99 Local-global visual processing in ASD

and global processing abilities requires measurements made across various levels of processing, ranging from low-level (perceptual-sensory) to high- level (conceptual). If individuals with ASD really present superior local and/ or inferior global processing, then this should be apparent at any level of processing. Moreover, the use of open-ended tasks has been promoted to capture processing style rather than ability (Booth, 2006; Happé & Frith, 2006).

1.2. A factorial design: local versus global, and lower versus higher level The present study aimed at investigating local and global visual processing in individuals with ASD and TD controls, while taking the abovementioned recommendations for task selection into account. Firstly, we made a distinction between processing ability (i.e., how well you can process information in a given way) and processing style (i.e., the spontaneous or natural tendency to process information in a particular way). Secondly, a distinction was made between the type of processing (local vs. global processing) and the level of processing (lower- vs. higher-level). Note that low-level and high-level processing are relative terms, referring to the level of processing in the brain, with low-level usually referring to simple stimulus H[[YPI\[LZLUJVKLKPU[OLÄYZ[Z[HNLZVMWYVJLZZPUNHUKOPNOSL]LSYLMLYYPUN to more complex aspects extracted and represented further downstream in the brain (Wagemans, Wichmann, & Op de Beeck, 2005). For instance, for visual stimuli one would refer to luminance, colour, line orientation, motion direction, etc. as low-level properties and fully integrated representations and meaning as high-level. Thirdly, we aimed to measure local and global WYVJLZZPUNHIPSP[PLZ^P[OV\[WSHJPUN[OLTPUKPYLJ[[YHKLVɈHUKH[IV[OSL]LSZ of processing (low- and high-level). Therefore, a factorial design was used, combining the two types of processing with the two levels of processing, with one task for each resulting combination.

;^V[HZRZYLX\PYPUNPU[LNYH[PVUVMPUMVYTH[PVUH[KPɈLYLU[SL]LSZ^LYL selected to measure global processing abilities: a lower-level Coherent Motion task (Milne et al., 2002) and a higher-level Fragmented Object Outlines Task (Torfs, Panis & Wagemans, 2010).

In the Coherent Motion (CM) task randomly moving dots appeared on the screen with a proportion of them moving coherently in a single direction, JYLH[PUN H ÅLL[PUN WLYJLW[PVU VM TV[PVU 7HY[PJPWHU[Z ^LYL YLX\LZ[LK to indicate the direction of coherent motion and the CM threshold was JHSJ\SH[LK YLÅLJ[PUN [OL ZTHSSLZ[ WYVWVY[PVU VM JVOLYLU[S` TV]PUN KV[Z required to perceive the global direction of motion. This task necessitates global processing, as the global direction of motion can only be inferred by pooling information of individual motion dots, allowing to segregate the signal (coherent motion direction) from the noise (randomly moving dots). The *4[HZRPZJVUZPKLYLKHSV^LYSL]LS[HZRZPUJLZ[PT\SPHYLKLÄULKIHZLKVU low-level properties (luminance and motion direction) and because it is more low-level compared to the Fragmented Object Outlines task.

100 Local-global visual processing in ASD

The -YHNTLU[LK6IQLJ[6\[SPULZ-66[HZR was selected as a higher- level global processing task. In this task, the outline of an object was gradually built-up in ten steps and participants were asked to identify the object as ZVVUHZWVZZPISL-YHNTLU[LKVIQLJ[PKLU[PÄJH[PVUYLX\PYLZKPɈLYLU[[`WLZVM global processing, since it involves bottom-up grouping of contour fragments (i.e., contour integration) as well as top-down matching of candidate object representations (stored in memory) with perceptual input (Panis & Wagemans, 2009). Previous studies have demonstrated that these processes of grouping HUKTH[JOPUNHYLPUÅ\LUJLKI`ZL]LYHSZ[PT\S\ZH[[YPI\[LZZ\JOHZVIQLJ[ complexity (or homogeneity), object category (natural vs. man-made), global symmetry and fragment curvature (curved vs. straight fragments) (Panis, De Winter, Vandekerckhove, & Wagemans, 2008; Panis & Wagemans, 2009; Torfs et al., 2010). Manipulating these attributes can therefore help to pinpoint which processes are altered in individuals with ASD.

To measure local processing abilities, we selected a low-level Visual Search (VS) task (based on O’Riordan, 2004, and O’Riordan, Plaisted, Driver, & Baron-Cohen, 2001). In this task, participants were instructed to search H WYLZWLJPÄLK [HYNL[ LTILKKLK ^P[OPU KPZ[YHJ[VYZ [OH[ KPɈLYLK MYVT [OL target in either colour or shape. This is considered to be a low-level task, ZPUJL PUKP]PK\HS Z[PT\SP HYL KLÄULK IHZLK VU JVSV\Y HUK ZOHWL ^OPJO HYL rather low- (to mid-) level stimulus attributes. Several studies have shown that individuals with ASD are faster at detecting the target compared to TD individuals (for a review, see Kaldy, Giserman, Carter, & Blaser, 2013). This OHZ ILLU HYN\LK [V YLÅLJ[ Z\WLYPVY SVJHS WYVJLZZPUN HIPSP[PLZ SPRL Z\WLYPVY unique item detection and perceptual discrimination. However, O’Riordan KLTVUZ[YH[LK[OH[NYV\WKPɈLYLUJLZVUS`LTLYNLKVUKPɉJ\S[ZLHYJO [HZRZH]VPKPUNJLPSPUNLɈLJ[Z4VYLV]LYI`ZOV^PUN[OH[ZLHYJOYH[LZ^LYL SLZZHɈLJ[LKI`THUPW\SH[PVUZVM[HYNL[KPZ[YHJ[VYZPTPSHYP[`6»9PVYKHU provided evidence for enhanced discrimination in adults with ASD. To ensure [OH[V\Y[HZR^HZZLUZP[P]LLUV\NO[VKL[LJ[NYV\WKPɈLYLUJLZ^L\ZLKH similar task as the one developed by O’Riordan (2004), i.e., a conjunction ZLHYJO[HZRPU^OPJOZLHYJOKPɉJ\S[`^HZHKKP[PVUHSS`THUPW\SH[LKI`]HY`PUN the number of distractors and the target-distractor similarity. Using this task also allows us to investigate the enhanced discrimination hypothesis in children and adolescents with ASD.

No task was selected to measure local processing abilities at a higher- level. At this higher conceptual level, a Gestalt percept naturally emerges and [O\ZPUÅ\LUJLZ[OLWLYJLW[PVUHUKWYVJLZZPUNVM[OLJVUZ[P[\[PUNWHY[Z;OL degree to which this happens, depends on the strength of a person’s global processing ability and/or processing style. We therefore contend that it is impossible to create a high-level task measuring local processing abilities unconfounded with global processing. For this reason, we selected an open- ended task, which enables us to evaluate the visual processing style instead, namely the Rey-Osterrieth Complex Figure (ROCF) task (Tsatsanis et al., 2011). In this task individuals had to copy a complex multi-part stimulus, without restrictions on how to do so. Afterwards, the degree of continuity or

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