LAMM Manuscript

Hayim Dar, Paul Miller

AbstractAbstract

Introduction

The existence, and nature, of a distinction between short and long term memory has been debated for over a century (James 1890, Jonides 2008). Associative buffer models, such as Raaijmakers and Shiffrin’s Search of Associative Memory (SAM, Raaijmakers 1981), present computational implementations of the highly influential theory of separate but interacting short and long-term memory processes (Atkinson 1968, Baddeley 1974).

The features of SAM illustrate the canonical dual-storage algorithm: Items to be recalled are entered into a temporary storage buffer, replacing prior items if the buffer is full, and therein form episodic associations with other buffer items, and perhaps also a context marker. Those associations then serve as cues for consecutive recalls, producing many of the familiar dynamics of list recall performance (Davelaar 2005).

The Linking via Active Maintenance Model (LAMM) aims to explore the biological feasibility of associative buffer memory theories by implementing them within simulated neural circuitry. Additionally, it advances the debate between dual- and single-memory models (Brown 2002, Howard 2002, Cowan 2008) by arguing that, when implemented neuronally, associative buffer networks incorporate temporal context dynamics **A KAHANA CITATION FOR TEMPORAL CONTEXT? (Usher 2008). Finally, LAMM’s architecture is based upon prior work showing that buffer-mediated associations are necessary to explain the effects of degraded stimuli on recall (Piquado 2010, Miller 2010, Cousins 2014).

Word list recall

  • Is an experimental paradigm with long history as a model system for short term memory and rich behavioural phenomena.

Recall for sequentially presented lists of items has long been a key experimental paradigm for revealing hidden structure in memory dynamics. Dependencies on list-position, presentation timing, temporal contiguity, effects of rehearsal, intra- and inter-list interference, categorical and acoustic relatedness, and scaling laws have all been observed (Murdock 1962, Rundus 1971, Kahana 1996, Golomb 2008, Grenfell-Essam 2012, Cowan 2008a, Farrell 2011) (more!). Disruptions and distractions have been used to probe the stages and modalities of memory encoding (Carroll 2010, Elliott 1998, Spataro 2013, Cowan 2008) including the finding of a retroactive effect indicating that encoding into short term memory continues beyond the removal of the stimulus (Rabbitt 1968, Cousins 2014).

The canonical result in free-recall experiments is the 'U'-shape performance curve for recall of items as a function of presentation order: the so-called serial position effects. Recall is stronger for both early items (denoted 'primacy'), and late items ('recency'), than for middle list items. Further, shorter lists seem to favour primacy over recency, and vice versa. Distractor tasks just before recall reduce recency but nor primacy, whereas distraction throughout both presentation and recall restore somewhat the concavity of the serial-position curve (Howard 1999). On the other hand, subjects tend to overtly rehearse early list items much more than late ones; and whilst recall increases with rehearsal, late items are recalled much more easily than early ones for a given proportion of allocated rehearsal time (Rundus 1971). It thus appears that different mechanisms may support the recall of early vs late items (but see discussions in Sederberg 2008, Usher 2008).

  • several models proposed, notably differential memory for late list items key support for the idea of the short term memory buffer.

A key point of disagreement within the literature -- and entwined in the early- vs late-items dissociation mentioned above -- is whether memory for recently perceived list items is computed by the same processes that encode and retrieve general long term memory (LTM) (the 'unitary view'), or by dedicated, temporally limited 'short-term' processes (the 'dual-store' or 'dual-process' view) (Raaijmakers 1981a, Davelaar 2005, Sederberg 2008). Generally, unitary models have relied on representations of distinctiveness between items to be remembered, which contain some invariance to timsecales (Brown 2002, Howard 2002), whilst dual-process models have posited a separate, short-term, actively maintained or reverberatory storage, or 'buffer' (Baddeley 1974, Raaijmakers 1981, Davelaar 2005) (more!). More recently, structural similarities have been pointed out between actual computational implementations of the different theories (Usher 2008), and modeling has suggested that both distinctiveness and buffer mechanisms may be necessary (Piquado 2010, Miller 2010, Cousins 2014). In most cases such short-term processes are assumed to contribute most strongly to memory for only very recently perceived stimuli, such as the ends of a word list, so that theoretical disagreement focuses on how these 'recent' items are remembered. On the other hand, few models provide an integrated account of the primacy effect, and even fewer still a biological one (Lansner 2013, Farrell 2012), and our model is guilty of this shortcoming. We argue that primacy should be a focus of future work in this area.

Memory theories, summary of trends

  • Buffers (active maintenance) vs similarity (interference)

The debate regarding the mechanisms operating in tasks of list recall is one front in the larger debate about short-term memory in general: a debate that contrasts buffer maintenance with attention-driven theories. Buffer theories posit that the most recently perceived stimuli are maintained in short term memory buffers via active mechanisms (Baddeley 1974, Baddeley 2010, Davelaar 2005, Grossberg 1978, Bradski 1994). This view has won empirical support, especially from electrophysiological recordings from behaving primates (Funahashi 1989, Fuster 1971, Fuster 1973, Miyashita 1988, Miyashita 1988a), coalescing into a 'standard model' of short term memory (Goldman-Rakic 1987, Goldman-Rakic 1990, Courtney 2004, Postle 2006).

Attention-driven theories propose that short-term memory is simply the process of temporarily re-activating, or 'attending' to, memory representations stored in LTM (Cowan 1993, Cowan 2008) (more?). Such theories reject Baddeley's (Baddeley 1974) physical separation of short- from long-term processing, and instead propose a functional separation only, consistent with Atkinson & Shiffrin's (Atkinson 1968) original proposal, and, in fact, the Hebbian idea of 'transient memory' (Hebb 1949). This approach avoids the undesirable requirement of duplicating representations of all stored knowledge in dedicated buffer networks organised by information type and stimulus modality, as implied in Baddeley's compartmental memory model (Baddeley 2010). Multiple LTM memories can instead be reactivated at the same time within their existing LTM media, requiring no duplicate networks. Only a subset of such reactivated memories are considered in the 'focus' of attention as generally understood, allowing for gradations in the level of focus and active maintenance (Cowan 2008): In the case of word list recall, the representation of the current word would be reactivated most strongly and be 'in focus', with less recent words fading to baseline.

In this way (Cowan 2008) leaves open the possibility for both decay of active maintenance and interference between stored representations as mechanisms of forgetting, which is a further point of conflict between buffer and attentional theories (Jonides 2008). In buffer theories recall fails when buffer activity decays. Attentional theories, by contrast, traditionally operate on memory encoding schemes in which memories which are most different, either in content, context, or time-of-perception, suffer least mutual interference from other encoded memories, and hence have greater recall (Hopfield 1982, Nairne 1997, Brown 2002, Brown 2007, Howard 2002). The Temporal Context Model (TCM) of (Howard 2002, Sederberg 2008) binds memories to an evolving 'current context' representation, which, by its nature, also includes the content of the memory itself. TCM is one of the most successful short term memory models, but in constructing their context representat