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Scaling of the extrastriate neural response to symmetry
Full text linkNeuroimaging work has shown that visual symmetry activates extrastriate brain areas, most consistently the lateral occipital complex (LOC). LOC activation increases with proportion of symmetrical dots (pSymm) in a degraded display. In the current work, we recorded a posterior ERP called the sustained posterior negativity (SPN), which is relatively negative for symmetrical compared to random patterns. We predicted that SPN would also scale with pSymm, because it is probably generated by the LOC. Twenty-four participants viewed dot patterns with different levels of regularity: 0% regularity (full random configuration) 20%, 40%, 60%, 80%, and 100% (full reflection symmetry). Participants judged if the pattern contained “some regularity” or “no regularity”. As expected, the SPN amplitude increased with pSymm, while the latency and duration was the same in all conditions. The SPN was independent of the participant’s decision, and it was present on some trials where people reported ‘no-regularity’. We conclude that the SPN is generated at an intermediate stage of visual processing, probably in the LOC, where perceptual goodness is represented. This comes after initial visual analysis, but before subsequent decision stages, which apply a threshold to the analog LOC response
Conditions for view invariance in the neural response to visual symmetry
No full textSymmetry detection is slow when patterns are distorted by perspective, perhaps due to a time-consuming normalization
process, or because discrimination relies on remaining weaker regularities in the retinal image. Participants viewed
symmetrical or random dot patterns, either in a frontoparallel or slanted plane (±50°). One group performed a color
discrimination task, while another performed a regularity discrimination task. We measured a symmetry-related eventrelated
potential (ERP), beginning around 300 ms. During color discrimination, the ERP was reduced for slanted
patterns, indexing only the remaining retinal structure. During regularity discrimination, the same ERP was view
invariant, and identical for frontoparallel or slanted presentation. We conclude that normalization occurs rapidly during
active symmetry discrimination, while symmetry-sensitive networks respond only to regularity in the retinal image when
people are attending to other features
Examining visual complexity and its effect on perceived duration
Full text linkWe investigated whether visual complexity of novel\ud
abstract patterns affects perceived duration. Previous\ud
research has reported that complex visual stimuli led\ud
to an underestimation of durations. However, to\ud
clarify the nature of the time estimation process, it is\ud
necessary to establish which component of image\ud
complexity, spatial or semantic, plays the critical role.\ud
Here we tested the impact of specific spatial\ud
properties. We used unfamiliar and abstract patterns\ud
made using black-and-white checkerboards in which\ud
the difference between stimuli was exclusively in\ud
configuration. Visual complexity was quantified by the\ud
GIF index based on a compression algorithm, which\ud
scanned the pattern in both horizontal and vertical\ud
directions. This metric correlated positively with\ud
subjective complexity (Experiment 1A). In the second\ud
study, we increased variability in the stimuli by\ud
changing the number of items across patterns while\ud
keeping overall size constant. A high positive\ud
correlation was found between objective and\ud
subjective complexity (r ¼ 0.95) (Experiment 2A). In\ud
Experiments 1B and 2B, observers estimated pattern\ud
durations in seconds using a continuous scale. A\ud
multilevel linear analysis found that perceived\ud
duration was not predicted by visual complexity for\ud
either of the two sets of stimuli. These results provide\ud
new constraints to theories of time perception,\ud
hypothesizing that complexity leads to an\ud
underestimation of duration when it reduces attention\ud
to time
Electrophysiological analysis of the affective congruence between pattern regularity and word valence
No full textAbstractReflection symmetry is an important property of human designs and biological organisms, and it is often judged to be beautiful. Previous reaction-time based studies have shown a congruency effect, where reflection symmetry facilitates processing of positive words, and random patterns facilitate negative words. But what is the neural basis of affective responses to symmetry? In Experiment 1 we recorded ERPs from posterior electrode clusters while participants viewed reflection or random patterns with either a positive or negative word superimposed. In the Discriminate Regularity task, participants categorized the patterns (reflection or random). In the Discriminate Word task, they categorized the words as positive or negative. In Experiment 2, participants classified words and patterns on each trial. We found a difference between ERP waves from congruent (reflection with positive word, random with negative word) and incongruent trials (reflection with negative, random with positive). This congruency effect began around 200ms, and persisted up to 1000ms post stimulus, and was only present in the Discriminate Word task. We suggest that when evaluating words, participants automatically evaluate the background pattern as well, and this alters early visual processing
Representation of symmetry in the extrastriate visual cortex from temporal integration of parts: An EEG/ERP study
Full text linkWhen symmetry is present in the retinal image, a symmetry-sensitive network in the extrastriate visual areas activates, and response magnitude scales with degree of regularity. Is this activation driven by the regularity in the image, or can the network recover regularity of an object? We investigated whether the network responds to bilateral symmetry for dynamically occluded shapes, and thus responds to symmetry in the object. The stimulus was an irregular shape partly occluded by a rectangle. After 500 ms, the rectangle was displaced to the other side, occluding the previously visible half, and revealing the other half for 1000 ms. Therefore, no symmetry was present in the image at any point in time. Exp.1 and Exp.2 used vertical and horizontal axis of reflection, and in Exp. 3 there was no occluder. Participants could detect symmetry with >80% accuracy. More importantly, ERP analysis showed a symmetry-specific response from ∼300 ms after presentation of the second half of the shape. When integration was made from halves of asymmetric whole shapes (Exp.4), and when symmetry was not task-relevant (Exp.5), no symmetry response was recorded. The results show, for the first time, an electrophysiological evidence of symmetry representation in the brain obtained by assembling information over time into a unitary gestalt. The integration process occurs when observers look for symmetric matches between the parts, and only if these are perceived as belonging to the same object
Do different types of dynamic extrapolation rely on the same mechanism?
Full text linkObservers can estimate the position of occluded moving objects, and this ability could be mediated by the oculomotor control system. However, other forms of motion cannot be tracked with pursuit eye movements. In Experiment 1, 2 kinds of motion extrapolation tasks were interleaved. In the Position extrapolation trials, participants observed a moving target that became occluded. They attempted to press a button when it arrived at the end of the occluder. In novel Accumulation extrapolation trials, they observed a matrix of Gabors with new elements appearing over time. The participants pressed when Gabors would have filled the entire matrix, had they continued accumulating at the same rate. In both tasks completion time estimates were related to presentation parameters in the same way. Moreover, there were robust intertask correlations: Participants who responded earlier in the position task also responded earlier in the accumulation task. Experiment 2 replicated these results with a third form of extrapolation. Although performance was not identical in all tasks, there were enough similarities to suggest that overlapping systems guide all forms of extrapolation. We propose that a common rate control mechanism guides extrapolation, like the velocity store in oculomotor control, but with a broader function than previously envisaged
Electrophysiological responses to symmetry presented in the left or in the right visual hemifield.
Full text linkSymmetry is a highly salient feature in the visual world, abundant in both man-made and natural objects. In particular, humans find reflectional symmetry most salient. Electrophysiological work on symmetry perception has identified a difference wave known as the Sustained Posterior Negativity (SPN) originating from extrastriate areas. Amplitude is more negative for symmetrical than random patterns, from around 200 msec after stimulus onset. For the first time, we report responses to patterns presented exclusively in one hemifield. Participants were presented with reflection or random dot patterns to the left and right of fixation (3.2°). They judged whether the patterns were light red or dark red in colour. In Experiment 1, the pair always included one symmetrical and one random pattern. In Experiments 2 and 3 we varied the information presented contralaterally. The SPN was generated separately in each hemisphere in response to what was presented in the contralateral visual hemifield (a lateralised SPN). We conclude that a symmetry-sensitive network of extrastriate areas can be activated independently in each cerebral hemisphere
Visual symmetry in objects and gap
Full text linkIt is known that perceptual organization modulates the\ud
salience of visual symmetry. Reflectional symmetry is\ud
more quickly detected when it is a property of a single\ud
object than when it is formed by a gap between two\ud
objects. Translational symmetry shows the reverse\ud
effect, being more quickly detected when it is a gap\ud
between objects. We investigated the neural correlates\ud
of this interaction. Electroencephalographic data was\ud
recorded from 40 participants who were presented with\ud
reflected and translated contours in one- or two-object\ud
displays. Half of the participants discriminated regularity,\ud
half distinguished number of objects. An event-related\ud
potential known as the Sustained Posterior Negativity\ud
(SPN) distinguished between reflection and translation.\ud
A similar ERP distinguished between one and two object\ud
presentations, but these waves summed with the SPN,\ud
rather than altering it. All stimuli produced\ud
desynchronization of 8?13 Hz alpha oscillations over the\ud
bilateral parietal cortex. In the Discriminate Regularity\ud
group, this effect was right lateralized. The SPN and\ud
alpha desynchronization index different stages of visual\ud
symmetry discrimination. However, neither component\ud
displayed the Regularity · Objecthood interaction that\ud
is observed in speeded discrimination tasks, suggesting\ud
that integration of visual regularity with objectness is\ud
not inevitable. Instead, both attributes may be\ud
processed in parallel and independently
Right-lateralized alpha desynchronization during regularity discrimination: Hemispheric specialization or directed spatial attention?
Full text linkWhen actively classifying abstract patterns according to their regularity, alpha desynchronization (ERD) becomes right lateralized over posterior brain areas. This could reflect temporary enhancement of contralateral visual inputs and specifically a shift of attention to the left, or right hemisphere specialization for regularity discrimination. This study tested these competing hypotheses. Twenty-four participants discriminated between dot patterns containing a reflection or a translation. The direction of the transformation, which matched one half onto the other half, was either vertical or horizontal. The strategy of shifting attention to one side of the patterns would not produce lateralized ERD in the horizontal condition. However, right-lateralized ERD was found in all conditions, regardless of orientation. We conclude that right hemisphere networks that incorporate the early posterior regions are specialized for regularity discrimination
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