What Is Optic Flow? Why Is It Important for People with Low Vision?

Optometry and Vision Science cover

New research from Indiana University is investigating how “optic flow,” or setting objects and scenes in motion, can help people who have low vision to interpret and comprehend the blurred images they typically encounter in everyday life. According to the researchers, the concept of optic flow “has important implications for understanding the daily functioning of [persons] with low vision.”

The research, entitled With an Eye to Low Vision: Optic Flow Enables Perception Despite Image Blur, was published in the October 2013 issue of Optometry and Vision Science. The authors are Jing Samantha Pan and Geoffrey P. Bingham from Indiana University, Bloomington.

Optometry and Vision Science, the official publication of the American Academy of Optometry, publishes current developments in optometry, optics, and vision science, and promotes interdisciplinary exchange among optometrists and vision scientists worldwide.

About Low Vision

The term “low vision,” also sometimes called “partial sight,” can be defined in several ways:

  • Low vision is a condition caused by eye disease, in which visual acuity is 20/70 or poorer in the better-seeing eye and cannot be corrected or improved with regular eyeglasses.
  • Low vision is uncorrectable vision loss that interferes with daily activities. In many cases, low vision is better defined in terms of function, rather than numerical test results.
  • In other words, low vision is “not enough vision to do whatever it is you need to do,” which can vary from person to person.
  • Even with the help of regular glasses, contact lenses, medication, or surgery, a person with low vision might find it challenging to perform many types of everyday tasks, such as reading mail, shopping, preparing meals, using a computer, or seeing the television screen.

About Optic Flow and Low Vision

Here’s more information about optic flow from the study authors (via subscription only):

Although the importance of motion perception has been recognized by clinicians, it has not been included as part of low vision evaluation, due to the lack of feasible assessment techniques. We investigate how moving objects [and events] may be perceived with limited image information (as in low vision) and how objects may continue to be perceived after motion stops.

Events are omnipresent in everyday experience. We typically live in environments densely populated with objects. Those objects are frequently in motion as a result of their interaction with other objects, [persons], and animals. Thus, objects may be perceived based on their image structures (e.g., edges, contours, shades, and colors) or in the context of events.

First, objects in the world project images to the eyes. [These images] … are grouped into primitive image structures, such as edges, bars, and blobs. These primitive image structures allow us to perceive some properties of objects in the [surroundings], for example orientation and position, and these properties are integrated to yield spatial relations among objects in the [surroundings].

For an observer to perceive an object from a static image, the observer must detect variations of intensity in the image. Thus, if the intensity change becomes hard to detect, either as a result of weakened signals (e.g., in blurred images) or as a result of insensitive detectors (e.g., the observer having low vision), the scene becomes difficult to perceive using image-based information alone.

Second, when objects are perceived in the context of events, motion generates optical information that allows an observer to perceive events. Motions of the observer and/or objects in the environment yield continuous changes in the structure of light, described as optic flow. Optic flow is generated by relative motions between an observer and surrounding surfaces and objects … Motion-generated optic flow, once detected, enables event perception and consequently, the perception of constituent objects.

More about the Research

From Setting Blurred Images in Motion Improves Perception at Newswise.com:

Blurred images that are unidentifiable as still pictures become understandable once the images are set in motion. That’s because of a phenomenon called “optic flow” – which may be especially relevant as a source of visual information in people with low vision.

The researchers designed a study to assess the contributions of static images and optic flow to identifying events in the environment. They took short videos of everyday activities, such as a woman pouring a drink, a man bowling, and two people dancing. The black-and-white videos were then blurred – similar to what might be seen by a person with low vision – and split into 20 frames.

Volunteers with [vision within normal limits] were then presented with the blurred images and asked to describe what they saw. First, static images were presented one at a time. Next, the images were set in motion by playing the frames in sequence.

When viewing the blurred, static images, the volunteers were usually unable to perceive what was going on. They correctly identify the event pictured in less than 30 percent of attempts.

In contrast, when participants viewed the moving images, the rate of correct identification increased to nearly 90 percent. For an example, see the video illustration at Supplemental Digital Content. In the moving images, it’s much easier to perceive the dancing couple, despite the blurring.

Conclusions and Future Directions

The results illustrate the importance of optic flow as a source of information for perceiving everyday events. As summarized by the study authors,

…visual functioning need not be limited solely by the quality of image-based information. Motion-generated information is also of key importance and such information is relatively unaffected by significant degradation in the quality of the image.

This finding has important implications for understanding of the daily functioning of [persons] with low vision. In low vision, the detection of high spatial frequencies [i.e., high image resolution] in images is poor. As a result, low vision [persons] often are unable to recognize their surroundings based on image-based information alone when everything is stationary.

However, because … high image resolution is not required for the detection of motion and motion-generated information, persons with low vision might be able to perceive events in their surroundings using optic flow that is generated either by their own motion or the motions of objects in surrounding events.

VisionAware will continue to provide updates on this low vision research as they become available.

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