Journal of Experimental Psychology: Human Perception and Performance 2014, Vol. 40, No. 6, 2252-2265
© 2014 American Psychological Association 0096-1523/14/$ 12.00 http://dx.doi.org/! 0.1037/a0038140
Enhanced Spatial Stimulus-Response Mapping Near the Hands: The Simon Effect Is Modulated by Hand-Stimulus Proximity Xiaotao Wang
Feng Du, Xiaosong He, and Kan Zhang
Institute of Psychology, Chinese Academy of Sciences and University of Chinese Academy of Sciences
Institute of Psychology, Chinese Academy of Sciences
Emerging evidence has revealed that visual processing of objects near the hands is altered. The present study shows that the visuomotor Simon effect when the hands are proximal to stimuli is greater than that observed when the hands are far from stimuli, thereby indicating stronger spatial stimulus-response mapping near the hands. The visuomotor Simon effect is robustly enhanced near the hands even when hand visibility and stimulus-response axis-similarity are controlled. However, the semantic Simon effect with location words is not modulated by hand-stimulus proximity. Thus, consistent with the dimensional overlap model and the known features of the bimodal visuotactile neurons, hand-stimulus proximity enhances spatial stimulus-response mapping but has no effect on semantic processing of location words. Keywords: hand-stimulus proximity, visuomotor Simon effect, bimodal visuotactile neurons, dimen sional overlap model, magnocellular visual pathway
letter recognition performance (Adam, Bovend’eerdt, van Dooren, Fischer, & Pratt, 2012). Moreover, stimuli near the hand receive extensive perceptual analysis because they are candidates for manipulation (Abrams, Davoli, Du, Knapp, & Pauli, 2008; Davoli & Abrams, 2009). For example, Abrams et al. (2008) found that hand-stimulus proximity (also known as hand proximity) results in a slower rate of visual search, a reduced inhibition of return and a greater attentional blink relative to the hand distal condition. When participants simply imagine their hands near the display, their search rate is slower than when they imagine their hands far from the display (Davoli & Abrams, 2009). Davoli, Brockmole, Du, and Abrams (2012), using a global/local switch task, reported a relatively slower shift of attention between global and local aspects of objects when stimuli are near the hands compared with when they are far from the hands. Abrams et al. (2008) proposed a delayed attentional disengagement hypothesis, which states that stimuli near the hands receive a more extensive perceptual analysis compared to items far from the hands. Consistent with this hypothesis, a study found that the P2 component of an irrelevant distractor (checkerboard) is attenuated when it is near the hands relative to when it is far from the hands (Qian, Al-Aidroos, West, Abrams, & Pratt, 2012), thus suggesting that stimuli near the hands benefit from attentional priority during later stages of perceptual processing. In addition, the distracting effect of peripheral flankers is reduced when they appear near the hands (Davoli & Brockmole, 2012). While most of the aforementioned studies indicate that hand-stimulus proximity influences various aspects of perceptual processing, it is still unknown whether hand-stimulus proximity influences stimulusresponse (S-R) mapping. To investigate this issue, the present study uses the visuomotor Simon task and the semantic Simon task to examine whether S-R mapping is modulated by hand-stimulus proximity. A visuomotor Simon task requires participants to respond to a spatially lateral-
Hand-stimulus proximity has been found to influence visual processing, especially that of visual attention (Brockmole, Davoli, Abrams, & Witt, 2013; Tseng, Bridgeman, & Juan, 2012). For example, stimuli near the hand are detected faster than those far from the hand (Reed, Grubb, & Steele, 2006). Additionally, the regions near the hand are more likely to be assigned as foreground figures than otherwise similar regions (Cosman & Vecera, 2010). As a consequence, hand-stimulus proximity modulates visual short-term memory (STM). For instance, holding the hands near a display increases the number of items that can be retained in the visual working memory (Tseng & Bridgeman, 2011) and improves
This article was published Online First October 13, 2014. Xiaotao Wang, Institute of Psychology, Chinese Academy of Sciences and University of Chinese Academy of Sciences; Feng Du, Xiaosong He, and Kan Zhang, Key Laboratory of Behavioral Science, Institute of Psy chology, Chinese Academy of Sciences. Xiaotao Wang and Feng Du contributed equally to this study. Xiaotao Wang and Feng Du developed the study concept and the study design. Testing and data collection were performed by Xiaotao Wang, and Xiaosong He provided technical support in programming and setting up apparatus. Xiaotao Wang performed the data analysis and interpretation under the supervision of Feng Du. Feng Du and Xiaotao Wang drafted the paper, and Xiaosong He and Kan Zhang provided critical revisions. All authors approved the final version of the paper for submission. This study was supported by the National Natural Science Foundation of China (31470982 and 31200766), the Chinese Academy of Sciences (KJZD-EWL04-4), and the scientific foundation of Institute of Psychology, Chinese Academy of Sciences (Y4CX033008). We thank Richard Carciofo for proofreading this paper. We thank Jiao Jun for providing technical support during programming. Correspondence concerning this article should be addressed to Feng Du, Key Laboratory of Behavioral Science, Institute of Psychology, Chinese Academy of Sciences, 16 Lincui Road, Beijing, China 100101. E-mail: [email protected] 2252
ENHANCED STIMULUS-RESPONSE MAPPING NEAR HANDS ized stimulus by pressing a left or right button. Participants are told to respond to a nonspatial feature of a stimulus, such as color or shape, while ignoring its spatial location (Simon, 1990; for re views, see Hommel, 2011, and Lu & Proctor, 1995). The Simon effect refers to the fact that responses to compatible stimuli (i.e., the stimuli are ipsilateral to the response) are faster and more accurate than responses to incompatible stimuli (i.e., the stimuli are contralateral to the response), while stimulus location is irrel evant to the task. Recently, many studies have used location words (e.g., left or right) to produce the semantic Simon effect (Khalid & Ansorge, 2013; Pellicano, Lugli, Baroni, & Nicoletti, 2009). In the semantic Simon task, location words are presented at the center of a display, and participants are told to respond to a feature of the word, such as color, while ignoring its meaning. Responses are faster and more accurate when the response location and the meaning of the word are compatible than when they are incom patible. It has been repeatedly demonstrated that the visuomotor Simon effect decreases as response times (RTs) increase, whereas the semantic Simon effect increases as RTs increase (Miles & Proctor, 2012; Pellicano et al., 2009; Proctor, Yamaguchi, Zhang, & Vu, 2009). The different temporal dynamics of the two Simon effects suggest that the semantic Simon effect differs from the visuomotor Simon effect. Thus, the present study examines whether hand-stimulus proximity leads to stronger S-R mapping in both the visuomotor Simon task and the semantic Simon task. According to Komblum, Hasbroucq, and Osman’s (1990) tax onomy of compatibility effects, the Simon effect is an S-R com patibility effect that involves a dimensional overlap between the task-irrelevant stimulus and the task-relevant response. The com patibility effects are generated from the common properties shared by both the stimulus and the response as the shared properties in the stimulus set automatically activate corresponding properties in the response set. Thus, the performance can either be facilitated when the automatically activated response is the required one, or it can be significantly impaired when the automatically activated response is not the required one. Furthermore, the magnitude of the compatibility effect is due to the degree of similarity (the dimen sional overlap) between stimulus and response features (Kom blum, 1994; Komblum et al., 1990). According to the Gestalt principle of perceptual organization, things proximal to each other are grouped together because of their spatial similarity (see Wagemans et al., 2012, for a review). As a result, putting the hands near the stimuli might increase the spatial similarity between the stim ulus and the response, thus leading to an increased visuomotor Simon effect near the hands relative to the effect far from the hands. Conversely, the semantic Simon effect is based on concep tual S-R similarity (Komblum, 1994; Komblum et al., 1990; Kornblum & Lee, 1995; Miles & Proctor, 2009), which remains the same whether the hands are near or far from the stimuli. Therefore, an enhanced semantic Simon effect in the handproximal condition is not expected. Alternatively, Nicoletti and L'milta (1989) proposed an attention-shift account with respect to the Simon effect, suggesting that attentional orienting is crucial to the Simon effect. Shifting attention to the target location generates a spatial code of the stimuli, which either facilitates corresponding responses or inter feres with conflicting responses (e.g., Abrahamse & Van der Lubbe, 2008; Nicoletti & UmiM, 1989, 1994; Van der Lubbe, Jaskowski, & Verleger, 2005). According to the delayed atten
2253
tional disengagement account, an attentional shift is slower when a stimulus is near the hands than when a stimulus is far from the hands (Abrams et al., 2008; Davoli et al., 2012; Vatterott & Vecera, 2013). Because both the visuomotor and the semantic Simon tasks require participants to make attentional shifts from fixation to periphery, hand-stimulus proximity might generally increase RTs irrespective of S-R compatibility in both tasks. Some studies suggest that the activation of bimodal visuotactile neurons might be the underling mechanism of the hand proximity effect. Single-cell recording in the primate brain has revealed that the visuotactile neurons in the premotor cortex and the putamen of macaques respond to both tactile stimulation on the hand and visual stimuli in the space immediately surrounding the hand (Graziano & Gross, 1993, 1994; Graziano, Yap, & Gross, 1994). It is well known that the receptive fields of the bimodal neurons are hand-centered rather than eye-centered. These bimodal neurons encode the spatial distance between the stimuli and the hand by enhancing their response to the stimuli as the stimuli get closer to the hand (Graziano, Hu, & Gross, 1997a; Fogassi et al., 1996). Recent fMRI studies of humans have also shown that bimodal neurons in the anterior part of the intraparietal sulcus, the inferior parietal lobe (supramarginal gyrus), and the dorsal and ventral portions of the premotor cortex encode the hand-centered space and arm ownership (Brozzoli, Gentile, & Ehrsson, 2012; Brozzoli, Gentile, Petkova, & Ehrsson, 2011). According to these known features of the bimodal visuotactile neurons, such as hand-centered spatial coding and enhanced response to visual stimuli near the hand, the spatial S-R mapping may become stronger when the stimuli come closer to the hands, thus resulting in an enhanced visuomotor Simon effect near the hands. However, as bimodal neurons are not selective for semantic meaning, they would not enhance S-R mapping to a location word near the hands, thus the semantic Simon effect remains the same under both hand-stimulus proximity conditions. Another possible neural mechanism might also contribute to the hand proximity effect on visual processing. Gozli, West, and Pratt (2012) reported better performance on the temporal-gap detection task and worse performance on the spatial-gap detection task when stimuli were presented near the hands compared to when they were far from the hands. A recent study also showed that temporal fusion near the hands is reduced (Goodhew, Gozli, Ferber, & Pratt, 2013). These studies consistently showed an improved temporal resolution to visual events near the hands. As the enhanced tem poral resolution and impoverished semantic processing near the hands (Davoli, Du, Montana, Garverick, & Abrams, 2010; Gozli et al., 2012) cannot be explained by activities of bimodal neurons per se, to account for these findings, Gozli et al. (2012) proposed that visual stimuli in the peri-hand space induce a bias toward the “action-oriented magnocellular visual pathway that supports pro cessing with high temporal resolution but low spatial resolution.” Conversely, objects far from the hands bias vision toward the perception-oriented parvocellular visual pathway with high spatial resolution but low temporal resolution. Some studies have sug gested that the visuomotor Simon effect relies on the visuomotor information transmission via the dorsal stream, which is special ized for visuomotor processing(Wascher, Schatz, Kuder, & Ver leger, 2001; Wiegand & Wascher, 2005, 2007). Thus, the bias to the action-oriented magnocellular visual pathway might help to transmit visuomotor information and facilitate the compatible re-
WANG, DU, HE, AND ZHANG
2254
sponse. As a result, an enhanced visuomotor Simon effect should be observed when hands are proximal to the stimuli. However, according to the magnocellular/parvocellular visual pathway com petition theory (Gozli et al., 2012), the bias to the magnocellular visual pathway when stimuli are near the hands would be antag onistic to the parvocellular visual pathway. Gozli et al. (2012) also postulated that, as the parvocellular neurons are responsible for the spatial grouping of letters, the reduced contribution of the parvo cellular neurons would result in the decreased semantic processing of words. Therefore, the magnocellular/parvocellular visual path way competition theory predicts that the semantic Simon effect would be reduced near the hands. In the present study, Experiments 1 and 2 are designed to test whether a spatially lateralized stimulus near the hands causes a larger Simon effect than the same stimulus far from the hands. Experiment 3 is designed to control for some potentially confound ing factors in Experiments 1 and 2. First, Experiment 3a is de signed to replicate Experiments 1 and 2 when the stimuli are symmetrically presented. Second, the hands are visible in the hand-proximal condition, but invisible in the hand-distal condition in Experiments 1 and 2. According to research on response discriminability (Koch, Schuch, Vu, & Proctor, 2011; Vu & Proctor, 2002), hand visibility near the hands may lead to higher hand discriminability, resulting in an enhanced Simon effect near the hands. Thus, in Experiment 3b the hands are visible in both hand-stimulus proximity conditions to rule out this possibility. Third, S-R axis-similarity may be another confounding factor in the previous experiments. The responses in the two hand-stimulus proximity conditions of Experiments 1 and 2 have the same horizontal separation but differ in their vertical height. Experiment 3c is designed to minimize the difference of S-R axis-similarity by controlling the responses in the two hand-stimulus proximity con ditions to have the same horizontal separation and vertical height. Experiment 4 uses location words to examine whether locating words near the hands enhance the semantic Simon effect compared to the same words located far from the hands.
were presented on a 17 in CRT display at a viewing distance of 43 cm. Participants steadied their head by resting on a chinrest. In the hand-proximal condition, subjects rested their elbows on cushions on the table and held each hand on a mouse that was attached to the edges of the monitor aligned with the middle of the CRT (see Figure 1A). In the hand-distal condition, subjects supported a lightweight 38-cm-long board on their laps upon which were mounted two mouses identical to those in the hand-proximal condition (see Figure IB). Subjects kept one hand on each mouse. The distance between the two mouses was the same in both conditions with approximately 32 cm separation. Each trial began with a 500-ms blank screen and was followed by a white fixation cross at the center of the screen for 500 ms. The target stimulus was then presented 11° left or right of the fixation cross (1.3° wide, 1.3° high) and remained visible until a response was made. The target was either a green (CIE 1931: Y = 73.98, x = 0.2944, y = 0.5719) or a red (CIE 1931: Y = 29.66, x = 0.5572, y = 0.3403) disk with a diameter of 1.7°. Participants were instructed to press one of the mouse buttons if a green disk appeared and the other button if a red disk appeared, irrespective of its position. The mapping between the stimulus color and the response buttons was counterbalanced across participants. The spatial mapping between stimulus locations and response buttons was varied across trials. For compatible trials, the correct response button was ipsilateral to the stimulus, whereas for incompatible trials, the correct response button was contralateral to the stimulus. Participants were instructed to respond as quickly and as accu rately as possible. If participants pressed an incorrect button, an error message was presented at the center for 1,000 ms. All stimuli were presented on a dark background.
E xperim ent 1 The visuomotor Simon task is used to test whether spatial S-R mapping is enhanced near the hands relative to far from the hands. The present experiment presents a colored disk (green or red) laterally and requires participants to report the color of the disk. If spatial S-R mapping is enhanced near the hands (i.e., in the hand-proximal condition) relative to that far from the hands (i.e., in the hand-distal condition), interference toward the incompatible response should become stronger, and thus cause a larger Simon effect. M ethod Participants. This study was approved by the institutional review board of the Institute of Psychology, Chinese Academy of Sciences. Thirty-two students (16 males and 16 females; 18- to 28-years-old), two of whom were left-handers, participated in the experiment for payment. All participants had normal or correctedto-normal vision. Apparatus, stimuli and procedure. The experiment closely followed the setting of the Abrams et al. (2008) study. Stimuli
Figure 1. Four postures used in the present study. (A) The hand proximal condition in all six experiments. (B) The hand distal condition in Experi ments 1, 2, 3a, and 4. (C) The hand distal condition in Experiment 3b. (D) The hand distal condition in Experiment 3c. See the online article for the color version of this figure.
ENHANCED STIMULUS-RESPONSE MAPPING NEAR HANDS
Design. There were two within-subject variables including hand-stimulus proximity and S-R compatibility between stimuli and response. Each trial was in one of two hand-stimulus prox imity conditions. In the hand-proximal condition, participants held each hand on a mouse that was attached to the edges of the monitor. In the hand-distal condition, participants supported a lightweight 38-cm-long board on their laps upon which were mounted two mouses. In addition, each trial was conducted under one of two S-R compatibility conditions such that the correct response button was ipsilateral to the stimulus (compatible), or the correct response button was contralateral to the stimulus (incom patible). The experiment consisted of 64 replications of each combination of two hand-stimulus proximity conditions and two S-R compatibility conditions, for a total of 256 trials. Participants first completed one block of 24 trials for practice. They then engaged in four blocks, each of which had 32 compatible trials and 32 incompatible trials. There were two blocks in each of the two hand-stimulus proximity conditions. The order of the two handstimulus proximity conditions was counterbalanced across partic ipants. Results and Discussion In the present and subsequent experiments, trials for which reaction time (RT) was less than 100 ms or greater than 1,500 ms were excluded from analysis (
© 2014 American Psychological Association 0096-1523/14/$ 12.00 http://dx.doi.org/! 0.1037/a0038140
Enhanced Spatial Stimulus-Response Mapping Near the Hands: The Simon Effect Is Modulated by Hand-Stimulus Proximity Xiaotao Wang
Feng Du, Xiaosong He, and Kan Zhang
Institute of Psychology, Chinese Academy of Sciences and University of Chinese Academy of Sciences
Institute of Psychology, Chinese Academy of Sciences
Emerging evidence has revealed that visual processing of objects near the hands is altered. The present study shows that the visuomotor Simon effect when the hands are proximal to stimuli is greater than that observed when the hands are far from stimuli, thereby indicating stronger spatial stimulus-response mapping near the hands. The visuomotor Simon effect is robustly enhanced near the hands even when hand visibility and stimulus-response axis-similarity are controlled. However, the semantic Simon effect with location words is not modulated by hand-stimulus proximity. Thus, consistent with the dimensional overlap model and the known features of the bimodal visuotactile neurons, hand-stimulus proximity enhances spatial stimulus-response mapping but has no effect on semantic processing of location words. Keywords: hand-stimulus proximity, visuomotor Simon effect, bimodal visuotactile neurons, dimen sional overlap model, magnocellular visual pathway
letter recognition performance (Adam, Bovend’eerdt, van Dooren, Fischer, & Pratt, 2012). Moreover, stimuli near the hand receive extensive perceptual analysis because they are candidates for manipulation (Abrams, Davoli, Du, Knapp, & Pauli, 2008; Davoli & Abrams, 2009). For example, Abrams et al. (2008) found that hand-stimulus proximity (also known as hand proximity) results in a slower rate of visual search, a reduced inhibition of return and a greater attentional blink relative to the hand distal condition. When participants simply imagine their hands near the display, their search rate is slower than when they imagine their hands far from the display (Davoli & Abrams, 2009). Davoli, Brockmole, Du, and Abrams (2012), using a global/local switch task, reported a relatively slower shift of attention between global and local aspects of objects when stimuli are near the hands compared with when they are far from the hands. Abrams et al. (2008) proposed a delayed attentional disengagement hypothesis, which states that stimuli near the hands receive a more extensive perceptual analysis compared to items far from the hands. Consistent with this hypothesis, a study found that the P2 component of an irrelevant distractor (checkerboard) is attenuated when it is near the hands relative to when it is far from the hands (Qian, Al-Aidroos, West, Abrams, & Pratt, 2012), thus suggesting that stimuli near the hands benefit from attentional priority during later stages of perceptual processing. In addition, the distracting effect of peripheral flankers is reduced when they appear near the hands (Davoli & Brockmole, 2012). While most of the aforementioned studies indicate that hand-stimulus proximity influences various aspects of perceptual processing, it is still unknown whether hand-stimulus proximity influences stimulusresponse (S-R) mapping. To investigate this issue, the present study uses the visuomotor Simon task and the semantic Simon task to examine whether S-R mapping is modulated by hand-stimulus proximity. A visuomotor Simon task requires participants to respond to a spatially lateral-
Hand-stimulus proximity has been found to influence visual processing, especially that of visual attention (Brockmole, Davoli, Abrams, & Witt, 2013; Tseng, Bridgeman, & Juan, 2012). For example, stimuli near the hand are detected faster than those far from the hand (Reed, Grubb, & Steele, 2006). Additionally, the regions near the hand are more likely to be assigned as foreground figures than otherwise similar regions (Cosman & Vecera, 2010). As a consequence, hand-stimulus proximity modulates visual short-term memory (STM). For instance, holding the hands near a display increases the number of items that can be retained in the visual working memory (Tseng & Bridgeman, 2011) and improves
This article was published Online First October 13, 2014. Xiaotao Wang, Institute of Psychology, Chinese Academy of Sciences and University of Chinese Academy of Sciences; Feng Du, Xiaosong He, and Kan Zhang, Key Laboratory of Behavioral Science, Institute of Psy chology, Chinese Academy of Sciences. Xiaotao Wang and Feng Du contributed equally to this study. Xiaotao Wang and Feng Du developed the study concept and the study design. Testing and data collection were performed by Xiaotao Wang, and Xiaosong He provided technical support in programming and setting up apparatus. Xiaotao Wang performed the data analysis and interpretation under the supervision of Feng Du. Feng Du and Xiaotao Wang drafted the paper, and Xiaosong He and Kan Zhang provided critical revisions. All authors approved the final version of the paper for submission. This study was supported by the National Natural Science Foundation of China (31470982 and 31200766), the Chinese Academy of Sciences (KJZD-EWL04-4), and the scientific foundation of Institute of Psychology, Chinese Academy of Sciences (Y4CX033008). We thank Richard Carciofo for proofreading this paper. We thank Jiao Jun for providing technical support during programming. Correspondence concerning this article should be addressed to Feng Du, Key Laboratory of Behavioral Science, Institute of Psychology, Chinese Academy of Sciences, 16 Lincui Road, Beijing, China 100101. E-mail: [email protected] 2252
ENHANCED STIMULUS-RESPONSE MAPPING NEAR HANDS ized stimulus by pressing a left or right button. Participants are told to respond to a nonspatial feature of a stimulus, such as color or shape, while ignoring its spatial location (Simon, 1990; for re views, see Hommel, 2011, and Lu & Proctor, 1995). The Simon effect refers to the fact that responses to compatible stimuli (i.e., the stimuli are ipsilateral to the response) are faster and more accurate than responses to incompatible stimuli (i.e., the stimuli are contralateral to the response), while stimulus location is irrel evant to the task. Recently, many studies have used location words (e.g., left or right) to produce the semantic Simon effect (Khalid & Ansorge, 2013; Pellicano, Lugli, Baroni, & Nicoletti, 2009). In the semantic Simon task, location words are presented at the center of a display, and participants are told to respond to a feature of the word, such as color, while ignoring its meaning. Responses are faster and more accurate when the response location and the meaning of the word are compatible than when they are incom patible. It has been repeatedly demonstrated that the visuomotor Simon effect decreases as response times (RTs) increase, whereas the semantic Simon effect increases as RTs increase (Miles & Proctor, 2012; Pellicano et al., 2009; Proctor, Yamaguchi, Zhang, & Vu, 2009). The different temporal dynamics of the two Simon effects suggest that the semantic Simon effect differs from the visuomotor Simon effect. Thus, the present study examines whether hand-stimulus proximity leads to stronger S-R mapping in both the visuomotor Simon task and the semantic Simon task. According to Komblum, Hasbroucq, and Osman’s (1990) tax onomy of compatibility effects, the Simon effect is an S-R com patibility effect that involves a dimensional overlap between the task-irrelevant stimulus and the task-relevant response. The com patibility effects are generated from the common properties shared by both the stimulus and the response as the shared properties in the stimulus set automatically activate corresponding properties in the response set. Thus, the performance can either be facilitated when the automatically activated response is the required one, or it can be significantly impaired when the automatically activated response is not the required one. Furthermore, the magnitude of the compatibility effect is due to the degree of similarity (the dimen sional overlap) between stimulus and response features (Kom blum, 1994; Komblum et al., 1990). According to the Gestalt principle of perceptual organization, things proximal to each other are grouped together because of their spatial similarity (see Wagemans et al., 2012, for a review). As a result, putting the hands near the stimuli might increase the spatial similarity between the stim ulus and the response, thus leading to an increased visuomotor Simon effect near the hands relative to the effect far from the hands. Conversely, the semantic Simon effect is based on concep tual S-R similarity (Komblum, 1994; Komblum et al., 1990; Kornblum & Lee, 1995; Miles & Proctor, 2009), which remains the same whether the hands are near or far from the stimuli. Therefore, an enhanced semantic Simon effect in the handproximal condition is not expected. Alternatively, Nicoletti and L'milta (1989) proposed an attention-shift account with respect to the Simon effect, suggesting that attentional orienting is crucial to the Simon effect. Shifting attention to the target location generates a spatial code of the stimuli, which either facilitates corresponding responses or inter feres with conflicting responses (e.g., Abrahamse & Van der Lubbe, 2008; Nicoletti & UmiM, 1989, 1994; Van der Lubbe, Jaskowski, & Verleger, 2005). According to the delayed atten
2253
tional disengagement account, an attentional shift is slower when a stimulus is near the hands than when a stimulus is far from the hands (Abrams et al., 2008; Davoli et al., 2012; Vatterott & Vecera, 2013). Because both the visuomotor and the semantic Simon tasks require participants to make attentional shifts from fixation to periphery, hand-stimulus proximity might generally increase RTs irrespective of S-R compatibility in both tasks. Some studies suggest that the activation of bimodal visuotactile neurons might be the underling mechanism of the hand proximity effect. Single-cell recording in the primate brain has revealed that the visuotactile neurons in the premotor cortex and the putamen of macaques respond to both tactile stimulation on the hand and visual stimuli in the space immediately surrounding the hand (Graziano & Gross, 1993, 1994; Graziano, Yap, & Gross, 1994). It is well known that the receptive fields of the bimodal neurons are hand-centered rather than eye-centered. These bimodal neurons encode the spatial distance between the stimuli and the hand by enhancing their response to the stimuli as the stimuli get closer to the hand (Graziano, Hu, & Gross, 1997a; Fogassi et al., 1996). Recent fMRI studies of humans have also shown that bimodal neurons in the anterior part of the intraparietal sulcus, the inferior parietal lobe (supramarginal gyrus), and the dorsal and ventral portions of the premotor cortex encode the hand-centered space and arm ownership (Brozzoli, Gentile, & Ehrsson, 2012; Brozzoli, Gentile, Petkova, & Ehrsson, 2011). According to these known features of the bimodal visuotactile neurons, such as hand-centered spatial coding and enhanced response to visual stimuli near the hand, the spatial S-R mapping may become stronger when the stimuli come closer to the hands, thus resulting in an enhanced visuomotor Simon effect near the hands. However, as bimodal neurons are not selective for semantic meaning, they would not enhance S-R mapping to a location word near the hands, thus the semantic Simon effect remains the same under both hand-stimulus proximity conditions. Another possible neural mechanism might also contribute to the hand proximity effect on visual processing. Gozli, West, and Pratt (2012) reported better performance on the temporal-gap detection task and worse performance on the spatial-gap detection task when stimuli were presented near the hands compared to when they were far from the hands. A recent study also showed that temporal fusion near the hands is reduced (Goodhew, Gozli, Ferber, & Pratt, 2013). These studies consistently showed an improved temporal resolution to visual events near the hands. As the enhanced tem poral resolution and impoverished semantic processing near the hands (Davoli, Du, Montana, Garverick, & Abrams, 2010; Gozli et al., 2012) cannot be explained by activities of bimodal neurons per se, to account for these findings, Gozli et al. (2012) proposed that visual stimuli in the peri-hand space induce a bias toward the “action-oriented magnocellular visual pathway that supports pro cessing with high temporal resolution but low spatial resolution.” Conversely, objects far from the hands bias vision toward the perception-oriented parvocellular visual pathway with high spatial resolution but low temporal resolution. Some studies have sug gested that the visuomotor Simon effect relies on the visuomotor information transmission via the dorsal stream, which is special ized for visuomotor processing(Wascher, Schatz, Kuder, & Ver leger, 2001; Wiegand & Wascher, 2005, 2007). Thus, the bias to the action-oriented magnocellular visual pathway might help to transmit visuomotor information and facilitate the compatible re-
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sponse. As a result, an enhanced visuomotor Simon effect should be observed when hands are proximal to the stimuli. However, according to the magnocellular/parvocellular visual pathway com petition theory (Gozli et al., 2012), the bias to the magnocellular visual pathway when stimuli are near the hands would be antag onistic to the parvocellular visual pathway. Gozli et al. (2012) also postulated that, as the parvocellular neurons are responsible for the spatial grouping of letters, the reduced contribution of the parvo cellular neurons would result in the decreased semantic processing of words. Therefore, the magnocellular/parvocellular visual path way competition theory predicts that the semantic Simon effect would be reduced near the hands. In the present study, Experiments 1 and 2 are designed to test whether a spatially lateralized stimulus near the hands causes a larger Simon effect than the same stimulus far from the hands. Experiment 3 is designed to control for some potentially confound ing factors in Experiments 1 and 2. First, Experiment 3a is de signed to replicate Experiments 1 and 2 when the stimuli are symmetrically presented. Second, the hands are visible in the hand-proximal condition, but invisible in the hand-distal condition in Experiments 1 and 2. According to research on response discriminability (Koch, Schuch, Vu, & Proctor, 2011; Vu & Proctor, 2002), hand visibility near the hands may lead to higher hand discriminability, resulting in an enhanced Simon effect near the hands. Thus, in Experiment 3b the hands are visible in both hand-stimulus proximity conditions to rule out this possibility. Third, S-R axis-similarity may be another confounding factor in the previous experiments. The responses in the two hand-stimulus proximity conditions of Experiments 1 and 2 have the same horizontal separation but differ in their vertical height. Experiment 3c is designed to minimize the difference of S-R axis-similarity by controlling the responses in the two hand-stimulus proximity con ditions to have the same horizontal separation and vertical height. Experiment 4 uses location words to examine whether locating words near the hands enhance the semantic Simon effect compared to the same words located far from the hands.
were presented on a 17 in CRT display at a viewing distance of 43 cm. Participants steadied their head by resting on a chinrest. In the hand-proximal condition, subjects rested their elbows on cushions on the table and held each hand on a mouse that was attached to the edges of the monitor aligned with the middle of the CRT (see Figure 1A). In the hand-distal condition, subjects supported a lightweight 38-cm-long board on their laps upon which were mounted two mouses identical to those in the hand-proximal condition (see Figure IB). Subjects kept one hand on each mouse. The distance between the two mouses was the same in both conditions with approximately 32 cm separation. Each trial began with a 500-ms blank screen and was followed by a white fixation cross at the center of the screen for 500 ms. The target stimulus was then presented 11° left or right of the fixation cross (1.3° wide, 1.3° high) and remained visible until a response was made. The target was either a green (CIE 1931: Y = 73.98, x = 0.2944, y = 0.5719) or a red (CIE 1931: Y = 29.66, x = 0.5572, y = 0.3403) disk with a diameter of 1.7°. Participants were instructed to press one of the mouse buttons if a green disk appeared and the other button if a red disk appeared, irrespective of its position. The mapping between the stimulus color and the response buttons was counterbalanced across participants. The spatial mapping between stimulus locations and response buttons was varied across trials. For compatible trials, the correct response button was ipsilateral to the stimulus, whereas for incompatible trials, the correct response button was contralateral to the stimulus. Participants were instructed to respond as quickly and as accu rately as possible. If participants pressed an incorrect button, an error message was presented at the center for 1,000 ms. All stimuli were presented on a dark background.
E xperim ent 1 The visuomotor Simon task is used to test whether spatial S-R mapping is enhanced near the hands relative to far from the hands. The present experiment presents a colored disk (green or red) laterally and requires participants to report the color of the disk. If spatial S-R mapping is enhanced near the hands (i.e., in the hand-proximal condition) relative to that far from the hands (i.e., in the hand-distal condition), interference toward the incompatible response should become stronger, and thus cause a larger Simon effect. M ethod Participants. This study was approved by the institutional review board of the Institute of Psychology, Chinese Academy of Sciences. Thirty-two students (16 males and 16 females; 18- to 28-years-old), two of whom were left-handers, participated in the experiment for payment. All participants had normal or correctedto-normal vision. Apparatus, stimuli and procedure. The experiment closely followed the setting of the Abrams et al. (2008) study. Stimuli
Figure 1. Four postures used in the present study. (A) The hand proximal condition in all six experiments. (B) The hand distal condition in Experi ments 1, 2, 3a, and 4. (C) The hand distal condition in Experiment 3b. (D) The hand distal condition in Experiment 3c. See the online article for the color version of this figure.
ENHANCED STIMULUS-RESPONSE MAPPING NEAR HANDS
Design. There were two within-subject variables including hand-stimulus proximity and S-R compatibility between stimuli and response. Each trial was in one of two hand-stimulus prox imity conditions. In the hand-proximal condition, participants held each hand on a mouse that was attached to the edges of the monitor. In the hand-distal condition, participants supported a lightweight 38-cm-long board on their laps upon which were mounted two mouses. In addition, each trial was conducted under one of two S-R compatibility conditions such that the correct response button was ipsilateral to the stimulus (compatible), or the correct response button was contralateral to the stimulus (incom patible). The experiment consisted of 64 replications of each combination of two hand-stimulus proximity conditions and two S-R compatibility conditions, for a total of 256 trials. Participants first completed one block of 24 trials for practice. They then engaged in four blocks, each of which had 32 compatible trials and 32 incompatible trials. There were two blocks in each of the two hand-stimulus proximity conditions. The order of the two handstimulus proximity conditions was counterbalanced across partic ipants. Results and Discussion In the present and subsequent experiments, trials for which reaction time (RT) was less than 100 ms or greater than 1,500 ms were excluded from analysis (
