Hippocampal neurons code individual episodic memories in humans

Chapter 2: Cellular properties of human hippocampal CA1 neurons. This chapter investigates the structural and functional properties of human hippocampal CA1 neurons, highlighting adaptations that support advanced cognitive processes and how these differ from mice. Human CA1 neurons are significantly larger and more complex than previously understood, with dendritic lengths reaching up to a millimeter. These larger neurons possess a greater number of oblique dendrites, which function as independent processing units, thereby substantially enhancing memory capacity. Additionally, nearly all human CA1 neurons exhibit a resonance frequency closely aligned with low theta rhythms, critical for memory encoding and spatial navigation. In contrast, mouse CA1 neurons are structurally simpler and display resonance less frequently, with a less precise alignment to theta rhythms. This suggests that mouse neurons are adapted for survival-related tasks rather than complex cognitive functions. Furthermore, the memory capacity of human CA1 neurons was found to be 11 times higher, a difference largely attributed to the role of oblique dendrites in humans. These findings provide strong evidence of species-specific adaptations in hippocampal neurons, which underpin humans' advanced cognitive abilities. Chapter 3: Synaptic communication in human pyramidal neurons compared to rodents. This chapter examines how synaptic communication in human pyramidal neurons of the cortex differs from rodents. Human neurons exhibit synaptic conductance and voltage changes three times greater than those of mice, resulting in significantly stronger and more reliable synaptic transmission. Connected human neurons respond nearly 100% of the time, compared to a 25% failure rate observed in rodents. Pharmacological studies confirmed that NMDA receptors play a critical role in this enhanced synaptic strength. Blocking NMDA receptors significantly reduced excitatory postsynaptic potentials (EPSPs) in human neurons, while the effect was less pronounced in rodents. NMDA receptor activation prolongs EPSPs and supports long-term potentiation (LTP), a fundamental mechanism for learning and memory. Interestingly, the enhanced synaptic strength and reliability are not due to a higher number of synapses per connection, as both species have an average of five synapses per pair. Instead, human neurons exhibit larger presynaptic active zones and postsynaptic densities, which enhance neurotransmitter release and receptor binding. Additionally, the human cortex contains approximately three times more total synapses than the rodent cortex, facilitating more long-range cortical connections. These features support complex and integrated neural networks that underlie human-specific cognitive functions. These adaptations illustrate how the human brain has evolved to manage more sophisticated cognitive tasks than the rodent brain. Chapter 4: Regional variability in morpho-Electric properties of human neocortical pyramidal neurons. This chapter explores the regional variability of superficial and deep pyramidal neurons in layers 2 and 3 (L2/L3) of the human neocortex. Superficial neurons, primarily involved in feedback processing, and deep neurons, responsible for feedforward projections, exhibit distinct regional patterns along the rostro-caudal axis. Superficial L2/L3 neurons show a gradient where structural complexity decreases and excitability increases from the frontal to the occipital lobe. This suggests that neurons in the frontal cortex are optimized for higher-order cognitive functions, such as decision-making and working memory. Conversely, neurons in the occipital cortex, with higher excitability, are better suited for rapid sensory information processing. Deep L2/L3 neurons exhibit less pronounced structural and excitability gradients but show a marked increase in HCN channel activity, indicated by a higher sag ratio, along the same axis. This suggests that deep neurons play a key role in modulating cortical excitability and supporting long-distance communication between cortical regions. These findings demonstrate that superficial and deep L2/L3 pyramidal neurons follow distinct specialization gradients. The regional variability of these neurons enables the human neocortex to meet diverse computational demands across different cortical areas.

The hippocampus is crucial for forming new episodic memories. While its role in encoding spatial and temporal information (where and when) is well understood, how it encodes objects (what) remains unclear due to the high dimensionality of object space. Rather than encoding each object separately, the hippocampus may encode object categories to reduce complexity. Here, an experimental‐modeling approach to investigate how the hippocampus encodes visual memory categories in humans is developed. Spikes are recorded from hippocampal CA3 and CA1 neurons in 24 epilepsy patients performing a delayed match‐to‐sample task involving five image categories. An interpretable memory decoding model is employed to decode memory categories from hippocampal spiking activity and identify the spatio‐temporal characteristics of hippocampal encoding. Using this model, the optimal temporal resolutions for decoding each visual memory category per neuron are estimated. Results indicate that visual memory categories can be decoded from hippocampal spike patterns, supporting the presence of category‐specific coding. Hippocampal neuron ensembles encode memory categories in a distributed manner, akin to a population code, while individual neurons use a temporal code. Additionally, CA3 and CA1 neurons exhibit similar and redundant memory category information, likely due to strong and diffuse feedforward synaptic connections from CA3 to CA1 regions.

Early cognitive neuroscientific research revealed that the hippocampus is crucial for spatial navigation in rodents, and for autobiographical episodic memory in humans. Researchers quickly linked these streams to propose that the human hippocampus supports memory through its role in representing space, and research on the link between spatial cognition and episodic memory in humans has proliferated over the past several decades. Different researchers apply the term "spatial" in a variety of contexts, however, and it remains unclear what aspect of space may be critical to memory. Similarly, "episodic" has been defined and tested in different ways. Naturalistic assessment of spatial memory and episodic memory (i.e., episodic autobiographical memory) is required to unify the scale and biological relevance in comparisons of spatial and mnemonic processing. Limitations regarding the translation of rodent to human research, human ontogeny, and inter-individual variability require greater consideration in the interpretation of this literature. In this review, we outline the aspects of space that are (and are not) commonly linked to episodic memory, and then we discuss these dimensions through the lens of individual differences in naturalistic autobiographical memory. Future studies should carefully consider which aspect(s) of space are being linked to memory within the context of naturalistic human cognition. This article is categorized under: Psychology > Memory.

Spiking Irregularity and Frequency Modulate the Behavioral Report of Single-Neuron Stimulation

What-Where-When (WWW) memory tasks have been used to study episodic(-like) memory in non-human animals. In this study, we investigate whether humans use episodic memory to solve such a WWW memory task. Participants are assigned to one of two treatments, in which they hide different coin types (what) in different locations (where) on two separate occasions (when). In the Active treatment, which mimics the animal situation as closely as possible, participants are instructed to memorize the WWW information; in the Passive treatment, participants are unaware of the fact that memory will be tested. In both groups, the majority of participants report using a mental time travel strategy to solve the task, and performance on a different episodic memory test significantly predicts performance on the WWW memory task. This suggests that the WWW memory task is a good test of episodic memory in humans. Participants remember locations and coins from the first hiding session better than they do those of the second hiding session, suggesting their memories may be reinforced during the second hiding session. We also investigated how well episodic memory performance predicted performance on the three aspects of the WWW memory task separately. In the Passive treatment, episodic memory performance predicts performance on all three aspects of the WWW memory task equally. However, in the Active treatment it only predicts performance on the what component. This could imply that during active encoding a different memory system is used for where and when information than during passive encoding. Encoding of what information seems to rely on episodic memory processing in both conditions.

The hippocampus is involved in episodic memory, which is composed of subjective experiences in the multisensory world; however, little is known about the subthreshold membrane potential responses of individual hippocampal neurons to sensory stimuli. Using in-vivo whole-cell patch-clamp recordings from hippocampal CA1 neurons in awake mice, we found that almost all hippocampal neurons exhibited a hyperpolarization of 1-2 mV immediately after the onset of a sound. This large-scale hyperpolarization was unaffected by the duration or pitch of the tone. The response was abolished by general anesthesia and a surgical fimbria-fornix lesion.

Progress on the hippocampal circuits and functions based on sharp wave ripples

Tulving (1972) defined episodic memory as memory for what, where, and when. Clayton and Dickinson’s (1998) behavioral model for animals was adapted to examine what, where, and when memory in humans. Participants viewed unique visual scenes of furnished homes in both blocked and mixed designs. They were tested separately for memory of objects (What), spatial configurations (Where), and temporal order of the scenes (When), and participants’ accuracy and reaction times were examined for each condition. Performance was highest and reaction time was fastest for the What condition. Participants were also faster and more accurate in the mixed design experiment compared to the blocked design experiment. This study established a behavioral analysis of episodic memory in humans based upon Clayton and Dickinson’s (1998) animal model, which will provide a basis for functional episodic memory studies to separately characterize the cortical mechanisms for processing episodic memory using the object, spatial, and temporal task with humans.

Induction and measurement of episodic memories in healthy adults

Mental time travel is defined as the human ability to remember unique personal past experiences (episodic memory) and to anticipate and plan future events. Considerable debate has arisen around the question of whether nonhuman animals are also capable of mental time travel, ranging from complete denial of the ability in nonhumans to the suggestion that they have episodic memory and readily plan for the future. We evaluate the current evidence available from comparative cognition experiments and human-developmental research. Studies of episodic-like memory in birds and nonhuman mammals have centered on their ability to remember what, where, and when a single event occurred. Although clear evidence for memory of what and where has been shown, memory of when does not always appear and may depend on both the species tested and the experimental design used. We argue for a clear distinction between remembering when in absolute time an event occurred and remembering how long ago it occurred. Studies of neural processes indicate that the hippocampus is necessary for episodic memory in humans and episodic-like memory in rats. It is argued that studies of nonhumans should not focus on presence versus absence of human traits but should examine alternative mental time travel abilities in animals that may have evolved independently as adaptations to a particular ecological niche.

Episodic memory: what can animals remember about their past?

A novel experimental approach to episodic memory in humans based on the privileged access of odors to memories

Two strategies used to uncover neural systems for episodic-like memory in animals are discussed: (i) an attribute of episodic memory (what? when? where?) is examined in order to reveal the neuronal interactions supporting that component of memory; and (ii) the connections of a structure thought to be central to episodic memory in humans are studied at a level of detail not feasible in humans. By focusing on spatial memory (where?) and the hippocampus, it has proved possible to bring the strategies together. A review of lesion, disconnection and immediate early-gene studies in animals reveals the importance of interactions between the hippocampus and specific nuclei in the diencephalon (most notably the anterior thalamic nuclei) for spatial memory. Other parts of this extended hippocampal system include the mammillary bodies and the posterior cingulate (retrosplenial) cortex. Furthermore, by combining lesion and immediate early-gene studies it is possible to show how the loss of one component structure or tract can influence the remaining regions in this group of structures. The validity of this convergent approach is supported by new findings showing that the same set of regions is implicated in anterograde amnesia in humans.

Episodic Memory: A Rat Model of Source Memory

Encoding New Episodes and Making Them Stick