Temporal and Spatial Navigation
Main Goals
To present a real time neural network capable of describing both TEMPORAL and SPATIAL learning in a unified fashion.
To test the aggregate prediction hypothesis of hippocampal function for control of associative learning. Aggregate predictions forecast WHAT event is going to occurr, WHEN in time, and WHERE in space.
Main Assumptions
Aggregate predictions would be represented in the activity of hippocampal pyramidal cells and disrupted by hippocampal lessions.
Temporal trace hypothesis: Stimuli give raise to temporal traces that increase over time to a maximum and then gradually decay back to zero.
Visual angle hypothesis: units specific to spatial landmarks which fire maximally when the current visual angle of the landmark equals a previously stored visual angle.
Presentation of the US tunes detectors to the amplitude of a temporal trace or a visual angle. Subsequently, the output of the tuned detectors can be associated with the US.
Architecture
The first stage of the network consists of a series of detectors that receive a combination of temporal traces and visual angles, the second stage associates the output of the detectors with another event (e.g. the US), and the third stage adds the output of the second stage and generates the output of the system
An additional neural element (H), assumed to to be the hippocampus, computes an EFFERENT copy used to control the associations between the output of the tuned detectors and the US (aggregate predictions).
After Hippocampal lessions each local circuit becomes independent of each other.
Achievements
Temporal Navigation: Experiments on temporal discrimination in classical conditioning (different ISI, mixed ISI).
Spatial Navigation: Place learning by building a generalization surface around both appetitive and aversive stimuli (e.g. Morris (1981).
Supporting data
Eichenbaum et al. (1986) and Breese et al. (1989) data suggest that place cell firing represents the aggregate prediction of biologically significant cues or places.
Paul et al. (1987) described both spatial and temporal correlates of hippocampal pyramidal units (firing of place cell was usually time-locked to arrival at a reward cup.
Limitations
No resemblance to firing maps
Prepared by Alex Guazzelli and Fernando Corbacho (aguazzel/corbacho@pollux.usc.edu)