2.4.1 Introduction
This experiment is designed to minimise assistance for categorical spatial relations by interfering with both categorical-verbal and categorical-perceptual codes in visuo-spatial processing. A set of stimuli with different colours was introduced to bind the stimuli’s position information with its identity in visuospatial memory. Dent (2009) also utilised coloured stimuli with articulatory suppression to interfere with visuospatial memory. The
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results showed that only when stimuli’s identity/colour was switched and its position was maintained (i.e. identity-position binding trials) was participants’ performance influenced by articulatory suppression. The categorical advantage effect remained when articulatory suppression was applied. That is, even though articulatory suppression affected the process of verbal recoding for objects during VSSTM, categorical spatial processing was still better than coordinate spatial processing. However, the articulatory suppression adopted in Dent’s study may not interfere with categorical spatial representations specifically. The current experiment utilised a more sophisticated design, which interferes with categorical-verbal and categorical-perceptual codes independently within categorical representations. The two subcomponents, categorical-verbal and categorical-perceptual codes, could then be
interrupted independently through different types of interference words. Specifically, categorical-verbal codes would be interfered with using spatially relevant words while perceptual codes would be interfered with via colour words. If categorical-perceptual codes contributed to the categorical advantage effect in Exp.1b, one would anticipate that the categorical advantage effect would vanish when colour words were played. Different encoding times and shift sizes were maintained to ensure that categorical information was attenuated and coordinate information was relatively intact during visual-spatial processing.
2.4.2 Methods 2.4.2.1 Participants
Forty-eight healthy young participants were included in the study. Participants were equally divided into four groups: spatial interference group (3 males and 9 females, mean age=23.3, SD=3.9), colour interference group (2 males and 10 females, mean age=24.7, SD=6.5), irrelevant interference group (4 males and 8 females, mean age=21.0, SD=1.9), and no interference/silent group (4 males and 8 females, mean age=21.8, SD=3.5). They were aged between 18 and 29 year-old, right-handed and had normal or correct-to-normal vision. The research was approved by the Faculty of Medical Sciences Ethics Committee in Newcastle University.
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2.4.2.2 Experiment material, design and procedure
The stimuli set was exactly the same as the previous experiments except the colour of the squares was changed from one colour (red) to multiple colours (red, green, blue, and
yellow). The assignment of colour to location was completely randomised. The colours of the four squares were consistent between the encoding and response image in the categorical, coordinate and the same conditions. An additional colour change/switch condition was included. Two of the colours were switched while locations of the squares remained the same in switched trials.
The study was a mixed design as each participant experienced three encoding times (250ms, 500ms, and 2500ms) and three shift sizes (15mm, 20mm, and 25mm) along with one type of interference words (spatially relevant words, colour words, irrelevant words, and no
interference). The types of interference were designed as a between-subject variable. The spatially relevant and irrelevant words were identical to Exp.1b and the colour words were
“red”, “blue”, “yellow”, and “green”. All of the words were pre-recorded and were played to participants repeatedly via headphones throughout the experiment. The experiment
procedure was identical to Exp.1b. An example trial procedure is illustrated in Figure 2-10.
The retention interval was fixed to 2000ms. There were 48 trials in a block: 12 categorical-change trials, 12 coordinate-categorical-change trials, 12 colour switched trials and 12 same trials. Each participant experienced 9 blocks since each retention interval contained three blocks. A short break was provided between blocks. Participants always performed the shortest encoding time first, followed by the medium encoding time, and the longest encoding time was performed last. They were also instructed to utilise their left or right index fingers to press the key “z” or “m” on the keyboard; the assignment of key to response was
counterbalanced over participants. Both accuracy and reaction times were recorded by E-Prime software. The experiment duration was approximately 50 minutes.
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Figure 2-10. Example stimuli and a trial procedure for Exp.1c. The green square on the top left is the reference and the blue square at the bottom left is the target in this example. In a categorical-change trial, the target would be shifted from the left of the reference to the right. In a coordinate-change trial, the target remains to the left of the reference but the distance has been changed. In a switched trial, the position of the target remains the same but the colour is switched to yellow. The colour and position of the squares remain the same in a same trial.
2.4.2.3 Data analysis
Similar to the previous experiments, two analyses were applied. A 4(condition: categorical vs.
coordinate vs. switched vs. same condition) × 3 (encoding: 250ms vs. 500ms vs. 2500ms) within-subject repeated measures combined with a between-subject variable (interference type: colour vs. spatial vs. irrelevant vs. no interference) ANOVA was adopted in the first analysis to investigate whether duration of encoding times and/or different types of verbal interference would affect performance on spatial judgments. Results report mainly on interactions between condition and encoding time, and condition and interference type as well as main effects of condition, encoding time, and interference type. The second analysis aimed to observe the impacts of shift size (15mm, 20mm, and 25mm), encoding time and
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interference type on the performance of categorical and coordinate spatial relations. A four-way ANOVA: 2 (spatial relation: categorical vs. coordinate relation) × 3 (retention interval:
500ms vs. 2000ms vs. 5000ms) × 3 (shift size: 15mm vs. 20mm vs. 25mm) within-subject repeated-measures combined with a between-subject variable (interference type: colour vs.
spatial vs. irrelevant vs. no interference) was performed. The same and switched conditions were excluded from this analysis as no spatial shift was manipulated. Interactions between spatial relations with the other three variables, encoding time, shift size, and interference type, and main effects of these variables are reported in the results section. Other
interactions will be provided in the Appendix.
2.4.3 Results
Figure 2-11 illustrates accuracy in performance of categorical-change, coordinate-change, the same and colour switched trials, with different interference types and different encoding times. The results showed that neither the interaction of condition and encoding time (F(6,264)=1.38, p=0.222) nor condition and interference type (F<1) was significant. Different encoding times and types of interference words did not affect performance on the
categorical, coordinate, same, or switched conditions. The main effect of condition was significant (F(3,132)=80.38, p<0.001). The same trials showed the best performance (79.5%), followed by switch trials (71.2%), categorical-change trials (54.0%), and coordinate-change trials were performed the worst (37.5%). The effect of encoding time was also significant (F(2,88)=11.24), p<0.001), indicating that longer encoding times resulted in better memory performance. The 2500ms encoding time showed better memory performance (63.2%) than the 500ms encoding time (60.0%) and the 250ms encoding time (58.4%) while there were no difference between the latter two. The types of interference did not affect performance (F<1). None of the interference types influenced spatial judgments. Especially, neither spatially relevant words nor colour words affected the superior role of categorical spatial representations in VSSTM.
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Figure 2-11. Mean accuracy of (A) the four conditions: categorical, coordinate, same, and switched condition, (B) the four conditions with the three encoding times (250ms, 500ms, 2500ms), and (C) the four conditions with the four interference types (colour interference, spatial interference, irrelevant interference, and no interference). ‘***’=p <0.001.
Figure 2-12 depicts reaction times for the four conditions (i.e. categorical-change, coordinate-change, same, and switched condition), performance in the colour, spatial, irrelevant, and no interference type and in the three different encoding times. The results revealed an interaction between condition and encoding time (F(6,258)=2.99, p=0.008). Post hoc analysis indicated that the four conditions consistently showed significant faster
responses in the 250ms and 500ms encoding time than the 2500ms encoding time (ps<0.001). However, responses in the 250ms and 500ms were not different from each other. There was no interaction between condition and interference type (F<1), indicating that type of interference did not affect categorical, coordinate, same, or switch judgments.
The main effect of encoding time was significant (F(2,86)=69.08, p<0.001). Post hoc analysis indicated that the longest encoding time showed the slowest response times (1386ms) while the medium encoding time and the shortest encoding time were not significantly different in
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response time (1084ms vs. 1125ms). However, there was no effect caused by condition (F(3,129)=1.88, p=0.136). Participants showed similar response times for categorical-change, coordinate-change, same, and switch trials. The effect of interference type was also not significant (F(3,43)=1.04, p=0.385). Types of interference words did not affect response times on the four conditions.
Figure 2-12. Mean reaction times of (A) the four conditions: categorical, coordinate, same, and switch condition, (B) the four conditions with the three encoding times (250ms, 500ms, and 2500ms), and (C) the four conditions with the four interference types (colour interference, spatial interference, irrelevant interference, and no interference). ‘***’=p <0.001.
The second analysis addresses performance of categorical and coordinate spatial relations in different encoding times (250ms, 500ms, and 2500ms), shift sizes (15mm, 20mm, and 25mm), and interference types (colour, spatial, irrelevant, and no interference) (See Figure 2-13). The results showed that there was a significant interaction between spatial relation and shift (F(2,88)=14.57, p<0.001). Post hoc analysis suggested that the greatest shift size
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(25mm) showed the best accuracy of performance, followed by the 20mm shift, and the smallest shift size was performed the worst in categorical-change trials (F(2,143)=3.76, p=0.026) and coordinate-change trials (F(2,143)=21.67, p<0.001). However, the interactions between spatial relation and encoding time (F(2,88)=1.79, p=0.173), and spatial relation and interference type (F<1) were not significant. There was no significant difference between performance in the categorical and coordinate condition with different encoding times or types of interference words. A significant main effect of spatial relation was found
(F(1,44)=111.09, p<0.001). The categorical condition (54.0%) was performed better than the coordinate condition (37.5%). Moreover, significant main effects of shift size and encoding time were also found (F(2,88)=78.84, p<0.001 and F(2,88)=3.40, p=0.038, respectively). Post hoc analysis indicated that the largest shift size was performed the best (54.3%), followed by the medium shift size (45.1%), and the smallest shift size showed the worst performance (37.9%). Greater shift sizes were easier to detect, resulting in better memory performance.
The effect of encoding time suggests that stimuli with longer presentation time result in better memory performance. Specifically, the 2500ms encoding time (48.5%) was memorised significantly better than the 500ms encoding time (44.8%) and the 250ms encoding time (44.0%) while performance for the latter two did not differ from each other.
There was no effect of interference type (F<1), which indicates that performance for categorical- and coordinate-change trials was not affected by types of interference words.
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Figure 2-13. Mean accuracy of (A) categorical and coordinate spatial relations, (B) the two spatial relations with the three encoding times (250ms, 500ms, and 2500ms), (C) the two spatial relations and the three shift sizes (15mm, 20mm, and 25mm), and (D) the two spatial representations with the four interference types (colour interference, spatial interference, irrelevant interference, and no interference). ‘***’=p<0.001.
In terms of reaction times, none of the interactions between spatial relation and shift size (F<1), spatial relation and encoding time (F<1), or spatial relation and interference type (F(3,30)=1.41, p=0.259) were significant (see Figure 2-14). Participants showed similar reaction times on judging categorical- and coordinate-change trials regardless of the duration of stimuli presentation, shift size or type of interference words. There was a main effect of spatial relation (F(1,30)=4.71, p=0.038). The responses for categorical-change trials (1165ms) were significantly faster than coordinate-change trials (1195ms). Moreover, the main effect of shift size was significant (F(2,60)=11.84, p<0.001). Participants responded much quicker to the greatest shift size (1139ms) than to the smallest and the medium shift size. However, there was no significant difference between the smallest and the medium shift size (1216ms vs. 1185ms). In addition, the main effect of encoding time was also
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significant (F(2,60)=29.93, p<0.001). Post hoc analysis showed that the longest encoding time resulted in the slowest response times. However, responses to the shortest and the medium encoding times did not differ from each other (1106ms vs. 1095ms). There was no significant effect of interference type, suggests that response times were not affected by types of interference words (F(3,30)=1.50, p=0.235).
Figure 2-14. Mean reaction times of (A) categorical and coordinate spatial relations, (B) the two spatial relations with the three encoding times:250ms, 500ms, and 2500ms, (C) the two spatial relations and the three shift sizes: 15mm, 20mm, and 25mm, and (D) the two spatial representations with the four interference types: colour interference, spatial interference, irrelevant interference, and no interference. ‘*’=p<0.05 and ‘***’=p<0.001.
Overall, the categorical advantage effect was found in the current experiment. Neither spatially relevant nor colour relevant words intruded on categorical representations since detection of categorical changes was still better and much faster than coordinate changes.
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2.4.4 Summary and discussion of Experiment 1c
The present experiment has further extended previous studies by adopting position-identity binding stimuli (i.e., coloured stimuli) to investigate the effects of the two categorical
subcomponents, categorical-verbal and categorical-perceptual codes, in the CATCOORD task.
The interference of spatially relevant words was designed to interrupt categorical-verbal codes whereas colour words were designed to interrupt categorical-perceptual codes.
Nonetheless, the categorical advantage effect was consistently found even when spatially relevant words or colour words were played to participants throughout the experiment. The current results demonstrate that categorical-verbal and categorical-perceptual interferences did not influence the superior categorical spatial processing in VSSTM. Importantly, even when categorical information was minimised, e.g. trials with the shortest encoding time and the smallest shift size, and verbal interference was maximised, e.g. interruptions started from the beginning of the encoding stage of the memory process, categorical-change trials were still performed better than coordinate-change trials in the spatial and colour
interference conditions.
In Experiment 1b, verbal interference of spatially relevant words did not affect performance of categorical spatial judgments more than coordinate. Yet the observed categorical
advantage may be caused by the categorical-perceptual codes which remained in use while categorical-verbal codes were interrupted. This experiment hence interfered with
categorical-verbal and categorical-perceptual coding independently with the coloured stimuli via spatially relevant and colour words in the CATCOORD task. The results still showed the categorical advantage effect, which suggests that categorical spatial relations are dominant in visuospatial representations. One possible reason for the finding may be due to weak interruption of categorical representations by the auditory verbal interference.
Future studies which intend to enhance the intervention of categorical spatial
representation and to examine the role of coordinate spatial representation in VSSTM could apply a stronger interference task to categorical representation, such as spatial tapping.
Another possibility is that the existence of the categorical advantage effect may be due to the design of the auditory verbal interference. Participants repetitively heard four pre-recorded interference words in the same order which may lead to them habituating and to the development of a memory strategy, such as to ignore the words; hence the impacts of
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the interference words were minimal. To play interference words less predictably may draw participants’ attention to the interference words throughout visuospatial processing and maximise the effect of interference in VSSTM.
Overall, the current study has extended previous findings by demonstrating the categorical advantage effect when the two subcomponents, verbal and
categorical-perceptual codes, were interrupted. The notion that categorical spatial representations are an intrinsic property of visuospatial memory whilst coordinate spatial representations are a supplementary visual-spatial process in VSSTM is supported by these results.
2.4.5 Summary of Experiment 1a-1c
This section will summarise the findings from the CATCOORD task. The implications will be addressed in a later section (see section 2.6). The three behavioural experiments in
Experiment 1 have consistently found a robust categorical advantage effect regardless of encoding times, retention intervals, and shift sizes. Moreover, when categorical-verbal and categorical-perceptual codes were interrupted during visuo-spatial processing, categorical changes between objects were still easier to detect than coordinate changes. Even though the behavioural experiments did not show sensitivity to time courses for coordinate spatial relations, separable spatial relations in VSSTM have been demonstrated with the whole visual-field task, the CATCOORD task. Although categorical spatial representations showed better memory performance than coordinate spatial representations in VSSTM, the
underlying neural mechanisms of the two spatial relations may reveal a different story.
Previous literature has demonstrated hemispheric specialisations for the two spatial representations, thus the following section will investigate lateralisation effects in the CATCOORD task with a neuroimaging technique.
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