CAS PS 222 Lecture Notes - Lecture 6: Receptive Field, Retinal Pigment Epithelium, Lightdark
18 May 2018
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Once the light is focused on the retina, the photoreceptors in different locations can detect the
light ad sigal it. But that is’t eough eause e eed ore iforatio tha just the
intensity.
How do we know the orientation of a bigger picture? In order to get it, we need to not only look
at individual pixels but also organize this information from the photoreceptors into behaviorally
relevant units. The first thing we need to do is figure out where the boundaries of the objects
are – difference in the light/dark reflective properties tells us this.
That’s here our first stop of the orgaizatio step oes i.
The image of RGC:
Light comes in through the cornea and focuses on the retina. Light has to go through RGCs etc
in order to get to the light sensitive area of the retina. Photoreceptors communicate the
information of light to RGC. There is not a necessarily one-to-one correspondence, it can be
getting info from multiple photoreceptors. The PTR when stimulated make up that RCGs
receptive field.
What kind of cells make up the receptive field of RCG: Photoreceptors.
RCG are wired up to get signals from multiple PR. Stimulation of the PR signaling the presence
of light to RCG do not necessarily ause the sae kid of atiity i RCG. They do’t uiforly
affect it. In fact the RCG is organized in a particularly useful way. We figure this out by doing the
simple experiment: take an animal typically a cat, stick an electrode into the RCG, and simulate
the sensory cell (the retina) in the visual field and measure the response of the cell.
We do’t ko hat photoreeptors is giig the ifo. We easure the ifo gie here the
light is being reflected. i.e. Location 1 and 2. Location 1 had an increase in activity whereas
Location 2 had a decrease. We did this with multiple spots. What we have is an organization of
the RGC receptive field. Its organized in a center surround manner.
You start to build up the representation of the boundaries of the world through this.
RGC preferably like light/dark boundaries.
When light falls everywhere equally: Inhibitory is cancelled out with exhibitory action.
What happens when a horizontal bar reflects in the eye: activity would go up but not as much.
A lot of activity exhibitory from the center, a little bit of inhibitory signal from the surround but
not enough to cancel out the action from inside. That horizontal or vertical bar, anything beside
a perfect disk is not the cells preferred stimulation.
What would happen if I had a disk of light that was slightly larger than the center region than
the Receptive field of the RGC? – There would be an increase in the activity but not as much as
it would be if it were just shining in the center. A little bit of inhibition.
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Does RGC bother about the orientation of the stimulus?- does it respond differently if there is a
horizontal or vertical bar? – Why not? Regardless of the orientation, the proportion of the
surround v/s the center is the same.
Here with this RGC receptive field organization, we cant really tell the kind of edge we have.
Which we will do next week.
What is the information that we are working with? What is going on in the sensory surface and
how is the visual system wired up to make use of the info?
After the RGC, we have some intermediary cells. After which is the photoreceptors with their
photosensitive parts embedded in the pigment epithelium.
Photoreceptors require the intermediary guides to help them with the firing rate.
How is a RGC wired up to have a center surround organization? –
What happes to the. Activity of the RGC if light falls in the cones right in the center?- it
increases/
The green arrows are excitatory signals. And the red ones are inhibitory signals.
Do all the cones have a direct line to the RGC? – no
Which cones have a direct line? – the one in the center. Center sends excitatory signals that
excite Bipolar cells when stimulated by light. Which then sends excitatory signals directly to the
RGC.
The Horizontal cells get some excitotry signals and gives some inhibitory signals.
When light falls on the surround-
The oes i the surroud do’t hae a diret ifluee o RGC. SO ho ill they ifluee the
RGC?
The horizontal cells only send inhibitory signals to the cones.
What about an off center on surround? –
If a group of bipolar cells are sending similar signals then it is likely to be very important. That
signal is amplified by amacrine cells.
RGC receptive field are organized to detect light/dark boundaries. They are organized this way
by differential responses. The photoreceptors in the surrounding region drive the activity of
RGC receptive field are organized in such a way that they can organize the light into light/dark
boundaries. It does’t like uifority. It likes he there is a oudary.
How are photoreceptors wired up to the RGC receptive field?
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Document Summary
Once the light is focused on the retina, the photoreceptors in different locations can detect the light a(cid:374)d sig(cid:374)al it. But that is(cid:374)"t e(cid:374)ough (cid:271)e(cid:272)ause (cid:449)e (cid:374)eed (cid:373)ore i(cid:374)for(cid:373)atio(cid:374) tha(cid:374) just the intensity. In order to get it, we need to not only look at individual pixels but also organize this information from the photoreceptors into behaviorally relevant units. The first thing we need to do is figure out where the boundaries of the objects are difference in the light/dark reflective properties tells us this. That"s (cid:449)here our first stop of the orga(cid:374)izatio(cid:374) step (cid:272)o(cid:373)es i(cid:374). Light comes in through the cornea and focuses on the retina. Light has to go through rgcs etc in order to get to the light sensitive area of the retina. Photoreceptors communicate the information of light to rgc. There is not a necessarily one-to-one correspondence, it can be getting info from multiple photoreceptors. The ptr when stimulated make up that rcgs receptive field.
