Object Perception: When Our Brain Is Impressed but We Do Not Notice It

Journal of Vision (2009) 9(1):7, 1–10 http://journalofvision.org/9/1/7/ 1

Object perception: When our brain is impressed but we do not notice it

Universitäts-Augenklinik, Freiburg, Germany,& Institut für Grenzgebiete der Psychologie, Jürgen Kornmeier Freiburg, Germany Michael Bach Universitäts-Augenklinik, Freiburg, Germany

Although our eyes receive incomplete and ambiguous information, our perceptual system is usually able to successfully construct a stable representation of the world. In the case of ambiguous figures, however, perception is unstable, spontaneously alternating between equally possible outcomes. The present study compared EEG responses to ambiguous figures and their unambiguous variants. We found that slight figural changes, which turn ambiguous figures into unambiguous ones, lead to a dramatic difference in an ERP (“event-related potential”) component at around 400 ms. This result was obtained across two different categories of figures, namely the geometric Necker cube stimulus and the semantic Old/Young Woman face stimulus. Our results fit well into the Bayesian inference concept, which models the evaluation of a perceptual interpretation’s reliability for subsequent action planning. This process seems to be unconscious and the late EEG signature may be a correlate of the outcome. Keywords: bistable perception, multistable perception, perceptual ambiguity, instable perception, ambiguous figures, Necker cube, Old/Young Woman, EEG, ERP, event-related potentials, object perception Citation: Kornmeier, J., & Bach, M. (2009). Object perception: When our brain is impressed but we do not notice it. Journal of Vision, 9(1):7, 1–10, http://journalofvision.org/9/1/7/, doi:10.1167/9.1.7.

cube (Figure 2b, Necker, 1832) are found in the chapters on Introduction conscious perception of nearly every neuroscience textbook. Many authors regard those figures as key stimuli with which What do you see in Figure 1: An old or a young to address the topics of object representation and conscious woman? Prolonged inspection suggests bothVbut not perception, since they enable us to separate neural activity simultaneously. When we observe an ambiguous figure, related to the visual input from activity related to the like the “Old/Young Woman” (Boring, 1930), our percept perceptual outcome (e.g., Blake & Logothetis, 2002). is unstable, changing spontaneously between two or more However, after research over more than 200 years possible interpretations. The information that enters via (Long & Toppino, 2004), the mechanisms underlying our eyes is very often ambiguous, not least due to the spontaneous perceptual reversals are still poorly under- projection of the 3D world onto our 2D retinae. Visual stood (Blake & Logothetis, 2002). perception can thus be viewed as an incessant attempt to Recent physiological studies concentrated on the question solve the visual “inverse problem” of how several of when and how perceptual reversals take place. Several perceptual interpretations can result from one and the authors thus investigated the time intervals, close to same retinal image. Given this uncertainty, how is the perceptual reversals (e.g., Bas¸ar-Eroglu, Stru¨ber, Schu¨rmann, brain at all able to construct a stable percept of the world, Stadler, & Bas¸ar, 1996; Kaernbach, Schro¨ger, Jacobsen, & reliable enough for us to act successfully in the world and Roeber, 1999; Kornmeier & Bach, 2004, 2006; O’Donnell, survive in the long run? There is a general consensus Hendler, & Squires, 1988; Pitts, Nerger, & Davis, 2007; regarding the earliest steps of visual perception, such as Roeber et al., 2008;Stru¨ber & Herrmann, 2002). In the the visual areas’ retinotopic and columnar organization present study, however, we examined how the processing of (Holmes & Lister, 1916; Schira, Wade, & Tyler, 2007), visual objects, resulting in enduring stable percepts, differs but a sizable gap remains if we approach object in general from object processing, which in turn leads to representation and consciousness. It is vividly discussed unstable percepts that spontaneously break down time and whether an object is represented by a small sample of time again. What is the difference between the neural grandmother-like neurons (e.g., Barlow, 1972; Quiroga, processing and representation of ambiguous and unambig- Reddy, Kreiman, Koch, & Fried, 2005) or by population uous stimuli? One surprising result is this: Although the codes of distributed neural assemblies (Pouget, Dayan, & figural changes rendering ambiguous figures unambiguous Zemel, 2000). Ambiguous figures, like Boring’s Old/ (Figure 2b) and enabling observers to identify them with Young Woman (Figure 1, Boring, 1930) or the Necker 994% accuracy are tiny (Figures 2c and 2d), certain EEG

doi: 10.1167/9.1.7 Received June 6, 2008; published January 12, 2009 ISSN 1534-7362 * ARVO

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2004) and two unambiguous variants with depth cues (shading, central projection, and aerial perspective, OpenGL lighting model, Woo, Neider, & Davis, 1998). The semantic stimuli were a line drawing of an ambiguous face proﬁle (Figure 2b, F1; Gale & Findlay, 1983), a variant of Borings “Old/Young Woman” (Boring, 1930). We added two unambiguous variants (Figure 2b,F2and F3). The lattice stimuli were presented at a viewing angle of 7.5- 7.5-. The face stimuli were presented at a viewing angle of 7.5- 12.5-. The stimuli were created with a Macintosh G4 computer and displayed on a Philips GD 402 monochrome CRT screen at a refresh rate of 75 Hz.

Procedure

The stimuli were presented discontinuously with 800-ms presentation time T 91-ms uniformly distributed random time jitter to prevent potential habituation effects (Figure 2a). Successive stimulus presentations were Figure 1. Old/Young Woman (Boring, 1930): This face profile can intermitted by 27-ms inter-stimulus intervals with a blank be seen as an old or a young woman looking to the right. The screen. EEG was recorded while the participants com- pronounced nose of the old woman can also be interpreted as the pared successive stimuli (Si). They indicated in go/nogo- chin of a young girl. The wart on top of the old woman’s nose can tasks in the reversal condition perceived orientation be seen as the nose of the young girl. And the old woman’s eye reversals in the two opposite directions (e.g., from lattice can be interpreted as the young woman’s right ear. front side bottom right to front side top left or vice versa) correlates of the perception of these stimuli differ dramat- with different hands pressing different keys. In a separate ically, by up to factor 4. Apparently the huge differences in stability condition they indicated stability of each of the neural activity underlying these EEG results are not two possible percepts (either two consecutive lattices consciously noticed. This finding occurred with stimuli perceived with their front side as bottom right or as top from two completely different categories, namely the left) with different hands pressing different keys. In the geometric Necker cube and the semantic Old/Young case of ambiguous stimuli, perceptual changes had an Woman. endogenous origin inside the brain, while in the cases of the unambiguous variants the changes were induced exogenously in a random order with a probability of P = 0.5. Methods The geometric lattice stimuli (Figure 2b, L1–L3) and the face stimuli (Figure 2b, F1–F3) were presented in Participants separate experiments with the same experimental protocol but with different participants. Each experiment (lattice and face) consisted thus of four conditions, namely Nine women and five men aged from 20 to 27 with a unambiguous stimulus/indicate reversal (“unambiguous/ median age of 24 years took part in the lattice experiment. reversal”), unambiguous/stability, ambiguous/reversal, Eight women and five men aged from 21 to 30 with a and ambiguous/stability (see Table 1). Each experiment median age of 25 years took part in the face experiment. consisted of 24 experimental runs with six repetitions of All participants were naive as to the specific experimental each of the four conditions in an ABCDDCBA sequence. question and gave their informed written consent. All of Each experimental run stopped after participants had them had normal or corrected-to-normal visual acuity. The responded 16 times at the latest after 7 minutes. study was performed in accordance with the ethical standards laid down in the Declaration of Helsinki (World Medical Association, 2000) and was approved by the local ethics review board. EEG recordings and data analysis EEG was recorded from nine gold-cup scalp electrodes Stimuli at Oz, P3, P4, Pz, T7, T8, Cz, Fz, Fpz (American Clinical Neurophysiology Society, 2006) with a linked ear refer- The geometric stimuli were a “Necker lattice” (Figure 2b, ence. Horizontal and vertical EOG detected blinks and eye L1), consisting of 3 3 Necker cubes (Kornmeier & Bach, movements. Impedance at each electrode location was

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Figure 2. (a) Paradigm: EEG was recorded while participants compared successive briefly presented stimuli (Si) and indicated in a go/ nogo task in one condition perceptual changes and in a separate condition perceptual stability (two consecutive stimuli perceived as identical). In the case of ambiguous stimuli, perceptual changes had an endogenous origin inside the brain, while in the case of the unambiguous variants changes were induced exogenously. Lattices and faces were presented in separate experiments with different participants. In each experiment ambiguous stimuli (L1 or F1) and unambiguous variants (L2 and L3 or F2 and F3) were presented in separate experimental blocks. (b) Stimuli: L1: Necker lattice; L2, L3: Unambiguous variants. F1: Ambiguous Old/Young Woman; F2, F3: Unambiguous variants. (c) Normalized magnitude plots (spectra) of a 2-dimensional Fourier analysis of the stimulus pictures directly above in row (b). To enhance small details, the gray level represents the square root of the magnitude in spatial frequency space. The spectra of the lattice and the face clearly differ. For instance in the lattice the regular grid leads to equidistant-spaced harmonics in the spectrum. The difference between ambiguous and non-ambiguous, while obvious in row (b) is hard to detect in the spectra. This suggests that the ERP findings cannot be explained by low-level image statistics. (d) Total duration that the specific stimulus from above (b) was perceived relative to the total presentation time of that stimulus. The relative percentages indicate that the ambiguous versions are close at the 50:50% ratio and that small figural changes render the stimuli almost completely unambiguous. kept below 10 k4. Signals were band-passed at 0.1–70 Hz, (“channel”) and averaged separately into ERPs (“event- digitized at a sampling rate of 500 Hz, and written to disk. related potentials”, Bach, 1998; Luck, 2005) with respect In the case of artefacts from eye movements and amplitude to stimulus onset as time reference. excursions exceeding T100 2V, the EEG sweeps were To provide an analysis of the ERP data with high automatically rejected. They were averaged per condition temporal resolution and concurrently avoiding an inflation and digitally filtered with a latency-neutral low-pass at of first order errors due to multiple testing we used a 20 Hz. Peak amplitude was measured relative to baseline, method introduced by Blair and Karniski (1993). It is which was defined as the average from 80 ms before to based on randomization tests (Edgington, 1995), which 20 ms after stimulus onset. Four types of EEG data were generate their reference distribution by permuting the sorted according to stimulus type (ambiguous and unam- pooled original data. Randomization tests require fewer biguous), task (“response” and “nonresponse,” collapsing assumptions than, e.g., a student t-test. Blair and Karniski over reversal and stability), and electrode position (1993) extended the principle of randomization tests to

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Experiments Blocks Conditions

Lattice experiment Necker lattices (ambiguous) ambiguous/reversal

ambiguous/stability

Unambiguous lattices unambiguous/reversal

unambiguous/stability

Face experiment Ambiguous Old/Young Woman ambiguous/change

ambiguous/stability

Unambigous face variants unambiguous/change

unambiguous/stability

Table 1. Overview of the experimental structures.

EEG data. In the following we denote this method as positive deflection can be observed at around 400 ms after “BK-statistics.” For each stimulus type (geometric and stimulus onset (labeled here as “P400”) at all electrode semantic) a subsequent repeated-measurement MANOVA positions, maximal at Pz and Cz, in the case of the (in the following denoted as “rm-MANOVA”) analyzed unambiguous lattices. the resultant ERP components from the difference traces This P400 is almost absent in the case of the ambiguous (“dERPs”: unambiguous minus ambiguous) with the lattices (Figure 3c, P G 0.001, BK-statistics). The P400’s factors task (response and nonresponse) and channel latency decreases monotonously from occipital over (=electrode positions, including only those electrodes at central to frontal and frontopolar positions (Figure 4a, which the BK-statistics had indicated significance) regard- F(10,120) = 21.9, P G 0.001 for the factor channel ing the variables amplitude and latency (time interval from concerning the variable latency, rm-MANOVA). stimulus onset to the extremum of the regarded ERP component). The amplitudes from individual participants were defined as the extrema from the ERP-difference Lattices: Response versus nonresponse traces in specific time windows (100–200 ms after conditions stimulus onset for the early ERP components and 200– 600 ms for the later P400 ERP component; see below). The early frontopolar negativity is present in the The total durations of stable percepts of each stimulus response condition and absent in the nonresponse con- interpretation was reconstructed from the participants’ dition (Figure 3c, P G 0.05). The amplitude of the responses and averaged across participants. subsequent P400 is twice as high in the response trials as in the nonresponse trials at the parietal and central electrode positions (F(1,12) = 22.1, P G 0.001 for the factor task and F(10,120) = 13.7, P G 0.001 for the Results interaction channel task regarding the variable amplitude, rm-MANOVA). Lattices: Unambiguous versus ambiguous stimuli Faces: Unambiguous versus ambiguous stimuli Figure 3a shows the ERP traces for the lattice stimuli of both the response and nonresponse trials. The BK- Figure 3b depicts the ERP traces for the ambiguous face statistics identified a small negativity at around 180 ms and the unambiguous variants for the response and the after stimulus onset at the frontopolar electrode in the case nonresponse trials. The BK-statistics identified higher of the unambiguous lattices. It is absent in the case of the amplitudes for the unambiguous stimuli compared to the ambiguous Necker lattices. This early negativity is ambiguous stimuli (P G 0.001) as early as 130 ms after marked with a black arrow in the difference traces stimulus onset. (dERPs: unambiguous minus ambiguous lattices, averaged This amplitude difference can be observed more easily in across participants, Figure 3c, P G 0.05, BK-statistics). the dERPs (unambiguous minus ambiguous; averaged across Twenty milliseconds later, at 200 ms, a further occipital participants) as positive deflections at parietal, temporal, negativity appears. Due to its high variability, it was not central, and frontal electrode positions (Figure 3d, arrows). significant based on the BK-statistics. Further, a huge Further, a huge positivity can be observed at all electrodes

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Figure 3. ERP traces to the geometric lattice stimuli (a) and to the semantic face stimuli (b) on schematic scalps. The difference traces T SEMs (unambiguous minus ambiguous; c, d) show the strong deviations from the null hypothesis of flat traces and the similarity of P400 patterns for the two stimulus types. The arrows indicate the earliest significant dERP deflections.

with maxima at parietal and central positions (P400). Its rm-MANOVA). Further, the latency decreases monoto- amplitude is about four times as high as with the ambiguous nously from occipital over central to frontal and frontopolar stimuli (P G 0.001, Figure 3d). electrode positions (Figure 4b, P G 0.001 for the factor channel, regarding the variable latency, rm-MANOVA). The first dERP difference between unambiguous and Faces: Response versus nonresponse trials ambiguous face stimuli occurs 50 ms earlier (130 ms), at different locations, it is more prominent and has a different The early positivity is indicated as significant (BK- sign compared to the lattice stimuli. statistics) for the parietal, temporal, central, and frontal After 250 ms the results for the face stimuli are electrodes in the response trials but only at the parietal remarkably similar to those for the lattice stimuli: electrode in the nonresponse trials, where it is much Although the pictorial differences between ambiguous weaker (F(1,12) = 22.9, P G 0.001 for the factor task, and unambiguous stimuli are very small (Figure 2c), there F(4,48) = 19, P G 0.001 for the factor channel, and F(4,48) = is a pronounced difference in amplitude of the P400 7.5, P G 0.001 for the interaction task channel regarding dERP-component (this can be seen in the difference traces the variable amplitude, rm-MANOVA). in Figures 3c and 3d that would be flat in case of no The P400 amplitude is higher in the response trials than in difference). And although the lattice and face stimuli the nonresponse trials (F(1,12) = 123.3, P G 0.001 for the belong to different categories with substantial pictorial factor task, and F(10,120) = 8.3, P G 0.001 for the differences (comparing Figures 2b and 2c, right with left), interaction channel task regarding the variable amplitude, the spatial and temporal pattern of the P400 is very similar

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Figure 4. Mean latencies (a, b) and amplitudes (c, d) T SEMs of the dERP P400 component (unambiguous minus ambiguous) across central electrode positions for the geometric lattice stimuli (a, c) and the semantic face stimuli (b, d). (comparing Figures 3c with 3d and Figures 4a and 4c with drawings (face stimuli). Participants observed the ambig- Figures 4b and 4d shows very similar patterns concerning uous and unambiguous stimuli from each stimulus P400 differences). category in different experimental runs and indicated in separate conditions in a go/nogo-task perceptual changes between or perceptual stability across successively pre- Discussion sented stimuli. Altogether, four different types of dERP traces were analyzed, according to stimulus category (lattice and face We analyzed the ERP differences (dERPs) between stimuli) and task (response and nonresponse), which differ ambiguous stimuli and unambiguous stimulus variants markedly depending on the time window: from two different stimulus categories: Geometric wire- Up to 250 ms after stimulus onset the results for the frame cube lattices (lattice stimuli) and simpliﬁed face face stimuli are different from those for the lattice stimuli:

Time window Features Lattice Face

0–250 ms (earliest peak) Latency: 180 ms 130 ms Size: j0.9 2V 1.6 2V Polarity: minus plus Location: Fpz Pz, Cz, T7, T8, Fz 9250 ms (P400) Latency: 280–410 ms 340–440 ms Size: 2.6–12.5 2V 1.5–6.7 2V Polarity: plus plus Location: max. at Pz and Cz max. at Pz and Cz

Table 2. ERP differences (dERPs) “unambiguous minus ambiguous.”

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The ﬁrst dERP difference between unambiguous and were less interested in physiological differences between ambiguous lattice stimuli occurs at 180 ms after stimulus representations of different stimulus categories or states of onset, is small, and restricted to the frontopolar position. attention. We thus concentrate our discussion on the large The diverging early dERP results for the lattice and the difference of P400 amplitude between ambiguous and face stimuli (Table 2) may represent differences in unambiguous stimuli, which occurs for both the geometric perceptual processing of the very different object catego- (lattices) and the semantic (faces) category. ries. This is in accordance with recent evidence of The difference in P400 amplitude may be explained by a differences in neural processing of animate objects (e.g., temporal jitter of perceptual processing from trial to trial faces) and nonanimate objects (e.g., buildings, Kanwisher (with respect to stimulus onset) in the case of the & Yovel, 2006). Task-related attentional effects (Reynolds ambiguous visual information. This could result in reduced & Chelazzi, 2004) may explain the response-related dERP (or nearly absent) mean P400 amplitudes for the ambig- enhancements. uous stimuli from both stimulus categories. However the In the present study we were interested in physiological following observations speak against this explanation. differences between stimuli with different degrees of Figure 5a shows the present data selectively averaged ambiguity across two different stimulus categories. We across trials with change indicated and separately across

Figure 5. (a) EEG data selectively averaged across trials with change indicated and separately across trials with stability indicated. Grand means for the Pz electrode across participants. Left: Lattice experiment. Right: Face experiment with different participants. Stimulus onset at 0 s. Clear differences between reversal and stability can be observed, whereas the huge P400 difference between ambiguous and unambiguous stimuli is common to both reversal and stability. (b) Median reaction times across participants with respect to stimulus onset after the blank period. The boxes indicate the T25 percentiles, the lines indicate the T95 percentiles. UC: unambiguous change; US: unambiguous stability; AC: ambiguous change; AS: ambiguous stability.

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trials with stability indicated. Clear differences between nogo trials. In the case of the unambiguous stimuli both change and stability can be observed. However, these the go and the nogo trials occur with equal probability. aspects are not in the focus of the present study, they are Further ambiguous and unambiguous stimuli are presented discussed elsewhere (Kornmeier & Bach, 2004, 2005, in different experimental runs and have thus both a 2006; Kornmeier, Ehm, Bigalke, & Bach, 2007; O’Donnell probability of P = 1. Probability can thus be ruled out as et al., 1988;Stru¨ber & Herrmann, 2002). For the present an explanation. discussion it is important that the temporal jitter with The “stimulus meaning” (Johnson, 1986), which may be ambiguous stimuli should be reflected in longer median higher in the go than the nogo trials (due to “task reaction times and broader interquartile ranges (Sommer, complexity” and/or “stimulus value”) can only explain a Leuthold, & Hermanutz, 1993; Verleger, 1997). However, small part of the whole effect. The major P400 difference as can be seen in Figure 5, the differences in reaction time occurs between ambiguous and unambiguous stimuli in variability cannot predict the differences in P400 ampli- both the go (higher value) and the nogo trails (lower tudes between ambiguous and unambiguous stimuli. value). In a very similar experimental paradigm Kormeier and This major part of the difference may be explained by Bach (2004, 2005) recently found that endogenous Johnson’s ‘equivocation’ factor. Sutton, Braren, Zubin, perceptual changes of Necker lattices are synchronized and John (1965) already proposed that the P3 is a correlate with stimulus onset with a precision of T30 ms or less. of the resolution of prior uncertainty. Hillyard, Squires, One might interpret the P400 as a correlate of object Bauer, and Lindsay (1971) concluded that their P3 representation but this would not be compatible with the amplitude reflects the level of confidence in the detection sparse coding hypothesis (e.g., Barlow, 1972), which of a signal. Johnson regarded equivocation as the amount assumes that the information carried by neurons is of information loss during stimulus processing, expressed represented by relatively few active neurons within a as subject’s a posteriori uncertainty about their percept largely populated neuronal network: Amplitudes of over and reflected in a decrease of P3b amplitude. The present 10 2V imply the synchronous activity of many neurons P400 difference may thus indicate different degrees of (reviewed in Bach, 1998). The activation strength hypoth- information transmitted by ambiguous compared to esis (e.g., Zeki, 2005), which assumes perceptual aware- unambiguous stimuli. ness of an object as a function of the strength of neural These considerations remind on the “Bayesian Inference activation, would likewise be ruled out: The P400 absence concept,” a model of how the perceptual system resolves in the case of the ambiguous stimuli would imply low the visual inverse problem (e.g., Kersten, Mamassian, & neural activity and thus no conscious perception of those Yuille, 2004): The probability P(SªI) of a scene S, given stimuli. the retinal image I, is proportional to the product of the The P400 amplitude, its distribution, and its latency probability P(S) of the scene prior to the retinal image decrease from posterior to anterior positions (Figures 4a information based on prior knowledge, and the probability and 4b) may indicate a superposition of two well-known P(IªS) of the retinal image, given the scene S: “cognitive” ERP components, namely the P3a and the P3b (e.g., Polich, 2004). PðS k IÞ ò PðSÞPðI k SÞ: ð1Þ The P3a, or “novelty P300,” typically occurs with a distractor stimulus that appears randomly between target The core idea is that the part of the information, which and standard stimuli in “three-stimulus” paradigms. It has is unavailable to our eyes but necessary to construct a a more anterior central/frontal maximum, shorter laten- stable percept, can be estimated by combining the cies, and rapidly habituates in contrast to the P3b (Polich, available retinal information with prior perceptual expe- 2004). Since the present paradigm has no third distractor riences stored in our memory. stimulus, P3a can be ruled out as contributing to our P400. This Bayesian probability P(IªS) can be regarded as a The P3b is typically induced in oddball paradigms with a reliability indicator expressing “how much an evaluation randomly occurring low-probability target stimulus. Its instance is impressed by the visual input.” The P400 may amplitude is known to be maximal at parietal electrodes. be a direct correlate of this indicator, reflecting the In his very influential paper, Johnson (1986) suggested a quality of the perceptual outcome: the more ambiguous “triarchic model” integrating most of the experimental the visual information is, the less certain the inferences findings regarding the modulation of the P3b amplitude. drawn by the evaluation instance, the smaller the P400 He assumes two additively related factors, namely ‘sub- amplitude, the less stable the percept, and the more jective probability’ (global probability and sequential probable a spontaneous perceptual change. We assume expectancies) and ‘stimulus meaning’ (task complexity, this reliability estimation to be unconscious for the stimulus complexity, and stimulus value). Both factors are following reasons: scaled multiplicatively by a third factor, which he called ‘information transmission’ (equivocation and inattention). 1. After the experiment several participants expressed The present paradigm can be regarded as a typical go/ their disbelief about the purely endogenous nature of nogo paradigm, where the P400 occurs in both go and perceived changes. They assumed subtle stimulus

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manipulations as a cause even though the ambiguity Bach, M. (1998). Electroencephalogram (EEG). In G. K. of these stimuli was explained at the beginning. This von Schulthess & J. Hennig (Eds.), Functional corresponds well with numerous reports of surprise imaging (pp. 391–408). Washington, PA: Lippincott- of naive observers when they experience sudden Raven. perceptual changes and speaks against a general Barlow, H. B. (1972). Single units and sensation: A conscious uncertainty regarding the ambiguous neuron doctrine for perceptual psychology? Percep- stimuli, which should be also felt during stability tion, 1, 371–394. [PubMed] periods. 2. Verleger, Jaskowski, and Wascher (2005) assume Bas¸ar-Eroglu, C., Stru¨ber, D., Schu¨rmann, M., Stadler, M., the P3b to be a correlate of a conscious perceptual & Bas¸ar, E. (1996). Gamma-band responses in the decision process, dependent on certainty. Higher brain: A short review of psychophysiological ambiguity of the stimulus should thus cause higher correlates and functional significance. International uncertainty in the decision process, which should be Journal of Psychophysiology, 24, 101–112. [PubMed] reflected in longer median reaction times and higher Blair, R. C., & Karniski, W. (1993). An alternative variability. This, however, is not the case in the method for significance testing of waveform differ- present data (Figure 5b, AC/AS vs. UC/US). ence potentials. Psychophysiology, 30, 518–524. [PubMed] Blake, R., & Logothetis, N. K. (2002). Visual competi- Conclusions tion. Nature Reviews, Neuroscience, 3, 13–21. [PubMed] We must constantly construct percepts from limited Boring, E. G. (1930). A new ambiguous figure. American sensory information. It may thus have been an evolu- Journal of Psychology, 42, 444–445. tionary key advantage for survival to rely our (re)actions Edgington, E. S. (1995). Randomization tests (3rd ed.). on the result of the perceptual outcome’s reliability. The New York: Dekker. more reliable our percept is rated, the faster, more appropriately and more goal-directed we can prepare and Gale, A. G., & Findlay, J. M. (1983). Eye movement execute our actions. We assume the P400 as a correlate of patterns in viewing ambiguous figures. In R. Groner, an evaluation instance’s reliability estimation. The less C. Menz, D. F. Fischer, & R. A. Monty (Eds.), Eye reliable one interpretation is, the better it would be to come movements and psychological functions: Interna- up with an alternative interpretation or concept about the tional views (pp. 145–168). Hillsdale, NJ: Erlbaum. V environment. Such an evaluation instance from the visual Hillyard, S. A., Squires, K. C., Bauer, J. W., & Lindsay, V or other sensory domains may thus be a basic precondition P. H. (1971). Evoked potential correlates of auditory for invention and cognition. The late positive deflection at signal detection. Science, 172, 1357–1360. [PubMed] 400 ms may be a correlate of its outcome. Holmes, G., & Lister, W. T. (1916). Disturbances of vision from cerebral lesions with special reference to the cortical presentation of the macula. Brain, 12, Acknowledgments 34–73.

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