Biomedical Engineering Reference
In-Depth Information
An essential step in any teletaction system is to first determine the type of stimulus
that is to be investigated, such as sampling density or amplitude resolution, so that the
appropriate design and system requirements can be formulated from which psychophysical
experiments can eventually be conducted. By contrast, psychophysical studies of haptic
fidelity apply careful metrics to user task performance during the execution of simple tasks
[33]. Psychophysical study of teletaction systems is necessary for several reasons. Firstly,
the mechanism of how variations in a tactile display's actuator parameters change the
perception of shape and dynamical properties of contact, such as texture and friction, is
still not sufficiently understood. Secondly, suitable sensory resolution associated with the
teletaction system still needs to be obtained through psychophysical experiments. Thirdly,
for the application uniqueness of each teletaction system there is still insufficient data that
describes detection thresholds and associated psychophysical functions.
10.4.1 Steven's Power Law
A law relating the objective, instrument-measured intensity of a stimulus to its intensity
as perceived by a human, was enunciated in 1957 by Stanley S. Stevens. It addresses the
same question that Fechner did almost 100 years earlier, but while Fechner postulated
that the perceived intensity is always related logarithmically to the physical intensity,
Steven's law says the magnitude of the perceived intensity is related to the magnitude
of the physical intensity raised to some power and is sometimes simply called the Power
Law. Stevens' power law is often considered to supersede Fechner's law (known as the
Weber - Fechner law) upon which he expanded and included a wider range of sensations.
In it he was able to demonstrate, more convincingly than his predecessors, that mutually
consistent ratio measurements of loudness and brightness were possible. The principal
methods used by Stevens to measure the perceived intensity of a stimulus were magni-
tude estimation (MA) and magnitude production (MP). In magnitude estimation with a
standard, the experimenter presents a stimulus called a standard and assigns it a number
called the modulus. For subsequent stimuli, subjects report numerically their perceived
intensity relative to the standard so as to preserve the ratio between the sensations and
the numerical estimates (e.g., a sound perceived twice as loud as the standard should be
given a number twice the modulus). In ME without a standard (usually just ME), subjects
are free to choose their own standard, assigning any number to the first stimulus, and all
subsequent ones with the only requirement being that the ratio between sensations and
numbers is preserved. In magnitude production a number and a reference stimulus is given
and subjects produce a stimulus that is perceived as that number times the reference. Also
used is
cross-modality matching
, which generally involves subjects altering the magnitude
of one physical quantity, such as the brightness of a light, so that its perceived intensity is
equal to the perceived intensity of another type of quantity, such as warmth or pressure.
The initial experiments were performed on hearing, in which an individual was given a ref-
erence standard consisting of a tone at a predetermined intensity and a number to express
its subjective loudness magnitude. The subject was instructed to assign numbers to subse-
quently presented tones in proportion to their loudness magnitudes relative to the standard.
The numbers proved to follow a power function law for which the general form is:
ψ (I )
=
kI
a
[34]
