Coarse-to-Fine Construction for High-Resolution Representation in Visual Working Memory

<div><p>Background</p><p>This study explored whether the high-resolution representations created by visual working memory (VWM) are constructed in a coarse-to-fine or all-or-none manner. The coarse-to-fine hypothesis suggests that coarse information precedes detailed information in entering VWM and that its resolution increases along with the processing time of the memory array, whereas the all-or-none hypothesis claims that either both enter into VWM simultaneously, or neither does.</p> <p>Methodology/Principal Findings</p><p>We tested the two hypotheses by asking participants to remember two or four complex objects. An ERP component, contralateral delay activity (CDA), was used as the neural marker. CDA is higher for four objects than for two objects when coarse information is primarily extracted; yet, this CDA difference vanishes when detailed information is encoded. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057913#s2" target="_blank">Experiment 1</a> manipulated the comparison difficulty of the task under a 500-ms exposure time to determine a condition in which the detailed information was maintained. No CDA difference was found between two and four objects, even in an easy-comparison condition. Thus, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057913#s3" target="_blank">Experiment 2</a> manipulated the memory array’s exposure time under the easy-comparison condition and found a significant CDA difference at 100 ms while replicating <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057913#s2" target="_blank">Experiment 1</a>′s results at 500 ms. In Experiment 3, the 500-ms memory array was blurred to block the detailed information; this manipulation reestablished a significant CDA difference.</p> <p>Conclusions/Significance</p><p>These findings suggest that the creation of high-resolution representations in VWM is a coarse-to-fine process.</p> </div

The averaged CDA amplitudes for the four critical conditions in Experiment 2.

<p>*indicates the difference between the two conditions was significant. The CDA is a difference wave, constructed by subtracting the ipsilateral from the contralateral activity according to the cued hemifield. (Error bars denote standard error).</p

Results of Experiments 2.

<p>The mean accuracy (A), CDA waveforms (B), and averaged CDA amplitudes of the tested time window (C) for the exposure time of 100 ms and 500 ms. Error bars in Fig. 4A and 4C denote standard error. The CDA is a difference wave, constructed by subtracting the ipsilateral from the contralateral activity according to the cued hemifield. *indicates the difference between the two conditions was significant; whereas <i>n.s.</i> indicates the difference between the two conditions was non-significant. Grey areas of the CDA waveforms denote the tested time window.</p

The complex and simple shapes used in Experiment 1.

<p>The stimuli in Category 4 were from ref. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057913#pone.0057913-Alvarez1" target="_blank">[8]</a>, and the other complex stimuli were new.</p

Example of a change trial in the left hemifield in Experiment 1.

<p>Example of a change trial in the left hemifield in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057913#s2" target="_blank">Experiment 1</a>.</p

Results of Experiments 1.

<p>The mean accuracy (A), CDA waveforms (B), and averaged CDA amplitudes of the tested time window (C) for the simple shape change, cross-category change and within-category change. Error bars in Fig. 3A, and 3C denote standard error. The CDA is a difference wave, constructed by subtracting the ipsilateral from the contralateral activity according to the cued hemifield. *indicates the difference between the two conditions was significant; whereas <i>n.s.</i> indicates the difference between the two conditions was non-significant. Grey areas of the CDA waveforms denote the tested time window.</p

Results of Experiments 3.

<p>The mean accuracy (A), CDA waveforms (B), and averaged CDA amplitudes of the tested time window (C) for remembering the blur objects. Error bars in Fig. 7A and 7C denote standard error. The CDA is a difference wave, constructed by subtracting the ipsilateral from the contralateral activity according to the cued hemifield. *indicates the difference between the two conditions was significant; whereas <i>n.s.</i> indicates the difference between the two conditions was non-significant. Grey areas of the CDA waveforms denote the tested time window.</p