PSYC2071
Week 8
Lecture notes A
Visual Processing
The organization of visual in brain
Parallel and hierarchical processing- in the parallel processing streams, there are sequential
processing steps following a modular hierarchy.
Image-based stage –Early processing: Each photoreceptor generates electrical current
proportional to the amount of light falling on it.
Surface Based Stage –Intermediate processing: our perceptual system is actually geared to pay
attention to surfaces. This is how we arrive to our judgment of the object.
Object/Category Based Stage –Late processing: Retina are a layer of matter in the eye that focus
light to the photoreceptors, fovea is the only part in the eye that allows lights to hit
photoreceptors directly
Organisation of the Retina
Optic disk is one of the most important parts of the retina, it doesn’t have any photoreceptors. It
also associates with the blind spots. There are processing in the brain that fills out missing
information created by blind spots. After the photoreceptors layer (Input Layer), we have bipolar
cells layer (middle Processing Layer), then retinal Ganglion cells layer (Output Layer), this is
known as the vertical pathways of retina.
Everything in visual are made up by Modules, in the input layer there are Rod and Cone modules.
In the middle processing layer it is also classified as On pathway and Off pathway (Bipolar cells).
Only On pathway will be responded if lights go brighter, and Off pathway only if lights go dimmer.
In the output layer, we have Magno Ganglion Cells module and Parvo Ganglion Cells module.
Retinal is a light sensitive molecule
A large protein Opsin is presented in both Cones (Active only during day) that produces
photopsin and Rodes (Active only during night) that produces scotopsin. It is extremely hard to
have both working in the same time (mesopic)
Retinal Cones are divided into Red Green and Blue
Visual Pigment Bleaching and Regeneration (When exposed to bright light, the ability for the eye
to absorb light decreases)
Cone Pigment regenerates in 6 minutes
Rod Pigment takes over 30 minutes to regenerate
Diseases that affect retina
Macular degeneration
Cholesterol builds up in the centre of your eye, fovea and small surrounding area are destroyed
and it creates blind spot
Retinitis pigmentosa
Rods are destroyed first, night blindness.
Next lecture we are going to talk about the selective types of degeneration.
Lecture Notes B
Middle Layer
Half of Bipolar cells are specialized in increasing illuminant and half are specialized in decreasing
illuminant.
Bipolar cells are classified into Cone bipolar cells and Rod bipolar cells.
Cone Pathway: Direct link to ganglion cells via midget and diffuse cone bipolar cells
Rod Pathway: Rod bipolars: signals originating in rod photoreceptors reach the ganglion cells via
an indirect route via the axon terminals of cone bipolar cells
Peripheral Retinal Pathway will have convergence of 15-45 peripheral receptors to a single bipolar
cell.
Central Retinal Pathway will divergence, from each foveal cone to two bipolar cells.
The greater convergence, the greater the overall sensitivity to light, such as the case in periphery.
There is a trade off however, the resolution of telling which direction the information is coming
from is weaker compare to divergence foveal cone, which is less sensitive.
So far we been talking about vertical connections, there are also horizontal connections, they are
providing inhibition of lateral. receptive field is very important to perceive the information as a
whole, this is done through horizontal cells, which project information to each individual
receptive field.
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Center-Surround Receptive Fields of Ganglion Cells has Two types:
– Excitatory-center-inhibitory surround: ON center-OFF surround
– Inhibitory-center-excitatory surround: OFF center- ON surround
they are an extension of on channel and off channel.
For On center Ganglion cells, it will be extremely stimulated if light falls on it’s centre only, and no
stimulation if the light falls on it’s surrounding only. And a moderate stimulation if the light covers
the entire cell.
For Off center Ganglion cells, the opposite happens, no stimulation if light hits centre only and
extreme stimulation if light hits surroundings, and moderate stimulation if light covers the whole
cell.
This property of Ganglion cells ensured the spatial ability of human eyes.
Lateral inhibition (above effect) provided excellent explanation to a lot of properties of human
eyes such as brightness perception, the theory can be used to explain the phenomenon of
Simultaneous Contrast where the two center squares reflect the same amount of light into your
eyes but look different because of simultaneous contrast. It is seeing illusory brightness
differences due to differences in the intensity of adjacent surrounding areas. Explained by the
white surround generates more lateral inhibition, compared to the black surround
The Hermann Grid where you see dark spots at an intersection of a black grid or white spot at the
intersection of a white grid. Caused by superimposed surrounding of the cell to the dark boxes
but not the cetre of the cell, thus causing the black spot.
As well as Mach Bands, where we see the edge more sharply, such as on a color progressing
picture we actually see the border of the color being darker than they really are. The edge of
color means that some the surrounding field are not in the color when it’s outside the edge, that
means the centre is more inhibited, makeing it more darker.
Wertheimer-Benary Cross shows that we cannot just look at the theory, we need to take note
that the brain functions affects our perception, the spatial belongingness that the brain
recognizes the color to be in actually affect your color perception.