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Brain Functioning

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PSYC 100
Ingrid Johnsrude

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 stimulus. 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 can use.  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 ‘out there’.  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 odor.  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 yellow-gr
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