MRI of mental time travel.

Arzy et al. make the interesting observation that one's imagined self location influences the neural activity related to mental time travel. Slightly edited clips from the article:

A fundamental characteristic of human conscious experience is the ability to not only experience the present moment but also to recall the past and predict the future, or to "travel" back and forth in time, a facility that is called "mental time travel" (MTT)...Converging evidence from recent memory research suggests that re-experiencing and pre-experiencing an event rely on similar neural mechanisms. Similar strategies and the same brain regions are found to be used in imagining past and future events, as future predictions may be based on past memories... when changing the location of one's self in time to past or future, one does not only recall and predict, but one also changes one's mental egocentric perspective on life events. Moreover, from these new self-locations in time, other life events might be regarded differently with respect to their relations to past or future. Thus, when imagining oneself as 10 years younger, last year's events are in the future (relative future) in relation to the initially imagined self-location in time, and vice versa (relative past).

Since earlier studies had shown behavioral and electrophysiological differences between judgments about one's own body while taken from one's actual spatial self-location versus different imagined self-locations, and given evidence that shared mechanisms process time and space in the brain, the authors developed a behavioral paradigm to determine if differences are found not only between different self-locations in time (past, now, and future), but also while imagining events in the relative past or the relative future. They followed neural correlates of MTT using behavioral measures, evoked potential (EP) mapping, and electrical neuroimaging in healthy adult participants.

Stimuli and procedure. The three different self-locations in time (past, now, and future) are shown. Participants were asked to mentally imagine themselves in one of these self-locations, and from these self-locations to judge whether different self or nonself events (e.g., top row) already happened (relative past, darker colors) or are yet to happen (relative future, lighter colors).

Their work confirmed that:

...that MTT is composed of two different cognitive processes: absolute MTT, which is the location of the self to different points in time (past, present, or future), and relative MTT, which is the location of one's self with respect to the experienced event (relative past and relative future). These processes recruit a network of brain areas in distinct time periods including the occipitotemporal, temporoparietal, and anteromedial temporal cortices. Our findings suggest that in addition to autobiographical memory processes, the cognitive mechanisms of MTT also involve mental imagery and self-location, and that relative MTT, but not absolute MTT, is more strongly directed to future prediction than to past recollection.

Generators of MTT map are localized to the right temporoparietal, occipitotemporal, and left anteromedial temporal cortices.

When your brain Lies to You

Even when a lie is presented with a disclaimer, people often later remember it as true. A brief review in the OpEd section of the NYTimes shows how a well documented feature of our memory, source amnesia, might lead 10 % of us to thinking that Barack Obama is a Muslim.

Making Memories, Again

Lasry et al. , in a letter to Science, offer an interesting interpretation of work reported in a previous post, showing that testing of already learned words enhances long-term recall when assessed 1 week later, whereas repeated studying had no beneficial effects. Here are their comments:

In their Report, "The critical importance of retrieval for learning" (15 February, p. 966), J. D. Karpicke and H. L. Roediger III show that delayed recall is optimized, not with repeated studying sessions, but with repeated testing sessions. The authors conclude that "retrieval during tests produces more learning than additional encoding."

We suggest a complementary interpretation. Classically, encoded information becomes consolidated and can later be retrieved. The tacit assumption is that retrieval of a consolidated memory is a read-only mechanism, which does not affect the memory. Recent studies have shown that elicited memories are in fact labile and become reconsolidated following each retrieval. Labile elicited memories require de novo protein synthesis to be maintained, similar to that of newly acquired memories. Neurobiological differences between consolidation and reconsolidation processes were recently described in Science. On the psychological level, reconsolidation is useful for explaining false and biased memories. Reconsolidation also leads to a memory model called multiple-trace theory: Every time a memory is reactivated, a new version of it is reconsolidated, leaving multiple traces of the same memory.

With respect to Karpicke and Roediger's study, we hypothesize that repeated testing (retrieval) should lead to multiple traces (due to repeated reconsolidation), which facilitate recall. Reinterpreting Karpicke and Roediger's results from a multiple-trace reconsolidation perspective supports this hypothesis and provides a new framework for explaining the effectiveness of frequent in-class assessments in pedagogies such as Peer Instruction.

Young and old brains differ in encoding positive information

A number of studies have revealed a "positivity shift" with aging; whereas young adults are more likely to remember negative information than positive or neutral information, older adults may be at least as likely (or even more likely) to remember positive information compared with negative information. It has been proposed that this "positivity shift" may occur because older adults put more emphasis on emotion regulation goals than do young adults, with older adults having a greater motivation to derive emotional meaning from life and to maintain positive affect. In the service of these goals, older adults may focus their attention on things that will elicit pleasant feelings and may process positive information in a more self-referential fashion. Thus this work (slightly edited) from Kensinger and Schacter probing the issue is of interest:

Young and older adults are more likely to remember emotional information than neutral information. The authors performed a magnetic resonance imaging study examining the neural processes supporting young (ages 18–35) and older (ages 62–79) adults' successful encoding of positive, negative, and neutral objects (e.g., a sundae, a grenade, a canoe). The results revealed general preservation of the emotional memory network across the age groups. Both groups recruited the amygdala and the orbito-frontal cortex during the successful encoding of positive and negative information. Both ages also showed valence-specific recruitment: right fusiform activity was greatest during the successful encoding of negative information, whereas left prefrontal and temporal activity was greatest during the successful encoding of positive information. These valence-specific processes are consistent with behavioral evidence that negative information is processed with perceptual detail, whereas positive information is processed at a conceptual or schematic level. The only age differences in emotional memory emerged during the successful encoding of positive items: Older adults showed more activity in the medial prefrontal cortex and along the cingulate gyrus than young adults. Because these regions often are associated with self-referential processing, these results suggest that older adults' mnemonic boost for positive information may stem from an increased tendency to process this information in relation to themselves.

Figure - Regions that showed a stronger correspondence to subsequent general recognition (i.e., subsequently recognized > subsequently forgotten) for the positive items than for the neutral or negative items. Red regions showed this correspondence for both young and older adults. Green regions showed this correspondence for the older adults but not for the young adults. No regions showed this correspondence for the young adults but not the older adults, consistent with the behavioral finding that only older adults showed mnemonic enhancement for the positive items.

Dan Dennett: Ants, terrorism, and the awesome power of memes

My son Jonathan sent me this link to an engaging talk by Dan Dennett given some time ago. I heard it back then, and think it is worth passing on...

Spatial memory requires new nerve cells.

At least this appears to be the case in mice. Here is the abstract from Dupre et al.

The dentate gyrus of the hippocampus is one of the few regions of the mammalian brain where new neurons are generated throughout adulthood. This adult neurogenesis has been proposed as a novel mechanism that mediates spatial memory. However, data showing a causal relationship between neurogenesis and spatial memory are controversial. Here, we developed an inducible transgenic strategy allowing specific ablation of adult-born hippocampal neurons. This resulted in an impairment of spatial relational memory, which supports a capacity for flexible, inferential memory expression. In contrast, less complex forms of spatial knowledge were unaltered. These findings demonstrate that adult-born neurons are necessary for complex forms of hippocampus-mediated learning.

(More specifically, the experiments involved generating transgenic mice that selectively overexpressed the pro-apoptotic protein Bax in neural precursor cells in an inducible manner. Overexpression of Bax removed newly born cells in the adult dentate gyrus and caused a strong deterioration in the relational processing of spatial information in the Morris water maze. Animals were unaffected when tested on simpler forms of spatial knowledge; nor were they affected in tasks where memory could be acquired without the hippocampus.)

Enhance your working intelligence with simple exercises...

Bakalar points to an interesting study by Jaeggi et al. showing that fluid intelligence (the kind of mental ability that allows us to solve new problems without having any relevant previous experience) can be enhanced by simple working memory training. It turns out that carefully structured training of the kind of memory that allows memorization of a telephone number just long enough to dial it enhances performance on standard tests of fluid intelligence. This suggests that fluid intelligence and working memory depend on the same brain circuitry.

Emotion enhancing learning and memory - a mechanism

Emotion enhances our ability to form vivid memories of even trivial events. Eric Nestler points to a study by Hu et al. that links this behavioral outcome to its molecular cause. They

...elucidated a molecular mechanism by which emotional stress and arousal promote long-term memory formation. In doing so, they brought together two well-characterized phenomena: that noradrenaline stimulates memory formation in the brain's hippocampus, and that the trPublish Postafficking of a type of glutamate receptor is important for a form of plasticity in the same brain region.

Malinow's team shows that, by stimulating noradrenaline release in the hippocampus, emotional stress leads to phosphorylation of glutamate receptors. This boosts the incorporation of these receptors at the synapse — the junction between nerve cells — which, in turn, enhances synaptic function and improves memory formation. Crucially, mice with a mutation that prevents phosphorylation of the relevant part of the glutamate receptor do not show noradrenaline-mediated memory enhancement.

Impressively, this study begins with a clinically important phenomenon — memory enhancement by emotional stress — and establishes a detailed biological pathway that underlies a behavioural endpoint in an animal model.

Enhancing our memories with brain implants.

Here are some interesting speculations by Gary Marcus on enhancing our memory - possibly through the use of computer chips as brain implants which combine cue-driven promptings similar to human memory with the location-addressability of computers. He does a nice job of distinguishing the differences in memory storage between our brains and computers.

Episodic-like memory in rats - not like humans

Until recent experiments showing that scrub jays remember where and when they cached or discovered foods of differing palatability, it had been thought that episodic memory - defined as ability to remember an event (what) as well as where and when it happened - was confined to humans. Memory for 'when' observed in scrub jays has been taken to suggest that animals can mentally travel in time or locate a past event within a temporal framework of hours and days. Roberts et al. point out that:

An alternative possibility is that, instead of remembering when an event happened within a framework of past time, animals are keeping track of how much time has elapsed since caching or encountering a particular food item at a particular place and are using elapsed time to indicate return to or avoidance of that location. The cues of when and how long ago are typically confounded in studies of episodic-like memory. Thus, animals might be remembering how long ago an event occurred by keeping track of elapsed time using accumulators, circadian timers, their own behavior, or the strength of a decaying memory trace. If this is the case, then episodic-like memory in animals may be quite different from human episodic memory in which people can reconstruct past experiences within an absolute temporal dimension.

Their experiments show that this is the case.

Three groups of Long-Evans hooded rats were tested for memory of previously encountered food. The different groups could use only the cues of when, how long ago, or when + how long ago. Only the cue of how long ago food was encountered was used successfully. These results suggest that episodic-like memory in rats is qualitatively different from human episodic memory.

Study increases learning less than testing...

Karpicke and Roediger question the common assumption that learning increases as people study and encode material, while measuring that learning by testing does not by itself produce learning. They examined undergraduates tasked with learning the meanings of 40 words in Swahili. Repeated testing of already learned words enhanced long-term recall when assessed 1 week later, whereas repeated studying had no beneficial effects. Testing required the students to retrieve the English-Swahili word associations, which suggests that encoding, although critical for the formation of a memory, may not be sufficient for its retention or consolidation. Their abstract:

Learning is often considered complete when a student can produce the correct answer to a question. In our research, students in one condition learned foreign language vocabulary words in the standard paradigm of repeated study-test trials. In three other conditions, once a student had correctly produced the vocabulary item, it was repeatedly studied but dropped from further testing, repeatedly tested but dropped from further study, or dropped from both study and test. Repeated studying after learning had no effect on delayed recall, but repeated testing produced a large positive effect. In addition, students' predictions of their performance were uncorrelated with actual performance. The results demonstrate the critical role of retrieval practice in consolidating learning and show that even university students seem unaware of this fact.

Watching an anesthetic block emotional memory

Here is an intriguing observation by Alkiri et al. We usually recall emotional pictures or events better than neutral ones. They found that low levels of the inhalation anesthetic sevoflurane could block this effect, and with PET imaging found a corresponding suppression of amygdala to hippocampal effective connectivity. Here is their abstract and one summary figure:

It is hypothesized that emotional arousal modulates long-term memory consolidation through the amygdala. Gaseous anesthetic agents are among the most potent drugs that cause temporary amnesia, yet the effects of inhalational anesthesia on human emotional memory processing remain unknown. To study this, two experiments were performed with the commonly used inhalational anesthetic sevoflurane. In experiment 1, volunteers responded to a series of emotional and neutral slides while under various subanesthetic doses of sevoflurane or placebo (no anesthesia). One week later, a mnemonic boost for emotionally arousing stimuli was evident in the placebo, 0.1%, and 0.2% sevoflurane groups, as measured with a recognition test. However, the mnemonic boost was absent in subjects who received 0.25% sevoflurane. Subsequently, in experiment 2, glucose PET assessed brain-state-related activity of subjects exposed to 0.25% sevoflurane. Structural equation modeling of the PET data revealed that 0.25% sevoflurane suppressed amygdala to hippocampal effective connectivity. The behavioral results show that 0.25% sevoflurane blocks emotional memory, and connectivity results demonstrate that this dose of sevoflurane suppresses the effective influence of the amygdala. Collectively, the findings support the hypothesis that the amygdala mediates memory modulation by demonstrating that suppressed amygdala effectiveness equates with a loss of emotional memory.

Figure - The cerebral metabolic effects of 0.25% sevoflurane are shown. (A) Representative high-resolution PET scans. (B) Absolute (mean ± SD) regional metabolic changes (white bars, placebo, no anesthesia; dark bars, 0.25% sevoflurane; marked with * for P less than 0.05). (C) Relative percent decreases of regional metabolism. (D) Regional SPM results of sevoflurane induced metabolic suppression (Upper, sagittal; Lower, axial). E shows the regional thalamic finding (brain center) on a colorized MRI. The SPM effects are significant at P less than 0.001, uncorrected; displayed at P less than 0.005, with a 500-voxel extent.

Stronger or weaker brain synapses after sleep?

Why do we spend a third of our lives asleep? The answers suggested so far are varied and controversial. It is well documented that improvement in learning and memory accompanies a night of sleep. One idea is that most new information is discarded during sleep, as diurnal animals are bombarded by stimuli during the day, most of which we want to (or need to) forget. Synapses need to recover. If this is the dominant reason why we sleep, then decreased numbers of synapses or synapse weakening should be a prominent neuronal feature of sleep. Fountain points to an article by Tonini and colleagues (Nature Neuroscience 11, pp. 200 - 208, 2008) that provides evidence for this option. Tononi suggests that after sleep "“we get a leaner brain — there’s a gain in terms of energy, space and supplies, and you are ready to learn anew.” Here is their abstract:

Plastic changes occurring during wakefulness aid in the acquisition and consolidation of memories. For some memories, further consolidation requires sleep, but whether plastic processes during wakefulness and sleep differ is unclear. We show that, in rat cortex and hippocampus, GluR1-containing AMPA receptor (AMPAR) levels are high during wakefulness and low during sleep, and changes in the phosphorylation states of AMPARs, CamKII and GSK3beta are consistent with synaptic potentiation during wakefulness and depression during sleep. Furthermore, slope and amplitude of cortical evoked responses increase after wakefulness, decrease after sleep and correlate with changes in slow-wave activity, a marker of sleep pressure. Changes in molecular and electrophysiological indicators of synaptic strength are largely independent of the time of day. Finally, cortical long-term potentiation can be easily induced after sleep, but not after wakefulness. Thus, wakefulness appears to be associated with net synaptic potentiation, whereas sleep may favor global synaptic depression, thereby preserving an overall balance of synaptic strength.

The bouncer in the brain...

In the January issue of Nature Neuroscience a review with the title of this post by Awh & Vogel discusses experiments by McNab & Klingberg that may explain why there are significant differences between individuals in working memory, which is known to be limited to about three or four items. Individual differences in this memory capacity correlate robustly with measures of fluid intelligence and scholastic aptitude. The experiments explore the idea that variations in the efficiency with which information is selected to fill this limited workspace are involved. From Awh and Vogel:

One perspective on individual differences in memory capacity views variation in terms of the number of 'slots' that are available for short-term storage. However, apparent capacity differences might also be explained by variations in the efficiency with which information is selected to fill this limited workspace. A useful analogy for understanding the difference between these two ideas is the difference between the space that is available in an exclusive nightclub and the effectiveness of the bouncer who grants admission. From this perspective, high-capacity individuals may have a better bouncer rather than a larger nightclub...brain imaging evidence from McNab and Klingberg implicates a specific neural region that may serve as the bouncer for the mind.

This hypothesis is consistent with a growing body of evidence that shows tight links between attention and working memory. Some theorists have even suggested that they are essentially the same mechanism. This viewpoint is supported by the strong overlap in the cortical areas that are active during attention and working-memory tasks, as well as evidence that directly implicates attention in the active maintenance of information in working memory. Furthermore, an individual's working-memory capacity is highly predictive of his or her performance on a wide range of attention tasks

Here is the abstract from McNab and Klingberg:

Our capacity to store information in working memory might be determined by the degree to which only relevant information is remembered. The question remains as to how this selection of relevant items to be remembered is accomplished. Here we show that activity in the prefrontal cortex and basal ganglia preceded the filtering of irrelevant information and that activity, particularly in the globus pallidus, predicted the extent to which only relevant information is stored. The preceding frontal and basal ganglia activity were also associated with inter-individual differences in working memory capacity. These findings reveal a mechanism by which frontal and basal ganglia activity exerts attentional control over access to working memory storage in the parietal cortex in humans, and makes an important contribution to inter-individual differences in working memory capacity.

Why can't we perform perfectly?

Some fascinating experiments by Tumer and Brainar on songbirds inform me on why I am not able to perform a completely learned and exhaustively practiced piano piece the same way each time I bang it out.... from the Nature Editor's review of their article:

Why is it that even the best-trained athletes and musicians cannot perform perfectly? One thought is that residual variability in performance is 'noise' that reflects fundamental limits on our ability to control our movements. Experiments using the exceptionally well-rehearsed songs of adult songbirds as a model point to an alternative explanation. Computerized monitoring of the apparently stereotyped songs of adult Bengalese finches revealed minuscule variations in performance. When the birds were given corrections each time the song varied beyond a certain limit, they rapidly learned to adapt their vocalizations. The implication is that once learned, songs can be maintained despite subtle changes to the vocal system due to factors such as ageing. So behavioural 'noise', rather than simply being a nuisance, may reflect experimentation by the nervous system to refine performance.

The abstract from Rumer and Brainar:

Significant trial-by-trial variation persists even in the most practiced skills. One prevalent view is that such variation is simply 'noise' that the nervous system is unable to control or that remains below threshold for behavioural relevance. An alternative hypothesis is that such variation enables trial-and-error learning, in which the motor system generates variation and differentially retains behaviours that give rise to better outcomes. Here we test the latter possibility for adult bengalese finch song. Adult birdsong is a complex, learned motor skill that is produced in a highly stereotyped fashion from one rendition to the next. Nevertheless, there is subtle trial-by-trial variation even in stable, 'crystallized' adult song. We used a computerized system to monitor small natural variations in the pitch of targeted song elements and deliver real-time auditory disruption to a subset of those variations. Birds rapidly shifted the pitch of their vocalizations in an adaptive fashion to avoid disruption. These vocal changes were precisely restricted to the targeted features of song. Hence, birds were able to learn effectively by associating small variations in their vocal behaviour with differential outcomes. Such a process could help to maintain stable, learned song despite changes to the vocal control system arising from ageing or injury. More generally, our results suggest that residual variability in well learned skills is not entirely noise but rather reflects meaningful motor exploration that can support continuous learning and optimization of performance.

Repressed Memory - A recent cultural invention?

Literary references to depression, hallucinations, anxiety, and dementia can be found throughout history. A fascinating article in Harvard Magazine by Ashley Pettus describes the research of Harrison Pope, who reasoned that if dissociative amnesia were an innate capability of the brain it also should appear in ancient texts. An extensive search, and a $1000 reward, was able to find no reference earlier than Nina, an opera by Dalayrac and Marsollier performed in Paris in 1786. The absence of dissociative amnesia in works prior to 1800 suggests that the phenomenon is not a natural neurological function, but rather a “culture-bound” syndrome rooted in the nineteenth century. From the article:

What, then, accounts for “repressed memory’s” appearance in the nineteenth century and its endurance today? Pope and his colleagues hope to answer these questions in the future. “Clearly the rise of Romanticism, at the end of the Enlightenment, created fertile soil for the idea that the mind could expunge a trauma from consciousness,” Pope says. He notes that other pseudo-neurological symptoms (such as the female “swoon”) emerged during this era, but faded relatively quickly. He suspects that two major factors helped solidify “repressed memory” in the twentieth-century imagination: psychoanalysis (with its theories of the unconscious) and Hollywood. “Film is a perfect medium for the idea of repressed memory,” he says. “Think of the ‘flashback,’ in which a whole childhood trauma is suddenly recalled. It’s an ideal dramatic device.”

Learning from errors - genetic differences between humans

From Holden's brief summary of the work:

"Once burned, twice shy" works for most people. But some people are slow to learn from bad experiences.

This work shows that:

...people with a particular gene variant have more difficulty learning via negative reinforcement.
...demonstrates that a single-base-pair difference in the genome is associated with a remarkably different ability to learn from past mistakes is quite an accomplishment
...combines brain imaging with a task in which participants chose between symbols on a computer screen,
...centers on the A1 variant, or allele, of the gene encoding the D2 receptor, a protein on the surface of brain cells activated by the neurotransmitter dopamine. Earlier studies have hinted that this variant alters the brain's reward pathways and thereby makes people more vulnerable to addictions.

Brain activity was monitored (color) as a subject chose between two symbols (inset) and was rewarded with a smiley or frowny face. In the left panel the lower colors are hippocampus, the upper one the posterior medial frontal cortex.

Here is the abstract from Klein et al.

The role of dopamine in monitoring negative action outcomes and feedback-based learning was tested in a neuroimaging study in humans grouped according to the dopamine D2 receptor gene polymorphism DRD2-TAQ-IA. In a probabilistic learning task, A1-allele carriers with reduced dopamine D2 receptor densities learned to avoid actions with negative consequences less efficiently. Their posterior medial frontal cortex (pMFC), involved in feedback monitoring, responded less to negative feedback than others' did. Dynamically changing interactions between pMFC and hippocampus found to underlie feedback-based learning were reduced in A1-allele carriers. This demonstrates that learning from errors requires dopaminergic signaling. Dopamine D2 receptor reduction seems to decrease sensitivity to negative action consequences, which may explain an increased risk of developing addictive behaviors in A1-allele carriers.

Blue light changes our brains

A new light sensitive system has recently been discovered in the ganglion cells of the retinas, which send signals to the rest of the brain. These cells contain light sensitive melanopsin, most sensitive to blue wavelengths between 460 and 480 nm. The responses triggered by blue light takes seconds to develop and persist for minutes, unlike the rapid and transient responses of our rod and cone photoreceptor cells. Recent work has shown that brain activity related to a working memory task is maintained (or even increased) by blue (470 nm) monochromatic light exposure, whereas it decreases under green (550 nm) monochromatic light exposure. Vandewalle et al. now show that activation of this system causes changes in brain areas related to working memory:

Figure - activity increase in left thalamus.

"We exposed 15 participants to short duration (50 s) monochromatic violet (430 nm), blue (473 nm), and green (527 nm) light exposures of equal photon flux (1013ph/cm2/s) while they were performing a working memory task in fMRI. At light onset, blue light, as compared to green light, increased activity in the left hippocampus, left thalamus, and right amygdala. During the task, blue light, as compared to violet light, increased activity in the left middle frontal gyrus, left thalamus and a bilateral area of the brainstem consistent with activation of the locus coeruleus.

These results support a prominent contribution of melanopsin-expressing retinal ganglion cells to brain responses to light within the very first seconds of an exposure. The results also demonstrate the implication of the brainstem in mediating these responses in humans and speak for a broad involvement of light in the regulation of brain function."

Memories play back on fast forward during sleep

It is know that correlations of nerve activity that are observed during learning sequence tasks replay during sleep, presumably to enhance learning and retention of the sequence. (I always find that I can play a difficult piano passage better when I wake up in the morning than when I was practicing it the day before). McNaughton and his collaborators now show that the replay during sleep occurs much faster than during actual awake behaviors. Here is their abstract:

As previously shown in the hippocampus and other brain areas, patterns of firing-rate correlations between neurons in the rat medial prefrontal cortex during a repetitive sequence task were preserved during subsequent sleep, suggesting that waking patterns are reactivated. We found that, during sleep, reactivation of spatiotemporal patterns was coherent across the network and compressed in time by a factor of 6 to 7. Thus, when behavioral constraints are removed, the brain's intrinsic processing speed may be much faster than it is in real time. Given recent evidence implicating the medial prefrontal cortex in retrieval of long-term memories, the observed replay may play a role in the process of memory consolidation.

Observing rat brains as they look to the future

An emerging view is that the hippocampus is essential to imagining the future as well as remembering the past (which makes a lot sense, since we usually base our imagined future on our past experience). Johnson and Redich have now observed ensembles of cells in the CA3 region of the rat hippocampus whose firing transiently encodes paths forward of an animal at decision points in a maze, as if they are reflecting on possible futures and deciding what to do next. The figure, from Heyman's review of the work in Science, illustrates that as a rat looks in one direction, neurons representing that position (inset) fire over a half-second period. Here is the abstract of the work:

Neural ensembles were recorded from the CA3 region of rats running on T-based decision tasks. Examination of neural representations of space at fast time scales revealed a transient but repeatable phenomenon as rats made a decision: the location reconstructed from the neural ensemble swept forward, first down one path and then the other. Estimated representations were coherent and preferentially swept ahead of the animal rather than behind the animal, implying it represented future possibilities rather than recently traveled paths. Similar phenomena occurred at other important decisions (such as in recovery from an error). Local field potentials from these sites contained pronounced theta and gamma frequencies, but no sharp wave frequencies. Forward-shifted spatial representations were influenced by task demands and experience. These data suggest that the hippocampus does not represent space as a passive computation, but rather that hippocampal spatial processing is an active process likely regulated by cognitive mechanisms.