• Question: can you explain how illushion works and what makes you think of what is in your head

    Asked by wilkycfc to Anne, Carolyn, Joe, Mariana, Nick on 25 Mar 2010 in Categories: .
    • Photo: Carolyn McGettigan

      Carolyn McGettigan answered on 25 Mar 2010:


      Illusions are the properties of perception, so experiences where what we think we see or hear is not actually what is there in the world. Lots of percepts are like this, even things you wouldn’t expect, like how we hear loudness and pitch in sound. Also how we make sense of our visual world – we don’t just see colours and edges floating in space, we see real objects, and this aspect of our perception helps us make sense of our environment. Illusions are due to processes that happen in our brains rather than in the eye or the ear. There are powerful examples where these usually helpful perceptual processes lead to confusion – for example, if you watch a waterfall closely for a few minutes, then look at the stationary river bank, you might for a few seconds perceive that the bank is somehow moving upwards. That’s because the cells in visual cortex in the brain that detect downward movement have adapted and decrease their activity through watching the downward-falling water for so long. This adaptation disrupts the balance between ‘downward’ and ‘upward’ cells’ activity and hence leads to the percept of things moving up when they are actually stationary.

    • Photo: Joseph Devlin

      Joseph Devlin answered on 25 Mar 2010:


      A very good question! This is another one that is a standard topic in undergrad psychology courses and really needs much more space to do it justice… I’ll give it a shot though.

      The secret is that our brains don’t perceive the world as it is — it’s more like as we expect it to be. So we have tons of information about normally daily life and how things work that we bring to bear whenever we perceive something. If these rules-of-thumb are violated, then we misperceive things and that’s all an illusion is.

      So the classic movie one has to do with size. You know how big things normally are so if you see things like a person next to a very large wagon wheel without any cue that the wheel is extra large, it looks as if the person is small (think hobbits in Lord of the Rings). The image itself is ambiguous, so your brain applies extra information that nearly always helps. In this specific case, though, it doesn’t help — it leads to an illusion.

Comments