Abstract
Working memory (WM) involves three cognitive events: information encoding, maintenance, and retrieval; these are supported by brain activity in a network of frontal, parietal and temporal regions. Manipulation of WM load and duration of the maintenance period can modulate this activity. Although such modulations have been widely studied using the event-related potentials (ERP) technique, a precise description of the time course of brain activity during encoding and retrieval is still required. Here, we used this technique and principal component analysis to assess the time course of brain activity during encoding and retrieval in a delayed match to sample task. We also investigated the effects of memory load and duration of the maintenance period on ERP activity. Brain activity was similar during information encoding and retrieval and comprised six temporal factors, which closely matched the latency and scalp distribution of some ERP components: P1, N1, P2, N2, P300, and a slow wave. Changes in memory load modulated task performance and yielded variations in frontal lobe activation. Moreover, the P300 amplitude was smaller in the high than in the low load condition during encoding and retrieval. Conversely, the slow wave amplitude was higher in the high than in the low load condition during encoding, and the same was true for the N2 amplitude during retrieval. Thus, during encoding, memory load appears to modulate the processing resources for context updating and post-categorization processes, and during retrieval it modulates resources for stimulus classification and context updating. Besides, despite the lack of differences in task performance related to duration of the maintenance period, larger N2 amplitude and stronger activation of the left temporal lobe after long than after short maintenance periods were found during information retrieval. Thus, results regarding the duration of maintenance period were complex, and future work is required to test the time-based decay theory predictions.
Highlights
Working memory (WM) is defined as the capacity to hold in mind for brief periods of time small amounts of information that are no longer available in the environment (Baddeley, 2012)
Post hoc comparisons revealed that inverse efficiency (IE) scores were significantly larger in the high load (HL) than in the low load (LL) condition for both maintenance period durations (p < 0.001)
ELECTROPHYSIOLOGICAL RESULTS Effects of memory load on electrical brain activity during information encoding Analysis of the effects of memory load during WM encoding revealed significantly lower TF2 factor scores in the HL condition than in the LL condition [t(28) = 3.79, p ≤ 0.001], which corresponded to a lower P300 amplitude in the HL condition than in the LL condition
Summary
Working memory (WM) is defined as the capacity to hold in mind for brief periods of time small amounts of information that are no longer available in the environment (Baddeley, 2012). This capacity is supported by synaptic changes, neural firing and synchronous activity in a network of brain regions (Buzsáki and Draguhn, 2004), mainly involving frontal, parietal, and temporal lobes (Klingberg, 2006; Linden, 2007). A model linking cerebral activity and the cognitive events involved in WM has recently been proposed (Jonides et al, 2008) This model considers three cognitive events: information encoding in memory, information maintenance, and information retrieval. Jonides and colleagues discuss a time-based decay theory that was proposed to explain the mechanisms that lead to forgetting: in this theory, the mere passage of time is considered to produce the enfeeblement of memory traces and the disruption or complete loss of the memory
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