Level	Explanatory Factors	Description
Physical	External Obstruction	Physical obstructions completely or partially block the sightline and prevent retinal image formation: e.g. buildings A-pillars, windshields.
Physiological	Optical	Optical imperfections. Blur caused by mis-accommodation and spherical aberration. Cataracts and opacity.
	Neural	Retinal limitations, e.g., field size, photoreceptor spacing, and scatomata.
	Motor	Eye muscle movements also reduce vision, e.g., saccadic suppression and blinking.
Psychophysical	Contrast	Factors that determine whether an object differs sufficiently from the background to be visible: e.g., size, adaptation level, retinal eccentricity, etc.

Cognitive (Learned Adaptation)	Expectancy	Attention tuned to the location and objects of highest anticipated meaningfulness, so others not noticed.
	Automaticity	Tasks performed with minimal attentional control, so little attentional supervision to notice difficulties.
	Familiarity Blindness	Failure to notice scene information that has been irrelevant in the past.
Cognitive (Loading)	Tunnel vision	Attention concentrated in the center of the visual field, so information in peripheral vision is unnoticed.
	Inexperience	Novices have not yet learned to reduce foveal loading by chunking or by automatic control.
	Stress Hypervigilance	Stress focuses attention on a narrow set of information, so other information not noticed.
	Mental Workload	Attention consumed by one task leaves less capacity to perform others.
Cognitive (Selection)	Inattentional blindness	Failure to notice even highly visible objects located in the center of the visual field.
	Change blindness	Failure to notice a feature alternation in a scene that has changed over time.
	Mind wandering (Internal distraction)	Attention is focused internally rather than to the external world.
	Repetition blindness	Second in a sequence of two identical objects is not noticed.
	Foreground bias	Viewers tend to fixate objects in the foreground and to ignore objects in the background.
Cognitive (Background)	Clutter masking	Background contours interfere with attention to foreground objects. It is sometimes a psychophysical effect.
	Overshadowing	Attention attracted to the most salient scene object and away from other information.
	Cue generalization	Viewers direct attention to the most easily discriminated cue, such as color.
	Crowding	Visible objects in peripheral vision merge and cannot be identified.
	Motion induced blindness (MIB)	Object projecting retinally stable images tend to disappear when seen against moving backgrounds.
Cognitive (Search)	Satisfaction of search	Termination of serial search before reaching the critical information.
	Inhibition of return	Once a scene area is searched, the probability of making a return saccade to re-search is reduced.
	Visual space asymmetry	Different parts of the 3-D visual space are specialized for different attentional tasks.
	Switching Costs/Attentional blink	Switching attention from one focus to another takes time and effort.
Cognitive (Reduced Capacity)	Fatigue/Lack Of sleep	Difficult to define, but usually explained as lowered performance due to lack of sleep.
	Vigilance decrement	Ability to notice information falls within the first half hour in routine tasks.
	Low Workload/Boredom/Monotony	Low arousal level with longer time spent in a dull and unchanging environment.
	Circadian Rhythm	Arousal lower in troughs of the daily 24-hour arousal cycle.
	Age	Older viewers are both slower and have lower attentional capacity.
Cognitive (Decision)	Biases & Heuristics	Attention and decision are guided by mental short cuts designed to increase efficiency.
Cognitive (Decision)	Satisficing	Attention becomes unnecessary once a reasonable solution achieved.