Neural Limits to Representing Objects Still within View

Abstract

Visual working memory is an online workspace for temporarily representing visual information from the environment. The two most prevalent empirical characteristics of working memory are that it is supported by sustained neural activity over a delay period and it has a severely limited capacity for representing multiple items simultaneously. Traditionally, such delay activity and capacity limits have been considered to be exclusive for maintaining information about objects that are no longer visible to the observers. Here, by contrast, we provide both neurophysiological and psychophysical evidence that the sustained neural activity and capacity limits for items that are continuously visible to the human observer are indistinguishable from those measured for items that are no longer visible. This holds true even when the observers know that the objects will not disappear from the visual field. These results demonstrate that our explicit representation of objects that are still “in view” is far more limited than previously assumed.

What limits working memory capacity?

“…The resource hypothesis correctly predicts that the set-size effect is observed even at a negligible RI (A2: Tsubomi et al, 2013;Sewell et al, 2014): The competition for resources takes place as soon as a memory set is encoded and does not change while that set needs to be maintained.…”

“…Much of the evidence we have reviewed above, however, speaks against decay having a primary role in limiting WM capacity. For verbal memoranda the evidence is against decay playing a role in determining retention over the short term; for visual and spatial memoranda decay might play a role, but is unlikely to determine the capacity limit, because the rate of forgetting that could be attributed to decay is too slow to explain the severe capacity limit observed at RIs of just one or two seconds, or even in the absence of any RI (Oberauer et al, 2003;Tsubomi et al, 2013). …”

“…The set-size effect in visual WM cannot be explained as reflecting the race between decay and restoration, because the typical limitation of visual WM to about 2-3 objects has been found even at a retention interval of zero (A2). For instance, Tsubomi et al (2013) presented participants with arrays of a variable number of colored squares for one second, immediately followed by a single bi-colored square in the location of one of the squares in the memory array. The bi-colored square served as a visual mask and as the response probe: Participants had to decide which of the two colors of the bi-colored square matched the color previously seen in the same location in the memory array.…”

“…Second, the measured capacity for visual stimuli is the same immediately after encoding -before any decay could have happened -as it is after a 1 s delay (Tsubomi et al, 2013). Third, some of the most valid tasks for measuring WM capacity involve no retention interval (Chuderski, 2014;Oberauer et al, 2003).…”

The Role of Local and Large-Scale Neuronal Synchronization in Human Cognition

“…Both visual WM and attention have a limited capacity between two and five objects (Cowan 2001;Vogel 1997, 2013;Zhang and Luck 2008), although some studies suggest that this capacity is dependent on the complexity of objects (Xu and Chun 2006;Xu and Chun 2009). Several theoretical frameworks have been advanced to explain the systems-level neuronal mechanisms determining the individual capacity limitations of these functions as well as on the intertwined relationship of attention and WM (Chun 2011;Cowan 2001;Drew et al 2012;Luck and Vogel 2013;Tsubomi et al 2013). We have investigated the role of local and large-scale synchronization in these distinct functions.…”