Summary of Chapter 2

The current chapter consists of a series of experiments to observe categorical and

coordinate spatial representations in VSSTM with a whole visual-field task, the CATCOORD task. The three behavioural experiments demonstrated that regardless of different time courses during encoding or maintenance stage and types of verbal interference, the categorical spatial representations were always performed better than the coordinate

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spatial relations. The results support and develop Dent’s findings (2009) in which categorical spatial representations are an intrinsic property in visuospatial memory. Moreover, different shift distances between target and reference were found to consistently affect memory performance in both categorical and coordinate spatial relations, with more apparent sensitivity to distance changes in coordinate relations than categorical relations. The

manipulated categorical and coordinate changes in the CATCOORD task could be attributed to visuospatial representation. A characteristic of precise metric spatial changes for

coordinate spatial representations has been replicated in the experiment paradigm.

The neuroimaging evidence indicates that bilateral activations occur when processing both spatial relations. The hemispheric lateralisation effects for categorical and coordinate spatial representations that have previously been reported (Slotnick & Moo, 2006; van der Ham et al., 2009) are not found when stimuli are presented on the whole visual field. The

separability of categorical and coordinate spatial processes is supported by the broader neural network found in categorical but not coordinate spatial process in Experiment 2. It is important to note that the hemispheric lateralisation effects for spatial processing on a whole visual field task have been demonstrated in patient studies (Gallagher, Gray, Watson, Young, & Ferrier, 2014; Kessels et al., 2000). For example, Kessels and colleagues showed that among ten patients who had undergone intracranial tumour resection and presented secretive spatial memory impairments, two of them were impaired on positional memory performance (i.e. the ability to remember precise, metric spatial information) and another two showed a selective impairment on object location binding and the combined processes (e.g. the ability to remember relative, abstract spatial location) (Kessels et al., 2000). Taking both behavioural (Experiment 1a-1c) and neuroimaging (Experiment 2) evidence together, the results are in agreement with previous literature (Dent, 2009; Kosslyn & Chabris, 1992;

Kosslyn, 1987; Niebauer, 2001); categorical relations are primary processes in VSSTM while coordinate relations are subsequent processes. More importantly, present neuroimaging results suggest that the two spatial processes share a similar neural network.

The three behavioural experiments conducted in the current chapter have further extended Dent's findings and indicated that categorical spatial relations are a dominant spatial process in visuo-spatial representation. The categorical advantage effect has been consistently found

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even when experimental design is unfavourable for categorical spatial processing.

Coordinate spatial representation, on the other hand, is thought to be a supplementary visuo-spatial process. Even though previous studies (van der Ham et al., 2009, 2007) have demonstrated that short time courses would reveal coordinate advantage in VSSTM, coordinate relations did not reveal a similar or even better performance than categorical relations in a short encoding time and short retention interval in the current experiment. In the shortest encoding time and maintenance interval and in trials where the shift between objects was the smallest, where the accessibility of categorical information was minimised and coordinate was maximised, categorical-change trials were still performed better than coordinate-change trials. This finding suggests that the coordinate advantage in short time courses disappears when spatial judgments are implicit and when stimuli array is presented on the whole visual field.

Verbal information is one of the characteristics of categorical representations. Experiments 1b and 1c hence adopted verbal interference to intrude on categorical-verbal and

categorical-perceptual processes during visuo-spatial processing. Although articulatory suppression is a common interference methodology and has been adopted in previous studies, its effect on spatial memory is inconsistent (Dent, 2009; Kessels & Postma, 2002;

Postma & de Haan, 1996; Postma, Izendoorn, & de Haan, 1998; Postma, Winkel, Tuiten, &

van Honk, 1999). When utilising an object location memory task, some studies observed interference effects on all conditions; position-only information, object location binding, and combined processes (Postma et al., 1998, 1999) but others showed selective interference effects on some conditions (Kessels & Postma, 2002; Postma & de Haan, 1996). Therefore, the current study adopted a different interference approach. The passive auditory verbal interference is a relatively weak interference method compared to articulatory suppression;

however, it is likely to affect exact spatial components. When spatially relevant words and colour words are applied, which represent categorical-verbal and categorical-perceptual codes, the interference with categorical spatial representation is thought to be more precise.

Coordinate performance may be similar to categorical performance in VSSTM once the two subcomponents of categorical representations are interfered with. The result still showed better performance of categorical over coordinate spatial relations. Therefore, categorical representation is deemed to be a primary spatial process.

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The neuroimaging study was conducted to observe the underlying mechanisms of the two spatial representations. The finding did not support previous studies which have

demonstrated hemispheric lateralisation effects for categorical and coordinate spatial representations in fMRI studies (Slotnick & Moo, 2006; Trojano et al., 2002; van der Ham et al., 2009), an ERP study (van der Ham et al., 2010), and an rTMS study (Trojano et al., 2006).

Bilateral activation for categorical and coordinate spatial processing was found in the neuroimaging results. The inconsistent finding may be due to different experiment tasks;

hemispheric lateralisation effects have emerged when a half visual field task and explicit spatial judgments instructions were applied. The lateralisation effects do not occur in the CATCOORD task, when stimuli are presented on the whole visual field and spatial judgment clues are not provided to participants. It is possible that as soon as participants are aware of types of spatial judgments, a memory strategy is developed. The specificity of the left

hemisphere processing categorical relations and the right hemisphere processing coordinate relations is a consequence of the memory strategy. The latter methodology examines

whether the lateralisation effects are caused by hemi-field specificity. That is, the

lateralisation effects only appear with tasks that present stimuli at different half visual fields.

Bilateral activation was found in the CATCOORD task when processing categorical or coordinate spatial relations. The result indicates that the two spatial relations may share similar neural networks. However, it should be mentioned that similar accuracy of performance and reaction times for categorical- and coordinate-change trials were also found in the neuroimaging study. The robust categorical advantage effect was not found when the task was performed in the scanner. The behavioural result restricts interpretation for the neuroimaging findings. Different experiment conditions (e.g. whether the head is fixed or not) may induce an artefact (e.g. visual residuals on a screen) which benefit coordinate spatial relations. Thus coordinate spatial changes became as apparent as categorical spatial changes. Moreover, experiment environments (e.g. illumination,

background noise) could cause inconsistent findings between Experiment 1 and Experiment 2. Future studies could manipulate different environment factors to explore impacts of these factors to the visual-spatial process.

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A series of experiments were conducted in Chapter 2 to investigate a possible categorical boundary in VSSTM. The results showed a robust categorical advantage effect in visuospatial representation even when the two subcomponents of categorical representation were interrupted. When stimuli are presented on the whole visual field, the effect of time courses and the hemispheric specificity for categorical and coordinate spatial process does not occur.

The results suggest that categorical spatial representation may be a primary process in

visuospatial memory while coordinate representation is a supplementary process. Moreover, neuroimaging results demonstrate bilateral activations when processing categorical or

coordinate spatial judgments, which indicate that their underlying neural mechanisms may be similar.

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