Archive for January, 2008
Dissociable areas of memory (in MTL): Two or three?
Jan 9th
There has been some discussion in memory literature as to whether familiarity / novelty detection and recollection (contextual recognition of a stimulus or episodic recall) are independent processes or are the same processes, but only the memory strength varies.
In 2006, an fMRI study came around that showed that there were three dissociable areas in MTL that were associated with familiarity, novelty and recollection detection.
There have been indications that recollection, familiarity, and novelty involve different medial temporal lobe subregions, but available evidence is scarce and inconclusive. Within the medial temporal lobes (MTLs), they found a triple dissociation among the posterior half of the hippocampus, which was associated with recollection, the posterior parahippocampal gyrus, which was associated with familiarity, and anterior half of the hippocampus and rhinal regions, which were associated with novelty. Furthermore, multiple regression analyses based on individual trial activity showed that all three memory signals, i.e., recollection, familiarity, and novelty, make significant and independent contributions to recognition memory performance.
This appeared to be the established dogma to me, till I came across this new PNAS paper, which again strives to swing the pendulum back in favor of memory strengths and a single process for recollection and familiarity/novelty detection. The authors found that while a distinct group of neurons in hippocampus and anygdala was responsible for novelty and familiarity detection, recollection could just be ascertained by the strength of the neural firing of these groups of neurons. Here is the abstract of the study:
Episodic memories allow us to remember not only that we have seen an item before but also where and when we have seen it (context). Sometimes, we can confidently report that we have seen something (familiarity) but cannot recollect where or when it was seen. Thus, the two components of episodic recall, familiarity and recollection, can be behaviorally dissociated. It is not clear, however, whether these two components of memory are represented separately by distinct brain structures or different populations of neurons in a single anatomical structure. Here, we report that the spiking activity of single neurons in the human hippocampus and amygdala [the medial temporal lobe (MTL)] contain information about both components of memory. We analyzed a class of neurons that changed its firing rate to the second presentation of a previously novel stimulus. We found that the neuronal activity evoked by the presentation of a familiar stimulus (during retrieval) distinguishes stimuli that will be successfully recollected from stimuli that will not be recollected. Importantly, the ability to predict whether a stimulus is familiar is not influenced by whether the stimulus will later be recollected. We thus conclude that human MTL neurons contain information about both components of memory. These data support a continuous strength of memory model of MTL function: the stronger the neuronal response, the better the memory.
PNAS has made the article freely available, so go have a look. This is what they discuss:
We analyzed the spiking activity of neurons in the human MTL during retrieval of declarative memories. We found that the neural activity differentiated between stimuli that were only recognized as familiar and stimuli for which (in addition) the spatial location could be recollected. Further, we found that the same neural activity was also present during behavioral errors, but with reduced amplitude. This data are compatible with a continuous signal of memory strength: the stronger the neuronal response, the better the memory. Forgotten stimuli have the weakest memory strength and stimuli that are only recognized but not recollected have medium strength. The strongest memory (and thus neuronal response) is associated with stimuli that are both recognized and recollected.
One methodological flaw of the current study is that it didn’t take the earlier studies showing triple dissociation into account and did not differentiate between MTL neurons based on their location within hippocampus/ amygdala. If they had distinguished based on the location, they might have found some neurons that were selectively coding for recollection. In absence of such observations I find it hard to concur that recollection is not an independent process from familiarity/ novelty detection. Recollection involves binding the familiarity/ novelty cues with other neuronal cues in MTL like neurons that code for time and place . It may be that the current study completely missed out on those integrator neurons.
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Depression, Dreaming and Rehearsal learning
Jan 7th
We all know that depression is marked by an increase in REM Sleep or dreaming and their are various theories of why this increased dreaming may be a root cause of depression itself. One theory posits that having too much dreams or emotional negativity even while sleeping (most dreams have negative content) may lead to maintenance of downward spiral of depressive cognitive and emotional style. Another theory posits that having too much negative dreams may lead us to get exhausted and the morning weariness found in depression is due to this fact.
I’ll not comment regarding the purported mechanism and causal direction of link between depression and dreaming. Suffice it to note few pertinent facts:
- Depressives dream 3 -4 times more than normal people.
- Most Anti-depressants cause REM sleep to be suppressed and this may underlie their therapeutic action or may just be a side-effect.
- There is mixed evidence as to whether REM sleep is also altered in Mania (although a decrease in REM sleep is not mentioned in literature- if anything REM increases just like in depression)
Now I’ll like to highlight an important experiment conducted by Wisconsin Madison scientists. They deprived rats of REM sleep and found that such rats became idiots in terms of survival sense and failed all standard test of survival. I’ll first describe the procedure and their results and then theorize:
What happens when a rat stops dreaming? In 2004, researchers at the University of Wisconsin at Madison decided to find out. Their method was simple, if a bit devilish. Step 1: Strand a rat in a tub of water. In the center of this tiny sea, allot the creature its own little desert island in the form of an inverted flowerpot. The rat can swim around as much as it pleases, but come nightfall, if it wants any sleep, it has to clamber up and stretch itself across the flowerpot, its belly sagging over the drainage hole.
In this uncomfortable position, the rat is able to rest and eventually fall asleep. But as soon as the animal hits REM sleep, the muscular paralysis that accompanies this stage of vivid dreaming causes its body to slacken. The rat slips through the hole and gets dunked in the water. The surprised rat is then free to crawl back onto the pot, lick the drops off its paws, and go back to sleep—but it won’t get any REM sleep.
Step 2: After several mostly dreamless nights, the creature is subjected to a virtual decathlon of physical ordeals designed to test its survival behaviors. Every rat is born with a set of instinctive reactions to threatening situations. These behaviors don’t have to be learned; they’re natural defenses—useful responses accrued over millennia of rat society.
The dream-deprived rats flubbed each of the tasks. When plopped down in a wide-open field, they did not scurry to the safety of a more sheltered area; instead, they recklessly wandered around exposed areas. When shocked, they paused briefly and then went about their business, rather than freezing in their tracks the way normal rats do. When confronted with a foreign object in their burrow, they did not bury it; instead, they groomed themselves. Had the animals been out in the wild, they would have made easy prey.
The surprise came during Step 3. Each rat was given amphetamines and tested again; nothing changed. If failure to be an effective rat were due to mere sleep deprivation, amphetamines would have reversed the effect. But that didn’t happen. These rats weren’t floundering because they were sleepy. Something else was going on—but what?
To me it seems that what is happening in these dream-deprived rats is an unlearning of learned helplessness paradigm. In learned helplessness, one stops exploring the environment and becomes extremely cautious. Learned helplessness is an extremely influential theory of depression and I have blogged about it previously. In the dream-deprived rats something exactly the opposite is happening – they are becoming more exploratory and sort of unlearning the basic survival instincts .
To me all this seems to nicely fit together. Dreams may be instrumental in rehearsal learning and when the rat (or human) has been repeatedly exposed to inescapable shocks (unavoidable stress), then it may lose the desire to explore not only in the real world, but also in the dream world – the primary purpose of which is to generate alternative strategies to previously un-encountered negative situations. When one loses or fails to find creative solutions to the inescapable situations, one falls in a negative dream loop whereby one fails to explore adequately new strategies or to reassess the environment in light of new evidences. Instead as one has failed to find creative solutions earlier, one;s dreams become pre-occupied with failure- and with each failure prompts more vigorous search for answers in the dream -thus leading to more REM sleep. Also, as REM sleep is required for thus maintaining the new (unhealthy) associations hence as long as adequate REM sleep is available one stays stuck with the learned negative associations and the learned helplessness.
SSRIs and other anti-depressants , by blocking the REM sleep , may be providing one additional step whereby dreaming stops for some duration and the synapses that underlie negative associations (that were constantly strengthened during dreams) are given time to naturally become weak. Thus events no longer have automatic negative connotations, but can be appraised afresh with a new outlook. This may be one putative mechanism of how anti-depressants work. This may also explain why anti-depressants take so much time to become effective. When dreaming stops, the unlearning doesn’t happen in a day- the weakening of associations would take weeks and months to materialize and have an effect.
Cognitive – behavioral theory may also be working on the level of dreams and it would be interesting to note how much dreaming is reduced and brought to normal levels as CBT starts showing effects.
The behavior of the dream deprived rat seems almost manic- unconcerned with survival and unnecessarily risk-taking. One experiment that can be conducted is to first induce learned helplessness in rats (by exposing them to unavoidable shocks) and then dream-deprive them as per the above methodology. If dream-deprivation restores the normalcy in rats and removes the depressive symptomatology we have a new theory of how depression works. This is not a difficult experiment to do and can be easily performed. I’m sure it will lead to positive results. I look forward to hearing from some readers of this blog as to how the experiment actually went (I myself am in no position to conduct such experiments). Do let me know via comments the experimental results- even if they are negative.
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Depression not only has bland taste but bland sense of smell too
Jan 4th
In one of my earlier post on depression, I had commented on the fact that those suffering from depression have less sensitivity to sweet and bitter tastes and as such may compensate by eating more sugar thus leading to the well documented diabetes – depression linkage.
In a new study it has just been discovered that not only depressives have bland sense of taste, their sense of smell is also diminished and they may make compensations by using greater amounts of perfume. Overall it seems that those suffering from depression will have bland subjective experience of flavor(which is a combination of both smell and taste) and thus may even not really find what they eat to be tasty.
To me, this is an important finding. To my knowledge no research has been done in other sense modalities (like vision), but there is every reason to think that we may discover a bland sense of vision in depression. Why do I surmise so? this is because there is extensive literature available regarding the manic state and how things seem ‘vivid’ during that state including visual vividness. If depression is the converse of Mania, it follows that a corresponding blandness of vision should also be observed in those who are clinically depressed.
We also know that in extreme or psychotic forms of Mania, auditory hallucinations may arise. I am not suggesting that hallucinations are equal to vividness, but I would definitely love to see studies determining whether the auditory sense is heightened in Mania (maybe more absolute pitch perception in Mania) and a corresponding loss of auditory absolute pitch perception in depression. If so found, it may happen that music literally becomes subdued for people with depression and they sort of do not hear the music present in everyday life!
Whether other sense like touch, vestibular/ kinesthetic , proprioception (a heightened sense of which may give rise to eerie out-pf-body experiences in Mania) are also diminished in depression is another area where research may be fruitful.
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pretend play = creating new worlds?
Jan 2nd
Edge asks a question each year to prominent scientists and there question this year was “What have you changed your mind about and why?”. Well, they didn’t invite me to answer that question (not yet 
, so I wont force my thoughts on the readers of this blog; but instead would like to highlight one of the answers (by Alison Gopnik).
She mentions that she though earlier that pretend play (and related to that the adult fiction reading/creation) was a spandrel and had no evolutionary significance. Now she thinks that the capacity for pretend play is necessary for imagination to develop whcih is necessary for creating new worlds. I, to say the least, am firmly entrenched in the view that treats pretend play as of fundamental importance to development and it is heartening to get support from a prominent psychologist.
I reproduce below the response of Alison in its entirety. (emphasis added by me)
Imagination is RealRecently, I’ve had to change my mind about the very nature of knowledge because of an obvious, but extremely weird fact about children – they pretend all the time. Walk into any preschool and you’ll be surrounded by small princesses and superheroes in overalls – three-year-olds literally spend more waking hours in imaginary worlds than in the real one. Why? Learning about the real world has obvious evolutionary advantages and kids do it better than anyone else. But why spend so much time thinking about wildly, flagrantly unreal worlds? The mystery about pretend play is connected to a mystery about adult humans – especially vivid for an English professor’s daughter like me. Why do we love obviously false plays and novels and movies?
The greatest success of cognitive science has been our account of the visual system. There’s a world out there sending information to our eyes, and our brains are beautifully designed to recover the nature of that world from that information. I’ve always thought that science, and children’s learning, worked the same way. Fundamental capacities for causal inference and learning let scientists, and children, get an accurate picture of the world around them – a theory. Cognition was the way we got the world into our minds.
But fiction doesn’t fit that picture – its easy to see why we want the truth but why do we work so hard telling lies? I thought that kids’ pretend play, and grown-up fiction, must be a sort of spandrel, a side-effect of some other more functional ability. I said as much in a review in Science and got floods of e-mail back from distinguished novel-reading scientists. They were all sure fiction was a Good Thing – me too, of course, – but didn’t seem any closer than I was to figuring out why.
So the anomaly of pretend play has been bugging me all this time. But finally, trying to figure it out has made me change my mind about the very nature of cognition itself.
I still think that we’re designed to find out about the world, but that’s not our most important gift. For human beings the really important evolutionary advantage is our ability to create new worlds. Look around the room you’re sitting in. Every object in that room – the right angle table, the book, the paper, the computer screen, the ceramic cup was once imaginary. Not a thing in the room existed in the pleistocene. Every one of them started out as an imaginary fantasy in someone’s mind. And that’s even more true of people – all the things I am, a scientist, a philosopher, an atheist, a feminist, all those kinds of people started out as imaginary ideas too. I’m not making some relativist post-modern point here, right now the computer and the cup and the scientist and the feminist are as real as anything can be. But that’s just what our human minds do best – take the imaginary and make it real. I think now that cognition is also a way we impose our minds on the world.
In fact, I think now that the two abilities – finding the truth about the world and creating new worlds-are two sides of the same coins. Theories, in science or childhood, don’t just tell us what’s true – they tell us what’s possible, and they tell us how to get to those possibilities from where we are now. When children learn and when they pretend they use their knowledge of the world to create new possibilities. So do we whether we are doing science or writing novels. I don’t think anymore that Science and Fiction are just both Good Things that complement each other. I think they are, quite literally, the same thing.
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