Social context, the real or imagined presence of other people (Allport, 1954), is a ubiquitous psychological force. Cognition is enveloped by social context (Smith and Semin, 2004; Smith and Conrey, 2009). Yet the effects of social context upon cognition often fall between the cracks of social and cognitive psychology. In cognitive and perceptual laboratories, we typically place participants in an experimental quarantine, away from the confounds of social influence. As a consequence, we have many elegant demonstrations of the different behavioral and neurological responses to social versus non-social stimuli (e.g., Cacioppo et al., 2005; Birmingham et al., 2008; Senju and Johnson, 2009), but little idea of how these and other stimuli are processed in a social versus a non-social context. Increasingly, as this special edition shows, researchers are advocating that neuroscience and cognitive psychology directly address issues of social context and interaction (Schilbach, 2010; Obhi and Sebanz, 2011; Shibata et al., 2011).

In this paper, we asked what is the difference between perceiving something by your self and perceiving it at the same time as another person? When a student hovers over your shoulder while you read their paper, does it influence your evaluation? When someone sits down on the sofa while you are watching TV, does their presence intrude upon your experience of the show? What if you are watching a show alone, but know that a friend across town is also tuned in? We term this phenomenon joint perception: the changes that happen when people believe that they are experiencing something at the same time as another person. To isolate these effects from the demands of social interaction, we minimized the social content of joint perception. Participants could not see, hear or interact with each other. We presented images to participants, tracked their gaze, and manipulated—on a trial by trial basis—whether or not they believed that an unseen other person was looking at the same sets of images.

It is hard to discern, from the literature, whether such a minimal social context will have any influence visual attention, as the presence of a social context is often intertwined with social interaction. For example, language use requires a high level of social interaction. When two people talk, their eye movements can be highly sensitive to what they think each other knows and sees (Horton and Keysar, 1996; Bromme et al., 2001; Nadig and Sedivy, 2002; Hanna et al., 2003; Metzing and Brennan, 2003; Brown-Schmidt et al., 2008; Richardson et al., 2009). In contrast, other researchers have argued that people can be striking egocentric (Keysar et al., 2000; Keysar, 2007) in the way they deploy their gaze during language processing.

At a lower level of social context, there are experiments in which two people do not speak to each other but are engaged in the same task. For example, in a traditional stimulus-response compatibility task, participants make a judgment about one stimulus property (color) and ignore another stimulus property (location). If there is an incompatibility between the irrelevant property and the response (e.g., the stimulus is on an opposite side of the screen to the response button) then reaction times increase (Simon, 1969). Sebanz et al. (2003) divided such a task between two people. The participants sat next to each other, and each person responded to one color: in effect, each acting as one of the fingers of a participant in Simon's (1969) experiment. Though each person had only one response to execute, they showed an incompatibility effect when acting together. There was no incompatibility effect when performing the same single response task alone. When engaged in a task together, participants represent their partners' actions as if they were their own.

People will also attune to stimuli that an experimenter identifies as shared: if they are told that someone else is looking at a stimulus, that increases its salience (Shteynberg, 2010). More subtly, people configure their attentional state to that of others. Their ability to attend to global or local features in a Navon figure (Navon, 1977) is influenced by the knowledge that a co-actor is attending to local or global features (Böckler et al., 2012). Infants follow the gaze of others (Senju and Johnson, 2009), and if their attention is drawn to an event by another's gaze (compared to a non-social cue such as a arrow) they learn more about that event (Wu and Kirkham, 2010; Wu et al., 2011).

In short, people are highly responsive to where others are looking, if they are given that information. In the these experiments, we address a more rudimentary issue: if people simply know that others are looking, but not where, how do they change their gaze patterns?

At this lowest level of social context, the eye movement literature is largely agnostic. From early eye movement research it has been shown that differences in expertise (Buswell, 1935) and cognitive process (Yarbus, 1967; Just and Carpenter, 1976) exert a top-down effect on gaze. But social context itself has not been a variable of concern in eye movement research (e.g., Henderson, 2003), in the way that it has been studied elsewhere (Zajonc, 1965).

The studies we present contrast with those which explicitly give participants a task to perform with another (Sebanz et al., 2003), which explicitly tell participants what another person is attending to (Brennan et al., 2008; Shteynberg, 2010; Böckler et al., 2012) and experiments in which participants communicate with each other (Richardson et al., 2007, 2009; Dale et al., 2011). We presented participants with a set of normed images, knowing that that they would be biased to attend to some over others. Instruction, interaction and cooperation with another person were absent, and we focused on changes in perception that were brought about just by the knowledge that the images were experienced with another person or not. By focusing on this minimal social context, we can explore the shifts in perceptual processes that occur in response to the presence of others, prior to communication, joint action or cooperation taking place.

Pairs of participants who did not know each other, or interact during the experiment, sat in opposite corners of the lab. We presented them with sets of four images, on screen for eight seconds. On different trials, they each believed that the other participant was looking at the same images, or that the other was looking at a set of unrelated symbols (Figure 1). The four pictures were taken from a normed database (Lang et al., 2005). In each set, there was one picture with a negative valence (e.g., crying child), one with a positive valence (e.g., a smiling couple) and two neutral images with no strong valence (e.g., a person reading). Negative images are considered more potent than equivalently-valenced positive images (for reviews, see Baumeister et al., 2001; Rozin and Royzman, 2001). We anticipated, therefore, that the negative stimuli were more likely to receive participants' attention in line with previous work (Smith et al., 2003; Norris et al., 2004; Hajcak and Olvet, 2008). We tested the hypothesis that this attentional bias would be influenced by the minimal social context of the participants' belief that they were looking at the pictures jointly or alone.

We have demonstrated that eye movements are influenced by beliefs about social context. One could argue, however, that eye movements are indicative of lower level perceptual processing alone, and that in cognitive terms they are epiphenomenal. Although there are theoretical and empirical arguments against this view (Spivey et al., 2009), we wanted to investigate whether minimal social context differences were also reflected in a measure of cognitive performance: recognition memory. In this version of the paradigm, eye movement measures were not taken but, following presentation blocks, participants' memory for the images was tested. We hypothesized that minimal social context which affected attention in Experiment 1 would be sufficient to affect memory here.

There are at least two ways to interpret “looking together,” which up until now we have treated as a single idea. On the one hand, looking together could mean just experiencing a set of images at the same time. On the other hand, it could mean examining the same images, but also having the same goal, attitude or intention towards them.

Our joint perception paradigm was based on work in joint action (Sebanz et al., 2003; Galantucci and Sebanz, 2009). Joint action effects do not occur if the participant is simply sat next to another person (Tsai et al., 2006), or if that person's button pressing actions are not intentional (their finger is moved by a mechanical device). Also, if the participant is acting jointly, but with a computer program (Tsai et al., 2008) or a marionette's wooden hand (Tsai and Brass, 2007) there is no stimulus-response incompatibility effect. Participants only form representations of another when that person's genuine, intentional actions are engaged in the same task (Atmaca et al., 2011).

In the current experiment, we began to investigate whether the same sort of conditions circumscribing joint action also determine joint perception. Unlike those described above, in this experiment the participants always believed that they were examining the same images. What changed, trial-by-trial, was the task that they were doing, and the task that they believed their partner was doing. Sometimes they or their partner were memorizing the pictures, sometimes they were scanning them for the presence of a small X. We predicted that joint perception effects would be strongest when participants believed that they were not just passively sharing an experience, but also engaged in the same task.

Social context exerts a pervasive effect on perception. Even a minimal social context, when the difference between looking alone and looking jointly is as small as possible, produces distinct behavioral and cognitive effects. Shared exposure is not sufficient to produce these effects alone: participants must also believe that they are engaged in the same task when processing the shared stimuli.

This result is distinct from other findings in area between social and cognitive psychology. There are many interesting studies of joint action (e.g., Obhi and Sebanz, 2011), but our experiments are different because participants are not instructed to coordinate their behavior or act together. There are many interesting studies on joint attention and how people use information about each other's attentional state (Brennan et al., 2008; Shteynberg, 2010; Böckler et al., 2012), but our experiments are different because participants are given no knowledge of where the other is looking. And finally, there are many studies of attentional coordination during social interaction and language use (e.g., Richardson et al., 2007), but in our experiments there is no interaction between people at all. Nevertheless, despite the very minimal nature of this minimal social context, it produces a systematic shift in participants' attention.

In these first experiments, we have tried to understand the conditions under which joint perception influences attention. But we have not yet addressed the direction of these effects. Why is it that sharing images in our paradigm led to increased attention specifically to the negative pictures? Here we discuss four alternatives: social context modulates the strength of the negativity bias specifically, or it modulates attention and alertness more broadly; social context increases the degree to which there is alignment with emotions, or alignment with saliency.

It has been argued that the negativity bias exists because of a learnt or evolved priority to detect threats in the environment (Baumeister et al., 2001; Rozin and Royzman, 2001). If social context was associated with an increase in perceived threat or anxiety, then it would follow that joint perception could increase the negativity bias specifically. This is possible, but it seems unlikely that our participants would have felt increased threat from each other. All participants were first year undergraduate students at UCL, and so were members of similar or overlapping social groups. Even if they did feel some anxiety in each others' presence, it is not clear why that threat would change trial-by-trial according to the stimuli they believed each other could see. However, to fully discount this possibility, we would need to experimentally manipulate the anxiety felt by participants, perhaps by changing their in/out group relationship.

The second possibility is that the social context of joint perception increases some broad cognitive factor such as alertness, in the way that the presence of others can cause social facilitation (Zajonc, 1965). It has been shown, for example, that when participants are engaged in a dialogue, it can increase alertness and counter the effects of sleep deprivation (Bard et al., 1996). Perhaps the lower level of social context used in this experiment, and modulated trial-by-trial, also increased alertness. This increased engagement would presumably benefit the negative images first of all, since there is a pre-existing bias towards them. However, under this account, it remains a puzzle why there would be no corresponding increase in looks to positive items at all. One would expect a main effect of social context on look times to these two items (compared to the neutral items), but throughout our experiments we found an interaction between social context and valance.

A third possibility draws on work in social psychology showing that social interaction leads to emotional alignment. When people interact, they are motivated to form a “shared reality” (Hardin and Higgins, 1996): a speaker will adapt the content of their message to align with the beliefs and emotions of their audience (reviewed by Echterhoff et al., 2009). Similarly, when people collaborate in groups, they tend to align with the group emotion (Hatfield et al., 1993; Wageman, 1995; Barsade, 2002). Since individuals are attuned to negative stimuli, it is conceivable that in a group, this shared negativity bias would be amplified as people seek to align with each other. Over repeated experiences, perhaps this social alignment towards negative stimuli becomes ingrained. In this light, our joint perception phenomenon could be seen as a form of minimal, imagined cooperation that is sufficient to evoke a learnt alignment towards negative images.

The final alternative is that the joint perception effect is not driven by emotion, per se, but by salience. This account draws on observations of language use and the rich joint activity of social interaction. Language is remarkably ambiguous. “Please take a chair,” could refer to a variety of actions with a variety of chairs in a room. Conversations do not grind to a halt however, because people are very good at resolving ambiguous references by drawing on knowledge about the context and assumptions that they have in common (Schelling, 1960). For example, when presented with a page full of items, such as watches from a catalogue, participants agreed with each other which one was most likely to be referred to as “the watch” (Clark et al., 1983).

When we enter into any conversation, such coordination is all important (Clark, 1996), and can be seen at many levels of behavior. When we talk, we use the same names for novel objects (Clark and Brennan, 1991), align our spatial reference frames (Schober, 1993), use each others' syntactic structures (Branigan et al., 2000), sway our bodies in synchrony (Condon and Ogston, 1971; Shockley et al., 2003) and even scratch our noses together (Chartrand and Bargh, 1999). When we are talking and looking at the same images, we also coordinate our gaze patterns with each other (Richardson and Dale, 2005), taking into account the knowledge (Richardson et al., 2007) and the visual context (Richardson et al., 2009) that we share. In short, language engenders a rich, multileveled coordination between speakers (Shockley et al., 2009; Louwerse et al., in press).

Perhaps the instruction stating that images were being viewed together was enough to turn on some of these mechanisms of coordination, even in the absence of any actual communication between participants. When images were believed to be shared, participants sought out those which they imagined would be more salient for their partners. Since saliency is driven by the valence of the images in our set, paying more attention to the most salient means paying more attention to the negative image. In this way, it can be argued that the shifts brought about by joint perception are the precursors to the more richly interactive forms of joint activity studied in other fields.

Our experiments echo a point that social psychologists have made from the outset. The presence and actions of others can have a powerful effect on an individual's motivations, goals and judgments (Triplett, 1898; Sherif, 1935; Lewin, 1936; Festinger, 1950; Asch, 1951; Allport, 1954; Heider, 1958; Zajonc et al., 1969). Beliefs and judgments are not formed in cognitive isolation, but always in the context of the thoughts and opinions of those around us (Smith and Semin, 2004). Here we have shown that these lessons from social psychology can be applied to a simple perceptual process in a minimal social context. Merely the belief that stimuli are attended to alone or with another is enough to activate coordinative behaviours that are the basis of joint action, communication and social interaction. The pervasive effects of social context have theoretical implications for how we view cognition (Robbins and Aydede, 2009), adding to calls to consider social interaction at its heart (Smith and Semin, 2004; Barsalou et al., 2007).