Giulia Rampone

Electrophysiological responses to visuospatial regularity

Humans are quicker to detect reflectional than rotational or translational symmetry, despite the fact that these patterns are equally regular. We were interested in the neural correlates of these perceptual effects. Participants viewed random, reflection, rotation, and translation patterns while we recorded EEG from the scalp. Half the participants classified the pattern regularity overtly, the other half did not explicitly attend to pattern regularity but reported rare oddball trials, where two squares were embedded among the dots. The amplitude of a symmetry-related ERP known as the sustained posterior negativity was most pronounced for reflection, then rotation and translation. We suggest that reflectional symmetry, despite its biological significance, may not be processed by unique visual mechanisms, but instead it could be a preferred stimulus for a more general regularity-sensitive network in the extrastriate visual cortex.

Implicit association of symmetry with positive valence, high arousal and simplicity

Symmetrical visual patterns are preferred to random patterns. Studies using the Implicit Association Test (IAT) have shown that symmetry is associated with positive valence words (e.g. love), and random with negative valence words (e.g. hate). Valence is an important aspect of emotion, but equally interesting is the relationship between the symmetry–random dimension and the dimensions of arousal and complexity. Possible links have long been discussed but empirical evidence is limited. Using a series of four IAT experiments, we report that participants implicitly associate symmetrical patterns with words high in arousal, and random patterns with low arousal. We also found that symmetrical patterns were associated with simple mathematic expressions, while random patterns were associated with complex expressions. No link was found for another aspect of mathematical complexity (smaller or larger numbers). This pattern of results shows that aesthetic responses to symmetry involve both positive valence and high arousal and that these emotional responses arise from the perceptual simplicity of symmetry, in line with the fluency account of aesthetics.

Visual symmetry in objects and gaps.

It is known that perceptual organization modulates the salience of visual symmetry. Reflectional symmetry is more quickly detected when it is a property of a single object than when it is formed by a gap between two objects. Translational symmetry shows the reverse effect, being more quickly detected when it is a gap between objects. We investigated the neural correlates of this interaction. Electroencephalographic data was recorded from 40 participants who were presented with reflected and translated contours in one- or two-object displays. Half of the participants discriminated regularity, half distinguished number of objects. An event-related potential known as the Sustained Posterior Negativity (SPN) distinguished between reflection and translation. A similar ERP distinguished between one and two object presentations, but these waves summed with the SPN, rather than altering it. All stimuli produced desynchronization of 8-13 Hz alpha oscillations over the bilateral parietal cortex. In the Discriminate Regularity group, this effect was right lateralized. The SPN and alpha desynchronization index different stages of visual symmetry discrimination. However, neither component displayed the Regularity × Objecthood interaction that is observed in speeded discrimination tasks, suggesting that integration of visual regularity with objectness is not inevitable. Instead, both attributes may be processed in parallel and independently.

Conditions for view invariance in the neural response to visual symmetry

Symmetry 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 event-related 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.

An Electrophysiological Index of Perceptual Goodness

A traditional line of work starting with the Gestalt school has shown that patterns vary in strength and salience; a difference in “Perceptual goodness.” The Holographic weight of evidence model quantifies goodness of visual regularities. The key formula states that W = E/N, where E is number of holographic identities in a pattern and N is number of elements. We tested whether W predicts the amplitude of the neural response to regularity in an extrastriate symmetry-sensitive network. We recorded an Event Related Potential (ERP) generated by symmetry called the Sustained Posterior Negativity (SPN). First, we reanalyzed the published work and found that W explained most variance in SPN amplitude. Then in four new studies, we confirmed specific predictions of the holographic model regarding 1) the differential effects of numerosity on reflection and repetition, 2) the similarity between reflection and Glass patterns, 3) multiple symmetries, and 4) symmetry and anti-symmetry. In all cases, the holographic approach predicted SPN amplitude remarkably well; particularly in an early window around 300–400 ms post stimulus onset. Although the holographic model was not conceived as a model of neural processing, it captures many details of the brain response to symmetry.

Electrophysiological analysis of the affective congruence between pattern regularity and word valence

Reflection 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 200 ms, and persisted up to 1,000 ms 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.

Brain Activity in Response to Visual Symmetry.

Research on the neural basis of symmetry perception is making rapid progress. ERP studies reliably show a sustained posterior negativity (SPN), with lower amplitude for symmetrical than random patterns at occipital electrodes, beginning around 250 ms after stimulus onset. Functional MRI shows activity in extrastriate visual areas and in the lateral occipital complex (LOC). We summarise and integrate the evidence by answering six questions. (1) Is there an automatic and sustained response to symmetry in visual areas? Yes, and because this is broadly unaffected by the task it suggests automatic processing of symmetry. (2) Which brain areas are involved in symmetry perception? There is an extended symmetry-sensitive network in extrastriate visual areas and the LOC. (3) Is bilateral reflection special? Reflection is the optimal stimulus for a general regularity-sensitive network that responds also to rotation and translation. (4) Is the response to symmetry independent of view angle? When people classify patterns as symmetrical or random, the response to symmetry is view-invariant. When people attend to other dimensions, the network responds to residual regularity in the image. (5) Does the neural response to symmetry scale with degree of regularity? Yes, the proportion of symmetrically positioned elements predicts the size of SPN and fMRI responses (see attached Figure). Finally, (6) Are connections between the hemispheres especially important for symmetry perception? No. SPN amplitude increases with the number of axes, and is comparable for horizontal and vertical symmetry. Overall, these studies of the brain mechanisms involved in symmetry perception show a consistent link between brain activity and measures of sensitivity from psychophysical studies. Meeting abstract presented at VSS 2015.

The Role of Visual Eccentricity on Preference for Abstract Symmetry.

This study tested preference for abstract patterns, comparing random patterns to a two-fold bilateral symmetry. Stimuli were presented at random locations in the periphery. Preference for bilateral symmetry has been extensively studied in central vision, but evaluation at different locations had not been systematically investigated. Patterns were presented for 200 ms within a large circular region. On each trial participant changed fixation and were instructed to select any location. Eccentricity values were calculated a posteriori as the distance between ocular coordinates at pattern onset and coordinates for the centre of the pattern. Experiment 1 consisted of two Tasks. In Task 1, participants detected pattern regularity as fast as possible. In Task 2 they evaluated their liking for the pattern on a Likert-scale. Results from Task 1 revealed that with our parameters eccentricity did not affect symmetry detection. However, in Task 2, eccentricity predicted more negative evaluation of symmetry, but not random patterns. In Experiment 2 participants were either presented with symmetry or random patterns. Regularity was task-irrelevant in this task. Participants discriminated the proportion of black/white dots within the pattern and then evaluated their liking for the pattern. Even when only one type of regularity was presented and regularity was task-irrelevant, preference evaluation for symmetry decreased with increasing eccentricity, whereas eccentricity did not affect the evaluation of random patterns. We conclude that symmetry appreciation is higher for foveal presentation in a way not fully accounted for by sensitivity.

Exogenous cueing modulates preference formation

Attentional shifts can be produced by external (exogenous) cues. Here we focus on whether a shift of attention modulates affective responses. In five experiments, the peripheral onset of an uninformative cue was followed by a novel abstract pattern. We observed that exogenous cuing enhanced target discrimination (RTs) and preference formation (ratings) (Experiment 1a). We therefore distinguish two validity effects: VERTs and VEpreference. Validity effects are measured by the difference between valid and invalid conditions. Interestingly, VEpreference was sensitive to several parameters. When oculomotor responses were inhibited, cuing induced VERTs but not VEpreference (Experiment 1b). Also the cue-to-target (inter-stimulus interval, ISI) interval was critical. By increasing ISI we eliminated both VERTs and VEpreference (Experiment 2a and 2b). Further investigation revealed that the VEpreference originates from a combination of a validity benefit and an invalidity cost (Experiment3). We also investigated whether VEpreference indirectly results from the experience of fluency on valid conditions. The target in Experiment 4 was a simple circle and participants responded to its location. After this response, a non-target pattern appeared at fixation and was evaluated. Although VERTs was present for target detection, VEpreference for non-target patterns at fixation was absent. In Experiment 5 the non-target patterns were presented at the peripheral location of the target. Both VERTs and VEpreference were observed. Preference modulation may thus be determined by a sense of fluency associated with the cue-to-target contingency. However, reorienting attention with an additional saccade can reset the effect. Overall these studies document the relationship between exogenous attention and preference. Our results highlight the important role of attention and oculomotor responses in the mechanism that links visual and emotional responses. Meeting abstract presented at VSS 2015.