PSYC 212 Lecture 6: Lecture 6
6 Nov 2018
School
Department
Course
Professor
Visual acuity
Vision scientists: A smallest visual angle of a cycle of
grating
§
The smaller the visual angle at which you can identify a
cycle of a grating, the better your vision
§
The limits I determined primarily by the spacing of
photoreceptors on the retina
§
Cones in the fovea have center-to-center separation of
about 0.5 minute of arc (one minute of arc = 1
degree/60), which perfectly fits with the minimum
resolution of 1 minute of arc
§
Spatial frequency: cycles of a grating per unit of visual angle
(in degrees)
Another way to think of spatial frequency is as the
number of times a pattern repeats per unit
§
Gabor patches: (a) has a low spatial frequency, (b) has a
medium spatial frequency, and (c) has a high spatial
frequency
§
Contrast: intensity difference between the lightest and
darkest potions of the patch
§
Contrast sensitivity function: visibility of a pattern as a
function of spatial frequency and contract
§
○
Retinal ganglion cells and stripes
Ganglion cells with center-sound receptive fields
respond preferentially to certain frequencies
§
The phase is also important
Phase: the phase of a grating refers to its
□
§
○
Why sine gratings?
Any black-and-white images can be described in terms
of a weighted combination of different
Frequencies
□
Contrast
□
Phases□
Orientation□
§
Low frequencies = broad outline
§
High frequencies = details
§
Retinal ganglion cells are sensitive to everything, except
orientation
§
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•
Primary visual cortex
From eye to brain
Contralateral representation of visual space: information from
the nasal part of the retina crosses to the other side of the
brain, such that the left visual field is seen by the right
hemisphere
○
First relay is the lateral geniculate nucleus of the thalamus
The thalamus is the "way station"
§
The thalamus is the "way station" for most senses
§
○
Types of layers in the LGN
Magnocellular (1 and 2): large cells, bottom two layers
Receive input from M ganglion cells□
Responds best to large, fast-moving objects□
§
Parvocellular (3-6): smaller cells, top four layers
Receive input from P ganglion cells□
Responds best to fin spatial details of stationary
objects
□
§
Koniocellular
Was very cells in between the magnocellular and
parvocellular section
□
We are still not entirely sure what these do□
§
○
Properties of the LGN
Contralateral representation: left LGN receives info from
the right visual field and vice-versa
§
Each LGN layer receives signal from one eyes only
§
Within each layer of the LGN, the neurons are arranged
in a retinotopic map of the visual field
§
In other words, RGCs with adjacent neurons in the LGN
§
○
•
Primary visual cortex
Two important features of striate cortex
Retinotopic (topographical) mapping1.
Cortical magnification
Dramatic scaling of information from different
parts of visual field
□
Proportionally much more context devoted to
processing the fovea than to processing the
periphery
□
2.
○
Visual crowding: he deleterious effect of clutter on peripheral
object detection
Stimuli that can be seen in isolation in peripheral vision
become hard to discern when other stimuli are nearby
§
This is a major bottleneck for visual processing
When we can't see an object due to crowding, we
have to move our eyes to look directly at it with
out high acuity foveal receptive fields
□
§
○
Response properties of V1 neurons
By combining information from several retinal ganglion
cells, it is possible to detect the orientation of lines
§
Simple cells preferentially respond to lines that have a
certain orientation
§
Complex cells respond to both dark bar on white
background AND white bar on black background (simple
cells only respond to one of the two possibilities)
§
Complex cells also respond to a bar flashed anywhere
on its receptive field
§
○
Tilt after-effect
We have different populations of neurons that
specialize in certain frequencies and orientations
§
Adaptation: looking at a pattern of stripes for a certain
time will "tire" the neurons and shift the balance in the
opposite direction
Note: this also implies that orientation is encoded
by a population of neurons
□
§
Where does this adaptation occur?
Retina - LGN -V1?□
§
○
V1 organization
Cortex arranged into several layers
§
Ocular dominance columns: columns of neurons
receiving signal mainly from one eye
§
Input coming from the magnocellular layers of the LGN
arrive in layer 4Ca
§
Input coming from parvocellular layers of the LGN arrive
in layer 4Cb
§
Input from the koniocellular layers arrive in layers 2/3
§
Parvocellular (ventral/what) and magnocellular
(dorsal/where) pathways
§
Orientation columns: cortical columns consisting of
neurons with the same (or very similar) orientation
tuning
§
'pinwheel' structure
§
Hypercolumn: a 1-mm block of striate cortex containing
"all the machinery necessary to look after everything
the visual cortex is responsible for, in a certain small
part of the visual world"
Each hypercolumn contains cells responding to
every possible orientation (0-180 degrees), with
one set preferring input from the left eye and one
set preferring input from the right eye
□
§
○
•
Lecture 6
Thursday, January 25, 2018
12:56 PM
Document Summary
Vision scientists: a smallest visual angle of a cycle of grating. The smaller the visual angle at which you can identify a cycle of a grating, the better your vision. The limits i determined primarily by the spacing of photoreceptors on the retina. Cones in the fovea have center-to-center separation of about 0. 5 minute of arc (one minute of arc = 1 degree/60), which perfectly fits with the minimum resolution of 1 minute of arc. Spatial frequency: cycles of a grating per unit of visual angle (in degrees) Another way to think of spatial frequency is as the number of times a pattern repeats per unit. Gabor patches: (a) has a low spatial frequency, (b) has a medium spatial frequency, and (c) has a high spatial frequency. Contrast: intensity difference between the lightest and darkest potions of the patch. Contrast sensitivity function: visibility of a pattern as a function of spatial frequency and contract.
