Depth perception

Our research looked at how people with Williams syndrome (WS) and typically developing children perceived and acted in depth using one or both eyes. Being able to perceive depth and make movements in depth are important skills for interacting and navigating within the environment, for example being able to cross a road and judge the distance of cars, to being able to reach for and pick up a mug. We have had lots of reports from parents and people with WS that understanding depth can be a problem, which was confirmed in a questionnaire study that Emily conducted a few years ago. Despite these skills being so important our understanding of depth perception in WS is very limited and to date only one study has been published in this area. 

Our most interesting results came from two tasks that looked at how well people with WS and typically developing children can see in 3-dimensions (3D) using two eyes, which is called stereopsis. Stereopsis relies on both eyes working well together, and so we used a test called the Frisby Stereotest (Near) to test this. In this test there are some shapes printed onto a clear acrylic sheet, in one area of the sheet a circle appears to pop out which can only be seen if you can see using stereopsis. When we use two eyes we can extract information about how far away objects are because of the difference in images that our eyes receive (because of their differing locations in our heads).  You can notice this difference if you stare at an object and alternate opening and closing each eye, you will see a slightly different view of the object from each eye. A 3D image is created by matching the features that are shared between the images from both eyes, so if both your eyes are not working well together you will not see the circle pop out in the Frisby Stereotest (Near).  By using the Frisby Stereotest (Near) we found that a third of people with WS could not see using stereopsis. In people that could see the circle pop out the ability to do this was at a level expected for a typically developing three year old, despite the average age of people in the group being nearly 27 years old. So, this is clear evidence that seeing in 3D using stereopsis is a real problem for people with WS.

We used a test called the Lang Two Pencil test to look at how people make movements in depth with one or both eyes open. You can try this simple test yourself- you will need two pencils, get someone to hold one pencil with the non-writing end pointing upwards arms length away from you and at eye level, then you take the other pencil and as quickly as you can with both eyes open try to touch the end of the other person’s pencil with the non-writing end of your pencil. You can get the other person to move their pencil around too, which makes the task harder for you to plan your movements.  Now, the task gets much harder if you do the same but with just one eye open- do you notice how much more difficult this is when you have to just rely on 2D information from one eye, you probably missed the pencil completely a number of times? Another version of this test is to have the pencil held horizontally. Completing this horizontal version of the test with both eyes open was harder for people with WS that could not see with stereopsis than people with WS that could see using stereopsis and typically developing children. We found a relationship in WS between how well people saw with stereopsis and how well they did at touching the end of the horizontal pencil with both eyes open. This means that this could be a useful test to identify people with WS that have problems with seeing in depth and that might need more detailed tests of stereopsis, like the Frisby Stereotest(Near).

So, this project was very helpful in getting a better understanding of depth perception in WS and identifying a new screening measure for depth perception ability in WS. We are very grateful to everyone that took part in this study and we hoped that you enjoyed taking part.

Funder

Autour des Williams