APPsychologyChapter5Sensation




 * Distal Stimulus: **A distal stimulus, as opposed to the proximal, is the thing (itself) that we are looking at. An object, such as the "T" above, is the distal stimulus. The proximal stimulus is the image that is registered on the back of the eye.

 
 * Proximal Stimulus: ** A near stimulus that acts directly on an aspect of the nervous system. The effect of the light waves on the retina of the eye is an example of proximal stimulus.


 * Retina: ** The interior rear surface of the eye, containing light-sensitive cells, called photoreceptors, which collect information and transfer it to other parts of the brain for processing and comprehension.




 * Visual Cortex: ** The region of the brain's cerebral cortex that processes visual information.

Defintion for **__Absolute Threshold__** (curtousy of About.com Psychology) : **Definition:** A term often used in neuroscience and experimental research. An absolute threshold is the smallest detectable level of a stimulus. For example, in an experiment on sound detention, researchers may present a sound with varying levels of volume. The smallest level that a participant is able to hear is the absolute threshold. We've got to do taste tests when I return.

Another good example of the residual effect of motion on our visual perception. Check this one out (with explanation from [|www.aber.ac.uk/media/Modules/MC10220/visper08.html] )

**The limitations of seeing movement** Despite our amazing ability to recognise moving shapes, our perception of movement falls within certain thresholds. Movement which is either too fast of too slow is not detectable. At one extreme we are unable to see a bullet fired from a gun, at the other we are unable to detect the motion of an hour hand going round a clock face. It is also possible to fool the brain into thinking there is movement where actually none exists. Such phantom movement is created by lookin at something constantly in motion, like a waterfall, and then looking at something which is not moving. If you do this, you will find that the stationary scene appears to be moving in the opposite direction to the waterfall, i.e., up. This illusion is called "the waterfall illusion." One example was shown in session 6; another variation of it is illustrated in Fig 1. Fig 1 - Instructions: stare at the spiral for about 20 seconds, then stare at the pattern.
 * [[image:http://www.aber.ac.uk/media/Modules/MC10220/images/spiral1.gif width="320" height="256"]] || [[image:http://www.aber.ac.uk/media/Modules/MC10220/images/spiral_bg1.gif width="320" height="256"]] ||

In a study that looked at similar spiral discs Spigel (1962) it was reported that all observers saw the spiral move in the opposite direction. The after effect lasted ten seconds on average.

Next is the Phi Phenomenon which we will actually do more with in Chapter 6 (Perception). The reason the dot appears to move is because of a set of two illusions that come under the heading of phi phenomena. Phi phenomena were discovered in 1912 by Max Wertheimer, a pioneer of gestalt theory(Wikipedia, www) - see session 7 for more on gestalt theory. Wertheimer was experimenting with two lamps, turning them off and on in quick succession and noticed that the light appeared to 'jump' from one lamp to the other. 'Phi phenomena' is actually an umbrella term that describes two types of apparent movement. One of these is the one we have already seen in figures 4, 5 & 7; that is the phantom dot chasing around the circle. Confusingly this is called phi-phenomenon, the singular of a phi phenomena. The other type of movement that comes under the heading of phi phenomena (plural) goes by the name of beta movement. Beta movement is illustrated in fig. 8, which is exactly the same image as fig. 4, but with its motion slowed down. However, you will notice that instead of the while dots appearing to be stationary and blinking on and off, that now the dots themselves appear to move - jumping to fill the place place vacated by the previous dot. Also you will also notice that the phantom chaser dot appears to have almost disappeared.
 * Phi Phenomena**

. Fig 8 - beta movement The fact that persistence of vision is wrong can be easily proved with a simple demonstration. I you look at figure 4, you will notice that a series of white dots appear to blink on and off in sequence against a blue background. You should also notice that a "phantom" dot, the same colour as the background, appears to chase around the circle in an anti clockwise direction.

. Fig 4 white dot, blue background

The phantom dot always takes on the colour the background. So just to prove this, Fig. 5 shows a phantom white dot chasing around a blue circle:

. Fig 5 - blue dot, white background

Now if the persistence of vision is correct it would follow that the apparent movement of this image would be due to the after image of the white dot (Fig. 4) or blue dot (Fig. 5). After-image is actually a very real phenomenon. For example, if you state at the pink triangle of (Fig. 6) for a while and then stare at the grey space next to it and you should see the after-image of the triangle - it will turn green, as the after image appears as complimentary colour of the source image.

Fig 6 - Instructions: stare at the triangle for about 10 seconds, then stare at the grey square.
 * [[image:http://www.aber.ac.uk/media/Modules/MC10220/images/pink_triangle.gif width="320" height="256"]] || [[image:http://www.aber.ac.uk/media/Modules/MC10220/images/grey_bg.gif width="320" height="256"]] ||

Now look at this series of pink dots in a white circle of fig 7.

. Fig 7 - pink dot, grey background

The chaser dot is not pink but as in (fig. 4) it takes on the colour of the background. It is possible you see an after image of the pink dot in this example, but you find that it actually fights with the movement of the chaser dot. Thus the very hypothesis on which the persistence of vision rests, does not in fact actually account for the movement of this phantom dot - if anything the after image tends to gets in the way of the perception of movement and you have to try to ignore it. //"The mind does not simply record an exact image of the world but creates its own 'picture'// ."- Irvin Rock //"The perceptual apparatus... does not copy reality but symbolizes it."// - Teresa de Lauretis

Remember the jobs of the brain? It receives information, it intreprets information, it catalogs information, it stores information, and it recalls information. In those processes, there are alterations to reality (small alteration, I think, but alterations nonetheless).

PHANTOM LIMB 

These questions pertain to the two articles from SCIENTIFIC AMERICAN on phantom limbs and how to deal with phantom limb pain. If you need the articles the links are here - [|The Mirror Cure for Phantom Pain] and [|Phantom Limb Pain Retraining the Brain]. The questions are: 1. What is a "phantoim limb"? 2. Why do we care? Who could possibly be involved with this phenomenon that would cause us to care? 3. What area (or areas) of the brain is (or are) involved in this phenomenon and in the related "phantom limb pain"? 4. How does phantom limb pain work? How can a limb that does not exist anymore hurt? 5. What is pain? 6. Perhaps "mirror therapy" is a great distractor. How and why does distraction lead to pain relief? 7. Where do motor imagery and motor execution occur in the brain? 8. What do you think should be done? What is the best possible course of action for dealing with the phenomenon of phantom limb pain?