Lacking control over the trade-off between quality and quantity in visual short-term memory.

Alexandra M Murray,Mark G Stokes,Anna C Nobre,Duncan E Astle,Elkan Akyürek

doi:10.1371/journal.pone.0041223

Abstract

Visual short-term memory (VSTM) is limited in the quantity and quality of items that can be retained over time. Importantly, these two mnemonic parameters interact: increasing the number of items in VSTM reduces the quality with which they are represented. Here, we ask whether this trade-off is under top-down control. Specifically, we test whether participants can strategically optimise the trade-off between quality and quantity for VSTM according to task demands. We manipulated strategic trade-off by varying expectations about the number of to-be-remembered items (Experiments 1–2) or the precision required for the memory-based judgement (Experiment 3). In a final experiment, we manipulated both variables in a complementary way to maximise the motivation to strategically control the balance between number and the quality of items encoded into VSTM. In different blocks, performance would benefit most either by encoding a large number of items with low precision or by encoding a small number of items with high precision (Experiment 4). In all experiments, we compared VSTM performance on trials matched for mnemonic demand, but within contexts emphasising the quality or quantity of VSTM representations. Across all four experiments, we found no evidence to suggest that participants use this contextual information to bias the balance between the number and precision of items in VSTM. Rather, our data suggest that the trade-off may be determined primarily by stimulus-driven factors at encoding.

Highlights

Capacity limits in visual short-term memory (VSTM) can be characterised by two key parameters: the number of memory items and the quality with which they are remembered
If participants were able to use the cue information to optimise VSTM resource allocation, the precision of VSTM should vary as a function of the number of memory items that are likely to be presented [10]
We found no evidence that participants use the provided strategic information to control the trade-off between the precision and number of items in VSTM

Summary

Introduction

Capacity limits in visual short-term memory (VSTM) can be characterised by two key parameters: the number of memory items and the quality with which they are remembered. Increasing the number of memory items reduces the quality of individual memory representations [1,2] This trade-off is well established, it remains unclear whether the relative allocation of VSTM capacity is under top-down control. A related account further proposes that if the number of to-be-represented items is below the maximum number of slots, items can be represented by multiple slots This ‘‘slots + averaging’’ model would predict that precision can increase when only a few items are represented. [7] do not impose an upper item limit, but suggest that VSTM resources can be allocated flexibly to accommodate increasing numbers of behaviourally relevant items, albeit with increasingly less precision [1,8] Flexible resource models e.g. [7] do not impose an upper item limit, but suggest that VSTM resources can be allocated flexibly to accommodate increasing numbers of behaviourally relevant items, albeit with increasingly less precision [1,8]

Methods

Results

Conclusion