Intact navigation skills after bilateral loss of striate cortex

How to study subjective experience in an animal model of blindsight?

The nature of subjective conscious experience, which accompanies us throughout our waking lives, and how it is generated, remain elusive. One of the challenges in studying subjective experience is disentangling the brain activity related to the sensory stimulus processing and stimulus-guided behavior from those associated with subjective perception. Blindsight, a phenomenon characterized by the retained visual discrimination performance but impaired visual consciousness due to damage to the primary visual cortex, becomes a special entry point to address this question. However, to fully understand the underlying neural mechanism, relying on studies involving human patients alone is insufficient. In this paper, we tried to address this issue, by first introducing the well-known cases of blindsight, especially the reports on subjective experience in both human and monkey subjects. And then we described how the impaired visual awareness of blindsight monkeys has been discovered and further studied by specifically designed tasks, as verbal reporting is not possible for these animals. Our previous studies also demonstrated that many complex visually guided cognitive processes were still retained despite the impairment of visual awareness. Further investigation needs to be conducted to explore the relationship between visually guided behavior, visual awareness and brain activity in blindsight subjects.

Functional blindsight and its diagnosis

Even when brain scans fail to detect a striate lesion, functional evidence for blindsight can be adduced. In the aftermath of an automobile accident, JK became blind. Results of ophthalmic exams indicated that the blindness must be cortical. Nevertheless, multiple MRI scans failed to detect structural damage to the striate cortex. Prior to the accident JK had been an athlete; after the accident he retained some athletic abilities, arousing suspicions that he might be engaged in fraud. His residual athletic abilities-e.g., hitting a handball or baseball, or catching a Frisbee-coupled with his experienced blindness, suggested blindsight. But due to the apparent absence of striate lesions, we designed a series of tasks for temporal and spatial dimensions in an attempt to detect functional evidence of his disability. Indeed, test results revealed compelling neural evidence that comport with his subjective reports. This spatiotemporal task-related method that includes contrasts with healthy controls, and detailed understanding of the patient's conscious experience, can be generalized for clinical, scientific and forensic investigations of blindsight.

Communication as the Origin of Consciousness

Since the middle of the 20th century, more and more data have appeared on the limited role of consciousness in determining human behavior. In this opinion paper, we hypothesize that the basis of consciousness is precisely the communicative function, and discuss relations of consciousness to other cognitive processes such sensory detection, decision-making and emotions. Within the framework of the hypothesis, consciousness is considered as a highly specialized function of the brain, which ensures encoding of personal information as communication messages. On a subjective level, mental representation just means the state of information to be shared in a human group. Accordingly, consciousness affects only those components of human behavior that are associated with the transmission of messages. Sensory detection, decision-making, emotions and other processes are only projected into consciousness during the encoding of information of them. The communication hypothesis assumes that consciousness is an adaptation that increases the efficiency of a collective way of life, and the emergence of consciousness is inextricably linked with the development of language in human culture. In the future, our view of consciousness provides an opportunity for an objective analysis of subjective phenomena by means of a directed study of the formation of messages both at the level of brain processes and at the level of interactions between individuals.

Conscious awareness is necessary to assess trust and mimic facial expressions, while pupils impact trust unconsciously

People make rapid inferences about others' thoughts and intentions. For example, they observe facial movements and pupil size of others and unwittingly make use of this information when deciding whether to trust someone or not. However, whether spontaneous mimicry depends on visual awareness of the stimulus and whether these processes underlie trust decisions is still unknown. To investigate whether visual awareness modulates the relationship between emotional expressions, mimicry and trust, participants played a series of trust games and saw either their partners' faces with a neutral, happy or fearful expression, or their partners' eyes in which the pupil size was large, medium or small. Subjects' trust investments, facial movements and pupil responses were measured. In half of the trials, the stimuli were rendered invisible by continuous flash suppression. Results showed that facial expressions were mimicked and influenced trust decisions during the conscious condition, but not during the unconscious (suppressed) condition. The opposite was found for pupil size, which influenced trust decisions during states of unawareness. These results suggest that the neurobiological pathway linking the observation of facial expressions to mimicry and trust is predominantly conscious, whereas partner pupil size influences trust primarily when presented unconsciously. This article is part of the theme issue 'Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience'.

Consciousness matters: phenomenal experience has functional value

‘Why would we do anything at all if the doing was not doing something to us?’ In other words: What is consciousness good for? Here, reversing classical views, according to many of which subjective experience is a mere epiphenomenon that affords no functional advantage, we propose that subject-level experience—‘What it feels like’—is endowed with intrinsic value, and it is precisely the value agents associate with their experiences that explains why they do certain things and avoid others. Because experiences have value and guide behaviour, consciousness has a function. Under this hypothesis of ‘phenomenal worthiness’, we argue that it is only in virtue of the fact that conscious agents ‘experience’ things and ‘care’ about those experiences that they are ‘motivated’ to act in certain ways and that they ‘prefer’ some states of affairs vs. others. Overviewing how the concept of value has been approached in decision-making, emotion research and consciousness research, we argue that phenomenal consciousness has intrinsic value and conclude that if this is indeed the case, then it must have a function. Phenomenal experience might act as a mental currency of sorts, which not only endows conscious mental states with intrinsic value but also makes it possible for conscious agents to compare vastly different experiences in a common subject-centred space—a feature that readily explains the fact that consciousness is ‘unified’. The phenomenal worthiness hypothesis, in turn, makes the ‘hard problem’ of consciousness more tractable, since it can then be reduced to a problem about function.

Functional neurological disorder: Extending the diagnosis to other disorders, and proposing an alternate disease term—Attentionally-modifiable disorder

BACKGROUND: The term “functional neurological disorder,” or “FND,” applies to disorders whose occurrence of neurological symptoms fluctuate with the patient’s attention to them. However, many other disorders that are not called “FND” nonetheless can also follow this pattern. Consequently, guidelines are unclear for diagnosing “FND.” OBJECTIVE: To review the neurological conditions that follow this pattern, but which have not so far been termed “FND,” to understand their overlap with conditions that have been termed “FND,” and to discuss the rationale for why FND has not been diagnosed for them. METHOD: A systematic review of the PubMed literature registry using the terms “fluctuation,” “inconsistency,” or “attention” did not yield much in the way of these candidate disorders. Consequently, this review instead relied on the author’s personal library of peer-reviewed studies of disorders that have resembled FND but which were not termed this way, due to his longstanding interest in this problem. Consequently, this approach was not systematic and was subjective regarding disease inclusion. RESULTS: This review identified numerous, diverse conditions that generally involve fluctuating neurological symptoms that can vary with the person’s attention to them, but which have not been called “FND.” The literature was unclear for reasons for not referring to “FND” in these instances. CONCLUSION: Most likely because of historical biases, the use of the term “FND” has been unnecessarily restricted. Because at its core FND is an attentionally-influenced disorder that can respond well to behavioral treatments, the field of neurological rehabilitation could benefit by extending the range of conditions that could be considered as “FND” and referred for similar behavioral treatments. Because the term “FND” has been viewed unfavorably by some patients and clinical practitioners and whose treatment is not implied, the alternative term attentionally-modifiable disorder is proposed.

Rehabilitation of visual perception in cortical blindness

Blindness is a common sequela after stroke affecting the primary visual cortex, presenting as a contralesional, homonymous, visual field cut. This can occur unilaterally or, less commonly, bilaterally. While it has been widely assumed that after a brief period of spontaneous improvement, vision loss becomes stable and permanent, accumulating data show that visual training can recover some of the vision loss, even long after the stroke. Here, we review the different approaches to rehabilitation employed in adult-onset cortical blindness (CB), focusing on visual restoration methods. Most of this work was conducted in chronic stroke patients, partially restoring visual discrimination and luminance detection. However, to achieve this, patients had to train for extended periods (usually many months), and the vision restored was not entirely normal. Several adjuvants to training such as noninvasive, transcranial brain stimulation, and pharmacology are starting to be investigated for their potential to increase the efficacy of training in CB patients. However, these approaches are still exploratory and require considerably more research before being adopted. Nonetheless, having established that the adult visual system retains the capacity for restorative plasticity, attention recently turned toward the subacute poststroke period. Drawing inspiration from sensorimotor stroke rehabilitation, visual training was recently attempted for the first time in subacute poststroke patients. It improved vision faster, over larger portions of the blind field, and for a larger number of visual discrimination abilities than identical training initiated more than 6 months poststroke (i.e., in the chronic period). In conclusion, evidence now suggests that visual neuroplasticity after occipital stroke can be reliably recruited by a range of visual training approaches. In addition, it appears that poststroke visual plasticity is dynamic, with a critical window of opportunity in the early postdamage period to attain more rapid, more extensive recovery of a larger set of visual perceptual abilities.

Visuomotor control in mice and primates

We conduct a comparative evaluation of the visual systems from the retina to the muscles of the mouse and the macaque monkey noting the differences and similarities between these two species. The topics covered include (1) visual-field overlap, (2) visual spatial resolution, (3) V1 cortical point-image [i.e., V1 tissue dedicated to analyzing a unit receptive field], (4) object versus motion encoding, (5) oculomotor range, (6) eye, head, and body movement coordination, and (7) neocortical and cerebellar function. We also discuss blindsight in rodents and primates which provides insights on how the neocortex mediates conscious vision in these species. This review is timely because the field of visuomotor neurophysiology is expanding beyond the macaque monkey to include the mouse; there is therefore a need for a comparative analysis between these two species on how the brain generates visuomotor responses.

Neural Mechanism of Blindsight in a Macaque Model

Some patients with damage to the primary visual cortex (V1) exhibit visuomotor ability, despite loss of visual awareness, a phenomenon termed "blindsight". We review a series of studies conducted mainly in our laboratory on macaque monkeys with unilateral V1 lesioning to reveal the neural pathways underlying visuomotor transformation and the cognitive capabilities retained in blindsight. After lesioning, it takes several weeks for the recovery of visually guided saccades toward the lesion-affected visual field. In addition to the lateral geniculate nucleus, the pathway from the superior colliculus to the pulvinar participates in visuomotor processing in blindsight. At the cortical level, bilateral lateral intraparietal regions become critically involved in the saccade control. These results suggest that the visual circuits experience drastic changes while the monkey acquires blindsight. In these animals, analysis based on signal detection theory adapted to behavior in the "Yes-No" task indicates reduced sensitivity to visual targets, suggesting that visual awareness is impaired. Saccades become less accurate, decisions become less deliberate, and some forms of bottom-up attention are impaired. However, a variety of cognitive functions are retained such as saliency detection during free viewing, top-down attention, short-term spatial memory, and associative learning. These observations indicate that blindsight is not a low-level sensory-motor response, but the residual visual inputs can access these cognitive capabilities. Based on these results we suggest that the macaque model of blindsight replicates type II blindsight patients who experience some "feeling" of objects, which guides cognitive capabilities that we naïvely think are not possible without phenomenal consciousness.

A brain-inspired computational model for spatio-temporal information processing

Spatio-temporal information processing is fundamental in both brain functions and AI applications. Current strategies for spatio-temporal pattern recognition usually involve explicit feature extraction followed by feature aggregation, which requires a large amount of labeled data. In the present study, motivated by the subcortical visual pathway and early stages of the auditory pathway for motion and sound processing, we propose a novel brain-inspired computational model for generic spatio-temporal pattern recognition. The model consists of two modules, a reservoir module and a decision-making module. The former projects complex spatio-temporal patterns into spatially separated neural representations via its recurrent dynamics, the latter reads out neural representations via integrating information over time, and the two modules are linked together using known examples. Using synthetic data, we demonstrate that the model can extract the frequency and order information of temporal inputs. We apply the model to reproduce the looming pattern discrimination behavior as observed in experiments successfully. Furthermore, we apply the model to the gait recognition task, and demonstrate that our model accomplishes the recognition in an event-based manner and outperforms deep learning counterparts when training data is limited.

Is the primary visual cortex necessary for blindsight-like behavior? Review of transcranial magnetic stimulation studies in neurologically healthy individuals

The visual pathways that bypass the primary visual cortex (V1) are often assumed to support visually guided behavior in humans in the absence of conscious vision. This conclusion is largely based on findings on patients: V1 lesions cause blindness but sometimes leave some visually guided behaviors intact-this is known as blindsight. With the aim of examining how well the findings on blindsight patients generalize to neurologically healthy individuals, we review studies which have tried to uncover transcranial magnetic stimulation (TMS) induced blindsight. In general, these studies have failed to demonstrate a completely unconscious blindsight-like capacity in neurologically healthy individuals. A possible exception to this is TMS-induced blindsight of stimulus presence or location. Because blindsight in patients is often associated with some form of introspective access to the visual stimulus, and blindsight may be associated with neural reorganization, we suggest that rather than revealing a dissociation between visually guided behavior and conscious seeing, blindsight may reflect preservation or partial recovery of conscious visual perception after the lesion.

Conscious awareness modulates processing speed in the redundant signal effect

Evidence for the influence of unaware signals on behaviour has been reported in both patient groups and healthy observers using the Redundant Signal Effect (RSE). The RSE refers to faster manual reaction times to the onset of multiple simultaneously presented target than those to a single stimulus. These findings are robust and apply to unimodal and multi-modal sensory inputs. A number of studies on neurologically impaired cases have demonstrated that RSE can be found even in the absence of conscious experience of the redundant signals. Here, we investigated behavioural changes associated with awareness in healthy observers by using Continuous Flash Suppression to render observers unaware of redundant targets. Across three experiments, we found an association between reaction times to the onset of a consciously perceived target and the reported level of visual awareness of the redundant target, with higher awareness being associated with faster reaction times. However, in the absence of any awareness of the redundant target, we found no evidence for speeded reaction times and even weak evidence for an inhibitory effect (slowing down of reaction times) on response to the seen target. These findings reveal marked differences between healthy observers and blindsight patients in how aware and unaware information from different locations is integrated in the RSE.

Blindsight

For over a century, research has demonstrated that damage to primary visual cortex does not eliminate all capacity for visual processing in the brain. From Riddoch's (1917) early demonstration of intact motion processing for blind field stimuli, to the iconic work of Weiskrantz et al. (1974) showing reliable spatial localization, it is clear that secondary visual pathways that bypass V1 carry information to the visual brain that in turn influences behavior. In this chapter, we briefly outline the history and phenomena associated with blindsight, before discussing the nature of the secondary visual pathways that support residual visual processing in the absence of V1. We finish with some speculation as to the functional characteristics of these secondary pathways.

Injecting Information into the Mammalian Cortex: Progress, Challenges, and Promise

For 150 years artificial stimulation has been used to study the function of the nervous system. Such stimulation-whether electrical or optogenetic-eventually may be used in neuroprosthetic devices to replace lost sensory inputs and to otherwise introduce information into the nervous system. Efforts toward this goal can be classified broadly as either biomimetic or arbitrary. Biomimetic stimulation aims to mimic patterns of natural neural activity, so that the subject immediately experiences the artificial stimulation as if it were natural sensation. Arbitrary stimulation, in contrast, makes no attempt to mimic natural patterns of neural activity. Instead, different stimuli-at different locations and/or in different patterns-are assigned different meanings randomly. The subject's time and effort then are required to learn to interpret different stimuli, a process that engages the brain's inherent plasticity. Here we will examine progress in using artificial stimulation to inject information into the cerebral cortex and discuss the challenges for and the promise of future development.

Changes in peri-calcarine cortical thickness in blindsight

Blindsight is the ability of patients with primary visual cortex (V1) damage to process information in their clinically blind visual field in the absence of conscious awareness. In addition to those with localized V1 lesions, some patients exhibiting this phenomenon have had a cerebral hemisphere removed or disconnected from the rest of the brain for the treatment of drug-resistant epilepsy (hemispherectomy). Research into the underlying neural substrates of blindsight has long implicated the intact visual cortex in maintaining residual vision and supporting visuo-guided responses to stimuli presented ipsilaterally within the blind visual field while operating outside the geniculo-striate pathway. A recent study demonstrated functional reorganization in the dorsal visual areas of the intact hemisphere, thereby supporting its compensatory role in non-conscious vision. In this study, we used cortical thickness analysis to examine anatomical differences in the visual cortex of the intact hemisphere of three subjects with varying degrees of cortical damage and well documented blindsight: two with a right hemispherectomy (complete and partial), and one with a left V1 lesion. T1-weighted MRI data were obtained for the subjects while control data were chosen from publicly available NKI-dataset to match closely the acquisition parameters of our blindsight cases. Our results show significant increases in cortical thickness in the visual cortex of all blindsight subjects compared to healthy controls, irrespective of age-onset, etiology, and extent of the damage. Our findings add to accumulating evidence from behavioral, functional imaging, and tractography studies of cerebral compensation and reorganization.

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Examined anatomical changes in the intact visual cortex of rare blindsight patients.

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First comparison of hemispherectomy and lesion patients to a large control sample.

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Blindsight subjects show significant increases in peri-calcarine cortical thickness.

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Similar changes observed despite differences in etiology and age at time of lesion.

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Increases are possible morphological signs of compensation underlying blindsight.

The influence of subcortical shortcuts on disordered sensory and cognitive processing

The very earliest stages of sensory processing have the potential to alter how we perceive and respond to our environment. These initial processing circuits can incorporate subcortical regions, such as the thalamus and brainstem nuclei, which mediate complex interactions with the brain's cortical processing hierarchy. These subcortical pathways, many of which we share with other animals, are not merely vestigial but appear to function as 'shortcuts' that ensure processing efficiency and preservation of vital life-preserving functions, such as harm avoidance, adaptive social interactions and efficient decision-making. Here, we propose that functional interactions between these higher-order and lower-order brain areas contribute to atypical sensory and cognitive processing that characterizes numerous neuropsychiatric disorders.

The Age-Dependent Neural Substrates of Blindsight

Some patients who are considered cortically blind due to the loss of their primary visual cortex (V1) show a remarkable ability to act upon or discriminate between visual stimuli presented to their blind field, without any awareness of those stimuli. This phenomenon is often referred to as blindsight. Despite the range of spared visual abilities, the identification of the pathways mediating blindsight remains an active and contentious topic in the field. In this review, we discuss recent findings of the candidate pathways and their relative contributions to different forms of blindsight across the lifespan to illustrate the varied nature of unconscious visual processing.

Preserved extrastriate visual network in a monkey with substantial, naturally occurring damage to primary visual cortex

Lesions of primary visual cortex (V1) lead to loss of conscious visual perception with significant impact on human patients. Understanding the neural consequences of such damage may aid the development of rehabilitation methods. In this rare case of a Rhesus macaque (monkey S), likely born without V1, the animal's in-group behaviour was unremarkable, but visual task training was impaired. With multi-modal magnetic resonance imaging, visual structures outside of the lesion appeared normal. Visual stimulation under anaesthesia with checkerboards activated lateral geniculate nucleus of monkey S, while full-field moving dots activated pulvinar. Visual cortical activation was sparse but included face patches. Consistently across lesion and control monkeys, functional connectivity analysis revealed an intact network of bilateral dorsal visual areas temporally correlated with V5/MT activation, even without V1. Despite robust subcortical responses to visual stimulation, we found little evidence for strengthened subcortical input to V5/MT supporting residual visual function or blindsight-like phenomena.

Neuronal mechanisms of motion detection underlying blindsight assessed by functional magnetic resonance imaging (fMRI)

Brain imaging offers a valuable tool to observe functional brain plasticity by showing how sensory inputs reshape cortical activations after a visual impairment. Following a unilateral post-chiasmatic lesion affecting the visual cortex, patients may suffer a contralateral visual loss referred to homonymous hemianopia. Nevertheless, these patients preserve the ability to unconsciously detect, localize and discriminate visual stimuli presented in their impaired visual field. To investigate this paradox, known as blindsight, we conducted a study using functional magnetic resonance imaging (fMRI) to evaluate the structural and functional impact of such lesion in a 33-year old patient (ML), who suffers a complete right hemianopia without macular sparing and showing strong evidences of blindsight. We thus performed whole brain and sliced thalamic fMRI scan sequences during an event-related motion detection task. We provided evidence of the neuronal fingerprint of blindsight by acquiring and associating neural correlates, specific structures and functional networks of the midbrain during blindsight performances which may help to better understand this condition. Accurate performance demonstrated the presence of residual vision and the ability to unconsciously perceive motion presented in the blind hemifield, although her reaction time was significantly higher in her blind-field. When the normal hemifield was stimulated, we observed significant contralateral activations in primary and secondary visual areas as well as motion specific areas, such as the supramarginal gyrus and middle temporal area. We also demonstrated sub-thalamic activations within the superior colliculi (SC) and the pulvinar. These results suggest a role of secondary subcortical structures in normal spontaneous motion detection. In a similar way, when the lesioned hemifield was stimulated, we observed contralateral activity in extrastriate areas with no activation of the primary lesioned visual cortex. Moreover, we observed activations within the SC when the blind hemifield was stimulated. However, we observed unexpected ipsilateral activations within the same motion specific areas, as well as bilateral frontal activations. These results highlight the importance of abnormal secondary pathways bypassing the primary visual area (V1) in residual vision. This reorganization in the structure and function of the visual pathways correlates with behavioral changes, thus offering a plausible explanation for the blindsight phenomenon. Our results may potentially impact the development of rehabilitation strategies to target subcortical pathways.

Spontaneous in-flight accommodation of hand orientation to unseen grasp targets: A case of action blindsight

The division of labour between the dorsal and ventral visual pathways is well established. The ventral stream supports object identification, while the dorsal stream supports online processing of visual information in the service of visually guided actions. Here, we report a case of an individual with a right inferior quadrantanopia who exhibited accurate spontaneous rotation of his wrist when grasping a target object in his blind visual field. His accurate wrist orientation was observed despite the fact that he exhibited no sensitivity to the orientation of the handle in a perceptual matching task. These findings indicate that non-geniculostriate visual pathways process basic volumetric information relevant to grasping, and reinforce the observation that phenomenal awareness is not necessary for an object's volumetric properties to influence visuomotor performance.

Methods matter: A primer on permanent and reversible interference techniques in animals for investigators of human neuropsychology

The study of patients with brain lesions has contributed greatly to our understanding of the biological bases of human cognition, but this approach also has several unavoidable limitations. Research that uses animal models complements and extends human neuropsychology by addressing many of these limitations. In this review, we provide an overview of permanent and reversible animal lesion techniques for researchers of human neuropsychology, with the aim of highlighting how these methods provide a valuable adjunct to behavioural, neuroimaging, physiological, and clinical investigations in humans. Research in animals has provided important lessons about how the limitations of one or more techniques, or differences in their mechanism of action, has impacted upon the understanding of brain organisation and function. These cautionary tales highlight the importance of striving for a thorough understanding of how any intereference technique works (whether in animal or human), and for how to best use animal research to clarify the precise mechanisms underlying temporary lesion methods in humans.

Blindsight relies on a functional connection between hMT+ and the lateral geniculate nucleus, not the pulvinar

When the primary visual cortex (V1) is damaged, the principal visual pathway is lost, causing a loss of vision in the opposite visual field. While conscious vision is impaired, patients can still respond to certain images; this is known as 'blindsight'. Recently, a direct anatomical connection between the lateral geniculate nucleus (LGN) and human motion area hMT+ has been implicated in blindsight. However, a functional connection between these structures has not been demonstrated. We quantified functional MRI responses to motion in 14 patients with unilateral V1 damage (with and without blindsight). Patients with blindsight showed significant activity and a preserved sensitivity to speed in motion area hMT+, which was absent in patients without blindsight. We then compared functional connectivity between motion area hMT+ and a number of structures implicated in blindsight, including the ventral pulvinar. Only patients with blindsight showed an intact functional connection with the LGN but not the other structures, supporting a specific functional role for the LGN in blindsight.

Psychophysical and neuroimaging responses to moving stimuli in a patient with the Riddoch phenomenon due to bilateral visual cortex lesions

Patients with injury to early visual cortex or its inputs can display the Riddoch phenomenon: preserved awareness for moving but not stationary stimuli. We provide a detailed case report of a patient with the Riddoch phenomenon, MC. MC has extensive bilateral lesions to occipitotemporal cortex that include most early visual cortex and complete blindness in visual field perimetry testing with static targets. Nevertheless, she shows a remarkably robust preserved ability to perceive motion, enabling her to navigate through cluttered environments and perform actions like catching moving balls. Comparisons of MC's structural magnetic resonance imaging (MRI) data to a probabilistic atlas based on controls reveals that MC's lesions encompass the posterior, lateral, and ventral early visual cortex bilaterally (V1, V2, V3A/B, LO1/2, TO1/2, hV4 and VO1 in both hemispheres) as well as more extensive damage to right parietal (inferior parietal lobule) and left ventral occipitotemporal cortex (VO1, PHC1/2). She shows some sparing of anterior occipital cortex, which may account for her ability to see moving targets beyond ~15 degrees eccentricity during perimetry. Most strikingly, functional and structural MRI revealed robust and reliable spared functionality of the middle temporal motion complex (MT+) bilaterally. Moreover, consistent with her preserved ability to discriminate motion direction in psychophysical testing, MC also shows direction-selective adaptation in MT+. A variety of tests did not enable us to discern whether input to MT+ was driven by her spared anterior occipital cortex or subcortical inputs. Nevertheless, MC shows rich motion perception despite profoundly impaired static and form vision, combined with clear preservation of activation in MT+, thus supporting the role of MT+ in the Riddoch phenomenon.

Ventral and Dorsal Pathways Relate Differently to Visual Awareness of Body Postures under Continuous Flash Suppression

Visual perception includes ventral and dorsal stream processes. However, it is still unclear whether the former is predominantly related to conscious and the latter to nonconscious visual perception as argued in the literature. In this study upright and inverted body postures were rendered either visible or invisible under continuous flash suppression (CFS), while brain activity of human participants was measured with functional MRI (fMRI). Activity in the ventral body-sensitive areas was higher during visible conditions. In comparison, activity in the posterior part of the bilateral intraparietal sulcus (IPS) showed a significant interaction of stimulus orientation and visibility. Our results provide evidence that dorsal stream areas are less associated with visual awareness.

Consciousness and confidence

It is natural to see conscious perceptions as typically bringing with them a degree of confidence about what is perceived. So one might also expect such confidence not to occur if a perception is not conscious. This has resulted in the use of confidence as a test or measure of consciousness, one that may be more reliable and fine-grained than the traditional appeal to subjective report as a test for a perception's being conscious. The following describes theoretical difficulties for the use of confidence as a reliable test for consciousness, which show that confidence is less reliable than subjective report. Difficulties are also presented for the use of confidence ratings in assessing degrees of consciousness, which cast doubt on any advantage confidence might have from being more fine-grained than subjective report. And an explanation is proposed for the wide appeal of using confidence to assess subjective awareness, an explanation that also makes clear why confidence is less reliable than subjective report.

Subcortical pathways to extrastriate visual cortex underlie residual vision following bilateral damage to V1

Residual vision, or blindsight, following damage to the primary visual cortex (V1) has been investigated for almost half a century. While there have been many studies of patients with unilateral damage to V1, far fewer have examined bilateral damage, mainly due to the rarity of such patients. Here we re-examine the residual visual function and underlying pathways of previously studied patient SBR who, as a young adult, suffered bilateral damage restricted to V1 which rendered him cortically blind. While earlier work compared his visual cortex to healthy, sighted participants, here we consider how his visual responses and connections compare to patients with unilateral damage to V1 in addition to sighted participants.

Detection of drifting Gabor patches of different contrasts (1%, 5%, 10%, 50% and 100%) was tested in SBR and a group of eight patients with unilateral damage to V1. Performance was compared to the neural activation in motion area hMT+ measured using functional magnetic resonance imaging. Diffusion tractography was also used to determine the white matter microstructure of the visual pathways in all participants.

Like the patients with unilateral damage, patient SBR showed increased % BOLD signal change to the high contrast stimuli that he could detect compared to the lower contrast stimuli that were not detectable. Diffusion tractography suggests this information is conveyed by a direct pathway between the lateral geniculate nucleus (LGN) and hMT+ since this pathway had microstructure that was comparable to the healthy control group. In contrast, the pathway between LGN and V1 had reduced integrity compared to controls. A further finding of note was that, unlike control participants, SBR showed similar patterns of contralateral and ipsilateral activity in hMT+, in addition to healthy white matter microstructure in the tract connecting hMT+ between the two hemispheres. This raises the possibility of increased connectivity between the two hemispheres in the absence of V1 input.

In conclusion, the pattern of visual function and anatomy in bilateral cortical damage is comparable to that seen in a group of patients with unilateral damage. Thus, while the intact hemisphere may play a role in residual vision in patients with unilateral damage, its influence is not evident with the methodology employed here.

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Bilaterally hemianopic patient SBR has neural patterns like unilateral patients.

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hMT+ activity increases with stimulus contrast and better stimulus detection.

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Like in unilateral patients, the pathway between LGN and hMT+ is intact in SBR.

Body-scaled action in obesity during locomotion: Insights on the nature and extent of body representation disturbances

OBJECTIVE

Conscious perception of our own body, also known as body image, can influence body-scaled actions. Certain conditions such as obesity are frequently accompanied by a negative body image, leaving open the question if body-scaled actions are distorted in these individuals.

METHODS

To shed light on this issue, we asked individuals affected by obesity to process dimensions of their own body in a real action: they walked in a straight-ahead direction, while avoiding collision with obstacles represented by door-like openings that varied in width.

RESULTS

Participants affected by obesity showed a body rotation behavior similar to that of the healthy weighted, but differences emerged in parameters such as step length and velocity.

CONCLUSION

When participants with obesity walk through door-like openings, their body parts rotation is scaled according to their physical body dimensions; however, they might try to minimize risk of collision. Our study is in line with the hypothesis that unconscious body-scaled actions are related to emotional, cognitive and perceptual components of a negative body image.

Affective blindsight relies on low spatial frequencies

The human brain can process facial expressions of emotions rapidly and without awareness. Several studies in patients with damage to their primary visual cortices have shown that they may be able to guess the emotional expression on a face despite their cortical blindness. This non-conscious processing, called affective blindsight, may arise through an intact subcortical visual route that leads from the superior colliculus to the pulvinar, and thence to the amygdala. This pathway is thought to process the crude visual information conveyed by the low spatial frequencies of the stimuli. In order to investigate whether this is the case, we studied a patient (TN) with bilateral cortical blindness and affective blindsight. An fMRI paradigm was performed in which fearful and neutral expressions were presented using faces that were either unfiltered, or filtered to remove high or low spatial frequencies. Unfiltered fearful faces produced right amygdala activation although the patient was unaware of the presence of the stimuli. More importantly, the low spatial frequency components of fearful faces continued to produce right amygdala activity while the high spatial frequency components did not. Our findings thus confirm that the visual information present in the low spatial frequencies is sufficient to produce affective blindsight, further suggesting that its existence could rely on the subcortical colliculo-pulvino-amygdalar pathway.

Direct Electrical Stimulation in the Human Brain Disrupts Melody Processing

Prior research using functional magnetic resonance imaging (fMRI) [1-4] and behavioral studies of patients with acquired or congenital amusia [5-8] suggest that the right posterior superior temporal gyrus (STG) in the human brain is specialized for aspects of music processing (for review, see [9-12]). Intracranial electrical brain stimulation in awake neurosurgery patients is a powerful means to determine the computations supported by specific brain regions and networks [13-21] because it provides reversible causal evidence with high spatial resolution (for review, see [22, 23]). Prior intracranial stimulation or cortical cooling studies have investigated musical abilities related to reading music scores [13, 14] and singing familiar songs [24, 25]. However, individuals with amusia (congenitally, or from a brain injury) have difficulty humming melodies but can be spared for singing familiar songs with familiar lyrics [26]. Here we report a detailed study of a musician with a low-grade tumor in the right temporal lobe. Functional MRI was used pre-operatively to localize music processing to the right STG, and the patient subsequently underwent awake intraoperative mapping using direct electrical stimulation during a melody repetition task. Stimulation of the right STG induced "music arrest" and errors in pitch but did not affect language processing. These findings provide causal evidence for the functional segregation of music and language processing in the human brain and confirm a specific role of the right STG in melody processing. VIDEO ABSTRACT.

Basic Emotions in Human Neuroscience: Neuroimaging and Beyond

The existence of so-called ‘basic emotions’ and their defining attributes represents a long lasting and yet unsettled issue in psychology. Recently, neuroimaging evidence, especially related to the advent of neuroimaging meta-analytic methods, has revitalized this debate in the endeavor of systems and human neuroscience. The core theme focuses on the existence of unique neural bases that are specific and characteristic for each instance of basic emotion. Here we review this evidence, outlining contradictory findings, strengths and limits of different approaches. Constructionism dismisses the existence of dedicated neural structures for basic emotions, considering that the assumption of a one-to-one relationship between neural structures and their functions is central to basic emotion theories. While these critiques are useful to pinpoint current limitations of basic emotions theories, we argue that they do not always appear equally generative in fostering new testable accounts on how the brain relates to affective functions. We then consider evidence beyond PET and fMRI, including results concerning the relation between basic emotions and awareness and data from neuropsychology on patients with focal brain damage. Evidence from lesion studies are indeed particularly informative, as they are able to bring correlational evidence typical of neuroimaging studies to causation, thereby characterizing which brain structures are necessary for, rather than simply related to, basic emotion processing. These other studies shed light on attributes often ascribed to basic emotions, such as automaticity of perception, quick onset, and brief duration. Overall, we consider that evidence in favor of the neurobiological underpinnings of basic emotions outweighs dismissive approaches. In fact, the concept of basic emotions can still be fruitful, if updated to current neurobiological knowledge that overcomes traditional one-to-one localization of functions in the brain. In particular, we propose that the structure-function relationship between brain and emotions is better described in terms of pluripotentiality, which refers to the fact that one neural structure can fulfill multiple functions, depending on the functional network and pattern of co-activations displayed at any given moment.

Visual stimuli modulate frontal oscillatory rhythms in a cortically blind patient: Evidence for top-down visual processing

OBJECTIVE

We investigated neuronal correlates of faces versus non-faces processing in a cortically blind patient (TN) and a group of healthy age-matched controls in order to test electrophysiological correlates of the processing of pertinent stimuli in this patient.

METHODS

An EEG paradigm was used, in which intact and scrambled faces were displayed on a screen. First, time-frequency transforms were conducted on the patients' data alone. These oscillations were then compared to the frontal activity of six control participants.

RESULTS

Post stimulus oscillatory modulations (synchronisation in theta and alpha frequency bands) of both intact and scrambled faces at frontal scalp sites were observed in TN. These modulations were different for correct and incorrect responses. A more important increase in the theta band for incorrect responses was observed. The oscillatory rhythms highlighted in blindsight and in frontal regions differ from the ones observed in control participants.

CONCLUSION

Despite the destruction of the visual cortex, oscillatory rhythms are not cancelled out but are shifted to anterior regions, revealing the activity of an alternate pathway for residual visual function.

SIGNIFICANCE

The results provide evidence for a top-down cognitive control process in blindsight.

Blindsight and Unconscious Vision: What They Teach Us about the Human Visual System

Damage to the primary visual cortex removes the major input from the eyes to the brain, causing significant visual loss as patients are unable to perceive the side of the world contralateral to the damage. Some patients, however, retain the ability to detect visual information within this blind region; this is known as blindsight. By studying the visual pathways that underlie this residual vision in patients, we can uncover additional aspects of the human visual system that likely contribute to normal visual function but cannot be revealed under physiological conditions. In this review, we discuss the residual abilities and neural activity that have been described in blindsight and the implications of these findings for understanding the intact system.

Avoiding unseen obstacles: Subcortical vision is not sufficient to maintain normal obstacle avoidance behaviour during reaching

Previous research found that a patient with cortical blindness (homonymous hemianopia) was able to successfully avoid an obstacle placed in his blind field, despite reporting no conscious awareness of it [Striemer, C. L., Chapman, C. S., & Goodale, M. A., 2009, PNAS, 106(37), 15996-16001]. This finding led to the suggestion that dorsal stream areas, that are assumed to mediate obstacle avoidance behaviour, may obtain their visual input primarily from subcortical pathways. Hence, it was suggested that normal obstacle avoidance behaviour can proceed without input from the primary visual cortex. Here we tried to replicate this finding in a group of patients (N = 6) that suffered from highly circumscribed lesions in the occipital lobe (including V1) that spared the subcortical structures that have been associated with action-blindsight. We also tested if obstacle avoidance behaviour differs depending on whether obstacles are placed only in the blind field or in both the blind and intact visual field of the patients simultaneously. As expected, all patients successfully avoided obstacles placed in their intact visual field. However, none of them showed reliable avoidance behaviour - as indicated by adjustments in the hand trajectory in response to obstacle position - for obstacles placed in their blind visual field. The effects were not dependent on whether one or two obstacles were present. These findings suggest that behaviour in complex visuomotor tasks relies on visual input from occipital areas.

Lost in dissociation: The main paradigms in unconscious cognition

Contemporary studies in unconscious cognition are essentially founded on dissociation, i.e., on how it dissociates with respect to conscious mental processes and representations. This is claimed to be in so many and diverse ways that one is often lost in dissociation. In order to reduce this state of confusion we here carry out two major tasks: based on the central distinction between cognitive processes and representations, we identify and isolate the main dissociation paradigms; we then critically analyze their key tenets and reported findings.

Audiovisual Association Learning in the Absence of Primary Visual Cortex

Learning audiovisual associations is mediated by the primary cortical areas; however, recent animal studies suggest that such learning can take place even in the absence of the primary visual cortex. Other studies have demonstrated the involvement of extra-geniculate pathways and especially the superior colliculus (SC) in audiovisual association learning. Here, we investigated such learning in a rare human patient with complete loss of the bilateral striate cortex. We carried out an implicit audiovisual association learning task with two different colors of red and purple (the latter color known to minimally activate the extra-genicular pathway). Interestingly, the patient learned the association between an auditory cue and a visual stimulus only when the unseen visual stimulus was red, but not when it was purple. The current study presents the first evidence showing the possibility of audiovisual association learning in humans with lesioned striate cortex. Furthermore, in line with animal studies, it supports an important role for the SC in audiovisual associative learning.

Looming sensitive cortical regions without V1 input: evidence from a patient with bilateral cortical blindness

Fast and automatic behavioral responses are required to avoid collision with an approaching stimulus. Accordingly, looming stimuli have been found to be highly salient and efficient attractors of attention due to the implication of potential collision and potential threat. Here, we address the question of whether looming motion is processed in the absence of any functional primary visual cortex and consequently without awareness. For this, we investigated a patient (TN) suffering from complete, bilateral damage to his primary visual cortex. Using an fMRI paradigm, we measured TN's brain activation during the presentation of looming, receding, rotating, and static point lights, of which he was unaware. When contrasted with other conditions, looming was found to produce bilateral activation of the middle temporal areas, as well as the superior temporal sulcus and inferior parietal lobe (IPL). The latter are generally thought to be involved in multisensory processing of motion in extrapersonal space, as well as attentional capture and saliency. No activity was found close to the lesioned V1 area. This demonstrates that looming motion is processed in the absence of awareness through direct subcortical projections to areas involved in multisensory processing of motion and saliency that bypass V1.

Human blindsight is mediated by an intact geniculo-extrastriate pathway

Although damage to the primary visual cortex (V1) causes hemianopia, many patients retain some residual vision; known as blindsight. We show that blindsight may be facilitated by an intact white-matter pathway between the lateral geniculate nucleus and motion area hMT+. Visual psychophysics, diffusion-weighted magnetic resonance imaging and fibre tractography were applied in 17 patients with V1 damage acquired during adulthood and 9 age-matched controls. Individuals with V1 damage were subdivided into blindsight positive (preserved residual vision) and negative (no residual vision) according to psychophysical performance. All blindsight positive individuals showed intact geniculo-hMT+ pathways, while this pathway was significantly impaired or not measurable in blindsight negative individuals. Two white matter pathways previously implicated in blindsight: (i) superior colliculus to hMT+ and (ii) between hMT+ in each hemisphere were not consistently present in blindsight positive cases. Understanding the visual pathways crucial for residual vision may direct future rehabilitation strategies for hemianopia patients.

DOI:http://dx.doi.org/10.7554/eLife.08935.001

Visual information from our eyes projects to a region at the back of the brain called the primary visual cortex, which is where the information is processed to allow us to see the world around us. If a person suffers a stroke that affects this primary visual cortex, he or she can become blind on one side. However, some people can still detect images within this ‘blind’ area, even if they are not consciously aware of it. This phenomenon is known as ‘blindsight’, but it remains unclear which pathways and structures in the brain might allow this information to be detected.

Ajina et al. have now examined the brains of a large group of patients with damage to the visual cortex. The results for the patients with blindsight were compared to those without, and to a group of sighted control participants. This analysis identified a pathway that seems to underlie blindsight. This pathway (which runs between an area of the brain called the lateral geniculate nucleus and another called the motion area hMT+) was present in all patients with blindsight, but was missing or disrupted in those patients without blindsight.

Ajina et al. then examined other pathways that had previously been suggested to support blindsight and revealed that they were unlikely to do so. This is because the suggested connections were not identifiable in all patients with blindsight, and were often intact in those patients without blindsight.

So far, this work has addressed the structure of the pathways rather than their activity. Future work will attempt to determine whether it is possible to strengthen such pathways to improve visual ability.

DOI:http://dx.doi.org/10.7554/eLife.08935.002

To see or not to see--thalamo-cortical networks during blindsight and perceptual suppression

Even during moments when we fail to be fully aware of our environment, our brains never go silent. Instead, it appears that the brain can also operate in an alternate, unconscious mode. Delineating unconscious from conscious neural processes is a promising first step toward investigating how awareness emerges from brain activity. Here we focus on recent insights into the neuronal processes that contribute to visual function in the absence of a conscious visual percept. Drawing on insights from findings on the phenomenon of blindsight that results from injury to primary visual cortex and the results of experimentally induced perceptual suppression, we describe what kind of visual information the visual system analyzes unconsciously and we discuss the neuronal routing and responses that accompany this process. We conclude that unconscious processing of certain visual stimulus attributes, such as the presence of visual motion or the emotional expression of a face can occur in a geniculo-cortical circuit that runs independent from and in parallel to the predominant route through primary visual cortex. We speculate that in contrast, bidirectional neuronal interactions between cortex and the thalamic pulvinar nucleus that support large-scale neuronal integration and visual awareness are impeded during blindsight and perceptual suppression.

Rehabilitation of homonymous hemianopia: insight into blindsight

Strong evidence of considerable plasticity in primary sensory areas in the adult cortex, and of dramatic cross-modal reorganization in visual areas, after short- or long-term visual deprivation has recently been reported. In the context of patient rehabilitation, this scientifically challenging topic takes on urgent clinical relevance, especially given the lack of information about the role of such reorganization on spared or newly emerged visual performance. Amongst the most common visual field defects found upon unilateral occipital damage of the primary visual cortex is homonymous hemianopia (HH), a perfectly symmetric loss of vision in both eyes. Traditionally, geniculostriate lesions were considered to result in complete and permanent visual loss in the topographically related area of the visual field (Huber, 1992). However, numerous studies in monkeys, and later, in humans, have demonstrated that despite destruction of the striate cortex, or even following a hemispherectomy, some patients retain a certain degree of unconscious visual function, known as blindsight. Accordingly, there have recently been attempts to restore visual function in patients by stimulating unconscious preserved blindsight capacities. Herein we review different visual rehabilitation techniques designed for brain-damaged patients with visual field loss. We discuss the hypothesis that explicit (conscious) visual detection can be restored in the blind visual field by harnessing implicit (unconscious) visual capacities. The results that we summarize here underline the need for early diagnosis of cortical visual impairment (CVI), and the urgency in rehabilitating such deficits, in these patients. Based on the research precedent, we explore the link between implicit (unconscious) vision and conscious perception and discuss possible mechanisms of adaptation and plasticity in the visual cortex.

Visualizing the blind brain: brain imaging of visual field defects from early recovery to rehabilitation techniques

Visual field defects (VFDs) are one of the most common consequences observed after brain injury, especially after a stroke in the posterior cerebral artery territory. Less frequently, tumors, traumatic brain injury, brain surgery or demyelination can also determine various visual disabilities, from a decrease in visual acuity to cerebral blindness. Visual field defects is a factor of bad functional prognosis as it compromises many daily life activities (e.g., obstacle avoidance, driving, and reading) and therefore the patient's quality of life. Spontaneous recovery seems to be limited and restricted to the first 6 months, with the best chance of improvement at 1 month. The possible mechanisms at work could be partly due to cortical reorganization in the visual areas (plasticity) and/or partly to the use of intact alternative visual routes, first identified in animal studies and possibly underlying the phenomenon of blindsight. Despite processes of early recovery, which is rarely complete, and learning of compensatory strategies, the patient's autonomy may still be compromised at more chronic stages. Therefore, various rehabilitation therapies based on neuroanatomical knowledge have been developed to improve VFDs. These use eye-movement training techniques (e.g., visual search, saccadic eye movements), reading training, visual field restitution (the Vision Restoration Therapy, VRT), or perceptual learning. In this review, we will focus on studies of human adults with acquired VFDs, which have used different imaging techniques (Positron Emission Tomography, PET; Diffusion Tensor Imaging, DTI; functional Magnetic Resonance Imaging, fMRI; Magneto Encephalography, MEG) or neurostimulation techniques (Transcranial Magnetic Stimulation, TMS; transcranial Direct Current Stimulation, tDCS) to show brain activations in the course of spontaneous recovery or after specific rehabilitation techniques.

Speeded manual responses to unseen visual stimuli in hemianopic patients: what kind of blindsight?

Blindsight, i.e., unconscious visually guided behaviour triggered by stimuli presented to a cortically blind hemifield, has been typically found either by using direct (forced choice) or indirect (interhemispheric) methods. However, one would expect to find blindsight also in fast responses to suddenly appearing visual stimuli, a reminiscence of evolutionary ancient adaptive behaviour. In this study we provide preliminary evidence of this form of blindsight by using a conservative method for assessing blindsight based on a comparison between the cumulative probability functions (CPFs) of simple reaction times to blind and intact field stimuli. Furthermore, in two patients with blindsight we provided evidence that their above-chance unconscious responses were likely to be triggered by the intact hemisphere.

On the use of continuous flash suppression for the study of visual processing outside of awareness

The interocular suppression technique termed continuous flash suppression (CFS) has become an immensely popular tool for investigating visual processing outside of awareness. The emerging picture from studies using CFS is that extensive processing of a visual stimulus, including its semantic and affective content, occurs despite suppression from awareness of that stimulus by CFS. However, the current implementation of CFS in many studies examining processing outside of awareness has several drawbacks that may be improved upon for future studies using CFS. In this paper, we address some of those shortcomings, particularly ones that affect the assessment of unawareness during CFS, and ones to do with the use of "visible" conditions that are often included as a comparison to a CFS condition. We also discuss potential biases in stimulus processing as a result of spatial attention and feature-selective suppression. We suggest practical guidelines that minimize the effects of those limitations in using CFS to study visual processing outside of awareness.

The coupling of vision with locomotion in cortical blindness

Maintaining or modifying the speed and direction of locomotion requires the coupling of the locomotion with the retinal optic flow that it generates. It is shown that this essential behavioral capability, which requires on-line neural control, is preserved in the cortically blind hemifield of a hemianope. In experiments, optic flow stimuli were presented to either the normal or blind hemifield while the patient was walking on a treadmill. Little difference was found between the hemifields with respect to the coupling (i.e. co-dependency) of optic flow detection with locomotion. Even in the cortically blind hemifield, faster walking resulted in the perceptual slowing of detected optic flow, and self-selected locomotion speeds demonstrated behavioral discrimination between different optic flow speeds. The results indicate that the processing of optic flow, and thereby on-line visuo-locomotor coupling, can take place along neural pathways that function without processing in Area V1, and thus in the absence of conscious intervention. These and earlier findings suggest that optic flow and object motion are processed in parallel along with correlated non-visual locomotion signals. Extrastriate interactions may be responsible for discounting the optical effects of locomotion on the perceived direction of object motion, and maintaining visually guided self-motion.

Lack of automatic attentional orienting by gaze cues following a bilateral loss of visual cortex

In social interactions, the location of relevant stimuli is often indicated by the orientation of gaze. It has been proposed that the direction of gaze might produce an automatic cueing of attention, similar to what is observed with exogenous cues. However, several reports have challenged this claim by demonstrating that the behavioral gain that arises with gaze cueing could be explained by shifts of attention, which are intentional and not automatic. We reasoned that if cueing by gaze was truly automatic, it should occur without awareness and should be sustained by subcortical circuits, including the amygdalae, independently of the main geniculo-striate visual pathway. We presented a cross-modal version of the Posner cueing paradigm to a patient (TN) with bilateral lesions of occipital cortex (Burra et al., 2013; Pegna, Khateb, Lazeyras, & Seghier, 2005). TN was asked to localize a sound using a key press. The location of the sound was congruent or incongruent with the direction of gaze of a face-cue. In groups of healthy young and age-matched participants, we observed significantly longer response times for incongruent than congruent sounds, suggesting that gaze direction interfered with processing of localized sounds. By contrast, TN׳s performance was not affected by sound-gaze congruence. The results suggest that the processing of gaze orientation cannot occur in the absence of geniculo-striate processing, suggesting that it is not automatic.