حرکات چشم در طول سفر ذهنی در زمان به دنبال یک خط مورب

Recent research showed that past events are associated with the back and left side, whereas future events are associated with the front and right side of space. These spatial–temporal associations have an impact on our sensorimotor system: thinking about one’s past and future leads to subtle body sways in the sagittal dimension of space (Miles, Nind, & Macrae, 2010). In this study we investigated whether mental time travel leads to sensorimotor correlates in the horizontal dimension of space. Participants were asked to mentally displace themselves into the past or future while measuring their spontaneous eye movements on a blank screen. Eye gaze was directed more rightward and upward when thinking about the future than when thinking about the past. Our results provide further insight into the spatial nature of temporal thoughts, and show that not only body, but also eye movements follow a (diagonal) “time line” during mental time travel.

The passing of time is not directly graspable by the human senses. The metaphorical mapping view assumes that abstract concepts are expressed in entities of concrete domains (e.g., Barsalou, 2008, Boroditsky, 2000, Gallese and Lakoff, 2005 and Lakoff and Johnson, 1980). In line with this view, humans often use the domain of space, which is thought to be concrete, in order to understand and structure the more abstract concept of time (Boroditsky, 2000 and Casasanto and Boroditsky, 2008). Especially, future and past events are often described by means of spatially distinct locations. For example, events in the past lie behind us, whereas we are looking forward to events in the future. In Western cultures, two spatial–temporal associations have been identified: past-back and future-front association ( Casasanto and Jasmin, 2012, Hartmann and Mast, 2012, Miles, Karpinska, et al., 2010, Miles, Nind, et al., 2010 and Ulrich et al., 2012; Walker, Bergen, & Núñez, 2013), as well as a past-left and future-right association ( Casasanto and Jasmin, 2012, Ouellet et al., 2010, Saj et al., 2014, Santiago et al., 2007, Torralbo et al., 2006, Ulrich and Maienborn, 2010 and Weger and Pratt, 2008). While the past-back and future-front association is in line with the metaphorical language use, there are no equivalent expressions in language associating past with the left and future with the right side of space ( Casasanto and Jasmin, 2012, Cienki, 1998 and Radden, 2004). It has been argued that temporal associations with the horizontal space can be explained by reading and writing direction from left-to-right in Western cultures (e.g., Casasanto and Bottini, 2014, Fuhrman and Boroditsky, 2007, Santiago et al., 2007 and Torralbo et al., 2006). The “mental time line” has been established as an analogous concept to the “mental number line”: Not only numbers are represented in an ascending order from left to right (e.g., Dehaene et al., 1993, Fischer et al., 2003, Hartmann et al., 2012, Hubbard et al., 2005, Lourenco and Longo, 2010 and Wood et al., 2008), but also time flies from left to right (e.g., Santiago et al., 2007). Most empirical evidence for the past-left and future-right association comes from temporal categorization tasks where future and past-related stimuli need to be categorized as future or past in two-alternative forced-choice reaction time tasks. Results show that left-sided responses are faster for past than for future-related stimuli, whereas the opposite is true for right-sided responses (e.g., Santiago et al., 2007, Torralbo et al., 2006 and Ulrich and Maienborn, 2010). Because these tasks require participants to categorize stimuli to the past or future, the concepts of past and future as well as the response options left and right are explicitly part of the task representation. As a consequence, these results do not provide direct evidence that past events are inherently represented in the left, and future events in the right side of space. Rather, these congruence effects can also be explained by stimulus–response compatibility such as the polarity correspondence account (Proctor & Cho, 2006). This account would suggest that past and left are coded with a negative, and future and right with a positive polarity, thus leading to faster responses when theses polarities are congruent. Therefore, shorter response times in the congruent conditions can be explained without making the assumption of a direct link between the two concepts (past-left, future-right). In fact, when the same stimuli needed to be categorized as word or non-word (also with a left and right key), the past-left and future-right advantage disappeared (Flumini & Santiago, 2013; see also Ulrich & Maienborn, 2010). The nature of the past-left and future-right association and the conditions under which it occurs remain unclear. It is an open question whether the mental time line is automatically activated during temporal processing (Flumini & Santiago, 2013), and the polarity correspondence account points to the possibility that the horizontal mental time line is merely an epiphenomenon of temporal categorization tasks. Mental time travel is an alternative paradigm that allows investigating spatial–temporal associations by avoiding stimulus–response compatibility effects. In episodic mental time travel, participants are asked to mentally displace themselves into their subjective past or future (i.e., remembering vs. autobiographical temporal imagining; see Stocker, 2012 and Tulving, 2002). Mental time travel has been used to study the past-back and future-front association. For example, Miles, Nind, et al. (2010) found that participants leaned slightly forward when they imagined how their life circumstances would look like in the future, and their body swayed slightly backward when thinking about their life circumstances in the past. Miles, Karpinska, et al. (2010) also analyzed the future and past-related contents of spontaneous mind-wandering during a vigilance task. More future-related thoughts came to participants’ minds when the task included visually induced forward motion (vection), whereas backward motion induced more past-related thoughts. Thus, thinking about the past and future seems to be intertwined with backward and forward motion, following the metaphorical language use (see also Hartmann & Mast, 2012). In this study, we investigated the horizontal mental time line by asking participants to mentally displace themselves into the past and future (i.e., episodic mental time travel). If mental time travel has a behavioral correlate in the horizontal dimension of space, this would strengthen the evidence that the horizontal mental time line is more than an epiphenomenon of temporal categorization tasks. We used eye movements as an indicator of the location of spatial attention (e.g., Corbetta et al., 1998 and Sheliga et al., 1994). Participants were asked to mentally displace themselves into their subjective past and future while spontaneous eye movements on a blank screen were measured (see Loetscher, Bockisch, Nicholls, & Brugger, 2010 for a similar approach in the domain of numerical cognition). If past-related events were inherently associated with the left, and future-related events with the right side of space, we would expect more rightward directed eye gaze behavior when participants think about the future than when they think about the past. This paradigm also allows for assessing whether there is a vertical component in the spatial–temporal association. The mental number line not only runs from left to right but also from bottom to top (Grade, Lefèvre, & Pesenti, 2012; Hartmann, Gashaj, Stahnke, & Mast, 2014; Hartmann et al., 2012, Ito and Hatta, 2004, Loetscher et al., 2010, Schwarz and Keus, 2004 and Winter and Matlock, 2013). Given the analogy between the mental number line and the mental time line (Arzy et al., 2009 and Bonato et al., 2012), it is conceivable that past is associated with the lower, and future with the upper space, which could also influence eye gaze behavior during mental time travel.