Examining the neural basis of visual working memory
How is visual working memory (VWM) instantiated in the brain? Previous studies have demonstrated that there are several brainwave correlates of VWM, but we may ask why there are several correlates and not just one. One possibility is that these multiple correlates are simply different measures of the same thing. For instance, you can measure how ‘big’ a TV is by measuring the length of the diagonal line from corner to corner (e.g., 50” TV), or you could calculate the surface area of the TV. Although these two measures are numerically different, they are correlated and reflect the same physical property (i.e., size) of how ‘big’ the TV is.
Alternatively, these multiple correlates of VWM could instead be indicators of dissociable aspects of VWM. For example, to assess how ‘good’ a TV is, one can assess its screen size and types of input/output (e.g., HDMI, DVI etc.). Unlike the measures of TV size, these factors reflect different aspects of TV quality and therefore they are not necessarily related to one another.
Our recent work has shown that two of these multiple correlates are dissociable, and therefore are not different reflections of the same aspect of VWM (Fukuda, Mance, & Vogel, 2015; Fukuda, Kang, & Woodman, 2016). This suggests that VWM is not instantiated by a single neural process, but rather by multiple neural processes that are dissociable.
What do these dissociable correlates of VWM reflect? We demonstrated that the dissociation has to do with the nature of VWM representation (Fukuda, Kang, & Woodman, 2016). One of the measures (the Contralateral Delay Activity (CDA)) reflects lateralized VWM representations, and the other measure (Alpha power suppression) reflects spatially-global VWM representations.
Uncovering the basis of individual differences in visual working memory capacity
How much visual information can we hold in mind at a given time? Despite our intuition, studies show that we can only hold a limited amount of information (i.e., 3 or 4 simple objects worth of information) at a given time (Fukuda, Awh, & Vogel, 2010)! This limitation is called visual working memory (VWM) capacity. Studies also show that individuals vary in this capacity limit; some individuals can maintain 4 or more objects while others can maintain fewer than 2 objects. Why, and how is this the case?
We showed that a major portion of these individual differences arise from people’s ability to control what gets access to their capacity-limited VWM (Fukuda & Vogel, 2009; Fukuda, Woodman, Vogel, 2015). To illustrate this, consider two travellers with the same suitcase going to a ski resort. One traveler is very good at selecting what to take to the trip, so her suitcase is packed with necessary items to enjoy the trip (e.g., ski jacket and pants, camera, and gloves). The other traveler, however, is not very good at choosing what to take, so instead packs items that are not necessary for skiing (e.g., a pair of flip flops from summer vacation) and may not have room for clothes that keep him warm and cozy during skiing… As a result, although they have the same suitcase, the functional storage space for the ski trip is very different between them.
So why did that traveler leave his flip flops in the suitcase? We showed that this is because he is slower at disengaging his attention from task-irrelevant information after his attention is accidentally captured by it (Fukuda & Vogel, 2011). When he opened his suitcase to pack for the ski trip, maybe he found the flip flops left over from his summer vacation. They brought back the fun memories form the trip to the beach, and he indulged in them for a bit, forgetting about what he was doing. As a result, the flip flops were left in the suitcase. What about the other traveler? When she found the flip flops too, and they also brought back the fun memories, but she quickly realized that she was packing for the ski trip instead and was able to remove them from the suitcase. As a result, her suitcase is packed with all she needs to enjoy skiing comfortably!
But does this mean the flip flop packing traveler is always bad at attentional control? No, not always! Most of the time, he can go back on track to what he was doing. This failure of attentional selection seems to occur occasionally (Adam, Mance, Fukuda, & Vogel, 2015), and when given proper feedback, he can reduce the frequency of such a failure (Adam & Vogel, 2016). We did this work with our amazing friends at the Awh Vogel Lab!