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Human information processing:Chapters 4-9

Computer as a metaphor for human performance Misses role of emotion and distributed cognition

Receptors Perception

Long-term memory

Decision and response selection

Response execution

Controlled system

Workingmemory

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Visual sensory system

Perceptually relevant characteristics of light The receptor system: The eyeball Basic sensory limits of the receptor cells (rods and cones) Perceptual processing

• Depth perception• Visual search• Detection• Discrimination• Absolute judgement

Bottom-up and top-down processes combine to guide visual perception

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Perceptually relevant characteristics of light

Humans sensitive to wavelengths between 400 and 700 nanometers

Hue=wavelength (composed of Red, Green, Blue) Brightness=amplitude Saturation=purity of color, undiluted by achromatic

gray or white light

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CIE color space

http://www.linocolor.com/colorman/colorman_frames.htm

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The receptor system: The eyeball

http://www.adobe.com/support/techguides/color/colormodels/cie.html

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Distribution of rods and cones

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Implications of eyeball characteristics

Foveal vision of 2 degrees (cones) surrounded by peripheral vision (rods)

Fovea (cones) sensitive to detail, greater acuity/ Periphery (rods) sensitive to motion

Rods sensitive to light provide night vision, but lose sensitivity when exposed to bright light

Cones sensitive to color Differential sensitivity to red light makes it possible to

engage cones and preserve sensitivity of rods

8http://www.usd.edu/psyc301/CSFIntro.htm

9

Adjustable contrast ratio and spatial frequency

http://webphysics.ph.msstate.edu/jc/library/30-7/

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Basic sensory limits

Contrast ratio (ratio of light to dark) and spatial frequency (level of detail) determine visibility

Color deficiency (red==green) affects 7% of males therefore design for monochrome

Sensitivity of rods to peripheral motion and cones reduced sensitivity to detail in low light “overspeed” at night

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Applications of basic perceptual limits to design

Contrast ratio and text conspicuity (not black on black for warnings as on many VRCs)

Optimal size/level of detail 3 cycles/degree (11 point font)

Use of mixed case in print conveys information through low frequency aspects of word shape

Avoid excessive detail in icons

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Perceptual processing: Bottom up/Top down

Bottom up: Interpretation of stimulus driven by data in the world

Top down: Interpretation of stimulus driven by knowledge in the head

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Bottom-up and Top-down processing

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Perceptual processing: Depth perception

Bottom up• Accommodation: Activation of ciliary muscles• Convergence: “cross-eyedness• Binocular disparity: Different image in each eye

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

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Depth cues at work

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Traffic circle application of depth cues

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Perceptual processing: Visual search

Unstructured visual search time= (N*I)/2• N=number of targets• I=search time per target

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)

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A visual search challenge

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Perceptual processing: Visual search

Bottom up factors affecting search time:• Conspicuity (color, size, brightness, motion, simplicity,

automaticity) Top down factors affecting search

• Expectancies (knowledge of organization, knowledge of likely location)

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Some Interesting Demos

http://www.usd.edu/psyc301/psyc301main.htm

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Perceptual processing: Detection

Described in terms of signal detection theory (SDT)

SDT defined by two parameters• Sensitivity (d’): Ability to differentiate signal from noise• Response style (Beta): Risky vs. conservative

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Signal detection theory

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Affecting detection bias and sensitivity

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

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Perceptual processing: Discrimination and Absolute judgement

Going beyond detection to categorize or to judge magnitude

3.3 vs. 33 Norflox vs. Norflex (typed and handwritten) 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

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Absolute judgement in value estimates

Judgement accuracy limited to 1/2 the distance between markers

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Key concepts

The computer as a metaphor for human information processing, and not

Rods,cones, and their capabilities and limits The relationship between contrast sensitivity,

spatial frequency, and visibility Perception depends on relative values than on

absolute values

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More key concepts

Depth perception and the one-eyed soccer player Sensitivity and bias and their effect on detection Discrimination and absolute judgement (limited to

at most 5 levels)

Bottom-up and top-down processes combine to guide visual perception