Monitoring and VIsual Working Memory

Prefrontal cortex is known to be important for working memory processes, but there is debate surrounding whether it is important for the online maintenance of information, the online monitoring of information, or both. Some theories hold that prefrontal cortex (and dorsolateral PFC in particular) are important for the "executive" aspects of working memory, such as monitoring, evaluation, or retrieval/maintenance strategies, as opposed to maintenance itself.

A 2000 J Neurosci paper speaks to this point. Michael Petrides gave monkeys a visual working memory task in which either the delay between display and test or the number of items was varied. These monkeys either had lesioned mid-dorsolateral prefrontal (DLPFC) or anterior inferotemporal (aIT) regions, the latter being a part of the brain that is known to be involved in object processing and could potentially be the neural locus of the active maintenance of visual objects.

The author found a double dissociation between the effects of DLPFC and aIT damage, such that aIT damage affected the robustness of memory accuracy to increased delay, whereas medial DLPFC damage affected the robustness of memory accuracy to increasing set sizes. According to Petrides, this evidence suggests that aIT is more critical for maintenance, whereas medial DLPFC is more critical for the monitoring of multiple stimuli and responses. (Although Petrides makes claims that DLPFC representations are symbolic, the evidence presented here really can't speak to issues of representational format.)

One idea that may tie all of this evidence together is that representations become more abstract in more anterior regions, and that maintenance of a representation in an anterior region is used to select among representations in more posterior regions. According to this perspective, executive functions are accomplished by maintenance within more anterior regions, and the types of executive functions required in this task may involve deciding which of the items in each set size to attend to, or they may involve selectively biasing each of these items' representations so that they are maximally different from one another. Consistent with much previous work, this kind of activity would occur in dlPFC. In contrast, a more posterior region (such as aIT) might contain the object or item representations biased by more anterior regions; activity here should be particularly important for robustness across delays, since this is where the actual item information per se is being maintained.

Several previous posts have discussed the anatomy that is relevant to visual short term memory. For example, Xu and Chun determined that inferior intraparietal sulcus activity is minimally sensitive to set size (pretty much regardless of object complexity), while superior intraparietal sulcus activitiy is sensitive to set size only for simple objects. Likewise, Vogel & Machizawa found parietal and lateral occipital areas were indicative of visual WM span (though these were ERP waves, and could certainly have their source in other regions of the brain).

On the other hand, at least one study has shown that differences in span may be accounted for by differences in "selection efficiency." This function certains suggests a strong role for dlPFC, which is known to be involved in processes like selection, and overcoming interference. And studies mentioned in this post, using TMS, found that disruption of neural activity in right & left PFC in humans resulted in lower visual recognition memory.

Related Posts:
Multiple Capacity Limitations for Visual Working Memory


Anonymous Anonymous said...

I just want you to know that all of your talk about the prefrontal cortex and working memory has been fascinating, and has ultimately lead to me deciding to start my own experiment... I've been training my PFC via digit span tasks every day for the last month with fascinating results. I plan on doing this until I reach some outrageous reverse digit span (i.e. 20) to see what happens. Kind of a longterm non-doubleblind experiment.

- Dan (synovexh --@-- gmail.com)

8/11/2006 10:33:00 AM  
Blogger Chris Chatham said...

Sounds very interesting! I know that it is definitely possible to increase the forward and backwards digit span to very high levels. But a lot of this ability may come from learning to organize numbers differently - ideally, you would probably want to exercise this "re-organization" or chunking ability not just for number, but for words too. To this end, take a look at the "word span" and "operation span" measures also.

In theory, if you practice a wide enough variety of these tasks for long enough, you might develop better "chunking" abilities overall, which could be very very useful!

You should chart your span measures over time - week by week at least, or even day by day, to keep an eye on your own progress.

Good luck, and do stop in every now and then to let me know how it's going!

8/11/2006 01:56:00 PM  
Anonymous Anonymous said...

The biggest thing I've noticed is I tend to *see* the numbers more clearly when I visualize them. My actual goal with the whole practice is mostly to exercise my attention control. I want to be able to get "in the zone" more easily in hopes to supress social anxiety. I've also seen some studies that correlate low digit spans with aggression, depression, and anxiety... So it makes sense that expanding your digit span would effectually remedy those problems.

I've attempted to chart my progress in the past, and the software I use (brainbuilder.com) happens to keep track but for the most part I try to disregard my actual span altogether. I find it distracting. Instead, I try to focus on it simply being "exercise" to eliminate goal-oriented frustration.

I'm up to about 12 reverse from an original ~9.

- Dan

8/11/2006 06:05:00 PM  
Blogger Chris Chatham said...

Wow. 9 reverse is a big number to begin with, and 12 is almost unheard of, in naive populations.

i am not aware of the research you mention, but I am not completely surprised. given your observation of increased "visualization" abilities, you ought to see if you get better visual working memory span, in addition to better digit span, as a result of this practice. that would be quite something. good luck, once again!

8/11/2006 07:24:00 PM  
Anonymous Anonymous said...

How would I test my visual working memory span? There is a function in the same software (brainbuilder) to use letters instead of digits.

8/12/2006 02:59:00 AM  
Blogger Chloe said...

So Cool ~ :D

8/12/2006 07:34:00 PM  
Blogger Jerng said...

This is a great site. My own research was actually in something related: I'm interested in a systematic understanding of all possible human experience. I've been quite successful at developing a framework which allows for the analysis and complete quantification of the above, but I don't think I'll spend too much time thinking about it between now and retirement (whereupon I'd like to go back to school and stay there :). So nowadays, I just give tutorials in 'philosophy' and teach a sort of mental yoga. Drop me a line if you'd like to chat or something. http://cyberpunk101.blogspot.com

8/13/2006 09:11:00 AM  
Anonymous Anonymous said...

That is rather interesting

8/13/2006 03:43:00 PM  
Blogger Mariesaintmichel said...

You know, Chris, I have been delving on human intelligence behavior and aspects for long years departing from an inquisitive philosophic perspective. I am deeply interested in all that concerns intelligence and I'll be reading your excellent blog and your links from now on. When you want to relax, you can go to my blog and get a much humbler and simpler perspective :0). I have just started putting some insights there, and soon I'll be dealing with some very controversial topics. Your opinion is more than welcome.

8/13/2006 05:33:00 PM  
Blogger Chris Chatham said...

THanks guys!@

To Dan: to test visual WM span you'd want something that can't be verbalized - but if you've got only brain builder at your disposal, you can probably try the letter/digit tasks but use articulatory suppression. in other words, do the same task, but repeat "coca cola" out loud, 2 times per second, as you do the task. This way, your "phonological loop" will be occupied by uninformative information, and you will have to rely on visual processing in order to do well on the span tasks.

8/13/2006 09:41:00 PM  
Anonymous Anonymous said...

I've actually researched quite a bit regarding digit-span, both visual and auditory. Digit-span appears to be the most fundamental of cognitive processes, directly linked to conceptualizing information while manipulating separate pieces into a correct sequential order within the mind. Like a filtered channel of learning ability, improving this "decoding" process does indeed greatly improve overall learning, as proof of my own personal experience.
Upon learning the importance and vast effects towards strengthening and expanding visual and auditory digit-span, I've began exercising both about 3 months ago. Evaluating forward and reverse digit-span, I initially began at a span of 8 digits. I would never consider this of any extraordinary level, but continued to exercise daily, evaluating my gains each day. I considered only 100% completion of either 6/8 or 8/10 runs as actual achievement of a level gained in order to maintain consistent and accurate evaluations towards general inconsistency we all share day-to-day. Today, roughly 3 months later, I've now reached solid ground at a span of 16 digits, forward and backwards, visual and auditory, and on average, 8/10 times. With such gain, I can vouch vast and notable improvements included in almost every cognitive task - mental acuity, mental agility, transition of information to long-term memory, computing/calculating efficiency, and most of all, articulate and eloquent speech under most circumstances. I feel that many have great latent and intellectual potential, but before any of this can flourish, a fundamental skill must establish a strong foundation. Any intelligent mind with an average digit-span is like being taught everything from a second-language only scarcely picked up on. Just as in any physical sport, without an emphasis of a strong, lean, and flexible body, all skills, techniques, and experience are limited.
When I train my digit-span, I'm weary of the disadvantages chunking provides, inhibiting most as a common habit. As a result, I make sure I treat each series of digits as separate pieces of information.
As a side note, I also notice brainbuilder.com displays forward digits all at once, while backwards one at a time. If you go to the memory gym at memorise.org, you can select either flashing numbers or spoken numbers, and each digit displays one at a time, with a selection of different time intervals (I prefer 1.5 seconds). Digits displayed one at a time appear serve greater challenge rather than shown all at once, as well as providing greater exercise on visualizing and conceptualizing abilities.

10/13/2006 02:11:00 PM  
Blogger Chris Chatham said...

I think that WM capacity limits are pretty much "set in stone," and that what you are in fact doing is practicing chunking. I'm not sure what you meant by "inhibiting" but what's wrong with chunking? This seems to me to be a very good strategy for making functional increases in short term memory.

1/13/2007 08:10:00 AM  

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