1. HERMANN VON HELMHOLTZ
(1821-1894)
Born in Potsdam
Mediocre student, though he spent his spare time reading
science.
Father could not afford to pay for scientific training.
Interested in natural sciences, but found a program for free
medical school if he served eight years as an army surgeon.
Studied under Johannes Muller.
2. Stand against vitalism
Vitalism : The belief that life cannot be explained
solely as the interaction of physical and chemical
forces.
-> Johannes Muller was a vitalist.
Materialism: the belief that there is nothinf
mysterious about life and assumed that it could be
explained in terms of physical and chemical processes.
-> no reason to exclude the study of life from the realm
of science.
3. Principle of conservation of energy
The energy within a system is constant; therefore , it
cannot be added to or subtracted from but only
transformed from one form to another.
Applies to living organisms as well.
The energy from food and oxygen will equal the energy
expended by muscles and organs.
Clearly a materialistic statement.
4. What is the speed of conduction of
information in the nervous system?
measured speed of nervous transmission in frogs and
humans.
In 1852 he measured the speed of a nerve impulse.
Helmholtz stimulated a frog’s nerve near a muscle and
then farther away . Impulse velocity within the
nervous system was calculated at one tenth speed to
the speed of sound or 26 meters per second.
5. Of frogs and Men
In comparison with his frog experiments, Helmholtz’s
work on human presented special challenges.
Simple Reaction Time Experiments
A very electric shock was applied to the skin.
Subjects were ask to react with hand movement which
registered time to react.
He repeated experiments in different subjects. Results
varied between a mean of 0.12 and 0.20 seconds.
6. Parts of Reaction Time
1. One part of the time between stimulation and
reaction was consumed by the ‘sending of the signal’
(i.e. the stimulus) through the sensory nerves.
2. Another portion of time ( which Helmholtz assumed
to be the same time as the first) was needed to
transmit the ‘message’ through the motor nerves to
the muscles.
3. The remaining part of time, Helmholtz concluded,
was the time required ‘in the brain for the process of
perceiving and willing’.
7. Speed of nerve conduction in
humans
The procedure Helmholtz then used was to stimulate the
human body in different places (e.g. in the toe and the
thigh).
He could then measures the relative differences in time to
response to a stimuli at different points on the body-the
processing time within the sensory nerves (assuming brain
pr0cesses and motor processes to be constant).
By this method, Helmholtz calculated that RTs were slower
when the toe was stimulated compared to the thigh-nerve
impulses were estimated to travel 50-60 meters per second.
8. How do we use RTs to quantify
mental processes?
An early mental chronometer invented by F. C. Donders
to measure reaction time. The paper tape given could be
used to measure the time between a stimulus sound and a
reaction sound. The top line shows the oscillations of a
tuning fork (which take a known amount of time); the
bottom shows the time, in distance, between the spoken
stimulus and spoken responses.
9. F. C. Donders
Donders’ logic of cognitive subtraction allows one to infer
the speed of internal mental processes that are not directly
observable.
Donders reasoned that the time needed for a simple
detection task consists of the time it takes to perceive the
stimulus plus the time it takes to generate the response. He
then used a “subtractive method” to infer how much time
was needed for intervening tasks, such as identification,
comparison, or other higher-level judgments.
10. Logic of Cognitive Subtraction
identification time= discrimination task RT- detection task RT
Decision process :
what color was the
light that I just saw?
Press button 1 when
you see a red light, but
press button 2 when
you see a green light.
Press a button when
you see a light.
11. Theory of color vision
Newton had discovered that pure orange wavelengths
were indistinguishable from orange created by mixing
red and yellow. -> the property of color cannot be in
the wavelengths themselves.
Young-Helmholtz theory of color vision: separate
receptors systems on the retina are responsive to each
of the three primary colors: red, green, and blue-
violet. Also called the trichromatic theory. -> an
extension of the doctrine of specific nerve energies.
Not just a single nerve energy for vision, but three
types of receptors on the retina.
