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Figure 3.1 Transfer of information in the body
hand, an obtained stimulation is produced by our own actions or in the course of an
action; for example, when a person moves a limb, moves an object, looks at light,
hears a sound, smells an odour, etc. In this way, an imposed stimulation would take
place with a passive observer, while an obtained stimulation would take place with an
active observer.
Researchers like Sherrington (1906) suggest a strict distinction between extero-
ceptors, proprioceptors and interoceptors. Exteroceptors (eyes, ears, nose, mouth and
skin) inform us about the changes in the surroundings and serve as the basis of percep-
tion. Proprioceptors (tips of organs, muscles, joints, internal ear) give us the sensations
related to the position and movements of the body: they create the sensation of move-
ment. Interoceptors (nerves leading to viscera) give vague sensations about the internal
organs. Contrary to the position taken by Sherrington, Gibson (1966) suggests that
sensing an action and sensing a movement do not depend on specialised receptors.
The eyes, ears or the skin can note the behaviour of an individual as also the exter-
nal events. For example, eyes note the movements of the head (front, back, rotation)
through the movements of the surrounding light (exteroception). Similarly, the move-
ments of the joints or the internal ear can note the movements imposed on the body
as well as the movements initiated by the individual. Hence, the proprioception -
considered to be the means of obtaining information about our own movements -
does not necessarily depend on proprioceptors. And exteroception - considered to
be the means of obtaining information about external events - does not necessarily
depend on exteroceptors.
All receptors code sensory stimulation in the same manner: The stimulus applied
creates a potential difference in the membrane of the transducer site. This membrane
depolarisation constitutes the receptor potential, whose intensity depends on the value
of the stimulation. The receptor potential then leads to the formation of action poten-
tials which the nerve fibres transfer in the form of a series of potentials at the speed of
1 to 100m/s. Their amplitude and duration remain constant (some dozens of millivolts
of 1 to 2 milliseconds) and their number depends on the value of the receptor potential.
The message is thus coded in frequency and transferred to the nerve centres. The read-
ers should note the similarities (binary signals) and the differences (digital or frequency
coding) between the transmission of signals in a computer and that in a human body.
There is a minimum level below which any stimulation does not create any effect
on a sensory organ. Above this level, the minimum perceptible variation of a stimulus
is proportional to the absolute value of the intensity of the stimulus (Weber's law).
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