The document discusses human information processing systems and their components of input-output, memory, and processing. It describes the human eye and vision, including the retina, rods, cones, and blind spot. It also covers interpreting visual signals related to brightness, color, expectations, and optical illusions. The document discusses reading processes, hearing mechanisms and sound perception, touch receptors, human movement and Fitts' Law, and the different types of human memory including sensory, short-term, and long-term memory.

1. Human Computer Interaction
Lecture 2
The Human

2. The Human
• Think of human as an information processing
system, which contains input/output, stores
information and processes information
• We will therefore consider three components
of this system: input-output, memory and
processing

3. The Eye - physical reception
• mechanism for receiving light and
transforming it into electrical energy
• light is reflected from objects
• images are focused upside-down on retina
• retina contains rods for low light vision. Rods
are responsible for vision in darkness.
Approximately 120 million rods.

4. The Eye - physical reception
• Retina also contains cones for colour vision.
They are responsible for vision in light.
• Cones are concentrated on fovea and rods are
concentrated on retina
• Blind spot contains neither rods nor cones.
• Ganglion cells (brain!) detect pattern and
movement

5. Design Focus
• A user concentrating on the middle of the
screen cannot be expected to read help text
on the bottom line.
• So if an error message is to be shown to user,
what to do???
• Better use flashing error message
• What about clever moving icons.

6. Interpreting the signal
• Brightness
– subjective reaction to levels of light
– affected by luminance of object, which is the
amount of light emitted by an object
– Contrast is luminance of object and luminance of
its background
– visual acuity increases with luminance as does
flicker.
– High display luminance systems are seen to flicker
even above 50 Hz.

7. Interpreting the signal (cont)
• Colour
– made up of hue, intensity, saturation
• Hue is determined by the spectral wavelength of the light
– Approximately 150 different hues can be discriminated by the average person
• Intensity is the brightness of color
• Saturation is the amount of whiteness in color
– Cones are sensitive to colour wavelengths. Three
types of cones (red, green and blue)
– blue acuity is lowest, because only 3-4% of the
fovea is occupied by cones which are sensitive to
blue light
– 8% males and 1% females colour blind

8. Interpreting the signal (cont)
• Our expectations affect the way an image is
perceived.
• Context is used to resolve ambiguity
• However, Optical illusions sometimes occur
due to above factors

9. Optical Illusions
the Ponzo illusion
the Muller Lyer illusion

10. Reading
• Several stages:
– visual pattern perceived
– decoded using internal representation of language
– interpreted using knowledge of syntax and semantics
• Reading involves saccades(jerky movements) and
fixations
• Perception occurs during fixations
• Word shape is important to recognition
• Negative contrast (dark character on a light
screen) improves reading from computer screen

11. Design Focus
• Standard font sizes of 9 to 12 are equally legible,
given proportional spacing between lines.
• Similarly line lengths of between 2.3 and 5.2
inches (58 and 132 mm) are equally legible.
• Nevertheless, reading from a computer screen is
slower than from a book. However, this fact can
be controlled by careful design of textual
interfaces.

12. Hearing
• Sound can convey a remarkable amount of information
• Provides information about environment:
distances, directions, objects etc.
• Physical apparatus:
– outer ear – protects inner and amplifies sound
– middle ear – transmits sound waves as
vibrations to inner ear
– inner ear – chemical transmitters are released
and cause impulses in auditory nerve
• Sound
– pitch – sound frequency
– loudness – amplitude
– timbre – type or quality

13. Hearing (cont)
• Humans can hear frequencies from 20Hz to 15kHz
– less accurate distinguishing high frequencies than low.
• Auditory system filters sounds
– can attend to sounds over background noise.
– for example, the cocktail party phenomenon.
• Sound could be used extensively in interface design to
convey information about the system state

14. Touch
• Provides important feedback about environment.
• May be key sense for someone who is visually impaired.
• Stimulus received via receptors in the skin:
– thermoreceptors – heat and cold
– nociceptors – pain
– mechanoreceptors – pressure
• Some areas more sensitive than others e.g. Fingers and hair

15. Movement
• Fitts' Law describes the time taken to hit a screen target:
Mt = a + b log2(D/S + 1)
where: a and b are empirically(gained by means of observations)
determined constants
Mt is movement time
D is Distance
S is Size of target
targets as large as possible distances as small as possible

16. Memory
There are three types of memory function:
Sensory memories
Short-term memory or working memory
Long-term memory

17. Sensory Memory
• Buffers for stimuli received through senses
– iconic memory: visual stimuli
– echoic memory: aural stimuli
– haptic memory: touch stimuli
• Examples: Move your finger fastly before
you, “sparkler” trail Continuously
overwritten
• Information is passed to STM by attention

18. Short-term memory (STM)
• What is the result of 35 * 6???
• Scratch-pad for temporary recall
– rapid access
– rapid decay
– limited capacity - 7± 2 Principle

19. Examples
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ATM Card example