Coherence and recurrency: maintenance, control and integration in working memory

Working memory (WM), including a ‘central executive’, is used to guide behavior by internal goals or intentions. We suggest that WM is best described as a set of three interdependent functions which are implemented in the prefrontal cortex (PFC). These functions are maintenance, control of attention and integration. A model for the maintenance function is presented, and we will argue that this model can be extended to incorporate the other functions as well. Maintenance is the capacity to briefly maintain information in the absence of corresponding input, and even in the face of distracting information. We will argue that maintenance is based on recurrent loops between PFC and posterior parts of the brain, and probably within PFC as well. In these loops information can be held temporarily in an active form. We show that a model based on these structural ideas is capable of maintaining a limited number of neural patterns. Not the size, but the coherence of patterns (i.e., a chunking principle based on synchronous firing of interconnected cell assemblies) determines the maintenance capacity. A mechanism that optimizes coherent pattern segregation, also poses a limit to the number of assemblies (about four) that can concurrently reverberate. Top-down attentional control (in perception, action and memory retrieval) can be modelled by the modulation and re-entry of top-down information to posterior parts of the brain. Hierarchically organized modules in PFC create the possibility for information integration. We argue that large-scale multimodal integration of information creates an ‘episodic buffer’, and may even suffice for implementing a central executive

A cortical mechanism for binding in visual working memory

Luck and Vogel (1997) showed that the storage capacity of visual working memory is about four objects and that this capacity does not depend on the number of features making up the objects. Thus, visual working memory seems to process integrated objects rather than individual features, just as verbal working memory handles higher-order "chunks" instead of individual features or letters. In this article, we present a model based on synchronization and desynchronization of reverberatory neural assemblies, which can parsimoniously account for both the limited capacity of visual working memory, and for the temporary binding of multiple assemblies into a single pattern. A critical capacity of about three to four independent patterns showed up in our simulations, consistent with the results of Luck and Vogel. The same desynchronizing mechanism optimizing phase segregation between assemblies coding for separate features or multifeature objects poses a limit to the number of oscillatory reverberations. We show how retention of multiple features as visual chunks (feature conjunctions or objects) in terms of synchronized reverberatory assemblies may be achieved with and without long-term memory guidance

Interference in implicit memory caused by processing of interpolated material

This study addresses the susceptibility of implicit memory to interference. Interference is manipulated by presenting interpolated lists of words that do or do not have word stems in common with previously studied target words (e.g., target word paragraph followed by interpolated words such as paradise or vicinity). Interference in a word stem completion task occurred only when words had similar word stems (Experiment 1). Increasing the number of interpolated words with corresponding word stems (e.g., not only paradise but also parking, pardon, and parliament) produced increasing amounts of interference (Experiment 2). Interference in implicit memory appears to be a simple response competition phenomenon that occurs when cues simultaneously activate primed targets and primed competing responses. The amount of interference can be explained by a quantitative model of the relative strengths of target and competing responses