Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Memory — Recent Research


Science 16 November 2007: Vol. 318. no. 5853, pp. 1147 - 1150

Fast-Forward Playback of Recent Memory Sequences in Prefrontal Cortex During Sleep

David R. Euston, Masami Tatsuno, Bruce L. McNaughton

Arizona Research Laboratories Division of Neural Systems, Memory and Aging, University of Arizona, Tucson, AZ 85724–5115, USA.

According to memory-consolidation theory, the hippocampus is necessary for the retrieval of recently encoded episodic memories. For remote memories, in contrast, the neocortex is sufficient for recall. The transfer of memories from hippocampal to neocortical control is widely believed to involve replay during sleep of the neural patterns representing the memory. Consistent with this hypothesis, patterns of brain activity during a task appear to be repeated during subsequent sleep in rats, birds, monkeys, and humans. In rats, multielectrode hippocampal recordings have shown that the temporal order of replay during sleep is preserved.

Among cortical areas, the medial prefrontal cortex (mPFC) apparently plays a unique role in mediating retrieval of consolidated, remote memories.



Nature 444, 559-560 (30 November 2006)

Neuroscience: A memory boost while you sleep

Robert Stickgold

Robert Stickgold is at the Center for Sleep and Cognition, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, Massachusetts 02215, USA.

It is generally agreed that sleep aids memory consolidation; researchers have produced a wealth of evidence for at least one function of slumber — the consolidation of memories. A wide range of converging data show that memories are replayed, modified, stabilized and even enhanced as we snooze, but an understanding of how this occurs remains elusive. The proposed mechanisms rely on two effects that are observed in the brain only during sleep: alterations in the levels of chemical neuromodulators and distinctive oscillations of electrical activity. Marshall et al. provide evidence that the direct induction of 'cortical slow oscillations' during sleep can improve the recall of word-pairs memorized the previous night.

Human sleep is divided into rapid-eye-movement sleep (REM) and non-REM sleep (NREM), with NREM sleep further divided into stages 1 to 4. These sleep phases are distinguished, in part, by well-defined patterns of oscillatory electrical activity in the brain, as measured by electroencephalography. Theta waves oscillate at 4–8 Hz, and are characteristic of REM sleep; NREM stage 1 is a transitional phase between full wakefulness and sleep, and is characterized by mixed-frequency waves; sleep spindles (12–14 Hz) typify NREM stage 2; and delta waves (1–4 Hz) distinguish NREM stages 3 and 4, which are known collectively as slow-wave sleep. Sleep spindles and delta waves rise and fall in concert with a yet slower (<1 Hz) oscillatory pattern known as cortical slow oscillations — it is these patterns that Marshall et al.  have studied.



Nature 444, 610-613 (30 November 2006)

Boosting slow oscillations during sleep potentiates memory

Lisa Marshall, Halla Helgadóttir, Matthias Mölleand Jan Born

There is compelling evidence that sleep contributes to the long-term consolidation of new memories. This function of sleep has been linked to slow (<1 Hz) potential oscillations, which predominantly arise from the prefrontal neocortex and characterize slow wave sleep. However, oscillations in brain potentials are commonly considered to be mere epiphenomena that reflect synchronized activity arising from neuronal networks, which links the membrane and synaptic processes of these neurons in time. Whether brain potentials and their extracellular equivalent have any physiological meaning per se is unclear, but can easily be investigated by inducing the extracellular oscillating potential fields of interest. Here we show that inducing slow oscillation-like potential fields by transcranial application of oscillating potentials (0.75 Hz) during early nocturnal non-rapid-eye-movement sleep, that is, a period of emerging slow wave sleep, enhances the retention of hippocampus-dependent declarative memories in healthy humans. The slowly oscillating potential stimulation induced an immediate increase in slow wave sleep, endogenous cortical slow oscillations and slow spindle activity in the frontal cortex. Brain stimulation with oscillations at 5 Hz—another frequency band that normally predominates during rapid-eye-movement sleep—decreased slow oscillations and left declarative memory unchanged. Our findings indicate that endogenous slow potential oscillations have a causal role in the sleep-associated consolidation of memory, and that this role is enhanced by field effects in cortical extracellular space.



Nature 456, 102-106 (6 November 2008)

Entrained rhythmic activities of neuronal ensembles as perceptual memory of time interval

Germán Sumbre, Akira Muto, Herwig Baier & Mu-ming Poo

Division of Neurobiology, Department of Molecular and Cell Biology, Helen Wills Neuroscience Institute, University of California, Berkeley, California 94720, USA

Department of Physiology, University of California, San Francisco, California 94158, USA

Present address: Laboratoire de Neurobiologie, UMR 8544, École Normale Supérieure, 46 rue d'Ulm, 75005 Paris, France.

The ability to process temporal information is fundamental to sensory perception, cognitive processing and motor behaviour of all living organisms, from amoebae to humans. Neural circuit mechanisms based on neuronal and synaptic properties have been shown to process temporal information over the range of tens of microseconds to hundreds of milliseconds. How neural circuits process temporal information in the range of seconds to minutes is much less understood. Studies of working memory in monkeys and rats have shown that neurons in the prefrontal cortex, the parietal cortex and the thalamus exhibit ramping activities that linearly correlate with the lapse of time until the end of a specific time interval of several seconds that the animal is trained to memorize. Many organisms can also memorize the time interval of rhythmic sensory stimuli in the timescale of seconds and can coordinate motor behaviour accordingly, for example, by keeping the rhythm after exposure to the beat of music. Here we report a form of rhythmic activity among specific neuronal ensembles in the zebrafish optic tectum, which retains the memory of the time interval (in the order of seconds) of repetitive sensory stimuli for a duration of up to approx20 s. After repetitive visual conditioning stimulation (CS) of zebrafish larvae, we observed rhythmic post-CS activities among specific tectal neuronal ensembles, with a regular interval that closely matched the CS. Visuomotor behaviour of the zebrafish larvae also showed regular post-CS repetitions at the entrained time interval that correlated with rhythmic neuronal ensemble activities in the tectum. Thus, rhythmic activities among specific neuronal ensembles may act as an adjustable 'metronome' for time intervals in the order of seconds, and serve as a mechanism for the short-term perceptual memory of rhythmic sensory experience.



Nature 454, 167-168 (10 July 2008)

'Convergence–Divergence Zones' (CDZs) – (‘mirror neurons’)

Antonio Damasio & Kaspar Meyer

Antonio Damasio and Kaspar Meyer are neuroscientists at the Brain and Creativity Institute of the University of Southern California, Los Angeles, USA.

Signals are conveyed within the brain in both forward and backward directions. (reentrant and recursive action) For example, signals are obviously sent from the eye to the visual cortex and on to areas of higher-level processing in the brain. But these high-level areas also send signals back to the visual cortex, and even to the visual thalamus, below the level of the cortex. The same forward–backward signalling arrangement is found for the hippocampus involved in making factual memories.


A Model: 'Convergence–Divergence Zones' (CDZs).

Sensory neural ensembles collect signals from separate sites, and signal back to those sites. When several signals converge on a CDZ, the ensemble creates an abstract record of the coincident activations — a memory trace. The model contained two broad types of CDZ. 'Local CDZs' are proposed to coordinate information within regions close to a sensory cortex, such as the visual cortex. The local hubs are proposed to converge on 'non-local CDZs' in higher-order sectors of the brain.

In this view, when a monkey breaks open a peanut, for example, local CDZs collect information about various sensory inputs, and feed these to a non-local CDZ that records the coincident information about the sound, sight and feel of this action. The CDZ does not hold all the details of this information; rather, it contains the potential to retroactivate the separate auditory, visual, tactile and motor sites, and thus reconstitute the original distributed set of memories and information.

In future, hearing a peanut being broken without seeing or feeling it triggers a series of events. First, it activates the auditory cortices and local auditory CDZ; second, it activates the non-local CDZ that previously collected the memory trace associated with this sound; third, it precipitates simultaneous signalling outwards from this non-local CDZ to all the local CDZs involved in the original event (motor, visual, auditory); fourth, it reactivates all or some of these sites. This leads to a more-or-less successful replay of the coincident set of separate brain activities that accompanied the monkey breaking open a peanut.

Looked at in this way, mirror neurons correspond to non-local CDZs. Their connections to other CDZs, and their ability to collect and distribute signals based on learned experience, allow the brain to reconstruct an action from only part of the story. A whole neural network underlies the understanding of action, rather than a single anatomical site or even a single cell. The monkey's comprehension of the sound of a cracking nut is not created just by mirror-neuron sites, but also by the nearly simultaneous triggering of widespread memories throughout the brain.

The neurons at the heart of this process, and at the heart of non-local CDZs, are not so much like mirrors.. They are more like puppet masters, pulling the strings of various memories.

Recent findings are in line with this view. Studies in humans and monkeys show that the neural network stimulated by watching an action goes beyond the original mirror-neuron sites; it encompasses more widespread sensorimotor cortices. Conversely, carrying out an action recruits sensory cortical areas even when subjects can neither see nor hear the actions they perform. This lends further support to the notion that the neural description of an action goes far beyond its motor components. At least one other study has invoked the necessity of convergent signals into mirror-neuron areas to explain such observations. The CDZ model, and our interpretation of mirror neurons, adds the aspect of divergence.

CDZ neurons induce widespread neural activity based on learned patterns of connectivity; these patterns generate internal simulation and establish the meaning of actions. CDZ neurons establish the connectivity pattern for a mental image or action pattern, but the neural pattern itself is made of a large brain subnetwork.

The ultimate test of the convergence–divergence model, and its explanation of how mirror neurons do what they do, depends on the ability to record brain activity simultaneously from neurons in separate sites, and on probing the underlying connectivity between neural areas.



Science 31 August 2007: Vol. 317. no. 5842, pp. 1230 - 1233
Localization of a Stable Neural Correlate of Associative Memory

Leon G. Reijmers, Brian L. Perkins, Naoki Matsuo, Mark Mayford

Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.

Do learning and retrieval of a memory activate the same neurons? Does the number of reactivated neurons correlate with memory strength? We developed a transgenic mouse that enables the long-lasting genetic tagging of c-fos–active neurons. We found neurons in the basolateral amygdala that are activated during Pavlovian fear conditioning and are reactivated during memory retrieval. The number of reactivated neurons correlated positively with the behavioral expression of the fear memory, indicating a stable neural correlate of associative memory. The ability to manipulate these neurons genetically should allow a more precise dissection of the molecular mechanisms of memory encoding within a distributed neuronal network.



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