Download 2320Lecture8

Seeing
• READING ASSIGNMENT
• Discussion of Gregory’s Article on
Visual Illusions – Tues Feb 17
• Available in your course pack
The Visual System
• What kind of energy does the visual
system sense and perceive?
Light
• Light is an oscillation (a wave) in the
electromagnetic field
Properties of light
• What are some characteristics of light
waves?
Properties of light
• What are some characteristics of light
waves?
– amplitude/intensity - how big of a
fluctuation in the field/how many waves
– frequency - how many fluctuations
(waves) pass by a certain point in a
given period of time
– polarization - the orientation of the waves
Properties of light
•What perceptions are associated with
those physical properties?
Properties of light
•What perceptions are associated with
those physical properties?
–Intensity is associated (loosely) with
brightness
–Frequency is associated (loosely) with color
–Polarization isn’t sensed by humans (but it is
by some insects and birds!)
Light
•
The spectrum is the
range of
frequencies of
electromagnetic
waves
Using Light
• The eye
– curved
cornea
– lens
– retina
– fovea
– optic disk
Using Light
• Light is focused on the retina by the curvature
of the cornea and the lens
– Flat lens (relaxed muscles) focuses distant points
Using Light
• Light is focused on the retina by the curvature
of the cornea and the lens
– Accomodation: Bulged lens (muscles flexed)
focuses on near points
Using Light
• Light is focused on the retina by the curvature
of the cornea and the lens
• Constriction of the pupil limits how much light
gets in AND reduces the amount of focusing
required of the lens
The Retina has layers of cells
•
•
photoreceptors transduce incoming light
ganglion cells send signals along to the brain
Two kinds of Photoreceptors
• 2 types of photoreceptors: rods and
cones
• rods are very sensitive - useful in dim
light
•
The Retina
•
Rods and cones are distributed differently across
the retina
The Retina
• visual acuity (ability to
see detail) depends
on cones - thus acuity
varies across the
visual field
The Retina
• Why don’t you notice your blind spot?
The Retina
• Why don’t you notice your blind spot?
– Blindspots don’t overlap!
– Your brain “fills in” the missing information
– The specific information in the blindspot isn’t
much more missing than the rest of the
periphery!
The Retina
•
•
three types of cones: short, medium, and long
different absorptions enable color vision
Neurons “collect” information
• each ganglion cell integrates
information from a particular spot on
the retina called its receptive field
Receptive Fields
Action potentials
Stimulus is in
receptive field
Stimulus is
near receptive
field
Stimulus is
outside receptive
field
Visual Pathways
• Ganglion cells
project to the
brain via the
optic nerve
• information is
projected to
contralateral
cortex!