Lab 6 (Chapter 8): The Retina
(Graded out of 50, worth 5% of final grade)
This lab consists of two parts and is done in the lab or at home. Part 2 requires the assistance
of a partner.
Equipment
- A computer with a web browser that is Java enabled (almost any modern computer will have
such software)
- A ruler
- A printed copy of the last page of this lab, which contains the stimuli need to do Part 2
(Provided by us)
Handing In
1. Visit the course website
2. Attach your completed lab 6
Background
In class we discussed the anatomy of the retina. Here, we will explore to aspects: the receptive
fields of retinal ganglion cells and the blind spot. Recall that retinal ganglion cells have centresurround receptive fields. This was discovered using single-cell recording techniques. You will
be doing a simulation of the same kinds of experiments here.
As for the blind spot, we do not normally notice it because the brain “fills in” the space where it
would normally appear. In part two of the lab, we will look at this phenomenon. In preparation for
this lab, you should read Chapter 8, sections A1 and B1.
Part 1: Mapping Receptive Fields
Researchers map out receptive fields by placing micro-electrodes in a neuron then searching
the area of a sense organ for a zone that affects the cell’s baseline firing rate. Here, we will
simulate that activity using a Java Applet.
Procedure
You will be acting as a researcher and technically the computer will simulate being the
participant.
Go to the following website: http://tinyurl.com/6tzd9ta
After a moment, you should see a window open up with a blue background. There is a large
dark area on the left and a set of axes on the right. At the bottom are buttons labeled “+”, “-“,
“Remove Last”, and “New Cell” (and also “Show Cell” but we don’t use it).
The axes on the right show the baseline-firing rate of a simulated retinal ganglion cell. It is
currently firing at a low but non-zero level.
The dark area to the left is a simulated patch of retina. You will see a white circle in the upper
left corner of that area. This represents a lift stimulus you will use to map out the receptive field
of the simulated retinal ganglion cell. Click and drag to move it around (or, use the horizontal
and vertical sliders beside and below the dark area).
As you move it around the simulated patch of retina, you will see the level of activity of the
simulated retinal ganglion cell change. For some areas, the retina ganglion cell activity level
drops below baseline (i.e., inhibitory areas), for others it jumps far above baseline (i.e.,
excitatory areas). Your job is to map out the inhibitory and excitatory areas.
To map out the receptive field, move the light slowly around the patch of retina while watching
the firing rate of the retinal ganglion cell. When it drops, you are in an inhibitory area and should
click the “-“ button. When it rises, you are in excitatory areas, and should click the “+” button.
Keep doing this, adding plus and minus symbols to the retina patch, until you feel you have
sufficiently mapped out the receptive field. You should figure out some kind of systematic
approach and follow it. You may want to review Chapter 9, section B to see what shape the
receptive field is likely to have.
Once you are satisfied you have mapped out the receptive field, do a screen capture.
If using Windows, press “Alt - PrntScrn” to do this. This will place a screen image in your
clipboard. Paste this image into this document (Control - V). Once pasted crop the image to
contain only the receptive fields.
If using OS X, press “Shift - Control - Command - 4” and select the area to capture. This will
place an image on your Desktop. Drag this image into this document.
Go back to the Java web application, and click the “New Cell” button and repeat the procedure.
Perform the above steps until you have mapped out a total of 4 retinal ganglion cells’ receptive
fields, copying and pasting each map below.
Paste your three retinal ganglion field maps below
RC 1
RC 2
RC 3
RC 4
Discussion
1. Compare and contrast the three RF’s you mapped. Were they the same shape? The same
size? Were they arranged as you expected?
The RF’s mapped were all the roughly the same size and shape. However, they differed
in the activity of the center of the field. Some of the receptive fields showed excitatory centers
whereas others showed inhibitory centers. The receptive fields were arranged as expected in a
circular manner with similar activation (ie inhibitory or excitatory) in the centre of the circle and
the opposite activation around the periphery.
2. What kind of systematic approach did you take in moving the light stimulus around when you
were mapping out the receptive field? Why?
When moving the light stimulus around the field, I began by noting the major
activity level of the field as a whole. Generally, the receptive fields showing excitatory activation
in its center tended to show overall inactivation on the field as a whole. The opposite is also
true. Furthermore, once a point on the field that represented the minority activation level was
mapped, I worked my way away from it to determine where the center meets the periphery. This
was usually indicated by an abrupt change in ganglion activation in the opposite direction (i.e.
above or below the base line). Once I had a center and periphery point mapped, I made my way
around what I thought was the center and continued to define the periphery. I chose to use this
method because I predicted that each receptive field would take on a similar circular shape.
3. Did the cell ever reach its maximum possible firing rate? Why or why not? What
characteristics of the stimulus would you need control over to elicit a maximum response?
No, the cell never reached its maximum possible firing rate. In order for a maximum
response to be elicited, a dark spot must fill the center of the receptive field entirely and a light
spot would need to surround it. For this to occur the stimulus must represent a dark center and
a light periphery.
Part 1: Mapping Receptive Fields
In class we discussed the fact that where the optic nerve leaves the eye (i.e., the optic disc),
there are no photoreceptors. This is, therefore, a “blind spot” in your visual field, where you do
not see anything. In this lab you will, with the help of a partner, map out your blind spot and
make some observations of what it does to your vision.
Equipment
For this part of the lab, you will need a method to take notes, a ruler and the last page of this lab
handout, which contains the stimuli for this lab.
Procedure
Practice: First, practice finding your blind spot. You may find this a little tough at first, so keep
trying. See “tips” below if you’re having trouble.
1. Hold the stimulus page about 40 cm from your face with the top pair of dots horizontal, and
close (or cover) your left eye.
2. Look at the left dot with your right eye (your left eye should be closed or covered) and
move the page slowly towards you. You should see the right dot disappear at some point.
Remember to keep your eye on the left dot or this will not work!
3. Now, keep moving the sheet towards you until the dot reappears.
If you’re having trouble, keep trying! Here are a couple of tips if you are really struggling:
a) Make sure you are not moving your eye. It can be tricky to keep fixated on the left dot. Just
focus on that and do not worry about the right dot. Just “let it disappear”. Have you partner
watch you and let you know if you are unconsciously moving your open eye.
b) Try tilting the page a bit so the dots are not quite horizontal. Some people’s blind spot is a
little higher or lower than horizontal.
c) Try switching to the left eye (i.e., close/cover your right eye instead). It may be easier to find
your blind spot in one eye than the other. If you do this, you want to look at the right dot
and watch for the left dot to disappear.
d) If you really can’t do it, use your lab partner’s data. If you both can’t do it, you’ll have to
persuade someone to help you out.
Data Gathering, Quantitative Measurement
Once you’ve practiced finding your blind spot a couple of times, it is time to map it out
quantitatively.
1. First, have someone measure the distance at which the dot disappears (in cm). Typically
this will be around 30 cm, but it varies considerably from person to person. Measure it 5
times and take the average.
Mean distance at which dot disappears: ______28,27,31,27,32___avg: 29_
2. Next, have your partner measure the distance at which the dot re-appears (in cm) as you
continue to bring the sheet closer to your face. Typically this is around 20 cm, but again
varies a lot from person to person. Again, take the average of 5 measurements.
Mean distance at which dot re-appears: _23,24,25,23,25___avg: 24___
3. Now calculate the visual angle (in degrees) of those two points using the following formula (it
may look daunting at first, but don’t worry, take it easy):
A = arctan (4 / X) * 2
Arctan (aka “atan” or “tan-1”) is the arctangent. Real easy to do on Excel or Numbers,
simply use the =ATAN( ) function.
Visual angle at which dot disappears:
Visual angle at which dot reappears:
____15.71_____
_____18.92_____
4. Take the average of these two values to get the centre point of your blind spot.
Centre point of blind spot:
______17.32________
Take the difference between these two values to get the width of your blind spot.
Width of your blind spot:
______3.21_________
5. How do your calculations compare with the average human values of 17 degrees (centre)
and 5 degrees (width)? If they differ, why do you think this might be?
The centre point of my blind spot is similar to the average human centre point value of
17 degrees. However, the width of my blind spot is less than the average width of human’s blind
spots. The difference is not too great so this might to be due to individual difference of blind spot
size.
Data Gathering, Quantitative Measurement
1. Find your blind spot as in the practice instructions above. Now, take a pencil and swipe it in
front of you, through the blind spot. Try this tip first, sideways, etc. What do you observe?
When the pencil was passed through my blind spot, I did not find any part of the
pencil to disappear despite several different approaches to passing it by my blind spot.
2. Next, find your blind spot using the second set of points on the stimulus sheet (in the middle
of the page). The right one has a vertical line through it. What happens to this vertical line
when the dot in the middle of it is in your blind spot?
When the dot was in the middle of my blind spot, I found that the vertical line
remained constant. There appeared to a break in the line where the dot was supposed
to be.
3. Finally find your blind spot using the third set of points (at the bottom of the page). The right
one is surrounded by a grid. What happens to this grid when the dot in the middle of it is in
your blind spot?
When the dot was in the middle of my blind spot, I found that the surrounding grid
remained constant. There appeared to be only one missing connection in the grid and
that was where the dot once was.
4. What do these observations mean in terms of what happens to visual stimuli in the blind
spot?
In terms of what I observed while placing the dot in my blind spot under a few
varying conditions, I found that the nature of the blind spot is very precise. Only the dot
disappeared under the varying conditions. This speaks to the specificity of the surface
area of the blind spot and the close proximity of the photoreceptors surrounding it.