Plasticity versus chronicity: Stable performance on category fluency 40 years post-onset.

What is the long-term trajectory of semantic memory deficits in patients who have suffered structural brain damage? Memory is, per definition, a changing faculty. The traditional view is that after an initial recovery period, the mature human brain has little capacity to repair or reorganize. More recently, it has been suggested that the central nervous system may be more plastic with the ability to change in neural structure, connectivity, and function. The latter observations are, however, largely based on normal learning in healthy subjects. Here, we report a patient who suffered bilateral ventro-medial damage after presumed herpes encephalitis in 1971. He was seen regularly in the eighties, and we recently had the opportunity to re-assess his semantic memory deficits. On semantic category fluency, he showed a very clear category-specific deficit performing better that control data on non-living categories and significantly worse on living items. Recent testing showed that his impairments have remained unchanged for more than 40 years. We suggest cautiousness when extrapolating the concept of brain plasticity, as observed during normal learning, to plasticity in the context of structural brain damage.

The contribution of single case studies to the neuroscience of vision.

Visual neuroscience is concerned with the neurobiological foundations of visual perception, that is, the morphological, physiological, and functional organization of the visual brain and its co-operative partners. One important approach for understanding the functional organization of the visual brain is the study of visual perception from the pathological perspective. The study of patients with focal injury to the visual brain allows conclusions about the representation of visual perceptual functions in the framework of association and dissociation of functions. Selective disorders have been reported for more "elementary" visual capabilities, for example, color and movement vision, but also for visuo-cognitive capacities, such as visual agnosia or the visual field of attention. Because these visual disorders occur rather seldom as selective and specific dysfunctions, single cases have always played, and still play, a significant role in gaining insights into the functional organization of the visual brain.

Direct encoding of orientation variance in the visual system.

Our perception of regional irregularity, an example of which is orientation variance, seems effortless when we view two patches of texture that differ in this attribute. Little is understood, however, of how the visual system encodes a regional statistic like orientation variance, but there is some evidence to suggest that it is directly encoded by populations of neurons tuned broadly to high or low levels. The present study shows that selective adaptation to low or high levels of variance results in a perceptual aftereffect that shifts the perceived level of variance of a subsequently viewed texture in the direction away from that of the adapting stimulus (Experiments 1 and 2). Importantly, the effect is durable across changes in mean orientation, suggesting that the encoding of orientation variance is independent of global first moment orientation statistics (i.e., mean orientation). In Experiment 3 it was shown that the variance-specific aftereffect did not show signs of being encoded in a spatiotopic reference frame, similar to the equivalent aftereffect of adaptation to the first moment orientation statistic (the tilt aftereffect), which is represented in the primary visual cortex and exists only in retinotopic coordinates. Experiment 4 shows that a neuropsychological patient with damage to ventral areas of the cortex but spared intact early areas retains sensitivity to orientation variance. Together these results suggest that orientation variance is encoded directly by the visual system and possibly at an early cortical stage.

Exogenous attention to unseen objects?

Attention and awareness are closely related phenomena, but recent evidence has shown that not all attended stimuli give rise to awareness. Controversy still remains over whether, and the extent to which, a dissociation between attention and awareness encompasses all forms of attention. For example, it has been suggested that attention without awareness is more readily demonstrated for voluntary, endogenous attention than its reflexive, exogenous counterpart. Here we examine whether exogenous attentional cueing can have selective behavioural effects on stimuli that nevertheless remain unseen. Using a task in which object-based attention has been shown in the absence of awareness, we remove all possible contingencies between cues and target stimuli to ensure that any cueing effects must be under purely exogenous control, and find evidence of exogenous object-based attention without awareness. In a second experiment we address whether this dissociation crucially depends on the method used to establish that the objects indeed remain unseen. Specifically, to confirm that objects are unseen we adopt appropriate signal detection task procedures, including those that retain parity with the primary attentional task (by requiring participants to discriminate the two types of trial that are used to measure an effect of attention). We show a significant object-based attention effect is apparent under conditions where the selected object indeed remains undetectable.

The contribution of LM to the neuroscience of movement vision.

The significance of early and sporadic reports in the 19th century of impairments of motion vision following brain damage was largely unrecognized. In the absence of satisfactory post-mortem evidence, impairments were interpreted as the consequence of a more general disturbance resulting from brain damage, the location and extent of which was unknown. Moreover, evidence that movement constituted a special visual perception and may be selectively spared was similarly dismissed. Such skepticism derived from a reluctance to acknowledge that the neural substrates of visual perception may not be confined to primary visual cortex. This view did not persist. First, it was realized that visual movement perception does not depend simply on the analysis of spatial displacements and temporal intervals, but represents a specific visual movement sensation. Second persuasive evidence for functional specialization in extrastriate cortex, and notably the discovery of cortical area V5/MT, suggested a separate region specialized for motion processing. Shortly thereafter the remarkable case of patient LM was published, providing compelling evidence for a selective and specific loss of movement vision. The case is reviewed here, along with an assessment of its contribution to visual neuroscience.

Color constancy for an unseen surface.

The illumination of a scene strongly affects our perception of objects in that scene, e.g., the pages of a book illuminated by candlelight will appear quite yellow relative to other types of artificial illuminants. Yet at the same time, the reader still judges the pages as white, their surface color unaffected by the interplay of paper and illuminant. It has been shown empirically that we can indeed report two quite different interpretations of "color": one is dependent on the constant surface spectral reflectance of an object (surface color) and the other on the power of light of different wavelengths reflected from that object (reflected color). How then are these two representations related? The common view, dating from Aristotle, is that our experience of surface color is derived from reflected color or, in more familiar terms, that color perception follows from color sensation. By definition, color constancy requires that vision "discounts the illuminant"; thus, it seems reasonable that vision begins with the color of objects as they naively appear and that we infer from their appearances their surface color. Here, we question this classic view. We use metacontrast-masked priming and, by presenting the unseen prime and the visible mask under different illuminants, dissociate two ways in which the prime matched the mask: in surface color or in reflected color. We find that priming of the mask occurs when it matches the prime in surface color, not reflected color. It follows that color perception can arise without prior color sensation.

Auditory-visual localization in hemianopia.

OBJECTIVE: Beyond visual field defects, patients with hemianopia have been suggested to perceive horizontal visual space in a distorted manner. However, the pattern of these distortions remained debatable. The aim of this study was to estimate the geometry of the visual representation of space in hemianopia using an auditory marker. METHOD: Patients with pure left or right hemianopia (without neglect) were tested in tasks requiring them to bring a visual stimulus into spatial alignment with a target sound (Experiment 1) or vice versa (Experiment 2). RESULTS: In Experiment 1, patients adjusted the location of a light such that it was displaced toward the anopic side with reference to the physical sound position. In Experiment 2, patients adjusted the location of a sound such that it was displaced opposite to the anopic side with reference to the actual position of the visual target. Both experiments consistently indicated that hemianopic patients perceived a sound and a light to be in spatial alignment when the physical position of the light deviated by several degrees from the sound toward the side of the anopic hemifield, that is, to the contralesional side. CONCLUSIONS: Given that auditory localization in patients with hemianopia has been previously shown to be only slightly biased toward the anopic side, the observed distortion of visual space with reference to auditory space can be explained by assuming that visual positions were, in absolute terms, perceived as shifted toward the intact side. As a result, HA patients may perceive visual space as compressed on their ipsilesional (intact), in comparison with their contralesional (anopic) side.

Object-based attention without awareness.

Attention and awareness are often considered to be related. Some forms of attention can, however, facilitate the processing of stimuli that remain unseen. It is unclear whether this dissociation extends beyond selection on the basis of primitive properties, such as spatial location, to situations in which there are more complex bases for attentional selection. The experiment described here shows that attentional selection at the level of objects can take place without giving rise to awareness of those objects. Pairs of objects were continually masked, which rendered them invisible to participants performing a cued-target-discrimination task. When the cue and target appeared within the same object, discrimination was faster than when they appeared in different objects at the same spatial separation. Participants reported no awareness of the objects and were unable to detect them in a signal-detection task. Object-based attention, therefore, is not sufficient for object awareness.

Rehabilitation of reading and visual exploration in visual field disorders: transfer or specificity?

Reading and visual exploration impairments in unilateral homonymous visual field disorders are frequent and disabling consequences of acquired brain injury. Compensatory therapies have been developed, which allow patients to regain sufficient reading and visual exploration performance through systematic oculomotor training. However, it is still unclear whether the reading and visual exploration impairments require specific compensatory training for their improvement. We present the first cross-over rehabilitation study to determine whether the training-related performance improvements are task-specific, or whether there is a transfer of training-related improvements between reading and visual exploration. We compared the therapeutic effects of compensatory oculomotor reading and visual exploration training in 36 patients with unilateral homonymous visual field loss in a cross-over design. In addition, we explored whether the training sequence determines the overall treatment outcome. Our findings demonstrate that the training-related improvements in reading and visual exploration are highly specific and task-dependent, and there was no effect of training sequence.

Is the origin of the hemianopic line bisection error purely visual? Evidence from eye movements in simulated hemianopia.

It is still unclear whether the contralateral line bisection error in unilateral homonymous hemianopia is caused by the visual field defect, strategic oculomotor adaptation or by additional extrastriate brain injury. We therefore simulated hemianopia in healthy participants using a gaze-contingent display paradigm and investigated its effects on manual and ocular line bisection performance and eye-movements. Although simulated hemianopia impaired line bisection and induced the adaptive oculomotor eye-movement pattern of hemianopic patients, it did not induce the contralateral bisection error, suggesting that neither the visual field defect nor oculomotor adaptation to it are the primary causes of the hemianopic bisection error.

Adaptation of eye-movements to simulated hemianopia in reading and visual exploration: Transfer or specificity?

Reading and visual exploration impairments in unilateral homonymous hemianopia are well-established clinical phenomena. Spontaneous adaptation of eye-movements to the visual field defect leads to improved reading and visual exploration performance. Yet, it is still unclear whether oculomotor adaptation to visual field loss is task-specific or whether there is a transfer of adaptation-related improvements between reading and visual exploration. We therefore simulated unilateral homonymous hemianopia in healthy participants and explored the specificity with which oculomotor adaptation to this pure visual-sensory dysfunction during uninstructed reading or visual exploration practice leads to improvements in both abilities. Our findings demonstrate that there is no transfer of adaptation-related changes of eye-movements and performance improvements between reading and visual exploration. Efficient oculomotor adaptation to visual field loss is highly specific and task-dependent.

The prioritization of feature singletons in the change detection paradigm.

Six experiments are reported investigating whether a discontinuity in colour can accrue attentional priority. In addition to a standard visual search paradigm, we examined the degree to which colour singletons and nonsingletons are susceptible to change blindness. Results showed that changes occurring at colour singletons were relatively more resistant to change blindness. Although suggestive of bottom-up marshalling of attention, no prioritization of the singleton occurred when the most stringent test of stimulus-driven attentional attraction was employed, that is, when attending to the singleton was detrimental to the task. We conclude that a discontinuity in colour will attract attention unless an attentional set is contrary to singletons.

Are hemianopic reading and visual exploration impairments visually elicited? New insights from eye movements in simulated hemianopia.

Hemianopic reading and visual exploration impairments are well-known clinical phenomena. Yet, it is unclear whether they are primarily caused by the hemianopic visual field defect itself or by additional brain injury preventing efficient spontaneous oculomotor adaptation. To establish the extent to which these impairments are visually elicited we simulated unilateral homonymous hemianopia in healthy participants, using a gaze-contingent display paradigm, and investigated its effect on reading and visual exploration. We demonstrate that simulated hemianopia induces the reading and visual exploration impairments of hemianopic patients. Over time, however, all participants showed efficient spontaneous oculomotor adaptation to the visual-sensory loss which improved their reading and visual exploration performance. Our results suggest that the hemianopic visual field defect is a major component of the chronic impairments of reading and visual and exploration found in hemianopic patients although it may not be their sole cause.

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.

Rehabilitation of hemianopic dyslexia: are words necessary for re-learning oculomotor control?

Unilateral homonymous visual field disorders after brain damage are frequently associated with a severe impairment of reading, called hemianopic dyslexia. A specific treatment method has been developed which allows patients to regain sufficient reading performance by re-learning eye-movement control in reading through systematic oculomotor practice. However, it is still unclear whether the treatment effect associated with this training procedure critically depends on using text material. We therefore evaluated the effectiveness of systematic oculomotor training with non-text material (Arabic digits) in comparison with conventional oculomotor training using text material (words) in 40 patients with unilateral homonymous visual field disorders and hemianopic dyslexia. Non-text training was found to be as effective as conventional text training in improving reading performance and associated eye-movements in these patients. Our results suggest that using words is not critical to the treatment effect of this training procedure. Thus, lexical-semantic processes seem not to be necessary for re-learning eye-movement control in hemianopic dyslexia.

The significance of visual information processing in reading: Insights from hemianopic dyslexia.

We present the first comprehensive review of research into hemianopic dyslexia since Mauthner's original description of 1881. We offer an explanation of the reading impairment in patients with unilateral homonymous visual field disorders and clarify its functional and anatomical bases. The major focus of our review is on visual information processing, visuospatial attention and eye-movement control during reading. An advanced understanding of the basis of hemianopic dyslexia and its rehabilitation also increases our knowledge about normal reading and its underlying neural mechanisms. By drawing together various sources of evidence we illustrate the significance of bottom-up and attentional top-down control of visual information processing and saccadic eye-movements in reading. Reading depends critically on the cortical-subcortical network subserving the integration of visual, attentional and oculomotor processes involved in text processing.

Voice recognition and the posterior cingulate: an fMRI study of prosopagnosia.

Voices, in addition to faces, enable person identification. Voice recognition has been shown to evoke a distributed network of brain regions that includes, in addition to the superior temporal sulcus (STS), the anterior temporal pole, fusiform face area (FFA), and posterior cingulate gyrus (pCG). Here we report an individual (MS) with acquired prosopagnosia who, despite bilateral damage to much of this network, demonstrates the ability to distinguish voices of several well-known acquaintances from voices of people that he has never heard before. Functional magnetic resonance imaging (fMRI) revealed that, relative to speech-modulated noise, voices rated as familiar and unfamiliar by MS elicited enhanced haemodynamic activity in the left angular gyrus, left posterior STS, and posterior midline brain regions, including the retrosplenial cortex and the dorsal pCG. More interestingly, relative to noise and unfamiliar voices, the familiar voices elicited greater haemodynamic activity in the left angular gyrus and medial parietal regions including the dorsal pCG and precuneus. The findings are consistent with theories implicating the pCG in recognizing people who are personally familiar, and furthermore suggest that the pCG region of the voice identification network is able to make functional contributions to voice recognition even though other areas of the network, namely the anterior temporal poles, FFA, and the right parietal lobe, may be compromised.

Object onset and parvocellular guidance of attentional allocation.

The parvocellular visual pathway in the primate brain is known to be involved with the processing of color. However, a subject of debate is whether an abrupt change in color, conveyed via this pathway, is capable of automatically attracting attention. It has been shown that the appearance of new objects defined solely by color is indeed capable of modulating attention. However, given evidence suggesting that the visual system is particularly sensitive to new onsets, it is unclear to what extent such results reflect effects of color change per se, rather than effects of object onset. We assessed attentional capture by color change that occurred as a result of either new objects appearing or already-present "old" objects changing color. Results showed that although new object onsets accrued attention, changing the color of old objects did not. We conclude that abrupt color change per se is not sufficient to capture attention.

Visual salience in the change detection paradigm: the special role of object onset.

The relative efficacy with which appearance of a new object orients visual attention was investigated. At issue is whether the visual system treats onset as being of particular importance or only 1 of a number of stimulus events equally likely to summon attention. Using the 1-shot change detection paradigm, the authors compared detectability of new objects with changes occurring at already present objects--luminance change, color change, and object offset. Results showed that appearance of a new object was less susceptible to change blindness than changes that old objects could undergo. The authors also investigated whether it is onset per se that leads to enhanced detectability or onset of an object representation. Results showed that the onset advantage was eliminated for onsets that did not correspond with the appearance of a new object. These findings suggest that the visual system is particularly sensitive to the onset of a new object.