What is memory for anyway?

It is almost inconceivable that a biological function would be dedicated to the past rather than the future of an organism. The only use for knowledge of the past is to prepare for a ‘good’ future by: learning from past experience, using the past to predict the future, judging choices by past outcomes, imagining possibilities and so on. A lot of research has gone into looking at how well memory records the past. Only a little research seems to ignore that and look at how well memory provides for a successful future. A recent review by D. Schacter (see Citation) looks at the research into the future aspect of memory.

 

The article points to some older research showing that the same core regions of the default network are used for memory of the past and imagining of the future and people with some types of amnesia also have difficulty imagining novel situations. But the body of the article deals with newer research.

Specifically, we have organized the literature with respect to four key points that have emerged from research reported during the past five years: (1) it is important to distinguish between temporal and nontemporal factors when conceptualizing processes involved in remembering the past and imagining the future; (2) despite impressive similarities between remembering the past and imagining the future, theoretically important differences have also emerged; (3) the component processes that comprise the default network supporting memory-based simulations are beginning to be identified; and (4) this network can couple flexibly with other networks to support complex goal-directed simulations. We will conclude by considering briefly several other emerging points that will be important to expand on in future research.

 

There is a very interesting distinction made about time. We have past, present and future; we can imagine various time relationships such as imagining some time in the future from the prospective of looking back at it from even further into the future. But we can also abandon identifying a particular time when we imagine. For example we can simulate what it would be like to be in another’s shoes or what it would be like to be in a different place. Instead of time-traveling, we can space-travel or identity-travel. It seem that the evidence so far implies that future and atemporal imagined events are represented similarly. But there are differences between temporal and atemporal imaginings. I find this distinction very interesting and something I had not really thought about before.

 

Another interesting idea (which I have thought about) is discussed with evidence for it.

The constructive episodic simulation hypothesis states that a critical function of a constructive memory system is to make information available in a flexible manner for simulation of future events. Specifically, the hypothesis holds that past and future events draw on similar information and rely on similar underlying processes, and that the episodic memory system supports the construction of future events by extracting and recombining stored information into a simulation of a novel event. While this adaptive function allows past information to be used flexibly when simulating alternative future scenarios, the flexibility of memory may also result in vulnerability to imagination-induced memory errors, where imaginary events are confused with actual events. … a process of ‘‘scene construction’’ is critically involved in both memory and imagination. Scene construction entails retrieving and integrating perceptual, semantic, and contextual information into a coherent spatial context. Scene construction is held to be more complex than ‘‘simple’’ visual imagery for individual objects because it relies on binding together disparate types of information into a coherent whole.

 

There is much more of interest in this review. If you are interested in memory or simulations or the default network – read the original paper.

 

Here is the abstract:

During the past few years, there has been a dramatic increase in research examining the role of memory in imagination and future thinking. This work has revealed striking similarities between remembering the past and imagining or simulating the future, including the finding that a common brain network underlies both memory and imagination. Here, we discuss a number of key points that have emerged during recent years, focusing in particular on the importance of distinguishing between temporal and non-temporal factors in analyses of memory and imagination, the nature of differences between remembering the past and imagining the future, the identification of component processes that comprise the default network supporting memory-based simulations, and the finding that this network can couple flexibly with other networks to support complex goal-directed simulations. This growing area of research has broadened our conception of memory by highlighting the many ways in which memory supports adaptive functioning.

 

ResearchBlogging.org

Schacter, D., Addis, D., Hassabis, D., Martin, V., Spreng, R., & Szpunar, K. (2012). The Future of Memory: Remembering, Imagining, and the Brain Neuron, 76 (4), 677-694 DOI: 10.1016/j.neuron.2012.11.001

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Correlates of memory

A review article by Suthana and Fried (citation below) has just been published on the nature of medial temporal lobe neurons. It is very informative.

 

The medial temporal lobe (MTL) includes the hippocampus, entorrhinal cortex, perirhinal cortex, parahippochampal cortex, and amygdala. It is associated with memory but not perception.

The ability to form new episodic memories, which can later be consciously accessed, relies on an intact hippocampus and surrounding MTL. However, other functions, such as visual perception, do not depend on an intact MTL . …The MTL receives multimodality sensory input from wide areas of the cortex, thereby offering the possibility of convergence and integration of information. Within the MTL, the hippocampus is positioned at the top of a multisensory hierarchy, receiving converging incoming sensory information through the entorhinal cortex that is either object identifying (via perirhinal cortex) or spatially informative (via parahippocampal cortex). For the MTL to encode a given episode in time properly, the event must be first perceived and processed in upstream sensory cortices.

 

The response of MTL neurons has been recorded and characterized.

(i) Responses are selective. MTL neurons can respond to particular stimulus categories (e.g. faces, outdoor scenes, animals, etc.) or to individual stimuli (e.g. a family member, a famous individual, or a particular landmark) during the passive viewing of visual stimuli.

(ii) Responses are invariant. An MTL neuron may respond to a particular stimulus, such as a particular face or landmark, but in addition it will also respond to other stimuli representing that particular face or landmark, even if these stimuli are distinctly different in stimulus features compared with the original stimulus. Thus, a neuron may respond to a picture of the Sydney Opera House and exhibit no response to 50 other landmarks, yet also respond to many permutations and physically different representations of the Sydney Opera House, seen in color, in black and white, or from different angles. In fact, the neuron may also respond to the iconic representation, namely the words ‘Sydney Opera’, which is obviously different in its visual properties compared with the image of this landmark. Recently, it was shown that this invariance crosses modalities, meaning that MTL neurons may exhibit a selective and ‘invariant’ response to a particular stimulus out of 100 images and do so independently of the sensory modality (visual image, audio, or written iconic representations) through which the stimulus was presented. Results were consistent with the anatomical hierarchy within the MTL; the highest percentage of neurons with modality-independent invariant responses was found in the hippocampus and entorhinal cortex, compared with the amygdala or parahippocampal cortex.

(iii) Responses are late. The selective and invariant responses described above are of relative long latencies, often in the 300–500 ms range. Interestingly and consistent with the anatomical hierarchy, responses in the parahippocampal cortex are significantly shorter than those in the hippocampus, amygdala, and entorhinal cortex, but still with longer latencies than those observed in animal studies.

(iv) Responses are associated with conscious perception. Using flash suppression and backward masking paradigms, it has been shown that selective MTL responses are mainly triggered when accompanied by the participants’ conscious perception or recognition of the stimulus. When varying the display time of an image between 33 and 256 m, MTL neurons selective for an image significantly increase in firing rate only if the image is recognized by the subject, even if the image is presented for as briefly as 33 ms. Conversely, if the subject reported no recognition of the image, the MTL neuron selective for that image is mostly silent.

  1. Responses can be internally generated. The act of re-experiencing a previous episode can be internally generated or cued by an external percept within the environment. Generating an internal percept of a stimulus through imagery in the absence of an external visual stimulus, recruits the same MTL neuron activated during viewing of the stimulus itself. Subjects’ ability to modulate these neurons was studied … Subjects were told to try and control which of two competing images would be projected on an external display; the displayed images were controlled by the firing rate of recorded MTL neurons selective for the two images. Subjects were able to control successfully which image was projected by altering the firing rate of two independently selective neurons independent of the visual input of the stimulus on the screen. These results highlight the power of internal representation to override sensory input. They also illustrate how human single neuron recordings can illuminate mechanisms of conscious perception, regardless of whether they are externally or internally generated.

 

This does appear to be the sort of response that could give us episodic memory.

Why are these responses present in the MTL, and in the hippocampus and entorhinal cortex in particular? Given the critical role of these regions in episodic memory, we posit that these responses are central in the transformation of novel stimuli to representations that can be later consciously retrieved as episodic memories. As such, these representations need to have detail but also abstraction (i.e. the loss of detail), so that they can be later summoned by internal as well as external cues. It is also possible that consciously perceived familiar stimuli trigger the recollection of an associated memory and, thus, reactivate MTL neurons. … Although patients with damage to the MTL can no longer form new episodic memories, their visual perceptual function remains intact. The correlation of single neuron responses in the MTL with specific conscious percepts does not imply that these regions are necessary for conscious percepts, yet these responses may reflect the link between conscious percepts and episodic memories that can be later

consciously accessed. … Strikingly, hippocampal and entorhinal neurons that were selectively active during the initial viewing of distinct episodes were similarly reactivated just before the verbal report of recall of those very same episodes. Although sustained selective responses during viewing of episodes was present in neurons of other brain regions, such as amygdala and frontal cortex, the specific reactivation before reported recall was only present in the hippocampus and entorhinal cortex.

 

Interestingly the synchrony of waves may imply a that memory and consciousness are working together.

Human intracranial studies have shown that the spiking rate of single hippocampal neurons predicts whether a recently learned item will be remembered. In addition, intracranial recording of local field potentials (LFPs) have yielded important insights. Theta LFP oscillations (3–8 Hz) have been widely implicated in human memory and the strength of their amplitude measured in the MTL has been shown to predict the success of episodic encoding in humans. Interestingly, the theta amplitude that predicted recall success was also strongly linked to the gamma oscillation (30– 100 Hz). The phase of theta oscillations and their relationship to gamma oscillations in monkeys and humans have been related to memory performance. … It has been hypothesized that the theta- spiking relation may reflect the cued recall of an upcoming item stored in memory. A recent study in humans showed that the relation between spiking and theta during encoding predicted memory success. … A tighter coordination between hippocampal single neuron spiking and the simultaneously recorded theta LFP oscillation during initial viewing of the image predicted the success of the formed memory for that image . These results implicate a direct role for theta-linked spiking activity in episodic memory.

 

ResearchBlogging.org

Nanthia Suthana, & Itzhak Fried (2012). Percepts to recollections: insights from single neuron recordings in the human brain Trends in Cognitive Sciences, 16 (8) DOI: 10.1016/j.tics.2012.06.006

News on working memory

Glia cells are the Cinderella’s of neuroscience – mostly ignored but sometimes making it, to be the hit of the party. In the cortex, the neurons live in a sea of glia cells. Lately there is another hint that they have important functions, an item in the Scientific American (here) says they are important to working memory. It may also explain stoned-type-logic-thinking.

To study how marijuana impairs working memory, Giovanni Marsicano of the University of Bordeaux in France and his colleagues removed cannabinoid receptors—proteins that respond to marijuana’s psychoactive ingredient THC—from neurons in mice. These mice, it turned out, were just as forgetful as regular mice when given THC: they were equally poor at memorizing the position of a hidden platform in a water pool. When the receptors were removed from astrocytes, however, the mice could find the platform just fine while on THC. … this is one of the first studies to suggest that glia play a key role in conscious thought. “It’s very likely that astrocytes have many more functions than we thought,” Marsicano says. “Certainly their role in cognition is now being revealed.”

Memory associations

ScienceDaily has an item (here) on a U of Pennsylvania press release about decoded neural patterns of memory recall. The research was done by M. Kahana, J. Manning and others. They took advantage of the opportunity presented by epileptic patients with implanted electrodes allowing experimentation on their brains during the wait for surgery. Recordings were made from the electrodes while the patients studied 15 words and than repeated them in any order.

The researchers examined the brain recordings as the participants studied each word to home in on signals in the participant’ brains that reflected the meanings of the words. About a second before the participants recalled each word, these same “meaning signals” that were identified during the study phase were spontaneously reactivated in the participants’ brains.

Because the participants were not seeing, hearing or speaking any words at the times these patterns were reactivated, the researchers could be sure they were observing the neural signatures of the participants’ self-generated, internal thoughts.

The subject’s patterns were individual. But there was a similarity between the how much patterns for different words overlapped and how close words were in order of their recall. The research implies that there is a neural signature in organizing of learned information by meaning.

“In addition to looking at memories organized by time, as in our previous study, or by meaning, as in our current study, one could use our technique to identify neural signatures of how individuals organize learned information according to appearance, size, texture, sound, taste, location or any other measurable property,” Manning said.

Separation of memory and belief

When we watch a show on stage, TV or movie, we do the little trick of suspending disbelief. We do not believe what we are experiencing but we treat the content ‘as if we believed it’ for the duration of the show. We can re-enter that disbelieved experience if we choose, as if it were a memory of something that actually happened. The show can have lasting effects on how we view the world and interact with others. It has all the hallmarks of a really personal experience except that we know it is fictional. Some books and story-tellers have enough power to activate the imagination in this way, even though we do not experience the sight and sound that we would in a show. What is the difference between this sort of memory and what we call false-memory? It is only the believe that the events remembered actually happened to us.

 

According to a recent paper (Clark, Nash, Fincham, Mazzoni – see citation), belief and memory are separate processes. We can have: memories that we believe were events, memories that we do not believe were events, beliefs about events that we do not remember, and events that we neither believe nor remember. So in the same way that sight is not like a camera, hearing is not like a microphone – memory is not like a video recording, and a good thing too, or we would have much less material to think with. But we have to be aware that there are such things as false-memories; they may even be quite common.

 

The authors found that belief is easier to modify than memory. It is easier to create a false belief in a subject that it is to create a false memory. And likewise, it is easier to destroy a false belief than a false memory. You can remain with a memory of an event, long after you have been convinced that the event did not happen and the memory is false.

 

The authors sound a note of caution:

Finally, our findings have broader implications for memory distortion research. To the extent that debriefing might not always completely ‘undo’ the effects of a suggestive manipulation, we might question the ethics of inducing false memories in experimental participants. Is it ethical for participants to leave research labs with remnants of nonbelieved false memory content in the forefront of their minds? A sensible approach to answering this question might be to consider whether the memories would likely be consequential. For example, it is conceivable that a person who ceased believing in a traumatic experience might nevertheless continue to be traumatised by intrusive mental images experienced as memories. We suggest that for most false-memory paradigms and study designs, this is highly unlikely to pose an ethical problem. Nevertheless, how participants might feel about any residual memory content should be an important question for researchers to consider when planning studies.

 

ResearchBlogging.org

Clark, A., Nash, R., Fincham, G., & Mazzoni, G. (2012). Creating Non-Believed Memories for Recent Autobiographical Events PLoS ONE, 7 (3) DOI: 10.1371/journal.pone.0032998

The mind wanders

I once had a very boring task that was part of my research (to do with rayon chemistry not the brain). I have never done anything as boring before or since. I had to time how long it took for 100 drops to fall from a tiny hole. So I had to keep a count without losing my place and I had to see and register each drop. It usually took between 5 and 10 minutes for 100 drops to fall. The problem was that there was no rhythm. There might be 2 mins between a pair of drops or there could be 3 or 4 drops almost touching one another. This meant that my whole attention was taken up with watching and counting. I could not let my mind wander at all. After a run, I was worn out with fatigue and had to recover before I could face another run. I often failed in this simple (mindless) procedure and ended up not knowing where I was in the count or whether I had see the last drops. I was only using a tiny bit of my brain and forcing the rest to sit quietly and not even ‘drum its fingers’. An item in ScienceDaily reminded me of that long ago task. (here)

Levinson, Smallwood and Davidson looked at the relationship between working memory and attention. When the task does not use all of someone’s attention – how do they use their idle resources?

The researchers asked volunteers to perform one of two simple tasks, either pressing a button in response to the appearance of a certain letter on a screen, or simply tapping in time with one’s breath – and compared people’s propensity to drift off… People with higher working memory capacity reported more mind wandering during these simple tasks, though their performance on the test was not compromised…. What this study seems to suggest is that, when circumstances for the task aren’t very difficult, people who have additional working memory resources deploy them to think about things other than what they’re doing.

Interestingly, when people were given a comparably simple task but filled with sensory distractors, the link between working memory and mind wandering disappeared. Giving your full attention to your perceptual experience actually equalized people, as though it cut off mind wandering at the pass….In essence, working memory can help you stay focused, but if your mind starts to wander those resources get misdirected and you can lose track of your goal. Many people have had the experience of arriving at home with no recollection of the actual trip to get there, or of suddenly realizing that they’ve turned several pages in a book without comprehending any of the words. It’s almost like your attention was so absorbed in the mind wandering that there wasn’t any left over to remember your goal to read.

Where your mind wanders may be an indication of underlying priorities being held in your working memory, whether conscious or not. But it doesn’t mean that people with high working memory capacity are doomed to a straying mind. The bottom line is that working memory is a resource. If your priority is to keep attention on task, you can use working memory to do that, too. (But it can be very tiring, I remember.)

Short-term memory capacity

What determines how much short-term memory can hold? ScienceDaily has an item on this (here). The paper is by J.Kaminski etal., Beta band oscillations engagement in human alertness process, in International Journal of Psychophysiology.

A human being can consciously process from five to nine pieces of information simultaneously. During processing these pieces of information remain in the short-term memory. In 1995 researchers from Brandeis University in Waltham suggested that the capacity of short-term memory could depend on two bands of brain’s electric activity: theta and gamma waves. However, only now, through carefully designed experiments conducted at the Nencki Experimental Biology Institute of the Polish Academy of Sciences (Nencki Institute) in Warsaw, it was possible to unambiguously prove that such a relationship really exists…The hypothesis formulated by Lisman and Idiart in 1995 assumes that we are able to memorise as many ‘bites’ of information, as there are gamma cycles for one theta cycle. Research to date provided only indirect support for this hypothesis…Only based on discovered correlations the ratio of the length of theta wave to gamma wave was determined and the likely capacity of verbal short-term memory was determined…Following the EEG recording, the volunteers, were subjected to classic short-term memory capacity test…(They) observed that the longer the theta cycles, the more information ‘bites’ the subject was able to remember; the longer the gamma cycle, the less the subject remembered… correlation turned out to be very high and it confirmed the hypothesis of Lisman and Idiart.

So we can manipulate as many of ‘bits’ of information in short-term memory as we can fit beta waves into a single theta wave. Each ‘bit’ would be carried by a beta wave.

Possible functions of consciousness 4 – place to imagine

People have noted often that we are conscious of our memory recall from episodic and semantic memory and also of imaginings. That is what it means to recall or to imagine – to be aware of the memory or the imagining. It is as if conscious awareness was a structure into which our awareness of ‘now’ or of some memory of past time or even some imagining of future or fictitious time, can be housed. It has been likened to a work space, a theatre, a model world – for now I am going to call it the conscious awareness structure. It is a 3D space centered on ‘here’, ‘now’ and ‘I’ but with the ‘here’, ‘now’ and occasionally the ‘I’ being ‘virtual’ when dealing with recalled memories or imaginings. We seem to be able to tell the difference between our awareness of the three different sources of the experience. It is not that the mechanics of recall or imagining are conscious but that the awareness of the result is in our conscious experience. I suspect that the same structure is used to house dreams.

It may be (and appears to me likely) that the process of recall is not a simple one. When I try to remember events from long ago, I find that they now incorporate changes or additions that make them fit with my current view of the world. I remember the pinkness of a particular dawn and it is placed in a particular farm yard. It is a familiar image from being recalled many times. Then I run across a photograph of the yard in question and I am surprised that my memory is physically impossible – there is no place in that yard to see the perspective I remember. Now when I recall the dawn it is seen corrected from its previous deviation. When we recall, we must fit the memory into our current conscious awareness structure. Throughout our lives, our bank of memories is reworked, consolidated, updated and so on to keep the bank useful and consistent. Some may think this is a poor way to arranged things. It would be better if evolution had produced an indelible, accurate trace of our past. But the purpose of our memories is not accuracy or permanence; it is usefulness because memory gives the elements used to think, solve problems, learn, plan the future, invent, avoid disaster. If I buy a new stove, I still want to use the lessons I learned on my old stove, not re-learn cooking with each change in my kitchen. Old memories have to be made to fit in the current structure of our awareness or they become useless. Little and large bits of memory are part of the stuff of thought.

We may (and again it appears to me likely) use fragments of memory to create imaginings. Stripped of their reference to a particular event, fragments can be furnish the pieces of an image – a tree here and a park bench there. Tulving’s notions of mental time travel and the possible connection of amnesia and inability to imagine are pointers to this sort of use of memory for imagining. These fragments used to form imaginings do not effect the original memory though, because the context of the original memory has to be part of the reworking that can occur with memory recall.

How important is the link between memory and consciousness? David Chalmers postulates the idea of a type of zombie that is indistinguishable from a conscious person but lacks consciousness. He says because we can envisage such zombies, they must be logically possible and therefore….well, many things follow. But it does not seem to me that his idea of zombies can be envisaged. People without consciousness would be very distinguishable from those without. A person without consciousness is likely to have no memory or a very poor one and no imagination or a very poor one. They would be very poor learners and speakers. The fluid easy flow of elements between consciousness and memory is extremely important to how the brain works. The conscious awareness structure is a very important component of the brain. It is well worth its cost in biological terms.

In the next post in the series, I am going to leave the memory connection and deal with other possible functions of consciousness. The reason for examining memory first is that I think it is neglected in discussions of consciousness.

There is more to come. Previous posts in this series:

Possible functions of consciousness 1 – leading edge of memory

Possible functions of consciousness 2 – gate to meaning

Possible functions of consciousness 3 – working memory

Possible functions of consciousness 3 – working memory

As well as the episodic and semantic types of explicit memory featured in the first two parts of this series (and the implicit or procedural memory which seems to have no connection with consciousness), there is another important type of memory – working memory. How working memory actually works is far from a settled question. It may not be one thing – but several separate working memories. The working memory that I am discussing has these characteristics: it has limited capacity of 7 or less items in memory; it holds those items for a limited time, about a second unless replaced by new items; items being held can be manipulated; the items appear to be in consciousness or extremely easily brought to consciousness; they seem to be the focus of attention or extremely easily made so. This is the working memory that is needed for intelligence and correlates with IQ. It is needed for certain types of accurate detailed calculations and similar cognition. It is needed for creating and interpreting complex sentences. It is required for keeping concentration on a task and some other executive functions. Most descriptions of consciousness include working memory as well as a larger less detailed world view (a global ‘gist’) and attention. Avoiding problems with exactly how both consciousness and working memory are defined and bounded, whether one is a part of the other or they are separate mechanisms – let us just consider them as inseparable under normal conditions.

Daniel Kahneman introduced the terminology of System 1 thinking and System 2 thinking. System 1 is unconscious, automatic, very fast while System 2 is conscious, does orderly reasoning, and is very slow. I think that all actual cognition is like System 1, and System 2 only differs by passing through consciousness.

More and more it appears that cognition is primarily an unconscious activity. We are aware of only a very tiny proportion of the cognition our brains do. And even for that tiny proportion we are not aware of the actual cognition but only of the changes it makes in the items held in working memory. If I add 5 to 17, I am aware of: task is to add 5 to 17, add 5 to 7, that is 12, but it is 17 not 7 so add 10 to 12, answer 22. But I have no awareness of how these steps are done – my awareness just jumps from one step to the next. Even given that the cognition itself is not a function of consciousness – we still seem to require consciousness to feed and hold items in working memory so that they can be manipulated. Tasks that are not sequential are difficult to do using working memory and tasks that are sequential are difficult without working memory. When working memory is used, the changes in what is held in the working memory are registered in consciousness.

Working memory (and therefore consciousness) is involved in doing mental arithmetic because we need the explicit semantic memory to retrieve facts like ’5 plus 7 equals 12′ which we have been taught by rote as small children. The same is true of mental logic, games with rules like bridge or chess, where much of the reasoning is sequential and tends to need explicit memory.

Sentences can be understood or created in a way that seems effortless but it does require a certain amount of juggling – holding this word until it is clear which meaning I should understand by it or which ending is grammatical for it and so on. The words and idiomatic phrases are in semantic memory to be retrieved as needed. Much of the juggling is unconsciously done but the results are past through consciousness for all those cases where working memory is involved.

We need working memory to learn motor skills. Once learned they can be done without working memory or even consciousness. But when learning such motor skills sequence and timing are important and it seems that learning the rough motor idea of a sequence takes the sort of manipulation that can be done on items held in working memory. Once the rough motor program is there, it can be honed and smoothed without consciousness or working memory. Soon there is a high skill level that is disrupted by conscious thought. Golfers must not think about the mechanics of their swing consciously or they will lose their skill.

There is no reason to think of this use of working memory as ‘the conscious mind’. The word ‘mind’ implies a system of cognition; and ‘conscious mind’ implies a system of cognition that is separate from unconscious cognition. However, all the manipulation, all the cognition is not part of awareness. We are not aware of how it is done. We are not aware of how 5 is found a retrieved from memory. It just pops in. All we are aware of is the flow of events, the stream of consciousness. In is more reasonable to think of having one undivided mind which does the work of cognition and a small part of the results rise to consciousness and so we are aware of them – one pop at a time.

So again we come to the function of consciousness. It is involved in a particular type of thinking in that it holds the keys to working memory. This type of thinking is very important to language, mathematics and logic. This function alone would be worth its cost.

Possible functions of consciousness 2 – gate to meaning

This is the second post in a series. The first post dealt with the importance of ‘experience’ in the sense of episodic memory, with consciousness supplying the moments making up an episode. Events that we are not conscious of simply do not get stored in episodic memory.

There is another type of memory which is not episodic, not autobiographical, carries no time and place information. It is usually called the semantic memory although it is not just about language. It stores facts, the sort of facts that you can state in natural language or some other representation (equations, musical scores, diagrams etc.). These are time-less, place-less, agent-less bits of understanding. This is memory of meanings, understandings, relationships, knowledge, ideas, concepts, and words with minimal context. The storage and retrieval processes of episodic and semantic memory appear to be separate with the exception that they are both declarative, or explicit, memory systems and therefore storage is from consciousness and retrieval is into consciousness.

What is the meaning of a word? Well, a word by itself does not have meaning. Meaning is gained by the relationships between words – they are defined in terms of other words/concepts.

Meaning is gained by how an entity takes part in the model of reality that we produce, by how it relates to other entities in the model. This model is what consciousness is derived from. Consciousness, in effect, brings the meaning from the model to the semantic memory and consciousness holds the meaning when it is retrieved from memory.

It is not clear whether semantic memory is stored as networks, matrices, hierarchies, chunks or some other embodiment. Whatever the structure, it supplies us with categories, contrasts, generalizations, and metaphoric analogs. Probably this structure is shared with other mammals, perhaps other vertebrates, but language has made it a powerful tool for humans by storing words so that they are retrievable into consciousness in ways that fit with our world model.

Language and other meaning systems are so important to us in communication with others and even within ourselves – their value cannot be overstated. The function of consciousness as a gateway to semantic memory is worth a great deal of biological cost.

In the three and a half years that I have been closely following scientific ideas about consciousness on the web, I have encountered a number of excellent ways to look at/ models of/ theories about consciousness. But I have not encountered anyone make a case for an intimate link between consciousness and declarative memory (except for the obvious definition of ‘declarative’ and ‘explicit’). I am surprised at this lack because the link seems so important in my view. If any readers know of such a argument having been made, I would appreciate a link to it.

There is more to come. The posts in this series to date:

Possible functions of consciousness 1 – leading edge of memory