by Matthew Cobb

The Jastrow illusion blows my damn mind https://t.co/GPKy3EbkQP pic.twitter.com/cfKMRQCTE1

— Kyle Hill (@Sci_Phile) February 22, 2016

Click on the arrow to watch it work. According to Greg Mayer’s favourite source, Wikipedia, this illusion was discovered by the American psychologist Joseph Jastrow (1863-1944) in 1889, when he was only 26 (some sources say 1891; the date of publication was 1892…). Jastrow’s explanation was follows. I think in fact it describes rather than explains the illusion:

“The lower figure seems distinctly the larger, because its long side is brought into contrast with the shorter side of the other figure. … In judging areas we cannot avoid taking into account the lengths of the lines by which the areas are limited, and a contrast in the lengths of these is carried over to the comparision oft the areas. We judge relatively even when we most desire to judge absolutely.”

You can apparently get the original paper, free, from JSTOR (just accept their unonerous terms and conditions). In it, Jastrow describes a series of illusions, most of which had already been discovered, and provides explanations/descriptions of them. The ‘Jastrow illusion’ illustrated above was a development of this illusion, in which the lower parallelogram looks smaller than the upper (they are, of course, the same size):

Jastrow appears to have been quite the skeptic, spending a lot of his time both studying and debunking various forms of occultism and woo. Another section of the article in which Jastrow described these illusions is devoted to ‘a study of involuntary movements’ – slight movements of the hand that occurred while subjects were focusing on another task, such as looking at different colours, with the hand involuntarily following the movements of the eyes. This is quite dramatic in the case of someone counting the oscillation of a pendulum, as shown by this figure (the arrow denotes time – recording began at A and ended at Z and covered 80 seconds:

Although Jastrow does not say so, this provides a nice explanation for how Ouija boards can work, even if everyone is absolutely honest and not trying to move the glass (or whatever) in any direction. Unconscious effects will produce slight movements.

Jastrow’s article is quite a find for me, as another part deals with my professional area of study, which is the sense of smell. Although I primarily study maggots, I’m becoming increasingly interested in cases of anosmia (people who have lost their sense of smell, or who never had one) and of phantosmia (where you can smell things that aren’t there). Phantosmia in particular is intriguing, as it is a form of olfactory illusion. For many people it is very distressing (smelling faeces everywhere, for example), but has a primarily physical origin (to do with damage/malfunction of a subset of our 4 million or so smell cells). I have met phantosmics (that is the word) who could smell things they could not describe and had never smelt before; I had a slight insight into this over Xmas, when I had a nasty case of sinusitis and ended up smelling what I can only describe as weird smoke all the time. It wasn’t unpleasant, but it was weird. It went after a while.

Jastrow describes the case of ‘Mr. E’ a 21 year-old man who was apparently a congenital anosmic (his mother had a similar defect, although she once remembered smelling things). They carried out various tests on Mr E, who appears to have been completely anosmic, although he could respond to stimulation of the trigeminal nerve by very high concentrations of ammonia and similar substances. Above all, Jastrow looked at Mr E’s sense of taste and found that although he was unable to distinguish some tastes (eg tea and coffee), this was entirely because his sense of smell was affected. Jastrow’s conclusion was perceptive and entirely accurate:

‘The results conclusively show that a great many of the mouth-sensations, which we ordinarily speak of as tastes, are really due to smell.’

In other work, Jastrow studied the dreams of the blind, and interviewed many blind people about their dreams, including the amazing Helen Keller. Born in Poland, his family emigrated to Philadelphia when he was only 3 years old, and he spent most of his career at the University of Wisconsin-Madison. I can’t find any reference to a biography of him, which is a shame as he sounds very interesting. My copy of Edward G Boring’s History of Psychology is at home, so I can’t check him out in there. If any reader knows any more, please chip in below.

Jastrow appears to have invented the famous rabbit-duck illusion. At the end of 2014 The New Yorker published this excellent cartoon by Paul Noth. I wonder if Noth knew how much Jastrow would probably have liked it?