Elsevier

Brain and Cognition

Volume 55, Issue 3, August 2004, Pages 415-426
Brain and Cognition

Imposing structure on a Corsi-type task: Evidence for hierarchical organisation based on spatial proximity in serial-spatial memory

https://doi.org/10.1016/j.bandc.2004.02.071Get rights and content

Abstract

Structure was imposed on a tapping task by requiring participants to reproduce sequences of responses to icons organised in spatial clusters. A first experiment featured sequences either segregated or not segregated by clusters. Accuracy was higher for sequences segregated by clusters. Moreover, inter-response times were longer at cluster boundaries than within cluster boundaries. To rule out possible confounding effects of movement length, this temporal pattern was replicated in a second experiment requiring a single response indicating the next sequential step, following the presentation of a portion of a previously practised sequence. These results suggest that sequence reproduction can be sustained by a hierarchical representation based on spatial proximity and provide a first indication of the role of spatial structure in serial-spatial memory.

Introduction

We are constantly faced with the problem of having to temporarily retain serial-spatial information in order to keep track of sequences of displacements of objects and to plan and execute sequences of movements in space.

The Corsi Tapping Test (CTT), or basic variations of it, is one of the most popular experimental tools for the assessment of serial-spatial temporary memory and the factors that influence it. Initially developed by Corsi (1972) for his unpublished dissertation, and presented to a wider scientific readership by Milner (1971), the CTT, as originally devised, required the participants to reproduce a sequence of tapping responses previously performed by the tester on an array of wooden blocks.

In recent years, the Corsi test has been used in a growing body of research on serial-spatial memory that has been accumulated following the development of the working memory model (Baddeley & Hitch, 1974). Within this framework, independent sub-systems have been identified that are specifically dedicated to the processing of either verbal (the phonological loop) or spatial information: (the visuo-spatial scratch-pad). In a recent review, Baddeley (2001) indicates the CTT as the test that is most closely associated with spatial short-term memory on the basis of both behavioural and neuropsychological evidence.

Although research on working memory has initially focused mainly on the verbal processing sub-system (as pointed out, for example, by Jones, Farrand, Stuart, & Morris, 1995; Logie, 1995), more recently, research on the visuo-spatial component of the model has enjoyed a renewed interest and major advancements have been made in our understanding of its characteristics and functions. For example, on the basis of careful experimental investigations, a sub-system of the visuo-spatial scratch-pad has been suggested, the “inner scribe,” that would be specifically responsible for the retention and processing of spatial information and would play an essential role in processing and planning movement sequences (Logie, 1995; Logie & Marchetti, 1991). The CTT seems to capture accurately the working of the inner scribe, as performance on this test seems to rely on a purely serial spatial component of working memory that is selectively affected by the concurrent presentation of spatial but not visual tasks (Logie, 1995; Reisberg & Logie, 1993; Salway & Logie, 1995) and tasks that make demand on spatial attention (Smyth & Scholey, 1994b). Performance on CTT shows, moreover, a double dissociation with visual tasks in neuropsychological patients (Della Sala, Gray, Baddeley, Allamano, & Wilson, 1999), and presents developmental fractionation with visual tasks (Logie & Pearson, 1997).

Movement per se has recently received further emphasis through a proposed distinction, within the visuo-spatial scratch-pad, of sub-systems specialised for processing static and dynamic information (Pickering, Gathercole, Hall, & Lloyd, 2001), the latter being conveyed by tasks with a strong serial component. CTT performance seem to rely on resources that are specifically allocated to the processing of movements to spatial targets and independent from the resources used for the processing of configurations of movements, such as hand configurations or body postures (Smyth, Pearson, & Pendleton, 1988; Smyth and Pendleton, 1989, Smyth and Pendleton, 1990).

Although the CTT has enjoyed an enormous popularity in both experimental research and clinical practice, it is unfortunate, as pointed out by Berch, Krikorian, and Huha (1998) in a recent review, that in different studies featuring the CTT almost every task parameter (ranging from the block arrangement to the scoring method) has been varied, often without providing an adequate description of important procedural details (Berch et al., 1998).

It seems particularly surprising that very little, if any, consideration has been given to the potential importance of the relative spatial position of the blocks and how spatial constraints might interact with the particular sequence to be reproduced. Berch et al. (1998) note that none of the 38 studies included in their review, including the original work by Corsi (1972), had reported the relative distance between the blocks.

When studying serial memory in non-spatial domains it is possible to devise relatively unstructured material, such as lists of non-sense words. By contrast, in the spatial domain structural constraints are always present and might play a role in how the information to be retained is organised in a serial representation. An observer can always detect structure in the spatial layout of the environment, provided, for example, by the relative spatial proximity of different objects or their location along spatial vectors.

An example of constraints that can be used in a spatial display is provided in Fig. 1A, which depicts a typical arrangement of blocks featured in a Corsi-type task.

Even in this relatively unstructured configuration, it is possible to see how some blocks (e.g., blocks B and D and blocks C and E) can be perceptually grouped on the basis of their spatial proximity and that the array can be segmented in three diagonal lines (i.e., blocks A–C; B–D–E–F; and G–H–I).

In those studies which have reported a graphical depiction of the block arrangement, it is possible to detect variations in the way in which the blocks form sub-groupings and clusters based on spatial proximity (see Berch et al., 1998, Fig. 2). Even when the spatial configuration was kept constant, it has been shown that the use of different tapping paths through the block-array selectively affects the ability of the participants to reproduce sequences of the same number of ordinal steps (Smirni, Villardita, & Zappala’, 1983).

However, the question of why some sequences should be easier to reproduce than others has not been specifically addressed. The fact that participants might find some sequences easier to remember has been mostly used as an argument highlighting the need for standardisation of the Corsi test for experimental and clinical testing (Kessels, Zandvoort, Postma, Kappelle, & Haan, 2000; Smirni et al., 1983). One possible explanation of the difference in performance according to the particular sequence presented is that even in the relatively unstructured configuration normally used in Corsi-type tasks, the serial structure of some sequences might collude with the spatial arrangement of the display to make it possible to use spatial structure as a memory offloading device. For example, a long sequence might be hierarchically encoded by segmenting it into sub-sequences defined by spatial clusters or linear structures.

It is, however, difficult to identify a posteriori what type of spatial constraints people might spontaneously use in order to reproduce given sequences in displays such as that depicted in Fig. 1A. It is, therefore, necessary to test specific hypotheses concerning the type of spatial constraint used to aid memory performance for given sequences by using configurations that explicitly afford a particular type of organisation and compare performance on sequences that are consistent with that particular type of organisation with sequences that are not. For example, the clustered display presented in Fig. 1B affords the segregation of a 9-item sequence into three 3-item sub-sequences defined by each spatial cluster. Thus, performance on a sequence such as ABC DFG EHI that is segregated by spatial clusters can be compared with performance on a sequence such as AHB DEC FEG that is not, in order to assess whether this type of constraint facilitates performance or not.

Some attention to the effects of different displays on spatial span in a Corsi-type task has been given by Smyth and Scholey (1994a). In an attempt to find an analogy to the articulatory loop in spatial memory, this study featured a set of experiments aimed at assessing the effect of movement time (time required to point to icons located at different distances on the screen and presumably related to the time required to rehearse their serial locations, in analogy to the word length effect observed in verbal working memory) on subjects’ performance. The first two experiments presented in the study failed to support the hypothesis that an increase in the length of movements had an effect on spatial memory span. However, the study featured a third experiment where icons were organised into clusters and participants were required to reproduce sequences featuring only one of the icons within each cluster. The rationale for grouping the icons in spatial clusters was to ensure that subjects were taking into account the precise location of the target (in order to distinguish it from other items in spatial proximity) when rehearsing the sequence. In this last experiment a negative relationship between the distance between the items in the sequence and memory span was observed and this result was used to support the claim that rehearsal time affects spatial span, similarly to what has been reported for serial verbal memory (Baddeley, Thomson, & Buchanan, 1975).

In the study of Smyth and Scholey (1994a) it was observed that generally memory span for items presented within an array featuring spatial clusters is impaired compared to sequences of similar length but presented within an array where icons are not organised into clusters. This result is interpreted by the Authors in terms of an increase in the confusability of icons presented in spatial proximity. However, as mentioned above and in line with the rationale of their study, in the Smyth and Scholey’s study (1994a) participants were not faced with sequences where all the items within a cluster had to be selected before moving onto another sub-set of icons grouped on the basis of spatial proximity.

Although very informative in highlighting analogies and differences in the rehearsal mechanisms underlying spatial and verbal working memory and although in one of their experiments icon displays featuring explicit spatial structure were used, the study by Smyth and Scholey (1994a) did not address the issue of whether spatial clustering in a Corsi-type task can support the emergence of hierarchically organised representations of the sequence which, in analogy with psychological chunking in other domains, can help the reduction of the memory load imposed by the task.

Hierarchical models of sequence reproduction in the verbal domain, have been supported by time analyses in studies requiring the participants to repeat, by speaking or typewriting sequences of words or grouped strings of letters presented on a computer monitor (Sternberg, Knoll, Monsell, & Wright, 1988; Sternberg, Knoll, & Turock, 1990). Nevertheless, due perhaps to the lack of interest in the role played by spatial constraints in the way material is organised in short-term memory or the failure to recognise its importance, very little seems to be known to date about the forms of representation that might underpin the efficient reproduction of serial responses to material featuring an explicit spatial structure.

The aim of the present study is to begin to address this issue by using a task similar to the CTT. However, in contrast with previous studies based on the CTT, here an explicit form of spatial organisation, based on spatial clustering was presented, and the serial structure of the sequence was manipulated so that some sequences were segregated by clusters and some were not.

The use of computerized procedures for the administration of Corsi-type tasks is becoming increasingly widespread and it is recommended rather than the use of the more traditional version featuring wooden blocks (Berch et al., 1998).

The obvious differences between computerised versions and traditional versions of the task (such as the use of two-dimensional as opposed to three-dimensional stimuli, the vertical instead of horizontal presentation of the stimulus array and the fact that changes in brightness or colour of the stimuli indicate the path sequence instead of the manual pointing of the tester) do not seem to play a significant role in the relevant response variables (Fischer, 2000). Furthermore, computerised versions of the task, especially if implemented using touch-sensitive computer monitors, offer, among others (see Berch et al., 1998; for a more extensive list) the advantage of enabling the recording of response latencies. Indeed a time analysis of the response of the participants has recently proved to be an indicator of important aspects of the way in which sequences of responses are planned and represented in Corsi-type tasks (Fischer, 2000).

By featuring an analysis of the pattern of RT shown by participants faced with a sequence reproduction task within an array of spatially clustered icons, the present study aimed to identify basic forms of organisation within serial-spatial memory.

Section snippets

Experiment 1

The aim of Experiment 1 was to assess whether sequences that are segregated by spatial clusters are reproduced more accurately than sequences that are not segregated by spatial clusters. Moreover, in order to test the hypothesis that a hierarchical representation underpinned the reproduction of sequences segregated by clusters, RT were analysed to ascertain whether longer latencies were observed for responses located at cluster boundaries.

The emergence of peaks of RT at critical points during

Experiment 2

Time to generate the next step in the sequence has been traditionally used to infer the nature of representations and cognitive processes underpinning serial memory in non-spatial domains. For example, Sternberg (1969) investigated the search processes operating upon the representation of serial-order information by requiring subjects to memorise a list of digits before being presented with a probe and asked to generate the next sequential step.

The analysis of time to generate the next step in

General discussion

The aim of the present study was to address the relatively unexplored issue of the role played by spatial structure in serial memory for locations. More specifically, it dealt with the issue of the role played by grouping by spatial proximity in the formation of hierarchical representations which enable the reduction of the demands associated with remembering sequences of nine ordinal steps.

The results of Experiment 1 confirm that in a task resembling Corsi’s tapping test participants’ ability

Acknowledgements

A semester of study leave granted by the University of Leicester is gratefully acknowledged. Many thanks to Tony Andrews for developing the software used in these experiments.

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