3/11/2006

Visualizing Working Memory

Shiffrin's modal model of memory includes three parts: a brief sensory store through which we experience the outside world, a working memory component with rehearsal processes, and a long-term store from which we can retrieve information into working memory. (If you haven't seen it before, please view the classic model before reading on.) Although hugely influential, the model has a few unfortunate shortcomings.

Most importantly, memory is not a static process. Memory decays unless it follows the cycle of learning, encoding, recall, and recoding. In contrast, the classic "modal" model distinguishes storage functions from processing functions - i.e., one is a "black box" and one is a "black arrow." Unfortunately, this distinction is artificial, because the brain contains only arrows: processing (activation) and memory (synaptic efficacy) are two sides of the same coin, and memory itself is a process.

Second, all arrows are not created equal. "Architectural" differences (such as the synaptic efficacy of recurrent connections) among different parts of the brain lead to different processing profiles. These profiles can differ in terms of bandwidth, or in other words, how much information they are capable of holding at a time. Orthogonal to bandwidth, these profiles can also differ in terms of decay rate (or alternately, vulnerability to interference): that is, some regions are capable of maintaining information indefinitely while others are capable of maintaining memories for less than one second.

Third, multiple processing paths require gating functions. Perhaps the largest advantage of removing the "boxes" from the traditional "box and arrow" diagram is that one must become explicit about the functions performed at every step. As a result, such diagrams will be more likely to provide a nice one-to-one relationship between cognitive functions and neuroanatomy. And pure-arrow diagrams remove the need for some elusive "executive functioning" box to coordinate and configure the processes; instead, one need only posit a gating function at the intersection of every arrow with another, in which some process allows information to pass along from one arrow to the next (probably based on dopamine fluctuations).

Accordingly, my new visualization of the modal model uses only arrows, and two simple visual metaphors. First, those arrows with a wider base represent increased bandwidth or initial capacity. Second, the change of the width of each arrow as it moves through space represents the rate of decay. Finally, at the intersection of each arrow, a simple gating function allows memory to pass through from one or more of the receiving paths. In the rest of this post, I'll narrate the flow of information through this structure.

Sensory Memory. As you can see from the image at the start of the article, incoming sensory information has an extremely high initial bandwidth, and a rapid rate of decay, as indicated by the fat, short arrows. Furthermore, there is no rehearsal process, and sensory memory is constantly overwriting itself. This is indicated visually by the multiple arrows which overlap, and quickly diminish in width.

Working Memory. From sensory memory, perceptual data feeds into (or is "sampled" by) a relatively low-capacity - and low-decay - polymodal short-term/working memory process, as indicated by convergence of sensory inputs into an arrow of smaller width in the middle-lower region of the diagram. Importantly, the width of this arrow does not shrink over time, indicating that the rate of decay is extremely low; indeed, we can hold somewhere between 4 and 7 items in working memory (depending on modality) without decay. This short-term/working memory process feeds two unique rehearsal processes (each is robust to decay and yet limited in bandwidth).

These rehearsal "loops" are capable of refreshing short-term/working memory with previous information. The phonological loop is capable of maintaining verbal information, and the "inner scribe" supports imagery functions (such as mental rotation). Oscillatory cycling of information through these loops allows it to be maintained over time, and short-term memory capacity is determined by the "cycle speed," or the amount of information that can be successively cycled through each loop.

Long Term Memory. Finally, the entire diagram rests above the base of one massive arrow that folds back on itself: this represents long-term memory. Long-term memory has an essentially unlimited capacity, as indicated by this arrow's enormous base width, but is subject to decay and interference over time. Items can be retrieved from long-term memory, at which point a "gating function" permits us to actively think about the past (i.e., rehearse it), or to merely let it recur and disappear again from our "mind's eye." Note that:

  • No arrow leads directly to long-term memory; one can only rehearse items and hope that this rehearsal process gives the memory long enough to seep into long-term memory.
  • All processing beyond the sensory memory rests "on top" of long term memory representations. This illustrates an important aspect of working memory: everything we consciously perceive is placed within the context of those things we remember happening previously.

In conclusion, I hope that this model provides an intuitive way of visualizing how synchronized oscillations within cortical circuits may play a role in cognitive functionl, but please be sure to visit this post at Zero Brane for an alternative view on working memory.

Related Posts:

Working Memory: A Black Box? [Zero Brane]
Models of Active Maintenance of Oscillation
Functional Anatomy of Visual Short Term Memory
An Informal Integration of Object Recognition Models
Perceptual Sampling: The Wagon Wheel Illusion

PS, If you liked this, don't forget to digg it.

3 Comments:

Anonymous Paul Kulchenko said...

Chris, great post (as always ;)). I posted some notes on an alternative model proposed by O'Reilly and his collaborators on my blog (http://notebook.kulchenko.com/intelligence/working-memory-black-box).

Paul.

3/14/2006 12:20:00 PM  
Blogger Chris Chatham said...

Hi Paul - thanks for stopping in - I noticed your post and I agree with you, Randy's model is a lot closer to how things really work. He teaches at my school and actually passed that paper to me, but I haven't gotten a chance to read it yet.

3/14/2006 12:33:00 PM  
Blogger Chris Chatham said...

BTW I don't know why your post isn't showing up in the links below. That may be a "blogspot-only" function, and since you use a different service, I guess you're being excluded.

3/14/2006 12:39:00 PM  

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