Interactive reportTracking the time-course of attentional involvement in spatial working memory: an event-related potential investigation
Introduction
Working memory and selective attention are cognitive constructs central to the dynamic interplay between perception, thought and action. Yet, only recently have the details of their relationship been explored [3], [10], [13], [14], [15]. One class of models has suggested that selective visual attention, defined as the set of processes by which relevant information is given higher priority in perceptual, decision, and response operations [37], is ideally suited to facilitate control over the contents of working memory [3], [10]. These control operations are thought to be subserved by the coordinated effort of specialized subregions within frontal and parietal cortex that have been reported in attention tasks (e.g. Refs. [11], [18], [19], [23], [33]) and working memory tasks (e.g. Refs. [7], [9], [12], [21], [20]).
To directly assess the degree of overlap in brain regions active during voluntary orienting of spatial attention and spatial working memory, Corbetta et al. [10] manipulated the duration of the interval between an attention directing cue and a subsequent attentional target in a recent fMRI study. They demonstrated that frontal and parietal regions activated during the initial allocation of spatial attention shortly after cue presentation, continued to be active when the cue-target interval was lengthened to 7 sec. They concluded that the circuit that is active during spatial attentional orienting is identical to the circuit activated during working memory maintenance operations at longer delays. Labar et al. [24] also reported a high degree of correspondence in frontal-parietal activity during working memory and attention when there was minimal overlap in task features.
Models of attention [35] and working memory [32] propose that these frontal-parietal control operations have their modulatory effects downstream within posterior perceptual regions. In an elegant series of studies, Awh and co-workers [2], [3], [4], [5], [6] tested a specific hypothesis regarding perceptual level effects relating spatial working memory and spatial attention. They proposed that the online maintenance (rehearsal) of information in spatial working memory necessitates directed allocation of spatial selective attention to memory locations. This attentional allocation was predicted to bias extrastriate activity in favor of memory locations. Such biasing within extrastriate cortex has been observed during attention tasks in the form of tonic effects [8], [22], [25] as well as stimulus-evoked enhancements for stimuli appearing in attended versus unattended locations (e.g. Refs. [27], [30]).
To test their prediction, Awh and co-workers conducted delayed-recognition experiments during fMRI [2] and ERP [6] recording. Subjects were to remember spatial locations, presented during the memory array (S1), over the course of a delay interval until a test item (S2) was presented. A continuous series of task-irrelevant probes with random onset asynchronies were presented during the delay. Activity was averaged over all delay-spanning probes. Consistent with their hypothesis, fMRI activity within extrastriate cortex as well as scalp-recorded P1 and N1 ERP components were greater in amplitude for probes appearing at memory versus non-memory locations.
One issue that remains unanswered from previous work is whether the engagement of selective attention during working memory is required throughout the entire delay interval, when maintenance operations are presumably ongoing. For example, if attention is most critical during mnemonic encoding, as has recently been suggested in studies of long-term memory [1], P1 and N1 enhancements would be expected to be greater for probes appearing shortly after the S1-memory cue is presented compared to probes presented later in the delay interval.
The aim of the current studies was to replicate and extend previous findings by tracking the time-course of perceptual-level attentional involvement in spatial working memory. ERPs were recorded to task-irrelevant probes presented during discrete time bins during the delay interval of a spatial delayed-recognition task. The extrastriate-generated [16] P1 and occipito-temporal N1 components were examined for early (400–800 msec following S1 offset) and late (2600–3000 msec) delay probes to address two specific questions. Firstly, are neural responses modulated during the delay interval in a manner similar to that obtained when attention is explicitly directed to a location in space? Secondly, if so, is the magnitude of attention-like neural activity comparable for early and late delay probes?
Section snippets
Participants
Ten females and nine males (ages 18–34) participated in one 5-hour session. Four of these participants were eliminated because they did not comply with instructions to maintain gaze at fixation. All participants were right-handed with normal or corrected-to-normal vision. All participants provided written consent and received cash compensation.
Stimuli and design
Subjects viewed an NEC 4FG computer monitor from a distance of 75 cm. They were instructed to maintain fixation on a small central cross throughout the
Experiment 2
The goal of Experiment 2 was to confirm that attentional modulations are observed for task-irrelevant stimuli presented during perceptual-based attention tasks as opposed to mnemonic tasks. Previous studies have manipulated delay length during delay-recognition tasks [21] and attentional orienting tasks [10] to assess the degree of overlap between brain regions involved in working memory and attention. Although the strength of such delay-manipulations is that stimulus properties can be held
Discussion
In the current set of studies, the time-course of attentional involvement in working memory was investigated. Task-irrelevant probes were presented early and late during the delay interval of a spatial working memory delayed-recognition task (Experiment 1). We reasoned that if continued allocation of spatial attention is required for ongoing maintenance, the magnitude of attention-like enhancements in the P1 and N1 ERP components should be comparable for early and late delay probes. ERPs were
Acknowledgements
The author wishes to thank G.R. Mangun for helpful comments on this manuscript.
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