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1 Sistema de Información Científica Red de Revistas Científicas de América Latina, el Caribe, España y Portugal María J. Blanca, Gema López-Montiel Hemispheric Differences for Global and Local Processing: Effect of Stimulus Size and Sparsity The Spanish Journal of Psychology, vol. 12, núm. 1, mayo, 2009, pp , Universidad Complutense de Madrid España Disponible en: The Spanish Journal of Psychology, ISSN (Versión impresa): psyjour@sis.ucm.es Universidad Complutense de Madrid España Cómo citar? Fascículo completo Más información del artículo Página de la revista Proyecto académico sin fines de lucro, desarrollado bajo la iniciativa de acceso abierto

2 The Spanish Journal of Psychology Copyright 2009 by The Spanish Journal of Psychology 2009, Vol. 12, No. 1, ISSN Hemispheric Differences for Global and Local Processing: Effect of Stimulus Size and Sparsity María J. Blanca and Gema López-Montiel Universidad de Málaga (Spain) The present experiment was designed to assess the hemispheric differences for global and local processing in healthy participants under different conditions of stimuli visibility, by means of varying the size and sparsity. Three different sizes and three different matrixes of hierarchical stimuli were introduced. Stimuli consisted of incomplete squares with one side missing. Participants were asked to carry out an orientation classification task (left/right), indicating the orientation of the square opening either at global or local levels. The results do not support the hemispheric differences for global and local processing, showing the same efficiency of right and left hemispheres for analyzing global and local information. Nevertheless, other results found are consistent with the hypothesis of right hemisphere superiority under degraded stimulus conditions. Keywords: global processing, local processing, hemispheric asymmetries, hemispheric specialization, task demands El objetivo del presente experimento ha sido analizar las diferencias hemisféricas en el procesamiento global y local de la información visual en participantes con cerebro intacto bajo diferentes condiciones de visibilidad del estímulo, Se introdujeron estímulos jerárquicos consistentes en cuadrados abiertos hacia la derecha o izquierda, variando el tamaño (3.23, 6.44 y 9.61 ) y la densidad estimular (matrices de 4 4, 5 5 y 6 6 elementos). Los participantes llevaron a cabo una tarea de clasificación de la orientación (izquierda/derecha), indicando la orientación de la apertura en el nivel global o en el local. Los resultados no muestran evidencias que apoyen la diferenciación hemisférica en el procesamiento global y local, aunque fueron consistentes con la hipótesis de una superioridad de hemisferio derecho bajo condiciones de degradación estimular. Palabras clave: procesamiento global, procesamiento local, asimetría hemisférica, especialización hemisféricas, demanda de la tarea This research is supported by a grant from the Ministerio de Ciencia y Tecnología (Proyect BSO ). Correspondence concerning this article should be addressed to María J. Blanca.Facultad de Psicología. Dpto. de Psicobiología y Metodología de las Ciencias del Comportamiento. Campus Universitario de Teatinos, s/n. Málaga, (Spain). blamen@uma.es. Phone: (95) Fax: (95) How to cite the authors of this article: Blanca, M.J., López-Montiel, G. 21

3 22 BLANCA AND LÓPEZ-MONTIEL The hypothesis of global precedence was enunciated by Navon (1977, 1981) stating that the processing of a visual form takes place hierarchically and sequentially, from global features to local ones. In his experiments, hierarchical stimuli were used, that is, large figures, which represent the global level, made up of smaller ones, which represent the local level. According to this hypothesis, two experimental results should be found in the research: global advantage and global interference. The first occurs when the global level is identified more quickly and more accurately than the local one. The second implies that global identity interferes when the local level is analyzed but local identity does not interfere when the global one is analyzed. Subsequent research with hierarchical stimuli has demonstrated that global and local levels can be processed in parallel (Hoffman, 1980; Kinchla, Solis-Macias, & Hoffman, 1983; Miller, 1981a, 1981b) and that global advantage and interference can be either attenuated or modified to local advantage depending on several experimental conditions. For this reason, the term dominance instead of precedence has been proposed (Blanca, Luna, López-Montiel, Rando, & Zalabardo, 2001). Hierarchical stimuli have also been used to study the functional hemispheric specialization for global and local processing. A superiority of right hemisphere (RH) for global processing and a superiority of left hemisphere (LH) for local one has been found in studies with brain-damaged patients (Delis, Robertson, & Efron, 1986; Hickok, Kirk, & Bellugi, 1998; Lamb, Robertson, & Knight, 1989; Robertson & Delis, 1986; Robertson, Lamb & Knight, 1988), and with healthy participants using positron emission tomography and functional magnetic resonance (Fink et al., 1996, 1997; Martínez et al., 1997). Delis et al., (1986) found that participants with lesions in the RH and LH had more difficulty in drawing and remembering the global and local form of the visual pattern, respectively. Later, Hickok et al., (1998), Lamb et al., (1989), Robertson and Delis (1986), and Robertson et al., (1988), obtained evidence in favor of the hemispheric specialization in subjects who had injured region adjacent parietal or temporal areas. From these results, Robertson (1995) concluded that the regions involved are posterior left or right areas including temporal area 22 and portions of adjacent caudal parietal areas 39 and 40. However, the results with neurologically healthy individuals using response time experiments have shown a lack of consistency. In these studies, the stimuli are randomly displayed at the right or left visual field during a very brief exposure time, with the purpose being that the visual information was represented in the contralateral hemisphere. With this procedure, Bedson and Turnbull (2002), Hübner (1997, 1998), Martin (1979b), Sergent (1982a), and Versace and Tiberghien (1988), obtained data that supports the hemispheric specialization. By contrast, other researchers failed to find evidence in favor of the predicted hemispheric asymmetry, suggesting that the RH and LH have the same ability to manage with global and local information (Alarcón & Blanca, 2000; Alivisatos & Wilding, 1982; Amirkhiabani, 1998; Arnau, Blanca, & Salvador, 1992b; Blanca, 1992; Blanca & Alarcón, 2002; Boles, 1984; Boles & Karner, 1996; Polich & Aguilar, 1990; Van Kleeck, 1989). On the other hand, hemispheric specialization has been obtained but depended on several experimental conditions (Blanca, Zalabardo, García-Criado, & Siles, 1994; Evert & Kmen, 2003; Hübner & Malinowski, 2002; Hübner & Volberg, 2005; Kimchi & Merhav, 1991; Volberg & Hübner, 2007; Yovel, Yovel & Levy, 2001). Thus, Kimchi and Merhav (1991) found it only with selective attention tasks, while Blanca et al. (1994) with 50 ms of stimulus exposure duration, but not with 100 and 200 ms. Blanca et al. (1994) concluded that the hemispheric specialization for global and local processing may be dependent upon the relative visibility of the stimuli in the experiment. Evert and Kmen (2003) referred to this conclusion as task demand hypothesis, suggesting that as task demands are increased and stimulus visibility is decreased, there is a greater likelihood of demonstrating hemispheric asymmetries. Boles and Karner (1996) used exposure duration of 33 and 100 ms, expecting to find a stronger hemispheric specialization at 33 ms. However, the results indicated no difference in global and local processing between the two hemispheres, but were consistent with the previous findings that the RH is superior to LH under degraded stimulus conditions (Bradshaw & Nettleton, 1983; Bryden & Allard, 1976; Hellige, 1980; Sergent, 1982b). Evert and Kmen (2003) referred to this finding as RH degradation hypothesis, suggesting degraded conditions favor RH specialization regardless of whether global or local processing is engaged. Evert and Kmen (2003) designed several experiments to test the two hypotheses above described, including a wider range of stimulus exposure duration across experiments: 27, 40, 53, 67, 80 and 147 ms. The results indicated that LH superiority for local processing was demonstrated more often than RH superiority for global processing and that asymmetries were most commonly found in the middle range of duration exposure (53-80 ms). The data are more interpretable from the task demand hypothesis and extend the findings of Blanca et al. (1994) by suggesting that there is a temporal range within which predicted asymmetries are found. However, the authors emphasized the need to further specify and clarify the conditions under which these asymmetries appear in healthy individuals. In this line of thinking, the present experiment was designed to assess hemispheric differences for global and local processing under different conditions of stimuli visibility, by means of varying the size. On the other hand, in order to find out if the number of local elements influence hemispheric differences, sparsity was also manipulated. There is a great deal of evidence showing the influence of these variables in global and local processing. Global advantage in reaction time (RT) has been found with figures

4 HEMISPHERIC DIFFERENCES FOR GLOBAL AND LOCAL PROCESSING 23 subtending 3-6 of visual angle, whereas local advantage with figures bigger than 9 (Antes & Mann, 1984; Arnau, Salvador, & Blanca, 1992; Kinchla & Wolfe, 1979). In relation to sparsity, global advantage has been found with hierarchical stimuli made up of many elements (7 6 and 7 5 matrixes) and local advantage with stimuli having a lower number of local elements (Arnau, Blanca, & Salvador, 1992a; Martin, 1979a). Three hypotheses are tested in this experiment. According to the global/local hypothesis, an LH local advantage and an RH global advantage are expected irrespective of size or sparsity. According to the task demand hypothesis, an LH local advantage and an RH global advantage are expected to appear only with small stimuli. According to the RH degradation hypothesis, RH advantage is expected with small stimuli regardless of level of processing. The experiment carried out was designed with hierarchical stimuli consisting of squares with the right or left side missing. The subjects were asked to carry out an orientation classification task, indicating the orientation of the square opening either at global or local levels. Three different sizes and three different levels of sparsity were introduced. Participants Method Figure 1. Stimuli used in the experiment. The global and local information in (A) and (B) are congruent while in (C) and (D) are incongruent. Thirty-three student volunteers (9 males and 24 females) from the Universidad de Málaga, Spain, from 18 to 25 years of age, participated in the experiment. All subjects had normal or corrected-to-normal vision and all were righthanded, right-footed and right-eyed as assessed by test of laterality dominance by Harris (1978). Apparatus Stimuli presentation was controlled by a personal computer fitted with a monitoriscope (patent oepm: p ). This is a box, with a viewfinder, around the screen which prevents head movements, isolates the subjects from distracting variables and maintains the visual angle and illumination constant. The stimuli were displayed at 62 cm from the computer screen, and were drawn in black on a white background. The stimuli were large incomplete squares, that is, squares with the right or left side missing, made up of small incomplete squares (Figure 1). Three global sizes were used: 9.61, 6.44 and The local size was in all cases seven times less than the global size. Three stimulus matrixes were also included: 4 4, 5 5 and 6 6. The distance between two consecutive elements was as big as local size in the 4 4 matrix, half the local size in the 5 5 matrix and 5 times smaller in the 6 6. The factorial combination between size and sparsity was introduced in the design. The participants were asked to indicate the opening orientation of the incomplete square (left or right) under two conditions of selective attention: globally and locally directed attention. In globally directed condition, subjects were asked to indicate the orientation of the global level and ignore the local one. In the locally directed condition, subjects were asked to indicate the orientation of the local level and ignore the global one. In both conditions, the patters could be congruent, if both the global and local levels had the same orientation, or incongruent, if the orientation was different for both levels (Figure 1). Procedure The experimental session consisted of two blocks of stimuli, one block for each attention condition. Each block was split into 20 practical trials and 288 experimental trials (144 for congruent condition and 144 for incongruent condition, including several size and matrixes). Half of these stimuli were presented in the right visual field (RVF-LH) and half in the left visual field (LVF-RH) at 2º (from the inner edge of the stimuli to the fixation point). Each trial began with the word ready in the center of the screen for 1 s. It was followed by the stimulus, which appeared for 50 ms randomly in the left or in the right visual field. After this, a plus sign appeared in the center of the

5 24 BLANCA AND LÓPEZ-MONTIEL screen as a fixation point and remained until the response. Next, the word ready appeared again and the sequence was repeated. The participants had to indicate the orientation of the level instructed by pressing a button with the index finger of the left hand for left orientation, or with the index finger of the right hand for right orientation. RT and response accuracy were recorded. The attention direction conditions were counterbalanced between subjects. Within these conditions, stimuli were randomly displayed across congruency, visual field, size and sparsity conditions. Results A repeated measures analysis of variance (ANOVA) was performed on the RT data. The factors were attention direction (globally and locally directed), stimulus size (small, medium and large), sparsity (4 4, 5 5 and 6 6), consistency between the global level and the local one (congruent and incongruent) and visual field (right and left visual field). The interpretation of the main effects and the interactions of smaller order are subordinated to the interaction of higher order. For this reason, a hierarchical approach is followed in the interpretation of results. The ANOVA table can be seen in Appendix 1. The statistically significant higher order interaction was the interaction between attention direction, stimulus size, sparsity and congruency, F(4,128) = 3.48; p =.01. In order to break down this interaction, the data for each stimulus size condition were analyzed separately, following a hierarchical approach in the interpretation. Bonferroni adjustment was used for post hoc comparisons. Small stimulus size. A repeated measures ANOVA was performed with the factors: attention direction, sparsity and congruency. The three-way interaction was not significant, F(2,64) = 1.78; p =.17, but there was a significant interaction between attention direction and sparsity, F(2,64) = 11.38; p <.001, and between attention direction and congruency, F(1,32) = 18.11; p <.001. The first indicates that global advantage increases as matrix size increases (Figure 2). The second is related to interference (Figure 3). Differences between congruent and incongruent stimuli were found for the globally directed condition, t(32) = 2.31; p =.01, as well as for the locally directed condition, t(32) = 4.98; p <.001, although the second was bigger than the first. So, interference is bidirectional and asymmetrical in favor of global interference. Medium stimulus size. A repeated measures ANOVA was performed with the factors: attention direction, sparsity and congruency. The three-way interaction was significant, F(2,64) = 10.50; p <.001. In relation to advantage, global advantage was found for congruent and incongruent stimuli for 4 4 matrix, t(32) = 4.33; p <.001, and t(32) = 2.9; p <.01, respectively; Figure 2. RT means for small stimuli as a function of attention direction and sparsity. Figure 3. RT means for small stimuli as a function of attention direction and congruency.s for 5 5 matrix, t(32) = 4.7; p <.001, and t(32) = 5.4; p <.001, respectively; and for 6x6 matrix, t(32) = 4.7; p <.001, and t(32) = 6.8; p <.001, respectively. However, as Figure 4 shows, global advantage increased as a function of the increment in matrix size. The data regarding interference revealed local interference, t(32) = 2.34; p <.01, but not global interference, t(32) = 1.23; p =.11, for 4 4 matrix; neither global, t(32) = 1.64; p =.06, nor local interference, t(32) = 1.38; p =.44, for 5 5 matrix; and global interference, t(32) = 2.83; p <.01, but not local interference, t(32) = 1.06; p =.15, for 6 6 matrix.

6 HEMISPHERIC DIFFERENCES FOR GLOBAL AND LOCAL PROCESSING 25 Large stimulus size. A repeated measures ANOVA was performed with the factors: attention direction, sparsity and congruency. The three-way interaction was not significant, F(2,64) = 1.65; p =.20, but there was a significant interaction between attention direction and sparsity, F(2,64) = 13.33; p <.001, indicating that global advantage increases as a function of the increase in sparsity (Figure 5). The interaction between attention direction and congruency was not significant, F(1,32) = 0.004; p =.95. However, there was an effect of congruency, F(1,32) = 19.9; p <.001, indicating that responses in the congruent condition were faster (M = ms) than those in the incongruent condition (M = ms). This main effect indicates a bidirectional interference. Figure 5. RT means for large stimuli as a function of attention direction and sparsity. Figure 4. RT means for medium stimuli as a function of attention direction, sparsity and congruency. Figure 6. RT means as a function of stimulus size and visual field (right visual field: RVF-LH, left visual field: LVF-RH).

7 26 BLANCA AND LÓPEZ-MONTIEL field (left and right visual field) and congruency (congruent and incongruent). The ANOVA table is showed in Appendix 2. The results revealed only two significant interactions: size congruency, F(1,32) = 19.75; p <.001, and sparsity congruency, F(2,64) = 3.18; p =.04. Regarding size, global interference was not found for medium stimuli, t(32) = 0.33; p =.34, but it was obtained for small ones, t(32) = 3.92; p <.001. Regarding sparsity, global interference did not appear for 4 4 matrix, t(32) = 0.74; p =.24, but it did appear for 5 5 matrix, t(32) = 3.24; p <.01, and for 6 6 matrix, t(32) = 2.57; p =.01. The means are showed in Figures 7 and 8. The only main effect not involved in the above interactions was visual field, which was statistically significant, F(1, 32) = 4.32; p =.04, indicating a superiority of right hemisphere (92 % vs. 90 % of correct responses). Figure 7. Percentage of correct responses for locally directed condition as a function of congruency and stimuli size. With respect to hemispheric differences, there was only an interaction between visual field and stimulus size (Figure 6). A further analysis revealed an RH advantage for small stimuli, t(32) = 1.98; p =.02. No differences between cerebral hemispheres were found for medium stimuli, t(32) = 0.91; p =.18, and large stimuli, t(32) = 1.42; p =.07. The data regarding accuracy presented a ceiling effect in the globally directed condition, exceeding 95% of correct responses, and in the locally directed condition for large stimuli. So, the analysis was only carried out for locally directed condition for medium and small stimuli. A repeated ANOVA was performed with the factors: stimulus size (small and medium), sparsity (4 4, 5 5 and 6 6), visual Figure 8. Percentage of correct responses as a function of congruency and sparsity. Discussion The aim of the present experiment was to assess the hemispheric specialization for global and local processing under different conditions of stimulus visibility, by means of varying the stimuli size. Moreover, the number of local elements was also manipulated to find out whether or not it has an effect on hemispheric differences. Three hypotheses were formulated on the basis of previous research. According to the global/local hypothesis, a global advantage should be found for RH and a local one for LH, irrespective of size or sparsity. According to the task demand hypothesis, the predicted asymmetries are expected to appear only with small stimuli. According to the RH degradation hypothesis, RH advantage is expected with small stimuli regardless of level of processing. The results did not show significant differences between the two cerebral hemispheres in the analysis of global and local features of hierarchical stimuli. The absence of difference was constant for all conditions of size and sparsity manipulated. Therefore, the results do not support either the first or second hypothesis, indicating that RH and LH have the same ability to manage with global and local information with the stimuli used here and with a task consisting of a categorical classification based on the orientation of the hierarchical stimuli. Recently, Hübner and Volberg (2005) have proposed that at an early stage of processing the global and local identities are represented independently of their respective levels and that the hemispheres do not differ with respect to these representations but they differ in their capacities for integrating identity and level information. Consequently, when response selection can be based on early stimulus representation, as it is the case for congruent stimuli, no hemispheric differences should be found. On the other hand, when response selection requires a more elaborated stimulus representation, as it is the case for incongruent stimuli, where the identities have to be integrated with their

8 HEMISPHERIC DIFFERENCES FOR GLOBAL AND LOCAL PROCESSING 27 respective levels, hemispheric differences should be shown. Although the current experiment was not conducted to test this hypothesis and the task and stimuli were different to those used by Hübner & Volberg (2005) and Volberg & Hübner (2007), the results found were not consistent with their explanation. Congruent and incongruent stimuli did not produce differences between hemispheres in the processing of global and local levels. Thus, the findings corroborate the previous research which did no find hemispheric differences with healthy participants (Alivisatos & Wilding, 1982; Amirkhiabani, 1998; Arnau et al., 1992b; Blanca, 1992; Blanca & Alarcón, 2002; Boles, 1984; Boles & Karner, 1996; Van Kleeck, 1989) although they contradict other authors who did find them (Bedson & Turnbull, 2002; Hübner, 1997, 1998: Martin, 1979b; Sergent, 1982a; Versace & Tiberghien, 1988). This lack of consistency among studies casts doubt on hemispheric specialization for global and local information. On the other hand, the results showed a superiority of RH over LH in the analysis of stimuli with small size. In figure 6, it can be observed that the responses to small stimuli were slower than to medium and large ones, suggesting a greater visual difficulty in the processing of this size. Consequently, the data obtained are consistent with the RH degradation hypothesis, which states that the RH has a general advantage over the LH under degraded stimulus conditions (Boles & Karner, 1996; Bradshaw & Nettleton, 1983; Bryden & Allard, 1976; Evert & Kmen, 2003). Studies which support this hypothesis have demonstrated an RH superiority with small stimuli (Boles, 1994, 1995; Boles & Rashid, 1993), short exposure duration (Sergent, 1982b), non-familiar stimuli (Morais & Ben-Dror, 1985), and visual mask (Hellige, 1980, 1983). Other results of interest were related to the effect of size and sparsity on global dominance. According to prior studies, global advantage should be found with small and dense stimuli, while local advantage should be found with large and sparse patterns (Antes & Mann, 1984; Arnau et al., 1992, 1992a; Kinchla & Wolfe, 1979; Martin, 1979a). The results regarding interference showed bidirectional interference for small and large stimuli but it depends on sparsity in medium ones. In this size, interference varied from local interference for 4x4 patterns to global interference for 6x6 patterns. In relation to advantage, the data obtained also indicate that size and sparsity modulate global processing, since global advantage increases as size decreases and as matrix size increases. However, a transition from global to local advantage as a function of these variables was not found, but global advantage in all experimental conditions was found. This lack of transition may be due to the presence of several variables that can facilitate global processing, such as spatial uncertainty (Grice, Canham, & Boroughs, 1983) and brief exposure duration (Rumiati, Nicoletti, & Job, 1989). Other studies have also found a global advantage regardless of the visual angle when concentric stimuli were used (Amirkhiabani, & Lovegrove, 1996; Luna, Marcos-Ruiz, & Merino, 1995). Recently, it has also been suggested the experimental task can modulate global and local processing. Thus, global dominance for tasks implying decisions on stimuli orientation has been proposed (Blanca et al., 2001; Blanca, Luna, López- Montiel, Zalabardo, & Rando, 2001; Han, Humphreys, & Chen, 1999). Blanca et al., (2001) found global advantage in divided and selective attention with an orientation classification task like the one used here, but not with the target detection task. Consequently, the task used can explain the presence of global dominance in all experimental conditions. However, this explanation is not consistent with Blanca and Alarcón (2002), who did not find global advantage with the same task and stimuli. They used 8.16 patterns and matrix of 5x5 elements, which are more similar to the large size used here. Nevertheless, the distance between two consecutive local elements was bigger in Blanca and Alarcón (2002) than the one used here. The ratio between global size and local distance in the present experiment was 14 against 8 in Blanca and Alarcón (2002). In addition to this, the local size here was bigger than the local distance while in the aforementioned experiment the local size was smaller than the local distance. These factors can weaken the grouping by proximity and impair the goodness of the global figure, provoking a vanishing of the global advantage. Therefore, the comparison of experiments suggests the need for further research to find out the effect of the relative local distance, while sparsity is maintained constant in orientation classification tasks with lateralized stimuli. In general, the present research did not support the hemispheric differences for global and local processing in healthy participants. The findings showed the same efficiency of RH and LH for analyzing global and local levels of hierarchical stimuli with several sizes and several numbers of local elements. Nevertheless, other results found here were consistent with the hypothesis of an RH advantage under degraded stimulus conditions. References Alarcón, R., & Blanca, M. J. (2000) Asimetría hemisférica en la dicotomía holística-analítica en tareas de atención focalizada. Psicothema, 12, Alivisatos, B. W., & Wilding, J. (1982). Type letter stimuli: hemispheric differences in matching stroop-type letter stimuli. Cortex, 18, Amirkhiabani, G. (1998). 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11 30 BLANCA AND LÓPEZ-MONTIEL APPENDIX I Results from the repeated measures ANOVA on RT, the factors being attention direction, stimulus size, sparsity, congruency and visual field. R SOURCE F p Attention (A) <.01 Size (S) <.01 Sparsity (SP) <.01 Congruency (C) <.01 Visual field (VF) A S <.01 A SP <.01 A C A VF S SP S C S VF SP C SP VF C VF A S SP A S C 9.45 <.01 A S VF A SP C A SP VF A C VF S SP C S SP VF S C VF SP C VF A S SP C A S SP VF A S C VF A SP C VF S SP C VF A S SP C VF

12 HEMISPHERIC DIFFERENCES FOR GLOBAL AND LOCAL PROCESSING 31 APPENDIX II Results from the repeated measures ANOVA on accuracy, the factors being stimulus size, sparsity, congruency and visual field. R SOURCE F p Size (S) <.01 Sparsity (SP) 9.32 <.01 Congruency (C) 9.29 <.01 Visual field (VF) S SP S C <.01 S VF SP C SP VF S SP C S SP VF S C VF SP VF C S SP VF C