PSYC271 Chapter 6: The Visual System
6.1 Light Enters the Eye and Reaches the Retina
Sometimes light behaves like particles, sometimes it behaves like waves
Our eyes respond to different wavelengths differently
Infrared waves ▯too long for humans to see
Wave length = colour perception
Wave intensity = brightness perception
The Pupil and the Lens:
Iris controls amount of light entering the retina
Light enters eye through pupil (hole in iris) ▯width of pupil changes based on
how much light its letting in and affects:
Sensitivity: in vision, the ability to detect the presence of dimly lit
Acuity: the ability to see the details of an object
Lens (located behind pupil), aims incoming light towards the retina. The lens
shape is controlled by ciliary muscles these are activated when we look at
something. By changing shape of lens, it ‘bends’ light and brings close objects
into sharp focus. Distant object ▯lens is flat.
Ciliary Muscles: the eye muscles that control the shape of the lenses
Eye Position and Binocular Disparity:
Eyes on front of head allow you to view 1 object from 2 angles, creating depth
perception. The closer the object is, the more your eyes ‘converge’ (move
inwards) to see the same object.
Binocular Disparity: the difference in the position of the retinal image of the
same object on the 2 retinas (your eyes will always see a single object from 2
slightly different points of view). Difference greater for closer objects.
6.2 The Retina and Translation of Light into Neural Signals
Lens sends light to retina, retina converts light to neural signals, conducts them towards
CNS, and participates in the processing of signals
Retina’s 5 layers (in order) of different types of neurons:
Retinal Ganglion Cells: retinal neurons whose axons leave the eyeball and
form the optic nerve
Amacrine Cells: a type of retinal neurons whose specialized function is lateral
Bipolar Cells: bipolar neurons that form the middle layer of the retina
Horizontal Cells: type of retinal neurons whose specialized function is lateral
Receptors: cells that are specialized to receive chemical, mechanical, or radiant
signals from the environment
Retinal cells communicate both chemically via synapses and electrically via gap
Retina works backwards. Light travels through all these layers until it hits back of eye
(cone & rod receptor cells) and then travels backwards to retinal ganglion cells.
2 issues with this system: 1. Light gets distorted through layers before reaching receptors
2. Area where bundles retinal ganglion cell axons leave the eye leaves a
gap in the layer ▯this is a blind spot.
Blind Spot: the area on the retina where the bundle of axons of the retinal ganglion
cells penetrate the receptor layer and leave the eye as the optic nerve
Fovea: the central indentation of the retina, which is specialized for highacuity vision.
Helps overcome problem of light distortion through layers because layer of retinal
ganglion cells is thin here, so less light distorted.
Completion: the visual system’s automatic use of information obtained from receptors
around the blind spot, or scotoma, to create a perception of the missing portion of the
We constantly use completion by taking in minimal information and filling in
the gaps to form our perception
Surface Interpolation: the process by which the visual system perceives large surfaces,
by extracting information about edges and from it, inferring the appearance of adjacent
CONE AND ROD VISION:
Cones: the visual receptors in the retina that mediate high acuity colour vision in
good lighting. Most are located in the fovea.
Rods: the visual receptors in the retina that mediate achromatic, lowacuity
vision under dim light
No rods in the fovea, but rods increase at boundaries of fovea and cones
More rods in the nasal hemiretina (half of eye closer to nose) than
temporal hemiretina (half of eye closer to temples)
Duplexity Theory: the theory that cones and rods mediate photopic and
scotopic vision, respectively.
Photopic Vision: conemediated colour vision, predominates in bright light
A few cones converge on each retinal cell (‘low convergence’). Cones
attach to RGC through bipolar cells.
Since fewer cones per RCG, easier to locate where stimulus came from
Scotopic Vision: rodmediated acuity vision, predominates in dim light
Several hundreds of rods converge on a single ganglion cell (‘high
convergence’). Rods attaches to RGC through bipolar cells.
Since many rods per RCG, very difficult for brain to locate where
stimulus came from
Spectral Sensitivity Curve: how bright a light appears when it is the same
intensity but has different wavelengths.
Photopic Spectral Sensitivity Curve: the graph of the sensitivity of
conemediated vision to different wavelengths of light
Tested by shining different wavelengths of light on fovea
Maximally sensitive to wavelengths of 560 nanometers. Thus,
intensity of 500 nanometer light would have to be increased to
appear as bright as 560 nanometer light. Scotopic Spectral Sensitivity Curve: the graph of the sensitivity of rod
mediated vision to different wavelengths of light
Tested by shining different wavelengths of light on boundary of
Maximally sensitive to wavelengths of 500 nanometers. Thus,
intensity of 560 nanometer light would have to be increased to
appear as bright as 500 nanometer light.
Purkinje Effect: in intense light, red and yellow wavelengths look brighter than
blue and green wavelengths of equal intensity; in dim light, blue and green
wavelengths look brighter than red and yellow wavelengths of equal intensity
Temporal Integration ▯eyes constantly scanning visual field, and our visual
perception at any instant is a summation of recent visual information
Fixational Eye Movements: involuntary movements of the eyes (tremors,
drifts, and saccades) that occur when a person tries to fix his gaze on a point.
Three types: tremors, drifts, and saccades
Saccades: the rapid movements of the eyes between fixations
Visual neurons respond to change. If retinal images somehow stopped from
moving, the image would start to disappear and reappear
VISUAL TRANSDUCTION: CONVERSION OF LIGHT INTO NEURAL SIGNALS:
Transduction: the conversion of one form of energy into another
Visual Transduction ▯conversion of light into neural signals by visual
Rhodopsin: the photopigment (pigment = light absorbing substance) of
If exposed to very bright light, it loses its colour and stops absorbing
light. If returning to dark, returns to normal.
In dim light, our sensitivity to certain wavelengths is a direct
consequence of Rhodopsin’s ability to absorb them
When rods in darkness, rhodopsin is inactive, sodium channels partially
open, keeping rods slightly depolarized, allowing steady outflow of
glutamate, and allows rods to function properly. When in bright light,
bleaching of rhodopsin leads to it binding with Transducin, activating an
enzyme that breaks down cGMP, leading to
sodium channels closing, rods get hyperpolarized, and inhibit glutamate
Absorption Spectrum: a graph of the ability of a substance to absorb light of
6.3 From Retina to Primary Visual Cortex
RetinaGeniculateStriate Pathways: the major visual pathway from each retina to the
striate cortex (primary visual cortex) via the lateral geniculate nuclei of the thalamus
About 90% of retinal ganglion axons are in the pathway Primary Visual Cortex: area of the cortex that receives direct input from the lateral
Lateral Geniculate Nuclei: 6layered thalamic structures that receive input from the
retinas and transmit their output to the primary visual cortex
All signals from the left eye reach the right primary visual cortex. Either ipsilaterally
(same side) from the temporal hemiretina, or contralaterally (oppositeside through optic
chiasm) from the nasal hemiretina of the left eye. Vice versa for right eye.
Retinotopic: organized like a map of the retina. Primary visual cortex organized
2 stimuli presented to adjacent areas of the retina excite adjacent neurons
at all levels of the system
Although fovea tiny part of retina, primary visual cortex dedicates 25%
to analyzing its input (highacuity vision)
THE M AND P CHANNELS:
Parvocellular Layers: layers of the lateral geniculate nuclei that are composed
of neurons with small cell bodies; the top 4 layers (‘P layers’)
Responsive to colour, detail, and stationary objects, mostly cones
Magnocellular Layers: layers of the lateral geniculate nuclei that are composed
of neurons with large cell bodies; the bottom 2 layers (‘M layers’)
Responsive to movement, mostly rods
These 2 different layers form 2 separate communication channels and project to
different parts of the visual cortex
6.4 Seeing Edges
Perception of edge is simply the perception of contrast between 2 adjacent areas of the
LATERAL INHIBITION AND CONTRAST ENHANCEMENT:
Mach Bands: nonexistent stripes of brightness and darkness running adjacent to
an edge. Enhance contrast and make edge easier to see
Contrast Enhancement: the intensification of the perception of edges
Lateral Inhibition: firing receptor inhibits lateral neurons/receptors around it
Contrast enhancement governed by firing rate of receptors on either side of an
edge. The receptor closest to the brighter side of the edge will fire more than the
other receptors on this side because it is experiencing less lateral inhibition.
Similarly, the receptor closest to the edge on the dimly lit side fires less than the
rest of the receptors on that side.
RECEPTIVE FIELDS OF VISUAL NEURONS:
Receptive Field: the area of the visual field within which it is possible for the
appropriate stimulus to influence the firing of a visual neuron
Hubel &Wiesel ▯placed electrodes near a single neuron in the visual system.
Paralyzed eye movements. Focus image on retina. Identify receptive field. Record
responses & identify which stimuli effect neural activity most. Repeat these steps and begin to move higher through the visual system to understand increasing
complexity of neuronal activity at each level.
RECEPTIVE FIELDS: NEURONS OF THE RETINAGENICULATESTRIATE
Hubel & Wiesel found little change in receptive fields throughout different areas
of the retinageniculatestriate system. Commonalities found:
Receptive fields in f