Episodic Working Memory

Visual events in the world have a rich episodic structure that must be encoded into working memory.  In a dynamic visual scene, such as when driving a vehicle, objects frequently emerge from occlusion or disappear.  Both the order and the duration of a sequence of such events carry important information.  Even during search in a static scene, the saccadic nature of fixational eye movements creates brief patterns of input to the visual system, and it is important to encode these objects as distinct representations. 

Experiments involving rapidly presented objects using rapid serial visual presentation (RSVP) provide valuable clues as to what tricks the visual system uses to create temporally accurate representations.  For example, if two instances of the same item are presented within a sequence of items, subjects often see only a single instance of the repeated item, a phenomenon known as repetition blindness (Kanwisher 1991, Mozer 1989). 

A similar (though distinct and dissociable) error is found if subjects are encoding two target items; subjects will frequently report only the first of the two items.  This deficit is known as the attentional blink (Raymond Shapiro & Arnell 1992). 

The attentional blink (AB) is a particularly odd phenomenon because only if the two targets are separated by about 200-400 milliseconds is the second item blinked.  If the two items are closer (~100 milliseconds), subjects frequently report them both!  More surprising is the catalog of findings which show how the attentional blink can be attenuated or even switched off.   The blink is reduced if subjects are distracted (Olivers & Nieuwenhuis 2006), perceive motion (Arend, Johnston, & Shapiro 2006), or is visually cued (Nieuwenstein, van del Lubbe, & Hooge 2005).  The blink seems entirely absent if subjects encode a whole string of consecutive items (Olivers, Van Der Stigchel & Hulleman 2007; Kawahara, Kumada,  & Di Lollo 2006, Nieuwenstein & Potter 2006).  

These results make it difficult to interpret the AB as the result of a processing limitation.   Rather, this attentional shutdown hints at a strategic attentional mechanism designed to enhance the episodic separation between target items that are presented separately.  This point is elaborated in recent modelling work (STST;   Bowman & Wyble 2007,  eSTST;  Wyble, Bowman & Nieuwenstein, In Press).

Another curious finding in visual perception, repetition blindness, also informs this modelling effort.  RB occurs as the result of a circuit that avoids encoding spurious instances of a single item.  In effect, the model is designed to err on the side of caution; it avoids encoding one item as two copies, at the expense of sometimes missing an actual repetition.