Performance on the ITPA following cerebral commissurotomy and hemispherectomy.
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Adolescent↗
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Results for “Perceptual Closure”
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[Perceptual organization from point fields].
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Automation↗
Global interactions in perceptual completion at the blind spot.
Nonlinearity and asymmetrical property in the filling-in process at the blind spot are investigated. Perceptual completion of a pattern with a gap is deteriorated, i.e. gap information inhibits completion across the gap. Filled-in patterns, observed from complementary patterns, are not the same in complementary sense. These suggest that perceptual completion cannot simply be predicted from a superimposition of completions, obtained from local stimulus conditions, and seems to be mediated by global processes.
Humans↗
Visual grouping without low spatial frequencies.
Gestalt psychologists have stressed the perceptual importance of grouping. On the basis of multichannel theory of early visual processing, grouping has been attributed to the operation of channels tuned to low spatial frequencies. If this explanation is correct and sufficient, grouping should not occur when looking at highpass filtered images, because they do not excite low spatial frequency channels. However this paper brings evidence that visual grouping also occurs in such a situation. Hence, a more comprehensive explanation of visual grouping is needed.
Filtration↗
The influence of gender, handedness, and performance level on specialized cognitive functioning.
Individual differences in performance on neuropsychological tests were analyzed across age (prepubertal to adult), gender, and handedness groups and examined for performance level as a moderating variable. No differences were observed for the factor structure of these tests across ages and between genders, suggesting similar cognitive structures among these groups. Significant differences in performance were observed between males and females and, to a lesser extent, between right and left handers. Of interest, were significant Gender x Handedness and Gender x Handedness x Level (of performance) interactions seen especially in the older (postpubertal and adult) subjects. High performing, right handed males and left handed females performed better on visuospatial tasks while left handed males and right handed females performed better on verbosequential tasks.
Adolescent↗
Speed of work and EEG asymmetry.
EEG from O1 and O2 was recorded from twelve subjects (6 male and 6 female) whilst they carried out a paced inspection task at five separate task speeds. Peak alpha activity was analysed using a spectral analysis technique. The results suggested that when subjects were working at a speed 25% faster than their self-chosen pace they were using predominantly their right hemisphere, whereas then they were working at 25% slower they were using mainly their left hemisphere. It was suggested, based on earlier literature, that subjects may be processing information about the task in different ways at different task paces: A sequential processing strategy is being used at the slow pace whilst a parallel or Gestalt processing strategy is being used at the fast pace.
Adolescent↗
Better living by not completing: a wonderful peculiarity of pigeon vision?
Whereas many non-human species have been demonstrated to visually complete partly occluded figures, pigeons have been repeatedly failed to do so. We asked whether this failure reflected the pigeons' lack of perceptual process for completion or their decision among completed and non-completed figures. Four pigeons searched for a red lozenge target having one of its four contours punched in a rectangular edge out among three intact lozenges. All of these four stimuli had a white square next to them. After obtaining consistent search performances, the pigeons were tested with the punched target in a variety of locations relative to the white square, including right at the edges. Humans tested in the same task needed longer times before detecting the target when the square was placed right at the punched edge, suggesting automatic completion in humans. In contrast, the pigeons showed no similar difficulty. This result has two important suggestions: first, pigeons fail to complete partially occluded objects at the perceptual level, and second, this lack of completion is sometimes advantageous for them.
Analysis of Variance↗
Different Gestalt processing for different actions? Comparing object-directed reaching and looking time measures.
Six experiments compared the Gestalt processing that mediates infant reaching and looking behaviors. Experiment 1 demonstrated that the positioning and timing of 8- and 9-month-olds' reaching was influenced by remembered relative motion. Experiment 2 suggested that a visible gap, without this relative motion, was not sufficient to produce these effects. Experiment 3 found that 6- and 7-month-olds required both remembered relative motion and a continuously visible gap to affect reaching; when two separate objects were placed adjacent to one another, they were treated as though they were connected. Experiments 4 and 5, using a looking time measure, identified a dissociation between looking and reaching performance. In Experiment 6, 6- and 7-month-olds' reaching was influenced when display parts differed in shape and color. In these experiments, Gestalt processing for looking was more influenced by remembered information than was Gestalt processing for reaching. For reaching, a low weighting of separation cues was also apparent.
Female↗
Interactions between proximity and similarity grouping: an event-related brain potential study in humans.
The current work examined neural substrates underlying interactions between two types of Gestalt grouping by recording event-related brain potentials (ERPs). Subjects were presented with stimulus arrays in which local circles and squares were grouped into columns or rows. The cues dominating grouping (proximity and similarity) were either congruent or incongruent. Subjects responded faster to the identification of orientations of perceptual groups based on proximity than similarity cues. Responses were slowed by incongruent cues and this effect was larger in the similarity than proximity conditions. Proximity grouping generated enhanced positivity over the posterior occipital cortex between 100 and 140 ms relative to similarity grouping. Relative to congruent grouping cues, incongruent cues elicited enlarged positivity over the temporal-parietal areas between 180 and 220 ms only in the similarity condition, but generated smaller P3 amplitudes with longer latencies in both proximity and similarity conditions. The results provide ERP indices of the dominance of proximity over shape similarity in guiding perceptual grouping when two grouping principles are present in stimulus displays.
Adolescent↗
Using afterimages to test neural mechanisms for perceptual filling-in.
Many theories of visual perception propose that brightness information spreads from edges to define the perceived intensity of the interior of visual surfaces. Several theories of visual perception have hypothesized that this filling-in process is similar to a diffusion of information where the signals coding brightness spread to nearest neighbors. This paper shows that diffusive mechanisms fail to account for the characteristics of certain afterimage percepts that seem to be dependent on the filling-in process. A psychophysical experiment tests a key property of diffusion-based filling-in mechanisms and finds data that rejects this class of models. A non-diffusive based filling-in mechanism is proposed and is shown to act much like the diffusive based mechanism in many instances, but also produces afterimage percepts that match the experimental data.
Afterimage↗
A model of contextual interactions and contour detection in primary visual cortex.
A new model of contour extraction and perceptual grouping in the primary visual cortex is presented and discussed. It differs from previous models since it incorporates four main mechanisms, according to recent physiological data: a feed-forward input from the lateral geniculate nucleus, characterized by Gabor elongated receptive fields; an inhibitory feed-forward input, maximally oriented in the orthogonal direction of the target cell, which suppresses non-optimal stimuli and warrants contrast invariance; an excitatory cortical feedback, which respects co-axial and co-modularity criteria; and a long-range isotropic feedback inhibition. Model behavior has been tested on artificial images with contours of different curvatures, in the presence of considerable noise or in the presence of broken contours, and on a few real images. A sensitivity analysis has also been performed on the role of intracortical synapses. Results show that the model can extract correct contours within acceptable time from image presentation (30-40 ms). The feed-forward input plays a major role to set an initial correct bias for the subsequent feedback and to ensure contrast-invariance. Long-range inhibition is essential to suppress noise, but it may suppress small contours due to excessive competition with greater contours. Cortical excitation sharpens the initial bias and improves saliency of the contours. Model results support the idea that contour extraction is one the primary steps in the visual processing stream, and that local processing in V1 is able to solve this task even in difficult conditions, without the participation of higher visual centers.
Feedback↗
Human MT+ mediates perceptual filling-in during apparent motion.
During apparent motion, spatially distinct items presented in alternation cause the perception of a visual stimulus smoothly traversing the intervening space where no physical stimulus exists. We used fMRI to determine whether the perceptual 'filling-in' that underlies this phenomenon has an early or late cortical locus. Subjects viewed a display comprised of concentric rings that elicited apparent motion (two concentric rings presented in alternation), flicker (the same rings presented simultaneously), or real motion. We independently localized the cortical regions corresponding to the path of apparent motion in early visual areas (V1, V2, VP, V3, V4v, V3A), as well as the human motion processing complex (MT+). Cortical activity in the path of apparent motion in early visual areas was similar in amplitude during both apparent motion and flicker. In contrast, cortical activity in MT+ was higher in amplitude during apparent motion than during flicker, but was lower in amplitude than during real motion. In addition, we observed overlap in the cortical loci of MT+ and the lateral occipital complex (LOC), a region involved in shape and object processing. This overlap suggests that these regions could directly interact and thereby support perceived object continuity during apparent motion.
Adult↗
Cortical responses to contextual influences in amodal completion.
Amodal completion refers to the process in the visual system that enables us to perceive partly occluded objects as whole objects. Both the overall shape of a visual object (global aspect) and the region immediately surrounding the occluder (local aspect) are known to determine the process of completion. We investigated the influence of overall shape context in completion on human brain activity using MEG recordings. Subjects were presented with two different types of shapes that were partly occluded by a rectangle. In the so-called convergent shapes, the local and global contexts trigger the same completion, and in the so-called divergent shapes, local and global contexts trigger different completions. The occluder was removed after 1 s, revealing the whole (completed) shape where expectancies based on the local and global context could either be fulfilled or violated. We found an early (from 200 ms on) left occipital component, which was predominantly sensitive to the figural aspects of the uncovered continuation, but was also modulated by the congruency between the physical shape and the perceived shape (based on the contextual cues). In contrast, an early right occipital component and a later (400 ms) anterior temporal component were sensitive only to the congruency between the physical and the perceived shape, showing the relevance of the global context in amodal completion.
Adult↗
Disappearance of memory fragments in patients with Alzheimer's disease: evidence from a longitudinal study of visual priming.
Previous studies suggested that perceptual memory as indexed by visual priming is normal in patients with Alzheimer's disease (AD). However, these studies did not specifically test the long-term effects of visual priming, which may differ significantly between Alzheimer's patients and normal subjects. To test this possibility, we examined long-term visual priming in AD patients, 1 hour, 1 month, and 3 months after training. Our results indicated a significant difference in visual priming between AD patients and normal subjects after 3 months, but not 1 month. For AD patients, there was a strong positive correlation between the 3-month priming effect and the Mini-Mental State Examination (MMSE) scores-severely demented patients were less likely to exhibit priming after 3 months. It appears that severe cortical degeneration may render AD patients unable to consolidate their perceptual memories. Our results suggest that lack of visual priming in AD patients is linked to the inability to maintain fragmented perceptual memories.
Aged↗
Evidence for a dissociation of structural and semantic knowledge in dementia of the Alzheimer type (DAT).
Object recognition and naming deficits in dementia of the Alzheimer type (DAT) have typically been attributed to deficits in semantic processing, with only a few studies proposing loci of deficits other than semantic. One possible cause of DAT object recognition impairments could involve deficits in processing structural aspects of visually presented items. In this paper, we assess the performance of a group of mild DAT patients on two tasks of structural access, object decision, and the complete/incomplete task (based on part-whole matching task), as well as on a semantic probes task, designed to assess the patients' semantic knowledge of the same items for which structural knowledge had earlier been assessed. The DAT patients were substantially impaired in their performance on tasks of structural access. Further, no evidence for item-to-item consistency in the DAT patients' errors for the structural and semantic tasks was found, raising the possibility that structural and semantic knowledge may become differentially impaired in DAT.
Aged↗
Unveiling the foveal blue scotoma through an afterimage.
The absence of short-wave-sensitive (S-) cones in the human foveola normally goes unnoticed, but the resulting foveal S-cone, or blue, scotoma can be visualized as the negative afterimage of a short-wavelength adapting field on a larger white background. The afterimage has an annular shape with a lighter inner region that corresponds to Maxwell's spot, and a small bright spot in the center corresponding to the foveal blue scotoma. We have shown that the visibility of the center spot in the afterimage approximately follows the spectral sensitivity curve of the S-cones. We further demonstrate that the central bright spot subtends a retinal area that is coincident with the tritanopic region of the foveola. The macular pigment distribution measured for the same observers also peaks in the central fovea, but has a relatively high density over a broader retinal region than the bright spot in the negative afterimage, and more closely corresponds to the lighter annular region of the afterimage. The results support the hypothesis of an active post-receptoral process for filling-in of chromatic scotomas.
Adaptation, Ocular↗
Contour interpolation revealed by a dot localization paradigm.
Contour interpolation mechanisms allow perception of bounded objects despite incomplete edge information. Here, we introduce a paradigm that maps interpolated contours as they unfold over time. Observers localize dots relative to perceived boundaries of illusory, partly occluded, or control stimuli. Variations in performance with dot position and processing time reveal the location and precision of emerging contour representations. Illusory and occluded contours yielded more proficient dot localization than control stimuli containing only spatial cues, suggesting performance based on low-level representations. Further, illusory contours exhibited a distinct developmental time course, emerging over the first 120 ms of processing. These experiments establish the effectiveness of the dot localization paradigm for examining interpolated edge representations, contour microgenesis, and the underlying processing mechanisms.
Adolescent↗
Illusory contours and surfaces without amodal completion and depth stratification.
Cognitive and figural cues were studied in modified Ehrenstein figures made from letters of the alphabet instead of radial lines. Capital letters with and without terminators (L, J vs O, D) were used, oriented towards or away from the central gap. Three groups, of 14 subjects each, estimated the magnitude of either (i) the illusory contour, (ii) brightness enhancement, or (iii) apparent depth. Strong illusory contour formation and brightness enhancement, but no depth stratification, were perceived in figures devoid of apparent occlusion and amodal completion. These results demonstrate that the Ehrenstein illusion can arise from line ends--with no need for perceptual completion, showing that illusory boundaries and surfaces can be dissociated from apparent depth. Results support a bottom-up explanation in terms of end-stopped neurons in the visual cortex. Conversely, top-down processes appear to be responsible for depth stratification.
Cognition↗