10/27/2006

Shared Mechanisms for Long and Short-term Memory: An Episodic Buffer?

In the new issue of Trends in Cognitive Sciences, Hasselmo & Stern explore the possible mechanisms involved in the short-term maintenance of novel information.

Many such "short-term maintenance" tasks involve familiar stimuli, such as digits in the case of digit span. This has led some to propose that most measurements of working memory are artificially inflated, since these tasks rely on stimuli that may also be represented in long-term memory. The implication is that novel stimuli may eliminate the role of long-term memory, and thus result in a more pure measurement of maintenance capacity.

The nearly canonical perspective is that this short term maintenance is enabled by sustained firing in the prefrontal cortex - regardless of whether that sustained activity actually represents the maintained information, or merely acts as a kind of "pointer" to where that information is actually represented in cortex. In both cases, prefrontal activity is assumed to convey an excitatory bias on more posterior regions.

In their article, Hasselmo & Stern take a radically different view. They suggest that while prefrontal cortex may act as a "pointer" for familiar stimuli (which may actually reside in distrbuted representations across sensory cortex), the prefrontal cortex is only partly responsible for maintenance of novel information. In this case, they suggest that medial temporal lobe structures (MTL; in particular the entorhinal and perirhinal cortices) may play a large role in working memory for novel information, because of that structure's ability to rapidly encode and almost arbitrarily bind various sensory representations.

According to this perspective, damage to the MTL should cause a larger performance decrement for tasks with novel stimuli than with familiar stimuli - and this is exactly what is found in lesion studies. PFC is sufficient for normal performance on digit span (even in the absence of the MTL) while it is not sufficient for normal performance on delay-match-to-sample and nonmatch-to-sample tasks with novel stimuli (these are tasks in which one views a stimulus and after a delay must either pick another identical or non-identical stimulus). Likewise, fMRI studies show differential activity in the MTL for WM tasks involving novel and familiar stimuli.

The authors point to fascinating aspects of cellular neurophysiology in the MTL to support this hypothesis. For example, cells in the parahippocampal cortices are able to maintain sustained firing for periods up to 15 minutes in complete isolation - in other words, this activity is not a result of excitation from other neurons or even self-exictatory recurrent connections. This bizarre phenomenon appears to be enabled by something they call the "Alonso current" - also known as afterdepolarization or plateau potential - thought to result from a calcium-sensitive cation current that relies on acetylcholine. Experiments with scopolamine (an acetylcholine antagonist) suggest that acetylcholine activity is necessary for this self-sustained activity.

Interestingly, delay activity in the entorhinal but not perirhinal cortex is resistant to distractors. Some entorhinal neurons also demonstrate "match enhancement" in which the subsequent presentation of a maintained item increases the rate of firing, while others show "repitiation suppression" where the opposite trend occurs.

The authors conclude that MTL delay-period activity could underlie Baddeley's recent proposal for a third component of working memory: the episodic buffer. The episodic buffer is presumed to tie together arbitrary events, such as might be involved in binding tasks. However, this proposal has been met with skepticism, largely because this seemed very unlike the other components of Baddeley's original working memory model (which included the phonological loop and a visuo-spatial store). However, if Hasselmo & Stern are right, the episodic buffer may soon become a more widely-accepted aspect of working memory.

Related Posts:
Working Memory Capacity: 7 +/- 2, around 4, or ... only 1?
EEG Signatures of Successful Memory Encoding
Working Memory and Convulsions
Valid Dimensions of Memory: Strength, Endurance and Capacity?
Multiple Capacity Limitations for Visual Working Memory

2 Comments:

Blogger Paul said...

From my understanding, Baddeley's episodic buffer is largely based upon the findings of a study by Prabhakaran et al. (2000, ref below). In this study, the hypothesis that the frontal cortex (human) is specialised for the maintenance of integrated information in working memory. It was an fMRI imaging study, and used both spatial and verbal cues for the subjects to maintain (in either integrated or unintegrated configurations). The results showed a clear dissociation between the maintenance of verbal and spatial information (right frontal and bilateral superior parietal regions for the latter, and left inferior frontal, left inferior parietal and temporal regions for the former) when presented separately. When the maintenance of integrated information was required, greater activation was observed in the right frontal cortex, particularly the right middle and superior frontal gyri, across all subjects. They concluded that the right prefrontal cortex seems to have modality free representational (or maintenance) capabilities, in contrast to the left prefrontal cortex, and posterior regions, which seemed to be more modality specific. Based on this right prefrontal activation reflecting the nature of the representation to be maintained, they proposed a (what was then) new type of buffer, which enabled the maintenance of integrated information. Baddeley then used this as the basis for the episodic buffer, which provides the previously absent link between long-term memory and the tripartite working memory system.

On Hasselmo & Stern's paper, their suggestion of the involvement of MTL structures in working memory seems reminiscent of O'Reilly et al's biologically based computational model of working memory (in Miyake and Shah's " models of working memory", 1999, for e.g.). In this model, the prefrontal regions allow the active maintenance of relevant (in this context novel) information which may influence 'processing' in other brain regions, while the hippocampus does exactly what Hasselmo and Stern suggest: rapid encoding and arbitrary associations. Is the work/authors linked in any way?

Just some thoughts - sorry for being a little long-winded. I enjoy reading your blog btw, keep up the good work!

Prabhakaran, V. et al (2000), Nature Neuroscience, 3(1), 85-90

10/27/2006 09:12:00 AM  
Blogger Chris Chatham said...

Hi Paul - Good to hear from you! Thanks for the clearly articulated and very interesting comments.

Just to be clear on the who-says-what, Hasselmo and Stern themselves suggest a connections between their findings in the MTL and the episodic buffer. On the other hand, the phrase about "arbitrary bindings" is not one they used, but merely the way I interpreted it - which is definitely strongly influenced by the O'Reilly et al work.

Anyway, pulling from Baddeley's TICS article from right before where he mentioned the Prabhakaran paper: "I would not expect the buffer to have a single unitary anatomical location; given its putative importance, some redundancy would be biologically
useful in making the system more robust." Hasselmo & Stern's results are compatible, in the sense that the buffer may rely on the MTL more in the case of completely novel stimuli, and more on frontal regions for familiar stimuli.

The real reason I thought this paper was cool, however, is that self-sustained neural firing is typically something that screams out "frontal cortex." In this case, that would be inaccurate. Pretty neat!

You have a great blog too, btw; hadn't seen that before. I'll be adding it to my blogroll shortly..

10/27/2006 01:05:00 PM  

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