A Dissociation of Attention and Awareness in Phase-sensitive but Not Phase-insensitive Visual Channels

The elements most vivid in our conscious awareness are the ones to which we direct our attention. Scientific study confirms the impression of a close bond between selective attention and visual awareness, yet the nature of this association remains elusive. Using visual afterimages as an index, we investigate neural processing of stimuli as they enter awareness and as they become the object of attention. We find evidence of response enhancement accompanying both attention and awareness, both in the phase-sensitive neural channels characteristic of early processing stages and in the phase-insensitive channels typical of higher cortical areas. The effects of attention and awareness on phase-insensitive responses are positively correlated, but in the same experiments, we observe no correlation between the effects on phase-sensitive responses. This indicates independent signatures of attention and awareness in early visual areas yet a convergence of their effects at more advanced processing stages

Investigating the role of right parietal cortex in multistable perception using non-invasive brain stimulation

Multistable perception describes the spontaneous fluctuation between two or more perceptual states when sensory input is ambiguous. An example hereof is bistability, which occurs when a stimulus has two competing interpretations that perceptually alternate over time. For instance, in structure- from-motion (SFM) bistable perception, the coherent movement of dots creates the illusion of a rotating sphere, where the direction of movement is uncertain. Another example is binocular rivalry (BR), which occurs when the two eyes are presented with dissimilar visual stimuli in the same retinal space, leading to an alternation of conscious awareness between the two stimuli. Multistable perception has been used to investigate the neural correlates of conscious experience, since an unchanging stimulus leads to a change in awareness, hence dissociating consciousness from sensory processing. Functional magnetic resonance imaging (fMRI) has consistently shown activity of the right intraparietal sulcus (IPS) and right superior parietal lobule (SPL) during perceptual transitions in multistable perception. Previously, transcranial magnetic stimulation (TMS) and in particular inhibitory theta burst stimulation (cTBS) has been used on the IPS to probe its causal role in multistable perception. That endeavour has produced inconsistent results on whether IPS inhibition shortens or lengthens multistable dominance durations. Problematically, the neural effects of cTBS over IPS during multistable perception are unknown, as is indeed the causal role of IPS in mediating perceptual reversals. Chapter 1 cTBS was applied over IPS or over vertex control site, between two sessions of fMRI, to illuminate the changes in neural activity accompanied by IPS cTBS. During the fMRI sessions, participants viewed alternating blocks of a bistable SFM stimulus or a replay condition using depth-cue disambiguated SFM. Behaviourally, it was found that IPS cTBS lengthened dominance durations when comparing pre vs post cTBS as well as when comparing IPS with vertex stimulation. Neurally, IPS cTBS led to a decrease in blood-oxygen-level dependent (BOLD) response in thalamus, foveal V1, right superior parietal lobule and middle frontal gyrus compared to vertex cTBS. Moreover, a decrease of functional connectivity between activity in IPS and ipsilateral hippocampus was observed. The present results suggest that the combined effects of a reduction of sensory processing as well as decoupling between IPS and the memory site hippocampus allows inhibitory TMS over parietal cortex to stabilise the current perceptual content. Together, these results provide a hitherto unreported insight into the brain networks that subserve the resolution of bistable perception and how IPS stimulation modulates them to bring about a behavioural effect. Chapter 2 Next to the IPS, also the more posterior SPL has been indicated as serving a causal role in multistable perception. TMS has been used to modulate bistable dominance durations for both sites, but in opposite directions. This led to the proposal that parietal cortex is fractionated, such that IPS and SPL serve opposing functions. However, neuroimaging evidence also suggests that higher cortical activity, including parietal cortex, is diminished when BR percept switches are either unreported or unreportable. This suggests that parietal regions have no causal role in multistable perception, but are active only as consequence thereof. To resolve this conflict, chapter 2 investigates whether cTBS to the IPS as well as the SPL affects the temporal dynamics of BR using regular button press rivalry as well as no-report and invisible rivalry paradigms. Specifically, it was hypothesised that cTBS would lead to a change in BR dominance when it was visible or unreported, but not when invisible. However, contrary to expectation, not only was it not possible to replicate the previously observed functional fractionation of parietal cortex, but also no difference was found between any cTBS condition. To verify if cTBS had its desired inhibitory effect, also motor-evoked potentials (MEP) were recording prior and following cTBS to primary motor cortex. It was found that cTBS to M1 decreases MEP amplitude. However, this effect did not correlate with the main findings over parietal cortex, leading to the conclusion that cTBS is not an apt neurostimulation technique to answer the present research question. Chapter 3 Relative intensities of steady-state responses (SSRs) over early visual cortex have been reported to correlate with conscious perception in paradigms like BR and have even be used to predict the content of consciousness. However, their causal role in perception remains uninvestigated despite their common use. Are modulations of SSRs mere epiphenomena of perception or do they aid in determining its content? To test this, it was enquired if interference with the SSR by means of transcranial alternating current stimulation (tACS) would affect conscious perception. Sham or real tACS across left and right parieto-occipital cortex was applied at either the same or a different frequency or in and out of phase with an SSR eliciting flicker stimulus, while participants viewed either BR or tried to detect stimuli masked by continuous flash suppression (CFS). It was found that tACS did not differentially affect conscious perception in the forms of BR predominance, CFS detection accuracy, reaction time, or metacognitive sensitivity. Importantly, the present null-findings are supported by Bayesian statistics. In conclusion, the application of tACS at frequencies and phases of stimulus-induced SSRs does not have perceptual effects. The relationship of tACS with SSRs and the possibility that SSRs are epiphenomenal to conscious perception is discussed. Chapter 4 One reason for the difference between findings of studies, which attempted to modulate multistable dominance durations thought cTBS to the IPS, may be that different stimuli were used, dissimilar properties of which modulated the TMS effect direction. To test this, cTBS was applied to the IPS between two sessions of SFM bistable perception (chapter 1), random dot motion BR (chapter 2), as well as checkerboard BR (chapter 3). It was foremost hypothesised that the findings of the first two chapters would be replicated, and moreover that the TMS effect would correlate between stimuli. Contrary to this hypothesis, cTBS neither consistently affected dominance durations in any of the stimuli, nor were effect sizes correlated across participants. This is supported by Bayesian statistics. Baseline dominance durations prior to TMS correlated across the three stimuli, suggesting a common mechanism to resolve multistability. However, the lack of correlation pertaining to the cTBS effect points towards the absence of any cTBS effect. Considering the present results, the small samples and effect sizes of previous studies, as well as recent literature of variable cTBS effects on motor cortex, this chapter concludes that there is good reason to cast general doubt over the ability of parietal cTBS to modulate dominance durations in multistable perception. Chapter 1 pointed towards the importance of multiple brain networks including the IPS in the resolution of multistability. Chapters 2 & 4 by contrast presented with null results that do not allow inference as to the causal role of IPS. Similarly, the use of tACS to modulate SSRs in chapter 3 was not able to demonstrate conclusively whether SSRs have a causal role in multistability. The search for the contribution of IPS by use of cTBS or tACS has been hindered by methodological concerns over whether these methods have an interpretable or even any effect on IPS activity. In summary, the causal role of IPS activity in multistable perception remains elusive

An assessment model and implementation of stereo image quality

In the past decade, many display hardware manufacturers have initiated research into the construction of stereo display devices. Currently, the use of such displays is limited to the computer-aided design; research, military and medical applications. However, it is anticipated that as display hardware becomes cheaper, gaming companies and desktop application software developers will realise the potential of using stereo to provide more realistic user experiences. To provide realistic stereo user experience it is necessary to utilise good quality stereo images in addition to suitable hardware. The growth of the Internet has resulted in an increase in the availability of stereo images. However, most have been captured using uncontrolled procedures and have questionable quality. The quality of stereo images is important since the viewing of poor quality stereo images can result in adverse viewing effects. A formal definition of stereo quality has not been achieved in current day research. This means that the factors which cause a stereo image to be perceived as poor quality have not been defined nor is a system available to detect its occurrence. This thesis attempts to address this problem by postulating a definition of stereo image quality based on detecting level of excess disparity levels, intensity differences and the occurrence of frame cancellation. An implementation system able to detect these identified factors is discussed and formulated. The developed system is utilised to test 14 stereo images of varying quality levels. The results of these tests are reported and are used to evaluated and refine the system. Using this image analysis, benchmarks for natural intensity difference in images, changes due to JPEG compression and comparisons with generated and ground truth disparity maps are formulated. Additionally, a

Do early neural correlates of visual consciousness show the oblique effect? A binocular rivalry and event-related potential study

When dissimilar images are presented one to each eye, we do not see both images; rather, we see one at a time, alternating unpredictably. This is called binocular rivalry, and it has recently been used to study brain processes that correlate with visual consciousness, because perception changes without any change in the sensory input. Such studies have used various types of images, but the most popular have been gratings: sets of bright and dark lines of orthogonal orientations presented one to each eye. We studied whether using cardinal rival gratings (vertical, 0°, and horizontal, 90°) versus oblique rival gratings (left-oblique, –45°, and right-oblique, 45°) influences early neural correlates of visual consciousness, because of the oblique effect: the tendency for visual performance to be greater for cardinal gratings than for oblique gratings. Participants viewed rival gratings and pressed keys indicating which of the two gratings they perceived, was dominant. Next, we changed one of the gratings to match the grating shown to the other eye, yielding binocular fusion. Participants perceived the rivalry-to-fusion change to the dominant grating and not to the other, suppressed grating. Using event-related potentials (ERPs), we found neural correlates of visual consciousness at the P1 for both sets of gratings, as well as at the P1-N1 for oblique gratings, and we found a neural correlate of the oblique effect at the N1, but only for perceived changes. These results show that the P1 is the earliest neural activity associated with visual consciousness and that visual consciousness might be necessary to elicit the oblique effect

Processes influencing visual awareness during motion-induced blindness

Visual illusions, such as motion-induced blindness, arise when the visual system, balancing speed and efficiency, summarizes the information it receives to form a percept. Motion-induced blindness (MIB) is characterized as the disappearance of a salient target when surrounded by a moving mask. Efforts to determine the mechanism have focused on the role of target characteristics on perceived disappearance by a coherently moving mask. In this dissertation, I take another approach, paying specific attention to the role of motion characteristics of the mask. In Experiments 1, 2, 3, and 4, I investigate whether the property of common fate influences target disappearance by manipulating coherence of the mask elements. Results showed that as mask coherence increased, perceived target disappearance decreased. This pattern was unaffected by the lifetime of the moving dots, the dot density of the motion stimulus, the target eccentricity, or the number of motion trajectories contained in the mask. The finding that motion coherence preferentially affected MIB prompted Experiments 5, 6, 7, and 8. Specifically, these experiments sought to investigate the spatial influence of mask motion on target disappearance. MIB was measured during conditions where opposing areas of motion coherence were confined locally surrounding the target or in more global areas across the display. The results revealed that motion coherence at more global locations, particularly at the area of fixation, had more influence on target disappearance compared to the coherence in proximity of the target and may reflect inability of the visual system to form a global motion pattern. The possibility that global motion may give rise to MIB, motivated the final research avenue. Experiment 9 addressed the possibility that variability in motion processing of the mask affects target disappearance. Specifically, the strength of motion processing was measured using the motion aftereffect. Results showed a strong connection between the amount of motion processing dedicated to the mask and the amount of MIB. As a whole, the findings show that motion processing, particularly during MIB, may be a driving force in our ability to perceive other objects in our environment

Finding prejudice in unexpected places: racially biased perception

A series of experiments reveal that people are more likely to mistake black men as holding a gun than white men (Payne 2001; Payne et al., 2002; Correll et al., 2002). These data suggest that real-world cases of racially biased object-identification occur, such as in cases of police killings of unarmed black men. The aim of this paper is to correctly model what goes on in people’s heads, leading them to misidentify objects in these instances. One possibility is that people are making the wrong judgment about the object in question; perception might proceed as it should, but the viewer may think that they’re seeing a gun due to a cognitive error. Instead, I present a model which construes the error as a result of a visual illusion: even though the object is a hand tool, erroneous visual processing causes them to have the illusory experience of a gun

A search asymmetry for interocular conflict

When two different images are presented to the two eyes, the percept will alternate between the images (a phenomenon called binocular rivalry). In the present study, we investigate the degree to which such interocular conflict is conspicuous. By using a visual search task, we show that search for interocular conflict is near efficient (15 ms/item) and can lead to a search asymmetry, depending on the contrast in the display. We reconcile our findings with those of Wolfe and Franzel (1988), who reported inefficient search for interocular conflict (26 ms/item) and found no evidence for a search asymmetry. In addition, we provide evidence for the suggestion that differences in search for interocular conflict are contingent on the degree of abnormal fusion of the dissimilar images

Rivalry and interference with a head-mounted display

Perceptual factors that affect monocular, transparent (a.k.a see-thru ) head-mounted displays include binocular rivalry, visual interference, and depth of focus. We report the results of an experiment designed to evaluate the effects of these factors on user performance in a table look-up task. Two backgrounds were used. A dynamic moving background was provided by a large screen TV and an untidy bookshelf was used to provide a complex static background. With the TV background large effects were found attributable to both rivalry and visual interference. These two effects were roughly additive. Smaller effects were found with the bookshelf. In conclusion we suggest that monocular transparent HMDs may be unsuitable for use in visually dynamic environments. However when backgrounds are relatively static, having a transparent display may be preferable to having an opaque display

Anti-Fat Bias and Attentional Capture

Explicitly-rated anti-fat attitudes are correlated with weight-based discrimination, which is rampant in society today as many countries grapple with soaring rates of obesity. Early perceptual processes, such as conscious awareness and visual attention, may be biased based on the weight of the perceived or the perceiver, or any number of individual perceiver characteristics regarding weight-biased attitudes and experiences. The three experiments presented used continuous-flash suppression (CFS) to mask body stimuli, thereby hoping to gain insight into attentional capture of unseen images and its relation to anti-fat attitudes. The pattern of findings in the three experiments presented suggest that what makes a stimulus likely to capture spatial attention may be distinct from the characteristics that afford it conscious perceptual processing initially. Stimulus-level features interacted with participant characteristics to bias the effectiveness of CFS. All three studies demonstrated significant differences in stimulus breakthrough based on stimulus weight, where larger images broke through to conscious awareness more readily than smaller images. Study 2 controlled for size by including inverted bodies as primes. Analyses suggest that heavy bodies are more susceptible to suppression than their overall size would predict. This effect interacted with gender and BMI; overweight participants and female participants displayed the significant effect of stimulus weight on breakthrough rate. In contrast, findings regarding the relationship between explicit anti-fat bias and attentional capture were inconsistent across studies

A population study of binocular function.

As part of a genome-wide association study (GWAS) of perceptual traits in healthy adults, we measured stereo acuity, the duration of alternative percepts in binocular rivalry and the extent of dichoptic masking in 1060 participants. We present the distributions of the measures, the correlations between measures, and their relationships to other psychophysical traits. We report sex differences, and correlations with age, interpupillary distance, eye dominance, phorias, visual acuity and personality. The GWAS, using data from 988 participants, yielded one genetic association that passed a permutation test for significance: The variant rs1022907 in the gene VTI1A was associated with self-reported ability to see autostereograms. We list a number of other suggestive genetic associations (p<10(-5)).This work was supported by the Gatsby Charitable Foundation (GAT2903). J.B. was supported by a fellowship from Gonville and Caius College.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.visres.2015.02.01