System analysis of the goldfish olfactory bulb: Spatio-temporal transfer properties of the mitral cell granule cell complex

In this paper the goldfish olfactory bulb is described from a systems theoretical point of view. A chain of nine interacting circuits, each one mitral cell and one granule cell, is modelled. Glomerular synapses are assumed to have variable strengths. The analysis of the model system leads to the following conclusions:

1. The temporal input pattern of a mitral cell—granule cell circuit is either maintained by the circuit or inverted (lateral inhibition effect). This property together with available receptor data allows the theoretical explanation of experimentally recorded mitral cell patterns.
2. The sensitivity of a mitral cell—granule cell circuit is a function of the input signal's frequency. This provides an explanation for mitral cell cluster activity patterns measured in experiments.
3. Given a spatial input pattern to adjacent mitral cell—granule cell circuits, the output pattern depends largely upon the ratio between the feedback parameter p and the similarity β of the inputs to adjacent circuits. For appropriate p and β a local order between the responses of single neighbouring circuits is established. This local order can lead to a globally ordered mapping of odours onto mitral cell activities, thus providing a coding concept for the bulb. Some consequences of this concept coincide well with the spatial activity patterns found in 2-DOG-studies.
4. Glomerular synapses endowed with plasticity could account for long term effects such as degeneration and sensitivity changes with respect to certain odours.