Experiment Components in Context-cells Simulations

This documentation provides further information about experiment components originally designed for context-cells experiments. These experiments were designed to test a neuromodulation theory by Dr. Hasselmo.

Overview

Some methods are specific to implementing an adapted version of Dr. Hasselmo’s Acetylcholine (ACh)-neuromodulation of an object association theory (Hasselmo, 2013). Some methods for this purpose are context cells, lateral inhibition, and neuromodulation-based inhibition described below. These methods may not need to be used in non-context cell versions of Oblomem experiments, but they can be included in those versions if wanted.

Context Cells

Lateral Inhibition

See lateral inhibition documentation on methods used in implementing lateral inhibition.

Neuromodulation Affecting STP

Dr. Hasselmo’s theory in (Hasselmo, 2013) includes that there should be an ACh-based reduction in “the spread of activity across all synapses” (Figure 6.7). Also, that neurons should be able to “selectively respond to the new afferent input”. Neuromodulating short-term plasticity (STP) in concept cell connections in a way that significantly depresses their synapses can help accommodate the “spread of activity” being inhibited condition.

Notably, neuromodulating STP to depression, in contrast to inhibition through interneurons controlled by neuromodulation, can still allow for the “respond to the new afferent input” condition to occur. Interneurons would lower the exitability of concept cells in general, which can interfere with responses to afferent input. STP-based “functional inhibition” still allows the concept cells to remain excitable to the afferent input.

See the “Usage” and “Basic explanation” sections of nm4cstp documentation to find information about how to use neuromodulation of STP in the Hippocampome branch.

The term “functional inhibition” refers to STP-based depression not perhaps being considered a classical form of inhibition in the way that effect from interneurons is created. Depressing synapses has a “functional” effect of inhibition because it reduces the signaling between neurons in the synapses. This distinction may be helpful when one describes the effect of STP in contrast with interneurons when creating inhibition. However, STP can also target interneurons, and these descriptions are not meant to exclude that activity, but instead to clarify the idea of “inhibition” from STP or interneurons themselves.

Neuromodulation Acting as Inhibitory Neurons

Neuromodulation acting as inhibitory neurons can be enabled by the parameter enable_ach_inhib set to 1. Setting that to 0 disables the parameter. Current can then be supplied to the interneuron group representing neuromodulation by using the setExternalCurrent() function. For instance, sim.setExternalCurrent(ACh_CA3_Basket, 1000.0f);.

The theory in (Hasselmo, 2013), e.g., in Figure 6.7’s caption describing “presynaptic inhibition”, describes that ACh can provide a role as inhibition. One way to simulate this is to provide inhibitory signaling through interneurons that are controlled by ACh levels. Specifically, the level of current supplied to interneurons that are connected to concept cells, e.g., context cells or place cells, can be managed by the level of ACh present at a given time.

Given that Dr. Hasselmo’s theory specifically describes “presynaptic” inhibition, in contrast to general inhibition, then STP-based neuromodulation may better represent this form of inhibition. CARLsim does not currently offer presynaptic as opposed to general inhibition simulation. Code has been added to the Hippocampome CARLsim branch to support STP-based neuromodulation. STP can allow depressing synapses specifically, compared to neural excitability, which (in the opinion of this doucment’s author) closer recreates presynaptic inhibition, compared to interneuron-based inhibition. Additional ways that STP-based inhibition through depression fit presynaptic inhibition better than interneuron-based inhibition are described in the STP-based neuronmodulation section above. Therefore, STP-based neuromodulation can be a better choice for representing presynaptic inhibition, but interneuron-based inhibition is available as an option if wanted.

References

Hasselmo, M. E. (2013). How we remember: Brain mechanisms of episodic memory. MIT press.