Psychology (Functional Area: Brain, Behaviour & Cognitive Science)

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Open Access
A Novel Neurorehabilitation Model Designed to Examine the Neural Plasticity Involved in Disease
(2022-03-03) Bearss, Karolina Anna; DeSouza, Joseph FX
Parkinson's disease (PD) is a progressive neurodegenerative disorder that that is most often characterised for its motor impairments. However, people with PD (PwPD) often experience a range of mental health and non-motor issues alongside their physical symptoms. Exercise has shown to positively impact and improve PD motor symptoms, less research observations have been shown in PD mental health and non-motor symptoms. Dance is a great form of exercise which provides both aerobic and anaerobic movements. Dance is constantly changing providing a creative outlet, dance provides flexibility and balance/coordination, develops social skills thereby improving mental health, and lastly dance with music combination allows this form of exercise to be unique in that it encompasses a multisensory component that exercise alone cannot provide. My dissertation aims to understand how dance impacts PD motor, non-motor symptoms and if the changes are associated to specific brain related alterations. Using behavioral, motor and EEG approaches, I will present three separate experiments to test the effects of dance on people with PD by first studying the potential impacts of dance on short-term behavioral changes in PwPD and their overall Quality of Life (QoL) after a 12-week dance intervention. Second I will present a novel examination of the interaction of dance on both behavioural measures and electroencephalography (EEG) activity before and after the short-term (1.25 hour) course of a single dance class. The third study is a novel examination of the interaction of dance on the progression of both behavioural measures and non-motor symptoms over the long-term course of participating in multiple dance classes over a 3-year period of time. Finally, EEG activity changes over the long-term course of participating in multiple dance classes over a 3-year period of time is presented. The results of these studies strengthen the idea of dance being an alternative or additional therapy for PwPD and also provides putative neuroplastic changes in the diseased brain.
Open Access
A Role for Hippocampal Sharp-wave Ripples in Active Visual Search
(2017-07-27) Leonard, Timothy S.; Hoffman, Kari
Sharp-wave ripples (SWRs) in the hippocampus are thought to contribute to memory formation, though this effect has only been demonstrated in rodents. The SWR, a large deflection in the hippocampal LFP (local field potential), is known to occur primarily during slow wave sleep and during immobility and consummator behaviors. SWRs have widespread effects throughout the cortex, and are directly implicated in memory formation their occurrence correlates with correct performance, and their ablation impairs memory in spatial memory tasks. Though SWRs have been reported in primates, their role is poorly understood. Whether or not SWRs play a role in memory formation, as they do in rodents, has yet to be confirmed. This work encompasses three separate studies with the goal of determining whether there is a link between SWR occurrence and memory formation in the macaque. Chapter 2 establishes the validity of the modified Change Blindness task as a memory task which is sensitive to normal hippocampal function in monkeys. Chapter 3 establishes that SWR events occur during waking (and stationary) activity, during visual search, in the macaque. Until this work, the prevalence of SWRs in macaques during waking exploration was unknown. Chapter 4 shows that gaze during SWRs was more likely to be near the target object on repeated than on novel presentations, even after accounting for overall differences in gaze location with scene repetition. The increase in ripple likelihood near remembered visual objects suggests a link between ripples and memory in primates; specifically, SWRs may reflect part of a mechanism supporting the guidance of search based on experience. The amalgamation of this work reveals several novel findings and establishes an important step towards understanding the role that SWRs play in memory formation in predominantly-visual primate brains.
Open Access
Age Estimation from Children's Faces
(2015-08-28) Harrington, Alexandra Elizabeth; Wilkinson, Frances E.
In this thesis, we addressed the question of whether or not people could estimate age from children’s faces 7 to 11 years of age. We found that undergraduates were able to make accurate relative age judgments for males and females, even in faces as little as two years apart, and that their performance improved as the age differences between the faces being compared increased. They were also able to make accurate absolute age judgments that increased with increasing face age for both genders. We also looked at estimate bias and while estimates were generally low in bias, the bias was in direction of the mean age of the stimuli. Additionally, we found that there is generally an advantage for male faces presented in frontal view. Finally, we looked at one possible factor influencing age estimates– facial expression. It was unlikely that facial expression was a primary cue informing age estimates.
Open Access
Altered White Matter Structure in Adults Following Early Monocular Enucleation
(2018-03-01) Wong, Nikita Ann; Steeves, Jennifer
Visual deprivation from early monocular enucleation (the surgical removal of one eye) results in a number of long-term behavioural and morphological adaptations in the visual, auditory, and multisensory systems. This thesis aims to investigate how the loss of one eye early in life affects structural connectivity within the brain. A combination of diffusion tensor imaging and tractography was used to examine structural differences in 18 tracts throughout the brain of adult participants who had undergone early monocular enucleation compared to binocularly intact controls. We report significant structural changes to white matter in early monocular enucleation participants that extend beyond the primary visual pathway to include interhemispheric, auditory and multisensory tracts, as well as several long association fibres. Overall these results suggest that early monocular enucleation has long-term effects on white matter structure throughout the brain.
Open Access
Atypical Lateralization of Language and Face Processing in Autism Spectrum Disorder
(2022-08-08) Solomon-Harris, Lily Marissa; Stevens, Dale
Autism spectrum disorder (ASD) is characterized by persistent deficits in communication and social interaction, as well as repetitive behaviours and restricted interests. Language and face processing are areas of domain-specific dysfunction impacting social interaction in ASD. Brain function associated with these cognitive domains is typically lateralized across the cerebral hemispheres, such that language and face processing are dominant within the left and right hemispheres, respectively. Furthermore, the degree of lateralization is related to behavioural proficiency in both domains. Converging evidence suggests that the development of lateralization in these separate domains is typically interrelated. Neuroimaging literature demonstrates reduced leftward lateralization of language in ASD, but the existing literature on face processing is inconsistent. Mixed findings are partly due to discrepancies in how regions of interest (ROIs) are localized for neuroimaging analyses. The present work aims to test the hypothesis that ASD is related to atypical lateralization of both language and face processing. First, a quantitative fMRI meta-analysis was conducted to resolve inconsistencies in the literature by identifying the most reliable, concordant patterns of differences in language and face processing in the brain associated with ASD across previous fMRI studies. The findings of the meta-analysis were then used to inform a rigorous analysis of category-related brain activation using individually localized ROIs and task-related fMRI data. However, the question remains regarding whether brain activation differences demonstrate a sustained, fundamental difference in the way language and faces are processed in the brain, rather than reflecting moment-to-moment differences in attention to these stimuli, motivating the final analysis of intrinsic functional connectivity between category-related brain regions. The findings from the meta-analysis, task-related fMRI analyses, and RSFC analysis of intrinsic functional connectivity converged, demonstrating that ASD is indeed related to reduced functional specialization and hemispheric lateralization of both language and face processing, particularly in posterior lateral temporal cortex. This is an important contribution to the mixed existing literature, and further demonstrates the importance of individual ROI localization when studying neurodiverse populations. Atypical development of hemispheric asymmetry for language and face processing in posterior lateral temporal cortex might be a fundamental factor underlying the behavioural presentation of ASD.
Open Access
Context-Dependent Dual Adaptation to Opposing Visuomotor Rotations
(2015-12-16) Ayala, Maria Nadine; Henriques, Densie
When reaching towards objects, the human central nervous system (CNS) can actively compensate for two different perturbations simultaneously (dual adaptation), though this does not simply occur upon presentation. Dual adaptation is made more difficult when the desired trajectories and targets are identical and hence do not cue the impending perturbation. In cases like this, the CNS requires contextual cues in order to predict the dynamics of the environment. Not all cues are effective at facilitating dual adaptation. In two experiments we investigated the efficacy of hand and body posture contextual cues that are intrinsic to the CNS. For the hand posture experiment, we also look at the role of extended training. We found that how people held the tool or oriented their body while reaching is sufficient for concurrently adapting separate visuomotor mappings such that over time, reach errors significantly decrease. Extended practice did not lead to further benefits though.
Open Access
Cueing Visual Spatial Working Memory: Effects of Cue Modality, Cue Type, and Age
(2018-03-01) Curtis, Ashley Ferne; Murtha, Susan J. E.
In general, attentional control and spatial working memory (WM) decline with increasing age. It is well known that relative to non-cued targets, spatially informative visual (uni-modal) cues quicken response time in target detection attention tasks, and improve feature and spatial WM performance. Spatially informative auditory and vibrotactile (cross-modal) cues provide additional benefit in more difficult attention tasks, but their effects on spatial location WM are unknown. This dissertation presents two studies that investigated effects of uni-modal visual cues and cross-modal auditory and vibrotactile cues on visual spatial location WM in younger adults (YA) and older adults (OA), and under various conditions that modulated WM task demands. In study one, we found that both spatially informative uni-modal and cross-modal cues improved spatial location WM performance to a similar degree for YA and OA. This benefit was generally greater under higher WM load (i.e., six-item vs. four-item memory arrays) and longer maintenance delays, whereas centrally presented alerting cues generally impaired performance. Individuals with lower spatial spans also benefitted most from spatially informative cross-modal cues. Study two assessed the impact of maintenance interference on spatially informative cue effects. In contrast to study one, we found age-related cue effects, which were moderated by WM maintenance interference type. When interference was to be ignored, OA benefitted from visual, auditory, and vibrotactile cues for lower WM loads (i.e., four-item arrays), whereas YA only benefitted from vibrotactile cues at higher WM loads (i.e., six-item arrays). When interference was to be compared, OA showed increased benefit to WM performance from cross-modal auditory and vibrotactile cues, whereas YA benefitted from all cue modalities. Taken together, these findings suggest spatially informative cross-modal cues can improve spatial location WM in both YA and OA, particularly when demands on spatial attention and attentional control are high. Furthermore, OA show more consistent benefit from cross-modal cues in resource demanding conditions. These results provide insight into cognitive underpinnings of cross-modal cue effects, and age-related differences in use of environmental support. They also provide a rationale for real world applications using cross-modal cues, aimed at improving cognitive function in complex visual environments, particularly for OA.
Open Access
Development of Low-Frequency Repetitive Transcranial Magnetic Stimulation as a Tool to Modulate Visual Disorders: Insights from Neuroimaging
(2018-11-21) Rafique, Sara; Steeves, Jennifer
Repetitive transcranial magnetic stimulation (rTMS) has become a popular neuromodulation technique, increasingly employed to manage several neurological and psychological conditions. Despite its popular use, the underlying mechanisms of rTMS remain largely unknown, particularly at the visual cortex. Moreover, the application of rTMS to modulate visual-related disorders is under-investigated. The goal of the present research was to address these issues. I employ a multitude of neuroimaging techniques to gain further insight into neural mechanisms underlying low-frequency (1 Hz) rTMS to the visual cortex. In addition, I begin to develop and refine clinical low-frequency rTMS protocols applicable to visual disorders as an alternative therapy where other treatment options are unsuccessful or where there are simply no existing therapies. One such visual disorder that can benefit from rTMS treatment is the perception of visual hallucinations that can occur following visual pathway damage in otherwise cognitively healthy individuals. In Chapters 23, I investigate the potential of multiday low-frequency rTMS to the visual cortex to alleviate continuous and disruptive visual hallucinations consequent to occipital injury. Combining rTMS with magnetic resonance imaging techniques reveals functional and structural cortical changes that lead to the perception of visual hallucinations; and rTMS successfully attenuates these anomalous visual perceptions. In Chapters 45, I compare the effects of alternative doses of low-frequency rTMS to the visual cortex on neurotransmitter levels and intrinsic functional connectivity to gain insight into rTMS mechanisms and establish the most effective protocol. Differential dose-dependent effects are observed on neurotransmitter levels and functional connectivity that suggest the choice of protocol critically depends on the neurophysiological target. Collectively, this work provides a basic framework for the use of low-frequency rTMS and neuroimaging in clinical application for visual disorders.
Open Access
Differentiating Visual Search Efficiencies for Symmetry Type and Texture Regularity
(2023-03-28) Moreau, Rachel Carmen Earleen; Kohler, Peter
Symmetry is believed to be a fundamental gestalt that aides in our day-to-day ability to interact with the visual world. The goal of this thesis was to investigate the differential processing of types of symmetry when embedded in texture or when viewed as individual objects. Across four experiments, I used the behavioural paradigm of the visual search task to measure processing efficiency across types of symmetry and texture regularity. I used stimuli called “wallpaper groups” which allowed for manipulations of symmetry type while holding constant low and midlevel visual cues. My results indicated that reflection symmetry was processed more efficiently than rotation symmetry and when these symmetries are embedded in a regular texture, they are processed more efficiently than not. The results of this research extend previous findings across behavioural psychology and visual neuroscience.
Open Access
Do Great Apes Choose to Choose?: An Investigation of Preference for Computer-Provided Choice in Orangutans (Pongo abelii)
(2021-03-08) Ritvo, Sarah Elizabeth; MacDonald, Suzanne
This dissertation examined orangutans preference for computer touchscreen-provided choice and their capacity to recognize the content of 2-D pictures. Investigation of these factors is important for advancing our understanding of orangutan cognition and the development of Animal-Computer Interaction systems that provide captive great apes environmental enrichment through provision of choice. Using a concurrent chain procedure presented on a touchscreen computer, the first experiment examined three orangutans intrinsic valuation of choice by assessing preference for free- or forced- choice when neither choice options nor outcomes vary. Initial results indicated a preference for free-choice across all participants. However, in two control conditions, preferences varied, suggesting a weaker tendency to exercise choice than species previously tested. Motivated by subjects difficulty learning associations between application icons and food rewards, a series of three experiments investigated five orangutans capacity to spontaneously recognize the content of novel pictorial stimuli by assessing if they demonstrated the same hierarchical preferences for food and pictures of food. Results indicated that orangutans only recognized picture content in certain formats and that they were more proficient in print than in digital mediums. Having confirmed that orangutans could recognize digital food images in a single format, this format was employed in the final pair of experiments to examine whether increasing the fidelity of the experimental choice paradigm elicited stronger free-choice preferences. This research question was investigated in a stepwise manner: Experiment 1 provided varied choices options that led to a single outcome and Experiment 2 varied both the choice options and outcomes. Results indicated a preference for free-choice in orangutans, but one that can be overwhelmed by competing factors and depend on the advantage afforded by it. Moreover, findings indicated that for orangutans, the strength and quality of preference for free-choice can be affected by the fidelity of the choice paradigm and vary between individuals. In light of these results, I suggest that preference for choice may be more accurately conceptualized along a spectrum rather than a dichotomy of preference for choice or lack thereof.
Open Access
Do the Subjective Visual Vertical and the Subjective Haptic Vertical Probe a Common Estimate of Gravitational Upright?
(2015-01-26) Fraser, Lindsey Ellen; Harris, Laurence
In the subjective visual vertical (SVV) and the subjective haptic vertical (SHV) task, participants must judge the alignment of a probe rod with perceived gravitational vertical by sight or by touch. Previous research suggests that as the body is roll-tilted, SVV and SHV show a systematic, distinct pattern of errors, although reports vary. The purpose of this research was to identify whether SVV and SHV probe the same underlying representation of gravity. In Experiment 1, I confirmed that SVV and SHV errors are divergent by comparing the two measures in the same participants. In Experiment 2, I varied the tilt of the head and body separately and applied galvanic vestibular stimulation to introduce vestibular noise. I found these manipulations had differential effects on SVV and SHV. Experiment 3 showed that when neck afferents were stimulated, SVV and SHV integrate optimally, pointing to two distinct underlying estimates of gravity vertical.
Open Access
Dynamic Neural Correlates of Mental Attention Capacity
(2021-03-08) Zarie, Amir Abbas; Stevens, Dale
Mental attention capacity (M-capacity) the maximum amount of information one can process simultaneously is a predictor of academic and professional success. However, its neural underpinnings are not well understood. Here, a novel implementation of dynamic functional connectivity (dFC) analysis of resting-state functional MRI data with parcellation of individual brains into a common set of functional areas (Group Prior Individual Parcellation, GPIP) was used to determine whether dFC is 1) related to individual differences in M-capacity and 2) modulated by prior performance of a demanding M-capacity task. Additionally, the novel dFC approach using GPIP was validated against a well-established dFC method (Group ICA of fMRI Toolbox, GIFT). While one measure of dFC accounted for individual differences in Mcapacity, dFC was not modulated by prior task performance. The novel dFC analysis using GPIP produced similar results to GIFT dFC, demonstrating the validity and potential advantages of this novel approach to dFC analysis
Open Access
Effects of bottom-up versus top-down cueing on conjunction search in 3-month old infants
Fuda, Maria Christina; Adler, Scott A.; Shanker, Stuart; DeSouza, Joseph Francis
Previous research with infants have suggested that they are fully capable of performing a feature search in a manner nearly identical to adults (Adler & Orprecio, 2006), but are developmentally immature in localizing a target in a conjunction search (Fuda & Adler, 2012). An explanation for the difference in infants performance between feature and conjunction searches was attributed to Wolfe's (1989) Guided Search model of visual search, in which feature searches are thought to rely mainly on bottom-up attentional resources to localize a target, whereas conjunction searches are theorized to require both bottom-up and top-down attentional resources. Because infants have been shown to perform a feature search like that of adults but have been shown not to be able to perform a conjunction search in a similar manner, the current study attempted to show that bottom up attentional mechanisms develop before top-down mechanisms. To this end, 3-month-old infants were presented with two types of cues prior to a conjunction search array that ·will provide them with prior bottom-up or top-down information that might facilitate their performance in a conjunction search task. The bottom-up cue consisted of four rectangular frames indicating where the possible location of the target will be, while the top-down cue consisted of the flashing what the target will be in the center of the array. Infant saccadic eye movement latencies were recorded for three different set sizes of conjunction search arrays (5, 8, & 10) when the target was either present or absent. When the target was present, the eye movement latency that localized the target was measured, while when the target was absent the first eye movement latency was measured. Results showed that the top-down cue, but not the bottom-up cue, facilitated the exhibition of a more adult-like conjunction search function in which latencies increased with increasing set sizes. More specifically, the bottom-up cue resulted in relatively flat search functions for both the target-present and target-absent trials. In contrast, the top-down cue results showed that in the second half of all infant trials, target-present latencies increased with increasing set sizes, while target-absent latencies decreased with increasing set sizes. These results show that infants are developmentally mature in their bottom-up processing, but immature in their top-down processing abilities, and as such the top-down cue provided the facilitation that they needed in order to localize a target in a conjunction search. The current study is the first of its kind to show that 3-month-old infants' top-down processing mechanisms are developmentally immature compared to their bottom-up mechanisms in visual search tasks.
Open Access
Effects of Gaze Position on Touch Localization
(2016-09-20) Pritchett, Lisa Marie; Harris, Laurence
Previous research has shown that the direction of gaze relative to the body affects the perceived location of touch, and has argued that these effects indicate that a gaze-centered reference frame is used for touch localization. In this dissertation I examine a discrepancy in the existing literature: why do different studies report opposite directions of effects when eye and head positions are manipulated separately? I resolve this discrepancy by showing that it is not due to whether eye or head position is manipulated (chapter 2) but is in fact due to the nature of the task (chapter 3). I also find that the effect occurs on the back of the body (chapter 4), a body part that is not normally in view and thus would be less likely to use gaze as a reference point. I test theories for why these effects occur (chapter 5), and find that results are compatible with the perceived location of a touch being attracted towards the location of gaze, at least for perceptual measures. When location was reported by pointing, an action-based measure, I find no effect of gaze direction on touch localization, suggesting that a gaze-independent reference frame is used for action. These behavioral results are complementary to recent neurophysiological and neuroimaging findings indicating that spatial locations are coded in a range of different reference frames, and indicate that gaze-related reference frames are behaviorally relevant in tactile localization.
Open Access
Effects of Lateral Motion on Stereoacuity Thresholds for Physically Moving Targets
(2016-09-20) Cutone, Matthew Daniel; Wilcox, Laurie M. Wilcox
The goal of this thesis was to determine the impact of lateral retinal motion on stereoacuity under natural viewing conditions. I found that stereoacuity thresholds remained stable when target velocities varied between 0 and 16 /s. These results do not agree with previous literature (Ramamurthy, Bedell & Patel, 2005) which found that stereoacuity degraded at higher velocities (greater than 3 deg/s). I suggest that depth is acquired very rapidly at target onset when targets are relatively broadband and have not been distorted by motion smear. Subsequent experiments ruled out the potential effects of monocular cues, retinal smear size and inter-stimulus delay enhancing perceived depth. I conclude that artefacts introduced by the graphical displays used by Ramamurthy et al. (2005) were responsible for the observed elevation of thresholds at higher velocities.
Open Access
Effects of music and dance training on executive functions in children
D'Souza, Annalise Aleta; Wiseheart, Melody
A body of evidence demonstrates superior executive functions in musicians, although most previous studies are cross-sectional. Dance, being similar to music, also offers potential for transfer to cognitive performance. A controlled experiment was used to isolate the causal influence of music and dance training on the executive functions of working memory, interference control, and task switching. Children between 6-9 years old were randomly assigned to music or dance groups, and tested on executive functions before and after training. Following training, significant decreases in global and local switch cost were observed, but no change on measures of working memory or interference control. Results show 15 days of training transferred to improved task switching, with no difference between groups. The current study provides early evidence on the influence of dance training on task switching in children, and the similarity between music and dance on executive functions.
Open Access
Effects of the Brain Derived Neurotrophic Factor Val66met Polymorphism on the Structural and Functional Architecture of the Human Brain
(2022-12-14) Alba Suarez, Vicente Alejandro; Stevens, Dale
Brain Derived Neurotrophic Factor (BDNF) is an important neurotrophin enabling synaptogenesis at the dendrites of neurons. Several studies have implicated the Val66Met single nucleotide polymorphism of the BDNF gene as a factor affecting cortical thickness and resting-state functional connectivity (RSFC) of the human brain. In this thesis, I investigated the effects of Val66Met on cortical thickness and RSFC among individual cortical regions and at the level of large-scale functional networks in all genotype groups (Val/Val, Val/Met, Met/Met, and Met carriers). Cutting-edge techniques were used to individually localize anatomical and functional brain regions in a large sample of healthy young adults from the Human Connectome Project. A comprehensive series of analyses revealed no significant group differences in cortical thickness or RSFC across the brain. These results suggest that, contrary to previous reports, the Met allele does not confer differences in structural or functional integrity of the healthy young adult brain.
Open Access
Experimental Evidence for Top-Down Attentional Selection in the Selective Tuning Model of Visual Attention
(2020-05-11) Yoo, Sang-Ah; Fallah, Mazyar
To overcome limited processing capacity, our visual system facilitates information that relates to the task at hand while inhibiting irrelevant information via selective attention. Among various attention models and theories, the Selective Tuning model of visual attention (ST) is a computation model of visual processing that is based on biological mechanisms. This model emphasizes the role of top-down feedback processing in visual perception and has predicted its unique consequences, such as an attentional surround suppression in which the attentional focus is accompanied by an inhibitory surround. The previous studies have experimentally validated STs predictions, indicating that the components in ST do reflect actual visual processing in the brain. Nevertheless, many aspects of ST still need to be elaborated and several predictions and assumptions remain untested. The series of works in this dissertation investigate different aspects of top-down feedback processing in visual perception that ST has proposed to corroborate this model and to broaden our understanding of visual attention. The first study examined whether top-down feedback processing is necessary for an attention-demanding, fine-grained visual localization (Chapter 2). The subsequent two studies focused on the properties of different types of the attentional surround suppression, the end-result of top-down feedback processing. The second study suggested the interplay between the location-based and feature-based surround suppression and tested the potential factors that could manipulate the spatial extent of the location-based suppressive surround (Chapter 3). The last study demonstrated feature-based surround suppression in motion processing and its neurophysiological mechanism (Chapter 4). Collectively, this work reinforces functional significance of top-down, attention-mediated feedback for visual processing and supports the validity of ST as well.
Open Access
Explicit Attention to Allocentric Landmarks Improves Memory-Guided Reaching
(2021-11-15) Musa, Lina; Crawford, John Douglas
The presence of an allocentric landmark can have both explicit (instruction-dependent) and implicit influences on reaching performance However, it is not known how the instruction itself (to rely either on egocentric versus allocentric cues) influences memory-guided reaching. Here, 13 participants performed a task with two instruction conditions (egocentric vs. allocentric), but with similar sensory and motor conditions. In the allocentric condition, participants were instructed to remember the initial location of the target relative to a landmark, and to reach relative to a shifted landmark. In the egocentric condition, participants were instructed to ignore the landmark and point toward the remembered location of the target. The allocentric instructions yielded significantly more accurate pointing than the egocentric instruction, despite identical visual and motor conditions and regardless of the final pointing side. This suggests that explicit attention to a visual landmark better recruits allocentric coding mechanisms that can augment implicit egocentric visuomotor transformations.
Open Access
First and Second Order Stereoscopic Processing of Fused and Diplopic Targets
(2015-08-28) Stransky, Debi; Wilcox, Laurie M.
Depth from stereopsis is due to the positional difference between the two eyes, which results in each eye receiving a different view of the world. Although progress has been made in understanding how the visual system processes stereoscopic stimuli, a number of questions remain. The goal of this work was to assess the relationship between the perceptual, the temporal and the 1st- /2nd- order dichotomies of stereopsis and in doing so, determine an appropriate method for measuring depth from large disparities. To this end, stereosensitivity and perceived depth were assessed using 1st- and 2nd- order stimuli over a range of test disparities and conditions. The main contributions of this research are as follows: 1) The sustained/transient dichotomy proposed by Edwards, Pope and Schor (2000) is best considered in terms of the spatial dichotomy proposed by Hess and Wilcox (1994). At large disparities it is not possible to categorize performance based on exposure duration alone; 2) There is not a simple correspondence between Ogle's (1952) patent / qualitative perceptual categories and the 1st- /2nd- order dichotomy proposed by Hess and Wilcox (1994); 3) Quantitative depth is provided by both 1st- and 2nd- order mechanisms in the fused range, but only the 2nd- order signal is used when stimuli are diplopic; 3) The quantitative depth provided by a 2nd- order stimulus scales with envelope size; and 4) The monoptic depth phenomenon may be related to depth from diplopic stimuli, but for conditions tested here when both monoptic depth and 2nd- order stereopsis are available, the latter is used to encode depth percepts. The results reported here expand on earlier work on 1st- and 2nd- order stereopsis and address the issues in the methodologies used to study depth from large disparities. These results are consistent with the widely accepted filter-rectify-filter model of 2nd- order processing, and 1st- and 2nd- order stimuli are likely encoded by disparity-sensitive neurons via a two-stream model (see Wilson, Ferrera, and Yo (1992); Zhou and Baker (1993)).