lecture 7 - encoding of real-world locations Flashcards
receptive field
- regions in a sensory input space (like the retina for vision or the skin for touch) where stimulation leads to activation of a neuron.
- exists in sensory coordinates
sensory coordinates
how the brain represents sensory information
receptive fields in sensory coordinates
- RFs are localized on the surface of the sensory input organ
- RFs of sensory/abstract dimensions are well-described by gaussian models
- qualities of maps: RF Location-Size Correlations: RFs can vary in size and location depending on their sensory roles. Some RFs are broadly tuned, meaning they respond to a wide range of stimuli
What is the brain’s task regarding sensory input?
- the brain has to infer the state of the world from sensory input
- all information enters the brain in sensory coordinates: e.g., sensory coordinates based on the position of light on the retina
- the brain needs to transform these into world coordinates
why does the brain need to transform sensory coordinates into world coordinates
To achieve a stable, global understanding of where objects are in the real world, not just their location on sensory organs like the retina.
What is an example of the brain using world coordinates?
Knowing a flower is in front of you, not just that its image fell on a specific part of your retina.
the challenge of transforming sensory coordinates into world coordinates
sensory input is highly dynamic and subject to changes from movement.
How does eye movement challenge sensory input processing?
Eye movements can completely jumble up incoming sensations, making it difficult to interpret stable visual information.
How does the brain create world-centered representations?
- By integrating across actions, time, modalities, and other factors.
- how the brain performs this integration from low-levels to high-levels is not clear.
- it is clear, however, that the brain does represent information in world-centred coordinates
What did behaviorists believe about how rats learn to find food in a maze?
Behaviorists believed that rats learn to find food only by memorizing stimulus-response (reward) pairings.
What did Tolman discover about how rats learn mazes?
Tolman found that rats learn the layout of a maze even without rewards and would be faster at finding food in a maze they had already traversed earlier.
What does Tolman’s discovery suggest about rats’ learning?
It suggests that rats’ ability to find rewards in a maze is better explained by the idea of a cognitive map rather than simple stimulus-response learning.
What is a cognitive map
A cognitive map is a mental map of the world around us, used to represent spatial relationships between objects in world-centered terms.
How does the cognitive map differ from stimulus-response learning?
The cognitive map implies that animals (and humans) can represent spatial relationships in world-centered terms rather than relying solely on local cues.
In what kind of coordinates do receptive fields exist?
sensory coordinates
What is a place field?
A place field is a specific spatial location where neurons in the hippocampus fire when a rat is in that location, discovered by O’Keefe et al. in the 1970s.
What can a population of place cells do
Report the animal’s location in the surroundings to the rest of the brain
How does the hippocampus represent paths?
- The hippocampus represents paths as a sequence of activations of place fields, allowing the reconstruction of space and time from hippocampal activity.
- this means we can deduce where a mouse is from its hippocampus activation
What happens during sleep in terms of hippocampal activity?
During sleep, the sequence of hippocampal activity is replayed in compressed time, either forward or backward (sped up or reversed).
What is preplay, and what are its functions?
Preplay is hippocampal replay that occurs before events and serves two functions:
- Memory formation: Linking experiences for learning.
- Planning wakeful behaviors: we therefore can see what a rat is going to do based on the replay we see in the hippocampus
What happens if the hippocampal replay mechanism is (task) inaccurate?
Taks-focused replay predicts accurate decisions, so if replay mechanism is inaccurate, decision-making will be inaccurate
How can disrupting replay affect behavior?
Disrupting hippocampal replay at decision points can reduce performance on tasks by impairing accurate decision-making.
What do memory formation and wakeful behaviors together demonstrate?
They show that replay represents what the rat is thinking and is crucial for both learning and decision-making.
What is remapping in place cells?
Remapping refers to how place cells adapt to track changes in surroundings by activating different place fields for different environments.
Can a single neuron have multiple place fields?
Yes, a single neuron can have different place fields in different surroundings, allowing it to adapt without needing a unique place field for every location.
What happens to the place field structure when a cell returns to a previous environment?
The cell picks up its previous place field structure when returning to the same environment.
Why do we study hippocampal inputs and outputs?
To understand the mechanism for remapping by examining hippocampal place fields in relation to afferent (incoming) signals.
Where do hippocampal inputs come from?
Hippocampal inputs come from the entorhinal cortex (EC).
What is the role of the entorhinal cortex (EC)?
The EC acts as a gateway, transmitting spatial and sensory information from other brain regions to the hippocampus.
Why is the entorhinal cortex important for place fields in the hippocampus?
Place fields in the hippocampus might emerge from the integration and transformation of inputs from the EC.
What are grid cells in the entorhinal cortex (EC)?
Grid cells are neurons in the EC with world-coordinate receptive fields that fire in a grid-like pattern across multiple locations, helping animals track their position in the environment.
How are grid cells different from hippocampal place cells?
Unlike hippocampal place cells, which fire at specific locations, grid cells fire at regularly spaced intervals across the entire environment.
What pattern do grid cells form in space?
Grid cells form a hexagonal, crystalline structure in world-centered space.
What properties can differ among EC grid cells?
- Frequency of repetition: size of the field
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Phase: location of the field
- Grid cells with the same frequency can have offsets (different phases), meaning their activation peaks occur at different locations.
What does the frequency of a grid cell define?
The frequency defines the size of the grid, indicating how far apart the hexagonal points of activation are.
- Larger spacing between grid points (low frequency).
- Smaller spacing between grid points (high frequency).
What does the phase of a grid cell determine?
The phase determines the exact location of the grid points in space.
What are the different bases for spatial coding?
- Gaussian basis: Used in sensory systems to encode locations in sensory coordinates (e.g., x and y).
- Fourier basis: Used in navigational systems to encode locations in world coordinates (e.g., phase and frequency).
How do receptive fields in sensory systems encode spatial information?
They use a Gaussian basis to encode location in sensory coordinates (x and y).
How do place fields in navigational systems encode spatial information?
They use a Fourier basis to encode location in world coordinates (phase and frequency).
What is the significance of linear combinations in spatial coding?
Linear combinations of grid cells with specific weights can encode precise locations, either in sensory or world coordinates.
What is the relationship between grid cells and place cells?
A single bank of grid cells can be reweighted to produce different place fields in the hippocampus.
What did Doeller et al. aim to observe in their VR experiment on grid cells?
They aimed to observe if humans show grid-like periodic patterns of activation during spatial navigation.
What is the hypothesis of the VR experiment on grid cells in humans?
- Strong activation when moving in your grid direction (over grid bumps).
- Weak activation when moving off-grid direction (over grid troughs).
What prediction was made about grid cell firing in humans?
Grid cells were expected to show periodic peaks in activity every 60°, as they move in directions aligned with grid bumps.
What were the key results of Doeller et al.’s VR experiment?
- Voxels with 60° periodic activation confirmed grid-like behavior.
- Positive correlation between directional coherence and spatial memory performance.
- Faster navigation speeds resulted in stronger periodic signals.
How does speed influence the grid-like response in humans?
Faster navigation speeds resulted in stronger periodic signals, suggesting speed strengthens grid-like responses.
What was the hypothesis of Constantinescu et al. (2016) regarding grid-like coding in humans?
They hypothesized that abstract conceptual knowledge is encoded using grids, predicting 60° periodic signals in abstract conceptual tasks.
What cognitive map space did Constantinescu et al. (2016) investigate?
They studied grid cells encoding an abstract space of neck and leg lengths of birds.
What was analyzed during each trial of the abstract conceptual task?
The analysis focused on the vector in 2D space, representing changes in the bird’s layout, rather than its specific location.
What type of grid-like coding result was observed in the abstract task?
Hexagonal coding in the entorhinal cortex (EC) and other regions, such as the default mode network.
How were 60° periodic signals related to trajectories and bird layout?
Stimuli aligned with grid axes showed stronger periodic activation, linking abstract dimensions (neck and leg lengths) to spatial grid coding.
What is not well understood about transforming sensory information into world-centered representations?
How the brain processes localized sensory information (Gaussian basis) into abstract, world-centered representations (Fourier basis).
What does the brain need to integrate to achieve world-centered representations?
Navigational cues such as self-motion, scene perception, and navigational cues.
What does the diagram of different connected brain areas suggest about navigational processing?
Different brain regions, including the entorhinal cortex and association areas, converge to integrate navigational information.
How are grid-like coding concepts being used in AI research?
Researchers are recreating grid-like coding patterns in neural network models to mimic brain navigation systems.
what is the contast of a stimulus
the difference between the max and min value relative to its mean value
increasing the contrast of the image
- increases the strength of the response
- done by amplifying the difference between the maximum and minimum pixel values relative to their mean
- Mathematically, this means that every pixel value in the image is multiplied by a factor greater than 1
- i.e., we can vary the contrast of a stimulus by multiplying the image itself
mosers
found that many EC neurons have repetitive place fields. they activate whenever the animal is in any of a whole set of locations
