Memory time

In this issue of Neuron, MacDonald et al. describe hippocampal "time cells" that fire during specific delay periods as rats performed a memory task. Converging results in monkeys suggest that the hippocampus encodes episodes by signaling events in time.     

In Memory

Abstract

Dr. Mathias P. Mertes, well-known medicinal chemist, member of the Editorial Advisory Board of Nucleosides & Nucleotides, and a good friend, died unexpectedly earlier this year. An obituary written by Drs. Gary Grunewald and Robert Hanzlik of the University of Kansas is presented below followed by a list of the publications of Dr. Mertes. Many of us have pleasant memories of stimulating interactions with Matt. We can hope that the spirit and enthusiasm with which Matt approached science has rubbed off on some of us. He will be missed.     

Memory Consolidation

Conscious memory for a new experience is initially dependent on information stored in both the hippocampus and neocortex. Systems consolidation is the process by which the hippocampus guides the reorganization of the information stored in the neocortex such that it eventually becomes independent of the hippocampus. Early evidence for systems consolidation was provided by studies of retrograde amnesia, which found that damage to the hippocampus-impaired memories formed in the recent past, but typically spared memories formed in the more remote past. Systems consolidation has been found to occur for both episodic and semantic memories and for both spatial and nonspatial memories, although empirical inconsistencies and theoretical disagreements remain about these issues. Recent work has begun to characterize the neural mechanisms that underlie the dialogue between the hippocampus and neocortex (e.g., “neural replay,” which occurs during sharp wave ripple activity). New work has also identified variables, such as the amount of preexisting knowledge, that affect the rate of consolidation. The increasing use of molecular genetic tools (e.g., optogenetics) can be expected to further improve understanding of the neural mechanisms underlying consolidation.Memory consolidation refers to the process by which a temporary, labile memory is transformed into a more stable, long-lasting form. Memory consolidation was first proposed in 1900 (Müller and Pilzecker 1900; Lechner et al. 1999) to account for the phenomenon of retroactive interference in humans, that is, the finding that learned material remains vulnerable to interference for a period of time after learning. Support for consolidation was already available in the facts of retrograde amnesia, especially as outlined in the earlier writings of Ribot (1881). The key observation was that recent memories are more vulnerable to injury or disease than remote memories, and the significance of this finding for consolidation was immediately appreciated.

In normal memory a process of organization is continually going on—a physical process of organization and a psychological process of repetition and association. In order that ideas may become a part of permanent memory, time must elapse for these processes of organization to be completed. (Burnham 1903, p. 132)

It is useful to note that the term consolidation has different contemporary usages that derive from the same historical sources. For example, the term is commonly used to describe events at the synaptic/cellular level (e.g., protein synthesis), which stabilize synaptic plasticity within hours after learning. In contrast, systems consolidation, which is the primary focus of this review, refers to gradual reorganization of the brain systems that support memory, a process that occurs within long-term memory itself (Squire and Alvarez 1995; Dudai and Morris 2000; Dudai 2012).Systems consolidation is typically, and accurately, described as the process by which memories, initially dependent on the hippocampus, are reorganized as time passes. By this process, the hippocampus gradually becomes less important for storage and retrieval, and a more permanent memory develops in distributed regions of the neocortex. The idea is not that memory is literally transferred from the hippocampus to the neocortex, for information is encoded in the neocortex as well as in hippocampus at the time of learning. The idea is that gradual changes in the neocortex, beginning at the time of learning, establish stable long-term memory by increasing the complexity, distribution, and connectivity among multiple cortical regions. Recent findings have enriched this perspective by emphasizing the dynamic nature of long-term memory (Dudai and Morris 2013). Memory is reconstructive and vulnerable to error, as in false remembering (Schacter and Dodson 2001). Also, under some conditions, long-term memory can transiently return to a labile state (and then gradually stabilize), a phenomenon termed reconsolidation (Nader et al. 2000; Sara 2000; Alberini 2005). In addition, the rate of consolidation can be influenced by the amount of prior knowledge that is available about the material to be learned (Tse et al. 2007; van Kesteren et al. 2012).Neurocomputational models of consolidation (McClelland et al. 1995; McClelland 2013) describe how the acquisition of new knowledge might proceed and suggest a purpose for consolidation. As originally described, elements of information are first stored in a fast-learning hippocampal system. This information directs the training of a “slow learning” neocortex, whereby the hippocampus gradually guides the development of connections between the multiple cortical regions that are active at the time of learning and that represent the memory. Training of the neocortex by the hippocampus (termed “interleaved” training) allows new information to be assimilated into neocortical networks with a minimum of interference. In simulations (McClelland et al. 1995), rapid learning of new information, which was inconsistent with prior knowledge, was shown to cause interference and disrupt previously established representations (“catastrophic interference”). The gradual incorporation of information into the neocortex during consolidation avoids this problem. In a recent revision of this framework (McClelland 2013), neocortical learning is characterized, not so much as fast or slow, but as dependent on prior knowledge. If the information to be learned is consistent with prior knowledge, neocortical learning can be more rapid.This review considers several types of evidence that illuminate the nature of the consolidation process: studies of retrograde amnesia in memory-impaired patients, studies of healthy volunteers with neuroimaging, studies of sleep and memory, studies of experimental animals, both with lesions or other interventions, and studies that track neural activity as time passes after learning.     

Memory.

The key interrelated issues in the neurobiology of memory are to identify the neural circuitries essential for memory formation, localize sites of memory storage and analyze mechanisms of memory formation, storage and retrieval. Several circuits have now been identified in vertebrates and researchers are investigating their properties, in particular the role of glutamate receptors and long-term potentiation, in memory formation. Invertebrate preparations continue to be of value and recent studies suggest that changes in gene expression and protein synthesis may be important in long-term sensitization.     

Active Memory

Collective memory is a phenomenon that receives a different interpretation depending on the discipline within which it is examined. In my research, I will employ ideas and concepts developed within the framework of a sociocultural approach (Wertsch, 2002; Cole, 1996). In particular, I will proceed from the definition of collective memory developed by James Wertsch (2002), who treats collective memory as textually mediated memory, conditioned by various—mainly historical—narratives. Historical narratives (annals, chronicles, history textbooks, etc.) are considered to be cultural instruments promoting collective remembrance. A category I borrow from Wertsch's work (2002) is the concept of the "schematic narrative template," which I will apply as analytic tool in my investigation of the phenomenon of collective memory phenomenon.     

Memory grows

Summary The thesis is reaffirmed that form memory (recognition) resides in the morphology of neuronal arborescences, the latter constituting physiological counterparts of local phase portraits of the infinitesimal transformation groups involved. At birth the brain comes equipped with essentially its full complement of neurons. These are initially in a very primitive, almost neuroblast form, but subsequently rapidly proliferate and branch, thus keeping pace with the growth of memory and learning. The Neuron Doctrine is equivalent to the assertion that the neuron constitutes the infinitesimal generator of our perceptions and cognitions. Memory thus consists, in the present view, simply of invariant recognition under time changes. The usual mathematical structure governing invariance in the presence of an infinitesimal operator, namely, Lie transformation groups, together with their prolongations to establish higher differential invariants, then indicates how the engrain is laid down. Learning takes place via differential refinements of already existing neuropsychological invariances, and is embodied in growth of the neuronal arborescence. Empirical support for this hypothesis is discussed: Ribot's law of psychological regression, the characteristics of short-term memory consolidation, agreement between neuron morphology and local phase portrait, neuronal packing density, and persistence of memory through topological lesions. The view advanced here is in no essential conflict with the currently fashionable idea of memory molecules. The generation of such macromolecules is incidental to the neuroplasmic flow process, which acts to extend the neuronal arborescence in the presence of a stimulus.     

Memory systems

Two recent findings are summarized here that bear on the organization of memory and brain systems. First, the capacity for simple recognition of familiarity (a form of declarative memory) depends on the hippocampal region in both humans and nonhuman primates. Second, probabilistic classification learning (a form of nondeclarative memory akin to habit learning) depends on the caudate nucleus and putamen. These findings are related to the classification of long-term memory and current understanding of the participating brain systems.     

Culture and Memory

This article presents the results of an investigation of cross-cultural mnemic processes among Moroccan and Ukrainian students, specifically, the effect of placement within a series (including on the right or left side of the visual field), the type of stimulus material (numbers, pictures, geometric figures, color, images of people), and exposure time on recall. The study identified universal and specific features of the recall process and demonstrated that cultural practices and types of activities—reading and writing left to right versus right to left, traditions of visually depicting humans, the form in which texts are displayed (different letterings, calligraphy) and so forth—can shape certain features of memory and recall.     

Memory Detection 2.0: The First Web-Based Memory Detection Test

There is accumulating evidence that reaction times (RTs) can be used to detect recognition of critical (e.g., crime) information. A limitation of this research base is its reliance upon small samples (average n = 24), and indications of publication bias. To advance RT-based memory detection, we report upon the development of the first web-based memory detection test. Participants in this research (Study1: n = 255; Study2: n = 262) tried to hide 2 high salient (birthday, country of origin) and 2 low salient (favourite colour, favourite animal) autobiographical details. RTs allowed to detect concealed autobiographical information, and this, as predicted, more successfully so than error rates, and for high salient than for low salient items. While much remains to be learned, memory detection 2.0 seems to offer an interesting new platform to efficiently and validly conduct RT-based memory detection research.     

Memory and Learning

The main principles of information processing in the nervous system of animals and man may be defined as follows:

1. A stimulus acting upon the input of a system is transformed in receptors, where it forms a combination of receptor excitations—the EM-dimensional receptor vector of excitation.