Cognitive Neuroscience Lecture 5: Visual Systems/Pathways

*L5: Visual systems/pathways*

Visual
system

Optics/eyeballs

Retina

Pathways

Cortex

Overview of
the visual pathway

There
is energy in the world, in the form of light

o
Light
is an electromagnetic wave

o
Light
travels in a straight line

o
We
see objects because they reflect light

§  But light reflects in all directions
from each point of objects

§  And it arrives from all points

§  So the pattern of light and dark is
destroyed

o
Solution:
pinhole camera: select one direction per point

§  The problem with this is that when
the pinhole is too small, not enough light

§  When the pinhole is too large, it is
too blurry

o
Speed
of light is slower in some materials than others

o
**Refraction:**

§  Because it travels at a diff speed in
diff materials, light bends when crossing the boundary between materials

§  The refraction is proportional to the
ratio of speeds

o
Prisms:
divergent > <, convergent <>

o
Lenses

§  Light from objects scatters in many
directions

§  Lenses gather light from a single point
in world and focuses it on a single point on the screen (at a certain distance
beyond the lens, say on a screen, or your retina)

o
Human
eye

§  Ciliary body: muscles that control
the shape of the lens

§  Cornea: most of the focusing power

§  Lens: mainly for fine tuning

§  Retina: no blood vessels on cornea or
lens, but plenty of blood vessels right on retina

·       We don’t see them because visual
system interested in change

·       If you were to shine a bright light,
you’d see the shadow of the retina

§  Fovea: blood vessels and nerves
pushed away, densely packed receptors, cones only

o
Accommodation
(focusing)

§  Emmetropia: normal

§  Myopia: nearsightedness

§  Hyperopia: farsightedness

Light
enters the eye, an image is formed on the retina

o
The
retina has receptors that react to light and convert into neural signal to use
in the brain

o
Photoreceptors:
rods and cones

o
Cones
= fovea

§  Daylight, packed in fovea, good for
high-res vision and color

§  Short wavelength, medium wavelength,
and long wavelength

§  S:M:L = 1:5:10

§  Dearth of blue receptors in fovea
(middle of image)

§
Retinal color blindness: lose L cones
(**protanopia**) or M cones (**deuteranopia**)

o
Rods
!= fovea

§  Night time, not in fovea, everywhere
but blind spot, low-res vision, no color, good for low-light conditions

o
NO
RECEPTORS AT ALL in blind spot

Retina
has receptors that react to light and transmit signals to the brain

o
Convergence
of cones and rods

§  In fovea: low convergence (one to one
mapping; high spatial res, high acuity)

§  In periphery: high convergence (many
to one mapping; low spatial res, low acuity)

·       1 RGC à many bipolar à many more rods

o
Optic
nerve

§  No photoreceptors: blind spot

§  All axons from RGC come together

§  Form optic nerve bundle and exit eye

Signals
get transmitted down the visual pathway from eye to cortex

o
Retina
à optic tract à thalamus LGN à V1/striate cortex

o
More
depth: retina àoptic nerve àoptic chiasmà optic tractàLGNàV1

o
Your
visual system is “cross-wired”

§  Signals from nasal half of each
retina cross at the optic chiasm

§  Left VF àright v1

§  Right VFàleft v1

§  NOT ‘left eye to right cortex, right
eye to left cortex’

§  “what happens if”….do it the way you
did, but then switch left to left after

o
Subcortical
waystations

§  LGN receives 90% of retinal input and
transmits info to cortex

§  Superior colliculus receives
remaining retinal input, important for eye movements

o
Parallel pathways

§  Two types of RGC: magnocellular
(rods), parvocellular (cones)

§  Temporal frequency higher for
magnocellular

The
brain tries to determine what is where in the world

o
**Receptive fields**: the area over which stimulation
affects the response of a cell

§  \mapping a receptive field: allowed
recording from otherwise normal animals

§  Use micro-electrode

§  Screen, amplifier, loudspeaker,
oscilloscope, micro-electrode, anesthetize animal

o
What
makes a V1 neuron fire? **Hubel &**
Wiesel

§  Take a cell in v1

§  Stimulate photoreceptors on retina

§  Determine which pattern of light the
neuron fires for

§  Talk about a needle in a haystack!

§  They found out it was straight
lines/bars of light

§  Simple v1 cells: bar of light

o
**Receptive fields in RGC**: center-surround (‘Mexican Hat’)

§  Not just single spot, small region
with concentric ON/OFF responses

§  On/off = whether middle region
“likes” light or dark

§  Excitation and inhibition cancel out
if diffuse light, and NEITHER turns on if 0 light

o
What does the retina care about?

§  Sensitive to changes in local
contrast

§  Only when it doesn’t cancel (light
only hits concentric circle or middle )

§  Retina signals edges, not uniform
luminance

§  Sensitive to changes in local
contrast : exaggerates it (actual luminance distribution is more sudden
than  perceived luminance distribution)

o
Not
only stronger responses

§  **ON**: neuron
fires more when light shown

§  **OFF**: neuron
stops firing when light shown

o
V1:
calcarine fissure

§  Adjacent regions of space are coded
for by adjacent neurons

§  Retinotopic mapping in humans with
fMRI  (polar and eccentricity)

o
Cortical
retinotopic map: cortical magnification (more for fovea/center)

§  Convergence of rods and cones: why is
there cortical magnification?

§  Density of RGC encoding visual info
decreases as you go from center to periphery

§  Ganglion cells representing central
vision provide finer grained info (high res) from central vision

o
Receptive
fields of simple v1 cells: edge detectors

§  Receptive fields become larger and
more complex in concentric cells

§  Orientation selectivity and tuning
(matters \ | - /)

§  Detect edges

§  Sensitive to specific orientation in
specific position

§  Orientation “mapped” along cortex:
preferred orientation changes as you move

o
Orientation
columns

§  V1 neurons tuned to same orientation
are stacked in vertical columns

§  Adjacent columns hav similar
orientations, resulting in “pinwheels”

o
Complex
v1 cells

§  Receptive fields larger and more
complex

§  Oriented bar in general location (not
as specific as in simple v1 cells)

o
Hierarchy
of processing up to v1

§  ‘building’ progressively complex representations
as you move up the visual hierarchy

§  Photoreceptors à LGN à V1

§  Size of receptive field increases as
you go from V1 to v4

o
‘Preferred’
stimuli

§  We say a neuron codes for whatever
makes it respond most

§  **Simple v1 cells:** Line orientation, in one spatial location      /

§  **RGC**: a small
point in space  .

§  **Complex v1 cells**: line orientation, in a general location /////////

Further processing in dorsal/ventral
pathways

o
Dorsal:
“where”

o
Ventral:
“what”

Pigmented
cell à Rods/cones à horizontal cell (integrates and
adjusts for light/dark) àbipolar cellà amacrine cell (intercept RGC/bipolar; directional
motion, light adaption) àRGC