"The Neural Basis of Recollection"
Neuphi
May 15, 2007, 4-6PM
Note-taker: E. I.
Last Updated: June 15, 2007
What we know about phenomenology of conscious recollection
Many parts of the brain contribute to information processing of memory (e.g. Frontal, paraetal, temporal), but we’ll focus on the hippocampus (medial temporal lobe) because it is known that selective loss of memory occurs with hippocampal damage/removal (e.g. H.M.).
What do we mean by recollection? We will refer to humans and animals.
(So, valid animal model is required, which is a challenge. There is much debate on whether animals even have a conscious recollection.)
Which features of conscious recollection are supported by hippocampus?
Familiarity vs Recollection. Aristotle may have been the first to distinguish this.
Familiarity: Simpler, extrapolation of perception. persists and can be resurrected again. Like wax inprint, or photograph.
Recollection: Episodic memory. Has subjective features as well as objective features. Has feature of mental time travel. Sense of self, or autonoetic consciousness (i.e. sense of I’ve-been-there-before), is crucial part of memory. [i.e. Autonouetic—I’ve experienced this before and I “really” remember. Familiarity—I’ve seen this before, but I can’t really say that I remember.]
Example describing Familiarity vs Recollection, & Objective features of recollection
After poster session of scientific meeting. Young women who looked vaguely familiar approached me. I knew that I know this person, but didn’t really know who she was. But, the moment she spoke, I got a rush of information, immediately knew who she was, and knew the context of how I knew her (remembered her standing in front of the poster). Then, I was able to do mental replay of her telling me about her experiment. It seemed that different episodes are connected to one another to form network of memories.
Thus, three objective features of conscious recollection seem to be:
1. Information is encoded as an item in its context. There is a threshold level of information that is recollected, which is fundamentally different from familiarity.
2. Multiple types of information involved in memory.
3. Mental replay and networks. Spacio-temporal features, as well as associativity of our experiences are preserved.
Experiment about feature #1 (Items are contextual in recollection):
Question: Do rats with hippocampal damage differ in their ability to recall events?
Procedure: Recognition memory test (typical in psych) with rats & odors. Let them dig. Cups and sand. Dig if it’s new, but not if not. Variables were height of cup, # of fruit loops, type of odor (cups with new odors have food, old odors have no food).
Observation: Mice w/ hippocampal damange behaves differently from controls. Rats w/ hippocampal damage have graphs that do not have the curve representative of “recollection” tasks.
Conclusion: Hippocampus supports recollection, but not familiarity.
Experiments about feature #2 (Multiple types of information are recollected):
Question: What kind(s) of memories do hippocampal cells encode? Are hippocampal cells responsible for encoding location, type of oder/information, or some/all of the above?
A) Rats—Record individual nerve cell activity in the hippocampus during odor recognition tasks.
Do hippocampal nerve cells encode type of odor and location of odor?
Answer: Yes. Some cells encode the type of odor, some encode location, some encode a combination of information.
B) Humans
UCLA study. Virtual reality game taxi cab. Find different shops, find 1 of three shops.
Concl: Same as rats. Some cells respond to where you are, some respond to what shop you’re going to, some fire only with a certain combination.
C) fMRI study
Horizontal sections of brain shows level of activation in hippocampus. Human word recollection task. Only when “truly remember”—hippocampus activation.
Conclusion: Hippocampal cells encode a variety of information, including: location, type of stimulus/object, combinations
Experiment about feature #3 (Recollection involves sequential encoding of information as well as networking of different information):
A) Regarding event sequence:
Aristotle’s claim that: Episodes are represented as sequences of events.
Look at spacial memory folding over time. When I stroll from MIT to restaurant in Fenway Park, hippocampus will encode each of these as sequence of events. Does it only encode what’s going on at that moment (ongoing event), or does it care about what happened before and after?
Instantiate in rats T-maze alteration experiment. (Rat starts at base of the “T” and gets reward for alternating turns.)
Hypothesis: If hippo encodes events as sequence of events, then, left turns and right turns should be encoded differently. While it’s running at the base of the maze.
Observation: there are cells that fire only when rats are about to do left turns.
Conclusion: Sequences of events are encoded in the hippocampus.
Same thing has been shown in human brain imaging. Subjects are presented with images of route view vs bird’s-eye view. Hippocampal activity has been observed for route type view but not for bird’s-eye view.
B) Regarding Network of memories:
Can be observed via associative transitive inference task. (If people are shown pairs of stimuli. A=B, B=C then, does the hippocampus say A = B = C?)
Procedure: Train rats to associate A&B, X&Y… Observe, where does the rat dig? (% time spent digging in correct location)
Observation: Rats without hippocampus cannot do association/inference task.
Conclusion: Hippocampus is involved in building networks between memory.
Same result observed with humans. (e.g. With association task about college dorm living situations.)
So, how does recollection emerge from circuitry?
Cortical association areas→ medial temporal region outside of hippo→ Hippocampus
Process of remembering events as an item in a context
Studies so far suggest: Visual information in V1 goes to two different cortical areas. Information about “what” particular objects are go to the ventral striatum, then to the “blue regions” (in the slide) in the parahippocampal area. Information about “where” the object is goes to the Dorsal stream, then to the “green regions” of the parahippocampal area. The two streams of information converge in the hippocampus.
Consequent hypothesis: The hippocampus is where different information (i.e. the object identity information from the “what” stream and the contextual information from the “where” stream) converges to become an event/episode of something occuring.
Evidence[1.19]:
Rats and monkey hippocampal areas are known to respond more to familiar stimuli.
fMRI: For object identity, perirhinal (blue) cortex responds more to novel than familiar stimuli. Reacts less to stimulus a second time. For spacial context, the green area in parahippocampus neurons responds to scenes, but not to objects. In perirhinal cortex, no extra stimulation if context is remembered in addition to the item. In hippocampus, item and context gets activation, but not for only one. i.e. The hippocampus responded only when people remembered words as well as where they saw this object.
Perihippocampal region get strong activation for objects in context, and for objects strongly associated with context (e.g. fridge), than objects with less contextual association.
Thus, Possible Mechanism of Recollection of an Event [1:23:11]:
Retrival cue (e.g. see a woman’s face) goes into the blue part of the parahippocampal region. If I recover only the extra features of the face, I might just be “familiar” with her. (Goes to frontal lobe. ) But, if it goes to Hippocampus (can recover context information) , then the information might get sent out to Parahippocampus where context is resurrected. Then, it could go to the lower cortex where the details of the episode can be retrieved which leads to the phenomena of “I recall!”
So hippocampal region acts more like a pointer to information rather than storage of information.
Finally, on subjective features of recollection: Data so far shows that medial cortical networks, and not hippocampus, seems to be the self-referential parts. (e.g. “I remember thinking about this…”) So, perhaps objective properties of recollection are shared with animals, but not the self, subject aspects (supported by cerebral cortex). People have the same “when/where” system as animals, but have an additional cortical input that encodes subjective/self “why” information. Subjective features are part of recovered system.
Q&A
Q. Episodic vs Semantic memory?
A. Episodic—Remembering learning about the capital of France in grade school. Semantic—Capital of France = Paris Hippocampus linked more with episodic memory. Cortex often correlated to semantic memory. Hippocampus learns more rapidly compared to the cortex. Computer network model simulation can show same feature. (Hippocampus is a fast learner, but has catastrophic interference when working by itself. Cortex doesn’t learn as quickly, but can work in conjunction with hippocampus to receive/provide information to help hippocampus recognize things it has observed in the past. Prefrontal cortex is working memory.)
Q. Does plasticity create a generalized sense of memory, separate from the hippocampal? A. Plasticity is feature of all parts of the brain and is responsible for familiarity—which can be unconscious--which I will talk less of today. For example, motor skill learning is largely unconscious, but still memory. The motor memory seems dependent on that general plasticity.
Q. Are you saying that even if rats had phenomenal consciousness, it would not have self-consciousness b/c it doesn’t have the neural structure??
A. Yes. An interesting recent speculation about cortical evolution is that what we usually associate as evolutionarily more simple and “lower” animals have only the “higher” cortical areas, undifferentiated association areas (e.g. snake recognition), whereas higher animals have additional “lower” more specialized functions in addition to the higher ones.
C(comment). Still that doesn’t seem to answer whether there is something to be like Mr. Rat (phenomenal consciousness)?
Q. Other than the hippocampus, what other brain structures mix information (e.g. sensations, emotions)?
A. It is know that stages such as the visual cortex collapse information. Even higher areas (e.g. Wernickes) collapse together higher order information. Even higher areas such as the Hippocampus, put together figure with ground even higher order information which seems to be linked to long-term storage of information. The prefrontal cortex is the other major area (runs show. working, thinking, processing). Hippocampus is probably evolutionarily older. It’s the final association pathway. All mammals have it. For all mammals, the major cortical-hippocampal connection is the same.
Q. Birds?
A. Birds seem to have an analogue of hippocampus. But unlike mammals, they don’t seem to have a cortex.
Q. Is it true that the evaluation itself (e.g. of “this is X”) affects the content of the memory?
A. Yes.
Q. So, then memory cannot be completely objective. Because the subjective evaluation would affect the memory.
A. True. There is no such thing as a pure retrieval event because every retrieval affects/contributes to the memory. Experiments can tend to simplify this complicated process.
Reading: http://www.neuphi.com/images/readings/CompCog.pdf_.pdf