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| • The perception of elements in the environment within a volume of time and space, the comprehension of their meaning, and the projection of their status in the near future |
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• Wavelength = Color/hue
• Amplitude = Brightness
• Purity = Saturation |
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| • white membrane visible to us. Shape maintained by pressure exerted by fluids it contains. |
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• transparent covering; protects the eye, bends light to provide focus
o Astigmatism – misshapen cornea that causes blurring |
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• Light enters the eye through the pupil, a small adjustable opening
o Size of the pupil adjusted by iris, a circular colored muscle (gives us our eye color)
o Whytt’s reflex – dilates in dark; contracts in light |
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| transparent structure behind pupil that changes shape to focus an image on the back of the eye (accomodation) |
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| • watery fluid of the eye filling the anterior chamber. Clear fluid=healthy eyes. |
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• Light-sensitive inner surface of the eye
o Rods
o Cones
o Fovea
• Rods and cones connected to ganglion cells
o Ganglion cells’ axons create the optic nerve ( a bundle of nerve fibers that carry messages from retina to the thalamus)
o No rods or cones at spot where optic nerve leaves eye (blind spot) |
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| • retinal cells/receptors that detect black, white, and gray; necessary for peripheral and twilight vision |
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| • retinal cells that detect colors and fine detail; function in daylight and well-lit conditions |
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| • central part of the retina (2 degrees of visual angle) with highest density of cones and highest resolution (contains virtually no rods) |
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| Visual Receptor System (VRS) |
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• Image characterized by its:
1) Intensity (luminance)
2) Size
3) Wavelength |
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| • Greatest at fovea due to high cone density; motion sensitivity higher at the periphery. |
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| • Rods have great sensitivity in low-light conditions |
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• Rods rapidly lose sensitivity when stimulated by light
• Cones can become hypersensitive they receive little stimulation. |
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| VRS – Differential wavelength sensitivity |
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| • Rods insensitive to long wavelengths (i.e., red). Use to illuminate objects at night so not to destroy rods dark adaptation. |
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| Vision Problems - Presbyopia |
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-Old Eye
• thickening and hardening of the lens that occurs naturally with age.
• Usually starts in mid-40s. |
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-nearsightedness
• Long eyeball; lens cannot flatten to focus objects that are far away. |
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| Vision Problems - Hyperopia |
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-farsightedness
• Short eyeball; lens cannot accommodate (curve) to near objects. |
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| Vision Problems - Glaucoma |
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| • Caused by damage to the optic nerve (in the retina) from pressure build-up within the eye |
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| Sensory Processing Limitations (4) |
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1) Contrast Sensitivity
2) Reading Print
3) Color Sensation
4) Night Vision |
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• Ability to distinguish object from its background
• Functional Vision
C=(L-D)/(L+D)
L=luminance of light
D=luminance of dark
• Optimal Spatial Frequency: 3 cycles/degree |
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| Factors that Affect Contrast Sensitivity and Visibility |
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• Spatial frequency
• Lower contrast
• Polarity
• Level of illumination
• Other Factors: Age – loss of accommodation (less light passing through cornea; Motion |
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• Spatial Frequency Goal: 3 c/d
• Maximize contrast between letters and background
• Use typical character fonts
• Use UPPERCASE for single words only |
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| Color Sensation – Protanopia |
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• Color blindness
• Most common is ‘red-green’ color blindness where 2 hues are indistinguishable
• 8% of American men and 1% American women effected
• Solution: Design for monochrome first and use color redundant backup
o Ex) stoplights, road signs |
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| Perception – Bottom-up vs. Top-down |
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• Top-Down: Experience/knowledge based
o Expectancy, Learning, Values
• Bottom-up
o Flow from lower level processing (eye) towards higher level (brain) |
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1) Proximity – Objects that are close together are more likely to be perceived as belonging to the same group
2) Similarity – objects that are similar in appearance are more likely to be perceived as belong to the same group
3) Continuity – objects that form a continuous form are perceived as belonging to the same group
4) Closure – objects that make up a recognizable image are more likely to perceived as belonging to the same group even if the image contains gaps that the mind needs to fill in |
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| Depth Perception – Near Vision |
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• Accommodation (lens shaping)
• Convergence (‘cross-eyedness’)
• Binocular disparity (stereopsis)
o All of these operate on bottom-up processing
• All are effective for judging distance, slant, and speed for objects that are within a few meters from viewer |
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| Depth Perception – Distance Vision |
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• Use of pictorial cues including:
o Linear perspective
o Relative size
o Interposition
o Light and Shading
o Textural gradients
o Relative motion |
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• Developed by age 3
• Use monocular cues (one eye)
o Interposition, texture, gradient, relative size, linear persepective
• Use binocular cues (both eyes)
o Retinal disparity, convergence |
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• Study of people’s tendencies to make correct judgments in detecting the presence of a stimuli
• Detection thresholds – involving recognizing some stimulus against a background of competing stimuli |
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• Humans have limited ability to judge absolute value of stimulus
• As the number of levels exceeds 5, accuracy drops
o Miller’s Number = 7+ 2 (1956)
o Ex) Pitch, saltiness |
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