Physiological Fundamentals

1
Physiological Fundamentals
2
Acknowledgements

• The presentations and assignments have been
  develop by the Georgia Tech HCI faculty over a
  period of years, and continue to evolve.
  Contributors include
• Gregory Abowd, Jim Foley, Diane Gromala,
  Elizabeth Mynatt, Jeff Pierce, Colin Potts, Chris
  Shaw, John Stasko, Bruce Walker
• Feedback is most welcome!

3
Physiological Fundamentals
4
Physiological Fundamentals

• Retina has
• 6.5 M cones (color vision), mostly at fovea
  (1/3)
• About 150,000 cones per square millimeter
• Fewer blue sensing cones than red and green at
  fovea
• 100 M rods (night vision), spread over retina,
  none at fovea
• Adaptation
• Switching between dark and light causes fatigue

5
Flicker
Fovea
10º from Fovea

• Measured as Critical Flicker Frequency (CFF)
• CFF decreases as intensity increases
• Longer persistence phosphor decreases CFF
• CFF lower on fovea than away from fovea (may have
  peripheral flicker)

Critical Flicker Frequency, Hz
Brightness
6
Color/Intensity Discrimination

• Absolute discrimination identify without any
  reference
• HUE 9
• INTENSITY 5
• SATURATION ?

7
Color/Intensity Discrimination

• The 9 hues most people can identify are
• Color Wavelength
• Red 629
• Red-Orange 596
• Yellow-Orange 582
• Green-Yellow 571
• Yellow-Green 538
• Green 510
• Blue-Green 491
• Blue 481
• Violet-Blue 460

8
Just Noticeable Difference (JND)

• Just noticeable difference (JND) is the smallest
  change detectable 50 of the time
• HUE/JND 128
• Intensity JND 32 64
• Saturation JND
• Minimum of 16, for yellow
• Maximum of 23, for red violet
• About 9 of males are red-green colorblind!
• Apparent color changes with surrounding color

9
Color Surround Effect

• Our perception of a color is affected by the
  surrounding color

10
Color Surround
11
Acuity

• Determines smallest size we can see
• About one minute of arc in normal light
• Less for blue and yellow than for red and green
• Measured with

12
Acuity

• Defined as reciprocal of minutes of subtended
  angle

• Viewing angle (a) in minutes
• 57.3 X 60 X d
• D
• Acuity decreases as intensity decreases