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.
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.
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 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.Descriptors: Symmetry, Event-related potentials, Sustained posterior negativity, View invariance, Perspective distortionThe two-dimensional retinal projection of an object changes dramatically as the observer adopts different vantage points. This produces novel inputs to the recognition system, but objects are nevertheless identified reliably and rapidly, and this formidable computational feat occurs unconsciously. Logothetis and Sheinberg (1996) concluded that some neurons are view invariant (firing to their preferred stimulus independent of view angle), some are view selective (firing more for some view angles), and that view invariance is more common for familiar objects than novel objects. It is also known that the neural response to faces becomes increasingly view invariant in higher visual regions (Axelrod & Yovel, 2012). Here, we measured whether the neural response to abstract visual symmetry is view invariant or view selective.Many visual systems are highly sensitive to symmetry, perhaps because it helps to identify objects against a background (Machilsen, Pauwels, & Wagemans, 2009), to achieve shape constancy (Pizlo & Stevenson, 1999), or because it indicates reproductive fitness (Tyler, 1995). Bilateral symmetry perception has been demonstrated in insects (Plowright, Evans, Leung, & Collin, 2011), birds (Møller, 1992), and humans (Julesz, 1971; Mach, 1886 Mach, / 1959. In humans, symmetry perception interacts with other figural cues (Bertamini, 2010;Treder & van der Helm, 2007), while models have been developed that extract symmetry from spatial filters (Dakin & Hess, 1997).Most psychophysical and neuroimaging researchers have used symmetric patterns in the frontoparallel plane, which produce a symmetrical retinal projection (e.g., Bertamini, Friedenberg, & Kubovy, 1997;Jacobsen & Höfel, 2003;Royer, 1981;Sasaki, Vanduffel, Knutsen, Tyler, & Tootell, 2005;Wenderoth, 1994). However, the benefits of symmetry perception presuppose the ability to recognize symmetrical objects from multiple viewpoints: After all, an observer will almost never enco...
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.
The brain can organize elements into perceptually meaningful gestalts. Visual symmetry is a useful tool to study gestalt formation, and we know that there are symmetry-sensitive regions in the extrastriate cortex. However, it is unclear whether symmetrical gestalt formation happens automatically, whatever the participant's current task is. Does the visual brain always organize and interpret the retinal image when possible, or only when necessary? To test this, we recorded an ERP called the sustained posterior negativity (SPN). SPN amplitude increases with the proportion of symmetry in symmetry + noise displays. We compared the SPN across five tasks with different cognitive and perceptual demands. Contrary to our predictions, the SPN was the same across four of the five tasks but selectively enhanced during active regularity discrimination. Furthermore, during regularity discrimination, the SPN was present on hit trials and false alarm trials but absent on miss and correct rejection trials. We conclude that gestalt formation is automatic and task-independent, although it occasionally fails on miss trials. However, it can be enhanced by attention to visual regularity.
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