What does visual agnosia tell us about perceptual organization and its relationship to object perception?

The authors studied 2 patients, S.M. and R.N., to examine perceptual organization and its relationship to object recognition. Both patients had normal, low-level vision and performed simple grouping operations normally but were unable to apprehend a multielement stimulus as a whole. R.N. failed to derive global structure even under optimal stimulus conditions, was less sensitive to grouping by closure, and was more impaired in object recognition than S.M. These findings suggest that perceptual organization involves a multiplicity of processes, some of which are simpler and are instantiated in lower order areas of visual cortex (e.g., collinearity). Other processes are more complex and rely on higher order visual areas (e.g., closure and shape formation). The failure to exploit these latter configural processes adversely affects object recognition.     

Prosopagnosia and visual object agnosia: A behavioral study

A left-handed woman developed visual object agnosia, prosopagnosia, and visual disorientation after resection of the right occipital lobe. Color agnosia and alexia were absent. When asked to identify objects presented visually, the patient's errors represented visually related objects (underspecifications) or perseverations. Identification was facilitated when she observed the object being used in a natural way. Identification was impaired by surrounding the object with unrelated objects, decreasing the background illumination, decreasing the duration of exposure of the object to the patient, and probably also by decreasing the visual angle subtended by the object. In addition, there were disturbances of visualization (i.e., imaging in the absence of a visual stimulus) that paralleled the perceptual difficulties. We conclude that: (1) A deficit in visual perception, characterized by insufficient feature analysis of visual stimuli, was the basis of the visual agnosia in this case. (2) The visual agnosia could not be explained by (a) a vision-language disconnection syndrome, (b) decay of visual memory traces, or (c) deficiencies in the visual fields (pathologic Funktionswandel). (3) The ability to visualize (visual imagery) probably utilizes some of the same neural pathways used in perception. (4) The results in this case probably can be generalized to some but not all cases of visual agnosia; in particular, the deficit in most previously reported patients with prosopagnosia is similar to that of our case. However, agnosic alexia and color agnosia usually have a different neuropsychological basis.     

Self-monitoring behavior in a case of severe auditory agnosia with aphasia

This case report describes an unusual combination of speech and language deficits secondary to bilateral infarctions in a 62-year-old woman. The patient was administered an extensive series of speech, language, and audiologic tests and was found to exhibit a fluent aphasia in which reading and writing were extremely well preserved in comparison to auditory comprehension and oral expression, and a severe auditory agnosia. In spite of her auditory processing deficits, the patient exhibited unexpected self-monitoring ability and the capacity to form acoustic images on visual tasks. The manner in which she corrected and attempted to correct her phonemic errors, while ignoring semantic errors, suggests that different mechanisms may underlie the monitoring of these errors.     

Perceptual grouping and distance estimates in typical and atypical development: comparing performance across perception, drawing and construction tasks

Perceptual grouping is a pre-attentive process which serves to group local elements into global wholes, based on shared properties. One effect of perceptual grouping is to distort the ability to estimate the distance between two elements. In this study, biases in distance estimates, caused by four types of perceptual grouping, were measured across three tasks, a perception, a drawing and a construction task in both typical development (TD: Experiment 1) and in individuals with Williams syndrome (WS: Experiment 2). In Experiment 1, perceptual grouping distorted distance estimates across all three tasks. Interestingly, the effect of grouping by luminance was in the opposite direction to the effects of the remaining grouping types. We relate this to differences in the ability to inhibit perceptual grouping effects on distance estimates. Additive distorting influences were also observed in the drawing and the construction task, which are explained in terms of the points of reference employed in each task. Experiment 2 demonstrated that the above distortion effects are also observed in WS. Given the known deficit in the ability to use perceptual grouping in WS, this suggests a dissociation between the pre-attentive influence of and the attentive deployment of perceptual grouping in WS. The typical distortion in relation to drawing and construction points towards the presence of some typical location coding strategies in WS. The performance of the WS group differed from the TD participants on two counts. First, the pattern of overall distance estimates (averaged across interior and exterior distances) across the four perceptual grouping types, differed between groups. Second, the distorting influence of perceptual grouping was strongest for grouping by shape similarity in WS, which contrasts to a strength in grouping by proximity observed in the TD participants.     

Sequential resolution of fragmented visual percepts: experimental investigation of a subject's perceptual experience after a right medial temporal stroke

This report concerns the fragmented visual percepts in a woman, TR, following a right entorhinal-perirhinal infarct. In a previous report, Weddell [Weddell, R. A. (2005). A visual disorder producing highly selective deletion of recurring letters. Cortex, 41, 471-485] linked TR's highly selective tendency to delete recurrent letters with her fragmented percepts. The conflation of same-identity form elements was attributed to anterior extrastriate damage, which reduced the amount of information sustainable in fully resolved visual percepts, and the present experimental investigation of her subjective account of segment formation and resolution completes the story. She said that complex objects and long words first appeared as blurred regions, which sometimes included form elements. It is argued that figure-centred attentional mechanisms subdivided this blurred region into up to 3-4 parts. String length, lexical status (word vs. nonword), and background colour and/or luminance determined fragment length. Two rules described the fragment resolution sequence: largest segments usually resolved first, left-to-right resolution accounting for a few sequences. This resolution sequence occurred when stimuli were exposed too briefly for saccadic exploration, implicating endogenous attentional shifts. Experiment 4 confirmed TR's assertion that spatial, orthographic, and phonological information were stored during the fragment resolution process. Moreover, TR exerted considerable voluntarily control over the fragment resolution sequence and some influence over fragment length. Finally, these findings were interpreted in terms of an extended version of a neural network model of vision largely derived from nonhuman primate studies.     

Developmental trends in utilizing perceptual closure for grouping of shape: effects of spatial proximity and collinearity

In two experiments, visual search was used to study the grouping of shape on the basis of perceptual closure among participants 5-23 years of age. We first showed that young children, like adults, demonstrate an efficient search for a concave target among convex distractors for closed connected stimuli but an inefficient search for open stimuli. Reliable developmental differences, however, were observed in search for fragmented stimuli as a function of spatial proximity and collinearity between the closure-inducing fragments. When only closure was available, search for all the age groups was equally efficient for spatially close fragments and equally inefficient for spatially distant fragments. When closure and collinearity were available, search for spatially close fragments was equally efficient for all the age groups, but search for spatially distant fragments was inefficient for younger children and improved significantly between ages 5 and 10. These findings suggest that young children can utilize closure as efficiently as can adults for the grouping of shape for closed or nearly closed stimuli. When the closure-inducing fragments are spatially distant, only older children and adults, but not 5-year-olds, can utilize collinearity to enhance closure for the perceptual grouping of shape.     

The role of Gestalt grouping principles in visual statistical learning

A major issue in visual scene recognition involves the extraction of recurring chunks from a sequence of complex scenes. Previous studies have suggested that this kind of learning is accomplished according to Bayesian principles that constrain the types of extracted chunks. Here we show that perceptual grouping cues are also incorporated in this Bayesian model, providing additional evidence for the possible span of chunks. Experiment 1 replicates previous results showing that observers can learn three-element chunks without learning smaller, two-element chunks embedded within them. Experiment 2 shows that the very same embedded chunks are learned if they are grouped by perceptual cues, suggesting that perceptual grouping cues play an important role in chunk extraction from complex scenes.     

Visual Object Agnosia without Alexia or Prosopagnosia: Arguments for Separate Knowledge Stores

We review the points raised by Farah (this issue) in her discussion of the paper by Rumiati, Humphreys, Riddoch, and Bateman (1994), in which a case study was presented of a patient, Mr W., with visual object agnosia without either prosopagnosia or alexia. We reiterate the argument that Mr W. had impaired stored knowledge about the visual characteristics of objects, and that this was more severe than his deficit at a semantic level. In addition, we summarize supporting evidence from another patient (Humphreys & Rumiati, submitted), who showed the same pattern of breakdown as Mr W. We suggest that stored visual knowledge can be specific to the form of stimulus (words, objects, and faces), that these different forms of knowledge may be represented in distinct neural areas, and that agnosia can result after damage to stimulus-specific sites of knowledge representation.     

Visual grouping by motion precedes the relative localization between moving and flashed stimuli

A flashed stimulus is perceived as spatially lagging behind a moving stimulus when they are spatially aligned. When several elements are perceptually grouped into a unitary moving object, a flash presented at the leading edge of the moving stimulus suffers a larger spatial lag than a flash presented at the trailing edge (K. Watanabe. R. Nijhawan. B. Khurana, & S. Shimojo. 2001). By manipulation of the flash onset relative to the motion onset, the present study investigated the order of perceptual operations of visual motion grouping and relative visual localization. It was found that the asymmetric mislocalization was observed irrespective of physical and/or perceptual temporal order between the motion and flash onsets. Thus, grouping by motion must be completed to define the leading-trailing relation in a moving object before the visual system explicitly represents the relative positions of moving and flashed stimuli.     

Covert reading of letters in a case of global alexia

This study describes the case of a global alexic patient with a severe reading deficit affecting words, letters and Arabic numbers, following a left posterior lesion. The patient (VA) could not match spoken letters to their graphic form. A preserved ability to recognize shape and canonical orientation of letters indicates intact access to the representation of letters and numbers as visual objects. A relatively preserved ability to match lowercase to uppercase letters suggests partially spared access to abstract letter identities independently of their visual forms. The patient was also unable to match spoken letters and numbers to their visual form, indicating that she could not access the graphemic representations of letters from their phonological representations. This pattern of performance suggests that the link between graphemic and phonological representations is disrupted in this patient. We hypothesize that VA’ residual reading abilities are supported by the right hemisphere.     

On the perception of shape from shading

The extraction of three-dimensional shape from shading is one of the most perceptually compelling, yet poorly understood, aspects of visual perception. In this paper, we report several new experiments on the manner in which the perception of shape from shading interacts with other visual processes such as perceptual grouping, preattentive search (“pop-out”), and motion perception. Our specific findings are as follows: (1) The extraction of shape from shading information incorporates at least two “assumptions” or constraints—first,that there is a single light source illuminating the whole scene, and second, that the light is shining from “above” in relation to retinal coordinates. (2) Tokens defined by shading can serve as a basis for perceptual grouping and segregation. (3) Reaction time for detecting a single convex shape does not increase with the number of items in the display. This “pop-out” effect must be based on shading rather than on differences in luminance polarity, since neither left-right differences nor step changes in luminance resulted in pop-out. (4) When the subjects were experienced, there were no search asymmetries for convex as opposed to concave tokens, but when the subjects were naive, cavities were much easier to detect than convex shapes. (5) The extraction of shape from shading can also provide an input to motion perception. And finally, (6) the assumption of “overhead illumination” that leads to perceptual grouping depends primarily on retinal rather than on “phenomenal” or gravitational coordinates. Taken collectively, these findings imply that the extraction of shape from shading is an “early” visual process that occurs prior to perceptual grouping, motion perception, and vestibular (as well as “cognitive”) correction for head tilt. Hence, there may be neural elements very early in visual processing that are specialized for the extraction of shape from shading.     

On the perception of shape from shading.

The extraction of three-dimensional shape from shading is one of the most perceptually compelling, yet poorly understood, aspects of visual perception. In this paper, we report several new experiments on the manner in which the perception of shape from shading interacts with other visual processes such as perceptual grouping, preattentive search ("pop-out"), and motion perception. Our specific findings are as follows: (1) The extraction of shape from shading information incorporates at least two "assumptions" or constraints--first, that there is a single light source illuminating the whole scene, and second, that the light is shining from "above" in relation to retinal coordinates. (2) Tokens defined by shading can serve as a basis for perceptual grouping and segregation. (3) Reaction time for detecting a single convex shape does not increase with the number of items in the display. This "pop-out" effect must be based on shading rather than on differences in luminance polarity, since neither left-right differences nor step changes in luminance resulted in pop-out. (4) When the subjects were experienced, there were no search asymmetries for convex as opposed to concave tokens, but when the subjects were naive, cavities were much easier to detect than convex shapes. (5) The extraction of shape from shading can also provide an input to motion perception. And finally, (6) the assumption of "overhead illumination" that leads to perceptual grouping depends primarily on retinal rather than on "phenomenal" or gravitational coordinates. Taken collectively, these findings imply that the extraction of shape from shading is an "early" visual process that occurs prior to perceptual grouping, motion perception, and vestibular (as well as "cognitive") correction for head tilt. Hence, there may be neural elements very early in visual processing that are specialized for the extraction of shape from shading.     

Closure of salient regions determines search for a collinear target configuration

Grouping operations offer an effective mechanism by which to structure complex visual scenes. Among the various principles that support element integration, closure may be regarded as a main factor in shape extraction. In five experiments, we investigated the impact of grouping by closure on search for target configurations made up of collinearly arranged corner junctions. Systematic variations in the configuration of distractor elements were explored to investigate how the figural information of distractors interferes with target detection. The results showed no search interference for distractor configurations that were open forms. By contrast, distractors making up closed forms reduced the efficiency of search performance, indicating closure as a major contributor to form detection in multielement configurations. In conclusion, the effects reported may be considered to arise from mechanisms of unit formation that support the rapid extraction of salient regions to guide search.     

Visual object agnosia without prosopagnosia or alexia: Evidence for hierarchical theories of visual recognition

Abstract

A single case study is presented of a patient, Mr. W, with a selective deficit in recognizing pictures and real objects, linked to impaired stored visual knowledge about objects. Despite this, Mr. W maintained a preserved ability both to read aloud printed words and to recognize famous faces, when compared with age-matched control subjects. In addition, his access to semantic information from words was superior to that from pictures. The data provide evidence that visual agnosia can occur without alexia or prosopagnosia, contrary to recent proposals (Farah 1990, 1991). This finding is consistent with a hierarchical model of visual object recognition in which agnosia can reflect impaired stored knowledge of objects without accompanying perceptual deficits. The selective recognition deficit for objects further indicates that stored knowledge concerning different classes of visual stimuli (common objects, faces, and words) is separately represented in the brain.     

Late influences on perceptual grouping: Amodal completion

Perceptual grouping is generally assumed to be an early visual process that operates on a previously unorganized image-based representation. The present experiment shows that elements perceived as occluded by a closer surface tend to be grouped with elements having the same shape as the amodally completed percept rather than with those having the same retinal shape as the incomplete stimulus. It is therefore concluded that perceptual grouping by shape similarity either occurs after amodal completion or is a temporally extended process that occurs both before and after it.     

知觉组织影响时序知觉的认知机制:ERP研究的证据

以往研究显示,知觉组织影响时序知觉,采用ERP技术拟探讨知觉组织影响时序知觉的认知机制。采集23名被试完成同时判断任务的脑电数据。行为结果发现,知觉组织影响时序知觉,具体表现为知觉组织条件的同时判断频率显著高于非知觉组织条件。ERPs结果发现,在P1和P2成分上,知觉组织条件与非知觉组织条件引发的波幅没有显著差异; 而在N1成分上,知觉组织条件诱发的N1波幅显著大于非知觉组织条件。这表明,知觉组织对时序知觉的影响可能发生在视觉刺激加工的早期阶段,视觉刺激的知觉组织先于时序信息加工。     

Grouping reduces visual extinction: Neuropsychological evidence for weight-linkage in visual selection

Abstract

We tested two subjects following damage to right parietal cortex to see if their failure to detect a left visual stimulus in the presence of a simultaneous right stimulus (visual extinction) could be modulated by perceptual grouping between the left and right stimuli. Subjects performed a simple detection task for brief displays in which items could appear in the left or right visual field, both fields, or neither field. On trials in which items appeared in both fields, we found that left omissions (extinction errors) were dramatically reduced when the two items formed a good perceptual group, either on the basis of Gestalt factors such as similarity and symmetry (Experiment 1). or by forming a familiar configuration (Experiment 2). We suggest that extinction may be a spatially specific exaggeration of a normal attention limitation, in which the contralesional item is disadvantaged in the competition for selection. However, this obstacle to selection can be overcome if, as a result of grouping, ipsilesional and contralesional items become allies rather than competitors for selection.     

Role of Attention and Perceptual Grouping in Visual Statistical Learning

Statistical learning has been widely proposed as a mechanism by which observers learn to decompose complex sensory scenes. To determine how robust statistical learning is, we investigated the impact of attention and perceptual grouping on statistical learning of visual shapes. Observers were presented with stimuli containing two shapes that were either connected by a bar or unconnected. When observers were required to attend to both locations at which shapes were presented, the degree of statistical learning was unaffected by whether the shapes were connected or not. However, when observers were required to attend to just one of the shapes' locations, statistical learning was observed only when the shapes were connected. These results demonstrate that visual statistical learning is not just a passive process. It can be modulated by both attention and connectedness, and in natural scenes these factors may constrain the role of stimulus statistics in learning.     

Structural salience and the nonaccidentality of a gestalt

We perceive structure through a process of perceptual organization. Here we report a new perceptual organization phenomenon-the facilitation of visual grouping by global curvature. Observers viewed patterns that they perceived as organized into collections of curves. The patterns were perceptually ambiguous such that the perceived orientation of the patterns varied from trial to trial. When patterns were sufficiently dense and proximity was equated for the predominant perceptual alternatives, observers tended to perceive the organization with the greatest curvature. This effect is tantamount to visual grouping by maximal curvature and thus demonstrates an unprecedented effect of global structure on perceptual organization. We account for this result with a model that predicts the perceived organization of a pattern as function of its nonaccidentality, which we define as the probability that it could have occurred by chance. Our findings demonstrate a novel relationship between the geometry of a pattern and the visual salience of global structure.