The title of this section could be "babies babble". All human motor systems are spontaneously active. We emit behaviors and then analyze the sensory consequences. In babies such emissions include literal babbling, and they begin long before birth. The results include the spontaneous muscular movements known as "kicking". The directionality of motor emissions leading to sensory consequences, is hard wired into the portion of the brain that equates with the sensorimotor timeline. However it is completely missing from the embedding network. The embedding network works on joint probability distributions, it has no sense of time other than what is encoded into the data streams.

At a high level a learning scheme for sensorimotor consequences might include the "TOTE" paradigm of Miller, Galanter, and Pribram (1960). TOTE means "test, operate, test, exit". The idea is you're going to keep trying till you get it right. Babies, however, don't have to try very hard. They make use of an enormous amount of information, when analyzing the sensory consequence of random emissions.

There are four important pieces to the developmental sequence for any neural network. First, the neurons must be able to identify and locate themselves. Second, axons must sprout and find their targets. Third, the synaptic connections must form and properly adjust themselves in relation to environmental input. And fourth, as information begins to flow the network must continually adjust itself to accommodate the characteristics of incoming data. To this we can add a fifth element for motor systems - a "generative" capacity in the form of spontaneous emissions.

In contrast to historical belief, our brains do in fact generate new neurons, constantly, all the time - and it is quite possible that in many cases spontaneous emissions could be linked to the introduction of new neurons and new synaptic pathways into the network. The process of adult neurogenesis is best studied in the hippocampus and surrounding areas. The rate of new neuron growth in the human entorhinal cortex exceeds 1500 per day. Why are new neurons needed? What's wrong with the old ones?

Mechanisms of Spontaneous Activity

Considering the brain in relation to a mapping of electrical activity, synapses are relatively easy to model as long as they're isolated. If we can take every synapse in isolation, we can conveniently use matrix multiplication to calculate the influence of synaptic weights on neural firing. However the time scales are limited to the msec range, and it turns out there is something more precise. When it comes to spontaneous activty, one interesting avenue of investigation is its relationship with external electromagnetic fields. Information about this comes from two areas of research: one is transcranial stimulation, which can be focused onto small groups of neurons but is rather non-specific within those groups, and the other is electroreception, which is a capability of some sharks, skates, and eels. Electroreception is especially interesting because it entirely involves wetware, the ionic currents indicating electric fields travel in the ocean itself, and the Ampullae of Lorenzini containing the electroreceptors have an internal milieu much like seawater.

Electro-reception works through a voltage gradient between one side of the receptor cell and the other. This in turn triggers calcium channels that generate nerve impulses. In the species Apteronotus leptorhynchus (the weakly electric brown ghost knifefish), electroreception is sensitive down to 5 nV/cm (about a billionth of a volt). Contrast this with the trans-membrane electric field in a typical human neuron, which is millions of volts per meter. A neuron near threshold can be easily influenced by external electromagnetic field, and especially by fields in other areas of the brain, generated by the geometric alignment of neurons and transmitted through volume conduction. There is some evidence that this occurs in the transmission of hippocampal theta to the dorsolateral prefrontal cortex, in the form of phase locking in cortical neurons that can't be explained by direct connections from the hippocampus.

The possibility of direct electromagnetic effects on calcium channels can not be ruled out. Even more intriguing is the relationship between calcium and astrocytes, which are connected by gap junctions. Ultimately the exposure to extracellular fields is linked to currents in the extracellular milieu, and one can calculate the field generated by a neuron when it fires. If one really wanted to model the volume conduction between the hippocampus and prefrontal cortex, one could do so. Perhaps there will be a need for that someday, once we figure out how the cortex works.