Subliminal Gestalt grouping: Evidence of perceptual grouping by proximity and similarity in absence of conscious perception

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Highlights

  • Previous studies supported perceptual grouping outside the focus of attention.

  • Our work provides direct evidence of subliminal perceptual grouping processes.

  • Grouped patterns by proximity and by similarity were displayed as masked primes.

  • The results showed a reliable priming effect in absence of prime awareness.

  • Our data support that Gestalt grouping can occur under subliminal conditions.

Abstract

Previous studies making use of indirect processing measures have shown that perceptual grouping can occur outside the focus of attention. However, no previous study has examined the possibility of subliminal processing of perceptual grouping. The present work steps forward in the study of perceptual organization, reporting direct evidence of subliminal processing of Gestalt patterns. In two masked priming experiments, Gestalt patterns grouped by proximity or similarity that induced either a horizontal or vertical global orientation of the stimuli were presented as masked primes and followed by visible targets that could be congruent or incongruent with the orientation of the primes. The results showed a reliable priming effect in the complete absence of prime awareness for both proximity and similarity grouping principles. These findings suggest that a phenomenal report of the Gestalt pattern is not mandatory to observe an effect on the response based on the global properties of Gestalt stimuli.

Introduction

The world we perceive consists of objects and their interrelations coherently arranged in scenes. The processes of perceptual organization are responsible for structuring the retinal mosaic into the global stimuli of perceived objects (Palmer, 1999). One century ago, the Gestalt school of psychology was the first to deal with the problem of perceptual organization (see Wagemans et al., 2012, for a review). According to Max Wertheimer, organization is basically composed of grouping and segregation processes. The well-know principles of grouping describe the stimulus factors that determine the visual grouping of discrete elements, including proximity, similarity, common fate, good continuation, closure (Wertheimer, 1923), and, more recently, other new principles such as common region, connectedness or synchrony have been proposed (Palmer, 1992, Palmer and Rock, 1994).

Classical cognitive theories of visual processing have assumed that perceptual grouping occurs preattentively in absence of attention, producing segmented perceptual units or ‘proto-objects’ for analysis by subsequent processes (Julesz, 1981, Kahneman and Henik, 1981, Neisser, 1967, Treisman, 1982, Treisman, 1985, Treisman, 1988). This assumption was challenged by the results of studies showing that perceptual grouping is not perceived under strict conditions of inattention (Mack and Rock, 1998, Mack et al., 1992, Rock et al., 1992). However, Moore and Egeth (1997), using inattentional conditions similar to that of Mack and colleagues but crucially introducing on-line measures of unattended processing, showed that grouping could occur outside the focus of attention. Moore and Egeth suggested that grouped patterns could have been perceived without attention but not encoded in memory. Later studies using similar methodological strategies have provided further evidence that perceptual grouping does not require attention (Chan and Chua, 2003, Driver et al., 2001, Kimchi and Razpurker-Apfeld, 2004, Lamy et al., 2006, Mitroff and Scholl, 2005, Russell and Driver, 2005).

Interestingly, in a similar way, two recent studies have reported evidence for subliminal processing of other types of perceptual organization such as contour integration (Rosenthal & Humphreys, 2010) and illusory contour completion (Wang, Weng, & He, 2012). Rosenthal and Humphreys found a significant subliminal learning of global contours generated by integration of Gabor elements, despite being irrelevant to the main task. Using a suppression time paradigm, Wang et al. (2012) have recently observed that Kanizsa-like stimuli can be processed without awareness. These results are impressive examples of unconscious organization of the discrete elements to form emergent patterns that are qualitatively different from the sum of their parts. In addition, previous works provide a theoretical basis for a grouping effect occurring at an unconscious level. Several models for contour processing have analyzed the possible mechanism underlying grouping effects and have suggested that grouping could occur at an unconscious level. For example, Grossberg and Mingolla, 1985a, Grossberg and Mingolla, 1985b proposed a model of recurrent networks that considers unconscious computation of boundary contours. The model distinguishes between a Boundary Contour System (BCS) and a Feature Contour System (FCS) that extract different information concerning contour at an early processing stage. Information on edges and boundaries (i.e. the form outline of an object) is processed in the BCS whereas information on surface features (i.e. color or brightness) that fills in the area delimited by the BCS is processed in the FCS. The model assumes that the processing of form in BCS, which extracts edge information, occurs at non-conscious levels. Neumann and Sepp (1999) developed a neuropsychological model for contour processing based on interactions between V1 and V2. The model suggests an early occurrence for grouping in the stream of these interactions. Further studies in the field of visual neuroscience (see Breitmeyer & Tapia, 2011, for a review) assuming separate pathways for the processing of form and surface and using a metacontrast masking procedure showed that the conscious processing of form requires previous fill in of surface properties. The results also suggest that form processing proceeds faster than surface processing at unconscious levels whereas at conscious level it proceeds slower than surface processing.

However, to our knowledge, no previous study has examined the organization of visual elements into global patterns by means of Gestalt principles of grouping in absence of conscious perception. The present work steps forward in the study of perceptual grouping processes examining the possibility of a subliminal processing of Gestalt patterns generated by the action of grouping principles. This issue is relevant, as recent theoretical contributions have differentiated between consciousness and attention, and, by extension, between unconscious and inattentional processes (Dehaene and Changeux, 2011, Dehaene et al., 2006, Koch and Tsuchiya, 2007, Lamme, 2003). In this line, Dehaene and collaborators have proposed a tripartite distinction of subliminal, preconscious and conscious processing (see Dehaene & Changeux, 2011, for a review). According to this taxonomy, a subliminal stimulus is an invisible, undetectable one, even with focused attention. In contrast, a preconscious stimulus is potentially visible but is not consciously perceived due to distraction or inattention. From this view, the results of studies supporting perceptual grouping without attention are samples of preconscious processing because their procedures involve supraliminal patterns that could be reported if they were attended (e.g., see Full Attention condition in Moore and Egeth (1997)’s experiments). This implies that the findings of grouping without attention cannot be directly generalized to a hypothetical subliminal processing of grouping. Consequently, an investigation on the possibility of Gestalt grouping under exhaustive subliminal conditions is necessary to extend our knowledge of perceptual organization operations in the human visual system. Our aim is to determine whether a phenomenal report of the patterns is mandatory to observe an effect on the response based on the global properties of grouped stimuli. This goal is especially relevant considering that Gestalt psychologists focused their attention on phenomenology and, coherently, proposed their classical principles of grouping based exclusively on subjective measures obtained by phenomenal demonstrations (Lamy et al., 2006, Spillmann, 2009).

In order to achieve our purpose, we had to make two important decisions relative to (1) the subliminal method to be implemented as well as (2) the representative principles of perceptual grouping to be included in the study. Regarding the subliminal method, we used a masked priming task, the most frequently used technique for studying the extent of processing during subliminal perception (see Kim and Blake, 2005, Kouider and Dehaene, 2007, for reviews). Specifically, a double masking procedure combining forward and backward masks was applied to the grouped pattern. As principles of grouping, we selected proximity and luminance similarity (see Fig. 1), two classical Gestalt laws already described by Wertheimer (1923) and widely studied in numerous previous works (e.g., Ben-Av and Sagi, 1995, Kubovy et al., 1998, Luna and Montoro, 2011, Quinlan and Wilton, 1998). Note that these principles base their capacity for grouping on the analysis of different properties of the elements, i.e., spatial relationships between single units in the case of proximity and relative levels of luminance concerning similarity.

Due to the temporal and computational differences between grouping principles showed by previous studies (Han et al., 2002, Han et al., 2001, Kurylo, 1997, Luna and Montoro, 2008), this selection might provide us with a more extensive exploration of the boundary conditions for observing a subliminal processing of Gestalt patterns. In two different experiments, the grouping principles were applied to disk lattices to form horizontally or vertically oriented global patterns, which could be congruent or incongruent with the orientation of the target stimuli displayed afterwards. Thus, the priming effect was based on the global orientation of the stimuli and not on any local characteristic. Several kinds of targets with different physical appearances were displayed in order to force participants to base their response exclusively on the orientation of the stimuli.

A crucial control parameter included in our priming procedure was to avoid displaying the Gestalt patterns as visible targets, and we used them exclusively as masked primes. Previous studies have suggested that subliminal priming effects could be due to the fact that the masked primes had been also displayed as conscious targets in other phases of the experiment (Abrams and Greenwald, 2000, Damian, 2001, Eimer and Schlaghecken, 1998, Neumann and Klotz, 1994). However, other studies have disconfirmed this hypothesis by showing subliminal priming effects for primes that were never presented as visible targets (Abrams et al., 2002, Kiesel et al., 2006, Naccache and Dehaene, 2001, Ortells et al., 2013, Van den Bussche et al., 2009). Therefore, we considered that it was necessary to control this experimental confound in order to determine whether genuine subliminal Gestalt grouping is possible. Thus, the Gestalt patterns were exclusively displayed as primes under masked conditions of presentation.

Section snippets

Participants

Thirty-eight undergraduate students (five men; age range = 19–59 years, M = 32.02, SD = 10.86) from the UNED participated in the main experiment consisting of two consecutive tasks: (1) a masked priming task and (2) a prime visibility discrimination task. All of them had normal or corrected-to-normal vision and received course credits for their participation.

A different sample of twenty-three undergraduate students (ten men; age range = 20–64 years, M = 35.5, SD = 12.1) from the UNED were recruited for an

Participants

Thirty-eight undergraduate students (three men; age range = 19–51 years, M = 29.24, SD = 10.5) from the UNED participated in the main experiment consisting of two consecutive tasks: (1) a masked priming task and (2) a prime visibility discrimination task. All of them had normal or corrected-to-normal vision and received course credits for their participation.

A different sample of twenty-three undergraduate students (ten men; age range = 20–64 years, M = 35.5, SD = 12.1) from the UNED were recruited for an

Combined analysis of Experiments 1 and 2

Finally, a combined analysis of Experiment 1 and Experiment 2 was conducted in order to compare the effect of each grouping principle. Previous studies have suggested temporal and computational differences between grouping principles (Han et al., 2001, Han et al., 2002, Kurylo, 1997, Luna and Montoro, 2008). Thus, we examined whether grouping by proximity versus by similarity also differ when they are processed under subliminal conditions. To test this possibility, mean RTs and mean accuracy

Discussion

Can perceptual grouping occur unconsciously in absence of a phenomenal report of the Gestalt pattern displayed? Previous studies making use of indirect processing measures have shown that perceptual grouping can occur even under the strictest conditions of inattention proposed by Mack and Rock’s (1998) paradigm (Chan and Chua, 2003, Kimchi and Razpurker-Apfeld, 2004, Lamy et al., 2006, Mitroff and Scholl, 2005, Russell and Driver, 2005). However, those results do not allow us to determine

Acknowledgment

This work was supported by Grant 2012V/PUNED/0009 from the UNED to PRM and DL.

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