Keywords: Oblique effect; Gestalt; Event-related potentials; P300

Article Outline

1. Introduction

1.1. Psychophysical oblique effect

1.2. Oblique effects in evoked potentials

1.3. Oblique effects on different processing levels

1.4. The oblique effect in higher-level event-related potentials

1.5. Gestaltness and ERPs

2. Methods

2.1. Stimuli and tasks

2.2. Subjects and EEG recording

2.3. Data analysis

3. Results

4. Discussion

4.1. General characterization of the ERPs as compared to earlier studies

4.2. Absence of low-level responses, but correlates of Gestaltness in mid-level processing

4.3. A visuo-cognitive oblique effect

5. Conclusion

Acknowledgements

Appendix A. Supplementary data

References

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Fig. 1. (A) Stimulus paradigm. Starting from random positions, 24 line elements continuously moved and rotated during a 3-s-epoch. After 2 s, all elements had either moved into a position to form one of two Gestalts (0° or 45° Gestalt, respectively), or they remained in a gestaltless (random) configuration. (B) Samples of the stimulus sequence taken at 200 ms intervals. For better reproduction, the contrast has been enhanced here as compared to the actual stimulus used in the experiment.

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Fig. 2. Grand-mean ERPs (±SEM) of all electrodes topographically arranged, showing the results from all target conditions (i.e. the stimuli corresponded to the task). While the largest difference is between Gestalt and gestaltless stimuli, there is also a clear orientation effect for the two Gestalt stimuli. Fig. 3 provides significance estimates.

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Fig. 3. Significance of ERP differences between all three target stimulus conditions. Each number indicates the P value for the comparison between the two stimuli depicted next to it. The values were obtained with a permutation test applied to the time interval from −1.0 to +1.0 s. Since 5000 random permutations were evaluated, the smallest possible P is 0.0002. The asterisks give the significance levels in standard notation (*, groupwise α=0.05; **, groupwise α=0.01), based on a sequential Bonferroni adjustment. Except for one combination at Fpz, all comparisons were indicating significance, suggesting that ERP differences exist between both Gestalt conditions, and between Gestalt and gestaltless conditions.

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Fig. 4. Time-resolved significance of differences between the three target conditions. Curves show P values as a function of time and indicate that primarily the differences in the P300-like component account for the small P value found with the permutation test for the 0° vs. 45° comparison at the Pz electrode (cf. Fig. 3). Gestalt vs. gestaltless comparisons, in particular for the 45° stimulus, revealed robust significant differences already around −200 ms at occipito-temporal locations.

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Fig. 5. Grand-mean Pz ERPs for different stimulus/task combinations, overlaid for direct comparison. The most dominant structure is a positive deflection peaking after physical Gestalt completion (time 0), but building up before it. With the Gestalt stimuli it is larger and occurs earlier than with the non-Gestalt stimuli. Compared to these differences, the effect of the task is relatively small.

Median reaction time (RT) and corresponding interquartile range (i.r.) for the three different tasks (0°/45° Gestalt, gestaltless stimuli) and abrupt vs. continuous presentation, as obtained in a supplementary behavioral experiment

	0° Gestalt				45° Gestalt				Gestaltless stimuli
	Abrupt		Continuous		Abrupt		Continuous		Abrupta		Continuous
Subject	RT	i.r.	RT	i.r.	RT	i.r.	RT	i.r.	RT	i.r.	RT	i.r.
S1	427	103	−76	127	469	86	43	105	744	375	477	357
S2	440	132	−101	137	432	109	8	143	400	280	219	227

In the trials without Gestalt, the movement of the elements was always continuous. Six blocks (3 tasks × abrupt/continuous presentation), each with 30 trials, were presented. The block order was reversed between subjects. Subjects were instructed to press a button as quickly as possible when they were sure that they had identified the target Gestalt. In the continuous conditions, subjects pressed the button about 400–500 ms earlier than in the abrupt conditions, but the variability as indicated by the interquartile range was only increased by about 20 ms. Responses for the 45° Gestalt occurred more than 100 ms later than those for the 0° Gestalt. RTs are given relative to the time when the Gestalt was presented (abrupt) or completed (continuous). Even though the trials without Gestalt were identical in the two presentation conditions, the reaction times differed by about 200 ms.
^a Note that the presentation of the gestaltless stimuli was always continuous; the column heading ‘abrupt’ refers to the block type and indicates that the Gestalt stimuli in this block were presented abruptly.

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