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Visual Strategy
by Juan C. D√ľrsteler [message nļ 178]

Visual strategy, understood as the coordination of eye and head movements in order to perform a visual task when looking at our environment, turns out to be a feature as personal as the way we walk or write. Information visualisation enables the creation of a visual map that makes the way we see easily understandable.
s7_p1.jpg (34526 bytes) s11_p1.jpg (45157 bytes)
Visual Map¬ģ of two different individuals performing the same task. The map to the left corresponds to a person that moves his eyes slightly and keeps them placed near the centre of his field of view. He mainly moves his head when scanning the world surrounding him.¬†
On the contrary the map to the right reveals a person that moves his head very little since he uses predominantly the movement of the eyes as a way to scan the world. Notice that the redder the colour, the higher the frequency of use. Different people produce different maps. 
Source:  Courtesy of Industrias de Optica S.A.
Click on the images to enlarge them.

Have you ever wondered whether the way we move our eyes and head when looking at our environment is the same for everybody or everyone has his/her own way to see, his/her own visual strategy?

This was the question that the R&D dept. of the lens division of Industrias de Optica S.A. (INDO), a Spanish  company devoted to ophthalmic optics that actively does research on Progressive Addition Lenses* (PALs), when they began to perform eye and head tracking tests trying to find mobility patterns in the coordination of eye and head of PAL users. The goal was to find clues that led to the personalisation of PALs. 

The users performed visual tasks with targets placed at different distances, from infinity up to reading distance. A Polhemus Fastrak electromagnetic system registered the head and chest while an ASL 504 infrared camera followed the line of sight of their eyes.  From the analysis of the data consisting of coordinates and rotation angles of head and eyes that the system produces you can derive a lot of information, such as velocities, accelerations fixation points and many others.

Nevertheless the graphical representation of the frequency of use of the visual space that got the name of Visual Map¬ģ, resulted specially clarifying in terms of the patterns people use to approach the world they are looking at.¬†

Effectively, if we represent in the abscissa axis the horizontal angle of rotation of the eye, and its vertical equivalent in the ordinates axis, assigning to each couple of angles i.e. each possible gaze direction in the visual space, a colour representing the frequency of use of this particular part of the space, we end up with a map that allows us to quickly understand the visual strategy of a particular person.

Once you know the power of the visual map, many questions arise, the first of them being; to what extent is the visual map an intrinsic feature of the person as their fingerprints are?.

The results of the studies carried out jointly by INDO and the Instituto de Biomec√°nica de Valencia (IBV) indicate that this is the case. Not only are the maps very repetitive for each person, within the logical variations of any biometric measurement, but there is a large variety of shapes. This fact allowed INDO to create a customised progressive addition lens taking the visual map as input for the personalisation. This PAL has been in the Spanish market since last September.

INDO_VisualMap_Developer.jpg (39321 bytes) Diadema.jpg (262370 bytes)
Visual Map Developer, measuring system of the Visual Map¬ģ
Source:  Courtesy of Industrias de Optica S.A.
Click on the image to enlarge it.
The user wears a wireless diadem
Source:  Courtesy of Industrias de Optica S.A.
Click on the image to enlarge it.

In order to produce a visual map in the optometrist's or in the ophthalmologists' shop it was necessary to create a compact version of the research lab system that could provide in a quick, comfortable and non invasive way the map of the visual strategy, key element for the optimisation of the progressive addition lens surface. 

The result, again a collaboration of the two institutions (INDO & IBV), has been a compact measuring machine that is made up of two screens that show the user a luminous stimulus moving through them. Each screen stimulates the visual system around two basic ergonomic postures:

  • the natural posture where the head stands up looking around the horizontal line of sight

  • the flexion posture, that we use when looking at our inferior field, for example for reading

A couple of digital stereoscopic cameras perform the following of the movements of head and eyes, with the help of a wireless diadem the simplifies the image treatment algorithms. The process is controlled by means of a touch screen that shows the software controlling the test, which consists of four simple stages:

  • Placing the user within the optimum field of vision of the cameras. The system detects the position of the diadem, suggesting to the operator the movements to perform in order to correctly place the user within the cameras optimum field with enough tolerance to ensure the validity of the measurement.¬†

    Situacion1.jpg (31498 bytes) Situacion2.jpg (33772 bytes)
    Placement of the user: in this case the user is outside the optimum field. He/she has to move in the direction indicated by the arrows. 
    Source:  Courtesy of Industrias de Optica S.A.
    Click on the image to enlarge it.
    Placement of the user: in this case the placement is correct and we can proceed to the next step of the test
    Source:  Courtesy of Industrias de Optica S.A.
    Click on the image to enlarge it.
  • Checking the situaction of the pupils and the readiness of the diadem. Once the user is properly placed we get a photograph from each of the cameras that helps us to locate the pupils and check the proper function of the LEDs of the diadem.

    CheckVMD.jpg (34236 bytes)
    Checking the location of the pupils and the readiness of the diadem.
    Source:  Courtesy of Industrias de Optica S.A.
    Click on the image to enlarge it.
  • Carrying out the test. A luminous stimulus crosses the two screens stimulating all the user's visual field in a homogenous way. The user only has to follow the stimulus with their eyes.

    IniTestVMD.jpg (29364 bytes) TestVMD.jpg (18654 bytes)
    Begining the test. Once everything is OK you get the beginning screen. 
    Source:  Courtesy of Industrias de Optica S.A.
    Click on the image to enlarge it.
    Performance of test. The user has to follow with the gaze the luminous stimulus.
    Source:  Courtesy of Industrias de Optica S.A.
    Click on the image to enlarge it.
  • Production of the visual map. Once the test is finished, after approximately two minutes, you get the map of the visual strategy, that is sent along with the users clinical prescription to the factory for its conversion into a personalised PAL.¬†
    MapaVMD.jpg (36791 bytes)
    Production of the visual map, after a few seconds from the end of the test.
    Source:  Courtesy of Industrias de Optica S.A.
    Click on the image to enlarge it.

it's most interesting to stress the fact that, obviously, what is used to compute the design of the lens is not the map itself but the biomechanic parameters that follow from the treatment of its information. Nevertheless, the visual map allows both practitioners and users alike to understand very quickly the particularities of the user's way of gazing.

Most probably each one of us leaves our stamp on everything we do, from the way we walk and wear our shoes up to our visual strategy. During World War II the intelligence services were able to identify individual Morse code operators by the way and the rhythm each one used in their transmissions. This was called  recognising the "fist of the sender" and it allowed them to detect the movement of troops just by knowing from where a particular operator was transmitting.  (See the article What You Know, What You Have, What You Are in the December issue of Health Management Technology). A variant of this is the BioPassword system that allows the computer to authenticate the password by analysing the keystroke dynamics. 

There are several eye-tracking systems like those of Tobii or the ones from Eyetracking Inc. Its main objective is to know where a person is looking to increase usability or to study how the gaze wanders through a particular scene. The study of visual strategy creates many new unknowns that go beyond mere biometry or usability studies since it probably affects our cognitive patterns that maybe is different for different activities. In any case the above mentioned studies imply that it is a personal stamp.  Information visualisation plays an important role in the detection of those patterns. 

* Progressive addition lenses aim to correct farsightedness, technically known as presbyopia. This conditions appears physiologically around the age of 45  and progressively impairs the focusing of objects up close. Progressive addition lenses bend their surface in such a way that it offers the optical power needed for any distance which the user is looking at. Other, less sophisticated solutions are the reading spectacles and bifocals.

Links of this issue:

http://www.indo.es   INDO website
http://www.polhemus.com/fastrak.htm   Fastrak by Polhemus
http://www.a-s-l.com/504_home.htm   ASL Model 504
http://www.ibv.org/   IBV website
http://www.healthmgttech.com/archives/1205/1205what_you.htm   Article about biometry
http://www.biopassword.com/bp2/welcome.asp   BioPassword website
http://www.tobii.com/   The Tobii eyetracker
http://www.eyetracking.com   Eyetracking.com
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