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.