PS263 – Chapter 6: The Visual System
Visual Illusions: The eye is tricked into seeing things the way they aren’t.
Fortification Illusion: Begins with gray area of blindness near center of visual field –
they gray area expands into a horseshoe with a zigzag pattern of flickering lines at
its advancing edges – Mrs. Richards
Light Enters the Eye and Reaches the Retina
Light can be thought of as photons travelling through space at 300,000km per sec
(186,000 miles) or as a wave of energy
Electromagnetic Energy: This is what light is sometimes defined as between 380 and
760 nanometers in length; humans and animals see different wavelengths;
rattlesnake = infrared waves; too long for humans.
Wavelength: Plays an important role in the perception of colour, while Intensity:
plays an important role in the perception of brightness.
o Refer to intense light with a WV of 700 nm as being a bright red light.
Pupil & Lens: The amount of light reaching the retina is regulated by the donut
shaped bands of contractile tissue; irises – gives our eye its colour. Light enters
through the hole in the iris; pupil – the adjustment of the size in response to changes
in light represents:
o Sensitivity: Ability to detect the presence of dimly lit objects
o Acuity: Ability to see fine details of objects.
o When the illumination is high the visual system constricts the pupils – the
image falls on each retina and there is a greater depth of focus (range of
depths kept simultaneously on the retinas) when illumination is low, the pupils
dilate to let in more light, sacrificing acuity and D.O.F.
When the illumination is high the visual system constricts the pupils – the image falls
on each retina and there is a greater depth of focus (range of depths kept
simultaneously on the retinas) When illumination is low, the pupils dilate to let in
more light, sacrificing acuity and depth of focus
Behind each pupil is the lens, focusing the incoming light on the retina - tension on
ligaments holding the lens in place is adjusted by ciliary muscles
o Increases the ability of the lens to refract light (bend) and sharpens close
images, when we focus on a distant object the lens is flattened.
o Accommodation: Process of adjusting the configuration of the lenses to
bring images into focus on the retina.
Eye Position: The fact that what we view can be seen simultaneously through both
eyes is important for our ability to create 3D perceptions from 2D retinal images –
vertebrates with eyes in front = predators vs. prey = sides.
o Binocular Disparity: The difference in position of the same image on 2
retinas – greater for close objects than distant objects – 3D from 2D.
The Retina & Translation of Light to Neural Signals
After light passes the pupil and lens it enters the retina where it is converted into
neural signals and conducted toward the CNS, processing the signals.
The retina is composed of 5 layers of neurons: receptors, horizontal cells, bipolar
cells, amacrine cells, and retinal ganglion cells (over 50 types)
o Amacrine and horizontal are specialized for lateral communication;
communication across the major channels of sensory input
Retinal neurons communicate chemically through synapses and electrically through
gap junctions – light reaches receptor layer after passing through other 4, it then is
activated, transmitting the neural message back through the layers to the retinal PS263 – Chapter 6: The Visual System
ganglion cells whose axons project across the inside of the retina before bundling in
o Problems: 1) Incoming light is distorted by the retinal tissue it passes en
route to receptors, 2) The bundle of retinal ganglion cells need a gap in the
receptor area (blind spot) in order to leave the eye.
o The fovea minimized problem 1 – it in an indentation (0.33cm) at the center
of the retina – the area that is specialized for high-acuity vision, the thinning
of the R.G.L. reduces the distortion of incoming light.
o The blind spot – the visual system uses info provided by the receptors
around the blind spot to fill in the gaps in your retinal images (completion)
o Surface Interpolation: the process by which we perceive surfaces; the visual
system extracts info about edges & infers the appearance of large surfaces.
Species that are active in the day tend to have cone-only retinas, while those active
at night tend to have rod-only retinas.
o Duplexity Theory: Theory that cones and rods mediate different kinds of
vision Photopic Vision: cone-mediated vision that predominates in good
lighting and provides high acuity colour perceptions of the world. Scotopic
Vision: When there is not enough light, these are more sensitive (rod-
mediated) however, it lacks detail and colour.
In the scotpoic there is an output of several hundred rods converging
on a single R.G.C where in the photopic only a few cones converge =
the effects of dim light stimulating many rods can add to the firing of
retinal ganglion cells. (ONLY CONES IN THE FOVEA and more in the
NASAL HEMIRETINA (closest to nose) vs. TEMPORAL HEMIRETINA)
Spectral Sensitivity: A graph of the relative brightness of lights of the same
intensity presented at different wavelengths (curve)
o Photopic Spectral Sensitivity Curve – determined by having subjects judge
the relative brightness of different wavelengths of light on the fovea.
o Scotopic Spectral Sensitivity Curve – determined by asking subjects to judge
the relative brightness of different WV of light shone on the periphery of the
retina at a intensity too low to activate the few peripheral cones located there.
o ** Photopic conditions are maximally sensitive to WV of 560 nm while
scoptopic is maximally sensitive to WV of 500nm **
Purkinje Effect: Noticed the flowers in his garden that were red & yellow were very
bright compared to the blue, a few minutes later it was shades of gray with blue
being the brighter gray shade
Eye Movement: Our eyes are constantly scanning the visual field, our visual
perception at any instant Is a summation of recent visual information (temporal
integration) that the world does not vanish when we blink
o When we fix our gaze on an object our eyes continuously move involuntarily
fixational eye movements: tremors, drifts and saccades.
Transduction: conversion of one form of energy to another – visual is when light is
converted into neural signals by visual receptors – noted when a red pigment was
extracted from rods rhodopsin and exposed to tense light, it was bleached and it
lost its ability to observe light – when returned to the dark it regained it and redness
Rhodopsin is a G-Protein coupled receptor that responds to light rather than to NT
molecules which initiate the cascade of chemical events when activated – in
darkness rods Na channels are slightly open, keeping rods depolarized, but when
bleached these are closed – signals are transmitted by inhibitions PS263 – Chapter 6: The Visual System
From Retina to Primary Visual Cortex
Retina Geniculate Striate Pathways: conduct signals from each retina to the primary
visual cortex or striate cortex via the lateral geniculate nuclei of the thalamus.
90% of the axons of the R.G.C. become part of the pathway – the signals from the left
VF and the right primary visual cortex either ipsilaterally from the temporal hemi
retina of the right eye or contra laterally via the optic chiasm
Retinotopic Organization: Each level of the system is organized like a map of the
retina – 2 stimuli presented to adjacent areas of the retina excite adjacent neurons
Parvocellular Layers (P Layers): Composed of neurons with small cell bodies
(parvo means small) – the first 4 layers; colour, fine details, patterns, stationary or
slowly moving objects – receive input from Cones
Magnocellular Layers (M Layers): Composed of neurons with large cell bodies –
the bottom 2 layers – responsive to movement – receive input from Rods
Edges are important because they define the extent & position of various objects – a
visual edge is a place where 2 areas of a visual image meet.
Mach Bands: they enhance the contrast at each edge and make the edge easier to see
Contrast Enhancement: Every edge we look at is highlighted for us by the contrast
enhancing mechanisms of our nervous system – perc