One of the difficulties in exploiting natural neuronal structure and function for engineering applications is the lack of understanding between the microscopic physiology and macroscopic behavior. Neuroscience is the study of neurons at the microscopic level, while psychophysics is the study of correlations between specific physical stimuli and the sensations that result. Neuroscience is somewhat focused on the chemistry of the neuron and psychophysics is more focused on the macroscopic behavior of the complete organism. The story of how and why neurons are connected the way they are continues to unfold and will continue for centuries to come.
2.2.1 Massive Interconnections
Neurons are highly interconnected to form the information channels in sensory systems. For example, the human brain contains about 1010 to 1012 neurons, each making up to 103 to 104 connections to other neurons. Small groups of neurons are called ganglia, which typically controls specific behaviors of an animal. Many invertebrates have large neurons and small ganglia, which allows researchers an opportunity to investigate neuronal signaling and primitive neuronal networks.
2.2.2 Hebbian Learning
Natural neuronal connections are often strengthened with continued use, known as Hebbian Learning. The result is an adaptation of the network to the most frequent signal sequences from external stimuli. Artificial neural networks (ANN) are now commonly used to solve computational problems when direct analytical methods are difficult or impossible. These networks are inspired by the natural neuronal paradigm, and many have taken on variations that diverge from these original examples. This is quite acceptable since the typical goal is to solve some engineering or computational problem, not necessarily to mimic the natural paradigm.
2.2.3 Physical Types of Natural Sensors
There are different ways to categorize the natural neuronal systems designs that are available for engineering exploitation. The method chosen here is based on the physics of the stimulus. The ones given the most attention are the ones most commonly found in nature:
- Photo-sensory systems, stimulated by photons
-- vision systems in vertebrates and invertebrates
- Mechano-sensory systems, stimulated by physical motion in the environment
-- touch systems in vertebrates and invertebrates
-- auditory systems in vertebrates and invertebrates
-- kinesthesia, which is knowing the relative positions of body parts
- Chemo-sensory systems, stimulated by changes in chemical content of stimuli
-- olfactory systems providing the sense of smell
-- gustation systems providing the sense of taste
Other physical senses occasionally found in biology include those sensitive to heat, infra-red radiation, polarized light, electric fields, and magnetic fields.
There are three basic types of stimulus reception in biological sensory systems:
- Exteroception is the receiving of signals from outside the organism, such as photons of light for the vision system, sound waves for the auditory system, and chemical traces for the olfactory system. Sensory systems in this group are the subject of this text and most of the bio-inspired sensory system research that has been done.
- Proprioception is the receiving of signals that relate position of body segments to one another and the position of the body in space, which involves kinesthesia mentioned earlier.
- Interoception is the receiving of signals from conditions inside the organism, such as blood glucose level and blood pressure level.
There are three basic maps of sensory receptive fields to portions of the brain:
- Somatotopic Map is a map of the body surface in the somatosensory cortex.
- Retinotopic Map is a map of the visual field (as focused onto the retina) in the primary visual cortex in the occipital lobe of the brain.
- Tonotopic Map is a map of the basilar membrane in the primary auditory cortex in the temporal lobe of the brain.
The amount of brain surface area dedicated to various regions of reception varies dramatically, as certain reception areas are more important and require more dedicated processing. For example, the allocation in the brain on the somatotopic map for the sensation of touch in the index finger is much larger than the same relative skin surface area of the back. Another interesting point is the nearest-neighbor receptor mapping is generally preserved in the cortex. That is, adjacent receptors in the peripheral sensory system tend to stimulate adjacent neurons in the cortex.