The influence of "blind" distractors on eye movements trajectories in visual hemifield defects.

There is evidence that some visual information in blind regions may still be processed in patients with hemifield defects after cerebral lesions ("blindsight"). We tested the hypothesis that, in the absence of retinogeniculostriate processing, residual retinotectal processing may still be detected as modifications of saccades to seen targets by irrelevant distractors in the blind hemifield. Six patients were presented with distractors in the blind and intact portions of their visual field and participants were instructed to make eye movements to targets in the intact field. Eye movements were recorded to determine if blind-field distractors caused deviation in saccadic trajectories. No deviation was found in one patient with an optic chiasm lesion, which affect both retinotectal and retinogeniculostriate pathways. In five patients with lesions of the optic radiations or the striate cortex, the results were mixed, with two of the five patients showing significant deviations of saccadic trajectory away from the "blind" distractor. In a second experiment, two of the five patients were tested with the target and the distractor more closely aligned. Both patients showed a "global effect," in that saccades deviated toward the distractor, but the effect was stronger in the patient who also showed significant trajectory deviation in the first experiment. Although our study confirms that distractor effects on saccadic trajectory can occur in patients with damage to the retinogeniculostriate visual pathway but preserved retinotectal projections, there remain questions regarding what additional factors are required for these effects to manifest themselves in a given patient. © 2008 Massachusetts Institute of Technology

Eye cannot see it: The interference of subliminal distractors on saccade metrics

AbstractThe present study investigated whether subliminal (unconsciously perceived) visual information influences eye movement metrics, like saccade trajectories and endpoints. Participants made eye movements upwards and downwards while a subliminal distractor was presented in the periphery. Results showed that the subliminal distractor interfered with the execution of an eye movement, although the effects were smaller compared to a control experiment in which the distractor was presented supraliminal. Because saccade metrics are mediated by low level brain areas, this indicates that subliminal visual information evokes competition at a very low level in the oculomotor system

The global effect: what determines where the eyes land?

In certain situations, the endpoint of an eye movement is not positioned on the centre of a target element, but deviates in the direction of another element. This phenomenon has been termed 'the global effect' and has proven to constitute a valuable measure of various processes that control and influence our oculomotor behavior. The goal of the current review is to provide insight in the factors that determine where the eyes land. We will focus on the fundamental characteristics of the global effect and discuss the various domains in which the global effect has been applied. The global effect appears to be best explained in terms of a weighted average of activity in a saccade map

Post-lesional functionality of the visual system in hemianopic patients

Hemianopic patients suffer for a loss of conscious vision in part of the visual field. The present work aimed to investigate the functionality of the visual system after lesions to visual cortices, by studying the spontaneous electrophysiological activity and the residual visual processing. The first three studies revealed the presence of alterations in the spontaneous alpha oscillatory activity during resting-state. Specifically, hemianopic patients showed a slowdown of the speed of alpha oscillations and a reduction of the amplitude of alpha activity in the lesioned hemisphere, resulting in an interhemispheric imbalance of the activity in the alpha range. Moreover, hemianopics showed also a reduction of alpha functional connectivity in the posterior regions of the lesioned hemisphere. However, the residual activity in the alpha range seemed functionally reactive, since hemianopics showed the typical alpha desynchronization in the transition from the eyes-closed to the eyes-open resting-state. More importantly, the spontaneous alpha activity predicted the visuospatial performance, suggesting that the resting-state activity in the alpha range, might be a biomarker for the functionality of the visual system. Notably, oscillatory patterns were more severely impaired in hemianopics with right lesions, suggesting a central role of the right posterior cortices in coordinating the spontaneous oscillatory activity. In the last study, unseen distractors presented in the blind visual field were able to interfere with the execution of saccades toward seen targets presented in the intact field, suggesting the presence of an implicit visual processing for stimuli presented in the blind visual field. However, only left-lesioned hemianopic patients showed implicit processing for the unseen distractors, suggesting that the right hemisphere might also contribute to this interference effect. Overall, the post-lesional oscillatory patterns and the implicit visual processing in the absence of awareness seem to reflect an impaired but residual functionality of the visual system in hemianopic patients

Virtual dissection of subcortical non-image forming visual pathways in the human brain with DTI tractography

This thesis presents novel evidence in support of a functional role for the anatomy of subcortical non-image forming white matter connections in the human brain. Observations of preserved ability to process threat in the absence of visual awareness in cortically blind patients suggest the existence of an older, primitive and phylogenetically conserved visual circuit that processes orientation to threat. Three pathways including the retinotectal tract, the stria terminalis and a putative pathway for processing visual threat connecting the superior colliculus and the amygdala via the pulvinar were virtually dissected with DTI tractography in healthy participants in vivo. Additionally, connectivity strength of dissected pathways were tested as predictors of performance on behavioural eye-tracking paradigms designed to optimise reflexive orienting responses to salient peripheral visual stimuli including abrupt onsets and threat. Behavioural findings demonstrated a temporal hemifield advantage for short latency reflexive saccades performed within a ‘gap’ paradigm, supporting the prediction that the superior colliculus receives retinal afferents via the retinotectal pathway that summon reflexive saccades. Saccadic bias to orient to threatening stimuli on a temporal order saccade decision task also demonstrated an advantage in the temporal hemifield, consistent with predictions that retinal afferents to the superior colliculus contribute to a subcortical threat mediating circuit connecting the superior colliculus, pulvinar and amygdala. Connectivity strength of anatomical demonstrations of a putative threat mediating connection between the superior colliculus, pulvinar and amygdala predicted a bias to orient to visual threat, thereby validating anatomical dissections presented here with functional evidence. Topography of the SC-amygdala pathway demonstrated a trajectory from the pulvinar to the lateral amygdala. The anatomy of the outflow of the amygdala connecting to the bed nucleus of the stria terminalis was also considered, demonstrating anatomically distinct trajectories of the two pathways despite occupying topographies in close proximity. Stria terminalis connectivity strength also predicted a bias to threat, providing novel evidence for a functional role in selective orienting to threat, potentially driven by superior-colliculus afferents to amygdala nuclei

Eye movements in hemianopia and the rehabilitation of hemianopic dyslexia

This thesis is a study of the nature and rehabilitation of the functional impairments in unilateral homonymous hemianopia (HH), with a major focus on hemianopic dyslexia. The reading, visual exploration and line bisection impairments associated with homonymous visual field loss are frequent and well-established clinical phenomena. Yet, it is still unknown whether the reading and visual exploration impairments are caused by the visual field defect or by additional extrastriate injury preventing efficient spontaneous oculomotor adaptation. It is also unclear whether the line bisection impairment directly arises from the visual field defect or its adaptive oculomotor consequences, or whether it indicates an associated visual-spatial deficit that is caused by injury to regions involved in visual-spatial perception (Introduction). Based on a critical review of research into hemianopic dyslexia since its original description in 1881, it is suggested that the visual field defect is a major component of hemianopic dyslexia but possibly not its sole cause (Chapter 1). This assumption was confirmed in six experiments whose purpose was to establish the extent to which the reading, visual exploration and line bisection impairments associated with HH are purely visually elicited. To study the behavioural changes associated with the visual field defect that are not caused by brain injury, a gaze-contingent display paradigm was used to simulate HH in healthy participants. Simulated HH induced the reading and visual exploration impairments of hemianopic patients. However, all participants showed efficient spontaneous oculomotor adaptation to simulated HH which was associated with highly specific and task-dependent improvements in reading and visual exploration (Chapters 2 and 3). Moreover, simulated HH did not induce the main feature of the hemianopic line bisection impairment, i.e., the contralateral line bisection error, albeit it nevertheless impaired line bisection performance (Chapter 4). The final study investigated the basis and specificity of the therapeutic effect of an efficient compensatory oculomotor treatment method for hemianopic dyslexia in patients with unilateral homonymous visual field loss. The results demonstrate that using text-material and, thus, lexical-semantic processes, is not critical to the treatment effect, which was also found to be specific to reading (Chapter 5). The concluding chapter reviews the main findings and suggests that the functional impairments associated with visual field loss may not simply be failures of vision. Although the hemianopic visual field defect is a major component of hemianopic dyslexia and possibly contributes to the visual exploration and line bisection impairments, additional injury to specific extrastriate regions seems to be the critical causative factor. The implications for understanding, assessing and rehabilitating functional impairments in homonymous visual field disorders are discussed. The important future research directions arising from this thesis are also identified and presented (Conclusion)

Saccades in the absence of binocular vision

The mechanism of suppression in strabismus is unclear and contribution of the suppressing eye to the generation of eye movements has received little attention. A series of nine experiments tested how the strabismic eye contributes to saccade generation in the presence of suppression and also considered the effect of the strabismic eye in the presence of abnormal retinal correspondence (ARC). These data were compared with data from subjects with normal binocular single vision (BSV). Chapters 2 and 3 describe the equipment, laboratory set-up and testing of the equipment used in the thesis for measuring eye movements, Skalar IRIS 6500 infrared limbal tracker, and presenting stimuli to each eye separately. The design of a novel method for dissociation of the eyes using four liquid crystal polymer shutters is presented. Chapter 4 compares the characteristics of saccades made by subjects with normal BSV (n=5) and strabismus (n=8). The effect of distractors on saccades is explored in Chapter 5 in subjects with normal BSV (n=5). The experiment documents the distractor effect produced in the described laboratory set-up, and compares it with that previously reported by Walker et al (1997). This is investigated further by comparing the effect of distractor presentations to the dominant eye, non-dominant eye or both eyes. There was no difference in the effect on saccade latency or gain with distractors presented to the dominant or non-dominant eye. The effect of binocular distractors on saccade gain was greater than monocular presentations. Chapter 6 repeats the experiment of Chapter 5 in subjects with constant strabismus and suppression (n=6) and constant strabismus with ARC (n=2) and found that distractors in the strabismic eye did affect saccades however the response differed from normal BSV. This was true even though it was shown that the distractor was not perceived by the strabismic eye. Chapter 7 investigates the influence of the central fixation target in the strabismic eye on saccade generation by inducing disconjugate saccade adaptation in subjects with normal BSV (n=8) and constant strabismus and suppression (n=6). The findings were that in the presence of suppression, disconjugate adaptation similar to that in normal BSV was possible. The conclusion of this thesis is to suggest that information from the suppressed eye is available to the saccadic system by either a sub-cortical pathway or processed cortically without conscious awareness

Neuroplasticity of the visual system after early brain lesion

I possibili mecccanismi di riorganizzazione funzionale dopo lesione cerebrale sono molto complessi e influenzati da un gran numero di fattori. In particolare, quelli che sembrano giocare il ruolo più importante nella determinazione della prognosi, sono i) il timing della lesione, ii) la sua localizzazione e iii) la sua estensione. Fino ad ora, sono stati fatti molti studi riguardanti le correlazioni anatomo-funzionali con le lesioni cerebrali sugli animali o sugli adulti, mentre poco si conosce riguardo ai bambini con lesioni cerebrali (congenite o acquisite) e spesso i dati riportati sono tra loro discordanti. Negli ultimi anni sono state messe a punto, però, tecniche di neuroimaging ad alta definizione in grado di fornirci informazioni più precise sul tipo di lesione, sulla sua estensione e sull’interessamento delle varie aree corticali e sottocorticali coinvolte nel processamento visivo. In questo modo è stato possibile capire meglio la correlazione tra struttura e funzione e il fatto che lo sviluppo normale del sistema visivo dipenda dall’integrità di una ricca rete di connessioni che comprende le radiazioni ottiche e la corteccia visiva primaria, ma anche aree corticali e sottocorticali come quelle dei lobi frontali e temporali o dei gangli della base. Tenendo presenti le conoscenze attuali, nel corso di questo progetto di dottorato, abbiamo cercato di approfondire l’influenza delle lesioni cerebrali, di estensione diversa e avvenute in tempi diversi, sulla riorganizzazione della funzionalità visiva. Ciò è stato possibile grazie alla messa a punto di nuovi test, realizzati in parte in collaborazione con l’Istituto di Neuroscienze-CNR di Pisa, che abbiamo poi potuto somministrare ad un gran numero di bambini (nati a termine o pretermine) con differenti tipi di danno cerebrale. In particolare, il nostro interesse è stato rivolto a due principali aspetti: studiare le abilità visive di bambini con lesioni cerebrali congenite bilaterali (scegliendo come modello di lesione quello della leucomalacia periventricolare) e studiare le capacità visive in bambini con lesioni cerebrali unilaterali avvenute in epoche diverse: lesioni congenite o acquisite. Ciò è stato possibile grazie all’utilizzo di test visivi ben strutturati e standardizzati e al confronto dei risultati ottenuti con i diversi quadri di danno cerebrale riportato alla RMN. Il lavoro di questi anni ci ha permesso di confermare la più alta potenzialità di plasticità in caso di lesioni unilaterali rispetto a quelle bilaterali, riconoscendo alcune strutture chiave nella messa a punto di meccanismi di riorganizzazione adattiva quale per esempio il collicolo superiore. Abbiamo inoltre riscontrato un maggiore potenziale di plasticità in caso di lesioni unilaterali congenite piuttosto che in caso di lesioni unilaterali acquisite seppur precocemente (in età pediatrica). Nel caso di lesione unilaterale acquisita abbiamo tuttavia dimostrato l’efficacia, nella messa a punto di meccanismi di compenso, di trattamenti riabilitativi basati sulla stimolazione multisensoriale (audio-visiva). I nostri studi confermano quindi che i meccanismi di risposta ad un danno avvenuto precocemente sono sicuramente diversi da quelli riscontrati nell’adulto, ma non meno complessi. L’influenza dell’ambiente, il momento il cui avviene la lesione e molti altri fattori giocano un ruolo fondamentale che è ancora lontano dall’essere completamente capito. Per tale motivo, siamo consapevoli di aver fornito solo un piccolo contributo e che è necessario ancora molto lavoro per continuare questa missione infinita che è quella di capire il cervello umano

Applied Vision Association Christmas Meeting, Leuven, Belgium 19–20 December 2013

For about 15 years, we have been attempting to create a software system (EvoFIT) that produces an identifiable composite when constructed of an unfamiliar face after a long retention interval (as is the usual case in police investigations). EvoFIT has now been shown to be effective in police field trials and has been used in 20 police forces. More recently, some developments have been very effective on their own to improve composite quality—these include: (a) encouraging constructors to focus on the overall (global) properties of the face during the initial interview (Holistic CI, H-CI), (b) constructing internal features first (masking external features), and (c) presenting composites for naming using a perceptual image-stretch format. In the current project, we explored whether these techniques would produce even more identifiable composites when combined. Forty participants looked at a video of an unfamiliar target face and, 24 hours later, received an interview to obtain a description of the face (either CI/ H-CI) and then constructed a composite using EvoFIT (either blurry externals/internals-only). Further participants were presented with the resulting composites to name, first front-on (the usual way to view the face) and then by looking at the face sideways (perceptual stretch). The three techniques were effective independently, replicating past research, but together produced composites with an effect that was more than just additive—to yield a mean naming level of 74% correct. A composite system using all three of these techniques would therefore appear to be effective for police-work