From Cortical Blindness to Conscious Visual Perception: Theories on Neuronal Networks and Visual Training Strategies

Anton-Babinski Syndrome: A Visual Anosognosia

Anton-Babinski syndrome is a rare extension of cortical blindness following injury to the occipital lobe. The patient behaves as sighted but has visual function derangements. The posterior cerebral artery (PCA) stroke represents 5% to 10% of total strokes. The COVID-19 pandemic has shown a rise in stroke cases. We present a case of this rare PCA stroke, first diagnosed by an ophthalmologist. This case had an inconsistent initial presentation, but subsequent computed tomography of the brain and other neurological investigations confirmed the diagnosis. If such cases are diagnosed early, they could have better management. Timely intervention can decrease morbidity as well as mortality.

The nature of blindsight: implications for current theories of consciousness

Blindsight regroups the different manifestations of preserved discriminatory visual capacities following the damage to the primary visual cortex. Blindsight types differentially impact objective and subjective perception, patients can report having no visual awareness whilst their behaviour suggests visual processing still occurs at some cortical level. This phenomenon hence presents a unique opportunity to study consciousness and perceptual consciousness, and for this reason, it has had an historical importance for the development of this field of research. From these studies, two main opposing models of the underlying mechanisms have been established: (a) blindsight is perception without consciousness or (b) blindsight is in fact degraded vision, two views that mirror more general theoretical options about whether unconscious cognition truly exists or whether it is only a degraded form of conscious processing. In this article, we want to re-examine this debate in the light of recent advances in the characterization of blindsight and associated phenomena. We first provide an in-depth definition of blindsight and its subtypes, mainly blindsight type I, blindsight type II and the more recently described blindsense. We emphasize the necessity of sensitive and robust methodology to uncover the dissociations between perception and awareness that can be observed in brain-damaged patients with visual field defects at different cognitive levels. We discuss these different profiles of dissociation in the light of both contending models. We propose that the different types of dissociations reveal a pattern of relationship between perception, awareness and metacognition that is actually richer than what is proposed by either of the existing models. Finally, we consider this in the framework of current theories of consciousness and touch on the implications the findings of blindsight have on these.

Good Visual Outcomes After Pituitary Tumor Surgery Are Associated With Increased Visual Cortex Functional Connectivity

BACKGROUND

Patients presenting with visual impairment secondary to pituitary macroadenomas often experience variable recovery after surgery. Several factors may impact visual outcomes including the extent of neuroaxonal damage in the afferent visual pathway and cortical plasticity. Optical coherence tomography (OCT) measures of retinal structure and resting-state functional MRI (rsfMRI) can be used to evaluate the impact of neuroaxonal injury and cortical adaptive processes, respectively. The purpose of this study was to determine whether rsfMRI patterns of functional connectivity (FC) distinguish patients with good vs poor visual outcomes after surgical decompression of pituitary adenomas.

METHODS

In this retrospective cohort study, we compared FC patterns between patients who manifested good (GO) vs poor (PO) visual outcomes after pituitary tumor surgery. Patients (n = 21) underwent postoperative rsfMRI a minimum of 1 year after tumor surgery. Seed-based connectivity of the visual cortex (primary [V1], prestriate [V2], and extrastriate [V5]) was compared between GO and PO patients and between patients and healthy controls (HCs) (n = 19). Demographics, visual function, and OCT data were compared preoperatively and postoperatively between patient groups. The threshold for GO was visual field mean deviation equal or less than -5.00 dB and/or visual acuity equal to or better than 20/40.

RESULTS

Increased postoperative FC of the visual system was noted for GO relative to PO patients. Specifically, good visual outcomes were associated with increased connectivity of right V5 to the bilateral frontal cortices. Compared with HCs, GO patients showed increased connectivity of V1 and left V2 to sensorimotor cortex, increased connectivity of right and left V2 to medial prefrontal cortex, and increased connectivity of right V5 the right temporal and frontal cortices.

CONCLUSIONS

Increased visual cortex connectivity is associated with good visual outcomes in patients with pituitary tumor, at late phase of recovery. Our findings suggest that rsfMRI does distinguish GO and PO patients after pituitary tumor surgery. This imaging modality may have a future role in characterizing the impact of cortical adaptation on visual recovery.

Association Cortex Is Essential to Reverse Hemianopia by Multisensory Training

Hemianopia induced by unilateral visual cortex lesions can be resolved by repeatedly exposing the blinded hemifield to auditory-visual stimuli. This rehabilitative "training" paradigm depends on mechanisms of multisensory plasticity that restore the lost visual responsiveness of multisensory neurons in the ipsilesional superior colliculus (SC) so that they can once again support vision in the blinded hemifield. These changes are thought to operate via the convergent visual and auditory signals relayed to the SC from association cortex (the anterior ectosylvian sulcus [AES], in cat). The present study tested this assumption by cryogenically deactivating ipsilesional AES in hemianopic, anesthetized cats during weekly multisensory training sessions. No signs of visual recovery were evident in this condition, even after providing animals with up to twice the number of training sessions required for effective rehabilitation. Subsequent training under the same conditions, but with AES active, reversed the hemianopia within the normal timeframe. These results indicate that the corticotectal circuit that is normally engaged in SC multisensory plasticity has to be operational for the brain to use visual-auditory experience to resolve hemianopia.

Where is my arm? Investigating the link between complex regional pain syndrome and poor localisation of the affected limb

Background

Anecdotally, people living with Complex Regional Pain Syndrome (CRPS) often report difficulties in localising their own affected limb when it is out of view. Experimental attempts to investigate this report have used explicit tasks and yielded varied results.

Methods

Here we used a limb localisation task that interrogates implicit mechanisms because we first induce a compelling illusion called the Disappearing Hand Trick (DHT). In the DHT, participants judge their hands to be close together when, in fact, they are far apart. Sixteen volunteers with unilateral upper limb CRPS (mean age 39 ± 12 years, four males), 15 volunteers with non-CRPS persistent hand pain (‘pain controls’; mean age 58 ± 13 years, two males) and 29 pain-free volunteers (‘pain-free controls’; mean age 36 ± 19 years, 10 males) performed a hand-localisation task after each of three conditions: the DHT illusion and two control conditions in which no illusion was performed. The conditions were repeated twice (one for each hand). We hypothesised that (1) participants with CRPS would perform worse at hand self-localisation than both the control samples; (2) participants with non-CRPS persistent hand pain would perform worse than pain-free controls; (3) participants in both persistent pain groups would perform worse with their affected hand than with their unaffected hand.

Results

Our first two hypotheses were not supported. Our third hypothesis was supported —when visually and proprioceptively encoded positions of the hands were incongruent (i.e. after the DHT), relocalisation performance was worse with the affected hand than it was with the unaffected hand. The similar results in hand localisation in the control and pain groups might suggest that, when implicit processes are required, people with CRPS’ ability to localise their limb is preserved.

Visual Field Test With Gaze Check Tasks: Application in a Homonymous Hemianopic Patient Unaware of the Visual Defects

Gaze control is required for applying visual stimuli to a particular area of the visual field. We developed a visual field test with gaze check tasks to investigate hemianopia. In this test, participants must report the presence or absence of visual stimuli when a small object at the fixation point vibrates. Trials in the absence of visual stimuli were used as gaze check tasks, since the vibration could be observed only when the gaze was directed at the fixation point. We evaluated the efficacy of our test in four control participants and one patient with homonymous hemianopia who was unaware of the defects in the left visual field. This patient presented hemianopia in the test with gaze check tasks, but not when the gaze check tasks were omitted. The patient showed spontaneous gaze movements from the fixation point to the upper left direction, as well as scanning of the left visual field during the test without gaze check tasks. Thus, we concluded that the visual defects in this patient were compensated in daily life by spontaneous eye movements coordinated with visual information processing. The present results show the usefulness of the visual field test with gaze check tasks.

Functional MRI of visual cortex predicts training-induced recovery in stroke patients with homonymous visual field defects

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Damage to the visual brain typically leads to vision loss.

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Vision loss may be partially recovered with visual restitution training (VRT)

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Cortical responses to visual stimulation do not always lead to visual awareness.

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A mismatch between Humphrey and neural perimetry predicts training outcome.

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This finding has important implications for better rehabilitation strategies.

Post-chiasmatic damage to the visual system leads to homonymous visual field defects (HVDs), which can severely interfere with daily life activities. Visual Restitution Training (VRT) can recover parts of the affected visual field in patients with chronic HVDs, but training outcome is variable. An untested hypothesis suggests that training potential may be largest in regions with ‘neural reserve’, where cortical responses to visual stimulation do not lead to visual awareness as assessed by Humphrey perimetry—a standard behavioural visual field test. Here, we tested this hypothesis in a sample of twenty-seven hemianopic stroke patients, who participated in an assiduous 80-hour VRT program. For each patient, we collected Humphrey perimetry and wide-field fMRI-based retinotopic mapping data prior to training. In addition, we used Goal Attainment Scaling to assess whether personal activities in daily living improved. After training, we assessed with a second Humphrey perimetry measurement whether the visual field was improved and evaluated which personal goals were attained. Confirming the hypothesis, we found significantly larger improvements of visual sensitivity at field locations with neural reserve. These visual field improvements implicated both regions in primary visual cortex and higher order visual areas. In addition, improvement in daily life activities correlated with the extent of visual field enlargement. Our findings are an important step toward understanding the mechanisms of visual restitution as well as predicting training efficacy in stroke patients with chronic hemianopia.

Rapid Recovery From Cortical Blindness Caused by an Old Cerebral Infarction

When the primary visual cortex (V1) is damaged, cortical blindness results. However, visual information obtained from the superior colliculus (SC) or direct thalamic afferents to higher visual cortices produces unconscious visual functions called blindsight. Alarming visual stimuli suggesting the approach of a predator are known to trigger escape behaviors via visual information mediated by the SC and amygdala in small animals, and salient and dynamic visual stimuli also produce some conscious visual experience even in patients with blindsight. Fresh cortical blindness sometimes recovers spontaneously in patients with fresh cerebral damages, and recovery can be accelerated by early rehabilitation. However, the mechanisms underlying recovery are not well-known. We analyzed a patient with cortical blindness caused by an old cerebral infarction. After repeated presentation of alarming visual stimuli, the ability to detect visual stimuli in the impaired visual field showed behavioral short-term improvement (STI) within a few minutes. Repeated behavioral STI induction was followed by behavioral long-term improvement (LTI) lasting more than several days. After behavioral LTI, the patient partially recovered the ability to read letters presented in the impaired visual field. The behavioral STI experiment, which can be performed within 10 min, may serve as a clinical screening test for anticipating recovery from cortical blindness.

Dissociations between perception and awareness in hemianopia

The most common visual defect to follow a lesion of the retrochiasmal pathways is homonymous hemianopia (HH), whereby patients are blind to the contralesional visual field of each eye. Homonymous hemianopia has been studied in terms of its deleterious consequences on perceptual, cognitive and motor tasks as well as because it represents an interesting model of vision loss after a unilateral lesion of the occipital lobe. From a behavioral perspective, in addition to exhibiting a severe deficit in their contralesional visual field, HH patients can also exhibit dissociations between perception and awareness. Firstly, HH patients suffering from anosognosia may be unaware of their visual field defect. Secondly, HH patients can present with unconscious visual abilities in the blind hemifield, a phenomenon referred to as blindsight. Thirdly, recent reports demonstrate that HH patients can suffer from a subtle deficit in their ipsilesional visual field that they are unaware of, a condition called sightblindness (i.e. the reverse case of 'blindsight'). Finally, HH patients may also exhibit visual hallucinations in their blind field; however, such patients are not systematically aware that their perceptions are unreal. In this review, we provide an overview of the visual-field losses in HH patients after a left or right unilateral occipital lesion. Furthermore, we explore the implications of these four phenomena for models of visual processing and rehabilitation of visual field defects in HH patients. Finally, in contrast to the traditional view that HH is solely a visual-field defect, we discuss why this deficit is an interesting model for studying the dissociation between perception and awareness.

Barbosa RT, Valenti VE, Oliveira FR, Roque AL, Júnior HMFES, Garner DM, Raimundo RD, de Abreu LC. The effects of cataract surgery on autonomic heart rate control: a prospective cross-sectional and analytical study

OBJECTIVES

We aimed to evaluate the effects of cataract surgery on cardiac autonomic modulation.

METHODS

A cross-sectional and analytical study was conducted at the Hospital Maria Braido in the city of São Caetano do Sul, São Paulo, between 2015 and 2016. We investigated 19 patients of both sexes who were all over 50 years old; all patients had a diagnosis of senile or bilateral cataracts and were recommended to undergo implantation of the intraocular lens. Heart rate variability (HRV) was evaluated before, during and after cataract surgery.

RESULTS

There were no significant changes in the time and geometric domains of HRV before, during or after surgery. The high-frequency (HF) band in normalized units (nu) on the spectral analysis significantly increased (p=0.02, Cohen's d=0.9, large effect size). However, the low-frequency (LF) band in nu significantly decreased during surgery (p=0.02, Cohen's d=0.9, large effect size).

CONCLUSION

Throughout the intraocular lens implantation cataract surgery, there was an increase in parasympathetic modulation and a decrease in the sympathetic component of the heart rate (HR). We propose that this result is attributable to the supine position of the patients during surgery and the trigeminal reflex.

Dissociation between objective and subjective perceptual experiences in a population of hemianopic patients: A new form of blindsight?

After a post-chiasmatic lesion, some patients may retain unconscious visual function, known as blindsight, in their contralesional visual field. Despite the importance of blindsight in the study of consciousness, little is known about the nature of patients' experience in their hemianopic field. To address this knowledge gap, we measured blindsight, and assessed the perceptual experience in the contralesional visual field, of seventeen homonymous hemianopic (HH) patients. To ensure that the stimuli were shown in a "blind" sector of the visual field, we selected a subgroup of eight complete-HH patients, as determined by automatic perimetry. Firstly, we measured blindsight through a forced-choice task in which the patients had to identify letters displayed on a screen. Secondly, we compared the patients' binary responses ("Something was presented" vs "Nothing was presented") to responses on a new, five-level scale, the Sensation Awareness Scale (SAS), which we designed to include visual as well as non-visual answers (e.g., "I felt something"). Interestingly, only one of the eight complete-HH patients met the criteria for blindsight. More importantly, our SAS enabled us to identify a previously unreported dissociation, which we have named blindsense, in four of the eight complete-HH patients. Specifically, these four patients exhibited better-than-chance sensitivity to the presence of a stimulus on the subjective scale, despite being unable to identify the stimulus during the forced-choice task. Our findings highlight the importance of awareness-assessment methods to investigate perceptual experiences in the contralesional visual field and suggest a low incidence of blindsight in post-stroke HH patients.

Residual vision activation and the brain-eye-vascular triad: Dysregulation, plasticity and restoration in low vision and blindness - a review

Vision loss due to ocular diseases such as glaucoma, optic neuropathy, macular degeneration, or diabetic retinopathy, are generally considered an exclusive affair of the retina and/or optic nerve. However, the brain, through multiple indirect influences, has also a major impact on functional visual impairment. Such indirect influences include intracerebral pressure, eye movements, top-down modulation (attention, cognition), and emotionally triggered stress hormone release affecting blood vessel dysregulation. Therefore, vision loss should be viewed as the result of multiple interactions within a “brain-eye-vascular triad”, and several eye diseases may also be considered as brain diseases in disguise. While the brain is part of the problem, it can also be part of the solution. Neuronal networks of the brain can “amplify” residual vision through neuroplasticity changes of local and global functional connectivity by activating, modulating and strengthening residual visual signals. The activation of residual vision can be achieved by different means such as vision restoration training, non-invasive brain stimulation, or blood flow enhancing medications. Modulating brain functional networks and improving vascular regulation may offer new opportunities to recover or restore low vision by increasing visual field size, visual acuity and overall functional vision. Hence, neuroscience offers new insights to better understand vision loss, and modulating brain and vascular function is a promising source for new opportunities to activate residual vision to achieve restoration and recovery to improve quality of live in patients suffering from low vision.

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.

Pulsed Stimuli Elicit More Robust Multisensory Enhancement than Expected

Neurons in the superior colliculus (SC) integrate cross-modal inputs to generate responses that are more robust than to either input alone, and are frequently greater than their sum (superadditive enhancement). Previously, the principles of a real-time multisensory transform were identified and used to accurately predict a neuron's responses to combinations of brief flashes and noise bursts. However, environmental stimuli frequently have more complex temporal structures that elicit very different response dynamics than previously examined. The present study tested whether such stimuli (i.e., pulsed) would be treated similarly by the multisensory transform. Pulsing visual and auditory stimuli elicited responses composed of higher discharge rates that had multiple peaks temporally aligned to the stimulus pulses. Combinations pulsed cues elicited multiple peaks of superadditive enhancement within the response window. Measured over the entire response, this resulted in larger enhancements than expected given enhancements elicited by non-pulsed (“sustained”) stimuli. However, as with sustained stimuli, the dynamics of multisensory responses to pulsed stimuli were highly related to the temporal dynamics of the unisensory inputs. This suggests that the specific characteristics of the multisensory transform are not determined by the external features of the cross-modal stimulus configuration; rather the temporal structure and alignment of the unisensory inputs is the dominant driving factor in the magnitudes of the multisensory product.