Fig 1. The hidden unit problem.

A. In a hypothetical circuit consisting of just two recorded neurons (no hidden neurons), we can measure the strength and time course of the directed interactions between neurons by measuring the response of the post-synaptic neuron’s membrane potential to a spike from the pre-synaptic neuron. B. Realistically, there are many more neurons in the network that are unrecorded and hence “hidden.” In this schematic, only two neurons are observed. The hidden neurons are driven by input from the presynaptic neuron labeled 1, and provide input to the recorded post-synaptic neuron labeled 2. Because the activity of the hidden neurons is not controlled, the membrane response reflects a combination of neuron 1’s direct influence on neuron 2 and its indirect influence through the hidden network. C. The “effective” 2 neuron network observed experimentally.