Preferential Looking Method


On this page, we explain the Preferential Looking Method (PLM) and various classic applications. In the second part, you will learn about PLM with the Lang-Stereopad for diagnosing stereoscopic vision: our innovation in the application of visual preference methods.

What is the Preferential Looking Method?

The Preferential Looking Method (Visual Preference Method) is a proven experimental procedure from visual perception research and developmental psychology.

The PLM was developed to study both the cognitive abilities and the attention and preference for visual stimuli in very young children, as well as in animals.

Since infants are unable to follow verbal instructions or provide verbal feedback, researchers use the natural drive of babies to focus their attention on things they find interesting or novel.

The PLM has many advantages. It is non-invasive, requires no verbal communication, and can be used to answer a variety of questions about cognitive development in very young age groups.

There are certain limitations, such as the possibility of misinterpretations. Nevertheless, experienced examiners achieve reliable results and, in case of doubt, make a correct interpretation through repetition of the examination.

How the preferential looking method works:

  1. Presentation: Two (or more) visual stimuli are presented to the infant at the same time. These stimuli can vary in complexity, colour, pattern or other visual characteristics. This can be done on screens, cards or by other means.

  2. Observation: An observer (or a camera) follows the infant's eye movements and gaze behaviour. The researcher notes which stimulus the child looks at for longer.

  3. Interpretation: Looking at one of the stimuli for longer (compared to the other) is interpreted as a sign that it is more interesting, surprising or different for the baby. From this preference, researchers can draw conclusions about what the child perceives, recognizes or favours.

  4. Extension: The recording of perception thresholds
    E.g. the testing of visual acuity or the stereo visual threshold

Classic examples of the application of the preferential looking method

The Preferential Looking Method has been used in numerous developmental psychology studies to explore different aspects of cognitive and perceptual development in infants and toddlers.

These applications of the PLM provide insights into the developmental stages of visual perception and cognitive processing in infants and toddlers. It is fascinating to see how quickly these abilities develop over the first months and years of life.

The PLM allows us to study this development in a non-invasive and child-friendly, playful manner.

The following 10 examples show the variety of questions that can be investigated with the Preferential Looking Method. PLM is a versatile tool that offers deep insights into the sensory and cognitive development of the youngest population groups.

►Language Discrimination: To determine if infants can distinguish their native language from other languages, they were presented with individuals speaking different languages simultaneously. Results showed that infants gazed longer at the screen with a person speaking their native language than at the monitor with a person speaking a foreign language.

►Language Development and Word Recognition: Toddlers are presented with the word for a specific object as an auditory stimulus. At the same time, multiple images are shown to them, including an image of the object just mentioned. If toddlers recognize and understand the word, they direct their gaze to the image of the mentioned object. This indicates whether children can associate the heard word with the corresponding object.

►Categorization: To test if infants can differentiate categories of images (e.g., animals versus vehicles), they were shown different groups of pictures. The results suggested that infants can recognize different categories of images.

►Social Preferences: To examine infants' social preferences, they were shown videos or images of individuals with different behaviors, ethnicities, or genders. These studies can provide insights into whether and how infants perceive social differences.

►Causal Perception: In some experiments, infants were shown a series of events to determine if they develop specific expectations about the physical properties of objects. For example, they might be surprised (and gaze longer) if an object appears to pass through a wall or floats in the air without visible support.

►Motion Perception: Patterns or objects in motion are presented alongside static patterns or objects. It is investigated whether the child perceives motion and whether they are more attracted to moving stimuli than static ones.

►Depth Perception: Studies on the "visual cliff" have shown that babies who can crawl avoid the deep side of the visual cliff (a simulated abyss), indicating a development of depth perception.

►Face Preference: A well-known example is the study of infants' preference for human faces over other visual stimuli. These experiments have shown that infants tend to look at human faces or face-like patterns longer than other visual patterns.

►Visual Acuity Testing: Visual acuity testing for children: Young children often cannot undergo traditional vision tests. That's why "Teller Acuity Cards" were developed, gray cards with variable black and white stripes and a solid-colored area. Narrower stripes indicate better visual acuity. Two images are presented to the child: one with a pattern and one without a visual stimulus. Children typically look at the patterned image. By reducing the details, the perception threshold is determined. This allows for child-friendly visual acuity testing. Early detection of vision problems can prevent permanent damage.

►Number Understanding: By showing groups of objects, researchers aimed to determine if infants have a rudimentary understanding of numbers. Some studies have shown that infants are surprised when the number of objects changes inexplicably between viewings.

The innovation: PLM and Lang-Stereopad®

The Preferential Looking Method for diagnosing stereoscopic vision

Preferential Looking Method with Lang-Stereopad®

The Lang-Stereopad represents a revolutionary innovation in the field of random dot stereo tests. Using a red test plate and six magnetic test cards, the child's spatial perception is examined without the need for special glasses, following the proven principle of the Lang-Stereotest.

The Preferential Looking Method (PLM) in combination with the Lang-Stereopad allows for the assessment of spatial visual ability in babies, toddlers, and individuals with speech impairments.

The examinee is presented with two random dot test cards of identical brightness, structure, and size. One of the cards displays a spatial figure. This card is preferentially viewed during presentation.

The test figure on the card is only visible when the lens grid is oriented vertically. When viewed with only one eye or with the card rotated 90 degrees, the test figure remains invisible.

The six test figures have horizontal disparities in the gradations of 1,000, 600, 400, 200, 100, and 50 arcseconds, allowing for the verification of the perception threshold.

With the Lang-Stereopad and PLM, professionals have a powerful tool to reliably test spatial visual ability, even in the youngest patients.

The advantages of the Lang-Stereopad in combination with the PLM lie primarily in the child-friendly examination:

  • Glasses-free: Unlike other stereo vision tests, the Lang-Stereopad does not require glasses. This makes the testing process easier, especially for babies and very young children.
  • Interactive elements: By rotating the test cards or the entire pad, as well as using popular test figures, the testing procedure becomes interesting and exciting for children.
  • Direct reactions: As many children not only look, but also point or tap on the test cards, this provides additional confirmation of their perception.
  • Flexibility: The six different magnetic cards allow for a variety of test combinations and the ability to explore the stereo threshold.

Procedure of PLM with the Lang-Stereopad®

  1. Preparation:

    • Familiarise yourself with the test method in advance.
      Detailed instructions are available.
    • Place two magnetic cards on the red test plate.
    • Turn one of the test cards by 90° so that the figure becomes invisible through the horizontal lenticular grid.
    • Check the correct visibility of one test figure in advance.
  2. Presentation:

    • Place the child frontally in front of you
    • Present the base plate with the 2 cards to the child at a reading distance of 25 to 40 cm.
    • Once the child has recognised the visible figure, turn the entire pad 90 degrees. The child should now recognise the second figure.
    • Repeat the process with cards of different gradations to determine the stereo vision threshold.
  3. Observation:

    • Track eye movements and gaze behaviour.
    • Pay attention to which card the child looks at, points at or taps for longer.
    • Note your results and the visible test figure used after each step.
  4. Interaction:

    • Encourage the child to point to recognised figures.
    • Instead of two, three test cards can be used at the same time. Random hits are reduced and the children's interest is increased.
  5. Evaluation:

    • Analyse the collected data to draw conclusions about the child's stereo vision ability.
    • If the child looks clearly and consistently at the test card with the 3D figure, it can be assumed that he/she perceives the figure and has stereo vision ability.
    • Inconclusive, searching or wandering gaze behaviour can be an indication that the child has difficulty recognising the spatial figure

This is the generally known procedure of the preferential looking method - applied here specifically to the examination of stereoscopic vision in babies, small children and speech-impaired people using the Lang-Stereopad.

Reasons for the Development of the Lang-Stereopad

Limitations of the Lang Stereotest I for PLM: In principle, two identical test cards from the Lang Stereotest I can be used for Preferential-Looking examinations, presenting one card in a horizontal and the other in a vertical orientation to the subject. However, this method has its limitations. A child might quickly ignore the vertical card since it doesn’t present 3D objects, making alternative test options non-existent and complicating valid interpretations.

Other Stereotests Unsuitable: Many stereo vision tests do not offer templates both with and without a stereoscopic stimulus, which makes them less than ideal for the Preferential Looking procedure.

The Inspiration for the Development of our innovative test system came primarily from the feedback of orthoptists: They desired a Random Dot Stereotest that does not require glasses. It should present various separate test objects in an unpredictable manner, suitable for examining preverbal children as well as individuals with speech barriers.
We embraced this request: Starting in 2016, we commenced the development of the innovative Lang-Stereopad, equipped with 6 square cards and test figures in 6 different disparities. It was launched in 2018 and received an extremely positive response.

Scientific Studies on PLM with the Lang-Stereopad

Two independently conducted scientific studies with the prototype of the Lang-Stereopad® confirmed that we met the expectations of orthoptists:

Dott. Andrea Piantanida 2017 (Preview as PDF download)
A comprehensive exploratory study with 240 normally sighted children from a large pediatric ophthalmology practice showed that children in the youngest age group of 6 to 12 months could consistently be tested with the Lang-Stereopad® using the Preferential Looking method, but not with any of the other conventional stereo vision tests.

M. Röthlisberger and A. Frick 2018 (See article on Wiley):
Using two test cards, one blinded, the second with the test object of the greatest cross-disparity (STAR 800'), an expected increase in stereopsis between the first measurement at 4 months and the second measurement at 7 months was demonstrated in a longitudinal study of 41 infants. The gaze behavior was documented through video recording and analyzed by independent observers who were unaware of the arrangement of the test cards (blinded card vs. card with stereo stimulus).

In practice, the superiority of our innovative stereo vision test in combination with the Preferential Looking method over other methods was confirmed in the first year of life.

The videos demonstrate the playful use of the Lang-Stereopad® during the paediatrician's examination. The children enjoy the examination and actively interact by tapping with their fingers. The girl was 4 years old, the boy 11 months old.

Product details: Detailed information on the Lang-Stereopad®

In the detailed product description, you will find:

  • Comprehensive product description
  • Test setup and objectives
  • PLM in preverbal toddlers
  • Testing the stereo vision threshold
  • Clinical studies on the testing procedure
  • Photo gallery with product images
  • Examination and application
  • Challenges and errors in examination
  • Technical specifications, care, and warnings
  • Download user manuals and presentations

► Lang-Stereopad Product Details

Do you have questions or need information?

We are happy to advise you.

Contact us 

Our website uses cookies to improve user experience. More information in our privacy policy.
Accept all cookies Necessary only