In general, consciousness includes all mental activities, such as human thoughts, feelings, cognition, and memory, while unconsciousness contains deeper mental processes that are not perceived by consciousness. Therefore, unconsciousness is the brain activity that occurs below the level of consciousness. In human behavior, a visual or facial message that flashes for tens of milliseconds can still have an impact on human consciousness. However, when a word, a figure, or a face is close to other visual stimuli in space and time, it becomes indistinguishable, even beyond human perception of the information. This is a visual phenomenon called masking (Dehaene et al., 2001). Evett and Humphreys (1981) first used the masked priming paradigm to measure unconscious perception. Their masked priming paradigm has four levels of structure: forward mask – priming stimulus – target stimulus – backward mask. However, this method controls the presentation time of the target stimulus and prime stimulus of 50 ms, which is prone to produce a fusion effect: the target stimulus and prime stimulus cannot be distinguished at all (Kouider and Dehaene, 2007).

With increased unconscious perception research, researchers have improved the four-level structure of the masked priming paradigm to improve its credibility (Greenwald et al., 1996). To date, the masked priming paradigm may be divided into three types: the forward masking paradigm, the backward masking paradigm, and the sandwich masking paradigm, including the forward and backward masking paradigms. The forward masked priming paradigm was proposed by Forster and Davis (1984) as a three-level structure because there is no backward masking. The target stimulus itself is seen as a very powerful masking, and this structure consists of forward masking, a short prime word (60 ms or below), and a target word. The principle of the backward masking paradigm is to reduce the visibility of the prime stimulus through the backward pattern mask so that the subjects can neither recognize the prime (McCauley et al., 1980) nor make the existence-absence judgment significantly more than the opportunity (Fowler et al., 1981; Balota, 1983; Marcel, 1983; Cheesman and Merikle, 1984). Since sandwich masking includes a forward mask and backward mask, it was considered to have a more effective masking effect (Greenwald et al., 1996; Gao and Zhang, 2014). However, regardless of the masking paradigm, the spatial and temporal presentation of the stimulus could be restricted to prevent the participants from consciously processing the stimulus cue.

In unconscious perception studies, another important concept is unconscious priming. Namely, “invisible” stimuli can nonetheless affect behavioral responses to subsequent probe stimuli presented shortly after the mask (Henson et al., 2008). For example, Greenwald et al. (1995) conducted a typical experiment in which the subjects were instructed to complete a classification task, where the targets were divided into positive (e.g., “happy”) or negative (e.g., “vomit”) words, and the front of these words had a congruent prime (i.e., a word from the same category, such as the “love” preceding the target “happy” or an incongruent prime “vomit” preceding “happy”). Subjects were faster in congruent trials than in incongruent trials (Greenwald et al., 1995). Therefore, the priming effect depends on whether the prime and the target categories are congruent or incongruent, which means that unconscious priming originates from competition between the prime stimulus category and the target stimulus category, thus reflecting categorical congruity (Kouider and Dehaene, 2007). Numerous functional magnetic resonance imaging (fMRI) studies have shown that the incongruent trial produced a larger increase in BOLD signals in brain regions than the congruent trial, although this was unconscious, possibly because the incongruent trial involved more cognitive resources (Dehaene et al., 2001; Nakamura et al., 2007; Kouider et al., 2010).

Numerous studies have used the masked priming paradigm to prove that people can process emotions, words, graphics, and other information presented at the unconscious level (Dehaene et al., 2001; D’Ostilio and Garraux, 2012a; Prochnow et al., 2013; Sato et al., 2016; Tacikowski et al., 2017). However, due to the differences in experimental purposes, different researchers mostly adopted different experimental materials and different masked priming paradigms for the study of a single experimental purpose and did not comprehensively analyze the unconscious information processing mechanism itself. With the application of fMRI technology in the study of unconscious information processing and the emergence of activation likelihood estimation (ALE) analysis methods (Brooks et al., 2012; Meneguzzo et al., 2014), it is possible to explore cross-experimental materials, cross-experimental paradigms, and cross-experimental purposes.

Most studies have accepted the view that the processing of cognitive information is mainly carried out by the ventral and dorsal pathways (Weiskrantz et al., 1974; Mishkin et al., 1983; Rees et al., 2002; Weiller et al., 2011). Previous studies have believed that the dorsal pathway is an unconscious perceptual process pathway that can directly connect sensory information and response parameters without consciousness (Neumann and Klotz, 1994). Although dorsal pathways can explain some simple unconscious motor processes, most cognitive activities cannot be summarized by a single visual pathway (Weiller et al., 2011). Therefore, researchers began to focus on the joint role of the ventral and dorsal pathways in unconscious perceptual processes (van Polanen and Davare, 2015). Ulrich and Kiefer (2016) used functional connectivity analysis to demonstrate that the ventral and dorsal pathways jointly regulate unconscious perceptual processes and pointed out that the two pathways mainly interact in the inferior occipital gyrus and posterior parietal lobe. However, it is well known that functional connectivity analysis relies on strong experimental hypotheses, and this study aimed to explore whether unconscious perceptual processes are mediated by both ventral and dorsal pathways at the whole brain level via ALE analysis.

In summary, this study aims to solve three key problems through cross-paradigm, cross-material, and cross-purpose experiments. First, we explored whether we can find the key brain regions specific to unconscious perceptual processes and unconscious priming effects in the masked priming paradigm. Second, we examined whether unconscious perceptual processes are regulated by both ventral and dorsal pathways. Finally, we examined whether the priming effect needs more brain regions to be involved because of the longer response time in incongruent compared to neutral or congruent conditions.

We conducted a preliminary meta-analysis of fMRI studies comparing the activation of unconscious processing versus baseline and unconscious priming effects in the masked priming paradigm. In the unconscious condition, the prime stimulus and the target stimulus have longer latency periods in the incongruent condition than in the neutral and congruent conditions. We considered two studies that indicated differences in activation between incongruent and neutral conditions, since we assumed that incongruent conditions invoked more cognitive resources than both neutral and congruent conditions. However, caution must be taken in interpreting these findings in terms of categories and differences in visual stimuli, as they may influence the observed data (Meneguzzo et al., 2014). It is worth noting that, in consideration of the caution of interpreting cross-task research, we adopted a strict literature screening method in this meta-analysis in an effort to obtain more reliable research results. In fact, Brooks et al. (2012) conducted an ALE analysis based on fMRI for subliminal stimuli for the first time, but their study focused more on the brain activity level induced by different types of subliminal stimuli. However, Meneguzzo et al. (2014) also conducted an ALE analysis based on fMRI, which paid more attention to the differences in brain activities induced by supraliminal and subliminal stimuli. Since neither of these two studies only targeted at the unconscious masked priming paradigm, this study is also the first study to reveal the unconscious information processing mechanism underlying the masked priming paradigm through an ALE meta-analysis based on fMRI. The results of the meta-analysis also provided sufficient evidence for the three questions we planned to answer. Surprisingly, the right IFG triangular part plays a crucial role in unconscious information processing. In the following section, we will discuss the latest findings in this study.

The bilateral IFG is a heterogeneous region in two pathways of cognitive information processing, with the opercular part (BA44) connected mainly via the dorsal pathway, and the triangular part (BA45) and BA47 are mainly connected via the ventral pathway (Petrides and Pandya, 2007, 2009). Although the involvement of the bilateral IFG in the semantic process at the lexical level is unclear and controversial (Bookheimer, 2002), the triangular part (BA45) and orbital part (BA47) are related to the relevant features of semantic knowledge (Thompson-Schill et al., 1997). There is neuropsychological evidence that this region is involved in the control of semantic processing (Sharp et al., 2010). In the results of this meta-study, clusters were found in the IFG triangular part not only in the comparison between unconscious processing and baseline but also, more importantly, in the analysis of unconscious priming effects. In the case of incongruent processing, an unconscious perceptual process not only has a longer latency, but when considering that incongruent processing has been shown to activate more IFG regions than congruent and neutral processing (Ulrich et al., 2014; Jiang et al., 2016), this coincidence seems to provide evidence that an unconscious perceptual process can reach the semantic level. However, Ulrich et al. (2014) found in an analysis of the interaction between unconscious perceptual induction processing and semantic association that compared with perceptual induction processing, semantic induction processing had greater activations in the bilateral IFG (BA45) and the boundary part of the left anterior insula. It was further confirmed that the unconscious semantic priming effect only occurs after semantic induction processing but not after perceptual induction processing.

However, why is it the right IFG, not the left IFG, that shows the activation likelihood? It is well known that the left IFG triangular part is part of Broca’s area and is an important motor speech center (Thompson-Schill et al., 1997; Tremblay and Dick, 2016). Jackson (2021) conducted a meta-analysis of the neural correlativity of semantic control and found that semantic control depended on a distributed network composed of the IFG, post middle temporal gyrus, post inferior temporal gyrus, and dorsomedial prefrontal cortex. This network is left-dominant, with the left IFG more involved than the right IFG, and produces the strongest activation likelihood in the left IFG triangular part (Gonzalez et al., 2019; Jackson, 2021). However, although the left IFG generated more activations than the right IFG in the semantic control network, activations were still detected in the right IFG, and the role of the right IFG triangular part in semantic control remains unknown. Thus, when considering the mechanism of unconscious information processing in this study, the right IFG triangular part is likely to be a specific brain region for unconscious information processing in the masked priming paradigm. The ventrolateral prefrontal cortex (VLPFC) is known for its cognitive control functions (Miller and Cohen, 2001; Niendam et al., 2012) and plays an important role in the learning, retrieval, and maintenance of stimulate-response (S-R) rules (Toni and Passingham, 1999; Murray et al., 2000; Toni et al., 2001; Bunge et al., 2002, 2005; Crone et al., 2006), and the right IFG triangular part is a part of the VLPFC. On the one hand, in congruent trials, the relevant S-R rules and related responses have been preactivated by the prime, and the right IFG triangular part regulates response consistency in the congruent condition. On the other hand, in incongruent trials, priming activated inappropriate S-R mapping, and it is necessary to retrieve the appropriate S-R rules for the target response in a more detailed way (Ulrich and Kiefer, 2016). However, it has been shown that the IFG is recruited when competitive task representations are simultaneously activated (Bunge et al., 2002; Zhang et al., 2004; Badre et al., 2005; Moss et al., 2005; Souza et al., 2009). Thus, during incongruent trials, when considering that the prime preactivated an S-R rule inappropriate for the upcoming target, the triangular part may be involved in resolving the conflict (Ulrich and Kiefer, 2016).

In general, the dorsal (occipito – parietal) pathway is used for spatial perception (Mishkin et al., 1983; Rees et al., 2002), which is often called the “where” pathway. It has the function of “action vision” and provides strong input for the motor system to guide action (Michael et al., 2013). In this meta-analysis, we found that clusters formed in the dorsal pathway, such as the left SPG, the right superior occipital gyrus, and the right precuneus. As mentioned above, previous studies have suggested that dorsal pathways can explain some simple unconscious motor processing. Therefore, our results are consistent with previous studies (Neumann and Klotz, 1994). However, in information processing, the IFG is a heterogeneous area, and its triangular part is connected with the ventral pathway. The ventral (occipito-temporal) pathway is used for object perception and recognition (Mishkin et al., 1983; Rees et al., 2002). It is often called the “what” pathway and has the function of “recognizing vision” (Michael et al., 2013). Most studies suggest that the ventral pathway is a conscious processing pathway (Weiller et al., 2011; van Polanen and Davare, 2015). This meta-analysis found that the ventral pathway was involved in unconscious perceptual processing in the masked priming paradigm, such as the bilateral insula, right IFG triangular part, and right fusiform clusters. Although some studies have also shown that the ventral pathway participates in unconscious perceptual processes, their studies are based on the analysis of a single experimental purpose and a single experimental paradigm (Dehaene et al., 1998, 2001; Ulrich and Kiefer, 2016). In this study, ALE analysis provided strong evidence for the involvement of the ventral pathway in the regulation of unconscious perceptual processes from cross-purpose, cross-paradigm, and cross-material evidence. However, for most functions, both pathways are not mutually exclusive but rather work in parallel (Makris and Pandya, 2009; Rauschecker and Scott, 2009), constituting a loop that has to be passed at least once (Weiller et al., 2011). This loop may explain our findings that the two pathways cooperate to complete unconscious perceptual processing in the masked priming paradigm. Moreover, van Polanen and Davare (2015) proposed that the dorsolateral prefrontal cortex regulates the interaction between ventral and dorsal pathways so that the grasping action is completed. Thus, considering that the IFG is a heterogeneous region, we have reason to speculate that the interaction of the two pathways is mediated by the IFG in unconscious perceptual processing. The supplementary motor area and the precentral gyrus may participate in the preparation, planning, and execution of unconscious motor responses (Hanakawa et al., 2003; Witt et al., 2008).

However, in the regulation of the priming effect, we have different findings. Consistent with previous studies, our meta-analysis also found that the ventral pathway regulated the unconscious priming effect, such as the formation of clusters in the right IFG triangular part, the left fusiform gyrus and the left inferior occipital gyrus, but no significant clusters were found in the dorsal pathway. Moreover, to the best of our knowledge, only a few fMRI studies have shown that the unconscious prime effect involves the dorsal pathway (Aron et al., 2003; D’Ostilio and Garraux, 2012a; D’Ostilio et al., 2013). However, due to the use of arrow stimulation as a prime and target, the explanation of the priming effect is limited. Thus, there is no sufficient evidence to show that the two pathways coordinate in the unconscious priming effect in the masked priming paradigm (Ulrich and Kiefer, 2016).

The other two findings of this study are also critical. One is that the SPG formed clusters. The general theory holds that unconscious perceptual processes do not involve attention resources, and the appearance of SPG clusters challenges this view. The SPG receives attentional modulation signals from the prefrontal cortex and then applies appropriate attentional control to task-related sensory brain regions (Ulrich et al., 2013). Top–down attention control is not only a feature of spatial position and visual object but can also use the same neural mechanism for semantic information (Kiefer, 2008; Hoenig and Scheef, 2009; Grandjean et al., 2012). Thus, the discovery of SPG suggests that attention resources are involved in unconscious perceptual processes. Although SPG was found to form clusters across experiments, considering that different experimental designs have different control over conscious intervention, this result should be interpreted carefully. Another finding is that the insula forms clusters in unconscious perceptual processes and priming effects. Although the insula is known to be sensitive to both conscious and unconscious visual stimuli (Salomon et al., 2016), few studies have focused on the effect of claustrum hiding in the insula on unconscious information processing. The claustrum is hidden under the general area of the insula, which is thought to act as an on-off switch for consciousness (Crick and Koch, 2005; Brooks et al., 2012; Gattass et al., 2014; Meneguzzo et al., 2014; Chau et al., 2015). Whether the clusters formed in the insula in this study extend to the claustrum remains to be considered. However, there is a two-way connection between the claustrum and almost all areas of the cerebral cortex (Crick and Koch, 2005). Considering its bidirectional connectivity, whether or not claustrum may connect bilateral pathways and modulate unconscious perceptual processes is still a question worthy of more researchers’ attention.

This study preliminarily confirmed our hypothesis. We found that ventral and dorsal pathways jointly regulate unconscious perceptual processes, but only the ventral pathway is involved in the regulation of unconscious priming effects. The right IFG triangular part is the key brain regions in unconscious information processing in the masked priming paradigm, which is involved in the regulation of unconscious perceptual processes and priming effects. These preliminary data provide the conditions for further investigation of the neural correlates of unconscious information processing.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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