The Doctrine of Specific Nerve Energies
The sensory quality experienced, such as from a sound, touch or light, depends
on which nerve is stimulated, not on HOW it is stimulated.
It is not the form of physical energy that determines the nature of the sensation
but rather the specificity of the neurons, receptors and nerves activated by the
Our brain has no access to the physical stimulus itself. (!!!)
Johannes Müller (1835)
We’re not entirely sure, but we know that there are different codes that neurons
Place (or labeled-line) code. Neurons in difference places in the body
signal different qualitative features. For example, where the nerve cell is located
in the retina says something about where in the visual field the stimulus must be
since light travels in a straight line – if the eyes are straight ahead, the more off to
the side an object is, the more off to the side the image of it will be on the retina.
The retina is a bit like a page scanner, in that every place on the scanner bed is
sensitive to a different place on the page being scanned. The particular cells in
the retina that are activated by a stimulus tell the organism where the stimulus is
Population (or pattern) code. Instead of information being conveyed by
single nerve cells, or a small group of cells, it is conveyed in the activity across a
whole population – a lot of cells. As an analogy, think of the
3 children’s toy, Lite Brite. This toy is a dense rectangular array of holes, into
which you stick lights. You can arrange the lights to make a pattern. Think of the
dense array of holes as a whole nerve bundle, with thousands and thousands of
nerve fibers (axons). The lights are the nerve fibers that are firing. The image of
Mr Potato Head isn’t in any one axon, but depends a pattern of activity across
the whole array. This seems to be the way that the olfactory system in your
nose works. Specific smells (coffee, banana, vanilla, lavender) are not coded by
a particular set of cells firing (there are no banana cells or coffee cells). Instead,
the pattern of activity across a whole array of olfactory cells codes the particular
Temporal code. Neurons can fire quickly, or they can fire slowly. There is
an upper limit on how fast they can go: different neurons fire at different speeds,
but a rough estimate is that a neuron can fire once every 5 milliseconds, or about
200 times a second. The frequency of a sound – perceived as pitch, can be
coded in the firing rate of a group of neurons, as you’ll learn next week.
Loudness, the psychological correlate of a sound’s intensity, is also coded in
firing rate, as is brightness, the psychological correlate of the intensity of light.
Neurons do fatigue though – like anything going as fast as it can go, eventually it
gets tired, and stops. It takes only a few seconds of steady, continuous
stimulation for a neuron to fatigue. This is a very important feature of sensory systems that we’ll come back to – the neural mechanism underlying the
phenomenon of adaptation.
There’s also a minimum rate of neuronal firing. When there is no stimulus
present, nerve cells still fire randomly, at some spontaneous rate. If a neuron
isn’t fatigued, then the rate of firing indicates the intensity of a stimulus – how
strong (e.g., bright; loud) it is. Weak stimuli will increase the firing rate a little bit;
strong stimuli will increase it a lot. A stimulus will be detectable if it causes the
rate of firing to increase beyond what is the typical spontaneous rate.
Purkinje Shift. As twilight approaches, our sensitivity to different colours
changes. During bright (midday) conditions, we find yellows and reds to be most
brilliant. When twilight comes, blues and greens become more brilliant. This was
first described by a physiologist named Purkinje who noted it when he was
gardening one evening. This is the third piece of evidence supporting the idea
that there are two photoreceptor systems.
Trichromatic Theory of Colour Vision. Based on his findings, Young, and later
Helmholtz, suggested that colour vision is the result of the activity of three
different colour receptors in the retina. In the 1940s Wald found that there were,
in fact, three different types of cones. Each one responds to a range of
wavelengths, but is most responsive to a different wavelength (what we see as