Yuste C17: Learning and Memory Flashcards
What do learning and memory both involve
gathering information from the world, storing it, and then recalling it, almost effortlessly.
Crick’s definition of learning
broadly defined leaning as “any change that makes a change”. anything that changes the function of the nervous system constitutes learning. anything that changes in the nervous system, which will change its future function.
Learning vs memory
Learning is the process of changing and memory is the change itself.
Memory definition
the encoding by the nervous system of some sort of information.
Information definition and conclusion
reduction in uncertainty, so the acquisition of information then can be seen as one of the essential functions of the nervous system, to reduce the uncertainty in the prediction of the future.
a lot of what the brain does is storing memories.
Why are memories essential
not just to interact with the world and to think, but also to build our personal identities.
Link between memories and thoughts
link between memories and thoughts or cognitive states is clear in disease (cf Borges). Without being able to acquire new memories, and knowledge, like Funes, we would only live second to second and would not be able to abstract, generalize, conceptualize, or build any “knowledge”.
Two different types of memory
Short term working memory and longterm memory; somewhat arbitrary, with short-term being seconds to minutes and long-term anything longer than that.
Relationship between short and long-term memories
short-term memories can get consolidated into long-term memory. And both types of memories can also disappear into forgetting, another process, or processes.
Different forms of short term memory
Verbal forms - things or objects you can describe in words; involve storage in the parietal and temporal cortical areas and rehearsal in Broca’s motor area.
Also visuospatial forms - object you see; which involve visual, parietal, IT, frontal, premotor and prefrontal cortex.
How is short term memory similar to the what and where streams
there are object and spatial knowledge subsystems
Delay match to sample task
the monkey has to pay attention to an object in a particular position on the screen (Figure 17.2). This is followed by a short delay and then the monkey is shown a selection of images. When the same object is shown again the monkey has to press a button to show that that was the object he first saw, and he gets a sip of juice, which motivates him to pay attention and remember the sample object.
What you see in recordings from the prefrontal cortex of monkey during the delay match to sample task
if you record from the prefrontal cortex of the monkey during this task, you find neurons that code for the initial object and are active in the delay period, as if they are holding the flame of its memory in its mind.
Interestingly, the neuron’s firing returns to its base-line firing as soon as the monkey has pressed the button and doesn’t need to remember the stimulus any more.
What are the conclusions from the delay match to sample task
These neurons are thought to encode working memory.
Neurons with delay period firing
Many neurons with these delay period firing are found in prefrontal cortex, grouped in territories where different groups of cells appear to code for the preferred object while others code for the preferred location, and others code at the same time for both the preferred object and the preferred location.
What does data regarding neurons with delay period firing suggest
This type of data suggests that the visuo-spatial working memory is encoded in the prefrontal cortex.
Other areas that have delayed firing neurons
There are many other locations in the cortex that have delayed firing neurons, similar to these: the parietal lobe, the infero-temporal, visual, pre-motor; all these cortical areas connect reciprocally to the prefrontal cortex.
Three mechanisms for cortical cells to code for a stimulus and maintain firing in response to a stimulus even when it has disappeared
- persistent intrinsic neuronal activity
- a circuit attractor due to excitatory connectivity
- flip flop circuit
Persistent intrinsic neuronal activity
If you record from a single cortical neuron in vitro (in a dish) and bathe it with
acetylcholine. If you stimulate the neuron with an electrode, it is depolarized and generates a burst of action potentials. Now if you stimulate the cell with a longer stimulus you see a longer burst of APs. If you stimulate it with an even longer stimulus, this seems to activate some type of switch that results in persistent firing, even after the stimulus is off — this mechanism is intrinsic, because it does not require other neurons
How are neurons able to generate persistent firing according tot eh persistent intrinsic neuronal activity mechanism
due to a positive feedback loop as a result of the channels
within the membrane of the neuron. The hypothesis is that
when an action potential is fired and the neuron depolarizes, it
opens calcium channels in its dendrites, permitting the influx of
calcium. The intracellular calcium binds and opens cation
channels, which further depolarizes the cells. And this brings in even more calcium, which opens yet more calcium-activated cation channels. Channels that feed off each other to generate a positive feedback loop of activation that acts as a cellular switch.
Circuit attractor due to excitatory connectivity
Recurrent neural networks; the neurons that are active in the delay-sample to match task are connected to others which connect back to them, in an excitatory loop that generate a reverberating activity—persistent endogenous activity.
Each of these attractors is a memory. The network of neurons is remembering a particular state that it has built previously, because we trip one neuron and we get the whole pattern — this is called pattern completion. This is very typical of human memory. We often only remember part of one thing, and that one part brings down the whole of the memory.
Recurrent neural networks
Excitatory connections within the neural circuit makes it so that stimulating one part of the circuit results in reverberating activity that has a snowball effect in persisting excitation within the neural circuit.
How the circuit attractor due to excitatory activity is formed
Imagine that we stimulate several neurons to make them fire at the same time. Imagine that there is synaptic plasticity and that the neurons that fire together end up being wired together (Hebb’s rule). If the experiment is repeated over and over then eventually you end up with a circuit of neurons that are continually stimulated together — they are interconnected. After these connections are formed, you have an ensemble and, if you now stimulate only one or two of the components of the circuit with a very brief pulse, because they are preferentially connected, they are going to generate a reverberating excitatory activity that flows within the interconnected neurons, allowing the neuron to continue to fire even in the absence of the stimulus; “valleys”, the points in the network where the energy is most stable are defined as attractors.
Circuit attractor and forgetting
This happens when the attractor is erased. So the connections in the neural network have been destroyed; flattens out the valley so that the activity flows though the area and does not stay in the valley where the attractor used to be.
Engram
Connects to the concept of attractors; word that has been used to refer to the physical substrate of a given memory in the brain.
Flip flop circuit
where you have two excitatory neurons, A and B, connected to
a neuron C. Now neurons A and B are also connected to two
inhibitory interneurons, lets call them Ai and Bi, respectively,
which inhibit the other side (Figure 17.5).
Imagine that excitatory neuron A
on the left is turned on, so inhibitory neuron Ai on the left is
activated by neuron A and it inhibits excitatory neuron B on
the right. So now only the left side fires. But then the left
neuron A stops firing (maybe it adapts or runs out of
neurotransmitter) so the right side is now disinhibited, causing
neuron B to fire, and subsequently inhibits the left A neuron,
via neuron Bi.
This is mutual disinhibition, analogous to the flip-flop circuit in the spinal
cord. The activity alternates between the two sides,
but because both neuron A and B connect to C, the result is that neuron C continues to fire indefinitely.
Where else has mutual disinhibition been found
in the oculomotor system: in the performance of saccades, we have an integrator that delivers coordinates to the eye muscles to keep the eye in a particular position. Mathematically homologous to the attractor model.
Patient HM
he had seizures that did not respond to medication; neurosurgery: surgeons extirpate the part of the brain generating the seizures. Removed the region of the brain that caused the seizures: a big part of his entorhinal cortex, subiculum and hippocampus on both sides. After the surgery, was left with an enormous deficit in his memory. He remembered everything before his operation, but was
unable to remember anything in long-term memory that occurred after his operation.
This is called“anterograde amnesia”.
Had preserved procedural memory.
Procedural memory
AKA implicit memory; the type of memory
associated with skills, enabling him to perform certain
tasks. For example, H.M. improved at the task of
tracing a star while viewing his hand in a mirror. However, on consecutive days he had no recollection of ever having done the task before.
Problem with patient HM
Problem with memory for events - declarative memory. He somehow could not “transfer” information from short to long-term memory.
What patient HM tells us about long term memory
two types of long- term memory: episodic (declarative, available to consciousness) and implicit (non-declarative, not available to consciousness), and that the hippocampus is necessary for the long-term consolidation of episodic memory.
By comparing him to other patients, determined that the process of consolidation, or transfer, of episodic memory occurs in the hippocampal formation.
Hippocampus
This area is also known as the archicortex (the evolutionarily oldest part of the cortex; receives connections, and connects to a large part of the cortex, so it is ideally suited to consolidating memories and transferring them there.
Hippocampus and LTP
the core hippocampal circuit is particularly prone to long-term synaptic plasticity (LTP); LTP is enhanced by repetition. This is just what you would need to consolidate short-term into long-term memory.
Lesions of the right hippocampus vs left hippocampus - revelations
For most patients, storage of spatial declarative information is affected in right hippocampus lesions whereas storage of verbal, or semantic, declarative information in left hippocampal lesions.
H.M. had normal long-term memories of events before the surgery. So once the memories were consolidated and transferred, they remained intact.
Final storage of all memories location/process
the final storage of all the memories is in the neocortex, striatum and cerebellum, even in the spinal cord and the brain stem (Figure 17.10). And that this storage involves transferring from declarative to implicit memories.
Location of basic and reflexive memories
the more basic reflexive memories, formed by habituation and sensitization, are found in the spinal cord and brain stem.
Location of more complex types of memory
those formed by associative learning, such as classical or operant conditioning, are found in the cerebellum (motor ones) or amygdala (emotional ones). Memories for complex procedures, skills, and habits involve the striatum, and memories of events and objects, the cortex, as well for sensory priming.
Priming
when previous exposure to a stimulus prepares you for it, making you more sensitive for future occurrences of it.
Memory as a constructive process
Recalling a memory reactivates the same regions involved in memory acquisition. So memory is a constructive process — when you are recalling a memory you are essentially making up the memory again
Place cells
O’Keefe and Nadel. Neurons in the hippocampus of rats would fire when the animal was in a particular position in space! Different neurons fired when the animal was in different positions. And argued that they code for a particular position in which the animal moved in space. So the hippocampus in rodents seemed to code for space, a map of the world.
Grid cells
Moser. Neurons in the enthorhinal cortex that fired specifically when the animal was located at regular positions in space, in the nodes of a spatial grid that extended around the area where the rat was. So a given neuron would fire if the animal was in a particular position, but also if it moved to another position at a precise distance from it. Grid cells seemed to build a spatial map too - allocentric.
Grid cells vs place cells
allocentric map of grid cells, meaning that it existed independently of the position of the animal, as opposed to the egocentric map built by hippocampal place cells, where the map is anchored by the position of the animal.
Grid cell map
mathematically regular, with the positions in which the neuron fire located a precisely regular intervals in space.
Place cells and time
recent studies are pointing out that place cells also code for time, firing specifically at a particular time during a behavioral task. It’s still early in the game, but it sounds as if the space- time continuum that forms the fabric of our world could have to do with the hippocampus.
Hippocampus and space and time
the hippocampus could be doing both things, memory storage and space/time encoding. An intriguing idea is that, perhaps, both functions are the result of the same circuit. Perhaps the hippocampus evolved to encode space and time, but the same hardware was then used to encode, and anchor in the brain, symbolic spaces, representing concepts, ideas or memories.
episodic memories could correspond to egocentric maps of our mental world, whereas semantic memories, more abstract, could correspond to mental allocentric maps. This all would fit well with the hypothesis that the evolution of the nervous system corresponds to the encephalization and abstract symbolic use of CPGs that generate fixed motor patterns.
