Richard Griffiths - Lecture Notes

Vision may be analysed as occurring in two stages:

1. Physical reception of the stimulus
2. Processing and interpretation of the stimulus

Physical reception of the stimulus

Optical Characteristics

Visual acuity (the ability to perceive fine detail)

Finest line that can be perceived has a minimum visual angle of 0.5 seconds of arc.

The smallest spaces between lines that can de detected have a minimum visual angle of from 30 seconds to 1 minute of arc.

Brightness Perception

The subjective reaction to levels of light.

Eye compensates for changes in brightness by adjusting the pupil size by expanding or contracting the iris.

In very dim light, rods cut in and suppress cones, decreasing perception of the centre of the field and increasing that of the periphery.

Visual acuity increases with increasing light, but perception of flicker also increases.  In very bright light, flicker faster than 50 Hz can de detected.  It is also more noticeable in peripheral vision.  So, the larger and brighter a display, the more likely that flicker will be perceived.

Colour Perception

Perceived colour consists of three components; hue, intensity and saturation.  By varying intensity and saturation around 7 million colours can be perceived (but only 10 or so can be named by untrained people).

Colour blindness affects approximately 8% of males and 1% of females, commonly by being unable to distinguish between red and green.

A resource for discovering how colourblindness affects viewing of web sites is at: http://colorfilter.wickline.org/

Hue

Determined by spectral wavelength of light:
short = blue, medium = green, long = red

Approximately 150 different hues can be distinguished.

Intensity

Brightness of the colour.

Saturation

Amount of whiteness (other wavelengths) in the colour.

Retina

The retina is the array of light receptor cells that lines the back of the eye.  It  contains two types of receptor cells, named after their shape; rod and cone cells, and interconnecting ganglion cells, of which there are two types with differing properties, X and Y cells.

An excellent source of reference on the Web, with photographs, of the neuronal structure of the eye is Lance Hahn's Retina Reference (Hahn, L.).

Receptor Cells

Rod Cells

120 million rod cells per eye.

Mainly at the periphery of the retina.

Sensitive to low light levels.

Do not resolve details.

May be saturated.

When active, they suppress cone receptors.

Cone Cells

6 million cone cells per eye.

Concentrated in the fovea (the pit in the retina at the centre of the visual field).

Less sensitive than rods.

May be saturated, which results in the complementary colour being seen in an after image.

Three types, each sensitive to light of different wavelengths — the colours red, green and blue.

Only 3-4% of receptors in the fovea are sensitive to blue light, so blue detail is more difficult to perceive.

Ganglion Cells

Ganglion cells interconnect with receptor cells, via intermediate bipolar cells, and the axons of the ganglion cells leave the retina to form the optic nerve.  There are approximately  eight hundred thousand per eye.  The ganglion cells also carry out early processing of the image.  They consist on two types:  X and Y cells.

X-Cells

In the fovea.

Pattern detectors.

Y-Cells

Widely distributed.

Movement detectors.

May be saturated (e.g., by looking at a flowing river from a bridge) which results in an after impression of movement in the opposite direction.

A viewer may not detect changes in pattern in peripheral vision, but will detect movement.

Connection to the Brain

Axons from the ganglion cells connect with either the lateral geniculate nucleus or the superior colliculus — both sub-cortical structures.  These structures perform further processing.  The lateral geniculate nucleus having a part to play in perceiving details and recognizing objects.  The superior colliculus processes the localization of objects in space.  Both of these structures are further connected with the visual cortex.

The Visual Cortex

There is a mapping between the receptors in the retina and cells in the visual context in that retina cells that are close together are connected to cortical cells that are close.  Connections from the same visual field of each eye map to either hemisphere, the cortical cells arranged in bands of cells associated with each eye.

Some cortical cells respond to large patterns in the visual field, detecting contrasting edges and bars, specific cells responding to position, orientation and width of the stimulus.

Cells respond if stimulation is presented to the corresponding visual region of either eye.

Processing and interpretation of the stimulus

Resolution of ambiguity

Optical illusion

Muller-Lyer illusion

\_______/
/       \

/_______\
\       /

Ponzo effect

     /_____\
    /       \
   /         \
  /           \
 /             \
/     _____     \

Laws of Gestalt.

Proximity

* *    * *    * *

Seen as three groups of two asterisks rather than simply six asterisks.

Similarity

X  O  X  O  X  O  X  O  X  O
X  O  X  O  X  O  X  O  X  O
X  O  X  O  X  O  X  O  X  O
X  O  X  O  X  O  X  O  X  O

Seen as columns of Xs and Os rather than rows of alternating Xs and Os.

Good Continuation

     *        *
      *      *
       *   *
        *
     *   *
  *       *
*          *

Seen as two lines crossing, rather than two bent lines touching, or just a collection of asterisks.

Closure / Good Form

**********
**********
*****************
*****************
*****************
       **********
       **********

Seen as two overlapping squares rather than one irregular grouping of asterisks.

These have crucial significance for the layout of information on screens and printed documents.

For example:  from the table below, did Brown pass or fail (score less than 40) module MC543?
 
 

	Module	Mark	Module	Mark	Module	Mark
Smith	RT154	55	AB356	42	AZ117	59
Brown	PF886	63	MC543	22	RT332	90
Jones	MC543	73	RT154	65	AZ117	72

 

An excellent resource for further information on vision and the eye is the 'The Joy of Visual Perception: A Web Book' by  Kaiser.  Detail of the neurology of the eye can be found in 'Lance Hahn's Retina Reference' by Hahn.

References

Anderson, J. R.  1985  "Cognitive Psychology and Its Implications" (Second Edition).  W.H. Freeman and Company, New York.

Dix, A., Finlay, J., Abowd, G., Beale, R.  1998  “Human-Computer Interaction” (Second Edition).  Prentice Hall.

Hahn, L.  "Lance Hahn's Retina Reference". http://retina.anatomy.upenn.edu/~lance/retina/retina.html

Kaiser, Peter K.  1996  "The Joy of Visual Perception: A Web Book"  http://www.yorku.ca/eye/
 
 
 
 

This page is maintained by Richard Griffiths and does not necessarily reflect the official position of the University of Brighton.

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