Abstract
One of the most fundamental, and most studied, human cognitive functions is working memory. Yet, it is currently unknown how working memory is unified. In other words, why does a healthy human brain have one integrated capacity of working memory, rather than one capacity per visual hemifield, for instance. Thus, healthy subjects can memorize roughly as many items, regardless of whether all items are presented in one hemifield, rather than throughout two visual hemifields. In this current research, we investigated two patients in whom either most, or the entire, corpus callosum has been cut to alleviate otherwise untreatable epilepsy. Crucially, in both patients the anterior parts connecting the frontal and most of the parietal cortices, are entirely removed. This is essential, since it is often posited that working memory resides in these areas of the cortex. We found that despite the lack of direct connections between the frontal cortices in these patients, working memory capacity is similar regardless of whether stimuli are all presented in one visual hemifield or across two visual hemifields. This indicates that in the absence of the anterior parts of the corpus callosum working memory remains unified. Moreover, it is important to note that memory performance was not similar across visual fields. In fact, capacity was higher when items appeared in the left visual hemifield than when they appeared in the right visual hemifield. Visual information in the left hemifield is processed by the right hemisphere and vice versa. Therefore, this indicates that visual working memory is not symmetric, with the right hemisphere having a superior visual working memory. Nonetheless, a (subcortical) bottleneck apparently causes visual working memory to be integrated, such that capacity does not increase when items are presented in two, rather than one, visual hemifield.
Highlights
Working memory is a key cognitive process in humans
The main result is that in all of the experiments the bilateral capacity was significantly lower than the sum of the capacities in the unilateral conditions, and in 4 of the 5 experiments this difference was significant (Exp.1: p = 0.006, Exp.2: p < 0.001, Exp.3: p < 0.001, Exp.4: p = 0.033, and Exp.5: p = 0.089)
On average, across Experiments 1–3, the bilateral capacity was slightly higher than the average unilateral capacity (i.e., 12.9% higher), whereas across Experiments 4 and 5, the bilateral capacity was 22.7% higher than average unilateral capacity. Note that these results are similar to what is found in healthy adults, in whom bilateral working memory capacity is slightly higher in the bilateral than the unilateral conditions but far from being the sum of the capacities per hemifield [12,13]
Summary
Working memory is a key cognitive process in humans. It is needed to achieve goals, such as navigating through traffic, understanding situations, and producing coherent language. When it comes to visual working memory the location of the neural basis is unclear. Some studies suggest that it resides mainly in the frontal cortical areas [1,2], whereas other research suggests that occipital areas are the primary source [3,4]. It is unclear what unifies visual working memory. If each hemisphere would have its own, independent, memory storage, items that are presented across the visual field capacity should be the sum of the capacity when items are presented in the left and the right hemifield—we will elaborate on this in more detail later on
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