Summary of Presentation by Prof. Thomas Scott, "Overview of the Senses and Characterization of the Chemical Senses."

Prepared by Jason Gorscak (additions and comments by Frawley)

The brain's sole purpose is to create thoughts and put them into action. Only through action does the purpose of the brain exist. But even with this simple characterization, the so called "simple" sense of touch, for example, is not so simple. Whenever you touch something, an electrical impulse is sent up to the brain and an impulse is sent back to the origin to create a reaction. This is called a reflex.

Note: in reflexes, nothing intervenes or mediates between input and response. But intelligence is built on neuronal mediation: i.e., there is something between input and response

Humans have approx. 100 billion neurons, of which only 1% are sensory: i.e., carry impulses to the central processing unit of the brain. Only 0.1% of those have motor funciton to perform the action: i.e., carry information out.

Note: 98% of the brain mediates input and response.

These mediating mechnisms limit intake and response. We have internal sensors that allows us to "know" the "information states" of our organs (e.g., hormone levels). We "know" our autonomic functions: heart beat, digestion, etc. We also have mechanisms to focus attention -- watch on wrist, cars passing by, etc. -- and hence further limit our information intake and output.

Note: the visual system is not just a light meter, internally recording the outside world in full, but a biased filter.

One way to think about nervous systems, then, is as limitation systems. The sensory systems, which are those neural strutcures that meet the world directly, can be thought of as biased energy detection mechanisms.

There are three types of sensory systems, depending on the energy/information to which they are sensitive:

1. Chemical Energy -- we take certain molecules into our body and then emit others

We can detect with reliability a specific chemical

Chemical signals do not need an external source of energy

Other species cannot hide; the molecules are spread out in an aqueous-like solution

Localization - You cannot tell where something is but you can tell what it is

Note how the two basic functions of an organism, food & sex, are served by chemical detection. Organisms have always performed these functions. They have always needed energy coming in and they always had to reproduce to keep their species in existence. [Drinking and sleeping, two functions we might also think essential, are rather recently developed biological needs.]

2. Mechanical Energy -- Organisms must displace molecules in the process of moving or emitting vibratory signals

Mechanical signals do not need an external source of energy

The disturbance passes in all directions. An organism cannot hide

Localization is still poor because the frequency of vibration is too low.

Note: Human range of frequency detection is 0 - 20,000 hertz using the skin and ear as vibration receptors. But bats have developed very high frequency systems that also have an external source: their own emissions.

3. Electromagnetic Radiation -- Organisms and objects reflect light.

Radiation travels in straight lines so you can tell the source of the radiation and therefore you have localization.

The object does not have to move or emit chemical energy, just reflect light

Need external energy source

Need long lines of sight to see the source of light. If something gets between you and the object, it is able to hide and you can't detect it.

Early mammals didn't have a highly developed system of electromagnetic radiation detection (sight) because they were nocturnal and low to the ground. They used chemical systems (smell, which is well developed in mammals) However, primates, which originated as arboreal creatures, became more dependent on the sense of sight, aiding upright posture. (Note: birds are bad smellers but good seers. Predators have front eyes to calculate depth via binocular vision, but they lose overall coverage of the visual field; seed-eating birds have eyes on the side, and so lose binocular vision, but gain overall coverage of the visual field.)

One way to think about electromagnetic detection is in terms of the scale of such energy -- from gamma rays (not occluded by soft tissue) to alternating current (wave too long to reflect usefully off an object) -- and adaptation thereto. Human vision is between the ultraviolet and infra-red, with other species extending a little into each of these.