Seeing, since childhood, without ventral stream: a behavioural study

A review of the impairments, preserved visual functions, and neuropathology in 21 patients with visual form agnosia - A unique defect with line drawings

We present a comprehensive review of the rare syndrome visual form agnosia (VFA). We begin by documenting its history, including the origins of the term, and the first case study labelled as VFA. The defining characteristics of the syndrome, as others have previously defined it, are then described. The impairments, preserved aspects of visual perception, and areas of brain damage in 21 patients who meet these defining characteristics are described in detail, including which tests were used to verify the presence or absence of key symptoms. From this, we note important similarities along with notable areas of divergence between patients. Damage to the occipital lobe (20/21), an inability to recognise line drawings (19/21), preserved colour vision (14/21), and visual field defects (16/21) were areas of consistency across most cases. We found it useful to distinguish between shape and form as distinct constructs when examining perceptual abilities in VFA patients. Our observations suggest that these patients often exhibit difficulties in processing simplified versions of form. Deficits in processing orientation and size were uncommon. Motion perception and visual imagery were not widely tested for despite being typically cited as defining features of the syndrome - although in the sample described, motion perception was never found to be a deficit. Moreover, problems with vision (e.g., poor visual acuity and the presence of hemianopias/scotomas in the visual fields) are more common than we would have thought and may also contribute to perceptual impairments in patients with VFA. We conclude that VFA is a perceptual disorder where the visual system has a reduced ability to synthesise lines together for the purposes of making sense of what images represent holistically.

Sex related biases for attending to object color versus object position are reflected in reaction time and accuracy

Processing of visual features related to objects and space relations occurs within separate cortical streams that interact with selective attention. Such separation has implications for cognitive development because the perception of ‘what’ and ‘where’ provide a neural foundation for the development of aspects of higher cognition. Thus, a small attentional bias in early development for attending to one aspect over the other might influence subsequent higher cognitive processing in tasks involving object recognition and space relations. We examined 134 men and women for evidence of an inherent sex-related bias for attending to basic perceptual features related to object discrimination versus object position. Each stimulus consisted of a circle located in one of 9 positions within a surrounding frame. Circles were one of three shades of blue or red. These stimuli were used in a match-to-sample paradigm where participants were required to match circles on the basis of color or spatial position. The first stimulus appeared in the center of the screen for 400 msec and the matching stimulus subsequently appeared for 400 msec oriented 5 degrees to the right or left of center. The same stimuli were used to test the perception of color and position, with order of testing counterbalanced across participants. Results showed significantly longer reaction times in females compared with males, with better accuracy to discriminate color when that color was tested before position. Males showed better accuracy when object position was tested before color discrimination. A second experiment employed the same procedure, but enhanced selective attention by adding an endogenous cue that predicted the right or left location for the appearance of the matching stimulus. This manipulation greatly attenuated the sex differences in reaction time and accuracy compared to Experiment 1, suggesting that the sex-related attentional biases are strongly coupled to bottom-up processing. Overall, the sex related attentional biases toward processing object characteristics versus object position location suggest a differential manifestation of biased competition between the weighted systems of dorsal and ventral stream processing. Results are discussed with how a developmental bias in the processing objects versus space relations may contribute to adult cognitive sex differences in humans and animals.

The dorsal "action" pathway

In 1992, Goodale and Milner proposed a division of labor in the visual pathways of the primate cerebral cortex. According to their account, the ventral pathway, which projects to occipitotemporal cortex, constructs our visual percepts, while the dorsal pathway, which projects to posterior parietal cortex, mediates the visual control of action. Although the framing of the two-visual-system hypothesis has not been without controversy, it is clear that vision for action and vision for perception have distinct computational requirements, and significant support for the proposed neuroanatomic division has continued to emerge over the last two decades from human neuropsychology, neuroimaging, behavioral psychophysics, and monkey neurophysiology. In this chapter, we review much of this evidence, with a particular focus on recent findings from human neuroimaging and monkey neurophysiology, demonstrating a specialized role for parietal cortex in visually guided behavior. But even though the available evidence suggests that dedicated circuits mediate action and perception, in order to produce adaptive goal-directed behavior there must be a close coupling and seamless integration of information processing across these two systems. We discuss such ventral-dorsal-stream interactions and argue that the two pathways play different, yet complementary, roles in the production of skilled behavior.

Functional outcomes following lesions in visual cortex: Implications for plasticity of high-level vision

Understanding the nature and extent of neural plasticity in humans remains a key challenge for neuroscience. Importantly, however, a precise characterization of plasticity and its underlying mechanism has the potential to enable new approaches for enhancing reorganization of cortical function. Investigations of the impairment and subsequent recovery of cognitive and perceptual functions following early-onset cortical lesions in humans provide a unique opportunity to elucidate how the brain changes, adapts, and reorganizes. Specifically, here, we focus on restitution of visual function, and we review the findings on plasticity and re-organization of the ventral occipital temporal cortex (VOTC) in published reports of 46 patients with a lesion to or resection of the visual cortex early in life. Findings reveal that a lesion to the VOTC results in a deficit that affects the visual recognition of more than one category of stimuli (faces, objects and words). In addition, the majority of pediatric patients show limited recovery over time, especially those in whom deficits in low-level vision also persist. Last, given that neither the equipotentiality nor the modularity view on plasticity was clearly supported, we suggest some intermediate possibilities in which some plasticity may be evident but that this might depend on the area that was affected, its maturational trajectory as well as its structural and functional connectivity constraints. Finally, we offer suggestions for future research that can elucidate plasticity further.

How do the two visual streams interact with each other?

The current consensus divides primate cortical visual processing into two broad networks or "streams" composed of highly interconnected areas (Milner and Goodale 2006, 2008; Goodale 2014). The ventral stream, passing from primary visual cortex (V1) through to inferior parts of the temporal lobe, is considered to mediate the transformation of the contents of the visual signal into the mental furniture that guides memory, recognition and conscious perception. In contrast the dorsal stream, passing from V1 through to various areas in the posterior parietal lobe, is generally considered to mediate the visual guidance of action, primarily in real time. The brain, however, does not work through mutually insulated subsystems, and indeed there are well-documented interconnections between the two streams. Evidence for contributions from ventral stream systems to the dorsal stream comes from human neuropsychological and neuroimaging research, and indicates a crucial role in mediating complex and flexible visuomotor skills. Complementary evidence points to a role for posterior dorsal-stream visual analysis in certain aspects of 3-D perceptual function in the ventral stream. A series of studies of a patient with visual form agnosia has been instrumental in shaping our knowledge of what each stream can achieve in isolation; but it has also helped us to tease apart the relative dependence of parietal visuomotor systems on direct bottom-up visual inputs versus inputs redirected via perceptual systems within the ventral stream.

Making memories: the development of long-term visual knowledge in children with visual agnosia

There are few reports about the effects of perinatal acquired brain lesions on the development of visual perception. These studies demonstrate nonseverely impaired visual-spatial abilities and preserved visual memory. Longitudinal data analyzing the effects of compromised perceptions on long-term visual knowledge in agnosics are limited to lesions having occurred in adulthood. The study of children with focal lesions of the visual pathways provides a unique opportunity to assess the development of visual memory when perceptual input is degraded. We assessed visual recognition and visual memory in three children with lesions to the visual cortex having occurred in early infancy. We then explored the time course of visual memory impairment in two of them at 2 years and 3.7 years from the initial assessment. All children exhibited apperceptive visual agnosia and visual memory impairment. We observed a longitudinal improvement of visual memory modulated by the structural properties of objects. Our findings indicate that processing of degraded perceptions from birth results in impoverished memories. The dynamic interaction between perception and memory during development might modulate the long-term construction of visual representations, resulting in less severe impairment.

Acquired prosopagnosia with spared within-class object recognition but impaired recognition of degraded basic-level objects

We present a new case of acquired prosopagnosia resulting from extensive lesions predominantly in the right occipitotemporal cortex. Functional brain imaging revealed atypical activation of all core face areas in the right hemisphere, with reduced signal difference between faces and objects compared to controls. In contrast, Herschel's lateral occipital complex showed normal activation to objects. Behaviourally, Herschel is severely impaired with the recognition of familiar faces, discrimination between unfamiliar identities, and the perception of facial expression and gender. Notably, his visual recognition deficits are largely restricted to faces, suggesting that the damaged mechanisms are face-specific. He showed normal recognition memory for a wide variety of object classes in several paradigms, normal ability to discriminate between highly similar items within a novel object category, and intact ability to name basic objects (except four-legged animals). Furthermore, Herschel displayed a normal face composite effect and typical global advantage and global interference effects in the Navon task, suggesting spared integration of both face and nonface information. Nevertheless, he failed visual closure tests requiring recognition of basic objects from degraded images. This abnormality in basic object recognition is at odds with his spared within-class recognition and presents a challenge to hierarchical models of object perception.

Size-weight illusion and anticipatory grip force scaling following unilateral cortical brain lesion

The prediction of object weight from its size is an important prerequisite of skillful object manipulation. Grip and load forces anticipate object size during early phases of lifting an object. A mismatch between predicted and actual weight when two different sized objects have the same weight results in the size-weight illusion (SWI), the small object feeling heavier. This study explores whether lateralized brain lesions in patients with or without apraxia alter the size-weight illusion and impair anticipatory finger force scaling. Twenty patients with left brain damage (LBD, 10 with apraxia, 10 without apraxia), ten patients with right brain damage (RBD), and matched control subjects lifted two different-sized boxes in alternation. All subjects experienced a similar size-weight illusion. The anticipatory force scaling of all groups was in correspondence with the size cue: higher forces and force rates were applied to the big box and lower forces and force rates to the small box during the first lifts. Within few lifts, forces were scaled to actual object weight. Despite the lack of significant differences at group level, 5 out of 20 LBD patients showed abnormal predictive scaling of grip forces. They differed from the LBD patients with normal predictive scaling by a greater incidence of posterior occipito-parietal lesions but not by a greater incidence of apraxia. The findings do not support a more general role for the motor-dominant left hemisphere, or an influence of apraxia per se, in the scaling of finger force according to object properties. However, damage in the vicinity of the parietal-occipital junction may be critical for deriving predictions of weight from size.

Development of a vocabulary of object shapes in a child with a very-early-acquired visual agnosia: A unique case

We report a longitudinal study of an exceptional child (S.R.) whose early-acquired visual agnosia, following encephalitis at 8 weeks of age, did not prevent her from learning to construct an increasing vocabulary of visual object forms (drawn from different categories), albeit slowly. S.R. had problems perceiving subtle differences in shape; she was unable to segment local letters within global displays; and she would bring complex scenes close to her eyes: a symptom suggestive of an attempt to reduce visual crowding. Investigations revealed a robust ability to use the gestalt grouping factors of proximity and collinearity to detect fragmented forms in noisy backgrounds, compared with a very weak ability to segment fragmented forms on the basis of contrasts of shape. When contrasts in spatial grouping and shape were pitted against each other, shape made little contribution, consistent with problems in perceiving complex scenes, but when shape contrast was varied, and spatial grouping was held constant, S.R. showed the same hierarchy of difficulty as the controls, although her responses were slowed. This is the first report of a child's visual–perceptual development following very early neurological impairments to the visual cortex. Her ability to learn to perceive visual shape following damage at a rudimentary stage of perceptual development contrasts starkly with the loss of such ability in childhood cases of acquired visual agnosia that follow damage to the established perceptual system. Clearly, there is a critical period during which neurological damage to the highly active, early developing visual–perceptual system does not prevent but only impairs further learning.

Cerebral Visual Impairment: which perceptive visual dysfunctions can be expected in children with brain damage? A systematic review

The current definition of Cerebral Visual Impairment (CVI) includes all visual dysfunctions caused by damage to, or malfunctioning of, the retrochiasmatic visual pathways in the absence of damage to the anterior visual pathways or any major ocular disease. CVI is diagnosed by exclusion and the existence of many different causes and symptoms make it an overall non-categorized group. To date, no discrimination is made within CVI based on types of perceptive visual dysfunctions. The aim of this review was to outline which perceptive visual dysfunctions are to be expected based on a number of etiologies of brain damage and brain development disorders with their onset in the pre-, peri- or postnatal period. For each period two etiologies were chosen as the main characteristic brain damage. For each etiology a main search was performed. The selection of the articles was based on the following criteria: age, etiology, imaging, central pathology and perceptive visual function test. The perceptive visual functions included for this review were object recognition, face recognition, visual memory, orientation, visual spatial perception, motion perception and simultaneous perception. Our search resulted in 11 key articles. A diversity of research history is performed for the selected etiologies and their relation to perceptive visual dysfunctions. Periventricular Leukomalacia (PVL) was most studied, whereas the main tested perceptive visual function was visual spatial perception. As a conclusion, the present status of research in the field of CVI does not allow to correlate between etiology, location and perceptive visual dysfunctions in children with brain damage or a brain development disorder. A limiting factor could be the small number of objective tests performed in children experiencing problems in visual processing. Based on recent insights in central visual information processing, we recommend an alternative approach for the definition of CVI that is based on functional visual processing, rather than anatomical landmarks. This could be of benefit in daily practice to diagnose CVI.

Holistic perception of the individual face is specific and necessary: evidence from an extensive case study of acquired prosopagnosia

We present an extensive investigation (24 experiments) of a new case of prosopagnosia following right unilateral damage, GG, with the aim of addressing two classical issues: (1) Can a visual recognition impairment truly be specific to faces? (2) What is the nature of acquired prosopagnosia? We show that GG recognizes nonface objects perfectly and quickly, even when it requires fine-grained analysis to individualize these objects. He is also capable of perceiving objects and faces as integrated wholes, as indicated by normal Navon effect, 3D-figures perception and perception of Mooney and Arcimboldo face stimuli. However, the patient could not perceive individual faces holistically, showing no inversion, composite, or whole-part advantage effects for faces. We conclude that an occipito-temporal right hemisphere lesion may lead to a specific impairment of holistic perception of individual items, a function that appears critical for normal face recognition but not for object recognition.

Superior voice recognition in a patient with acquired prosopagnosia and object agnosia

Anecdotally, it has been reported that individuals with acquired prosopagnosia compensate for their inability to recognize faces by using other person identity cues such as hair, gait or the voice. Are they therefore superior at the use of non-face cues, specifically voices, to person identity? Here, we empirically measure person and object identity recognition in a patient with acquired prosopagnosia and object agnosia. We quantify person identity (face and voice) and object identity (car and horn) recognition for visual, auditory, and bimodal (visual and auditory) stimuli. The patient is unable to recognize faces or cars, consistent with his prosopagnosia and object agnosia, respectively. He is perfectly able to recognize people's voices and car horns and bimodal stimuli. These data show a reverse shift in the typical weighting of visual over auditory information for audiovisual stimuli in a compromised visual recognition system. Moreover, the patient shows selectively superior voice recognition compared to the controls revealing that two different stimulus domains, persons and objects, may not be equally affected by sensory adaptation effects. This also implies that person and object identity recognition are processed in separate pathways. These data demonstrate that an individual with acquired prosopagnosia and object agnosia can compensate for the visual impairment and become quite skilled at using spared aspects of sensory processing. In the case of acquired prosopagnosia it is advantageous to develop a superior use of voices for person identity recognition in everyday life.

How cortical neurons help us see: visual recognition in the human brain

Through a series of complex transformations, the pixel-like input to the retina is converted into rich visual perceptions that constitute an integral part of visual recognition. Multiple visual problems arise due to damage or developmental abnormalities in the cortex of the brain. Here, we provide an overview of how visual information is processed along the ventral visual cortex in the human brain. We discuss how neurophysiological recordings in macaque monkeys and in humans can help us understand the computations performed by visual cortex.

Preserved Striate Cortex is Not Sufficient to Support the McCollough Effect: Evidence from two Patients with Cerebral Achromatopsia

The McCollough effect (ME) is a colour aftereffect contingent on pattern orientation. This effect is generally thought to be mediated by primary visual cortex (V1) although this has remained the subject of some debate. To determine whether V1 is in fact sufficient to subserve the ME, we compared McCollough adaptation in controls to adaptation in two patients with damage to ventrotemporal cortex, resulting in achromatopsia, but who have spared V1. Each of these patients has some residual colour abilities of which he is unaware. Participants performed a 2AFC orientation-discrimination task for pairs of oblique and vertical/horizontal gratings both before and after adaptation to red/green oblique induction gratings. Successful ME induction would manifest itself as an improvement in oblique-orientation discrimination owing to the additional colour cue after adaptation. Indeed, in controls oblique grating discrimination improved post-adaptation. Further, a subdivision of our control group demonstrated successful ME induction despite a lack of conscious awareness of the added colour cue, indicating that conscious colour awareness is not required for ME induction. The patients, however, did not show improvement in oblique-orientation discrimination, indicating a lack of ME induction. This suggests that V1 must be connected to higher cortical colour areas to drive ME induction.

Seeing the world dimly: the impact of early visual deficits on visual experience in schizophrenia

Deficits in early visual processing are well documented in schizophrenia, using methods such as contrast sensitivity. Higher, integrative stages of functioning, such as susceptibility to visual illusions, have been evaluated less extensively. For example, patients show increased susceptibility to (ie, are more easily affected by) the Muller-Lyer illusion but decreased susceptibility (ie, are less easily affected by) to stereopsis based upon binocular disparity. The basis for pattern of illusion response and interaction between sensory and integrative stages of processing, however, is unclear. We tested a group of 38 patients and 28 control subjects in contrast sensitivity, the Muller-Lyer and Poggendorff illusions, as well as a subgroup in stereopsis and the Ponzo illusion, Sander parallelogram, and Hermann grid illusions. We predicted that patients would be more susceptible to tests that become more apparent with increased contrast (Muller-Lyer illusion), less susceptible to tests that become less apparent with increased contrast (stereopsis, Ponzo illusion, Hermann grid), and equally susceptible to contrast-insensitive tests (Poggendorff illusion). Additionally, the Hermann grid was tested at varying levels of contrast. Patients demonstrated significant deficits in contrast sensitivity, especially to brief, low spatial frequency stimuli, and the predicted differential response to the tested illusions. Additionally, poor performance on stereopsis and the Hermann grid significantly correlated with decreased contrast sensitivity (all P's <.01). Muller-Lyer illusion and stereopsis performance were also inversely related (P < .01). This study replicates and expands upon previous findings with visual illusions. Our results offer a unifying explanation for disparate studies and suggest that deficits in early sensory gain affect subsequent integrative processes.

Visuoperceptive impairment in adult patients with occipital lobe epilepsies

The aim of the study was to verify whether adult patients with occipital lobe epilepsy (OLE) are at risk for cognitive impairment compared with controls. Twenty patients with OLE and 20 controls, matched as closely as possible to the epilepsy group in terms of gender, age, education, and intelligence, were administered a battery of tests based on visuoperceptive domains. None of the participants was affected by progressive pathologies, received any medication other than antiepileptic drugs, nor had a psychiatric background. We noted a statistically significant difference between patients with OLE and controls in the following tasks: Perceptive Differences Test, and Object Denomination Test, and Famous Faces Test (P<0.05). No difference was noted between symptomatic and nonsymptomatic patients with respect to neuropsychological results (P>0.05). The linear regression analysis performed did not show any statistically significant contribution by clinical variables. Our data confirm that patients with OLE manifest subtle difficulties in processing and mental manipulation of visual spatial data.

FMRI reveals a dissociation between grasping and perceiving the size of real 3D objects

BACKGROUND

Almost 15 years after its formulation, evidence for the neuro-functional dissociation between a dorsal action stream and a ventral perception stream in the human cerebral cortex is still based largely on neuropsychological case studies. To date, there is no unequivocal evidence for separate visual computations of object features for performance of goal-directed actions versus perceptual tasks in the neurologically intact human brain. We used functional magnetic resonance imaging to test explicitly whether or not brain areas mediating size computation for grasping are distinct from those mediating size computation for perception.

METHODOLOGY/PRINCIPAL FINDINGS

Subjects were presented with the same real graspable 3D objects and were required to perform a number of different tasks: grasping, reaching, size discrimination, pattern discrimination or passive viewing. As in prior studies, the anterior intraparietal area (AIP) in the dorsal stream was more active during grasping, when object size was relevant for planning the grasp, than during reaching, when object properties were irrelevant for movement planning (grasping>reaching). Activity in AIP showed no modulation, however, when size was computed in the context of a purely perceptual task (size = pattern discrimination). Conversely, the lateral occipital (LO) cortex in the ventral stream was modulated when size was computed for perception (size>pattern discrimination) but not for action (grasping = reaching).

CONCLUSIONS/SIGNIFICANCE

While areas in both the dorsal and ventral streams responded to the simple presentation of 3D objects (passive viewing), these areas were differentially activated depending on whether the task was grasping or perceptual discrimination, respectively. The demonstration of dual coding of an object for the purposes of action on the one hand and perception on the other in the same healthy brains offers a substantial contribution to the current debate about the nature of the neural coding that takes place in the dorsal and ventral streams.

Neuropsychological Findings in Idiopathic Occipital Lobe Epilepsies

PURPOSE

We reviewed the clinical charts of 22 patients (mean age 12 years) with idiopathic occipital lobe epilepsies (IOLE) to verify the presence of visuoperceptual difficulties.

METHODS

All 22 patients underwent a standard neuropsychiatric examination and had a sleep electroencephalogram (EEG). Eleven had normal development and adequate scholastic achievements, so no formal testing was performed. Psychological assessment was carried out in the remaining 11 patients who had been referred because of learning and behavioral difficulties.

RESULTS AND CONCLUSIONS

IQ was in the average/low-average range in most patients, with a cognitive profile characterized by relatively better verbal than performance abilities (8/11). There was a high incidence of scholastic disabilities (7/11); psychiatric disorders in the form of anxiety and depressive disorders were also frequent (6/11). In the six patients who had neuropsychological assessment with specific visuoperceptual testing, a deficit in facial discrimination was found in four patients, associated with a line orientation deficit in three. Although preliminary, and based on a tertiary-care clinical sample, these data suggest that children with IOLE, are at risk for lower intellectual performance, poor scholastic achievement and psychiatric disorders, as well as for specific deficits in the visuoperceptual domain, probably due to dysfunction of the occipitotemporal circuitries most often involved in seizure spread in epilepsies originating in the posterior brain.

Visual recognition and visually guided action after early bilateral lesion of occipital cortex: a behavioral study of a 4.6-year-old girl

We report the case of a 4.6-year-old girl born pre-term with early bilateral occipital damage. It was revealed that the child had non-severely impaired basic visual abilities and ocular motility, a selective perceptual deficit of figure-ground segregation, impaired visual recognition and abnormal navigating through space. Even if the child's visual functioning was not optimal, this was the expression of adaptive anatomic and functional brain modifications that occurred following the early lesion. Anatomic brain structure was studied with anatomic MRI and Diffusor Tensor Imaging (DTI)-MRI. This behavioral study may provide an important contribution to understanding the impact of an early lesion of the visual system on the development of visual functions and on the immature brain's potential for reorganisation related to when the damage occurred.

No double-dissociation between optic ataxia and visual agnosia: multiple sub-streams for multiple visuo-manual integrations

The current dominant view of the visual system is marked by the functional and anatomical dissociation between a ventral stream specialised for perception and a dorsal stream specialised for action. The "double-dissociation" between visual agnosia (VA), a deficit of visual recognition, and optic ataxia (OA), a deficit of visuo-manual guidance, considered as consecutive to ventral and dorsal damage, respectively, has provided the main argument for this dichotomic view. In the first part of this paper, we show that the currently available empirical data do not suffice to support a double-dissociation between OA and VA. In the second part, we review evidence coming from human neuropsychology and monkey data, which cast further doubts on the validity of a simple double-dissociation between perception and action because they argue for a far more complex organisation with multiple parallel visual-to-motor connections: 1. A dorso-dorsal pathway (involving the most dorsal part of the parietal and pre-motor cortices): for immediate visuo-motor control--with OA as typical disturbance. The latest research about OA is reviewed, showing how these patients exhibit deficits restricted to the most direct and fast visuo-motor transformations. We also propose that mild mirror ataxia, consisting of misreaching errors when the controlesional hand is guided to a visual goal though a mirror, could correspond to OA with an isolated "hand effect". 2. A ventral stream-prefrontal pathway (connections from the ventral visual stream to pre-frontal areas, by-passing the parietal areas): for "mediate" control (involving spatial or temporal transpositions [Rossetti, Y., & Pisella, L. (2003). Mediate responses as direct evidence for intention: Neuropsychology of Not to-, Not now- and Not there-tasks. In S. Johnson (Ed.), Cognitive Neuroscience perspectives on the problem of intentional action (pp. 67-105). MIT Press.])--with VA as typical disturbance. Preserved visuo-manual guidance in patients with VA is restricted to immediate goal-directed guidance, they exhibit deficits for delayed or pantomimed actions. 3. A ventro-dorsal pathway (involving the more ventral part of the parietal lobe and the pre-motor and pre-frontal areas): for complex planning and programming relying on high representational levels with a more bilateral organisation or an hemispheric lateralisation--with mirror apraxia, limb apraxia and spatial neglect as representatives. Mirror apraxia is a deficit that affects both hands after unilateral inferior parietal lesion with the patients reaching systematically and repeatedly toward the virtual image in the mirror. Limb apraxia is localized on a more advanced conceptual level of object-related actions and results from deficient integrative, computational and "working memory" capacities of the left inferior parietal lobule. A component of spatial working memory has recently been revealed also in spatial neglect consecutive to lesion involving the network of the right inferior parietal lobule and the right frontal areas. We conclude by pointing to the differential temporal constraints and integrative capabilities of these parallel visuo-motor pathways as keys to interpret the neuropsychological deficits.

Intact automatic avoidance of obstacles in patients with visual form agnosia

In everyday life our reaching behaviour has to be guided not only by the location and properties of the target object, but also by the presence of potential obstacles in the workspace. Recent evidence from neglect and optic ataxia patients has suggested that this automatic obstacle avoidance is mediated by the dorsal, rather than the ventral, stream of visual processing. We tested this idea in two studies involving patients with visual form agnosia resulting from bilateral ventral-stream damage. In the first study, we asked patient DF to reach out and pick up a target object in the presence of obstacles placed at varying distances to the left or right of the target. We found that both DF and controls shifted their trajectories away from the potential obstacles and adjusted their grip aperture in such a way as to minimize risk of collision. In a second study, we asked DF and a second patient, SB, to either reach between, or to bisect the space between, two cylinders presented at varying locations. We found that both patients adjusted their reach trajectories to account for shifts in cylinder location in the reaching task, despite showing significantly worse performance than control subjects when asked to make a bisection judgement. Taken together, these data indicate that automatic obstacle avoidance behaviour is spared in our patients with visual form agnosia. We attribute their ability to the functional intactness of the dorsal stream of visual processing, and argue that the ventral stream plays no important role in automatic obstacle avoidance.

Follow-up of neuropsychological function recovery in a 9-year-old girl with anoxic encephalopathy: a window on the brain re-organization processes

OBJECTIVE

To investigate comprehensive neuropsychological outcome, disabilities in daily life and individual recovery processes in a case of anoxic encephalopathy.

DESIGN

A 9-year-old child's functional outcome after anoxic coma was evaluated in a follow-up study with assessments at 5, 9 and 12 months post-injury. A comprehensive neuropsychological protocol was administered. Qualitative methods of analysis and ecological observation were associated with standard and non-standard quantitative measures.

RESULTS

The child presented pervasive functional deficits with prevalence of gnosic, praxic and self-regulatory dysfunction. Dissociated functional recovery was documented in 12 months time. Improvement of self-regulatory abilities was likely a 'propeller' of global system re-organization.

CONCLUSION

A descriptive longitudinal study of functional and ecological behavioural changes after anoxic coma provides insight into the re-adaptation processes in the brain connected to post-lesion ecological and training experiences. Contextual factors and their relations to functional improvements deserve further study.

Spatial representations and multiple-visual-systems hypotheses: evidence from a developmental deficit in visual location and orientation processing

AH, a young, well-educated woman, has a developmental deficit in processing visual location and orientation information. Her deficit manifests itself in a wide range of visual tasks, including visually-guided reaching, copying pictures and words, and responding verbally to the location or orientation of visual stimuli; however, her performance in non-visual localization tasks is intact. AH's visual location and orientation errors are systematic left-right or up-down reflections (e.g., reaching to the far right for an object on the far left). More specifically, the errors involve reflection across the point where AH's attention is focused, regardless of where her eyes are fixated. These results imply that at some level(s) of the visual system, locations and orientations of visual stimuli are represented in a spatial coordinate system with an origin defined by the focus of attention. In these attention-centered representations location is specified in terms of distance and direction of displacement from the attentional focus along horizontal and vertical reference axes. AH's errors, I argue, result from misrepresentation of displacement direction (e.g., left rather than right, down rather than up) along a reference axis. Several visual variables dramatically affected AH's performance in visual location and orientation tasks: She was much more accurate for stimuli that were brief, moving, flickering, low in contrast, or high in eccentricity, than for those that were long in duration, stationary, continuous, high in contrast, and low in eccentricity. These results suggest that location and orientation are computed in each of two visual subsystems, which I call transient and sustained, and that AH's deficit affects only the sustained subsystem. I argue that AH's performance poses challenges to multiple-visual-subsystems hypotheses proposed by Ungerleider and Mishkin (1982) and by Milner and Goodale (1995).

Visually guided reaching: bilateral posterior parietal lesions cause a switch from fast visuomotor to slow cognitive control

The visually guided reaching of two patients with bilateral optic ataxia was explored in two experiments. In Experiment 1 simple delayed pointing was compared with immediate pointing. In the immediate pointing task both variable and constant errors increased with target eccentricity. In contrast to the performance of control subjects and contrary to their own beliefs, the patients both showed improved accuracy in the delay condition. This improvement was manifest as a reduction in both pointing variability and in the constant angular error towards the point of fixation. Both angular errors and their improvement with the delay were proportional to target eccentricity. Experiment 2 used a task in which the target was pre-viewed 5s prior to its re-exposure for pointing ('delayed real pointing'). On some trials a conflict was introduced between the present and previous visual information by changing the target's location during the delay. In contrast to control subjects, who ignored the pre-viewed location and aimed directly at the current target, both patients with optic ataxia initiated their movements towards the previously viewed target location. Evidently they relied on off-line information in preference to on-line visual information. In addition, the patients often failed to detect the changes in target location. One of the patients sometimes even guessed incorrectly that the target had changed its location, and her movement trajectory was then more affected by her false belief than by the target's actual location. These findings confirm that posterior parietal lesions severely disrupt direct visuomotor transformations, and suggest that the residual performance is mediated indirectly by expectations or beliefs about target position.

Dual routes to action: contributions of the dorsal and ventral streams to adaptive behavior

More than a decade ago, Goodale and Milner proposed that our perceptual experience of the world depends on visual processing that is fundamentally distinct from that mediating the moment-to-moment visual control of our actions. They mapped this distinction between vision-for-perception and vision-for-action onto the two prominent visual pathways that arise from early visual areas in the primate cerebral cortex: a ventral "perception" pathway projecting to inferotemporal cortex and a dorsal "action" pathway projecting to posterior parietal cortex. In the years since these ideas were first put forward, visual neuroscience has advanced rapidly on several fronts. In this chapter, we examine the perception-action distinction in the light of some of these developments, giving particular emphasis to the differences in the way the two streams process visual information and the way they interact in the production of adaptive behavior.

Visuomotor performance in a patient with visual agnosia due to an early lesion

We tested a patient with visual agnosia who had suffered severe bilateral brain damage early in life, on a series of visuomotor tasks. The broad pattern of results confirms that S.B., like the extensively tested patient D.F., shows an impressive array of preserved skills, despite his severe perceptual problems. Also like D.F., S.B. shows certain subtle visuomotor difficulties that can be related to the idea that his partially intact occipito-parietal areas are unable to benefit from interactions with the apparently severely damaged occipito-temporal regions. Unlike D.F., however, he is able to make accurate discriminations of simple visual features, such as object width and orientation, albeit with very slow response times. We hypothesize that several factors such as the early onset of S.B.'s lesion and the long period since his brain lesion have allowed his brain to compensate to a degree what has been impossible in D.F., whose brain damage occurred in adulthood. This may include an element of 'rewiring' and self-monitoring of visuomotor processes that allow S.B. to achieve perceptual access to visual information processed in the dorsal stream: information that is normally only available for on-line visuomotor control.

An evolving view of duplex vision: separate but interacting cortical pathways for perception and action

In 1992, Goodale and Milner proposed a division of labour in the visual pathways of the primate cerebral cortex between a dorsal stream specialised for the visual control of action and a ventral stream dedicated to the perception of the visual world. In the years since this original proposal, support for the perception-action hypothesis has come from neuroimaging experiments, human neuropsychology, monkey neurophysiology, and human psychophysical experiments. Indeed, some of the strongest support for this hypothesis has come from behavioural experiments showing that visually guided actions are largely refractory to perceptual illusions. Although controversial, the findings from this literature both support the original hypothesis and suggest important modifications. The ongoing challenge for neurobiologists is to map these behavioural findings onto their corresponding neural substrates.

A higher order motion region in human inferior parietal lobule: evidence from fMRI

The proposal that motion is processed by multiple mechanisms in the human brain has received little anatomical support so far. Here, we compared higher- and lower-level motion processing in the human brain using functional magnetic resonance imaging. We observed activation of an inferior parietal lobule (IPL) motion region by isoluminant red-green gratings when saliency of one color was increased and by long-range apparent motion at 7 Hz but not 2 Hz. This higher order motion region represents the entire visual field, while traditional motion regions predominantly process contralateral motion. Our results suggest that there are two motion-processing systems in the human brain: a contralateral lower-level luminance-based system, extending from hMT/V5+ into dorsal IPS and STS, and a bilateral higher-level saliency-based system in IPL.

Optic ataxia revisited: visually guided action versus immediate visuomotor control

Optic ataxia and visual agnosia have been proposed to constitute a double dissociation which provides the main argument for the assimilation of the anatomical distinction between a dorsal and a ventral visual stream to the functional distinction between perception and action. In the present review, we argue that insufficient evidence has been collected to argue for this double dissociation. Several criteria are reviewed: (1) exploration of the visuomotor behavior in central versus peripheral vision has not been matched for the two types of patients; (2) the temporal constraints of visual processes that are impaired in the two neurological conditions appear to play a crucial role in the apparent dissociation; (3) the necessary reductionism of experimental conditions used to study action has led to an overconsideration of optic ataxia as a global deficit for action. Altogether optic ataxia appears to result from a specific impairment of immediate visuomotor control rather than of visually guided action as a whole. These results are discussed in the light of recent research on optic ataxia and on motor control, and directions for future research are proposed.

The effect of inversion on the encoding of normal and "thatcherized" faces

In the "Thatcher illusion" a face, in which the eyes and mouth are inverted relative to the rest of the face, looks grotesque when shown upright but not when inverted. In four experiments we investigated the contribution of local and global processing to this illusion in normal observers. We examined inversion effects (i.e., better performance for upright than for inverted faces) in a task requiring discrimination of whether faces were or were not "thatcherized". Observers made same/different judgements to isolated face parts (Experiments 1-2) and to whole faces (Experiments 3-4). Face pairs had the same or different identity, allowing for different processing strategies using feature-based or configural information, respectively. In Experiment 1, feature-based matching of same-person face parts yielded only a small inversion effect for normal face parts. However, when feature-based matching was prevented by using the face parts of different people on all trials (Experiment 2) an inversion effect occurred for normal but not for thatcherized parts. In Experiments 3 and 4, inversion effects occurred with normal but not with thatcherized whole faces, on both same- and different-person matching tasks. This suggests that a common configural strategy was used with whole (normal) faces. Face context facilitated attention to misoriented parts in same-person but not in different-person matching. The results indicate that (1) face inversion disrupts local configural processing, but not the processing of image features, and (2) thatcherization disrupts local configural processing in upright faces.

Processing of normal, inverted, and scrambled faces in a patient with prosopagnosia: behavioural and eye tracking data

In this study, we addressed the issue of a spared processing of faces in a patient (SB) with severe prosopagnosia. We designed an experiment comprising of two parts. In Part I, normal upright faces were entwined with scrambled faces, while in Part II normal upright faces were mixed with inverted faces, under unlimited time exposure. Performance, decision times, and eye movements were measured in both parts. The results indicated that SB categorised the normal faces better in the context of inverted faces than in the context of scrambled faces. Furthermore, SB's performance was better for the inverted faces than for the scrambled faces. Overall, SB performed better on the abnormal faces than on the normal faces, as did the control participants. Eye-tracking data showed that the pattern observed for the number of fixations and for exploration order was similar in SB and in controls. In the discussion, we propose that, despite his severe prosopagnosia, SB might have retained some kind of processing specific to face perception. Further investigations will be required, using limited time exposure, to determine the nature of this spared processing.

Implicit face perception in a patient with visual agnosia? Evidence from behavioural and eye-tracking analyses

This paper investigates face perception in a visual agnosic and prosopagnosic patient (SB). Despite very extensive lesions of visual areas, SB remains capable of some visual processing [Brain 125 (2002) 58]. However, in everyday situations SB does not exhibit signs of specific face recognition. To investigate how SB may process faces, we tested two hypotheses. According to the 'spared module hypothesis,' SBs abilities come from spared modules of implicit face processing. According to the 'general strategy hypothesis,' SB may have developed some deliberate compensatory strategies. A two-session experimental design was constructed. In both sessions, face and non-face pictures were shown to participants. In Session 1 (implicit condition), participants had to decide whether each picture was a vegetable. In Session 2 (explicit condition), participants had to decide whether each picture was a face. Verbal reports showed that SB was not aware of faces in Session 1. However, behavioural results showed that (1). SB could process faces; (2). even when SB was not aware of faces, he processed them differently than non-faces; (3). when knowing the presence of faces, he did not process faces better. In addition, eye-tracking data suggested that SB did not change the nature of his processing from Sessions 1 to 2. Pupil diameters showed that fixated facial features were processed similarly as in control participants. Together, these results are not compatible with a general compensatory strategy hypothesis and suggest sparing of an implicit face processing module in SB.

Visual agnosia

As was originally proposed by Lissauer, visual recognition may break down either at an apperceptive or at an associative level. At an apperceptive level, finer grain distinctions may be made; the authors distinguished here between disorders of shape recognition and perceptual integration. It is not the case, however, that all patients with visual recognition deficits have impaired perception: poor recognition and naming may also result from damage to stored perceptual knowledge (e.g., structural descriptions), from problems in accessing semantic knowledge, from perceptual knowledge, or from impairments to semantic knowledge itself. These represent different types of associative deficit. In some cases, mild damage to stored perceptual knowledge also generates problems that are more severe on naming than on recognition and more severe for some categories of objects than others.