Biomedical Engineering Reference
In-Depth Information
their neuroanatomical structure, it is an important question whether this similarity is par-
alleled by a similarity of processes. These issues are addressed by state-of-the-art research
methods in behavioral research, psychophysiology, and mathematical modeling [31, 32].
Psychophysics, as established by Fechner, aimed at finding pervasive quantitative rules
for relationships between the intensity of stimulus and the psychophysical sensation itself.
In physics, which derives a tremendous power from them, such rules are called scien-
tific laws. As mentioned, the first psychophysical law was established by Fechner. The
observation proved to hold not only for weight, but for other physical variables as well.
Although Fechner's logarithmic law was much later proved to be incorrect (notwithstand-
ing its association with Weber's law) it did, nevertheless, demonstrate the importance
of Fechner as a physicist and the contribution he made to scientific laws. They point to
relationships of great generality from which specific relationships can be derived and, in
so doing, they provided the basic structure of this science.
Most recently, psychophysics and physiology together have become essential parts of
environmental sciences by telling us how our environments affect us. Psychophysics may
be defined nowadays as a science of quantitative relationships between psychological
variables and the physical variables that elicit them. 'Physical' is used here as a generic
term that includes 'chemical.' Some of the relationships are so intimate that before suffi-
cient instrumentation was developed, the only way people knew about the physical events
was through their senses. This is probably the reason why, even today, the same word
is used for the physical light as for the sensation of it. The same is true for sound and
some other physical variables. We have to be clear in specifying whether we mean the
physical variable or its sensation. When we say 'the light is bright' we really mean our
sensory impression, not the physical quantity that we can know only through inference.
The inference may be quite inaccurate and depend on context variables. Optical illusions
are well known.
The laws constitute the backbone of a science from which other relationships can be
derived, and the effects of variables producing deviations from the relationships defined
by the laws studied. For example, Newton's law applies to objects falling in a vacuum.
When an object falls in air due to the effect of gravity, its acceleration is decreased by air
resistance. To establish a law empirically, all the variables not specified in the law must
be eliminated or their effects determined and accounted for.
In psychophysics, the determination of sensory characteristics occurs indirectly by mea-
suring one stimulus variable as a function of another. For example, the threshold of
audibility is determined as a function of sound frequency, the threshold of visibility as a
function of the wavelength of light, detectable vibration as a function of vibration dura-
tion, and so forth. It is also possible to measure magnitudes of different stimuli producing
equal subjective magnitudes such as, for example, sound intensities at two different fre-
quencies that produce equal loudness. While all such measurements have proven to be
useful they are, nevertheless, only limited to threshold values, or to subjective magnitudes
relative to others specified indirectly in terms of the stimulus values that produce them.
Such stimulus-oriented psychophysics provides only an incomplete image of our sensory
functioning that most often occurs at stimulus values that only have sufficient strength or
quantity to produce a barely perceptible physiological effect (suprathreshold) regardless
of specific reference standards.
