Depth map color constancy

The early stages of visual information processing, involving the detection and perception of simple visual stimuli, have been demonstrated to be sensitive to psychotropic agents. The present study investigated the effects of an acute dose of bromazepam (3 mg), compared with placebo, on the P100 component of the visual evoked potential and reaction time. The sample, consisting of 14 healthy subjects (6 male and 8 female), was submitted to a visual discrimination task, which employed the "oddball" paradigm. Results suggest that bromazepam (3 mg) impairs the initial stage of visual information processing, as observed by an increase in P100 latency.

A general impairment of cognitive performance occurs during acute insulin-induced hypoglycaemia, but little objective evidence is available for disruption of more specific cognitive processes. The effect of controlled hypoglycaemia on the early stages of visual information processing and contrast sensitivity was examined in a homogeneous group of 20 nondiabetic human subjects. Hypoglycaemia caused a significant disruption in general cognitive performance as assessed by a digit symbol task (P < 0.001) and the trail making B task (P < 0.05). Hypoglycaemia also produced a highly significant deterioration in performance on all of the visual information processing tasks, namely inspection time (IT) (P = 0.01), visual change detection (VCD) (P < 0.005) and visual movement detection (VMD) (P < 0.005). A significant deterioration in contrast sensitivity was observed during hypoglycaemia (P < 0.005). In contrast, no significant effect of hypoglycaemia was demonstrated on standard clinical measures of visual acuity or stereoscopic vision. Thus, although hypoglycaemia caused no detectable deterioration in visual acuity as measured by Snellen-type tests, a marked deterioration occurred in the speed of visual information processing and in contrast sensitivity. As many decisions are made under conditions of limited perceptual time and low visual contrast (e.g. when driving), the disruptive effect of moderate insulin-induced hypoglycaemia on visual perception will have important practical implications in diabetic humans exposed to this metabolic stress. The present results are congruent with other evidence which shows that the early stages of visual information processing are susceptible to deterioration by general cerebral insults.

Cross-diagnostic comparison of visual processing in bipolar disorder and schizophrenia

A spreadsheet was used to construct a computer simulation of the early stages of visual information processing, specifically, the processing taking place in the retina, lateral geniculate nucleus, and visual cortex. The simulation was based on a mathematical model of Marr (1982) that accounts for the process of locating edges or boundaries and discriminating them from other changes in intensity in the visual input. The simulation relied on two mechanisms suggested by Marr. One mechanism integrated inputs from a region of the retina in order to map intensity changes into a functional pattern called a zero-crossing. The other mechanism discriminated edges from other sorts of intensity changes by proposing that edges and only edges are apparent as spatially coincident zero-crossings across a range of retinal field sizes. The simulation embodied these principles in a spreadsheet model with three layers: one that mapped intensity changes to zero-crossings, another that detected these zero-crossings, and a third that detected spatial coincidence across retinal fields of diverse widths. The simulation was able to discriminate edges (patterns with abrupt changes in intensity) and edges buried in noise from uniform input patterns, patterns with gradual changes in intensity, and random noise patterns.

Humans and macaque monkeys, performing a Wisconsin Card Sorting Test (WCST), show a significant behavioral bias to a particular sensory dimension (e.g. color or shape); however, lesions in prefrontal cortical regions do not abolish the dimensional biases in monkeys and, therefore, it has been proposed that these biases emerge in earlier stages of visual information processing. It remains unclear whether such dimensional biases are unique to the WCST, in which attention-shifting between dimensions are required, or affect other aspects of executive functions such as 'response inhibition' and 'error-induced behavioral adjustments'. To address this question, we trained six monkeys (Macaca mulatta) to perform a stop-signal task in which they had to inhibit their response when an instruction for inhibition was given by changing the color or shape of a visual stimulus. Stop Signal Reaction Time (SSRT) is an index of inhibitory processes. In all monkeys, SSRT was significantly shorter, and the probability of a successful inhibition was significantly higher, when a change in the shape dimension acted as the stop-cue. Humans show a response slowing following a failure in response inhibition and also adapt a proactive slowing after facing demands for response inhibition. We found such adaptive behavioral adjustments, with the same pattern, in monkeys' behavior; however, the dimensional bias did not modulate them. Our findings, showing dimensional bias in monkey, with the same pattern, in two different executive control tasks support the hypothesis that the bias to shape dimension emerges in early stages of visual information processing.

How we perceive a visual stimulus can be influenced by its surrounding context. For example, the presence of a reference skews the perception of a similar feature in a stimulus, a phenomenon called reference repulsion. Ongoing research so far remains inconclusive regarding the stage of visual information processing where such repulsion occurs. We examined the influence of a reference on late visual processing. We measured the repulsion effect caused by an orientation reference presented after an orientation ensemble stimulus. The participants’ reported orientations were significantly biased away from the post-stimulus reference, displaying typical characteristics of reference repulsion. Moreover, explicit discrimination choices between the reference and the stimulus influenced the magnitudes of repulsion effects, which can be explained by an encoding-decoding model that differentiates the re-weighting of sensory representations in implicit and explicit processes. These results support the notion that reference repulsion may arise at a late decision-related stage of visual processing, where different sensory decoding strategies are employed depending on the specific task.

Exploring the short term visual store in schizophrenia using the attentional blink

In the present study, we investigated effects of phasic alerting on visual attention in a partial report task, in which half of the displays were preceded by an auditory warning cue. Based on the computational Theory of Visual Attention (TVA), we estimated parameters of spatial and non-spatial aspects of visual attention and measured event-related lateralizations (ERLs) over visual processing areas. We found that the TVA parameter sensory effectiveness a, which is thought to reflect visual processing capacity, significantly increased with phasic alerting. By contrast, the distribution of visual processing resources according to task relevance and spatial position, as quantified in parameters top-down control α and spatial bias windex, was not modulated by phasic alerting. On the electrophysiological level, the latencies of ERLs in response to the task displays were reduced following the warning cue. These results suggest that phasic alerting facilitates visual processing in a general, unselective manner and that this effect originates in early stages of visual information processing.

Visual processing in anorexia nervosa and body dysmorphic disorder: Similarities, differences, and future research directions

IntroductionEye movement disorder in patients with Parkinson's disease (PD) is less likely to appear when observing complex images compared with simple images. However, in case of auditory stimuli, meaningful stimuli can promote PD movement. Thus, we measured visual search movements with an eye tracker to investigate whether visual search changes in PD depending on the meaningfulness of visual stimuli; additionally, we measured event‐related potentials (ERPs) to neurophysiologically examine visual information processing in PD.MethodsData from 11 patients with cognitively unimpaired PD and 10 neurologically healthy individuals (controls) were used in this study. We simultaneously measured eye tracking and ERPs during the observation of three types of images: operation, noun, and meaningless figures. We compared intergroup differences in visual search parameters, such as saccade count, fixation time, and saccade amplitude, as well as in ERPs vertex‐to‐vertex amplitudes and latency among P1, N1, P2, and N2 at the posterior regions. These ERPs reflect different stages of visual information processing and provide insights into the underlying neural mechanisms of visual search in PD.ResultsAll visual search parameters were consistently smaller in the PD group than in the control group, regardless of visual stimulus. The saccade count in the PD group was significantly lower than in the control group for operation and meaningless figures but not for noun figures. Further, P1 and N1 amplitudes—bottom–up processing—were smaller in PD group when viewing operation figures and N2 amplitudes—top–down processing—were larger in PD group when viewing operation and meaningless figures.ConclusionsPD's visual search movements may change depending on the meaningfulness of visual stimuli. Further, the abnormal visual search movements in PD may be due to insufficient bottom–up processing and excessive top–down processing.Plain Language SummaryParkinson's disease (PD) is a neurodegenerative disorder that can cause eye movement disorders. People with PD may have difficulty searching for and processing visual information. In this study, we investigated how meaningfulness affects visual search movements and visual information processing in people with PD. We measured eye movements and brain activity in response to different types of visual stimuli, including meaningful and meaningless images, and compared them to a group of healthy control participants. Our results showed that PD patients had fewer eye movements and different brain activity patterns compared to healthy controls, regardless of the meaningfulness of the visual stimuli. However, we also found that the meaningfulness of visual stimuli had an effect on visual search movements in PD patients. This suggests that meaningfulness can impact the way people with PD process visual information. Understanding these differences could help in the development of new therapies to improve visual processing in people with PD.

Modulation of responses to visual stimulus onset and offset by chronic alcohol consumption and withdrawal in the rat visual cortex and lateral geniculate nucleus.

STANLEY, GORDON, and HALL, RODNEY. Short-Term Visual Information Processing in Dyslexics. CHILD DEVELOPMENT, 1973, 44, 841-844. 2 measures of visual information processing were compared for dyslexic and control samples of children. With the first measure 2 parts of a stimulus were exposed sequentially for 20 msec each at increasing interstimulus intervals (ISIs). Both separation and identification thresholds were longer for dyslexics than normals. With the second measure letters presented for 20 msec were masked by dots, the interstimulus interval between the letter and masker being incremented over trials. Dyslexics required longer ISIs than controls for correct identification of the letter. This difference was greater when the letters were visually confusable. Both experiments provide evidence that significant differences exist between dyslexics and normals at early stages of visual information processing.

This study used feature-integration theory as a means of determining the point in processing at which selective attention deficits originate. The theory posits an initial stage of processing in which features are registered in parallel and then a serial process in which features are conjoined to form complex stimuli. Performance of young and older adults on feature versus conjunction search is compared. Analyses of reaction times and error rates suggest that elderly adults in addition to young adults, can capitalize on the early parallel processing stage of visual information processing, and that age decrements in visual search arise as a result of the later, serial stage of processing. Analyses of a third, unconfounded, conjunction search condition reveal qualitatively similar modes of conjunction search in young and older adults. The contribution of age-related data limitations is found to be secondary to the contribution of age decrements in selective attention.

Despite the constant influx of visual information, observers are nonetheless able to segment this input into discrete objects and events. The perceptual system does so on the basis of spatial and temporal properties, thus allowing one to keep track of visual objects as they move to different locations across time. This process of object individuation is integral for visual awareness; when it is disrupted, stimuli are no longer perceived. Behavioural studies that have investigated object individuation in temporal and spatial domains converge on the idea that object individuation is a capacity-limited process that gates which items proceed for further analysis. Although there has been considerable theoretical and behavioural work on object individuation, we know relatively little about its neural substrates. The experiments in this thesis investigate the brain regions that support object individuation across multiple episodic contexts and processing stages, characterise its capacity limits and relationship to identification, and isolate the stage of processing at which individuation arises.Chapter 2 describes a functional magnetic resonance imaging (fMRI) study that isolated the neural bases of temporal individuation during perception, and the consequences that arise when its processing limit is reached (repetition blindness, RB). RB is a rapid serial visual presentation phenomenon characterised by reduced performance on trials with a target repetition compared with those in which the two targets have different identities (non-repetition trials). This failure of perceptual consciousness is thought to reflect a capacity limit of temporal individuation. I first verified that my RB paradigm elicited the standard behavioural effect (Experiment 1) and was specific to the temporal limits of individuation, rather than identification (Experiment 2). Using fMRI (Experiment 3), I found that multivariate patterns of blood-oxygen-level-dependent (BOLD) activity across a large number of occipital, parietal and frontal regions could discriminate between trials in which a repetition was correctly reported (a demanding individuation scenario), compared with correct non-repetition trials (a relatively easy individuation scenario). Consistent with current models of consciousness, and contrary to existing work on spatial individuation at the level of memory encoding, these findings suggest that temporal individuation is supported by a distributed set of brain regions. In terms of RB itself, I found greater activity in the left premotor cortex for incorrect versus correct repetition trials. This result suggests that the left premotor region is critical for the processing limitations that give rise to RB.In Chapter 3, I tested whether object individuation and identification can be dissociated in the brain at the level of visual short term memory (VSTM) encoding and beyond, as proposed in the neural object file theory. Participants completed a delaye VSTM task in which they had to remember the identity and spatial locations of one object, four identical objects or four different objects. To isolate object individuation regions, BOLD activity was compared between one object and four identical objects. By contrast, to identify object identification regions, BOLD activity was contrasted between four identical objects and four different objects. Across univariate and multivariate analyses, I found brain regions that were specific to individuation or identification processes, and others that were common between the two. These findings challenge the neural object file theory, and instead suggest that object individuation and identification processes have distributed and overlapping neural substrates.The aim of the experiments reported in Chapter 4 was to characterise the timecourse of object individuation for attended and unattended objects, and determine the extent to which this operation draws on early sensory cortices. Previous event-related potential (ERP) studies were unable to show definitive evidence of object individuation at early perceptual stages of analysis, because the paradigms in these studies confounded manipulations of individuation load (i.e., number of targets) with low-level visual features (e.g., luminance). I first developed a novel enumeration paradigm involving items defined by illusory contours, which held all physical properties constant across conditions (Experiment 1), and then used electroencephalography (EEG) to investigate the timecourse of object individuation for attended and unattended stimuli (Experiment 2). Both P1 (100-140 ms) and N2 (185-250 ms) amplitudes increased with the number of attended targets, but the number of unattended non-targets only modulated the N2. An fMRI study (Experiment 3) showed that early visual cortex (including V2) was sensitive to individuation load, which I hypothesised might underpin the observed P1 effect. These findings demonstrated that task-relevant individuation occurs at a relatively early stage of visual information processing, and voluntary spatial attention modulates the timecourse of this operation.Taken together, the experiments reported in this thesis offer novel insights into our understanding of the neural underpinnings of object individuation across various stages of processing. These findings have implications for current theoretical accounts of object individuation and its associated processes in the brain, and contribute to models of how individuation should be operationalised as a construct.

The cannabinoid system and visual processing: A review on experimental findings and clinical presumptions