SENSORY PROCESSING

This laboratory is intended to illustrate some basic principles of sensory processing by the brain, and to give you the background and some incentive to further explore them on your own. The general question under investigation is the nature of the central information processing steps which intervene between the initial stages of sensory transduction and the appreciation of a sensory percept.

This is very much an "open ended" laboratory, one intended to provide you with the wherewithal to rigorously explore some interesting questions and the opportunity to follow your own nose in directions that interest you. Accordingly, it includes some quite well-defined questions and concrete observations aimed at addressing them, as well as a number of less well-defined questions which you may personally find more interesting and prefer to spend more time on. At the outset, you should "get a feel" for all of the kinds of observations discussed below. You are then free to focus for the remainder of the lab session (and, of course, any additonal time you'd like to spend on your own) on any of the kinds of observations and questions which you would like to further explore.

If you think you might want to use this laboratory as one of your required write-ups, recall that science depends fundamentally on interesting questions, relevant, concrete,and well-characterized observations, and careful interpretation of those observations. One fairly certain route to this is to do the careful analysis of the "one eye" snapshot described under C, and the comparison to it of the "two eye snapshot" described under D. This will yield unambiguous quantitative data in the form of two "visual field records", sheets of white paper marked with the locations of foveae (assayed two different ways, once in terms of acuity and once in terms of some other parameter you must discover) and blind spots. The observations to verify the importance of eye movements mentioned in E will yield two additional "visual field records", one for a new fixation point and the other for the same fixation point viewed from a greater distance. A paper reporting these observations (you'll want to put the data from the large sheets of paper in some more manageable form) and discussing their significance would be an appropriate lab report. Alternatively, you might want to design your own observations to explore how the nervous system handles the blind spot (section F), some of the problems associated with movement and perceived direction (Section E), or any other of the matters discussed under the general rubric of accounting for "the picture in your head" in terms of brain processes. Whatever you choose, be certain you satisfy the requirements for clear observations rigorously interpreted in light of a well-defined question.

BACKGROUND

Sensory processing is usually presumed to begin when matter or energy in one form or another impinges on a set of specialized sensory neurons, which transduce that input into permeability changes and ultimately action potentials, the common currency of communication within the nervous system. In the case of the visual system, on which we will focus, sensory transduction occurs in a two-dimensional sheet of photoreceptors which covers the back of the eyeball. Transduced signals are further processed in additional overlaying sheets of neurons which together constitute the neural retina. The innermost of these neuronal layers consists of a sheet of ganglion cells whose axons group together, and collectively pass through the retina at the optic nerve head to form the optic nerve, which carries visual information from the eye to the brain. In addition to the optic nerve head, which lacks photoreceptors entirely, there is a second, distinctive region of the retina: the fovea. Photoreceptors are particularly narrow and closely packed in the fovea. For this and other reasons, visual images falling on the fovea are particularly clearly seen.

At the front of the eyeball are a series of refracting structures (the cornea and lens) which bend light rays in an effort to converge the rays of light diverging from each location in space so that they come together at a particular location on the retina. The effect of this, for relaxed vision and a distant scene, is that the light intensity of each point in the distant scene is accurately represented at a distinct location in the photoreceptor array, with neighboring points in the scene represented at neighboring points on the photoreceptor array. To put it differently, the optics of the eye create on the photoreceptor layer a very good picture of the scene: a pattern of variation in light intensity which quite faithfully replicates the variation of light intensity across points in the scene. Resolution is a measure of how close points in the scene can be and still be seen as separate points (with distinguishable intensities), and is closely related to how "clearly" one sees. The optics of the eye do not produce any large differences in the resolution of different parts of the picture produced on the retina.

Humans, like many but not all other animals, have frontally placed eyes. An important consequence of this is that much, though not all, of the visual scene is simultaneously seen and reported on to the brain not once but twice, once by each eye. The binocular visual field, that area seen simultaneously by both eyes, is a large central zone bordered by smaller peripheral areas seen only by one eye. Single-eyed (monocular) vision can, of course, be extended to the entire scene by the simple expedient of closing one eye.

OBSERVATIONS AND INTERPRETATIONS

A. The "picture in your head"

What needs to be accounted for is how the world looks to you. So before starting an analysis, relax, look around you, and try to get a sense of what you are currently seeing. Pay attention to what "it" looks like, what properties it has. This is the "picture in your head". Among other things, it is stable, continuous, and looks reasonably clear in all directions. What we will do is compare this normal end product with what is seen under more controlled conditions in an effort to infer some of the information processing events that must be occurring in the brain.

B. Preliminaries

Since the picture in your head "begins" with a pattern of light intensity on the retina, a good starting point for our analysis is the "one-eye snapshot", how the world appears when looked at using one eye directed at a particular point in the scene. Stand 57 cm from a large blank sheet of white paper with a cross marked on it and stare at the cross (the "fixation point") with your left eye closed. At 57 cm away, a distance of one centimeter on the paper corresponds to one degree of visual angle. On a large enough sheet of paper (more than about 60 cm from the cross to the edge), you should be able to locate the left border of the visual field of your right eye by determining where on the piece of paper you can and cannot see a black spot. Do so, marking the border on the paper. Now open your left eye and verify that the visual scene has become larger. Repeat the whole process to locate the right edge of the visual field of the left eye. Between your marks is the binocular visual field.

For reasons of simple geometry (the sheet of paper is flat rather than curved), one centimeter corresponds to one degree of visual angle only relatively close to the cross at which you are looking. You can more accurately (and simply) approximate the size of binocular and monocular visual fields by looking at (fixating) any distant point and, closing each eye in turn, determining where you can and cannot see some appropriate object (a finger, for example) at locations equidistant from your head around you. Do so, and then compare the size of the visual fields of each eye to the total field size you can see when looking at a point. What does this imply about what the brain must be doing to create the extent of the "picture in your head" when you are looking in one direction? Can you detect any signs of this process in the "picture in your head" itself under these circumstances? What is the relation between the total visual field size you measured and the size of the scene in the "picture in your head" when you (as normal) are free to look around? Does the measured visual field size increase if you move your eyes? your head? What additional things must the brain be doing to yield the extent of the "picture in your head?"

C. A closer look at the "one-eye snapshot"

Use a one centimeter black spot on a small piece of paper to start exploring the question of whether the "one-eye snapshot" is stable, continuous, and clear throughout its extent. Begin by placing the black spot at a fixation point on a large sheet of paper and noting its appearance, both immediately after you place it there and after it has been there for a while. Repeat the process for locations in different directions and at increasing distances from the fixation point. You should be able to detect significant differences in the appearance of the spot at difference locations in the one-eye snapshot, as well as a circumscribed region where the spot disappears entirely. Define the latter region, the "blindspot", as closely as possible, and mark its extent on the white paper. Where is it located? What is its size and shape? Points at this location in the visual field are imaged at the optic nerve head of the retina, and so light coming from these points are not transduced into neural signals. This raises some interesting questions which we will return to. The retina, as mentioned, contains a second reasonably circumscribed, distinctive region: the fovea. You should be able to detect such a region using the one centimeter black spot. Define some appropriate criterion for the appearance of the black spot within and outside a circumscribed region, and mark its extent on the white paper. Where is it located? What is its size and shape?

The fovea, as mentioned earlier, is usually defined as the retinal region responsible for the clearest, or highest acuity, vision. You can define a region of relatively high acuity by drawing two small dots, a half centimeter apart, and determining where in the visual field you can clearly see two separate dots and where they instead blur into one. Do so, marking the borders on the white paper. How does this region compare to that previously defined for the fovea? What happens if you use dots with a one centimeter or two centimeter separation?

The "one-eye snapshot" obviously differs from "the picture in your head" in some well-defined ways, related not only to the visual field extent but also to the stability and clarity of subregions of the pictures. Its properties largely reflect phototransduction and the initial stages of neural processing which occur in the synaptic layers of the retina. The question then is what must be done in the brain to signals coming from the optic nerve to turn the "one-eye snapshot" into the "picture in your head". Review the differences between the two pictures and think about the differences between the circumstances in which they are seen in an effort to come up with testable hypotheses about what is going on in the brain.

D. Role of binocular vision? The two-eye snapshot

An obvious difference in the circumstances which could contribute to observed differences between the "one-eye snapshot" and the "picture in your head" is that you are getting information from only one eye in the former case and simultaneously from two in the latter. This might suggest that the combining of information from the two eyes is an important part of what the brain does to create the more stable, continuous, and clear "picture in your head." You can and should test this by characterizing a "two-eye snapshot" in the same way you did the one-eye snapshot, fixating the marked cross but with two eyes open instead of one and testing various locations with the one centimeter black spot as well as pairs of spots.

How many blind spots do you expect to observe and where? How many do you find? What conclusions can you draw? How many circumscribed areas where the black spots appears different do you expect to find and where? Areas where acuity is higher? How many do you find and where? Can you explain what you've observed? What does it imply about what each eye does when you view scenes with both? How does it bear on the hypothesis that simultaneous input from both eyes is what accounts for the stable, continuous, and clear "picture in your head"?

E. Role of eye movements? Back to the one-eye snapshot

A second difference in the circumstances which could contribute to observed differences between the "one-eye snapshot" and the "picture in your head" is that your eyes are stationary in the first case and free to move in the second. This might suggest that the picture in your head depends not only on input to the brain from the retina but also on signals related to eye movement. You can and should test the possibility that eye movements play a significant role in creating the clarity of the picture in your head. Begin by staring at the cross as you did for the "one-eye snapshot" and positioning the one centimeter black spot outside the region where it can be clearly seen. Can you make it clearly visible by moving your eyes? What eye movement achieves this and why? Design and carry out an experiment to verify the hypothesis that effective eye movements are those which result in the black spot rather than the cross being imaged on the fovea. Be sure you understand this point by predicting and verifying changes in the one-eye snapshot if you stand twice as far from the white paper. What does all this imply about what must be going on in the brain to create the overall clarity of the picture in your head? Design and carry out an experiment to verify this general conclusion by showing that it is not the location of the image on the retina but rather signals related to eye movement which determine whether you perceive a spot to be to your left or to your right. (This is a quite general problem, and quite interesting related experiments can be done in other sensory modalities; your might want to conceive and carry out some if you have extra time). You should also be able to show that, in binocular vision, the apparent direction of a target between you and the screen varies depending on which eye you look at it with. Explore this, considering both what it might be good for and what additional problems it means must be solved in the brain (If you have time, you might also want to explore the dependence of your concept of "rightness" and "leftness" on whether you are looking with the right eye, the left eye, or both eyes).

F. Other factors? The one-eye snapshot and the blindspot

Clearly, both eye movements and binocular vision contribute to the clarity and continuity of the "picture in your head". By eliminating both, we can determine whether any other factors also contribute. Again stare at the cross (you should now understand why doctors checking your vision frequently ask you to fixate a point), and locate the blindspot. Place a large colored sheet of paper across the area of the blindspot. What do you see? What does this imply about what the brain must be doing to create the picture in your head? This observation raises some interesting questions about the "veracity" of the picture in your head. The underlying mechanisms can and should be further explored, depending on your inclination and available time. You might, for example, want to determine the minimum size and shape of the colored sheet needed to obtain the observed effect. Does the underlying mechanism constitute a simple "bleeding" into the blindspot of a surrounding homogenity or is it more sophisticated? Try positioning a pen across the blindspot and comparing what you see then with what you see when the pen is positioned so that it intrudes into but does not fully cross the blindspot. You can try a variety of patterns to determine what aspects of the surroundings do and do not influence what you see in the blindspot and the role that your conscious expectations do and do not play in this.

CONCLUSIONS

The general concern of this laboratory session was to better understand what must be going on in the brain after the sensory transduction processes in the retina in order to produce the "picture in your head". Critically review the observations you have made, and consider the extent to which they do or do not yield clear conclusions. What would you look for if you could make more direct observations on the brain itself? Would you expect this to verify your conclusions? alter them? extend them? What lessons from these observations would you expect to generalize to other sensory inputs? What implications do these observations have for sensory perception generally? For behavior generally? How could these be further explored?




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