Vision
The Visible Spectrum
Light is electromagnetic energy.
One nm = one billionth of a meter
Properties of light
• hue – determined by wavelength.
• saturation – relative purity of light.
• brightness – variation in intensity.
Wavelength and Frequency
As wavelength increase, frequency decreases
The Human Eye
The Human Eye
In order to see things in greatest detail our eyes are moved so that the
object being looked at falls on the fovea.
Fovea is a central portion of the retina with the greatest visual acuity.
Photoreceptors
• rods and cones contain photopigment that provides input to bipolar and
horizontal cells.
• photoreceptors and bipolar cells do not produce action potentials –
instead release neurotransmitters to the ganglion cells.
• ganglion cells connect with the optic nerve.
Blind spot
Optic disk – where the optic nerve joins the retina – transmits retinal
information to the occipital lobes
Blind spot
Close your LEFT eye and move head closer to or further away from the screen until
the central red circle disappears – always fixate the CROSS.
Visual Fields
VISUAL FIELD
temporal
nasal
temporal
nasal
RETINA
Primary geniculostriate visual pathway
Primary Geniculastriate Pathway
retina
optic nerve
optic chiasm
optic tract
dorsal lateral geniculate nucleus
optic radiations
striate visual cortex
Lateral Geniculate Nucleus
The LGNd has six layers each of which
gets independent input from either the
left or the right eye but not both.
There are two major classes of
projections, parvocellular (small) and
magnocellular
(large)
projections
(known as the P and M pathways).
Magnocellular
Parvocellular
Large ganglion cells
Small ganglion cells
Centre/Surround
Centre/Surround
Colour insensitive
Colour sensitive
Large RFs
Small RFs
Fast, transient
Slow, sustained
High contrast sensitivity
Low contrast sensitivity
Primary Visual Cortex
The LGNd projects to primary visual cortex (striate cortex or area V1) in
the occipital lobe.
The magno and parvo projections are still somewhat segregated in V1.
Retinotopic map in striate visual cortex
Visual receptive fields
• receptive fields of retinal
ganglion cells correspond to
specific regions in space –
hence a retinotopic map of the
world in the occipital cortex.
• receptive fields in visual cortex
also respond selectively to other
stimulus
properties
(e.g.,
orientation, brightness).
Centre – surround organization
• tuning – different types of cells
are “tuned” to respond to different
aspects of visual information
e.g., brightness, location, direction
of motion, colour etc…
Coding information at the retina - brightness
• centre / surround organization
• ON, OFF and OFF/ON cells
• ON/OFF cells project primarily to the superior colliculus (midbrain)
• the SC is important for directing reflexive saccades
Coding information at the retina - colour
Coding information at the retina - colour
• trichomatic sensitivity AND colour opponency
• red – green
• blue – yellow
• on/off surround organization
Yellow ON
Blue OFF
Blue ON
Yellow OFF
Green ON
Red OFF
Red ON
Green OFF
Coding information at the retina - colour
• impossibility of seeing a redish green colour!
Adaptation – negative afterimages
• after staring at the green Canadian flag you see a red one because the
“green” component of red/green cells has adapted to the stimulus.
• some red/green cells are inhibited for a long period.
• when looking at neutral light (white light) these cells “rebound” due to
the absence of inhibition creating the afterimage.
• Big Spanish Castle
• can get afterimages for motion – waterfall illusion .
Striate cortex
• 6 layers (bands or striations).
• input from magno and parvocellular information processed at layer IV.
• disproportionate representation of the fovea (brain would weigh over
30,000 pounds (≈13,600 kg) if the whole visual field had as many
neurons dedicated to it as are dedicated to the fovea!!!).
Orientation and movement
• cells in striate cortex sensitive to
specific orientations.
• simple cells – opponent system.
• complex cells – no inhibitory
surround – direction specific
movement detectors (also in MT).
• cells organized in columns.
Spatial frequency
• many of the cells in striate
cortex are actually tuned to
different spatial frequencies.
low spatial
frequency
• everything you see in the
world can be described in
terms of spatial frequency.
high spatial
frequency
Information not lost at low spatial
frequencies
Gender and can still be extracted from the low frequency image (right)
but identity requires the high frequency image (left).
Modularity in vision
• Different “modules” sensitive
to different visual processes
• V4 – colour
• MT – motion
• FFA – face perception
• PPA – place recognition
• IT – object recognition
Review Questions
1) A unique feature of the fovea is that it
A) contains mostly rods.
B) contains mostly cone photoreceptors.
C) is devoid of photoreceptors.
D) mediates vision in dim light.
E) has very poor acuity.
2) The reason for a "blind spot" in the visual field is that
A) rods are less sensitive to light than are cones.
B) blood vessels collect together and enter the eye at the blind spot.
C) the lens cannot focus all of the visual field onto the retina.
D) retinal cells die with age and overuse, resulting in blind spots.
E) there are no photoreceptors in the retina where the axons exit the eye.
3) Action potentials in the visual system are first observed in the
A) bipolar cells.
B) horizontal cells.
C) ganglion cells.
D) photoreceptors.
E) axons leaving the internal surface of the retina.
4) Select the correct sequence for processing of information in the primary
visual pathway.
A) Retina - > dorsal lateral geniculate (DLG) -> striate cortex
B) Retina -> striate cortex -> extrastriate cortex -> inferior temporal cortex
C) DLG -> retina -> striate cortex -> primary visual cortex
D) Retina -> DLG -> inferior temporal cortex -> amygdala
E) DLG-> frontal cortex -> amygdala -> extrastriate cortex
Recommended web page
http://www.tutis.ca/Senses/index.htm