Global Form Processing Research Articles

Lack of orientation specific adaptation to vertically oriented Glass patterns in human visual cortex: an fMRI adaptation investigation

The perception of coherent form configurations in natural scenes relies on the activity of early visual areas that respond to local orientation cues. Subsequently, high-level visual areas pool these local signals to construct a global representation of the initial visual input. However, it is still debated whether neurons in the early visual cortex respond also to global form features. Glass patterns (GPs) are visual stimuli employed to investigate local and global form processing and consist of randomly distributed dots pairs called dipoles arranged to form specific global configurations. In the current study, we used GPs and functional magnetic resonance imaging (fMRI) adaptation to reveal the visual areas that subserve the processing of oriented GPs. Specifically, we adapted participants to vertically oriented GP, then we presented test GPs having either the same or different orientations with respect to the adapting GP. We hypothesized that if local form features are processed exclusively by early visual areas and global form by higher-order visual areas, then the effect of visual adaptation should be more pronounced in higher tier visual areas as it requires global processing of the pattern. Contrary to this expectation, our results revealed that adaptation to GPs is robust in early visual areas (V1, V2, and V3), but not in higher tier visual areas (V3AB and V4v), suggesting that form cues in oriented GPs are primarily derived from local-processing mechanisms that originate in V1. Finally, adaptation to vertically oriented GPs causes a modification in the BOLD response within early visual areas, regardless of the relative orientations of the adapting and test stimuli, indicating a lack of orientation selectivity.

Faces and words are both associated and dissociated as evidenced by visual problems in dyslexia

Faces and words are traditionally assumed to be independently processed. Dyslexia is also traditionally thought to be a non-visual deficit. Counter to both ideas, face perception deficits in dyslexia have been reported. Others report no such deficits. We sought to resolve this discrepancy. 60 adults participated in the study (24 dyslexic, 36 typical readers). Feature-based processing and configural or global form processing of faces was measured with a face matching task. Opposite laterality effects in these tasks, dependent on left–right orientation of faces, supported that they tapped into separable visual mechanisms. Dyslexic readers tended to be poorer than typical readers at feature-based face matching while no differences were found for global form face matching. We conclude that word and face perception are associated when the latter requires the processing of visual features of a face, while processing the global form of faces apparently shares minimal—if any—resources with visual word processing. The current results indicate that visual word and face processing are both associated and dissociated—but this depends on what visual mechanisms are task-relevant. We suggest that reading deficits could stem from multiple factors, and that one such factor is a problem with feature-based processing of visual objects.

The interaction of global motion and global form processing on the perception of implied motion: An equivalent noise approach

Global motion and global form are proposed to be processed through functionally differentiated independent channels along dorsal (motion) and ventral (form) pathways. However, more recent studies show significant interactions between these pathways by inducing the perception of motion (implied motion) from presenting the independent frames of static Glass patterns. The mechanisms behind such interaction are not adequately understood with studies showing a larger contribution of either a motion or form processing mechanism. In the current study, we adapted the equivalent noise paradigm to disentangle the effect of internal noise (local processing) and sampling efficiency (global processing) on global motion, global form, and the interaction of both on the perception of implied motion using physically equivalent stimuli. Six visually normal observers discriminated the direction or orientation of random dot kinematograms (RDK), static Glass patterns (Glass), and dynamic Glass patterns (dGlass) whose directions/orientations were determined by the means of normal distributions with a range of direction/orientation variances that served as external noise. Thresholds (τ) showed a consistent pattern across observers and external noise levels, where τGlass > τdGlass > τRDK. Nested model comparisons where the thresholds were related to the external noise, internal noise, and the sampling efficiency revealed that the difference in performance between the tasks was best described by the change in sampling efficiency with invariable internal noise. Our results showed that the higher thresholds for implied motion compared to real motion could be due to inefficient pooling of local dipole orientation cues at global processing stages involving motion mechanisms.

The neural mechanisms underlying directional and apparent circular motion assessed with repetitive transcranial magnetic stimulation (rTMS)

Dynamic Glass patterns (GPs) are a class of visual stimuli that evoke apparent motion and are commonly used to investigate the interaction between global form and motion processing in the visual system. Neuroimaging studies showed that (complex) circular dynamic GPs mainly activate areas along both the ventral and the dorsal stream such as hMT+, V3b/KO, dorsal V4 and LOC, whereas directional motion from rotating random dot kinematograms (RDKs) mainly activates the human MT complex (hMT+) and area V6. However, despite the large number of correlational information from fMRI studies, there is scarce evidence about the causal involvement of these brain areas in the perception of dynamic circular GPs and rotating RDKs. The aim of this study is to compare the neural basis of dynamic circular GPs and rotating RDKs by temporarily interfering with repetitive transcranial magnetic stimulation (rTMS) delivered over two visual areas largely involved in form and motion processing, such as V1/V2 and hMT+. Our results showed that rTMS over hMT + interfered only with the processing of rotating RDKs but not with the processing of circular dynamic GPs. On the other hand, rTMS delivered over early visual areas (V1/V2) did not interfere with the processing of both visual stimuli. These results suggest that partially different neural substrates subtend the processing of circular directional motion and apparent non-directional motion.

Towards a unified understanding of lateralized vision: A large-scale study investigating principles governing patterns of lateralization using a heterogeneous sample

While functional lateralization of the human brain has been a widely studied topic in the past decades, few studies to date have gone further than investigating lateralization of single, isolated processes. With the present study, we aimed to arrive at a more unified view by investigating lateralization patterns in face and word processing, and associated lower-level visual processing. We tested a large and heterogeneous participant group, and used a number of tasks that had been shown to produce replicable indices of lateralized processing of visual information of different types and complexity. Following Bayesian statistics, group-level analyses showed the expected right hemisphere (RH) lateralization for face, global form, low spatial frequency processing, and spatial attention, and left hemisphere (LH) lateralization for visual word and local feature processing. Compared to right-handed individuals, lateralization patterns of left-handed and especially those who are RH-dominant for language deviated from this ‘typical’ pattern. Our results support the notion that face and word processes come to be lateralized to homologue areas of the two hemispheres, under influence of the RH- and LH-specializations in global form, local feature, and low and high spatial frequency processing. As such, we present a more unified understanding of lateralized vision, providing evidence for the input asymmetry and causal complementarity principles of lateralized visual information processing. The absence of correlations between spatial attention and lateralization of the other processes supports the notion of their independent lateralization, conform the statistical complementarity principle.

Global motion and form processing and attention deficits in multiple child cohorts with neurodevelopmental disorders: Dorsal vulnerability or dorsal/ventral integration?

Global motion and form processing and attention deficits in multiple child cohorts with neurodevelopmental disorders: Dorsal vulnerability or dorsal/ventral integration?

Different trajectories of decline for global form and global motion processing in aging, mild cognitive impairment and Alzheimer's disease

The visual processing of complex motion is impaired in Alzheimer's disease (AD). However, it is unclear whether these impairments are biased toward the motion stream or part of a general disruption of global visual processing, given some reports of impaired static form processing in AD. Here, for the first time, we directly compared the relative preservation of motion and form systems in AD, mild cognitive impairment, and healthy aging, by measuring coherence thresholds for well-established global rotational motion and static form stimuli known to be of equivalent complexity. Our data confirm a marked motion-processing deficit specific to some AD patients, and greater than any form-processing deficit for this group. In parallel, we identified a more gradual decline in static form recognition, with thresholds raised in mild cognitive impairment patients and slightly further in the AD group compared with controls. We conclude that complex motion processing is more vulnerable to decline in dementia than complex form processing, perhaps owing to greater reliance on long-range neural connections heavily targeted by AD pathology.

Electrophysiological evidence of altered visual processing in adults who experienced visual deprivation during infancy.

We examined the role of early visual input in visual system development by testing adults who had been born with dense bilateral cataracts that blocked all patterned visual input during infancy until the cataractous lenses were removed surgically and the eyes fitted with compensatory contact lenses. Patients viewed checkerboards and textures to explore early processing regions (V1, V2), Glass patterns to examine global form processing (V4), and moving stimuli to explore global motion processing (V5). Patients' ERPs differed from those of controls in that (1) the V1 component was much smaller for all but the simplest stimuli and (2) extrastriate components did not differentiate amongst texture stimuli, Glass patterns, or motion stimuli. The results indicate that early visual deprivation contributes to permanent abnormalities at early and mid levels of visual processing, consistent with enduring behavioral deficits in the ability to process complex textures, global form, and global motion.

Concurrent Investigation of Global Motion and Form Processing in Amblyopia: An Equivalent Noise Approach.

Directly comparing the motion and form processing in neurologic disorders has remained difficult due to the limitations in the experimental stimulus. In the current study, motion and form processing in amblyopia was characterized using random dot stimuli in different noise levels to parse out the effect of local and global processing on motion and form perception. A total of 17 amblyopes (8 anisometropic and 9 strabismic), and 12 visually normal subjects monocularly estimated the global direction of motion and global orientation in random dot kinematograms (RDK) and Glass patterns (Glass), whose directions/orientations were drawn from normal distributions with a range of means and variances that served as external noise. Direction/orientation discrimination thresholds were measured without noise first then variance threshold was measured at the multiples of the direction/orientation threshold. The direction/orientation and variance thresholds were modelled to estimate internal noise and sampling efficiency parameters. Overall, the thresholds for Glass were higher than RDK for all subjects. The thresholds for both Glass and RDK were higher in the strabismic eyes compared with the fellow and normal eyes. On the other hand, the thresholds for anisometropic amblyopic eyes were similar to the normal eyes. The worse performance of strabismic amblyopes was best explained by relatively low sampling efficiency compared with other groups (P < 0.05). A deficit in global motion and form perception was only evident in strabismic amblyopia. Contrary to the dorsal stream deficiency hypothesis assumed in other developmental disorders, deficits were present in both motion (dorsal) and form (ventral) processing.

Global form and motion processing in healthy ageing

The ability to perceive biological motion has been shown to deteriorate with age, and it is assumed that older adults rely more on the global form than local motion information when processing point-light walkers. Further, it has been suggested that biological motion processing in ageing is related to a form-based global processing bias. Here, we investigated the relationship between older adults' preference for form information when processing point-light actions and an age-related form-based global processing bias. In a first task, we asked older (>60years) and younger adults (19–23years) to sequentially match three different point-light actions; normal actions that contained local motion and global form information, scrambled actions that contained primarily local motion information, and random-position actions that contained primarily global form information. Both age groups overall performed above chance in all three conditions, and were more accurate for actions that contained global form information. For random-position actions, older adults were less accurate than younger adults but there was no age-difference for normal or scrambled actions. These results indicate that both age groups rely more on global form than local motion to match point-light actions, but can use local motion on its own to match point-light actions. In a second task, we investigated form-based global processing biases using the Navon task. In general, participants were better at discriminating the local letters but faster at discriminating global letters. Correlations showed that there was no significant linear relationship between performance in the Navon task and biological motion processing, which suggests that processing biases in form- and motion-based tasks are unrelated.

Local form interference in biological motion perception.

Replacing the local dots of point-light walkers with complex images leads to significant detriments to performance in biological motion detection and discrimination tasks. This detriment has previously been shown to be larger when the local elements match the global shape in object category and facing direction. In contrast, studies using Navon stimuli have demonstrated that local interference on global processing primarily occurs when local elements are dissimilar to the global form. In 3 experiments, we investigated this contradiction by replacing the local dots of a point-light walker with human images or stick figures. Participants were significantly faster and more accurate at discriminating the facing and walking direction of a walker when the local images were facing in the same direction as the global walker than when they were facing in the opposite direction. These results provide support for the idea that organization of biological motion depends on allocation of limited processing resources to the global motion information when the local elements are complex. However, there is more disruption to global form processing when the local elements and global form conflict in task-related properties.

The effect of blur on cortical responses to global form and motion.

Global form and motion sensitivity undergo long development in childhood with motion sensitivity rather than form being impaired in a number of childhood disorders and both impaired in adult clinical populations. This suggests extended development and vulnerability of extrastriate cortical areas associated with global processing. However, in some developmental and clinical populations, it remains unclear to what extent impairments might reflect deficits at earlier stages of visual processing, such as reduced visual acuity and contrast sensitivity. To address this, we investigated the impact of degraded spatial vision on cortical global form and motion processing in healthy adults. Loss of high spatial frequencies was simulated using a diffuser to blur the stimuli. Participants completed behavioral and EEG tests of global form and motion perception under three levels of blur. For the behavioral tests, participants' form and motion coherence thresholds were measured using a two-alternative, forced-choice procedure. Steady-state visual evoked potentials were used to measure cortical responses to changes in the coherence of global form and motion stimuli. Both global form and global motion perception were impaired with increasing blur as measured by elevated behavioral thresholds and reduced cortical responses. However, form thresholds showed greater impairment in both behavioral and EEG measures than motion thresholds at the highest levels of blur. The results suggest that high spatial frequencies play an important role in the perception of both global form and motion but are especially significant for global form. Overall, the results reveal complex interactions between low-level factors and global visual processing, highlighting the importance of taking these factors into account when investigating extrastriate function in low vision populations.

The impact of recreational MDMA 'ecstasy' use on global form processing.

The ability to integrate local orientation information into a global form percept was investigated in long-term ecstasy users. Evidence suggests that ecstasy disrupts the serotonin system, with the visual areas of the brain being particularly susceptible. Previous research has found altered orientation processing in the primary visual area (V1) of users, thought to be due to disrupted serotonin-mediated lateral inhibition. The current study aimed to investigate whether orientation deficits extend to higher visual areas involved in global form processing. Forty-five participants completed a psychophysical (Glass pattern) study allowing an investigation into the mechanisms underlying global form processing and sensitivity to changes in the offset of the stimuli (jitter). A subgroup of polydrug-ecstasy users (n=6) with high ecstasy use had significantly higher thresholds for the detection of Glass patterns than controls (n=21, p=0.039) after Bonferroni correction. There was also a significant interaction between jitter level and drug-group, with polydrug-ecstasy users showing reduced sensitivity to alterations in jitter level (p=0.003). These results extend previous research, suggesting disrupted global form processing and reduced sensitivity to orientation jitter with ecstasy use. Further research is needed to investigate this finding in a larger sample of heavy ecstasy users and to differentiate the effects of other drugs.

Orientation coherence sensitivity

We developed a global orientation coherence task for the assessment of global form processing along similar lines to the global motion coherence task. The task involved judgments of global orientation for an array of limited duration 1-D Gabors, some of which were signal (signal orientation) and some of which were noise (random orientation). We address two issues. First: Do motion and form global processing have similar dependencies? And second: Can global sensitivity be explained solely in terms of integrative function? While most dependencies (e.g., contrast, spatial scale, and field size) are similar for form and motion processing, there is a greater dependence on eccentricity for form processing. Sensitivity for global tasks involves more than just integration by filters broadly tuned for orientation. Results are best modeled by filters with narrowband orientation tuning that effectively segregate as well as integrate global information.

Local form–motion interactions influence global form perception

Object motion perception depends on the integration of form and motion information into a unified neural representation. Historically, form and motion perception are thought to be independent processes; however, research has demonstrated that these processes interact in numerous and complex ways. For example, an object's orientation relative to its direction of motion will influence its perceived speed (Georges, Seriès, Frégnac, & Lorenceau, Vision Research 42:2757-2772, 2002). Here, we investigated whether this local form-motion interaction influences global form processing. In Experiment 1, we replicated the effect of orientation-dependent modulation of speed. In Experiment 2, we investigated whether the perceived speed of local elements could influence the perceived shape of a global object constructed from grouping of those elements. The results indicated that the orientation of local elements indeed influenced the perceived shape of a global object. We propose that inputs from local form-motion processes are one of perhaps many neural mechanisms underlying global form integration.

Visual processing in Noonan syndrome: Dorsal and ventral stream sensitivity

Global spatial and motion processing abilities were assessed in 18 patients with Noonan syndrome (NS) and in 43 matched controls using form and motion coherence testing, respectively. We observed a discrepancy between the two groups since the study group had significantly lower performances than the control group for form coherence while there was no impairment on motion coherence. All the patients were also assessed on the Movement Assessment Battery for Children (M-ABC) to evaluate visuomotor skills. Thirteen of the 18 (72%) also had global poor performances on the M-ABC. The results show that children with NS have a specific impairment in the global processing of visuospatial information and are likely to have a specific ventral stream deficit as also suggested by the frequent visuomotor perceptual difficulties. Testing form and motion coherence thresholds may be a useful diagnostic tool for this group of patients, despite their normal cognitive abilities, since aspects of global form processing and visuomotor perceptual difficulties can be identified and potentially targeted for a specific rehabilitation program.

Perception of shapes targeting local and global processes in autism spectrum disorders

Several researchers have found evidence for impaired global processing in the dorsal visual stream in individuals with autism spectrum disorders (ASDs). However, support for a similar pattern of visual processing in the ventral visual stream is less consistent. Critical to resolving the inconsistency is the assessment of local and global form processing ability. Within the visual domain, radial frequency (RF) patterns - shapes formed by sinusoidally varying the radius of a circle to add 'bumps' of a certain number to a circle - can be used to examine local and global form perception. Typically developing children and children with an ASD discriminated between circles and RF patterns that are processed either locally (RF24) or globally (RF3). Children with an ASD required greater shape deformation to identify RF3 shapes compared to typically developing children, consistent with difficulty in global processing in the ventral stream. No group difference was observed for RF24 shapes, suggesting intact local ventral-stream processing. These outcomes support the position that a deficit in global visual processing is present in ASDs, consistent with the notion of Weak Central Coherence.

Global visual processing in macaques studied using Kanizsa illusory shapes

The ability to extract form information from a visual scene, for object recognition or figure-ground segregation, is a fundamental visual system function. Many studies of nonhuman primates have addressed the neural mechanisms involved in global form processing, but few have sought to demonstrate this ability behaviorally. In this study, we probed global visual processing in macaque monkeys (Macaca nemestrina) using classical Kanizsa illusory shapes as an assay of global form perception. We trained three monkeys on a "similarity match-to-sample" form discrimination task, first with complete forms embedded in fields of noncontour-inducing "pacman" elements. We then tested them with classic Kanizsa illusory shapes embedded in fields of randomly oriented elements. Two of the three subjects reached our criterion performance level of 80% correct or better on four of five illusory test conditions, demonstrating clear evidence of Kanizsa illusory form perception; the third subject mastered three of five conditions. Performance limits for illusory form discrimination were obtained by manipulating support ratio and by measuring threshold for discriminating "fat" and "thin" illusory squares. Our results indicate that macaque monkeys are capable of global form processing similarly to humans and that the perceptual mechanisms for "filling-in" contour gaps exist in macaques as they do in humans.

High-density VEP measures of global form and motion processing in infants born very preterm

High-density VEP measures of global form and motion processing in infants born very preterm

High-density VERPs show distinct mechanisms for global form and motion processing in adults and infants

High-density VERPs show distinct mechanisms for global form and motion processing in adults and infants