29/07/2010 by admin.

Memories move through stages when they are formed: they are ‘encoded’ or some other process of being prepared (perhaps working memory), they are held in an early stage (short term memory), changes occur in synapses in both the hippocampus and cortex forming a somewhat stable memory from minutes to hours later (synaptic consolidation), the memories are processed so that they are less dependent on hippocampus and more on the cortex from days to years after (system consolidation). Consolidated memories are fairly stable – but- when they are recalled into consciousness, they can be modified. This seems to be the main way in which memories are re-consolidated (or changed) over time. Passing a memory through consciousness means it is re-stored, either unchanged or updated depending on circumstances. Much of sleep is busy with the consolidation and re-consolidation of memories. This is a simplified outline of the current science on memory and it is important to realize that more is unknown than known.

An interesting item in BPS research digest (here) looks at experiments with a treatment for traumatic memories, EMDR, eye movement desensitization and reprocessing.

A controversial treatment for post-traumatic stress disorder involves the traumatised person holding a painful memory in mind while simultaneously following with their eyes the horizontal movements of their therapist’s finger… Raymond Gunter and Glen Bodner have tested three possible explanations…

(first) relative to staring straight ahead, eye-movements increased arousal levels. (undermining) the idea that eye movements activate an innate investigatory reflex that inhibits fear and provokes relaxation.

A second experiment showed that both horizontal and vertical eye movements reduced the vividness and emotionality of the students’ memories. (undermining) the idea that horizontal eye movements aid interhemispheric communication, thus allowing the more rational left hemisphere to process the right hemisphere’s traumatic memories.

(third) experiment showed that the students’ memories became less vivid and emotional, not only when they performed concurrent horizontal eye movements, but also if they instead performed a simultaneous simple hearing task. This undermines the idea that EMDR works specifically by taxing the so-called “visuo-spatial sketch-pad” of working memory. It suggests instead that the mechanism underlying EMDR is a more general effect based on taxing the big boss of short-term memory - the central executive.

…performing a concurrent task, be it eye movements or some other distraction, while also recalling a painful memory, allows a person to be exposed to that memory, without having the mental resources available to get too upset by it. Over time, this process acts like a form of gentle exposure to the memory, as the person learns that they can, after all, cope with their past.

This seems a clear case of a memory being forced to change during re-consolidation – by passing through consciousness under conditions that modify the memory.

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29/06/2010 by admin.

I knew a man once that only really thought he understood the meaning of a concept if he knew it through history, fiction or anecdotal narrative. I would not have credited such a way of understanding the world except for knowing him. Some people really ‘get’ algebra and they feel they really know something if they can describe it mathematically. Other people need their explanations to be graphic: an illustration, a map or a diagram. Still others need ideas to be verbal in order to easily grasp them. My friend needed a dramatic plot or a parable to have that feeling of understanding. Words were not enough; there had to be a plot. Most of us can use many or even all those vehicles to understanding, maybe more ways that I have never noticed. We use different ways to understand different things.

Most people I know understand their own lives in narrative form. G. Strawson argues that this is not true of all of us (here). The article starts with this abstract:

I argue against two popular claims. The first is a descriptive, empirical thesis about the nature of ordinary human experience: ‘each of us constructs and lives a “narrative” . . . this narrative is us, our identities’ (Oliver Sacks); ‘self is a perpetually rewritten story . . . in the end, we become the autobiographical narratives by which we “tell about” our lives’ (Jerry Bruner); ‘we are all virtuoso novelists. . . . We try to make all of our material cohere into a single good story. And that story is our autobiography. The chief fictional character . . . of that autobiography is one’s self’ (Dan Dennett). The second is a normative, ethical claim: we ought to live our lives narratively, or as a story; a ‘basic condition of making sense of ourselves is that we grasp our lives in a narrative’ and have an understanding of our lives ‘as an unfolding story’ (Charles Taylor). A person ‘creates his identity [only] by forming an autobiographical narrative – a story of his life’, and must be in possession of a full and ‘explicit narrative [of his life] to develop fully as a person’ (Marya Schechtman).

Strawson identifies four characteristics of this sort of life story narrative. First, the person has a diachronic rather than episodic viewpoint, or in other words, they live in a time continuum rather than in the present. Second, the person has to have a tendency to search for a unifying or form-finding construction. Third, the person uses story-telling conventions. Fourth, the person continues to revise the story.

Like episodic and verbal type memory, narrating a our life seems to me to require that the elements come from conscious experience. If we look at this from the view point of learning in general and learning about ourselves in particular than the reason for narrative seems clear.

1. We construct and remember experience in the form of memories of moments of consciousness. The memory has the setting, the on-going action and especially important things such as elements that are surprising or significant. To learn from experience, we must have experience.

2. We remember moments of consciousness in a time ordered sequence, making a little episode of the episodic memory. We can attach meaning to such episodes by associating them with cause and effect links. Causality allows us to use our experiences to understand and predict the world around us.

3. As memories get older, they are consolidated into larger and larger units. Many nearly identical episodes became one. All the trips to work in a first job, long ago, become one memory. The trip to the store yesterday is still an individual memory. It seems that memory is reworked in light of our present knowledge and interests over and over again.

4. If we want to tell someone about something that happened, we put these memories in words and those words than become associated with the memory. Often we talk to ourselves about a memory and by doing so make it a narrative to be remembered, partly in a narrative form.

Thus moments of experience become meaningful episodes which become summaries of life and finally become narratives.

Strawson believes that personal narrative is not a universal human characteristic and he may be right. But one thing we know is that long, long ago humans learned to make tools, to harass fire and use language - then, well fed, they sat around the fire and told stories into the night.

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11/06/2010 by admin.

Neurophilosophy has a posting on embodiment (here) that looks to a motor action/emotional memory link using K. Dijkstra’s work among others.

These results show that bodily movements can influence the rate at which autobiographical memories are recalled as well as the emotional content of the memories. The results of the first experiment demostrate that what we do with our bodies can affect how we think - memory recollection was more efficient when the direction of movement was congruent with the valency of the emotional content of the memory. The second experiment further demonstrates, for the first time, that meaningless bodily movements can also influence what we choose to think about, with upwards movements being associated with positive memories and downward movements with negative ones.

It is well known that memory recollection is facilitated when the context in which recollection occurs matches that in which encoding took place. Classical studies of context-dependent retrieval focused on aspects of the environment in which memory encoding and retrieval take place, and Dijkstra extended this to show that context also includes body posture. The new findings show that movements which are completely unrelated to the encoding of emotional memories can also influence their retrieval. They add to a growing body of evidence that supports the embodied cognition hypothesis; specifically, they provide evidence that thinking involves creating mental simulations of bodily experiences, and that knowledge is represented by partial re-enactments in the brain which activate the same systems associated with real experiences.

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30/05/2010 by admin.

The blog Neurophilosophy has a review of the connection between space and time, and between direction and past or future. (here) It is worth reading in full as I am only talking about a very little part of the review.

We know that imagining a future event is dependent on memory, because patients with amnesia cannot imagine new experiences. It involves piecing together fragments of past experiences to generate a plausible simulation of what might happen.

It seems that memory and imagination/planning use the same basic mental processes. There is the same framework for consciousness of now, of memories and of ‘what if’ scenes. Consciousness is used to create memory, memory is used to produce future imaginings and short term projections are use to produce the consciousness of now. Only what is found in consciousness is stored in episodic memory. We know that stimulations of the future and imaginings are constructed from memory scrapes. And our conscious experience is a slight prediction. The difference between our current consciousness and recalled memories or imaginings seems to be in the amount of detail and the vividness of the experience. I presume that both the detail and the vividness is due to the availability of the actual sensory data in the sensory cortex areas that is only there for a few moments.

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29/03/2010 by admin.

ScienceDaily reports on the dissertation of Kristina Kompus at Umea University on the entirely different signal paths for spontaneous (just happened to remember) and deliberate (try to remember) activation of memories. (here)

…these two different ways of remembering things are initiated by entirely different signal paths in the brain. Efforts to retrieve a specific memory are dealt with by the upper part of the frontal lobe. This area of the brain is activated not only in connection with memory-related efforts but also in all types of mental efforts and intentions, according to the dissertation. This part of the brain is not involved in the beginning of the process of unintentionally remembering something as a response to external stimuli. Instead, such memories are activated by specific signals from other parts of the brain, namely those that deal with perceived stimuli like smells, pictures, and words….memories do not need to be emotionally charged to be revived spontaneously, unintentionally. Nor do memories that are revived spontaneously activate the brain more than other memories…The studies also reveal that our long-term memory is more flexible that was previously believed. There is not just one single neurological signaling path for reliving old memories but rather several paths that are anatomically separate. …The dissertation uses a combination of two imaging methods for the brain: functional magnetic resonance imaging (fMRI) and electroencephalography (EEG). The methods yield different information about the function of the brain. By combining them, Kristiina Kompus has been able both to determine what part of the brain is activated and how the activation proceeds over extremely brief time intervals, on the order of milliseconds.

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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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