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Per c e p t ual Org an i za tion


Per c e p t ual Org an i za tion

by Megan Nadkarni


Visual Cognition Visual cognition can be broken down into 3 steps:

Step 1.

Basic Analysis, known as “bottom up processing�,

signals from the retina pass

through the visual cortex,

detecting raw information

such edges, lines, and shapes

Step 2.

Organization, by two different

kinds of grouping:

a) learned, previous knowledge

such as schema and context

b) assumptions made through

Gestalt principles

Step 3.

Meaning, achieved through

cognitive function, otherwise

known as top-down processing

during which the brain accesses

long-term memory and applies

previous knowledge


2 3

Perceptual Organization

Arriving at Meaning Through bottom-up and top-down processes, we convert basic visual information (green, sphere, etc.) into meaning (apple).

Bottom-up image processing begins when a patterned array of light from the environment strikes a complex network of cells that covers the inside back of the eye. This network of cells is called the retina and includes receptors that fire electrical signals in response to light.2 Electrical activity generated by the retinal image is initially registered in the cortical receiving area as edges, lines, and blobs. Collectively, these representations have been described as raw primal sketches.3 When the electrical activity reaches deeper levels of the visual cortex, it is combined with stored knowledge that helps humans interpret the ambiguous bottom-up visual information, represented as the electrical raw image information. By itself, bottom-up processing is not adequate because it does not contain enough detail and can often be ambiguous as to how it should be interpreted. Humans must combine the information that they have attained from bottom-up processing with prior knowledge about the visual world. 4,5,6 The application of this stored knowledge to the bottom-up stimulus information is known as top-down processing.


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4 5

Perceptual Organization

Types of Grouping Context and Schema One type of learned grouping is the use of context for image identification. When humans are familiar with the context of an image, they are often able to recognize this image at a faster rate (e.g., Palmer, 1975). For example, you might quickly recognize your dentist in his/her office but it might take longer if you see him/her at the mall. Another factor to consider is the schema that humans create about particular spatial scenes. A schema is a pattern of knowledge that describes what is typical in a particular situation.8 For example, all the associations connected to “mammals.� Context can in some ways be considered a type of schema (eg: things you find in an office setting). However, since these groupings are learned, it takes time for the brain to access long-term memory and categorize this way. Most of the time, this kind of grouping also requires a lot of localized information (details) which also takes time to process.


Mammals

Grouping by schema, we know that mammals are one category and amphibians another. Given the context of a forest, however, the giraffe would immediately look out of place.

Amphibian


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Perceptual Organization

Similarity

Proximity

Closure

Continuation

Figure-Ground (Pr채gnanz)

Gestalt Principles The brain is also able to group objects by making assumptions about a stimulus. These assumptions are described by Gestalt principles, namely similarity, proximity, closure, continuation, and figure ground (also known as Pr채gnanz). These all rely more heavily on the raw sketches from basic analysis and are processed relatively quickly. Gestalt principles are therefore useful for global grouping (broad grouping of information). The results of the reported experiments suggest that global, as compared to local, groupings are more accessible across stimulus durations. They also suggest that global groupings can be utilized faster than local groupings even in unlimited exposure times. So when Gestalt principles and learned grouping are integrated, we arrive at meaning more efficiently because the better we can organize at a broad level, the faster we can apply learned experience.


Color Similarity Color is our primary form of grouping so its particularly crucial that this is consistent with the most important information.

Proximity Blocks of text placed near each other group together. Similarly, a caption placed near the image it refers to groups with that image.

Continuation In typography, continuation translates to alignment. Indentation breaks up the implied line in a block of text, hence its use to indicate the grouping of thoughts into paragraphs. Other implied lines such as margins and gutters between columns also serve to group text.

Within Gestalt grouping, some kinds of grouping prevail over others. The earliest processed form of Gestalt is color similarity, followed by proximity and continuation. Other grouping mechanisms such as size and shape take longer to process. This makes exposure time an important factor in our use of grouping mechanisms. Therefore, the interactions between Gestalt grouping principles at different exposure levels are important considerations. In many cases, they are mistakenly used in opposition, giving rise to ambiguities. Designers should keep in mind that the amount of time the viewer spends looking at something is uncertain, which means grouping needs to be consistent at all levels.


8 9

Perceptual Organization

Exposure Time


Global & Local Information in Decision-Making Shorter duration exposures to stimuli may result in only gathering global information (shape, color) whereas if we had more time, we could process local information (details such as the time on the clock).

Whether we rely more on Gestalt principles or top-down grouping depends largely on the amount of time we are exposed to a visual stimulus. Depending on the exposure time, the ratio between using global and local information changes. If we have only a few fractions of a second, then we rely more on global information to identify an object (ie: basic characteristics like shape, color, etc.). Since top-down processing takes a little while to kick in, we don’t have enough time to access our memory and apply that knowledge to the situation. However, if we had longer or even unlimited exposure time, we would be able to use more local information to organize and understand the stimulus. But even in this case, the presence of clear, unambiguous global grouping would make the process faster.


10 11

Perceptual Organization

One of these things is not like the others. Which is the one that doesn’t belong? The dark disk, the higher disk, or the disk with the cat?

Depending on the goal, designers can choose to implement these principles for increased clarity to achieve meaning faster; or to exploit the brain’s reliance on grouping principles and subvert meaning by being ambiguous. However, situations arise where time is of the essence and its crucial that the viewer reach meaning as quickly and efficiently as possible. For example, in a pilot’s display, the goal is to compare current status measurements of both altitude and speed with a recommended number. The most important distinction is therefore current versus recommended. Though we can still complete the goal in the first display, the arrangement of the second is much more efficient.


Altitude

Speed

Status

Measure

Status

Measure

Current

1000 ft

Current

575 mpi

Recommended

1200 ft

Recommended

490 mpi

Though we can still decipher the charts, it takes time to decode the information they are intended to convey.

Altitude Speed

If the most important dichotomy (current and recommended) is globally grouped by color, information may be deciphered more quickly.

Current

Recommended

1000 ft

1200 ft

575 mpi

490 mpi


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Perceptual Organization


The Verdict on Ambiguity The London Underground map (top left) uses color as the primary way to distinguish different lines, which resonates well with passengers. The Spartan logo (bottom left) uses ambiguities of grouping by proximity and color in order to play off learned groupings (ie. Spartan helmets and common graphic representations of swinging motion).

Gestalt principles of organization are always at work—whether we incorporate them into design or not. Therefore, it’s important to consider these grouping qualities since they help simplify complex goals and add clarity to nebulous information. Perhaps most importantly, conflicting principles of grouping can lead to unintentional ambiguity. Applications of information design such as way-finding tools (top left) or the aforementioned pilot’s display require this efficiency. But ambiguity isn’t always bad. In logo design (bottom left), intentional ambiguity can be the contingent factor upon which the mark itself functions. Intentionality is the key.


About

References

This book was designed by Megan Nadkarni for Vision & The Brain

1. Schulz, Melissa F.; “Time Course of Perceptual Grouping in User Interface Displays�; May 2, 2004.

Fonts used: Interstate Light Vitesse Medium Illustrations from Florida Center for Instructional Technology at University of South Florida Printed on an IKON CPP 500

2. Goldstein, E. B. (1999). Sensation and perception. Pacific Grove, CA: Brooks/Cole Publishing Company. 3. Marr, D. (1982). Vision: A computational investigation into the human representation and processing of visual information. San Francisco: W. H. Freeman. 4. Biederman, I. (1987). Recognition-by-components: A theory of human image understanding. Psychological Review, 94, 115-147. 5. Peterson, M.A. (1994). Object recognition processes can and do operate before figure-ground organization. Current Directions in Psychological Science, 3, 105-111. 6. Shepard, R. (1983). Ecological constraints on internal representation: Resonant kinematics of perceiving, imagining, thinking, and dreaming. Psychological Review, 91, 417-447. 7. Palmer, S. E. (1975). The effects of contextual scenes on the identification of objects. Memory and Cognition, 3(5), 519-526. 8. Reisberg, D. (2001). Cognition: Exploring the science of the mind. (2nd edition). New York, NY: W.W. Norton & Company.


Perceptual Organization  

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