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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