16 The Hippocampus Index and Episodic Memory The

16 The Hippocampus Index and Episodic Memory

The Hippocampus and Episodic Memory The episodic memory system captures the content of our experiences in a form that permits us to recollect or replay them. When the hippocampus is significantly damaged, this capacity is lost and we become disconnected from our past. Thus, there is something special about the hippocampal formation and its connections with other brain regions that is fundamental to the episodic memory system.

Properties of Episodic Memory: The Episodic Memory System Supports Conscious Recollection Conscious recollection The episodic system is most often described as supporting memories that can be consciously recollected or recalled. The term conscious recollection has two meanings: • It means intentional in the sense that you actively initiated a search of your memory. In this case the reference would be to the manner in which retrieval is initiated. • It also means that you have an awareness of remembering—a sense that a memory trace has been successfully activated. In this case the reference would be to a subjective feeling that is a product of the retrieval process (Schacter, 1987).

Properties of Episodic Memory: Conscious Recollection and Contextual Information The feeling of remembering emerges when a retrieved memory trace contains information about the time, place, or context of the experience that established the memory. “Once the context is reconstructed, it may seem surprising how easy it is to recall the scene and what took place. In this way, one can become immersed in sustained recollection, sometimes accompanied by strong emotions and by a compelling sense of personal familiarity with what is remembered. ” (Squire & Kandel, 1999)

Properties of Episodic Memory: The Episodic Memory System Automatic Captures Information The episodic memory system automatically captures information simply as a consequence of our exploring and experiencing the environment. The hippocampus does not need to be driven by our intentions or goals to store information. Some researchers also say that it stores incidental information—information that was not the focus of your attention.

Properties of Episodic Memory: Captures Events that Only Occur Once The content of the episodic memory is also described as episodic—it captures information about single episodes of our lives. What constitutes the duration of an episode is vague. However, the gist of this idea is that the declarative system can capture information about an experience that only occurs once.

Properties of Episodic Memory: Stored Memories Are Protected from Interference The episodic system stores highly similarly episodes such as where you parked your car today versus where you parked it yesterday so that these memories do not interfere with each other. Our success in keeping memories of similar events separate suggests that an important property of the episodic system is that the representations it stores are somehow protected from interference.

The Neural System that Supports Episodic Memory On the left is a schematic representation of the flow of information from the neocortical unimodal and polymodal associative areas to the medial temporal lobe regions. Information flows to the highest level of integration and then loops back to the neocortical areas. The location of these regions in the primate brain is shown on the right.

Information from the Entorhinal Cortex Flows through the Hippocampus and Returns via the Subiculum This figure illustrates the flow of information into and out of the hippocampus, from and to the entorhinal cortex. The combination of hippocampal components and the subiculum is sometimes referred to as the hippocampal formation.

The Hippocampal Indexing Theory of Episodic Memory The important ideas represented in the indexing theory of the hippocampus. (A) In memory trace formation, the top layer represents potential patterns of neocortical activity, while the bottom layer represents the hippocampus. (B) A set of neocortical patterns (purple dots) activated by a particular experience is projected to the hippocampus and activates a unique set of synapses. (C) In memory retrieval, a subset of the initial input pattern can activate the hippocampal representation. (D) When this occurs, output from the hippocampus projects back to the neocortex to activate the entire pattern. Thus the hippocampus stores an index to neocortical patterns that can be used to retrieve the memory.

Index Theory and Memory Formation Animation of trace formation

Memory Retrieval: The Hippocampal Index Supports Pattern Completion Animation: A subset of the original experience can activate the memory for the entire experience. The hippocampus indexes the original input pattern. Partial Input Pattern

The Hippocampal Indexing Theory of Episodic Memory The hippocampus does not store content. It indexes patterns of neocortical activity. The content is in the neocortical regions.

Advantages of the Hippocampal Index If all that is required for the memory trace to be retrieved is to reactivate the neocortical patterns of activity activated by a memory producing experience, then why not just directly strengthen the connections between those patterns of activity in the neocortex itself? Why should the brain need an elaborate hierarchical system in which neocortical regions project to the hippocampus and then loop back to the neocortex to store a memory?

Advantages of the Hippocampal Index: The Associative Connectivity Problem The associative connectivity in the cortex may be too low to support the rapid changes needed to associate patterns of activation distributed widely across the neocortex (Rolls and Treves, 1998). Associative connectivity in the hippocampus is rich and connections are easily modified.

Advantages of the Hippocampal Index: The Interference Problem: The Hippocampal Indexing Supports Pattern Separation The hippocampus keeps memories of similar episodes separated. Two similar input patterns (ABCD and CDEF) activate their respective patterns of neocortical activity. Presenting the AC combination selectively activates the ABCD pattern, and the CE combination activates the CDEF pattern. In contrast, the neocortex has difficulty keeping the memories for similar episodes separated. Because the patterns share common features (CD), they are interconnected and the memories for the different episodes lose their identity. Any combination of inputs (e. g. , AC, CE, or BD) activates the entire blended network.

Indexing Theory and Properties of Episodic Memory Conscious awareness can emerge when pattern completion processes activate a representation of the entire experience, including the context in which it occurs, sufficiently to replay the memory. The automatic storage property emerges because the synapses in the hippocampus that support the memory are automatically strengthened just by the fact that experience generates new patterns of neural activity. Such a mechanism captures the contextual information in which the experience occurs because it binds the cortical representations of the entire experience.

Indexing Theory and Properties of Episodic Memory The episodic nature of the memory trace is due to single experiences, each generating unique patterns of neural activity in the neocortex and hippocampal formation. Interference among similar memory traces is reduced because the hippocampus supports pattern separation.

Evidence for Indexing Theory: Conscious Recollection Patients with significant damage to the hippocampus are selectively impaired in their ability to consciously recall episodes of their personal experiences and events that occurred at specific times and places.

Evidence for Indexing Theory: Do Rodents Have an Episodic Memory System? How Can You Tell? Indexing theory makes several predictions that can be tested with rodents: • The hippocampus captures the context in which events are experienced. • It does so automatically. • The hippocampus supports pattern completion and pattern separation. There a number of experiments with rodents that support these predictions.

The Hippocampal System Establishes the Memory Representation of Context Needed for Recall

The Hippocampal System Establishes the Memory Representation of Context Needed for Recall Retrieving a representation of context

The Hippocampal System Establishes the Memory Representation of Context Needed for Recall Retrieving a representation without a hippocampus

An Episodic Memory System in Rodents: Acquisition of Context Memories Depends on the Hippocampus How can they do this? A) The top of the figure illustrates the experimental design revealing that the rat acquires a representation of an explored context. Rats that are placed into a particular context and immediately shocked do not later fear that context. However, if they are preexposed to that context the day before the immediate shock experience, they do later show fear. (B) This graph shows that immediate shock itself does produce fear of the context, but that context preexposure markedly increases the fear produced by immediate shock. This result, called the context preexposure effect, indicates that the rat acquires a representation of the explored context.

An Episodic Memory System in Rodents: The Hippocampus Is Needed to Remember Context (A) Rats were allowed to explore two objects, a cube and a cylinder. Each object was explored in a different context. (B) The rats were then tested twice. Both objects were presented in each context. Control rats spent more time exploring the object that had not previously been experienced in the test context. In contrast, rats with damage to the hippocampus explored the objects equally. This means that control animals had a memory of the object and the context in which it occurred, but rats with damage to the hippocampus did not.

An Episodic Memory System in Rodents: Storage of the Spatial Location of Objects Storage of the spatial location of objects is automatically stored by normal rats but not by rats with damage to the hippocampus. (A) Rats were allowed to explore an arena containing two objects. (B) In the test phase the rat was returned to the arena but the location of one of the objects was changed. Control rats explored the moved object much more than they explored the unmoved object. They responded to it as if it were novel. Rats with damage to the hippocampus explored the two objects equally. Thus, automatically capturing information about the location of the objects depended on the hippocampus.

An Episodic Memory System in Rodents: Pattern Completion and Cued Recall Require the Hippocampus Rat with damage to the hippocampus

The Hippocampal Indexing Theory of Episodic Memory: Optogenetic Activation of the Index The hippocampus does not store content. It indexes patterns of neocortical activity. The content is in the neocortical regions.

Shining Light on the Index: Optogenetics Channelrhodopsins (Ch. Rs) are directly light-gated ion channels originating from microalgae. They are larger than any other rhodopsin with a 7 TM region and a long C-terminal extension. In algae they function as visual proteins directing the alga towards or away from a light source and to find light conditions that are optimal for photosynthetic growth. Video about optogenetics (click link to open in browser)

Optogenetic Activation of the Index In this experiment, Liu et al. (2011) used optogenetic methods to test indexing theory. Hippocampal neurons in the dentate gyrus that were active during a contextual fear conditioning experience expressed channelrhodopsin-2. When these neurons were activated by a light, the mice displayed fear in a context where they had never been shocked. When the light was off the mice displayed no fear. Mice that were not conditioned displayed no fear whether the light was on or off.