This series begins with The Machine in the Ghost

Back to Abstraction of the Neuron || On to Sensing and Perceiving

So far in this series, we have examined the need for a theory of brain and outlined our approach to such a theory, roughed out a theory and created a general model for an abstract organism that has a neuronal brain to better cope with the environment, and created a model of the neuron. In this chapter, we will focus in closely on the interface between the brain and its environment, and find some implications for the meaning of knowledge and learning, which is to say how the brain understands and models the structure of reality.

Our Hypothesis -3 states that a neuron is a detector. It detects an event and signals that detection to other neurons. Now we have to ask, "Just what is an `event'? What is it that a neuron detects, exactly?", "How can such a limited number of detectors detect the myriad phenomena that reality presents?", and "What is the significance of the detection?" These obvious and seemingly simple questions quickly expand into the wider, more philosophical questions of "What things exist?", "How can we know they exist?", "What is the relation between things that exist and our knowledge of them?" and What is the relation of our knowledge to our purposes?" Philosophers in the fields of ontology and epistemology have struggled with those questions for millennia. We will answer them in a pragmatic way within our theory in the following four sections.

Sensory neurons--the gates of perception

Sensory neurons are transducers; they convert some information about reality into a signal. In our abstract organism, all information about the external environment and the internal environment of the organism's own body enters the brain via the sensory neurons. Therefore, what events sensory neurons are able to detect and what they do with that information are elemental to brain operation and so are also basic constraints on brain theory. We need to understand two things about sensory neurons: how they are stimulated and what kind of signal they send on to the rest of the brain.

Definition: The sensory environment comprises all of the internal and external events that can be sensed by an organism.

Sensory neurons are stimulated either directly or indirectly. Directly stimulated neurons have their electrical potentials affected by the stretching or movement of tissues near them, or by specific chemicals to which they are exposed (other than the usual neurostransmitters issued from other neurons). Specific examples include the nocioceptors, which detect pain situations, the cells that are sensitive to pressure and vibration in the skin and deep tissues, and the olfactory receptors in the nose. Internal afferent (sensory) nerves also use this mechanism to detect changes in blood sugar level, dilation or contraction of blood vessels, tissue temperature, balance, changes in limb position and many other kinds of events that occur inside an organism.

There are types of events that neurons cannot detect directly, so they must work together with special sensory cells, which are somewhat like half-neurons. On the input side, the sensory cells have areas that are sensitive to certain stimulating events, but on the output side, they form the same kinds of synapses as do ordinary neurons. Those synapses relay the stimulus signal from the sensory cell to the dendrites of neurons. Light, sound, balance, and some tastes are detected by this mechanism.

Let's look at light sensing in more detail. Vision relies on special sensing cells, the rods and cones of the retina in our eye. When light that is focused on the retina strikes a rod cell, for example, the rod cell generates an electro-chemical signal that is communicated to the dendrite of a nerve cell via a synapse. The nerve cell integrates the signal from the rod with other signals and may then fire off a signal to a higher layer of neurons called ganglion cells, which do some further integration and finally carry the signal off to the lateral geniculate nucleus (LGN) in the brain via the optic nerve (Fig. 3).

Figure 3. Sensory transduction for vision in the retina

One clear fact that leaps out of this picture of sensation is that there is in all sensory modes a smallest unit of sensing. That elemental unit corresponds to a single sensory cell or sensory neuron. Less easily seen at this point is that a single unit of sensing may carry a great deal of import to the organism, or very little at all. This important fact will become very clear later, but for now imagine the difference between the stimulation of a single rod cell in the retina and the stimulation of a single olfactory sensor in the nose. The signal from the single rod cell may even reach all the way into consciousness, but, buy itself, conveys almost no information that the consciousness and subconciousness can use in isolation. Imagine a single pixel flashing once on an HDTV display. (Note 1) The signal from single olfactory sensory on the other hand may produce a conscious experience of smell and may be indicate something that is very important to the organism, such as the presence of food.

Note 1: Even an HDTV displays only two million pixels, whereas the human eye pixel equivalent may be on the order of 300 million pixels. (http://www.clarkvision.com/imagedetail/eye-resolution.html)

Back to Abstraction of the Neuron || On to Sensing and Perceiving

This series begins with The Machine in the Ghost