10/03/2010 by admin.

Here is the abstract of a paper (here) in PNAS, Phase-dependent neuronal coding of objects in short-term memory, by M. Siegel and others:

The ability to hold multiple objects in memory is fundamental to intelligent behavior, but its neural basis remains poorly understood. It has been suggested that multiple items may be held in memory by oscillatory activity across neuronal populations, but yet there is little direct evidence. Here, we show that neuronal information about two objects held in short-term memory is enhanced at specific phases of underlying oscillatory population activity. We recorded neuronal activity from the prefrontal cortices of monkeys remembering two visual objects over a brief interval. We found that during this memory interval prefrontal population activity was rhythmically synchronized at frequencies around 32 and 3 Hz and that spikes carried the most information about the memorized objects at specific phases. Further, according to their order of presentation, optimal encoding of the first presented object was significantly earlier in the 32 Hz cycle than that for the second object. Our results suggest that oscillatory neuronal synchronization mediates a phase-dependent coding of memorized objects in the prefrontal cortex. Encoding at distinct phases may play a role for disambiguating information about multiple objects in short-term memory.

Does this bring us any closer to understanding short-term memory? I think so, but it is not clear exactly how.

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24/01/2010 by admin.

ScienceDaily had an article on the research of B. Strowbridge and P. Larimer. (here) Their ‘first’ is to create stimulus-specific sustained activity patterns in brain circuits maintained in vitro using pieces of rodent hippocampus – memory in a petri dish.

Mossy cells are unusual because they maintain much of their normal activity even when kept alive in thin brain slices. The spontaneous electrical activity found in mossy cells was critical to their discovery of memory traces in this brain region.

When stimulating electrodes were inserted in the hippocampal brain slice the spontaneous activity in the mossy cells remembered which electrode had been activated. The memory in vitro lasted about 10 seconds, about as long as many types of working memories studied in people.

“This is the first time anyone has stored information in spontaneously active pieces of mammalian brain tissue. It is probably not a coincidence that we were able to show this memory effect in the hippocampus, the brain region most associated with human memory,” said Strowbridge.

The scientists measured the frequency of synaptic inputs onto the mossy cells to determine whether or not the hippocampus had retained memory…They also found the brain circuit that enabled the hippocampus to remember which input pathway had been activated. The memory effect occurred because of a rare type of brain cell called semilunar granule cells, described in 1893 by the father of neuroscience, Ramón y Cajal. The semilunar granule cells have an unusual form of persistent activity, allowing them to maintain memory and connect to the mossy cells.

Working memory is intimately involved with consciousness.

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16/12/2009 by admin.

Edge has a speech by S. Dehaene, Signatures of Consciousness. Here is a small part of it on the possible usefulness of consciousness. The whole speech is worth reading (here).

In several experiments, we have contrasted directly what you can do subliminally and what you can only do consciously. Our results suggest that one very important difference is the time duration over which you can hold on to information. If information is subliminal, it enters the system, creates a temporary activation, but quickly dies out. It does so in the space of about one second, a little bit more perhaps depending on the experiments, but it dies out very fast anyway. …. When you are conscious of information, however, you can hold on to it essentially for as long as you wish. It is now in your working memory, and is now meta-stable. The claim is that conscious information is reverberating in your brain, and this reverberating state includes a self-stabilizing loop that keeps the information stable over a long duration. Think of repeating a telephone number. If you stop attending to it, you lose it. But as long as you attend to it, you can keep it in mind.

Our model proposes that this is really one of the main functions of consciousness: to provide an internal space where you can perform thought experiments, as it were, in an isolated way, detached from the external world. You can select a stimulus that comes from the outside world, and then lock it into this internal global workspace. You may stop other inputs from getting in, and play with this mental representation in your mind for as long as you wish.

In fact, what we need is a sort of gate mechanism that decides which stimulus may enter, and which stimuli are to be blocked, because they are not relevant to current thoughts. There may be additional complications in this architecture, but you get the idea: a network that begins to regulate itself, only occasionally letting inputs enter.

It would be interesting to know how Dehaene separates the contents of consciousness from focus of attention. I do think that one of the reasons for consciousness has to do with being able to remember in episodic memory things that are in consciousness.

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04/12/2009 by admin.

JR Minkel has an interesting piece in the Scientific American site (here). He is commenting on the work of W. Zhang and S. Luck who have found that working memory does not fade away but disappears suddenly.

When you go from bed to bathroom on a dark night, a quick flick of the lights will leave a lingering impression on your mind’s eye. For decades evidence suggested that such visual working memories—which, even in daylight, connect the dots to create a complete scene as the eyes dart around rapidly—fade gradually over the span of several seconds. … (Zhang & Luck) tested subjects’ recall for the hues of colored squares flashed briefly on a screen up to 10 seconds earlier. Subjects marked their answer on a color wheel. If memories decay gradually, the guesses should have become increasingly imprecise as time wore on …. Instead subjects went straight from fairly accurate answers to random choices—no better than chance—indicating the memories were decaying all at once. According to Zhang and Luck’s mathematical analysis, most subjects’ memories went “poof” somewhere between four and 10 seconds after the stimulus. …Researchers say a sudden die-off is to be expected if working memories are stored in circuits that feed back on themselves.

I think that working memory and consciousness are related but that it is not clear what the detail of the relationship is. Clearly our conscious perceptions have more resolution, detail and accuracy then our later memories. Is this because of the detail in the working memory?

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08/05/2009 by admin.

A paper has been published by a large multidisciplinary group from Sweden and Spain, using computer simulations and fMRI scans to look at working memory. The research is reported in Science Daily (here) and (here).

The working memory, which is our ability to retain and process information over time, is essential to most cognitive processes, such as thinking, language and planning. It has long been known that the working memory is subject to limitations, as we can only manage to “juggle” a certain number of mnemonic items at any one time. Functional magnetic resonance imagery (fMRI) has also revealed that the frontal and parietal lobes are activated when a sequence of two pictures is to be retained briefly in the visual working memory. However, just how the nerve cells work together to handle this task has remained a mystery…

For their project, the researchers used techniques from different scientific fields, applying them to previously known data on how nerve cells and their synapses function biochemically and electrophysiologically. They then developed, using mathematical tools, a form of virtual or computer simulated model brain. The computations carried out with this “model brain” were tested using fMRI experiments, which allowed the researchers to confirm that the computations genuinely gave answers to the questions they asked.

With their model brain, the team was able to discover why the working memory is only capable of retaining between two and seven different pictures simultaneously. As the working memory load rises, the active neurons in the parietal lobe increasingly inhibit the activity of surrounding cells. The inhibition of the inter-neuronal impulses eventually becomes so strong that it prevents the storage of additional visual input, although it can be partly offset through the greater stimulation of the frontal lobes. This leads the researchers to suggest in their article that the frontal lobes might be able to regulate the memory capacity of the parietal lobes…This finding was also replicable in follow-up experiments on humans.

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11/04/2009 by admin.

In a recent ScienceDaily item (here) there is a report on a paper, Short-Term Memory for Figure-Ground Organization in the Visual Cortex, by P. O’Herron and R. von der Heydt. The problem they looked at was the stability of vision.

“As we look at the world around us, images flicker into our brains like so many disparate pixels on a computer screen that change every time our eyes move, which is several times a second. Yet we don’t perceive the world as a constantly flashing computer display.”
“…Recent studies have hotly debated whether the visual system uses a buffer to store image information and if so, the duration of that storage,…We found that the answer is ‘yes,’ the brain in fact stores the last image seen for up to two seconds.”
“The image that the brain grabs and holds onto momentarily is not detailed; it’s more like a rough sketch of the layout of objects in the scene. This may elucidate, at least in part, how the brain creates for us a stable visual world when the information coming in through our eyes changes at a rapid-fire pace: up to four times in a single second.”
“The study was based on recordings of activity in nerve cells in the V2 region of the brains of macaques, whose visual systems closely resemble that of humans…. discovering memory in this region was quite a surprise because the usual understanding is that neurons in the visual cortex simply respond to visual stimulation, but do not have a memory of their own.”

This fits with the idea that there are two levels of conscious images: a very detailed image of two or three aspects of the scene which changes often and a much, much less detailed image of the whole visual field that changes far less often.  In combining these two levels, we have the feeling (illusion) of seeing the whole visual field in detail all at once. What happens for vision is probably also happening in the other senses.

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15/03/2009 by admin.

An item in Science Daily, Echoes Discovered in Early Visual Brain Areas Play Role in Working Memory, talks about how we keep things in mind when they are no longer in sight (here).  

Vanderbilt University researchers have discovered that early visual areas, long believed to play no role in higher cognitive functions such as memory, retain information previously hidden from brain studies… The results were published Feb. 18 online by Nature.

“We discovered that early visual areas play an important role in visual working memory,” Frank Tong… said. “How do people maintain an active representation of what they have just seen moments ago? This has long been a conundrum in the literature.

“Before, we knew that early visual areas of the cerebral cortex that are the first to receive visual information were exquisitely tuned to process incoming visual signals from the eye, but not to store this information,” Tong said. “We also knew that the higher-order brain areas responsible for memory lack the visual sensitivity of early brain areas, but somehow people are able to remember a visual pattern with remarkable precision for many seconds, actually, for as long as they keep thinking about that pattern. Our question was, where is this precise information being stored in the brain?

“Using a new technique to analyze fMRI data, we’ve found that the fine-scale activity patterns in early visual areas reveal a trace or something like an echo of the stimulus that the person is actively retaining, even though the overall activity in these areas is really weak after the stimulus is removed,” Tong continued.

“Visual cortex has always been thought to be more stimulus driven and has not been implicated in cognitive processes such as memory or active maintenance of information,” Stephenie Harrison, lead author of the research … said. “By using a neural decoding technique, we were able to read out what people were holding in their visual memory. We believe this sustained visual information could be useful when people must perform complex visual tasks in everyday life.”

Research subjects were shown two examples of simple striped patterns at different orientations. They were then told to hold either one or the other of the orientations in their mind while being scanned using fMRI. … By analyzing responses over several trials, we were able to accurately read out which of the two orientation patterns a subject was holding in his or her mind over 80 percent of the time.”

The researchers found that these predictions held true even when the overall level of activity in these visual areas was very weak, no different than looking at a blank screen. This suggests that the act of remembering an image leaves some sort of faint echo or trace in these brain areas. These activity traces are weak but are quite detailed and rich in information.

“By doing these pattern analyses, we were able to find information that was hidden before. We do not know for sure, but it’s possible that a lot of information in the brain might be hidden in such activity patterns,” Tong said. “Using this decoding technique and others, neuroscientists might get a better understanding of how the brain represents specific cognitive states involving memory, reminiscing, or other visual experiences that do not obviously lead to a huge amount of activity in the visual areas.”

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13/02/2009 by admin.

Memories can be divided into two types: implicit and explicit. Only the explicit memories require conscious attention when they are formed and produce conscious memories when they are retrieved. Implicit memories are formed without our awareness and either not retrieved in the normal sense or retrieved as ‘guesses’. Implicit memories may come in various types: a procedural memory like the skill of riding a bicycle; a perceptual memory that allows us to guess at something we have perceived when we are primed with a small part of the perception given in the same sensory modality as the original encounter; and finally, emotional conditioning.

Perceptual implicit memory and explicit memory differ in a number of ways:

1.      They are different types of events in the brain. Implicit storage occurs 200-450 msec. after an event with a negative-going potential in the centroparietal region. Explicit storage occurs 900-1200 msec. after an event with a positive-going potential in the right frontal region.

2.      Damage to the hippocampus interferes with explicit but not implicit memory.

3.      Explicit memory retrieval is accompanied by feelings of remembering, recognition or familiarity and these feeling are absent when implicit memories are retrieved.

4.      Implicit memory is not affected by the depth of processing of the original event whereas explicit memory is stronger if the original event is attended to in more depth.

5.      Implicit memories can not be experienced or described in language. Explicit memories are experienced consciously, can be reported and are holistic with additional aspects that were not part of the retrieval specification.

6.      Implicit memories using priming can be more accurate then explicit memories. However implicit memories are more prone to the illusion-of-truth type of errors.

Explicit memory is semantic or episodic. The picture appears clear that in order to have a conscious memory, the original experience must have been part of consciousness. The formation of these memories probably is an important function of consciousness.

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10/02/2009 by admin.

A recent item in Science Daily (here) discusses work on how the hippocampus time-stamps memories.  

“Ironically, Gage and his team had not set out to explain how the brain stores temporal information. Instead they were interested in why adult brains continually spawn new brain cells in the dentate gyrus, the entryway to the hippocampus. The hippocampus, a small seahorse-shaped area of the brain, distributes memory to appropriate storage sections in the brain after readying the information for efficient recall… Each of these newborn neurons undergoes a prolonged maturation process, during which it changes from hyper-excitable to composed and reaches out to mature brain cells that are already well-connected within the established circuitry. Exercise, learning, and environmental enrichment increase proliferation and survival of new neurons, while pathological (chronic) stress and age send their numbers plummeting. Despite an increasing understanding of how new neurons become part of the existing dentate gyrus network, it is still unclear what their exact function is… It quickly became clear that overly excitable youngsters respond indiscriminately to incoming information… But nothing lasts forever. Even the most highly strung nerve cells that used to get excited by just about anything will eventually quiet down. As they mature into fully functional granule cells, they take their place in the existing circuitry while the next generation of newborn neurons takes their place firing away at new events.

Yet, independent events that had nothing in common but the fact that they occurred around the same time will now be connected forever in our minds—explaining why discussing the movie we saw a couple of months ago might bring back the name of the café we visited afterward but whose name has been eluding us.

“Current thinking holds that when we bring up a certain memory, it passes back to the dentate gyrus, which pulls all related bits of information from their offsite storage,” says Gage. “Our hypothesis suggests that cells that were easily excitable bystanders when the memory was formed are engaged as well, providing a hyperlink between all events that happened during their hyperactive youth.”

This would be the source of the life-long narrative that is our ‘life’.

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07/02/2009 by admin.

I try very hard to be open to ideas rather than fix on a good one to the exclusion of others. Most people try to do this to a certain extent, of course. I am not saying that I am particularly good at it even. Nor is an open mind always the most effective tool – sometimes people who are in stubborn, blind opposition to one another’s ideas can get further, faster.

But there are ideas that I find it hard to keep at arm’s length. One is that consciousness is the leading edge of memory. It is what becomes, in a fraction of a second, sensory memory, then working memory, then short-term memory, then long-term memories that become progressively more consolidated.

How do we know we have been unconscious? We know because there is a discontinuity in our memory. I was standing and now I am on the floor. It was evening and now it is morning. I was in the house and now am in the garden. I was watching the news and now I’m watching a movie. What happened? I must have lost consciousness – fainted or slept or something else but I definitely was not conscious.

What is the difference between my conscious experience and my memory of an experience? Not much seems different. There is the same feeling of space and time. There is the same feeling of self. There are the same colors and movement and sound. The only difference seems to be that the real thing is more vivid and compelling than the memory. Also the one seems to carry a label saying ‘now’ and the other a label saying ‘then’.

How do you make sure you are going to remember something? When we want to improve our memory we focus our conscious attention on the thing we want to remember. Also, we keep returning the thing to remember to our consciousness. We say a phone number over and over to ourselves. We look again at someone’s face and remember their name to try and make the connection firm. The route to memory is conscious attention.

If we understood more about memory than perhaps we would understand more about consciousness. 

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