Photograph closeup of a home inspector's eye The Nature of Vision
Inspecting Complex Systems, Why We See or Do Not See Things We Are Looking For - Or Should Be Looking For

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Visual perception errors in building, environmental & forensic investigations: this article discusses errors in visual perception during examination of complex systems and structures such as buildings or mechanical systems.

The purpose of this article is to improve the chances that any inspector of any complex system, charged with the visual examination of the system will find both the visually apparent and the visually present but more subtle clues which signal important costly repairs, unsafe conditions, or other targets of the inspection.

Most people believe that if our eyes are open, we are seeing. Cognitive scientists once thought our visual perception acted much like a videotape recorder. We know now that this is not the case. Perception studies are demonstrating how little people actually see when they are not paying attention.1.

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Surprises in the Study of Vision and Inspection of Buildings or Other Complex Systems

Study in a variety of fields (eye and brain, the psychology of error, the physics of light and optics, microscopy, and aerobiology) has led to some surprises and to recognition that forensic experts and inspectors should not be overconfident in their ability to "see" what they are looking "for." Even the ability to see what we are looking "at" is demonstrated to be incomplete.

"Inspection" as used here refers to a primarily-visual approach which may make limited use of special equipment such as moisture meters or electrical circuit polarity testers, but which does not involve demolition or similar "invasive" methods to evaluate the condition of a building or similar complex system and its subsystems such as building mechanicals.

Article Contents

Research on Vision Can Inform Inspectors of Complex Systems, Machines, Buildings, etc.

This article is a chapter in the authors "Developing your X-Ray Vision, A Promotion Theory for Forensic Observation of Residential Construction - Levels of Fear, and how to use them to find and report significant, hidden problems." See - The Nature of Vision - inspecting complex systems (this article, online) and see for the online version of "Developing your X-Ray Vision." As this is an ongoing study project these articles are updated periodically.

Also see LIGHT, GUIDE to FORENSIC USE and see about sinkholes in Florida.

Also see for free in-depth research on building defect recognition and repair, including both physical defects on buildings and environmental concerns such as mold and allergens.

Several areas of research can inform forensic inspectors and may improve their ability to recognize, record, and thus act on defects:

An enormous body of research pertinent to the human ability to consciously see, recognize, and record information exists among these areas. I have not attempted to recap everything about these areas, but I will cite a few ideas they suggest and which pertain to the use of an inspection methodology employing techniques to improve recognition of subtle clues of hidden defects.

What is needed in order to "see" any object or object-feature?

Presence: Information or Object Must Be Present

The object or feature must be present in the visual field of the observer so that all the observer needs to do is direct his or her eye there to be [potentially] able to see it. The size and other requisite aspects of the object or feature are discussed below.

For purposes of this discussion we refer to normal sighted-human vision, unaided by special equipment such as microscopes or telescopes, though the observer may [indeed should] make use of corrective eyeglasses or contact lenses if their vision has been found in need of those aids. In my longer X-ray Vision article my view is that "clues" which suggest an otherwise "hidden" defect are in fact visual "features" of that very defect or condition and can so can be recognized.

Light: Light Reflects From Object to Eye

Light reflected from a surface towards the eye of an observer carries incomplete information about the object and can be distorted. In any case it will not contain all of even the surface information about the object due to limitations of illumination, optical distortion, and of course the inability to "see around corners."

The human eye is sensitive to light (electromagnetic radiation) from around 350 to 750nm in wavelength. It is speculated that we evolved to be sensitive to radiation at these wavelengths because they are so common in sunlight. Some other animals have eyes which are sensitive to electromagnetic radiation at lower wavelengths (such as infra red).

In the process of "inspecting" objects and systems in the field, inspectors use a combination of local natural light, artificial indoor lighting, and flashlights. The flashlight is a more important tool than some inspectors realize. the level or intensity of illumination, angle of illumination, and color of the light source can make the difference between a humans ability to "see" or not see a feature.

The ultimate level of detection of visual features being illuminated by light is a function of the wavelength of the light source, since particles smaller than the wavelength may fail to refract or bend the light waves sufficiently to permit detection. In forensic microscopy the wavelength of the light source is a factor but more important is the ability of the microscopes lens system to direct light rays at the particle being examined. The particle refracts, or "bends" light rays, producing a visible image which is focused by the lens system.

Eye: Light Reaches Eye

The eye, the cornea and lens of the eye itself is the image-forming system for human inspectors working without a microscope or binoculars.

The image of the observed object is formed at the back of the eye on the retina where special cells in turn fire neurons which pass the visual image map to the visual cortex of the brain. The human eye can resolve particles down to a few hundredths of an inch or so in diameter.

Brain: Brain Interprets Information

The brain plays a considerable additional role in "seeing" beyond mere image mapping. Additional image data processing occurs in the brain in order to recognize what is being observed.

What a human "sees" is a representation of the objective world built-up in the brain from a combination of sensory inputs and significant additional processing in the brain. While I'm taking some literary license to say so, it would appear most likely that the brain constructs the "image" in the observers consciousness from a combination of visual inputs from the eye, a map of the eyes input in the visual cortex, and processing of that data along with stored maps of "expected images."

As human brains are designed to "make sense" of various sensory inputs, we may do so even if "making sense" means "making it up." Thus our observations are vulnerable to illusion.

Inattentional Blindness: Observer (Home Inspector) Must Pay Conscious Attention

In addition to the need for an object to be observed, a light source, the observers eye and brain to form an image, vision also requires a level of attention by the observer. The eye and brain are not a video camera faithfully recording all visual data towards which aimed.

The inspector may fail to see objects even when they are present and readily apparent in the center of the visual field. Perhaps less difficult to grasp is research about forms of blindness or an inability to "see" - change blindness and inattentional blindness in particular. In the forming conscious visual information in the brain, inattentional blindness refers to an observers blindness to the presence of an object right in the visual field, even though the observer is awake and directing her eyes directly towards it.

Because "seeing" at a conscious level requires both the processing of visual signals from the eye and a processing of the input by the brain to form an "image," research has demonstrated that conscious "attention" is absolutely required to "see" anything. In other words, a person can be "looking" right at something, but if his attention is not focused there as well, he will not see it. Perhaps not at all. [1] The observer is likely to fail to see something significant, even in the center of the visual field, if s/he is not looking for that particular thing - if its not in the observers "attention set."

Inspector learn quickly the importance of a comprehensive visual scan of everything. An inspector wont "inspect" a room by only looking at one wall. But now we understand that merely directing the eyes at every surface may be quite insufficient.

What's tricky about this is that the inspector believes s/he has "looked-at" a surface, but may not have "attended" to it.

Photo Manipulation Experiment in Change Blindness in Home Inspection Reports

In a similar experiment I (D Friedman) provided a few clients with home inspection reports that included photographs of the homes we had inspected together. Despite spending three or four hours "looking" at a home, and in most cases despite multiple visits to the home by the clients, not one of the clients observed that the report included multiple inconsistent photographs of the front of the home.

In the first photograph I used photo-retouching software to add or delete windows from the front of the home. In the second photograph later in the report I used an un-edited photograph that accurately described the number of windows. Only when I asked clients to carefully compare the two photographs side by side did they report the difference in the number of windows presented.

The information that enters our eyes and is transmitted to our brains is not necessarily identical to the data that enters our consciousness. Experts explain that change blindness occurs at least in part because more information falls on our eyes than our brain can efficiently process. In other words, our brains discard much of the visual information that is available in order to avoid brain overload.

Keeping change blindness in mind and developing strategies for coping with this source of inaccuracy in visual awareness can help any inspector perform better.

Kristins Cards: A Demonstration of Inattentional Blindness

The ease with which IB occurs is nicely demonstrated by Kristins Cards at -- UPDTATE: regrets, this website is obsolete and I have not found a current version online. I'm working on reproducing this exhibition.

I previously left the explanation of the viewers inability to "see" something important to Kristen. Because that online display has been lost, below I will explain this illusion of inattentional blindness:

This "card trick" shows a set of cards, say by projection onto a movie screen, and invites the audience to remember them, and to select ONE of the cards to pay attention to. "Pick your card and watch it" is more or less the instruction.

Then this illusion changes and displays a "new" set of cards. The audience is asked if the card they were concentrating on has "disappeared". Everyone will pretty much say yes, their card has vanished.

What most audiences fail to detect and report is that all of the cards have changed between the two displays.

Church points out that while this is an entertaining illustration, Kristin's Cards may not be the best example of inattentional blindness because we are directing the audience to attend a particular card. Good inattentional blindness examples demonstrate that the observer(s) fail to notice changes, even substantial changes, in two successive images of the "same" scene.

Factors Affecting Inattentional Blindness

"Seeing" involves conscious mental effort and focus, a task from which observers can be distracted or which can fail due to IB. Four factors affect inattentional blindness: conspicuity [both sensory and cognitive], mental workload [too much or too little], expectation [attention set], and capacity. [Reference 5]

An inspector is more likely to see what s/he is looking for, and outside of that set, to see that which is familiar or large.

Home inspectors are not "seeing" everything that is present, even when its right in the center of their field of view.

An inspector is less likely to see something that s/he is not expecting, even if it is important and visually distinct by characteristics such as shape, color, or motion.

An inspectors "attention set" of expectations filters out other stimuli. An inspector is less likely to "see" a defect in a structure or system being examined if s/he does not already have an in-depth and visceral understanding of the construction and operation of the structure or system, leading to a broad and deep "attention set."

Change Blindness and its Effects on the Inspection of Complex Systems

Researchers in this area describes "change blindness" - the inability to "see" a component change between two rapid presentations of an image, and "inattention blindness [IB]which describes an inability to register and use visual information if attention is not properly focused. "Inattention blindness" is most pertinent to inspectors and recalls earlier work on the psychology of errors, and capture errors.

An interesting article on Change Blindness appeared in the New York Times reported on a talk by Jeremy Wolfe (Harvard Medical School, May 2008). Wolfe addressed a symposium, (held at the Italian academy for Advanced studies in America, at Columbia University) on the relationship between the themes of Art and Neuroscience, and he explained how often people "see" inaccurately. For inspectors of complex systems (home inspectors or even more lofty space shuttle inspectors) the significance of Wolfe's report was the underscoring of the "frequent inability of our visual system to detect alterations to something staring us straight in the face."

This visual error occurs not only in small subtle details but also in significant items. Angier's article reports that the audience failed to notice entire stories disappearing from buildings!

The Natalie Angier's article and Jeremy Wolfe's presentation emphasize that change blindness is an important component of the list of caveats that face any inspector who needs to form an rapid, accurate impression of the system she is inspecting.

Wolfe defines two classes of brain information processing that control our conscious attention to details among the soup of items that appear in the visual field:

Bottom-up attentiveness: something grabs our attention, such as a moving object or a bright color against a different background. These items are usually noticed consciously.

Top-down attentiveness: the viewer makes a conscious act to attend to something in the visual field even though it is not contrasted by conditions such as color or movement.

-- "Blind to Change, Even as It Stares Us in the Face", Natalie Angier, New York Times, p. F2, 1 April 2008

Improving Vision: Tuning Up Your Ability to See During an Inspection

Improving Top-Down Attentiveness to Items in the Visual Field

In addition to taking advantage of an understanding of the mechanisms that our eyes and brain use to record and attend information (bottom-up attentiveness and top-down attentiveness for example) there is another step we can take to improve our top-down attentiveness:

Better training in content or significance recognition: It is possible that we can train ourselves to better manage what information our brains discard and what information our brains bring to consciousness by tuning our attention to details that pertain to specific needs, and by broadening our training in specific fields of diagnostic inspection so that more visual data leaps to our attention because we have learned that that data may be important for the task at hand.

Better training in attention management: because research shows (and Wolfe reports) that the human brain can only attend well to a limited number of items at a time, even simplistic mechanical or rote inspection procedures that force the inspector to spend a portion of his/her time attending to specific questions might improve inspection performance. For example, let's not just "inspect the heating system" rather, let's "look for evidence that the chimney may be blocked, having been trained to identify a larger set of clues that can indicate that condition. (Back pressure burns on the boiler, odors, soot, stains in the living space, occupant health complaints, etc.)

A Checklist of Coping Strategies to Improve Visual Attention during Inspections


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