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33 Reasons For Not Seeing

Marc Green

Viewers often fail to see objects, such as pedestrians and other vehicles. Such failures-to-see have great importance in safety as well as in many other domains such marketing, medicine, and sports. Elsewhere, I have discussed seeing in general and why failure-to-see is so common. Here, I provide a broad overview of the specific reasons for failures-to-see. I present these in tables that are adapted from the book Roadway Human Factors: From Science To Application (Green, 2017), which discusses them in detail. However, such categorizations inevitably begin to fray at the edges. They are didactic and an attempt at cognitively simplifying the large number of visual factors affecting seeing.

The reasons can very broadly be divided into visibility and cognitive. The first table shows 5 visibility factors. External physical factors are partial or complete sightline obstructions. Physiological factors can be optical, neural, or motor. Psychophysical factors usually boil down to those affecting contrast detection. Lastly, these five entries can readily be decomposed further. There are more than 16 subfactors in contrast alone (Green, 2017).

Level

Explanatory Factors

Description

Physical

External Obstruction

Physical obstructions the sightline completely or partially block retinal image formation: e.g. buildings A-pillars, windshields. Obstructions can be total or partial.

Physiological

 

Optical

Optical imperfections. Blur caused by mis-accommodation and spherical aberration. Cateracts 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.

In many cases, however, the cause of failure-to-see is cognitive - the viewer does not consciously see an object which is theoretically highly visible. The usual culprit is attention. The failure to see is especially likely in given circumstances. There is vast a literature on attention and cognition that provides many potential explanations for failure to see in a given set of circumstances. The table below boils this literature down to a single list of 28 cognitive failure-to-see "constructs" that are again grouped into rough categories purely for didactic purposes. Further, the chart constructs require further elaboration.

Level

Explanatory Constructs

Description

Psychophysical

Cognitive

Salience

Salience is the convergence of sensory conspicuity factors. Objects that are visible but have low sensory impact are less likely to be noticed.

Foreground

Objects in the foreground of the figure-ground segregation (e.g., bright) are more noticeable.

 

Cognitive

(Adaptation)

Expectancy

Attention tuned to the location and objects of highest anticipated meaningfulness, so others not noticed.

Automaticity

Tasks are performed with minimal attentional control, so little attentional is paid to "irrelevant" information.

Familiarity Blindness

The failure to notice scene information that has been irrelevant in the past.

Cognitive

(Overload)

Tunnel vision

Attention is concentrated in the center of the visual field, so information in peripheral vision is unnoticed.

Inexperience

Novices have not yet learned to reduce attention by developing schemata or by using automatic control.

Stress/Narrowing/ 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 scene change over time. The change may be very fast or occur gradually.

Mind wandering (Internal distraction)

Attention is focused internally rather than externally.

Repetition

Blindness

The second in a sequence of two identical objects is not noticed.

Cognitive

(Background)

Clutter masking

Background contours interfere with attention to foreground objects. (It has some sensory component.)

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. (It may a be sensory, not a cognitive effect.)

Cognitive

(Search)

Visual space asymmetry

Different parts of the 3-D visual space are specialized for different attentional tasks, e.g., peripersonal vs. extrapersonal space.

Switching Costs/ Attentional blink

Changing the focus of attention is slow and effortful.

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.

Cognitive

(Low Capacity and low arousal)

Fatigue/ Lack Of sleep

Fatigue is difficult to define, but usually explained as lowered performance due lack of sleep.

Vigilance decrement

The ability to notice information falls within the first half hour in routine tasks.

Low Workload/ Boredom/Monotony

Low arousal level with long time in dull and unchanging environments.

Circadian Rhythm

Arousal is lower in the troughs of the day 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.

Satisficing

Attention becomes unnecessary once a "good-enough" solution achieved, then search ceases.

The cognitive construct list is long, but the phenomena overlap considerably because they are different manifestations of our fundamental cognitive architecture and its consequences: 1) conscious awareness is limited by capacity, 2) attention is selective, 3) attention is efficient, trying to use the easiest and simplest selection criterion, and 4) attention generally selects information that is most meaningful based on past experience and on expectation of the future. This overlap is why I call them "constructs" rather than variables. The criterion for inclusion in the list is only that the authors frequently use the "construct" to explain a cognitive failure-to- see. In many cases, the construct name specifies little more than an experimental paradigm, e.g. "change blindness".

Lastly, the constructs also represent different granularities of explanation. "Inattentional blindness" (failure-to-see central objects) and "tunnel vision" (failure to see peripheral objects) are really just descriptive terms for phenomena that are ultimately caused by more specific constructs (expectation, hypervigilance, mental workload, etc.). They can be useful as umbrella terms when merely describing the functional loss, but they are not very specific. The appropriate level of analysis depends on the discussion context1.

Endnotes

1 The required level of analysis depends on circumstances. For example, amblyopia is an umbrella descriptive term for loss of vision in one eye with no retinal pathology. It is usually caused by childhood strabismus or hypermetropia. When discussing functional loss in some cases, it is the amblyopia - what will the viewer see - that is important and not the underlying cause. In other cases, the underlying cause is important - how to prevent the condition.

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