Layman’s summary In this literature review I discuss the temporal and spatial properties underlying our memory processes. Research of spatial processes has been focused on neurons in the rat hippocampus that fired at particular locations. These cells are now called place cells and together with other spatially sensitive neurons they are thought to underlie a great deal of our spatial processing. Recently, cells have been discovered that seem to do the same for our temporal processing, these cells fired at specific moments in time and were thus called time cells. I discuss the temporal and spatial processes in the hippocampus and the memory processes they are thought to underlie. Furthermore, I also discuss the limitations of these time and place cells and the gaps in the literature. Finally, I emphasize the importance of a network including the hippocampus and the entorhinal cortex and the need for more research into this network regarding spatiotemporal processing in our memory system. In a galaxy not too far away; Space and Time in the Hippocampal-Entorhinal System S. van Bijnen1 1 Experimental Psychology, Utrecht University, Utrecht, The Netherlands. Corresponding author: s.vanbijnen@students.uu.nl Abstract The long tradition of research on hippocampal involvement in spatial memory and spatial properties of episodic memory has often been focused on spatially sensitive neurons called place cells. Together with other spatially sensitive neurons they are proposed to function as an internal representation of the outside world (i.e. a cognitive map). More recently, cells were discovered that encode for a specific moment time. These ‘time cells’ have been found to parse temporally defined environments analogous to how place cells parse spatially defined environments. Similarities between time and place cells have led researchers to propose that they are not distinct cell types, but rather that cells become sensitive to either spatial or temporal information based on the context in which learning occurs. Some evidence supports this notion. However, if time cells, like place cells, are to be an important component of episodic memory, they should be able to code for multiple time domains. The hippocampalentorhinal system has been proposed to be able to do this using distinct, variable, neuronal representations. Interestingly, input from grid cells in the entorhinal cortex to CA1 cells in the hippocampus have also been suggested to combine linearly to create the place fields, emphasizing the importance of this network. However, the role of the hippocampal-entorhinal system in the creation and stability of place cells have been questioned. Concluding, evidence is beginning to support a role for the temporally sensitive neurons in episodic memory, analogous to the spatial sensitive neurons. However, the importance of the hippocampal-EC connections for the basic properties of both time and place cells needs further investigation. Key words: Hippocampus – Episodic memory – Entorhinal cortex – Place cell – Time cell Introduction As one of the neurocognitive memory systems, episodic memory is an impressive neuroevolutionary feat that is supposedly uniquely human. Tulving (2002) defined it as “the ability to remember personally experienced episodes in a spatial and temporal context”. It is often referred to as “what”, “where” and “when” memory because episodic memory makes it possible to re-experience personal events (what) in a spatial (where) and temporal (when) context. Importantly, episodic memory is a hypothetical memory system, assumed to have evolved from semantic memory; it is defined by its properties and functions and it shares many features with other memory systems. According to Tulving (2002) however, core features of episodic memory are the self, autonoetic awareness and subjectively sensed time. These features allow us to mentally travel through time; we consciously recollect and reexperience events while being aware of the experiencing self through time. This paper will focus on the spatial (where) and temporal (when) components and structures of episodic memory. The hippocampus has been implicated in a broad range of memory processes, including but not limited to: spatial memory, spatial properties of episodic memory, recollection and temporal order of events (Bird & Burgess, 2008; Eichenbaum, Yonelinas, Ranganath, 2007; Eichenbaum, 2013). However, as Tulving already stated, it is not fruitful to identify particular memories as being in one memory system or another (Toth & Hunt, 1999, p. 233 in Tulving, 2002). Therefore, I discuss the neural mechanisms proposed to underlie the spatial and temporal mechanisms of (episodic) memory. Importantly, they also subserve other cognitive (memory) processes. The debate about the neural mechanisms behind spatial properties of episodic memory has been strongly focused on a specific type of cells, called place cells, in the hippocampus. Recently, MacDonald and colleagues (2011) discovered cells that encode for specific moments in time and named them “time cells”. This focused the attention of researchers on the hippocampus and its proposed role in episodic memory once again. However, knowledge about the place cells greatly exceeds that of the time cells. First, I will summarize the findings and conclusions related to place cells and spatial processing. Next, I will move on to time cells and temporal processes in the hippocampus. Finally, I will discuss the spatio-temporal context that time and place cells are thought to underlie. Place cells and spatial processing In 1971, O’Keefe and Dostrovsky discovered pyramidal cells in CA1 and CA3 region of the rat hippocampus that increased their firing rate at particular locations. They recorded extracellular action potentials from the rat hippocampus which showed spatially localized firing. These cells characteristics make them very likely to function as a marker for certain locations in an environment and were thus termed “place cells”. Importantly, their firing rate was independent of the orientation of the rats head as well as which direction the rat was heading (O’Keefe & Dostrovsky, 1971). Numerous studies have been performed since, vastly increasing the knowledge of place cells as well as documenting these cells in monkeys and humans (Ono, Nakamura, Fukuda & Tamura, 1991; Ekstrom et al., 2003). Originally it was related to spatial cognition, however, the underlying mechanisms are very like to subserve multiple cognitive (memory) processes. In the recent decade studies have further investigated O’Keefe and Nadels’ (1978) proposal that the hippocampus is a neural substrate of a ‘cognitive map’ and how place-cell activity is linked to spatial memory traces. A cognitive map is usually defined as an internal representation of the outside world (Marozzi & Jeffery, 2012). It is a crucial part of spatial cognition and place cells are said to play an important role by representing specific locations in the environment. The discovery of other spatially sensitive neurons provided even more knowledge on how this internal representation might looks like. Head-direction cells, grid cells and border cells were named after their respective functions; head-directions cells showed preferred firing when the animal faced a particular direction, grid cells showed ‘firing fields’ that were evenly spaced representing distance information, finally border cells mark out boundaries of an environment (Marozzi & Jeffery, 2012). Figure 1 shows the firing patterns of the cell types underlying the cognitive map. Fig 1. Firing patterns of the different cell types underlying the cognitive map; (A) place cells show preferred firing at certain locations, (B) head direction cells when facing a particular direction, (C) grid cells provides distance information by evenly spaced ‘firing fields’ and (D) border cells mark out boundaries. Adapted from Marozzi & Jeffrey (2012) and Bird & Burgess (2008) without permission. An important question is how these cells function and interact to encode and represent spatial properties of the environment. Besides incoming perceptual information, place cells also rely on interoceptive signals related to self-motion (Bird & Burgess, 2008). Combined, the many sensory systems that provide input to the place cells converge to supramodal representations of our environment (Marozzi & Jeffrey, 2012). Marozzi & Jeffrey (2012) divided these representations into three broad categories; external positional information (e.g. landmarks, boundaries), diffuse non-positional contextual cues (e.g. color, task instructions) and selfmotion information. Place cells usually have one or two place fields (i.e. a ‘receptive field’ in this case the location to which a place cell responds). These place fields are thought to be generated by different receptive fields from the grid cells, called ‘grid fields’ (Moser, Kropff & Moser, 2008). Spatial properties of grid, head-direction and border cells As can be seen in figure 1, grid cells have multiple receptive fields ‘mapping’ the floor of an environment. Because grid fields have different spacing, ranging from approximately 25 cm for cells situated dorsally, to several meters for the ventral cells (Marozzi & Jeffrey, 2012), it has been proposed that the different grid fields combine linearly to generate the place fields (O’Keefe & Burgess, 2005; Moser, Kropff & Moser, 2008). Cells that are in close vicinity of each other have similar receptive fields regarding spacing and orientation but are offset relative to each other (Marozzi & Jeffrey, 2012). The combined activation of several grid cells would result in a peak of activation at the location where most of the grid cells are in phase, resulting in a place field (Moser, Kropff & Moser, 2008). The fact that grid cells are found in the entorhinal cortex (EC), which is the main cortical input to the hippocampus where the place cells reside, further supports this idea. Grid cells are also important in a process known as ‘path integration’. Path integration relies on proprioceptive, vestibular and efference copies from intended movements to provide information about self-motion. It contains information about the location of the animal, based on its own movement. Besides information on self-motion, grid cells also use environmental information to create and adjust their grid fields. Initially, the fields are determined by intrinsic self-motion information, however, grid fields can be altered after changes in a familiar environment (Marozzi & Jeffrey, 2012). Nevertheless grid cells maintain their firing pattern even when completely deprived of environmental cues (e.g. dark room). This ability to mediate between self-motion based information and information from the environment is why grid cells are likely to play an important role in path integration. Not only the grid cells but also the head-direction and border cells are important parts of the cognitive map as spatial sensitive neurons that help explain how space is represented in the brain. Head-direction cells act as a neural compass, and show the same mediating properties as grid cells that make them likely to be important in the path integration process. Although less is known about how border cells interact with the other spatial sensitive cells, it has been proposed be important for grid cells since the grid fields can be deformed after changes to the environment. Border cells do not show this remapping (Marozzi & Jeffrey, 2012). Reacting to changes is an important ability of these cells, since our environment is subject to constant changes. Updating the cognitive map Our environment is far from static and animals have to constantly adapt to changes. An impressive feat of place cells is the ability to adjust to alterations in our environment. The degree of change varies however, warranting the need for a system that can react to different environmental changes. If the environment has changed significantly, the cognitive map should be able to distinguish it as novel. Indeed Marozzi & Jeffrey (2012) stated that about 50% of the cells keep expressing place fields after significant changes. However, their place fields are unrelated to the locations they coded in the previous environment. Apparently the cells are used to generate a new map of the unfamiliar environment, a process called ‘remapping’. Small changes however, should not lead to complete remapping for obvious reasons. In 2005, Leutgeb and colleagues differentiated between two kinds of remapping: rate remapping and global remapping. Global remapping is a complete reorganization of hippocampal place cells which normally happens when an environment changes completely, for instance when moving from one environment to another (McNaughton, Battaglia, Jensen, Moser & Moser, 2006). It allows us to recognize similar experiences that occur in different spatial contexts (Leutgeb et al., 2005). In contrast, rate remapping allows us to distinguish different kinds of experiences that take place in the same environment. Place cells do this by either changing only the firing rates of the cells while maintaining the location of the place fields (rate remapping) or, in case of global remapping, changing both the firing rate and the location to statistically independent values (Leutgeb et al., 2005). Concluding, place cells have the ability to decide if environmental changes are substantial enough to warrant remapping, and if so, distinguish between rate remapping and only change firing rate and global remapping by changing both firing rate and location. To do so we need memory traces. Sensory information accompanying the experience has to be compared to the memory trace to decide if the environment is; constant (recognized), changed (familiar) or completely novel. The hippocampus is likely to be an important structure in this process, as small differences in cortical input are hypothesized to be amplified as they propagate through the hippocampal network (McNaughton et al., 2006). Importantly, there is evidence that strongly suggest that place cells and other spatially sensitive cells represent memory traces. Hippocampal memory system and spatially sensitive neurons A long tradition of research on hippocampal involvement in episodic memory combined with the discovery of place cells led to the suggestion that the physiology of spatial encoding allowed the cells to function as a memory trace (Moser, Kropff & Moser, 2008; Marozzi & Jeffery, 2012). Several findings supported this hypothesis, more or less indirectly. First, Lever and colleagues (2002) found that place fields are able to remain stable for several weeks. Second, by using ‘pattern completion’, place cells can hold this stability after minor changes to an environment. This allows the animal to stay orientated in familiar environments even when certain landmarks or features have changed. In contrast, when the environment changes too much, ‘pattern separation’ helps create distinct representations (Bird & Burgess, 2008). Bird & Burgess (2008) defined pattern separation as “a process by which small differences in patterns of input activity are amplified as they propagate through a network”, similar to the definition of McNaughton et al. (2006). Pattern completion/separation and memory processes have often been associated with an attractor network. This is a network of neurons that have recurrent connections and one or more preferred patterns of firing rates called ‘stable states’. The stable states depend on the strength of the connections. Pattern completion occurs when the network ends up in one of the stable states (Bird & Burgess, 2008). This is related to the earlier discussed rate remapping, the environment has to be compared to memory traces before they can signal whether or not the animal is in a novel or familiar environment. Nakazawa and colleagues (2004) listed four necessary attributes of memory traces. First, the memory trace should be experience dependent; place cells should only encode for spatial locations after an environment has been experienced. This is a relative straightforward attribute, but it has been difficult to prove; Nakazawa et al. (2004) stated that the code of a novel space improves rapidly with exploration of the environment but monitoring of single cells proves to be difficult because the speed of improvement in the coding is too fast. Furthermore, Marozzi & Jeffery (2012) described sequences of place cell activation during sleep that was highly correlated with the sequence of activation during the original experience in wakefulness. It was proposed that this is part of the consolidation process; by replaying the experiences, memories can be transferred or copied to the neocortex (Marozzi & Jeffrey, 2012). However, these sequences have also been observed even before an environment has been explored. This is a correlation between sleep and waking place cell activation sequences and the function of this phenomenon is not yet clear. The second, third and fourth attribute are supported by more convincing results. After re-exposure to an environment a unique set of place cells is reactivated (criterion 2). Furthermore, the firing patterns are persistent and remain stable after the animal is exposed to the information (criterion 3). Finally, when an animal is presented with a subset of the original cues, the same set of place cells are reactivated (criterion 4) (Nakazawa et al., 2002; Muller & Kubie, 1987; Nakazawa, McHugh, Wilson & Tonegawa, 2004). Another reason why place cells are suggested to be neural memory traces is the role of long term potentiation (LTP) in the formation place fields and the activity of place cells. The first clue was that NMDA receptor (NMDAR)-dependent LTP is essential in the hippocampal memory system (Nakazawa et al., 2004; Isaac, Buchanan, Muller & Mellor, 2009). LTP is an enhancement of signal transmission due to a synchronous stimulation of neurons and it has been widely proposed to play a crucial role in learning and memory. This synaptic plasticity is important because this chemical process allows for a strengthening of the synapse. Hebb (1949) already proposed memory might be encoded this way. It has been proposed that place fields emerge through LTP like mechanisms (Moser, Kropff & Moser, 2008; Isaac et al., 2009). Indeed, NMDAR-dependent LTP has been observed in place cell firing patterns, but only when the place cells showed overlapping place fields. The same study also showed that LTP in place cell firing patterns rely on cholinergic tone, which increases in behavioral conditions such as exploration (Isaac et al., 2009). Both pharmacological intervention and gene knock-out studies have been performed to study LTP and place cell activity with mixed results. Kentros et al. (1998) blocked NMDAR using an antagonist on CA1 place cell formation and maintenance called CPP. They found that the formation and short term stability of the place fields were independent of NMDAR activation. In contrast, long-term stability of newly formed place fields relied on NMDAR functioning (Kentros et al., 1998). This contrasts gene knock out studies who did find NMDAR dependent LTP in place field formation (Nakazawa et al., 2003). Nakazawa and colleagues (2004) explained this difference by highlighting the methodological differences of gene knock-out and pharmacological studies. Pharmacological studies show less regional specificity but have superior temporal control. The exact role of NMDAR dependent LTP in place fields formation and maintenance is not yet entirely clear. For now, the consensus is that new place cell activity can be initiated even when NMDARs are blocked, however, NMDAR-dependent plasticity is thought to be crucial for place cell map stability (Kentros et al., 1998; Isaac et al., 2009; Nakazawa et al., 2004). Importantly, these studies support the hypothesis that place cells are neural memory traces. Next, I will focus on the temporal properties of the place cells and discuss the recently discovered (and possibly related) ‘time cells’. Temporal processing in the hippocampus Already since Hebb (1949) described his cell assembly theory it has become clear that the temporal dimension of neural representations is important in our thought as well as our learning and memory capabilities. His theory stated that persistent or repetitive activity of cell assemblies induces lasting cellular changes that add to its stability. He emphasized that temporal precedence is a requirement in this process. The hippocampus has been implicated in both recollection and episodic memory making it a likely candidate to play an important role in the temporal processing of memories. Interestingly, evidence also suggests that place cells have a temporal aspect to them. Neuronal populations in the hippocampus show membrane oscillations in the theta range (612 Hz). Place cells show theta phase precession when animals run through its place fields. This means that place cells fire at progressively earlier phases of the theta rhythm when animals follow a fixed path (O’Keefe & Recce, 1993; Marozzi & Jeffery, 2012; Moser, Kropff & Moser, 2009). This phase precession is highly correlated with an animal’s location (Marozzi & Jeffery, 2012). Thus, using the phase of the theta oscillation the brain can determine the relative locations of the place fields. The precise neuronal mechanisms behind temporal processing of place cells are not yet clear, but temporally sensitive neurons have been discovered that are suggested to bridge the gap (MacDonald, Lepage, Eden & Eichenbaum, 2011). Time cells In 2011, MacDonald and colleagues discovered pyramidal cells in the same region of the hippocampus where the place cells are found. These cells responded to specific moments in a temporally structured sequence. They were dubbed ‘time cells’ because they are thought to be analogous to place cells in the sense that where place cells encode locations of a spatially structured environment, time cells encode moments in a temporally structured period (Eichenbaum, 2013). The task that MacDonald et al. (2011) used involved rats that had to learn associations between specific objects and certain odors with a 10 second delay. Initially, rats were presented with an object which they had to poke with their nose. When they poked the object a 10 second delay followed after which the rat had to respond to one of two odors to get a reward. Figure 2 shows the task and the hippocampal neurons that fire sequentially, together filling the 10 second delay period. Fig 2. In the task (left), rats had to poke the object and, after a 10 second delay, respond to an odor. The neuronal firing sequence (right) suggest that this assembly of neurons encode moments in time. Taken from Eichenbaum (2013) without permission. The firing sequence reflected the passage of time even after controlling for location, head direction and movement speed (MacDonald et al., 2011; Eichenbaum, 2013). Time cells seem to show the same characteristics of memory traces, described by Nakazawa et al. (2004), as place cells do. First, time cell activation patterns are experience dependent. In a typical experiment, rats are taught that a specific delay of time exists between two events (for example, exposure of an object followed by a scent; Eichenbaum, 2013). Time cells become sensitive for a specific moment in time within this delay. Second, time cell activation patterns are unique for each moment within this delay (MacDonald et al., 2011). Similarities and differences between place and time cells Time and place cells show some striking similarities. First, time cells parse temporally defined periods in specific chunks, just like place cells parse spatially defined environments. Thus, these cells are either responsive to a moment in time or a specific place in the environment. The spatial or temporal receptive fields of these cells are referred to as ‘placefields’ and ‘time-fields’ respectively. Although ‘re-timing’ has not yet been looked at so extensively as remapping, alterations in the temporal dimension of the task do seem to cause re-timing of hippocampal time cells (MacDonald et al., 2011). Eichenbaum (2013) also noted that that the properties of the time cells parallel the properties of place cells in other ways. He stated that place and time cells could have a similar input source; the medial entorhinal cortex, which is an important relay station for neocortical input to the hippocampus (Eichenbaum, 2013). Furthermore, time and place cells have been found to incorporate both temporal and spatial information. Whether a time/place cell becomes sensitive to spatial or temporal information is thought to depend on the context in which learning occurs (Eichenbaum, 2013). Finally, both place and time cells have been found in the CA1 sub region of the hippocampus. (MacDonald et al., 2011; O’Keefe & Dostrovsky, 1973). Eichenbaum (2013) concluded two important things that warrant more discussion. First, I will discuss his idea that time and place cells are not distinct cell type. Although some evidence has been brought forward, it remains circumstantial. The interaction with the spatially sensitive cells described above will be of special interest. Second, Eichenbaum (2013) claims that these cells establish a spatial and temporal framework for organizing episodic memories. The spatial processing of episodic memory and its neural underpinnings have been discussed and studied at length, the evidence for time cells as a neural mechanism for temporal aspects of episodic memories is weaker Distinct or similar cells Summarizing the similarities described above, Eichenbaum (2013) concluded that the distinction between temporal or spatial encoding of pyramidal cells in the hippocampus is caused by the experimental designs of the different studies. Although some evidence supports this notion, ‘pure’ time cells have been discovered in the hippocampus (Kraus, Robinson, White, Eichenbaum & Hasselmo, 2013). In an attempt to dissociate time and distance as much as possible, Kraus et al. (2013) used an experimental design in which rats ran through a modified version of the T-maze. At the start of maze, instead of roaming free, the rats had to run in a treadmill, while also holding in working memory whether to go left or right. Trials varied between ‘time-fixed’, where rats had to run for a certain amount of time and ‘distancefixed’, where they had to run a certain distance. Using different treadmill speeds, they argued that time and distance were sufficiently dissociated (Kraus et al., 2013). The majority of the cells (70%) activity was best explained by both time and distance. A minority of neurons responded exclusively to time (8%) or distance (11%), suggesting ‘pure’ time and/or place cells do exist. However, the majority of cells seem to have the characteristic that Eichanbaum (2013) proposed. It is not yet known how this switching or differentiating of cells occurs. Jezek, Henriksenm, Treves, Moser & Moser (2011) suggested a role for theta cycles and thought time and distance information could be represented by different phases of the theta oscillation, but Kraus et al. (2013) found no such distinction. As described above, a substantial part of our spatial processing relies on interactions between the spatially sensitive neurons. If time and place cells’ sensitivity to temporal or spatial information is interchangeable, it follows that time cells may also interact with the neurons that were thought to be exclusively sensitive to space. This begs the question whether grid/head-direction/border cells are also involved in temporal processing. Of these cells, grid cells are the most likely to be involved in temporal processes. Grid cells are also the most interesting because it has been suggested that grid fields combine linearly to directly inform the place cells about where to fire (Marozzi & Jeffery, 2012; Moser, Kropff & Moser, 2008). Importantly, if grid fields, through Hebbian learning, contribute to a certain place field and place fields can also be time fields depending on the context then grid fields are likely to contribute to time fields as well. If a given cell can switch between place and time cells, either the grid cells have to discriminate between signaling spatial and temporal information or they too can signal both spatial and temporal information depending on context. Nevertheless, somewhere the information needs to be classified as spatial or temporal. Where and how this happens has yet to be solved. After place and time cells’ supposed role in episodic memory, I will return to the importance of the hippocampal-entorhinal cortex system. Place & Time cells and episodic memory One network state of place cells activation in the hippocampus is suggested to play an important role in consolidating memories. Sharp-wave/ripple (SWR) events are sequences of place cell activation that are highly similar to the ones observed during wakefulness exploration (Marozzi & Jeffery, 2012). Perhaps this place cell ‘replay’ contributes to the transferring of these memories to the neocortex, enabling us to recall the environment without having to be there. However, this replay can occur both forwards and backwards (future exploration) and so the function of SWR-associated replay still has to be determined. Eichenbaum (2013) argues that time cells are able to bridge the gap in the current literature on hippocampal function in episodic memory and spatial mapping. The time cells are hypothesized to provide a temporal framework for episodic memory. This is a tempting conclusion since place cells show a prominent role in both spatial processing and episodic memory. However paradigms investigating time cells all have used relatively short delay periods and MacDonald et al. (2011) made the safer conclusion that it is mainly a way to bridge small temporal gaps in events. If time cells provide a temporal framework of episodic memory, then they should show the same stability over time but they should also extend their activation longer delay periods. Indeed Hassabis & Maguire (2007) already pointed out that when we talk about time in episodic memory, there are at least 2 types that are relevant. The moment by moment order of a certain event of sequence is called ‘micro-time’. The experiments used for time cell research have a relatively short temporal dimension, so when time cells are discussed in the light of episodic memory, it is most probably micro-time that is discussed. Hassabis and Maguire (2007) conclude it is an intrinsic property of episodic memory. Micro-time ensures that episodic memories are recalled in the same or reverse sequence in which they were encoded. The second type is called ‘macro-time’ concerns our subjective sense of time, closely related to the autonoetic awareness described by Tulving (2002) and explained above. It has been proven difficult to provide evidence that suggest a distinct neural mechanism for this kind of time in episodic memory (Hassabis & Maguire, 2007). Here again, the neural mechanisms seem to underlie multiple cognitive processes. At the moment, macro-time is somewhat elusive and sometimes viewed as a reconstructive process of a particular memory and its semantic knowledge (Hassabis & Maguire, 2007). For now, it is unlikely that time-cells are able to provide our subjective sense of time in episodic memories. One line of research argues that temporal processes and episodic memory is supported by the gradual changes of neural representation over time. Recovering this variable neural representation enables us to jump back in time, accompanied by our subjective experience of mental time travel. It is not entirely clear how the different timescales, episodic memory and the hippocampus are related. One study argued that the hippocampus is necessary for keeping track of elapsed time since an event, on fine temporal discriminations, between long intervals (8 vs. 12 min), but not for the same temporal resolution on a shorter timescale (1 vs. 1.5 min) (Jacobs, Allen, Nguyen & Fortin, 2013). So then, time cells are not critical for the timescales generally used in the time cell paradigm. Instead, Jacobs and colleagues (2013) state that they are important for “temporally segregating individual events within the context of an unfolding sequence of events”. Arguably, this is exactly what happens in the 10 second delay period described earlier. For longer intervals, hippocampal-entorhinal interactions could be important. The hippocampal-entorhinal cortex system Combining different studies we are left with three different mechanisms for the broad range of intervals of temporal processing in the medial temporal lobe. First, the time cells described above provide a way of temporal ordering of events on a second to minute’s timescale. Like Eichenbaum (2013) argued, the mechanisms that keep information about past memories, spatial processes and planned goals are also able to keep track of time and bridge noncontiguous events (Buzsáki & Moser, 2013). Time cells do not seem to just ‘mindlessly’ keep track of time, but also provide a temporal context. This is supported by findings that the firing sequences change when the delay between cue and response is changed. Combined with Kraus’ et al. (2013) finding that the majority of cells firing patterns are explained by both spatial and temporal processes, it provides support for Eichenbaum’s (2013) hypothesis that these are not (always) distinct cell types. However, other mechanisms are thought to be important for longer intervals. The second mechanism was described by Mankin and colleagues (2012). Here, they looked at firing consistency and similarity over time. This consistency over longer intervals is a necessity for Eichenbaum’s hypothesis that these cells function as a temporal framework for episodic memory. Mankin et al. (2012) looked at intervals of hours and days and found a distinction between the CA1 and CA3 region of the hippocampus. They found that neuronal activity in the CA1 diverges after longer intervals (i.e. 6-h compared to 1-h). This diverging provides a way for events that are separated by longer intervals to be represented by distinct neuronal representations. This contrasted cell assemblies in the CA3 regions which showed a high degree of similarity in their firing patterns over both longer and shorter intervals (Mankin et al., 2012). Thus, they concluded that the CA1 region of the hippocampus is required for the temporal coding over extended time periods. This is reflected by the decreasing similarity of firing patterns over longer delay intervals. The stability in firing patterns of the CA3 region is thought to provide a stable spatial and contextual representation, often associated with pattern completion (Mankin et al., 2012). So far, this supports Eichenbaum’s (2013) hypotheses, since time cells were discovered in the CA1 region of the hippocampus, just as the variability on the firing patterns were found in the CA1 region and not the CA3 region. Arguably, this could also subserve our re-experiencing of past events. Even though recollecting our personal past appears as a continuous process, it usually involves chunks or parts of an episode at any one time. Another study provided insight into how item (what) and timing (when) information are integrated for episodic memories (Naya & Suzuki, 2011). The hippocampus provides “a robust incremental timing signal” that anchors temporal information to events within an episode. This time information flows to the entorhinal cortex and perirhinal cortex. Here it is integrated with item information from visual cortices (Naya & Suzuki, 2011). However, the question remains on how the different (sub)structures and firing patterns are related. For instance, the variable firing patterns of the CA1 receive input from the CA3 region, which show similar firing patterns over time. Here, it is thought that input from the entorhinal cortex contributes to CA1’s variable firing pattern (Mankin et al., 2012). Indeed, the third layer of the entorhinal cortex has been associated with processing timing information, although possibly to a lesser extent than in the hippocampus (Suh, Rivest, Nakashiba, Tominaga & Tonegawa, 2011; Naya & Suzuki, 2011). However, the direction of flow is unclear. According to Naya & Suzuki (2011) timing information in the service of episodic memory flows from the hippocampus to the entorhinal cortex. In contrast, Mankin and colleagues (2012) suggested that information from the entorhinal cortex contribute to the firing patterns in the hippocampus. There are two closed loop networks present in the hippocampal-EC network. The preforant path (PP) is the main input source from the EC to the hippocampus. It has two major projections (figure 3). One originating in layer II of the EC which project to granule cells of the dentate gyrus (DG) and pyramidal cells of the CA3 region (trisynaptic pathway; TSP). The other originates in layer III of the EC and projects to the subiculum and the CA1 region (monosynaptic pathway; MSP). Furthermore, axons from CA3 region project to the CA1 region which in turn send the main Fig 3. The main input pathways of the hippocampus; the preforant path (PP). Layer III of the EC to subiculum (Sub) and CA1 (red line, MSP) and layer II of the EC to DG and CA3 (blue line, TSP). Axons from CA3 project to CA1, where output is being send back to the EC through the Sub using two pathways, forming two closed loops. Taken without permission from Suh et al. (2011). hippocampal output back to the EC through the subiculum via two pathways, forming two closed loop networks. Given these connections between the hippocampal-entorhinal cortex network, reciprocal communication is inevitable and brain systems involved in spatial processes, memory and temporal processes have been found to overlap to a great extent (O’Keefe & Nadel, 1978; Burgess, Maguire & O’Keefe, 2002; Buzsáki & Moser, 2012). However, how these connections and the direction of flow are connected to function is not entirely understood. Conclusion There has been a long tradition of research on hippocampal involvement in episodic memory, spatial memory processes and, more recently, temporal processes of memory. The discovery of space cells and other spatially sensitive neurons greatly boosted our knowledge on spatial processes in the hippocampus and episodic memory. The more recent discovery of time cells seems to do the same for temporal processes in the hippocampus. Similarities between time and place cell characteristics led to the proposition that they are not distinct cell types, but rather that these cells become sensitive to either spatial or temporal information based on the context in which learning occurs (Eichenbaum, 2013). This is an interesting suggestion since we know a lot more about place cells than we do about time cells. Although some evidence support this idea (Kraus et al., 2013), a notable difference is the range of spacing for spatially and temporally defined environments. Spatially defined environments are somewhat limited in range; place and or grid fields of centimeters to meters are sufficient to “map” the environment spatially. This is accounted for by the topographical organization of place and grid cells. In contrast, a temporal environment, especially in human episodic memory, has a larger reach and it is unclear how this can be accounted for. Furthermore, somewhere the information needs to be classified as spatial or temporal. For both these issues, the hippocampal-entorhinal cortex system might be interesting to look at for several reasons. First, it has been suggested that grid fields in the entorhinal cortex combine linearly to directly inform the place cells about where to fire (Moser, Kropff & Moser, 2008; Marozzi & Jeffery, 2012). However, in transgenic mice in which the layer III input of the entorhinal cortex to the hippocampus (i.e. the monosynaptic pathway) are specifically inhibited, place fields’ basic properties were unaffected (Suh et al., 2011). Arguably, the other afferent projection from the EC to the hippocampus (i.e. the trisynaptic pathway) is important for place cell properties. Second, the different temporal scales (e.g. minutes/hours/days) have been related to the variable EC input to the hippocampus. If place and time cells are the same cells that differentiate based on context of the learning phase, then time cells should also be unaffected by inhibiting the MSP providing entorhinal input to the hippocampus. This is a counterintuitive hypothesis, since the MSP provides the more direct route to the CA1; the region where time cells have been discovered. Figure 4 depicts the discussed findings. The colored arrows indicate the need for more evidence. Nevertheless, just like place cells’ topographical arrangement provides them with a way to represent multiple spatial domains, it can be argued that with varying their firing sequence, time cells also have this ability (Mankin et al., 2012). Although this mechanism does not exclude other mechanisms to account for the multiple domains it is a great step forward in arguing the role for these cells in episodic memory. As already stated, although the recollections of our episodic memories appear continuous, it often involves chunks or parts. This emphasized the need for these multiple domains. Together, it supports the idea that the multiple aspects of memories (what, where & when) coexist in the hippocampus. Even though time stamps over days or weeks or the integration of this time information with the other aspects of episodic memories might be explained by other mechanisms, it could still be able to account for a substantial amount of temporal processes in episodic memory. Especially since it has been argued that the order of a sequence (micro-time) is an intrinsic property of episodic memory (Hassabis & Maguire, 2007). In contrast, it has been questioned whether macro-time (subjective time) can be distinguished from other processes in episodic memory. Furthermore, temporal information has also been proven to be a poor retrieval cue for episodic memory (Hassabis & Maguire, 2007). Thus, a crucial part of temporal properties of episodic memories is the order of events, for which these cells are particular well suited. Hippocampus Domains Seconds – hours Time cells (CA1) Entorhinal interactions with HC Firing sequence variability Spatiotemporal properties of Episodic memory Context Centimeters - meters Place cells (CA1/3) Topographical arrangement Fig 4. Schematic overview of the discussed findings. The colored arrows indicate the need for more evidence. Entorhinal input to the hippocampus has been related to the creation of place fields, but this has been questioned by genetic studies. It has been proposed that cells become sensitive to either temporal or spatial information depending on the context in which learning occurs (red arrow). However, both cells seem to have the ability to represent multiple domains. Together they are thought to underlie a substantial part of the spatiotemporal properties of episodic memory Concluding, there is substantial evidence that supports the idea that temporally sensitive neurons in the hippocampus are an important component of episodic memory, reflected by mechanisms for multiple time domains. However, the interaction between the hippocampus and the EC needs to be investigated further. Studies investigating the importance of this interaction for the basic properties of time and place cells showed mixed results. Importantly, EC input to the hippocampus has been related to both the creation and stability of place fields as well as providing a way for time cells to parse events with longer intervals with distinct (variable) neural representations. 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