Abstract
Fast learning designates the behavioral and neuronal mechanisms underlying the acquisition of a long-term memory trace after a unique and brief experience. As such it is opposed to incremental, slower reinforcement or procedural learning requiring repetitive training. This learning process, found in most animal species, exists in a large spectrum of natural behaviors, such as one-shot associative, spatial, or perceptual learning, and is a core principle of human episodic memory. We review here the neuronal and synaptic long-term changes associated with fast learning in mammals and discuss some hypotheses related to their underlying mechanisms. We first describe the variety of behavioral paradigms used to test fast learning memories: those preferentially involve a single and brief (from few hundred milliseconds to few minutes) exposures to salient stimuli, sufficient to trigger a long-lasting memory trace and new adaptive responses. We then focus on neuronal activity patterns observed during fast learning and the emergence of long-term selective responses, before documenting the physiological correlates of fast learning. In the search for the engrams of fast learning, a growing body of evidence highlights long-term changes in gene expression, structural, intrinsic, and synaptic plasticities. Finally, we discuss the potential role of the sparse and bursting nature of neuronal activity observed during the fast learning, especially in the induction plasticity mechanisms leading to the rapid establishment of long-term synaptic modifications. We conclude with more theoretical perspectives on network dynamics that could enable fast learning, with an overview of some theoretical approaches in cognitive neuroscience and artificial intelligence.
Fast Learning Behaviors
Fast learning mechanisms are best characterized in one-shot or single-trial learning paradigms which lead to memory formation after a single and brief (few hundred milliseconds to few minutes) exposure to relevant stimuli (Figures 1A–D). Indeed, what distinguishes fast learning are the features of the encoding stage (or learning experience): fast learning refers here to situations where memory traces are rapidly formed without requiring repetitions of the learning experience.
Figure 1
In animal models, a single exploration of a new place or novel objects can evoke long-lasting memories, as assessed in rodents using the delayed-matching-to-place task in a watermaze or novel object recognition (Ennaceur and Delacour,
In humans, episodic memory, referring to the formation and maintenance of memory traces of unique experiences, illustrates well fast learning. It should be noted that the existence of episodic memory in non-human mammals has remained controversial in the literature (Clayton et al.,
Neuronal Activity During Fast Learning
In vivo recordings of spiking activity during a fast learning experience reveal sparse activity modulation and the occurrence of few bursting events (Figures 1E,F). During a ~1 s visual stimulation, few neurons from human or monkey anterior and medial temporal cortical lobes and basolateral amygdala (BLA) fire ~5–20 spikes (Messinger et al., 2001; Rutishauser et al., 2006; Ison et al., 2015). These brief presentations of a novel stimulation can however induce significant changes in neuronal activity lasting for at least 10 h (Fried et al., 1997; Xiang and Brown, 1998; Rutishauser et al., 2006) and even allow distinguishing between subjects having perceived the stimulus as a novel or familiar (Fried et al., 1997; Rutishauser et al., 2006). During active touch or passive sensing, sparse activity is also detected in the visual, somatosensory, and auditory cortices, with 0.5–5% of neurons increasing their firing rates in cats, ferrets, or rodents (Yao et al., 2007; Hromádka et al., 2008; Jadhav et al., 2009; O’Connor et al., 2010; Tang et al., 2018; Yoshida and Ohki, 2020; Figure 1E). Such sparse activity is accompanied by the emission of bursts in pyramidal cells, coupled to active dendritic events, in sensory cortices and in the hippocampus (Xu et al., 2012; Smith et al., 2013; Takahashi et al., 2016; Manita et al., 2017; Figure 1F). Moreover, a single presentation of a sensory stimulus induces short-term reverberatory patterns in spontaneous activity during at least a few minutes (Yao et al., 2007), and persistent changes in receptive fields, lasting for several hours (Fritz et al., 2003). Similarly, a single passage in a maze, inducing few bursting events (O’Keefe and Recce, 1993; Epsztein et al.,
Altogether, despite the low number of spikes or bursting events, the induction of persistent selective responses can be initiated after a single or few stimulus presentations across the brain and thus serve as neural indicators of acquired memory traces.
Fast Learning-Induced Neuronal Long-Term Changes
Long-term changes have been reported subsequently to various fast learning tasks. In particular, some studies identified such long-term changes in cells activated during the learning experience, thus giving privileged access to understanding the nature of fast learning engrams, that are the set of “enduring physical and/or chemical changes elicited by learning and that underlie a newly formed memory” (Josselyn et al., 2015; Tonegawa et al., 2015; Josselyn and Tonegawa, 2020).
Fast Learning-Induced Structural, Synaptic, and Intrinsic Plasticity Changes
One-shot learning tasks can be sufficient to activate some immediate-early genes or cellular transcription factors in hippocampal, BLA, or cortical neurons (Radulovic et al., 1998; Sananbenesi et al., 2002; Miyashita et al., 2009; Liu et al., 2012; Fellini and Morellini,
Long-term structural and synaptic plasticity changes have been reported in subcortical areas after a fast learning experience and specifically linked to engram cells in fear conditioning protocols: in vivo measurements of field-EPSPs reveal long-term potentiation (LTP) in rat dentate gyrus during spatial exploration, and at CA3-CA1 synapses after novel object recognition or an inhibitory avoidance task in mice (Moser et al., 1993; Whitlock et al., 2006; Clarke et al.,
Long-term intrinsic plasticity changes are also evoked by a one-shot learning experience: hippocampal neurons activated by a single fear conditioning protocol become more excitable during several days, thus potentially facilitating subsequent learning (Crestani et al.,
Fast Learning Neocortical Engrams
While fast learning mechanisms have historically been associated to allocortical and subcortical areas (McClelland et al., 1995; Buschman and Miller,
These experiments suggest that the neural correlates of one-shot learning experience engage molecular machinery and cellular processes similar to those reported after repetitive training, such as the long-term activation of the same genetic markers, the establishment of long-term synaptic and structural changes, or the requirement for NMDA receptors. Yet, several questions remain to be elucidated, such as the modalities of induction and the extent (both in terms of magnitude and number of cells engaged) of fast learning-induced changes compared with repetitive training.
Deconstructing Fast Learning Synaptic Plasticity Mechanisms
In light of the aforementioned results and given the neuronal activity patterns observed during fast learning, we now present some hypotheses on the induction mechanisms of fast learning-induced synaptic changes. Two natural hypotheses ensue: fast learning could constitute a condensed version of synaptic events similar to those occurring during repetitive learning and/or fast learning could be enabled by intrinsically stronger synaptic events. In the former scenario, the difference between the two learning processes would lie in the sensitivity of plasticity induction, a factor that could be modulated by the initial state of the synapses (with for instance more or less available membrane voltage-gated channels) or the efficiency of neuromodulatory systems. In the latter scenario, fewer synaptic events may be needed to initiate long-term changes, such that a one-shot exposure may be sufficient to drive the formation of a memory trace. The excitability of the active cells and/or the activation of some specific membrane channels could promote the generation of larger events, such as amplificatory dendritic phenomena or larger post-synaptic responses leading to stronger calcium influx.
Due to the sparsity of direct links between neuronal activity and synaptic changes of engram cells, we mostly refer here to in vitro electrophysiological experiments using brief stimulation protocols, mimicking activity patterns observed during a fast learning experience, and capable of inducing long-term synaptic plasticity in randomly selected neurons (Figures 1G,H, 2). We also examine the impact of additional factors such as short-term intrinsic changes and neuromodulators. The following experiments provide first insights into the putative induction mechanisms, but further work is needed to apply similar protocols on to-be engram cells and link their artificially-induced long-term changes to learning, as well as to observe in vivo natural plasticity rules.
Figure 2

Elementary cellular mechanisms of fast learning. Schematic diagram of the putative cellular mechanisms leading from a one-shot experience to a long term memory. A one-shot experience leads a small fraction of cells to fire a few spikes (sparse network activity, left), and/or few bursts accompanied by active dendritic events (right). Neuromodulators can also be released, particularly in the presence of salient elements in the stimulus (e.g., novelty or rewards). In vitro evidence showed that neuromodulators play a role in the selection of patterns, as well as in the induction of short-term changes in excitability that could prime neurons to become engram cells. Neuromodulators can also lower the synaptic plasticity induction threshold, thereby facilitating long-term plasticity. Such long-term plasticity can then be consolidated by specific gene expression, structural changes, or reverberatory activity, altogether leading to the emergence of long-term memory following a single experience.
Induction of Long-Term Synaptic Plasticity Under Sparse and Burst Firing
Sparse Firing
Only a few spikes may be transmitted between neurons during a one-shot experience: this constraint could potentially conflict with the classic Hebbian framework requiring repetition or persistence of a given activity pattern to induce stable long-term synaptic plasticity (Hebb, 1949). Yet, in vitro studies demonstrate that few coincident activities can be sufficient to induce spike-timing-dependent plasticity (STDP) in several brain areas. No more than ~10–15 spike-EPSP pairings between L2/3 pyramidal cells of the visual cortex are sufficient to induce Hebbian LTP, while LTD induction requires ~30 spikes (Froemke et al., 2006), with classical induction STDP protocols relying on 75–150 pairings. Interestingly, LTP magnitude is not affected by adding more pairings, suggesting a potentially rapid memory acquisition through the induction of an abrupt all-or-none LTP in response to minimal stimulation, as observed at CA3-CA1 synapses (Petersen et al., 1998). Moreover, in striatal projecting neurons and in L5 pyramidal cells of the somatosensory cortex, in vitro STDP paradigms involving ~5–15 cortico-striatal pairings induce an endocannabinoid-mediated LTP (eCB-LTP; Cui et al.,
Burst and Active Dendritic Events
Since bursts are transmitted more efficiently than isolated spikes, they could increase the signal-to-noise ratio of the network information transmission, and therefore appear as a privileged signal for inducing long-term changes during fast learning (Lisman, 1997; Krahe and Gabbiani, 2004; Figure 2). Indeed, triggering a single synaptic stimulation of L5 pyramidal neurons of the visual cortex induces LTD in vitro, only if the paired excitatory postsynaptic potential (EPSP) produces an NMDAR-dependent dendritic spike (Holthoff et al., 2004; Figure 1H). Similarly, a single burst of activity in Schaffer collaterals induces LTP, under the condition of triggering a postsynaptic dendritic spike and activating NMDAR and L-type voltage-gated calcium channels in CA1 pyramidal cells (Pike et al., 1999; Wittenberg and Wang, 2006; Remy and Spruston, 2007). Importantly, a pioneer in vivo study by Bittner et al. (
Factors Facilitating Long-Term Synaptic Plasticity Under Sparse and Burst Firing
Intrinsic Plasticity
Changes in neuronal excitability not only support synaptic changes as described above but can also act as a short-term priming mechanism (Figure 2). Indeed, intrinsic modulation of neuronal excitability generally has a lower induction threshold than synaptic plasticity and could contribute to induce early changes in neuronal activity (Titley et al., 2017) that will later favor the establishment of synaptic plasticity, even under sparse activity (Sah and Bekkers, 1996; Louise Faber et al.,
Neuromodulation
Exposure to novel or salient stimuli releases neuromodulators necessary for spatial memory, context, and object recognition or one-shot emotional learning (Duszkiewicz et al.,
Reverberating Activity, as a Bypass to Sparse Firing
In addition to plasticity rules activated by small numbers of spikes, spontaneous replications of the associated neuronal activity may contribute to consolidating one-shot memories according to the classical Hebbian framework (Figure 2). Reverberating activity is observed especially during slow-wave sleep up to 48 h after a transient tactile exploration of novel objects (Ribeiro et al., 2004), and replay of past trajectories, in awake or asleep animals, is associated to memory consolidation in the hippocampus, ventral striatum and neocortex (Wilson and McNaughton, 1994; Hoffman and McNaughton, 2002; Pennartz et al., 2004; Ólafsdóttir et al., 2018). Interestingly, the awake replay is more prevalent and precise for trajectories in novel environments or associated with salient elements (e.g., a reward, Carr et al.,
Reconstructing Fast Learning in Neuronal Networks
Fast Learning and Slow-Fast Network Dynamics
If fast learning is best epitomized in one- or very few-shots learning tasks, it may also apply to the initial stages of repetitive and sustained training (Karni et al., 1998; Muellbacher et al., 2002; Costa et al.,
How to Control Learning Speed?
If fast learning can confer strong advantages, e.g., in survival-threatening situations, this strategy is not always adapted and can potentially lead to detrimental responses, such as superstitious behaviors, when an outcome is too rapidly associated with the wrong cause. Typically, procedural learning based on trial-and-error, or reinforcement learning, in which a trade-off slowly balances exploration and exploitation, necessitates several sessions before reaching an optimal behavior. Hence, control of the learning speed or meta-learning should also be seen as a critical component of the learning process. Yet, little is known about how the brain is implementing and switching between different learning strategies. Recent studies highlight how an accurate tracking of feedback (Mao et al., 2019) or stimulus saliency (Ceballo et al.,
A Computational Perspective on a Fast Learning
Physiologically realistic models of fast learning have remained scarce and may require the development of new synaptic learning paradigms (Brea and Gerstner,
In machine learning, various methods were developed to learn from few examples (Botvinick et al.,
Conclusion
Fast learning is thus a crucial component in daily life memory acquisition that involves one-shot learning experiences. If fast learning can be characterized by the brevity and rarity (or even uniqueness) of the learning experience, the processes involved in memory acquisition and maintenance are embedded in multiple timescales, considering their interactions with meta-learning and consolidation systems. While current findings have lifted part of the veil on fast learning engrams, several mechanisms remain to be further elucidated. In particular, causal interactions between minimal activity patterns and the induction of long-term changes during one-shot learning task remain to be further explored in vivo, as well as the cellular and molecular determinants controlling the learning speed across brain areas and in different contexts. These elucidations could uncover additional components of cellular and synaptic-based learning rules and would allow the development of more physiological models of fast learning.
Statements
Author contributions
CP wrote the “Fast Learning Behaviors, Neuronal Activity During Fast Learning, Fast Learning-Induced Neuronal Long-term Changes, Deconstructing Fast Learning Synaptic Plasticity Mechanisms, and Reconstructing Fast Learning in Neuronal Networks” sections and designed (Figures 1, 2). JT wrote the “A Computational Perspective on a Fast Learning” section and designed Figure 2. LV edited all versions of the manuscript and designed the Figures 1, 2. All authors contributed to the article and approved the submitted version.
Funding
CP was supported by Ecole Normale Supérieure PhD fellowship. This work was supported by grants from the Fondation du Collège de France and the CRCPEN.
Acknowledgments
We thank Gaëtan Vignoud and Nicolas Gervasi for helpful suggestions and critical comments.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
- BLA
basolateral amygdala
- CTA
conditioned taste aversion
- eCB
endocannabinoid
- EPSP
excitatory postsynaptic potential
- LTD
long-term depression
- LTP
long-term potentiation
- mPFC
medial prefrontal cortex
- NMDAR
N-methyl-D-aspartate receptor
- NMDAR-LTP
NMDAR-mediated LTP
- STDP
spike-timing-dependent plasticity.
Abbreviations
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Summary
Keywords
fast learning, one-shot learning (OSL), memory engram, synaptic plasticity (LTP/LTD), neuromodulation, neurocomputational models, artificial intelligence
Citation
Piette C, Touboul J and Venance L (2020) Engrams of Fast Learning. Front. Cell. Neurosci. 14:575915. doi: 10.3389/fncel.2020.575915
Received
24 June 2020
Accepted
24 September 2020
Published
28 October 2020
Volume
14 - 2020
Edited by
Robert C. Froemke, New York University, United States
Reviewed by
Helen Barron, University of Oxford, United Kingdom; Andreas Toft Sørensen, University of Copenhagen, Denmark
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© 2020 Piette, Touboul and Venance.
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*Correspondence: Laurent Venance laurent.venance@college-de-france.fr
Specialty section: This article was submitted to Cellular Neurophysiology, a section of the journal Frontiers in Cellular Neuroscience
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