1 Human information processing: Chapters 4-9 n Computer as a metaphor for human performance n Misses role of emotion and distributed cognition ReceptorsPerception Long-term memory Decision and response selection Response execution Controlled system Working memory

2 Visual sensory system n Perceptually relevant characteristics of light n The receptor system: The eyeball n Basic sensory limits of the receptor cells (rods and cones) n Perceptual processing Depth perception Visual search Detection Discrimination Absolute judgement n Bottom-up and top-down processes combine to guide visual perception

3 Perceptually relevant characteristics of light n Humans sensitive to wavelengths between 400 and 700 nanometers n Hue=wavelength (composed of Red, Green, Blue) n Brightness=amplitude n Saturation=purity of color, undiluted by achromatic gray or white light

4 CIE color space

5 The receptor system: The eyeball

6 Distribution of rods and cones

7 Implications of eyeball characteristics n Foveal vision of 2 degrees (cones) surrounded by peripheral vision (rods) n Fovea (cones) sensitive to detail, greater acuity/ Periphery (rods) sensitive to motion n Rods sensitive to light provide night vision, but lose sensitivity when exposed to bright light n Cones sensitive to color n Differential sensitivity to red light makes it possible to engage cones and preserve sensitivity of rods

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9 Adjustable contrast ratio and spatial frequency n

10 Basic sensory limits n Contrast ratio (ratio of light to dark) and spatial frequency (level of detail) determine visibility n Color deficiency (red==green) affects 7% of males therefore design for monochrome n Sensitivity of rods to peripheral motion and cones reduced sensitivity to detail in low light “overspeed” at night

11 Applications of basic perceptual limits to design n Contrast ratio and text conspicuity (not black on black for warnings as on many VRCs) n Optimal size/level of detail 3 cycles/degree (11 point font) n Use of mixed case in print conveys information through low frequency aspects of word shape n Avoid excessive detail in icons

12 Perceptual processing: Bottom up/Top down n Bottom up: Interpretation of stimulus driven by data in the world n Top down: Interpretation of stimulus driven by knowledge in the head

13 Bottom-up and Top-down processing

14 Perceptual processing: Depth perception n Bottom up Accommodation: Activation of ciliary muscles Convergence: “cross-eyedness Binocular disparity: Different image in each eye n Top down Linear perspective Relative size Interposition Light and shading Texture gradients Relative motion Design applications: One-eyed soccer player and 3-D virtual reality Pavement markings and speed control

15 Depth cues at work

16 Traffic circle application of depth cues

17 Perceptual processing: Visual search n Unstructured visual search time= (N*I)/2 N=number of targets I=search time per target n Unstructured visual search is not exhaustive Design applications: Cost of clutter, eliminate unnecessary items (BU) Make critical items more salient (color, contrast) (BU) Structure visual field (alphabetical, consistent location) (TD)

18 A visual search challenge

19 Perceptual processing: Visual search n Bottom up factors affecting search time: Conspicuity (color, size, brightness, motion, simplicity, automaticity) n Top down factors affecting search Expectancies (knowledge of organization, knowledge of likely location)

20 Some Interesting Demos n

21 Perceptual processing: Detection n Described in terms of signal detection theory (SDT) n SDT defined by two parameters Sensitivity (d’): Ability to differentiate signal from noise Response style (Beta): Risky vs. conservative

22 Signal detection theory

23 Affecting detection bias and sensitivity n Sensitivity (d’): Ability to differentiate signal from noise Experience Visual template Redundant representations Alertness, frequent breaks Slow down rate of decisions n Response style (Beta): Risky vs. conservative Value and costs of hits, false alarms (motivation) Introduce false signals

24 Perceptual processing: Discrimination and Absolute judgement n Going beyond detection to categorize or to judge magnitude n 3.3 vs. 33 n Norflox vs. Norflex (typed and handwritten) n Absolute judgement: Attaching labels to levels Limited to 5 levels at most No more than seven colors Accuracy limited to half the distance between scale markers

25 Absolute judgement in value estimates n Judgement accuracy limited to 1/2 the distance between markers

26 Key concepts n The computer as a metaphor for human information processing, and not n Rods,cones, and their capabilities and limits n The relationship between contrast sensitivity, spatial frequency, and visibility n Perception depends on relative values than on absolute values

27 More key concepts n Depth perception and the one-eyed soccer player n Sensitivity and bias and their effect on detection n Discrimination and absolute judgement (limited to at most 5 levels) n Bottom-up and top-down processes combine to guide visual perception