Multimodal cue integration in balance and spatial orientation

The global objective of this thesis was to make a significant contribution to our understanding of how the human brain integrates multisensory, multimodal information to inform our motion through space. The primary objectives were to discern whether visual system differentially encodes visual motion coherence and how both allocentric visual cues interact with vestibular system to tell us where and when we are in physical space. A secondary objective was to develop current techniques for the recording and analysis of visuo-vestibular sensory information for the purpose of multisensory, multimodal integration. I studied the response of cortical visual motion area V5/MT+ to random dot kinematograms (RDK) of varying motion coherence, from complete coherence to random. I used the probability of observing TMS (transcranial magnetic stimulation) evoked phosphenes before and after the RDK as a measure of cortical excitability change. I could not show what I had hypothesised: that coherent and random motion elicited a similar net effect upon V5/MT+ excitability, with intermediary coherences of motion having comparatively less effect. However, I argue that a large factor was insufficient sample size to find the effects given the analyses used. The results do show trends consistent with coherent and random net effects being achieved by different modes of cortical activation, and the study will inform future investigation with the paradigm used. I also measured cortical excitability change at a range of relative TMS intensities. This elicited a significant differential effect consistent with the theory that TMS facilitates neurons as a function of the amount of signal they carry. In a separate TMS evoked phosphene study, I show an interaction between whole body rotation in yaw and the ability to observe phosphenes in V5/MT+; as a function of the TMS intensity used and the velocity of whole body rotation used, relative to perceptual thresholds. As I found no main effects, I could not show whether the findings were consistent with a model of reciprocal visual and vestibular cortical inhibition. My work can be considered a feasibility study to inform further investigation. I also used a visual-vestibular mismatch paradigm to probe how erroneous visual landmark cues update veridical vestibular estimates of angular position and motion duration. I used visual masking to reduce the reliability of the visual landmark cues, prevent visual capture and to also elicit subliminal encodement. I found that reversion to vestibular estimates of angular position was made as a function of the noise inherent in the masked visual landmark cues. I found that it was possible to subliminally encode visual landmarks to update vestibularly derived estimates of motion duration. Lastly, I investigated the combination of a two-interval forced choice technique to record estimates of vestibularly derived angular position and a Bayesian Inference technique to parameterize the characteristics of the angular position estimates. I show this combination provides accurate estimates at the subject level and is suitable for incorporation in a Bayesian inference model of multimodal integration. The hypothesis I aim to test in the future is that if visual landmark and vestibular cues of angular position operate within different spatial reference frames, they cannot be optimally integrated in the brain analogous to a Bayesian Inference model of the multimodal integration.Open Acces

The Attentional Mechanisms of Active Forgetting

Recent work has shown that intentional forgetting of distracting, erroneous, or irrelevant information aids memory, and relies on active, effortful processes. Two experiments investigated the underlying attentional mechanisms that are active during directed forgetting (DF). Across both experiments, participants completed a modified item-method DF task, in which they received memory instructions to remember or forget individual images for a subsequent memory test. Participants studied items associated with remember or forget instructions before they were shown a subliminal presentation of target items. Finally, participants responded to probes by identifying briefly shown letters to assess how attention and item identity information are inhibited following forget instructions. In Experiment 1, after studying items, participants completed either an explicit memory test (recognition) or an implicit memory task (perceptual identification). Experiment 2 extended the findings of Experiment 1 by examining how spatial information is inhibited following instructions to forget, given spatial components in many recent investigations of DF (e.g., Fawcett & Taylor, 2008, 2010; Taylor, 2005). Although it was predicted that active forgetting would be associated with attentional inhibition linked to both item identity and spatial location, results revealed no inhibitory effects during speeded probe responses across both experiments. However, clear forgetting effects were observed, with participants exhibiting better memory for items they were cued to remember, relative to items they were cued to forget. The results of both experiments support the hypothesis that some information is lost or degraded by instructions to intentionally forget, but raise further questions about the nature of attentional withdrawal proposed to occur during a DF task

THE MODULATION OF COVERT ATTENTION BY EMOTION: AUTOMATIC PROCESSING OF EMOTIONAL VERSUS NEUTRAL VALENCED CUES IN A COVERT ATTENTION PARADIGM

Selective attention has been studied extensively and it is shown, for example, that individuals with conditions such as anxiety show attention bias to threat-related stimuli. It has been proposed that humans are predisposed or that it is naturally adaptive to selectively attend to emotional stimuli (Lang, 2000). Similarly, LeDoux (1996) and others have proposed limbic brain networks allowing for quick and automatic, but sometimes inaccurate, processing of emotion which bypasses primary cortical areas. Along these lines, automatic attention bias to subliminal image cues in an adapted Posner Covert Attention Task was examined in the current study. A sample of 64 participants was used in each of three separate experiments to examine how individuals were cued subliminally by negative or positive emotional vs. neutral images and the modulation of covert attention by emotion. Due to automatic or motivated attention to emotionally salient stimuli, participants were expected to be facilitated in task performance by negative and positive emotional image cues, relative to neutral cues. Further, state anxiety and depression were expected to impact performance on emotional cueing as well. As expected in Experiment 1, subliminal images produced significant covert attentional cueing and only negative image cues compared to neutral ones produced response time (RT) reduction by valid cueing across both cue-target delay conditions. Further, cueing differences between neutral and negative images were seen only at short delays, supporting differential subliminal processing of emotional cues in attentional paradigms and supporting previous evidence of unconscious fear processing and specialized automatic fear networks. Moreover, in Experiment 2, when delays following subliminal cues were extended further, emotional cues did not differentially modulate covert attention, suggesting that subliminal emotional cueing seems to occur more immediately. Positive subliminal imagery in Experiment 3 was largely unsuccessful in differentially modulating covert attention compared to neutral cues, suggesting that positive information is either not effective in modulating covert attention or occurs over similar immediate time durations as negative cues in Experiment 1. Finally, the presence of self-reported state anxiety and depression affected task performance, especially in Experiment 1 negative for subliminal discrimination of negative vs. neutral image cues. Overall, the current study adds to the research literature which demonstrates that emotional information, especially negative imagery processed at short intervals, can be processed below awareness to modulate attention in a different manner than less salient neutral stimuli and this modulation is further influenced by state anxiety or depressive symptomatology. Implications of these findings and future directions for research are discussed

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

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

Regressive research: The pitfalls of post hoc data selection in the study of unconscious mental processes

Many studies of unconscious processing involve comparing a performance measure (e.g., some assessment of perception or memory) with an awareness measure (such as a verbal report or a forced-choice response) taken either concurrently or separately. Unconscious processing is inferred when above-chance performance is combined with null awareness. Often, however, aggregate awareness is better than chance, and data analysis therefore employs a form of extreme group analysis focusing post hoc on participants, trials, or items where awareness is absent or at chance. The pitfalls of this analytic approach are described with particular reference to recent research on implicit learning and subliminal perception. Because of regression to the mean, the approach can mislead researchers into erroneous conclusions concerning unconscious influences on behavior. Recommendations are made about future use of post hoc selection in research on unconscious cognition

Centre-periphery-difference in low-level vision and its interactions with top-down and sensorimotor processes

There is a profound difference in low-level vision between the retinal centre and the periphery (cpd). That contrast sensitivity declines from centre to the periphery is well established in humans. However, recently TMS on FEF was found to remotely affect visual cortex such that the cpd was reduced. No direct connections between FEF and occipital visual areas are known, but connections between FEF, the pulvinar and the occipital visual areas exist. I examined the cpd pattern in contrast sensitivity after real lesions in FEF and pulvinar areas by estimating visual thresholds. The results showed that real lesions of FEF do not have the same effect as TMS and are consistent with TMS causing subthreshold activation mimicking covert visuospatial attention. The cpd pattern in contrast sensitivity was different between FEF and pulvinar patients. Differences were prominent for foveal processing, while peripheral processing revealed parallel deficits, although these did not reach significance. In the second part of this work I focused on manual visuo-motor processes that have been found to differ between centrally and peripherally presented subliminal primes. For the periphery, when invisible primes are compatible with targets in their motor associations, RT‟s to targets speed up. However, for foveal primes, priming costs (negative compatibility effects (NCE)) can occur with compatible primes and targets. I examined the impact of perceptual sensitivity decline for the absence of NCE in the periphery by equating primes‟ strength via contrast threshold measurements. The results showed that perceptual equation does not equate priming effects. The critical factor, to trigger visuo-motor processes in periphery was found to be the prolonged time of the mask-target interval (SOA). This indicates that the functionally distinct retinal areas can both trigger visuo-motor processes, which are independent from visibility equation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo