Visual Form Agnosia Research Articles

Preserved grip scaling to visual size despite non-veridical haptic feedback in a patient with visual form agnosia

Preserved grip scaling to visual size despite non-veridical haptic feedback in a patient with visual form agnosia

Poster #154 THE CENTRAL ROLE OF ATTENTION AND INFORMATION PROCESSING DEFICITS IN THE PROFILE OF COGNITIVE DYSFUNCTION IN SCHIZOPHRENIA

Among the many dissociations describing the visual system, the dual theory of two visual systems, respectively dedicated to perception and action, has yielded a lot of support. There are psychophysical, anatomical and neuropsychological arguments in favor of this theory. Several behavioral studies that used sensory and motor psychophysical parameters observed differences between perceptive and motor responses. The anatomical network of the visual system in the non-human primate was very readily organized according to two major pathways, dorsal and ventral. Neuropsychological studies, exploring optic ataxia and visual agnosia as characteristic deficits of these two pathways, led to the proposal of a functional double dissociation between visuomotor and visual perceptual functions. After a major wave of popularity that promoted great advances, particularly in knowledge of visuomotor functions, the guiding theory is now being reconsidered. Firstly, the idea of a double dissociation between optic ataxia and visual form agnosia, as cleanly separating visuomotor from visual perceptual functions, is no longer tenable; optic ataxia does not support a dissociation between perception and action and might be more accurately viewed as a negative image of action blindsight. Secondly, dissociations between perceptive and motor responses highlighted in the framework of this theory concern a very elementary level of action, even automatically guided action routines. Thirdly, the very rich interconnected network of the visual brain yields few arguments in favor of a strict perception/action dissociation. Overall, the dissociation between motor function and perceptive function explored by these behavioral and neuropsychological studies can help define an automatic level of action organization deficient in optic ataxia and preserved in action blindsight, and underlines the renewed need to consider the perception-action circle as a functional ensemble.

No Dissociation between Perception and Action in Patient DF When Haptic Feedback is Withdrawn

Goodale et al. (1991) reported a striking dissociation between vision for perception and action. They examined DF, a human patient who had damage to her ventral visual stream and suffered from visual form agnosia. She was unable to perceive an object's size but could match the opening of her hand to the object's size during grasping. It was concluded that grasping relied on a separate representation of visual size in the dorsal stream and required no visual input from the ventral stream. This observation inspired the influential perception-action model, which claimed separate visual streams for perception and action. However, in grasping (but not in corresponding perceptual tasks), participants receive haptic feedback after each trial. Using this feedback, DF might compensate for her impaired size-vision. I reexamined DF's grasping behavior using a mirror apparatus to dissociate the image of an object from its physical presence. DF's grasping was only normal when she received haptic feedback. Thus, in grasping, DF can rely on haptic feedback to compensate for her deficit in size-perception. This can explain why her grasping is significantly better than her perceptual performance. The findings emphasize the extent of early interstream interactions and highlight the multimodal nature of sensory processing in the dorsal stream.

20 years later: A second look on DF's motor behaviour

The so-called action vs. perception model represents one of the currently dominating models addressing visual processing in primates. One of the crucial cornerstones of the action vs. perception model of visual processing is the dissociation of impaired perception versus intact visuomotor control in neurological patients with visual form agnosia (VFA). In fact, virtually all evidence related to VFA supporting the model was reported from only one patient: patient D.F. Through the last two decades D.F. became as important as only very few other exemplar cases in the neurosciences. However, a large corpus of experiments with this individual used methods that were insufficient to reveal less obvious impairments on a single subject level. We reanalysed the data of D.F. and identified basic visuomotor impairments that had been overlooked so far. Our reanalysis underlines the fact that the widespread and popular presentation of strong dissociations between distinct visual systems seems to be exaggerated.

Visuomotor performance based on peripheral vision is impaired in the visual form agnostic patient DF

The perception–action model states that visual information is processed in different cortical areas depending on the purpose for which the information is acquired. Specifically, it was suggested that the ventral stream mediates visual perception, whereas the dorsal stream primarily processes visual information for the guidance of actions ( Goodale & Milner, 1992). Evidence for the model comes from patient studies showing that patients with ventral stream damage (visual form agnosia) and patients with dorsal stream damage (optic ataxia) show divergent performance in action and perception tasks. Whereas DF, a patient suffering from visual form agnosia, was found to perform well in visuomotor tasks despite her inability to use vision for perceptual tasks, patients with optic ataxia show usually the opposite pattern, i.e. good perception but impaired visuomotor control. The finding that both disorders seem to provoke a mirror-reversed pattern of spared and impaired visual functions, led to the belief that optic ataxia and visual form agnosia can be considered as complementary disorders. However, the visuomotor performance of patients with optic ataxia is typically only impaired when they are tested in visual periphery while being often preserved when tested in central vision. Here, we show that DF's visuomotor performance is also only preserved when the target is presented centrally. Her reaching and grasping movements to targets in peripheral vision are abnormal. Our findings indicate that DF's visuomotor performance is quite similar to the visuomotor performance of patients with optic ataxia which undermines previous suggestions that the two disorders form a double-dissociation.

Color picture drawings without form and eye movements: A case report of visual form agnosia in a girl

The case of an 8-year-old girl who manifested cortical blindness and whose color drawings of faces and objects were without outlines is reported. Her birth was uneventful. When she was 10 months old, she fell down to the floor from a chair, resulting in a subarachnoidal hemorrhage. A repeat brain MRI revealed localized lesions in the visual cortices in the right and left hemispheres. As she grew older she was found to have visual imperceptions. She was found to have difficulties in learning visually the names of objects with form and letters, and in recognizing faces of her family. However, she was able to discriminate well the colors of faces and objects and learn easily the names of the objects with form by touching. She seemed to utilize subcortical vision for seeing colors of faces and objects.

The Functional Neuroanatomy of Object Agnosia: A Case Study

Cortical reorganization of visual and object representations following neural injury was examined using fMRI and behavioral investigations. We probed the visual responsivity of the ventral visual cortex of an agnosic patient who was impaired at object recognition following a lesion to the right lateral fusiform gyrus. In both hemispheres, retinotopic mapping revealed typical topographic organization and visual activation of early visual cortex. However, visual responses, object-related, and -selective responses were reduced in regions immediately surrounding the lesion in the right hemisphere, and also, surprisingly, in corresponding locations in the structurally intact left hemisphere. In contrast, hV4 of the right hemisphere showed expanded response properties. These findings indicate that the right lateral fusiform gyrus is critically involved in object recognition and that an impairment to this region has widespread consequences for remote parts of cortex. Finally, functional neural plasticity is possible even when a cortical lesion is sustained in adulthood.

A PDP model of the simultaneous perception of multiple objects

Illusory conjunctions in normal and simultanagnosic subjects are two instances where the visual features of multiple objects are incorrectly ‘bound’ together. A connectionist model explores how multiple objects could be perceived correctly in normal subjects given sufficient time, but could give rise to illusory conjunctions with damage or time pressure. In this model, perception of two objects benefits from lateral connections between hidden layers modelling aspects of the ventral and dorsal visual pathways. As with simultanagnosia, simulations of dorsal lesions impair multi-object recognition. In contrast, a large ventral lesion has minimal effect on dorsal functioning, akin to dissociations between simple object manipulation (retained in visual form agnosia and semantic dementia) and object discrimination (impaired in these disorders) [Hodges, J.R., Bozeat, S., Lambon Ralph, M.A., Patterson, K., and Spatt, J. (2000), ‘The Role of Conceptual Knowledge: Evidence from Semantic Dementia’, Brain, 123, 1913–1925; Milner, A.D., and Goodale, M.A. (2006), The Visual Brain in Action (2nd ed.), New York: Oxford]. It is hoped that the functioning of this model might suggest potential processes underlying dorsal and ventral contributions to the correct perception of multiple objects.

Depth-cue integration in grasp programming: No evidence for a binocular specialism

When we grasp with one eye covered, the finger and thumb are typically opened wider than for binocularly guided grasps, as if to build a margin-for-error into the movement. Also, patients with visual form agnosia can have profound deficits in their (otherwise relatively normal) grasping when binocular information is removed. One interpretation of these findings is that there is a functional specialism for binocular vision in the control of grasping. Alternatively, cue-integration theory suggests that binocular and monocular depth cues are combined in the control of grasping, and so impaired performance reflects not the loss of ‘critical’ binocular cues, but increased uncertainty per se. Unfortunately, removing binocular information confounds removing particular (binocular) depth cues with an overall reduction in the available information, and so such experiments cannot distinguish between these alternatives. We measured the effects on visually open-loop grasping of selectively removing monocular (texture) or binocular depth cues. To allow meaningful comparisons, we made psychophysical measurements of the uncertainty in size estimates in each case, so that the informativeness of binocular and monocular cues was known in each condition. Consistent with cue-integration theory, removing either binocular or monocular cues resulted in similar increases in grip apertures. In a separate experiment, we also confirmed that changes in uncertainty per se (keeping the same depth cues available) resulted in larger grip apertures. Overall, changes in the margin-for-error in grasping movements were determined by the uncertainty in size estimates and not by the presence or absence of particular depth cues. Our data therefore argue against a binocular specialism for grasp programming. Instead, grip apertures were smaller when binocular and monocular cues were available than with either cue alone, providing strong evidence that the visuo-motor system exploits the redundancy available in multiple sources of information, and integrates binocular and monocular cues to improve grasping performance.

Letter Posting and Orientation Matching: Two Equivalent Tasks in Action and Perception?

The finding that in a patient with visual form agnosia (DF), the performance level varies in a visuomotor letter-posting task and a perceptual orientation matching task was considered as part of the evidence for the perception-action model (Milner and Goodale, 1995). In this study we examined an alternative interpretation of these findings. We specifically tested whether orientation matching and letter posting can be accomplished using different strategies. Sixteen neurologically intact participants were asked to either put cards of different sizes through a target slot of a certain orientation or to simply indicate the slot's orientation. Letter-posting was performed in three different conditions varying the amount of visual feedback available. Results show that some participants apply a strategy of obstacle-avoidance in the posting task. That is, they oriented the card such that the safety margin between the edges of the target and the card was increased. This tendency became stronger with increasing card size. In contrast, in the orientation matching task, the end-orientation of the card was unaffected by its size and closer to the slot's actual orientation. The findings suggest that posting and matching can be solved using different visuo-spatial information. The perception-action dissociation reported for these tasks in DF might therefore simply indicate a difficulty in computing visual orientation, an ability that is needed for successful orientation matching but not for posting.

POD08 Abnormal saliency processing in a patient with visual form agnosia

BackgroundWhen we look at a visual scene our scanning eye movements are not random. We have previously compared the eye movements made by two patients with associative visual agnosia with...

Stereoscopic Vision in the Absence of the Lateral Occipital Cortex

Both dorsal and ventral cortical visual streams contain neurons sensitive to binocular disparities, but the two streams may underlie different aspects of stereoscopic vision. Here we investigate stereopsis in the neurological patient D.F., whose ventral stream, specifically lateral occipital cortex, has been damaged bilaterally, causing profound visual form agnosia. Despite her severe damage to cortical visual areas, we report that DF's stereo vision is strikingly unimpaired. She is better than many control observers at using binocular disparity to judge whether an isolated object appears near or far, and to resolve ambiguous structure-from-motion. DF is, however, poor at using relative disparity between features at different locations across the visual field. This may stem from a difficulty in identifying the surface boundaries where relative disparity is available. We suggest that the ventral processing stream may play a critical role in enabling healthy observers to extract fine depth information from relative disparities within one surface or between surfaces located in different parts of the visual field.

A tale of two agnosias: Functional differences between integrative and visual form agnosia

A tale of two agnosias: Functional differences between integrative and visual form agnosia

On-line reaching to perturbed targets in visual form agnosia (patient DF)

On-line reaching to perturbed targets in visual form agnosia (patient DF)

How would you catch a ball if you had visual form agnosia?

How would you catch a ball if you had visual form agnosia?

FMRI confirmation of a neurological dissociation between perceiving objects and grasping them

Some of the most compelling evidence for the distinction between vision-for-perception and vision-for-action has come from studies of patient DF who developed visual form agnosia following an anoxic episode. DF is able to use visual information about the size, shape, and orientation of objects to control her grasping movements despite the fact that she is unable to perceive those same object attributes. On the basis of these results and some early structural MR data, we proposed that the anoxia had interrupted the normal flow of object form information into her ventral stream without affecting the processing of object form information by her dorsal stream. Recently we confirmed this idea in an fMRI experiment. New anatomical MR images showed that the lateral occipital cortex (LOC), a ventral-stream area implicated in object recognition, is severely damaged in DF. A comparison of activation with line drawings of objects and scrambled versions of the same drawings revealed no differential activation in this or the surrounding cortex. Objects with color and visual texture did result in significant but atypical activation in areas outside of LOC. In contrast, when DF grasped objects that varied in size and orientation, she showed relatively normal activation in the anterior intraparietal sulcus (AIP), a dorsal-stream area implicated in the visual control of grasping. These findings provide additional support for the idea that visual perception and the visual control of action depend on separate visual pathways in the cerebral cortex, and confirm the respective roles of the ventral and dorsal visual streams in these functions.

Scene classification and parahippocampal place area activation in an individual with visual form agnosia

Scene classification and parahippocampal place area activation in an individual with visual form agnosia

Not all patients labeled as “prosopagnosia” have a real prosopagnosia

Since face recognition is the most powerful source of information for identifying familiar people, patients showing a multimodal defect in people recognition have been sometimes considered as affected by “prosopagnosia”—namely, by a form of visual agnosia, specifically affecting face recognition. In this note we report two anatomoclinical observations and a neuroanatomical study in which an inappropriate use of the term “prosopagnosia” was made, because the person recognition defect was not confined to the visual (face) modality, but also concerned voice and/or name of the target person. The dangers of this inappropriate use of the term prosopagnosia are briefly discussed.

Memory-guided saccade processing in visual form agnosia (patient DF)

According to Milner and Goodale's model (The visual brain in action, Oxford University Press, Oxford, 2006) areas in the ventral visual stream mediate visual perception and oV-line actions, whilst regions in the dorsal visual stream mediate the on-line visual control of action. Strong evidence for this model comes from a patient (DF), who suffers from visual form agnosia after bilateral damage to the ventro-lateral occipital region, sparing V1. It has been reported that she is normal in immediate reaching and grasping, yet severely impaired when asked to perform delayed actions. Here we investigated whether this dissociation would extend to saccade execution. Neurophysiological studies and TMS work in humans have shown that the posterior parietal cortex (PPC), on the right in particular (supposedly spared in DF), is involved in the control of memory-guided saccades. Surprisingly though, we found that, just as reported for reaching and grasping, DF's saccadic accuracy was much reduced in the memory compared to the stimulus-guided condition. These data support the idea of a tight coupling of eye and hand movements and further suggest that dorsal stream structures may not be sufficient to drive memory-guided saccadic performance.

Illusions, Demonstratives, and the Zombie Action Hypothesis

David Milner and Melvyn Goodale, and the many psychologists and philosophers who have been influenced by their work, claim that ‘the visual system that gives us our visual experience of the world is not the same system that guides our movements in the world’. The arguments that have been offered for this surprising claim place considerable weight on two sources of evidence — visual form agnosia and the reaching behaviour of normal subjects when picking up objects that induce visual illusions. The present article shows that, if we are careful to consider the possibility that a demonstrative gesture can contribute content to a conscious experience, then neither source of evidence is compelling.