Experiments 4 and 5 strongly suggested that domain-specific resources are not involved over and above the involvement of domain-general resources in the maintenance of cross-domain associations in working memory. Neither a verbal nor a spatial attention-demanding processing task had resulted in lower cross-domain span scores than a domain-neutral attention-demanding processing task. This result fits nicely with the conclusion of the preceding section that cross-domain associations are maintained integrated and not as
separated features. If this had been the case, maintenance of the verbal features should have suffered from the verbal processing task and maintenance of the spatial features from the spatial processing task. This is not what we observed. Several other studies had however shown evidence for domain-specific resources to be involved within memory for cross-domain associations (Allen et al., In press; Guérard et al., 2013; Guérard, Tremblay, & Saint-Aubin, 2009; Morey, 2009). Nonetheless, at least three differences between those studies and ours could be suggested to explain these different results. These will be discussed in turn.
A first difference between the above-mentioned studies and ours is the attention-demanding nature of our tasks. Our verbal and spatial processing task relied on both domain-specific and domain-general resources. The above mentioned studies all made use of
manipulations implying only domain-specific resources (i.e., articulatory rehearsal, phonemic similarity, path complexity) without any specific attentional involvement. The condition in
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our experiment that resembles most the above-mentioned studies is the low cognitive load condition, as this is the condition in which the processing task uses the least of the attentional resources. Even in the low cognitive load condition, we did not observe any differences between the neutral and the verbal or spatial processing task. We do hence not assume that the use of a task combining the manipulation of domain-general and domain-specific demands of the processing task has hidden any effect of the involvement of domain-specific resources.
A second difference concerned the evaluation of memory performance. Our recall scores in Experiment 4 and 5 were based only on the recall of the entire association, while the above-mentioned studies evaluated feature memory. Next to the maintenance of associations as a whole, on some occasions only a single feature is maintained. This was clearly reported by Cowan et al. (2013) and this was also observed in our experiments. It is hence possible that the associations as a whole are maintained through attentional resources, while features
without their counterpart are maintained by domain-specific resources. A verbal or spatial processing task would then merely affect recall of these single features. Hence, the cross-domain recall scores would not be affected by the verbal or spatial nature of our processing tasks. Nevertheless, it was observed by Morey (2009), Guérard et al. (2009) and Guérard et al.
(2013) that spatial features of cross-domain associations were affected by the manipulations of the verbal domain-specific resources. Single spatial features are however not affected by variations in verbal domain-specific resources. This cross-domain interference could hence only occur for those spatial features integrated with a verbal feature. Our cross-domain recall scores should thus have suffered from domain-specific interference as well.
A hypothetical explanation for this pattern of results would be in terms of the level of integration between the features. Morey (2011) has shown that incidental binding between features leads to small memory advantages, but intentional binding leads to even bigger advantages. This seems to suggest that several levels of integration of the features might be possible. If this is indeed true, then we might also assume features that are less well integrated to be more prone to domain-specific interference. Verbal interference would then act on these less integrated verbal features, resulting in their loss from working memory. As spatial
features have often been described as being attached to the verbal feature (e.g., Elsley &
Parmentier, 2014; Guérard et al., 2013; Maybery et al., 2009), the loss of this verbal feature may make the retrieval of the spatial feature impossible. Concerning our results, we have already shown that the verbal and spatial features are maintained integrated. Although it is difficult to methodologically determine the level of integration between features, we have an
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indication that they are rather well integrated. In an experiment not reported here, we
compared a pure recall paradigm with two paradigms combining recall and recognition (as in Experiment 8). In the first combination paradigm, recognition probes consisted always of single features (feature recognition, object recall). In the second combination paradigm, recognition probes always consisted of integrated letter-location probes and additionally, participants were obliged to maintain the link between the features in this second combination (object recognition, object recall). They were explicitly tested on the correct associations (answer YES to exact probes and NO to recombined probes). If the recall paradigm on its own does not or poorly incites an integrated maintenance, then the object recognition task should nevertheless result in this incitation. A higher recall score in this object object recall combination as compared to a pure recall paradigm or the feature recognition-object recall combination would hence be indicative of a poor integration of the features when participants have to perform only the recall task. We observed however no difference in recall scores between these three conditions. We can hence assume that our recall paradigm on its own resulted in a strong integration for the letter-location associations present within working memory. This strong integration might hence be the reason the cross-domain associations in our study were not prone to domain-specific interference.
A third difference between our studies, and the above-mentioned studies that did show evidence for the involvement of domain-specific resources, concerns the moment of
interference. In our study, the processing tasks were administrated during the retention intervals. Presentation of the memory items was rather long (in comparison to other studies) and this encoding/consolidation process was not hindered by other tasks. This was done to allow at least a correct encoding of the items, and subsequently investigate the maintenance process. Several other studies have however manipulated the availability of domain-specific resources from encoding on. For example, Morey (2009) used articulatory suppression from the moment of presentation until the appearance of the test probe. This might already have hindered the encoding process. Guérard et al. (2009, 2013) used phonemic similarity and path complexity in order to manipulate the domain-specific resources. While path complexity did not influence verbal recall, phonemic similarity had a clear effect on spatial recall. The study on the disappearance of the visuo-spatial bootstrapping effect when performed with spatial tapping at encoding (Allen et al., In press) confirms the involvement of visuo-spatial
resources at the encoding of cross-domain associations as well. Domain-specific resources do hence seem to play an important role at encoding of cross-domain associations. Our study
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showed that these domain-specific resources are however not implied for the further maintenance. The neuro-imaging study by Li, et al. (2014) confirms the role of domain-specific resources to be limited to encoding while domain-general resources are involved at encoding, maintenance and retrieval.
Several differences in the experimental design could thus be at the basis for the differences in results we have obtained in comparison with the studies mentioned above. A closer look at these differences showed however that none of these studies is explicitly contradicting our results.