the figures in perceptual judgements may be accomplished using selected stimulus dimensions. The model is designed to allow integration of visual information.

Attention↗

PubMed2003

[Physiological basis of visual acuity measurement].

The paper presents information about types of acuity tasks, visual acuity limitations and contrast sensitivity function. Measurements of resolution acuity may be interpreted as estimates of spatial density of the mosaic of photoreceptors and ganglion cells of the living human eye. Physiological basis such as Raleigh's criteria, sampling theory and filtering theory of visual resolution are described.

Form Perception↗

PubMed1998

Neural encoding of subject-object distance in a visual recognition system.

Domestic chicks follow a familiar (imprinted) object when it recedes. In behaving, imprinted chicks with no experience of objects at different distances, neuronal activity was recorded from the intermediate and medial part of the hyperstriatum ventrale (IMHV), a brain region crucial for the recognition memory underlying imprinting. We found that (i) some neurones responded equivalently, irrespective of the subject-object distance d (d-invariant); (ii) other neurones responded differently at different values of d (d-sensitive); (iii) these response types were found in monocular chicks and in chicks with both eyes exposed; (iv) the action potential shape of d-invariant neurones was different from that of other neurones and (v) the spontaneous firing rate of some neurones was correlated with locomotor activity. Taken together with previous findings, the results raise the possibility that IMHV has a major role to play in the sensory and motor-control aspects of imprinting in addition to its mnemonic functions.

Action Potentials↗

PubMed1977

Retraining in literal alexia: substitution of a right hemisphere perceptual strategy for impaired left hemispheric processing.

An adult patient with literal alexia, agraphia, slight anomia, and dyscalculia due to a left hemisphere infarct showed lack of sequential skills while pattern recognition remained intact. Some words were recognized as patterns, but could not be read phonetically. Therapy concentrated on forming an association of the visual pattern of the complete word with the retained auditory pattern. In this way the patient learned to read several hundred words and short phrases, even as anomia worsened. The patterns learned could not be generalized to noun declension or verb conjugation, or broken into smaller words. This learning process is characteristic of right hemispheric skills which were utilized as left hemispheric functions deteriorated.

Agraphia↗

PubMed1999

Long-term reversal of hemispheric specialization for visual memory in a split-brain macaque.

Accuracy of response and pattern of ocular fixations in three split-brain macaques were used to evaluate performance of each hemisphere in a continuous visual recognition task. The animal indicated by ocular fixation upon response points whether a displayed image or face was 'NEW' or 'OLD'. An inadvertent lesion of cingulate gyrus severely reduced contralateral fixations and impaired performance of the affected hemisphere in one animal, confirming the inferred relation between hemisphere and laterality of fixations. The hemispheres in the other two animals were initially remarkably similar in accuracy with human faces and with images; but the right hemisphere was significantly superior to the left for macaque faces. Parallel to this, in the one animal tested while simultaneously using both eyes/hemispheres, fixations were made primarily on the left half of human and macaque faces (right hemispheric control), whereas for images the ocular fixations were predominantly focused on the right half. However, after further, extensive training the left hemisphere performed with significantly greater accuracy than the right on all material and this shift was accompanied and further corroborated by a reversal of the fixational pattern to favor the right half of faces, as continued to be the case with images. Thus, over the long term both the pattern of ocular fixations and the accuracy of performance demonstrate a migration from right to left hemispheric dominance as familiarity with the task increased. Performance of the initially superior hemisphere actually diminished with this shift, presenting a uniquely puzzling question of hemispheric balance in the absence of the forebrain commissures.

Animals↗