Our lab organizes various vision demos each year for the annual IISc Open Day (typically the first Saturday of March). Click on the links below to learn more. 

Open Day 2017

posted May 23, 2017, 10:50 PM by Georgin Jacob   [ updated May 24, 2017, 3:48 AM by SP Arun ]

We displayed three fascinating illusions during IISc Open Day 2017. 

1. Pulfrich Effect
This is an extremely simple illusion to try out at home. Take any swinging object moving in a single plane and look at it with dark glasses covering one eye but not the other. It will now be perceived to be moving in an elliptical path.  

Why does this work?
    This effect occurs because the visual system responds more quickly for bright targets compared to dim targets. As a result, in the covered eye, the perceived position lags a little behind. Differences in position between the eyes in corresponding retinal locations are interpreted in our brain as different depth, because that's exactly what happens if objects really are at different distances from us. 

2. Ambiguous Cylinder Illusion
In this illusion the same shape seen from one angle looks like a circle and like a square from another angle. This optical illusion was placed second at the 2016 edition of the annual Illusion of the Year contest. It gives us insights into the way in which human brain identifies the geometry of an object from the available cues. 

3. Hollow face illusion

When a hollow face is rotated in one direction we perceive only convex faces twice switching direction of rotation every half cycle. 

Human visual system interprets 3D objects from its 2D projection on retina. From the figure, we can see that there are two different faces having the same 2D projection. Since both concave and convex surface have the same 2D projection, our brain is confused between the convex and concave shape. Now brain tries to solve the problem by interpreting other cues like shadow, colour. Human brain has a specialized face processing area (Fusiform face area) which is specially trained for convex faces. This strong prior to expect a convex face overrides all other weak cues from disparity, shadow in this case and make us perceive convex face even when viewing the concave face.

Sources: Brain Facts | Michael Bach

Open Day 2016: The Incredible Shrink Machine

posted May 23, 2017, 12:26 AM by Georgin Jacob   [ updated May 23, 2017, 10:35 PM by SP Arun ]

Left: Special viewpoint. Right: The same setup from a generic view. 

This illusion reveals several fascinating insights into how the brain does vision. Basically the brain always assumes that you are looking at things from a very generic viewpoint. At most viewpoints, the chair looks like the image on the right - the base is twice as large as the frame and is placed far away from the camera location.

When you see this setup from a particular viewpoint (left), the brain assumes that the base (and the people on it) are actually at the same distance away as the chair frame. But then this automatically means that the base (and person) that is creating a small image on your retina is actually close by. This in turn means that the base and person are actually small objects in comparison to the person standing beside the frame (who is producing a big image). When people stand beside the base of the chair, they too get shrunk, which means that the brain assumes that the entire visual space surrounding the base is also located near the frame. 

The illusion tells us that estimating the actual size of an object is actually a difficult problem. This is because objects produce a large image when they are closer and a small image when they are far away. So how do you figure out the actual size of an object? One idea is that you compute relative size i.e. the size of one object relative to another. The precise computations by which the brain calculates the real-world size of an object are still unknown. 

This illusion ALSO tells us that the brain first calculates WHERE things are in the scene, and then only assigns their actual real-world size. Scene understanding precedes object understanding! 

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