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Psychology
Neuropsychology and Physiological Psychology

Electrophysiological responses to visuospatial regularity

Profile image of Marco BertaminiMarco Bertamini

2013, Psychophysiology

https://doi.org/10.1111/PSYP.12082
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Abstract

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.

Figures (5)
Figure 1. Stimuli used in this experiment. In the EEG recording, all participants were presented with 60 repeats of each regular pattern and 60 repeats o sither the constrained or unconstrained random patterns. Patterns were visible for 3s before participants entered their judgment. Half the participant >xplicitly discriminated between reflection, rotation, random, and translation patterns. For the other participants, the task was to judge whether it was ; 10rmal trial (all dots) or an oddball trial (two squares among the dots).
Figure 1. Stimuli used in this experiment. In the EEG recording, all participants were presented with 60 repeats of each regular pattern and 60 repeats o sither the constrained or unconstrained random patterns. Patterns were visible for 3s before participants entered their judgment. Half the participant >xplicitly discriminated between reflection, rotation, random, and translation patterns. For the other participants, the task was to judge whether it was ; 10rmal trial (all dots) or an oddball trial (two squares among the dots).
Figure 2. Behavioral results. A: Results of the manipulation check experiment. Mean response time is shown for each stimulus type. Separate bars show data for the groups of participants presented with constrained or unconstrained random patterns. B: Explicit discrimination performance as a function of regularity and random constraints. C: Sensitivity in the oddball detection task for each participant. Greater d’ values reflect better performance. D: Response bias in the oddball detection task. Greater C values reflect bias towards reporting “no oddball.” Error bars =+ 1 S.E.M.
Figure 2. Behavioral results. A: Results of the manipulation check experiment. Mean response time is shown for each stimulus type. Separate bars show data for the groups of participants presented with constrained or unconstrained random patterns. B: Explicit discrimination performance as a function of regularity and random constraints. C: Sensitivity in the oddball detection task for each participant. Greater d’ values reflect better performance. D: Response bias in the oddball detection task. Greater C values reflect bias towards reporting “no oddball.” Error bars =+ 1 S.E.M.
Figure 3. Grand average ERP waveforms (N = 48). A: Grand average ERPs from the PO7/PO8 electrodes in the four regularity conditions. N1 and SPN are highlighted in purple. B: Topographic plots of the difference between the average of the regular patterns and the random patterns at N1 and SPN latencies The SPN is shown in two plots, demonstrating that it remains similar throughout. Here, red means higher amplitude in the regular patterns, blue means highet amplitude for the random patterns. C: Grand average ERP from the P1, PZ, and P2 electrodes. All ERPs were smoothed with a 10-point moving average filter in addition to data preprocessing procedures described above (the same applies to other figures). D: Difference plot showing the parietal P300 component Arrows on the topographic maps show the electrodes plotted in the corresponding ERP plots.
Figure 3. Grand average ERP waveforms (N = 48). A: Grand average ERPs from the PO7/PO8 electrodes in the four regularity conditions. N1 and SPN are highlighted in purple. B: Topographic plots of the difference between the average of the regular patterns and the random patterns at N1 and SPN latencies The SPN is shown in two plots, demonstrating that it remains similar throughout. Here, red means higher amplitude in the regular patterns, blue means highet amplitude for the random patterns. C: Grand average ERP from the P1, PZ, and P2 electrodes. All ERPs were smoothed with a 10-point moving average filter in addition to data preprocessing procedures described above (the same applies to other figures). D: Difference plot showing the parietal P300 component Arrows on the topographic maps show the electrodes plotted in the corresponding ERP plots.
Figure 4. Grand average ERPs in each condition (N = 24). A: Explicit discrimination task. B: Oddball detection task. C: ERPs from participants presente with constrained random patterns. D: ERPs from participants presented with unconstrained random patterns. N1 and SPN windows are highlighted in purple Insets show SPN amplitude for each regular pattern. Bars show the difference between the random wave and each regular wave from 300-1,000 m poststimulus onset. Asterisks highlight significant differences, so that asterisks under the bars denote that the amplitude was less than random, while other show differences between regularities. *p < .05; **p < .01; ***p < .001. (See online version for color figure.)
Figure 4. Grand average ERPs in each condition (N = 24). A: Explicit discrimination task. B: Oddball detection task. C: ERPs from participants presente with constrained random patterns. D: ERPs from participants presented with unconstrained random patterns. N1 and SPN windows are highlighted in purple Insets show SPN amplitude for each regular pattern. Bars show the difference between the random wave and each regular wave from 300-1,000 m poststimulus onset. Asterisks highlight significant differences, so that asterisks under the bars denote that the amplitude was less than random, while other show differences between regularities. *p < .05; **p < .01; ***p < .001. (See online version for color figure.)
Figure 5. Grand average ERPs in each subcondition (N = 12). A: Explicit discrimination task with constrained random patterns. B: Explicit discrimination task with unconstrained random patterns. C: Oddball detection task with constrained random patterns. D: Oddball detection task with unconstrained random patterns. Insets are based on the same conventions as Figure 4. (See online version for color figure.)
Figure 5. Grand average ERPs in each subcondition (N = 12). A: Explicit discrimination task with constrained random patterns. B: Explicit discrimination task with unconstrained random patterns. C: Oddball detection task with constrained random patterns. D: Oddball detection task with unconstrained random patterns. Insets are based on the same conventions as Figure 4. (See online version for color figure.)

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Marco Bertamini

Scientific Reports

Electrophysiological (EEG) studies of human perception have found that amplitude at posterior electrodes is more negative for symmetrical patterns compared to asymmetrical patterns. This negativity lasts for hundreds of milliseconds and it has been called sustained posterior negativity (SPN). Symmetry activates a network of visual areas, including the lateral occipital complex (LOC). The SPN is a response to presence of symmetry in the image. Given the sustained nature of this activation, in this study we tested the persistence of the SPN after stimulus offset. Two shapes were presented (for 0.5 s each) with a 1 s blank interval in between. We observed a sustained response after stimulus offset, irrespective of whether the task required processing of shape information. This supports the idea that the response to symmetry is generated by information in the image, independently of task, and that it is sustained over approximately one second post stimulus onset. Visual symmetry has an important role in the study of visual perception, and empirical work on perception of symmetry has a long history 1-6. Neurophysiological and neuroimaging work has found evidence, using different methodologies, of tuning to image regularity within ventral, lateral and dorsal occipital cortex, including the Lateral Occipital Complex 7. For example, several ERP studies have shown a regularity-related negativity over parieto-occipital areas (Fig. 1). In this paper, we briefly review this literature, and then present new data about the persistence of the symmetry response after stimulus offset. Observers compared patterns presented in two intervals (500 ms each) separated by a 1000 ms blank interval. This presentation is short compared to that used in previous EEG studies (typically more than 1 s). We use this procedure to (i) analyse persistent activity during the blank interval, and (ii) test whether this persistent activity is present also when shape information is not relevant for the task. In addition, we will analyse the response to the second pattern as a function of its relationship with the first pattern. Observers compared the patterns (symmetrical or asymmetrical) and decided whether they were identical or different. Note that this is a task requiring processing of shape, but not necessarily processing of symmetry. We compared this shape matching task to a different task in which observers were not required to process the shape relationship between patterns (i.e. colour task). It is possible that when participants are engaged in matching the stimuli based on shape information, response to symmetry continues after the first offset; on the contrary when colour is attended, symmetry-related activity might be reduced or absent. To anticipate our results, we report that the activation continues after stimulus offset in both tasks, and this activation does not change (it is not enhanced) when the task requires a comparison and therefore a processing of shape information. With respect to the response to the second stimulus, symmetry-response was enhanced in the second interval when a novel symmetrical exemplar was shown, more than when the same symmetrical pattern was repeated, suggesting a form of symmetry priming. The main focus of the study is the persistence of the response to a visual stimulus after offset, using electroencephalography (EEG) and the analysis of visual evoked potential (VEP). The International Society for Clinical Electrophysiology of Vision (ISCEV) defines three standard VEP protocols based on the type of stimuli used: (a) Pattern-reversal, (b) Pattern onset/offset and (c) Flash 8. For pattern reversal there is no distinction between onset and offset, and the typical VEP has an initial negative deflection (N70), a prominent positive peak (P100), and a later negative component (N155). For pattern onset/offset, the time course is similar but with reversed polarity.

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    Asymmetry for Symmetry: Right-Hemispheric Superiority in Bi-Dimensional Symmetry Perception
    Giulia Prete

    Symmetry

    A right-hemispheric superiority has been shown for spatial symmetry perception with mono-dimensional stimuli (e.g., bisected lines). Nevertheless, the cerebral imbalance for bi-dimensional stimuli is still controversial, and the aim of the present study is to investigate this issue. Healthy participants and a split-brain patient (D.D.C.) were tested in a divided visual field paradigm, in which a square shape was presented either in the left or right visual field and they were asked to judge whether a dot was placed exactly in the center of the square or off-center, by using the left/right hand in two separate sessions. The performance of healthy participants was better when the stimuli presented in the left visual field (LVF) were on-center rather than off-center. The performance of D.D.C. was higher than chance only when on-center stimuli were presented in the LVF in the left hand session. Only in this condition did his accuracy not differ with respect to that of the control group, whereas in all of the other conditions, it was lower than the controls' accuracy. We conclude that the right-hemispheric advantage already shown for mono-dimensional stimuli can be extended also to bi-dimensional configurations, confirming the right-hemispheric superiority for spatial symmetry perception.

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    Anticipatory Defocusing of Attention and Contextual Response Priming but No Role of Aesthetic Appreciation in Simple Symmetry Judgments when Switching between Tasks
    Thomas Jacobsen

    Symmetry

    Visual attention can be adjusted to task requirements. We asked participants to switch between judging the symmetry of vertically presented three-letter strings and identifying the central stimulus (i.e., Eriksen task) to investigate anticipatory adjustment of attention. Our experiments provide evidence for anticipatory adjustment of visual attention, depending on the cued task (i.e., focusing and defocusing of attention after the Eriksen task cue and after the symmetry task cue, respectively). Although, symmetry judgments were, overall, considerably slower than the identification of the central letter, the effects of response congruency between tasks were comparable in the two tasks, which suggested strong response priming from concurrent symmetry judgment in Eriksen task trials. Symmetry judgment performance was best for homogeneous letter strings (e.g., HHH), worst for strings that were symmetrical and inhomogeneous (e.g., XHX), and intermediate for asymmetrical strings (e.g., HH...

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    The Posterior Sustained Negativity Revisited—An SPN Reanalysis of Jacobsen and Höfel (2003)
    Thomas Jacobsen

    Symmetry

    Symmetry is an important cue for the aesthetic judgment of beauty. Using a binary forced-choice format in a cued mixed design, Jacobsen and Höfel (2003) compared aesthetic judgments of beauty and symmetry judgments of novel graphic patterns. A late posterior sustained negativity elicited by symmetric patterns was observed in the symmetry judgment condition, but not in the beauty judgement condition. Therefore, this negativity appeared to be mainly driven by the task.In a series of studies, Bertamini, Makin, and colleagues observed a comparable sustained posterior negativity (SPN) to symmetric stimuli, mainly taken to reflect obligatory symmetry processing independent of task requirements. We reanalyzed the data by Jacobsen and Höfel (2003) using similar parameters for data analysis as Bertamini, Makin, and colleagues to examine these apparent differences. The reanalysis confirmed both a task-driven effect on the posterior sustained negativity/SPN to symmetric patterns in the symmetry judgment condition and a strong symmetry-driven SPN to symmetric patterns. Differences between the references used for analyses of the electroencephalogram (EEG) had an effect. Based on the reanalysis, the Jacobsen and Höfel (2003) data also fit well with Bertamini's, Makin's, and colleagues' account of obligatory symmetry processing.

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    Haemodynamic Signatures of Temporal Integration of Visual Mirror Symmetry
    David Badcock

    Symmetry

    EEG, fMRI and TMS studies have implicated the extra-striate cortex, including the Lateral Occipital Cortex (LOC), in the processing of visual mirror symmetries. Recent research has found that the sustained posterior negativity (SPN), a symmetry specific electrophysiological response identified in the region of the LOC, is generated when temporally displaced asymmetric components are integrated into a symmetric whole. We aim to expand on this finding using dynamic dot-patterns with systematically increased intra-pair temporal delay to map the limits of temporal integration of visual mirror symmetry. To achieve this, we used functional near-infrared spectroscopy (fNIRS) which measures the changes in the haemodynamic response to stimulation using near infrared light. We show that a symmetry specific haemodynamic response can be identified following temporal integration of otherwise meaningless dot-patterns, and the magnitude of this response scales with the duration of temporal delay. ...

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