Multiple Capacity Limitations for Visual Working Memory

A fundamental debate concerns whether memory capacity limitations may be due to some central bottleneck, or whether there is an array of lower-level limitations that give rise to the simple "span" measures, or to the magic number "seven plus or minus two," frequently observed in behavioral experiments.

One aspect of this debate concerns whether a central bottleneck might be attention - that is to say, according to this perspective memory span is limited by how many things you can pay attention to. This debate has largely been resolved, thanks in part to a paper discussed last week, showing that both types of limitations have a role to play, such that task specific bandwidth limitations reduce memory capacity above and beyond the (substantial) role played by attentional limitations.

[In retrospect, it would have been incredibly surprising if memory capacity was determined solely by a single bottleneck. Even if the utility of memory capacity for ensuring survival plateaus at around 4 items, thus rendering natural selection incapable of selecting for a higher capacity limit, it seems unlikely that the cause of any limit would be centralized, simply because of the brain's massively parallel architecture. Instead, it seems much more likely that the capacity limitations arise as an emergent property of the type of information being processed and the architecture of the dynamic system - between the the "energy" and "medium" one might say - as in the picture accompanying this article, where the topology of the water funnel is dependent both on the viscosity of the water and the amount of rotational energy.]

If we agree that memory bandwidth is not limited by a single cause, the next step is to map and define the architectural features of the brain that give rise to each capacity limitations. In the March issue of Nature, Xu & Chun describe their work to dissociate the contributions of inferior and superior intraparietal sulcus to visual short term memory, based on a visual short-term memory task in which both set size and object complexity was manipulated. In the process, they discover independent capacity limits for each region.

However, some research has reported that capacity limits are lower if the complexity of the items is increased (although there is also evidence to the contrary). To further investigate whether VSTM capacity limits are due to item complexity, number of items, or both, Xu and Chun had subjects attempt to detect a change in the shape of 1 item from a display of 1, 2, 3, 4, or 6 "simple" or "complex" objects, while inside an fMRI magnet. Based on Cowan's K, the authors were able to estimate the visual span of each subject as a function of both their "hit rate" and "correct rejection" rate in detecting shape changes, and correlate these behavioral results with neural activity.

The authors found that VSTM capacity was around 4 for the simple shapes, and around 2 for the complex shape. Interestingly, superior intraparietal sulcus (as well as lateral occipital) activity closely tracked behavioral performance for the simple task alone, such that the neural activity in these regions increased with set size for simple shape features, but not for complex shape features. In contrast, inferior intraparietal sulcus activity only tracked neural activity from 1 to 2 objects, regardless of complexity. A subsequent experiment with different visual stimuli replicated the essential trends of this result, which is not surprising, given that these findings are basically the "fMRI version" of a study by Vogel and Machizawa with EEG.

Based on a final experiment, in which the authors controlled the location of visual stimuli during the display, the authors argue that inferior IPS representations are primarily spatial, while those in superior IPS combine spatial representations with the object identity information that appears to be maintained in the lateral occipital complex. Thus, based on Xu and Chun's work, it appears that there are independent capacity limits for stimulus complexity and for stimulus location, which become subsequently bound together in more superior (& anterior, presumably) regions.

And yet, a new study by Olsson and Poom in the Proceedings of the National Academy of Sciences suggests that memory span is as low as one (yes, 1) in the absence of long-term memory support. Could long term memory be yet another source of capacity limitation in working memory, paradoxical though it may seem? Tomorrow's post will further explore this issue and the implications that this surprising finding has for our understanding of memory architecture.

Related Posts:
Memory Bandwidth and Interference
Visualizing Working Memory
Neuroindices of Memory Capacity
Functional Anatomy of Visual Short Term Memory