Transcript The Visual System

The Visual System
Into. to Neurobiology
2010
Types of Retinal Ganglion Cells
M (magnocellular) ganglion cells
• Input primarily from rods.
• Achromatic (black and white center-surround receptive fields).
• Constitute about 10 % of the ganglion cell population.
• Larger receptive fields (low spatial frequencies).
• Sensitive to the directions of visual motion. High temporal frequencies.
• Sensitive to low contrasts (saturate when the the contrast is high).
• Most have linear response properties.
Types of Retinal Ganglion Cells
P (parvocellular) ganglion cells
• Input primarily from cones.
• Chromatic.
• Constitute about 70 % the ganglion cell
population.
• Small receptive field centers = high spatial
frequencies (resolution).
• More sensitive to the form and fine details
of the visual stimuli.
• Respond poorly to low contrast but do not
saturate at high contrasts.
• Low temporal frequencies.
• Linear response properties.
Types of Cat Retinal Ganglion Cells
X Cells
• Small receptive fields
X cell
• Linear response properties (positive transient
response to light, adaptation at steady state, drop
in firing rate in response to dark).
• Good at detecting contrasts.
Y cells
Y cell
• Large receptive fields (X cell * 3)
• Non-linear response properties (positive
transient response to light, zero steady state
response, no response to dark).
• On / off cells (respond only to light or only to
dark).
• Good at detecting motion.
Stimulus
Receptive
Field
The Lateral Geniculate Nucleus (LGN)
• Located in the Thalamus. Receives
convergent input from several optic nerve
fibers and feedback from V1.
• Left-right, top-bottom organization from
Ganglion cells is maintained.
• 6 layers: layers 1, 4, 6 = information from the
contralateral eye ; layers 2, 3, 5 = ipsilateral
eye.
• Magnocellular/Parvocellular distinction.
• Topographically organized projection to V1.
• Circular, center-surround receptive fields
similar to those of Ganglion cells, but more
finely tuned.
Primary Visual Cortex - V1
• The primary visual cortex (= the striate cortex
= Brodmann area 17 = V1) is divided into 6
layers, differing in cell type and connections
with other brain areas.
• LGN inputs primarily received at layer 4
- parvocellular to a lower subdivision (layer
4cβ) and magnocellular to an upper
subdivision (layer 4cα).
• Function: highly specialized for processing
information about static and moving objects and
excellent in pattern recognition (classification).
dorsal
• V1 outputs to 2 pathways: the dorsal stream
and the ventral stream.
ventral
V1 Organization
• Retinotopic map: a very well-defined map
of the spatial information. The fovea is
represented over a large portion = cortical
magnification.
• Smallest receptive field size of any visual
cortex region.
• Occular dominance columns (L4).
• Most cells are binocular.
• Orientation (most cells) and direction (2535% of cells) selectivity.
• V1 receives feedback connections from
higher regions, creating complex responses
(for example, to context).
V1 Organization : Ocular dominance columns
• Input from ipsilateral and
contralateral eyes is separated in
adjacent columns in layer 4
• Adjacent columns make up one
hypercolumn
• Within each ipsilateral and
contralateral ocular dominance
column all orientation columns
are represented
• Output from monocular cells in
layer 4C converges on binocular
cells in other layers
V1 Organization
• Another kind of columns was revealed using a
stain called cytochrome oxydase.
• They are called “blobs”, are spaced at regular
intervals and run through layers II, III, V, and VI.
• These blobs are arranged in lines, centered on
an ocular dominance band in layer 4C. Between
the blobs are areas called interblobs whose
neurons do not have the characteristics of these
blobs.
• The blob cells are sensitive to the wave length
of light (color) , they are monocular, and they do
not have any orientation selectivity; instead, they
have circularly symmetrical receptive fields.
• Some blob cells have the same centersurround color opposition structure as the P
ganglion cells, where this pathway originates.
Functional properties of V1 cells
Simple Cells:
Receive direct input from the LGN. Respond to points or bars
of light in a particular orientation and location.
Cortical Receptive Fields
Simple Cells: “Line Detectors”
B. Dark Line Detector
Firing
Rate
Horizontal Position
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Simple Cells: “Edge Detectors”
C. Dark-to-light Edge Detector
Firing
Rate
D. Light-to-dark Edge Detector
Firing
Rate
Horizontal Position
Horizontal Position
© Stephen E. Palmer, 2002
Constructing a Simple Cell
Functional properties of V1 cells
Complex cells:
Respond to bars of light in a particular orientation (but in any
location), moving in a specific direction.
Constructing a Complex Cell
Detection of motion:
A. When the axons of many simple cells with the same orientation and adjacent but not
identical receptive fields converge on a complex cell, it can detect movement from the
differences between these fields.
B. Temporal summation: if a cell that has already been excited once is excited again
shortly afterward, its membrane is still depolarized enough that a stimulus that would not
normally suffice to trigger another action potential can do so. Thus, when a moving light
beam activates several simple cells in succession, the temporal summation of the stimuli
applied to them causes the complex cell to respond to the movement
The Visual Cortex
V1 – primary visual cortex
V2 – prestriate cortex. First association area. Receptive
fields: orientation, spatial frequency, color. Has dorsal and
ventral regions
V3 – cells similar to V2, but some are sensitive to color and
movement. Has dorsal and ventral regions.
V4 – located in extrastriate cortex. First ventral region
showing strong attentional modulation. Tuned for object
features of intermediate complexity, like simple geometric
shapes. Receives info. From blobs and interblobs.
V5 / MT (middle temporal) – located in extrastriate cortex.
Perception of motion.
LO – Lateral Occipital complex
FFA - Fusiform Face Area
PPA – Parahippocampal Place Area
STS – Superior Temporal Sulcus
(Pl = places, O = objects,
F = faces, P-U = ventral,
P-D = dorsal )
Dorsal and Ventral Streams
• The dorsal, or M-pathway (WHERE) ends
up in the parietal cortex.
• The ventral, or P-pathway (WHAT) ends
up in the temporal cortex.
• The two pathways are not totally
independent – there are many crossconnections at every level, and information
also flows in the reverse direction.
Dorsal and Ventral Streams
Ventral visual pathway:
Concious perception, recognition and
identification of objects by processing “intrinsic”
propertis (shape, color, etc.)
Dorsal visual pathway:
Allows visual-motor control over objects by
processing their “extrinsic” properties necessary
to handle them (size, location, position and
orientation in space)
V1 and Beyond: Depth Perception
Depth is analyzed through a combination of monocular and binocular cues.
1. Monocular Cues:
• Perspective - The property of parallel lines
converging at infinity allows us to reconstruct the
relative distance of two parts of an object, or of
landscape features.
• Relative retinal size – more distant = smaller.
• Familiar size - use of previous knowledge.
• Loss of detail in distance – more distant = less
luminance contrast + accomodation (hard to focus).
• Occlusion - "ranking" of relative nearness.
• Relative apparent movement as you move
your head – more distant = more slowly moving.
V1 and Beyond: Depth Perception
2. Binocular Cues: Stereopsis
• When an observer fixates on a visual object the
image of this object is positioned on corresponding
regions of the two retinae.
• Human eyes are horizontally separated by about 5075 mm (between pupils). Thus, each eye has a
slightly different view of the world. Objects more
near or far than the fixation point fall on noncorresponding areas on both retinae.
• The degree to which the images are noncorresponding (as measured by difference scores in
retinal eccentricity for instance) is defined as
binocular disparity.
• The ability to use binocular disparity to determine
the distance of an object from oneself, and its
relation to the fixation plane, is called stereopsis, or
depth perception.
Optical Illusions
• Optical illusions give us a better understanding of how human visual perception
works.
• They demonstrate that what we see of the world is not a simple physical record,
like a photograph.
• Some of these mechanisms arise in the retina, but most of them result from the
way that the images captured with the eyes are reconstructed by the visual
cortex.
• Geometric Illusions:
Produced by the arrangement of points, lines,
and simple shapes in ways that make you
misinterpret these elements when you see
them. Many geometric illusions involve two
objects that are actually identical but look
different because of their surroundings.
Zöllner's illusion
Optical Illusions
• Size-relationship Illusions:
The proximity of a test element to larger /
smaller inducing elements causes the size of
the test element to be underestimated /
overestimated.
The result is that though two test elements are
identical, they can look different to us,
because of the context effect.
The presence of lines suggesting perspective
can also create size illusions. Given two
objects of equal size, if one of them looks
farther away because of perspective, we will
perceive it as being larger.
Optical Illusions
• Motion Illusions:
Some images can give the illusion that their
elements are moving when you move yourself
slightly relative to them.
For other motion illusions, the particular
arrangement of the graphic elements in the
picture is enough to create the appearance of
movement as you look at it, because the pattern
makes it hard for your eye to determine the
contours of the circle in the center.
Optical Illusions
• Artistic Illusions:
It is not that the human visual system interprets
reality incorrectly, but rather that the reality itself is
deliberately ambiguous. Using various tricks of
drawing, the artist creates an object that looks
realistic but could never actually be built in the real
world.
Optical Illusions
• Artistic Illusions:
Another type of artistic illusions uses ambiguous cues – the drawing can be
interpreted in more than one way.