Academic journal article Psychonomic Bulletin & Review

Encoding Strategy and Not Visual Working Memory Capacity Correlates with Intelligence

Academic journal article Psychonomic Bulletin & Review

Encoding Strategy and Not Visual Working Memory Capacity Correlates with Intelligence

Article excerpt

There is conflicting evidence on whether the capacity of visual working memory (VWM) reflects a central capacity limit that also influences intelligence. We propose that encoding strategy and, more specifically, attentional selection, underlie the correlation of some VWM tasks and IQ, and not variations in VWM itself. In Experiment 1, change detection measures of VWM were found to be contaminated by some cognitive process that depressed performance at higher set sizes, so that fewer items were remembered when eight rather than just four were presented. Measuring VWM using whole report instead gave a less variable estimate that was higher, particularly for larger set sizes. Nonverbal IQ did not correlate with this estimate of VWM capacity, but instead with the additional factor that contaminates change detection estimates. We propose that this phenomenon reflects a lack of selection during encoding. In Experiment 2, we investigated the role of rehearsal using articulatory suppression and showed that this could not account for the key differences between the procedures.

There is intense debate about the relationship between visual working memory (VWM) and intelligence, with conflicting evidence on whether or not VWM capacity reflects a central limit that is a bottleneck in higher order cognition. A procedure commonly used to measure VWM is change detection (CD; Luck & Vogel, 1997; Phillips, 1974). In a typical experiment, a memorandum comprising a number of colored squares is presented briefly, in order to minimize chunking or the piecemeal transfer of items into phonological working memory. This is followed by a delay period that is sufficiently long (around 1 sec) to ensure the decay of sensory representations responsible for iconic memory, and then by a probe display. The participant must determine whether the items in the probe have the same color and position as those in the sample. When measured in this way, VWM has been found to predict fluid intelligence and educational achievement in adults and children (Cowan et al., 2005), as well as top-down control assessed using electroencephalography (EEG) (Vogel, McCollough, & Machizawa, 2005), suggesting that it taps into a central capacity limit.

In contrast with this view is evidence from other studies that the capacity of VWM and measures of top-down control vary independently across individuals. Like in the classic studies by Sperling (1960), both Finke et al. (2005) and Peers et al. (2005) used a whole report (WR) paradigm to measure VWM. The memoranda were a set of letters presented briefly. Participants were asked to orally report all of the letters they could remember. This measure of VWM was compared with a measure of the efficiency of top-down control, which was derived from the ability to report target letters from among distractors of a different color (see Bundesen, 1990). It was concluded that VWM was independent of the efficiency of top-down control, across both a group of healthy controls (Finke et al., 2005) and a group of patients with parietal and frontal damage (Peers et al., 2005).

What might be the cause of these discrepant results? We focus on two possible explanations. The first is the different memoranda used in the VWM tasks: color and position (Cowan et al., 2005; Vogel et al., 2005) versus letters (Finke et al., 2005; Peers et al., 2005). A second is the different experimental procedures used: CD (Cowan et al., 2005; Vogel et al., 2005) versus WR (Finke et al., 2005; Peers et al., 2005).

Experiment 1

Method

The influence of procedure and memorandum type on estimates of VWM, and their relationship to intelligence and top-down control, were investigated. Forty-eight participants were tested (16 male, from the ages of 19 to 63; mean age 47). Approval for the experiment was given by the Cambridge Psychological Research Ethics Committee.

VWM was measured with a single WR and two CD procedures that were closely matched. …

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