Automatic response activation of implicit spatial information: Evidence from the SNARC effect

Abstract

In the present paper, we focus on how irrelevant implicit spatial information is processed. By irrelevant we mean information that is not required to fulfill the task and by implicit we mean information that is not directly available in the external stimulus. A good example of a task in which such information exists is the SNARC task [Dehaene, S., Bossini, S., & Giraux, P. (1993). The mental representation of parity and number magnitude. Journal of Experimental Psychology: General, 122, 371–396]. The SNARC effect shows that the magnitude of a number, although irrelevant to the task, activates spatial codes that may interfere with the task-related response. These spatial associations exist both for the horizontal and the vertical direction. In Experiment 1, response keys were discriminating in the vertical or the horizontal direction. It is shown that the impact of the numerical spatial codes on overt behavior, although automatic, depends on the response discrimination of the horizontal or the vertical dimension. In Experiment 2, response keys were assigned such that both the horizontal and the vertical direction of the response were discriminating. In this case, the horizontal and the vertical dimension of the irrelevant numerical spatial codes were shown to interact. In general, the results are in line with the response-discrimination account [Ansorge, U., & Wühr, P. (2004). A response-discrimination account of the Simon effect. Journal of Experimental Psychology: Human Perception and Performance, 30, 365–377].

Introduction

The perception of numbers elicits spatial codes that are associated with the magnitude of the number (for recent reviews see Fias and Fischer, 2005, Hubbard et al., 2005). Evidence for such spatial codes comes from the Spatial Numerical Association of Response Codes or SNARC effect (Dehaene, Bossini, & Giraux, 1993). Dehaene and colleagues had participants perform a parity judgment task (odd vs. even) in a bimanual response setting. Although magnitude information is irrelevant to perform parity judgment relatively small numbers were responded to faster with the left hand and relatively large numbers were responded to faster with the right hand (Dehaene et al., 1993). Writing direction has been described as a key determinant for the SNARC effect. French monoliterates who use a left-to-right writing system persistently showed a stronger SNARC compared to high skilled French-Persian biliterates who use both a left-to-right and a right-to-left writing systems (Dehaene et al., 1993; see also Zebian, 2005).

Since this influential paper (Dehaene et al., 1993), the SNARC effect was observed with a variety of tasks like magnitude comparison, orientation discrimination (Fias et al., 2001, Lammertyn et al., 2002) and phoneme monitoring (Fias, Brysbaert, Geypens, & d’Ydewalle, 1996). Additionally, the SNARC effect has been shown to be effector-independent as it was demonstrated with unimanual responses (Fischer, 2003), bimanual responses (Dehaene et al., 1993) and eye movements (Fischer et al., 2004, Schwarz and Keus, 2004).

Important for the present purposes, the SNARC effect is not restricted to a left-to-right orientation. Schwarz and Keus (2004) observed that downward saccades were initiated faster in response to relatively small numbers whereas upward saccades were initiated faster in response to relatively large numbers. Further evidence for the existence of a SNARC effect in the vertical dimension comes from a study by Ito and Hatta (2004). In a bimanual response setting, Japanese subjects showed an association with small numbers represented at the lower half of space and larger numbers at the upper half of space. These results corroborate the existence of spatial-numerical effects within the vertical dimension. However, unlike the SNARC effect in the horizontal dimension, the spatial association with the magnitude of a number in the vertical dimension does not seem to be the result of reading and writing habits. As Ito and Hatta (2004) pointed out, reading and writing is performed from top to bottom, which should result in an association of small magnitudes with up and large magnitudes with down whereas the opposite pattern is observed. We believe that a plausible explanation for the association in the vertical dimension is provided by daily life experience. Or as Lakoff (1987, p. 276) puts it quite clearly: “Whenever we add more of a substance—say, water to a glass—the level goes up. When we add more objects to a pile, the level rises. Remove objects from the pile or water from the glass, the level goes down. The correlation is overwhelming: more correlates with up, but less correlates with down”.

A number of recent studies investigated the locus of the SNARC effect by relating the SNARC effect to the Simon effect (Gevers, Caessens, et al., 2005, Keus and Schwarz, 2005, Mapelli et al., 2003). In a typical Simon task, subjects are asked to respond with a left or right response key to the color of a stimulus that is presented to the left or right of the fixation cross. In this case, only the color of the stimulus is relevant and the location of the stimulus should be ignored. However, RTs are faster when the irrelevant stimulus location corresponds with the response location (for an overview see Simon, 1990). Traditionally, the Simon effect is explained by dual-route models in which it is assumed that upon the presentation of the stimulus two routes are activated in parallel (e.g. De Jong et al., 1994, Kornblum et al., 1990). One unconditional route is activated by the irrelevant stimulus location and automatically primes an ipsilateral response. At the same time, the relevant information (e.g. color of the stimulus) is processed through a slower conditional and controlled route. This general processing architecture seems to be responsible for the SNARC effect as well.

Studying the SNARC effect together with the Simon effect, Gevers, Caessens, et al., 2005, Gevers, Ratinckx, et al., in press, Gevers, Verguts, et al., in press (see also Keus & Schwarz, 2005) found that the relationship between the Simon and the SNARC effect depends on the relevance of the magnitude information. Additionally, psychophysiological measurements showed that the Simon and the SNARC effect have the same ERP-signature (Gevers, Ratinckx, et al., in press, Keus et al., 2005).

The lateral presentation of a stimulus in the Simon task is usually thought to automatically activate the spatially compatible response (De Jong et al., 1994, Hommel, 1995, Kornblum et al., 1990). However, Valle-lnclán and Redondo (1998) showed that the automatic activation of explicit irrelevant spatial information is limited. Using electrophysiological measurements, they showed that ipsilateral response activation was absent in a Simon task if the response-key assignment was provided after the target stimulus was presented. Therefore, it was concluded that unconditional response activation requires a predefined stimulus–response mapping rule. Recently, Ansorge and Wühr (2004) confirmed and extended this interpretation by arguing that the spatial features associated with the response need to be represented in working memory for the unconditional response activation to occur. In addition, these spatial features only influence the response when they discriminate between alternative responses. A Simon task was conducted in which participants had to respond to the color of a stimulus while ignoring its position. Two modifications of the standard Simon design were introduced. First, the stimuli appeared along both a vertical (below or above the fixation point) and a horizontal axis (left or right of the fixation point). Second, the location of the response was manipulated such that it discriminated for one axis whereas it did not for the other (Ansorge & Wühr, 2004, Experiment 1). For instance, subjects were required to press a response key located at the bottom-left and the upper-left of a central home key. With this mapping, the responses discriminated along the vertical axis (e.g. up or down depending on the color of the stimulus), but not along the horizontal axis (e.g. all responses are to the left of the central home key). A Simon effect was observed in the discriminating vertical dimension (faster responses with top response key when stimulus was presented above fixation, than when stimulus was presented below fixation and faster responses with bottom response when stimulus was presented below fixation than when stimulus was presented above fixation), but not in the horizontal dimension (responses to left response key not faster than response to right response key), Similarly, if the horizontal response axis discriminated between responses (e.g. if red press upper-left key, if green press upper-right key), a Simon effect was observed in the horizontal dimension but not in the vertical dimension. Finally, when both axes discriminated between alternative responses (e.g. press bottom-left and upper-right key) a Simon effect was observed in both the horizontal and the vertical dimension.

These data show that the automatic response activation in the Simon task depends on response characteristics. Presenting a stimulus left of the fixation cross does not result in response priming of the left response, if no left–right discrimination is required. The response-discrimination account states that top-down controlled response representations (i.e. discriminating spatial information that is relevant for the task) are crucial for the response priming of explicit spatial information. Although the Simon and the SNARC effect are amenable to a mutual underlying processing architecture, they differ in one important aspect, namely the kind of irrelevant spatial information that is being processed. In a Simon task, this spatial information is explicitly presented by varying the position of a stimulus on the screen, whereas in a SNARC context, this information is implicitly conveyed in the magnitude representation of a number. Therefore, the key question of the present paper is whether the automatic activation of implicit irrelevant spatial information is governed by the same rules as its explicit counterpart. To this end, we applied the logic of the response-discrimination hypothesis to the SNARC task.