BHS14.1-01 Vision Science I
Note taker: Iwona Miltko
Date: 01/31/2013 2nd hour
Page 1
Begin with page 41.
I.
Spatial Summation
a. Rods summate signals over space and therefore rods can be more sensitive than cones.
b. Spatial summation occurs when simultaneous stimuli at different locations have a
cumulative effect on the transmembrane potential.
c. Example: For neuron to generate an action potential, it must be depolarized to a
threshold amount. Multiple adjacent neurons act simultaneously to reduce this
membrane potential more quickly. If the stimuli are more localized, their spatial
summation is much greater and more efficient
d. Video about neuron spatial summation of action potentials.
e. Many rods signal (are connected synaptically or converge) onto a single rod bipolar cell.
In the near peripheral retina, about 50 rods signal onto a rod bipolar cell. In the far
periphery, up to 300 rods signal onto a single rod bipolar cell. Signals from many rods are
summated by the rod bipolar cells.
f. In this picture, the larger circles represent rod bipolar
cells. These larger circles are the areas over which the
rod excitations are collected or summed. The small
circles are rods which converge onto the bipolar cells.
Some bipolar cells converge onto multiple rod bipolar
cells. In this picture, the rods are “end on”.
i. Two test flashes are presented to a human
subject.
ii. TF1 delivers 10 quanta to one summation area.
iii. TF2 also delivers 10 quanta but these quanta
are more spread out.
iv. Since 10 quanta of each test flash fall onto one
summation area, each test flash will produce a
signal of strength “10”.
g. As long as the threshold quanta are summed over the
summation area and treated as one signal, the spatial
distribution of the quanta does not matter.
h. The rods and cones form summation areas for
bipolar cells.
i. The bipolar cells form summation areas for
retinal ganglion cells.
BHS14.1-01 Vision Science I
Note taker: Iwona Miltko
II.
Date: 01/31/2013 2nd hour
Page 2
Worksheet #8: Spatial Summation and Visual Sensitivity
1. In the diagram at the right from
Cornsweet, suppose that in order for
a rod BP cell to generate a threshold
response 10 photons need to be
absorbed. Assume that all photons
delivered to the retina will be
absorbed. Will the summation area
shown give the same response to the
two different distributions of photons
(red and black)?
a. The black photons are
distributed in such a way that
10 quanta fall within one
summation area (Rod bipolar
cell) so this will produce a
strong response.
b. The red photons do not all fall within one summation area and so will not
produce a strong response.
2. In order for the summation area to reach threshold with the red photons, will the
test flash need more than ten quanta?
a. Yes
3. As long as the test flash results in 10 photon absorptions within one summation
area, threshold will be reached. True or False
a. True
4. Assume now that 10 photon absorptions occur in this pattern.
How many test flashes will be seen?
a. 1
5. Assume the test flash causes 10 absorptions as in this
diagram. Will the test flash be seen?
a. No.
6. Suppose that 10 quanta need to be absorbed within an area equal to 10’ in
diameter. If the TF delivered 10 quanta over an area equal to 25’ in diameter, would
the subject be able to detect the test flash?
a. No
7. In order for the subject to perceive the test flash when its diameter is equal to 25’ in
diameter, the number of quanta contained in the test flash must be ______.
a. Increased
Date: 01/31/2013 2nd hour
Page 3
BHS14.1-01 Vision Science I
Note taker: Iwona Miltko
Suppose that in order for a retinal ganglion cell to signal an event a total of
10 quanta must be absorbed by the photoreceptors that feed into it and
assume that each photoreceptor absorbs only a single quantum.
As shown below for the scotopic system, the 10
quanta are delivered in 2 separate flashed by distance x
and that each TF contains 5 quanta.
8. Will the RGC reach threshold?
a. yes
9. How many TFs will be perceived?
a. 1
Again, 10 quanta are delivered to a patch of retina containing 3 RBC Receptive Fields.
The 10 quanta are delivered in two TFs each containing 5 quanta separated by distance “x”.
10. Will any of the RGCs illustrated reach
threshold?
a. No
11. What if each TF in the photopic example
illustrated above is changed so that they
now deliver 10 quanta per flash. How many
photopic RGCs will reach threshold?
a. 2
12. How many TFs will be seen in the photopic
condition?
a. 2 total (1 in RGC 1 and 1 in RGC 23