Gestalt Hemispheric Differences 1 Running Header: Gestalt Hemispheric Differences Are There Hemispheric Differences in Visual Processes That Utilize Gestalt Principles?

Hemispheric differences in the visual system have been well-established, although their exact nature remains under study (Hellige 1990). Thus far, research has focused on the perception of whole objects or scenes. Van Kleeck and Kosslyn (1989) investigated the possibility of lateralization for Gestalt principles using an embedded figures task. They^ found an effect of lateralization, that parts of figures fitting Gestalt breaks were more quickly identified by the right than the left hemisphere. They concluded that Gestalt processing is lateralized to the right hemisphere. In this study, two experiments were conducted to more closely investigate this claim. The first experiment focused on the Gestalt principle of grouping by proximity using dot lattices. No effects of lateralization were found. In the second experiment, concentric circles of Gabor patches were used to test grouping by orientation. This did not result in any main effects of hemisphere, but in reaction time, those stimuli with the probe located in central vision created a large decrease in reaction time for the right hemisphere, while probes in the periphery caused a greater increase for the right hemisphere than the left. This seems to indicate that not all Gestalt principles are processed preferentially in the right hemisphere, but some Gestalt principles do show the same lateralization as found by Van Kleeck and Kosslyn. Gestalt Hemispheric Differences 3 Are there hemispheric differences in visual processes that utilize Gestalt principles? One of the early discoveries about how the brain functions was that the two hemispheres of the brain do not play identical roles. Broca and Wernicke both discovered language centers of the brain which were located in the left hemisphere, while damage to the analogous area in the right hemisphere did not affect language. The brain has several physical and functional hemispheric differences. One of the important physical divisions is the two visual fields that provide information to visual system. Each visual field is seen mostly by the eye on that side, so most of the right visual field (RVF) is processed by the right eye, and the majority of the left visual field (LVF) is processed by the left eye. When the information obtained in these visual fields transfers from the retina to the visual cortex, the information of each visual field crosses over to be used by the contralateral hemisphere. In other words, information from the RVF travels to the left hemisphere (LH) and information from the LVF is processed by the right hemisphere (RH). At first this may not seem significant, since the visual cortex is generally held to be nearly identical in both hemispheres. However, damage to one of the hemispheres will only affect the processing of the corresponding visual field. For example, some people have problems in or lesions to area V4 of the brain which results in achromatopsia (color-blindness). Usually a problem such as color-blindness affects the entire visual system, but when a person only has damage on one side, only one visual field appears without color (Gazzaniga 2000). There are also functional division in vision, as well as throughout the two hemispheres. Most current dichotomies have grown out of the suggestion that the LH processes information analytically and the RH processes information holistically. As researchers like Robertson (referenced in Hellige 1990) have pointed out, analytic versus holistic processing is very illdefined. Because of these ambiguous descriptions, many results have been argued to both Gestalt Hemispheric Differences 4 support and refute this dichotomy. In order to provide more specific terms, two possible dichotomies have arisen: local versus global processing and categorical versus coordinate processing. The first dichotomy stems from research done by Navon where he used stimuli that contained a global letter shape formed by several of a different letter, for example a 'H' made of small 's 's (referenced in Hubner 1997). In this dichotomy, the RH plays a larger role in processing global stimuli, such as the larger letter being formed, and the LH plays a primary role in processing local stimuli, such as the smaller letters in the stimuli. The other dichotomy has been suggested by Kosslyn and colleagues (i.e. Van Kleeck & Kosslyn 1989). Kosslyn proposes that instead of processing local information, the LH hemisphere has superior processing for categorical information like defining whether a specific dot is above or below or near to or far from a specific line. The role of the RH is processing coordinate information, such as how near the line, in millimeters, the dot is (i.e. the metric information). These explanations, while more concrete than the original, still lack full definitions that can easily be tested. In an attempt to both unite these theories and provide a physical description to use in creating stimuli, Robertson and Ivry (2000) have proposed a theory they refer to as Double Filtering by Frequency (DFF). This theory suggests that both global and coordinate distinctions are referring to lower spatial frequencies. In this way, the RH is faster to process the low spatial frequencies presented to the visual system and the LH is faster at processing high spatial frequencies. In this theory, the low versus high is still relative to each stimulus, creating a continuum, but it at least provides a measurable dimension. The majority of research in hemispheric differences up to this point has used complete objects or scenes as stimuli. Processing objects, however, is a complicated task for the visual system. While the hemispheric differences are often found with this higher level of processing, Gestalt Hemispheric Differences 5 relatively little research has looked at the aspects involved in lower levels to create this object representation. One important step in creating a representation of an object or a scene is to group the information according to various rules. These grouping principles, usually referred to as Gestalt principles, include grouping by color, shape, orientation, and proximity. While using these principles to create object representations can seem elementary, Bronson, Scott, Fox, and Pye (2004) found that not all people are able to make these representations naturally. They found that autistic children are much less likely to utilize these grouping principles than controls of the same mental age. Since this is an important step of visual processing, and is not something already embedded in the information, it is possible that this processing could play different roles in the two hemispheres. Gestalt plays an important role in the global understanding of an object or scene, and the autistic children had difficulty identifying possible and impossible shapes. To investigate the possibility of hemispheric differences, Van Kleeck and Kosslyn (1989) used an embedded figures task to test accuracy and reaction time for identifying whether a piece of a figure had been part of a figure that had previously been presented to the participant. They expected and found that the RH was faster at identifying embedded figures that followed important Gestalt rules of the larger object. For incorrect embedded figures, the LH was faster, suggesting that either local processing was required, and the breaking of the Gestalt rules made figure more difficult for the RH to process. This study, however, still uses objects to establish hemispheric differences. Research has well established that there are some hemispheric differences at higher levels of processing. Research has also shown that the processing in primary visual cortex, the first part of visual processing by the brain, appears to function identically in both hemispheres. The question arises as to when the hemispheres begin to process visual information differently. Based on the Gestalt Hemispheric Differences 6 findings by Van Kleeck and Kosslyn, hemispheric differences should appear as soon as Gestalt principles are utilized in making judgments about stimuli.