Month: July 2012

Three fun ways to have three hands – for you at home

Tired of having been born with only two hands?

Jealous of Indian goddesses?

Doubtful about Psychology and Neuroscience’s ability to replicate findings?

Then this set of exercises is for you. No need for any technical equipment. If all goes well you will grow* a hand as part of all this. You will have the strong feeling that you have three hands. You will feel and/or care about the illusory third limb. Let’s get started.
1) An additional cheeky hand
Equipment: none
Partners needed: 1
Time: 3 minutes
Success rate: 43% (70% report some sort of self-touch illusion)
Publication: Davies & White (2011)
a) Your partner and you place your hands on the same warm water bottle in order to have equally warm hands.
b) Sit down and close your eyes. Your partner sits opposite you. Her eyes are open.
c) Your partner takes your right hand and makes you stroke and tap yourself lightly on your right cheek. At the same time she herself administers synchronous, identical strokes and taps with her hand to the corresponding location on your face’s left side. Your own left hand simply rests.
d) Vary pressure and frequency of strokes and taps. Mind that on each side of the face timing and pressure have to match. Do so for three minutes.
Davies and White, 2011_cheeky hand illusion

Set-up to add a cheeky hand.

Outcome: The feeling that some sort of third hand, a disconnected one for example, strokes your left cheek.
2) Two rubber hands
Equipment: two rubber/plastic/wooden or otherwise somewhat realistic feeling right hands, a table, one double paint brush (see picture), one normal paint brush
Partners needed: 1
Time: 2 minutes
Success rate: not reported
Ehrsson, 2009_two rubber hands

Set-up to induce two rubber hand illusions simultaneously.

Publication: Ehrsson (2009)
a) Sit at a table. Place your right hand underneath the table, e.g., on your leg.
b) Place the rubber hand models of right hands in front of you over the area where your real right hand is. The models should be 10cm apart.
c) Look at the rubber hands. At the same time you partner uses the double paint brush to stroke the rubber hands and the single paint brush to stroke your real hand. These strokes need to be absolutely in synchrony.
Outcome: The feeling that both rubber hands are your right hands.
3) An additional rubber hand
Equipment: a rubber/plastic/wooden or otherwise somewhat realistic feeling right hand, a table, two paint brushes, a piece of cloth
Partners needed: 1
Time: unknown
Success rate: unknown
Publication: Guterstam, Petkova, & Ehrsson (2011)

Left: Set-up to add rubber hand to one’s own body. Right: Testing how real the ownership of the rubber hand really is (don’t do this test at home).

a) Sit at a table. Put your real right hand on the table in front of you.
b) Place the rubber hand in a similar position slightly to the left of your real hand, about 12cm apart. Cover the space from your real shoulder to the arm-bit of the rubber hand with the cloth.
c) Look at the rubber hand. At the same time your partner uses the two paint brushes to stroke both your real right hand and the rubber hand simultaneously on the index and middle fingers. She needs to do so absolutely synchronously, matching the strokes in time and speed. It is best to stroke irregularly but still synchronously.
Outcome: The feeling that both the real hand and the rubber hand are your right hands.
Did these illusions work for you? Let me know!
As a bonus for all those still in for some more, the following two techniques substitute your real hand for a fake one.
Bonus 1) A rubber hand (with vision)
Equipment: a rubber/plastic/wooden or otherwise somewhat realistic looking hand (it can be a bit bigger – but not smaller – than your real hand and it can also have a different ‘skin’ colour), two identical paint brushes, one standing screen (a big book would do as well), one table
Partners needed: 1
Time: 10 minutes
Success rate: 42%
Publication: Botvinick & Cohen (1998)
a) Sit at a table. Place the screen in front of you and hide your left hand behind it. Be sure you cannot see your left hand.
b) Place the rubber hand model of a left hand in front of you on the table.
c) Look at the rubber hand. At the same time your partner uses the two paint brushes to stroke both your hidden hand and the rubber hand simultaneously. She needs to do so absolutely synchronously, matching the strokes in time.
Outcome: The feeling that the rubber hand is your own hand.
Bonus 2) A rubber hand (without vision)
Equipment: a rubber/plastic/wooden or otherwise somewhat realistic feeling hand, three pairs of rubber gloves, a table
Partners needed: 1
Ehrsson et al., 2005: rubber hand illusion without vision

Top: Set-up for inducing rubber hand illusion without vision. Bottom: same in a MRI scanner.

Time:  60 seconds
Success rate: 78%
Publication: Ehrsson, Holmes, & Passingham (2005)
 a) Both you, your partner and the rubber hand need to wear rubber gloves.
b) Sit at a table. Place the rubber hand model of a right hand in front of you on the table. Close your eyes.
c) Your partner takes your left hand and makes you touch the rubber hand’s index finger’s knuckle. At the same time she herself administers synchronous, identical touches with her hand to the corresponding location on your own right index finger’s knuckle.
Outcome: The feeling that you are touching your own hand even though you are touching the rubber hand.
Finally, if you now wonder whether scientists have also found a way to make you lose a hand, just watch this video. Unfortunately, the techniqual requirements go beyond what is available in most homes and so your own private replication of this illusion will be rather difficult to implement.

Aimola Davies, A.M., & White, R.C. (2011). Touching my face with my supernumerary hand: A cheeky illusion Perception, 40, 1245-1247 DOI: 10.1068/p6956

Botvinick, M., & Cohen, J. (1998). Rubber hands ‘feel’ touch that eyes see. Nature, 391 (6669) PMID: 9486643

Ehrsson, H.H. (2009). How many arms make a pair? Perceptual illusion of having an additional limb. Perception, 38 (2), 310-312 PMID: 19400438

Ehrsson, H.H., Holmes, N.P., & Passingham, R.E. (2005). Touching a rubber hand: feeling of body ownership is associated with activity in multisensory brain areas. The Journal of neuroscience : the official journal of the Society for Neuroscience, 25 (45), 10564-10573 PMID: 16280594

Guterstam, A., Petkova, V.I., & Ehrsson, H.H. (2011). The illusion of owning a third arm. PloS one, 6 (2) PMID: 21383847


images: from respective journal publications

*in a psychological sense

Is it safe to talk while driving? – Partly depends on what you talk about.

World Health Organization reports about road safety are mind boggling: about 1.2 million people die on the world’s roads every year. For people of my age (15 to 29 year olds) it is the leading cause of death.

A rather recent addition to laws designed to reduce these numbers was the adoption of compulsory hands-free devices for mobile phones. Their safety value is easy to understand. When you look at a mobile phone display you cannot simultaneously look at the road. Similarly, using your hands for typing and using them for steering are at least partly incompatible actions.
mobile phone use while driving

How mobile phone use impairs sight and hands.

From a psychological point of view the current law tries to ensure that visual input channels (eyes) and motor output channels (hands) remain undisturbed. But what about the brain areas which control these channels?
This is the question recently investigated by Bergen from UC San Diego and colleagues. They put undergraduates in a driving simulator giving the impression of a motorway with steady traffic and a car in front of the driver breaking from time to time. Simultaneously, the driver had to judge simple true/false statements from the motor domain (e.g., “To open a jar, you turn the lid counterclockwise.”), the visual domain (e.g., “The letters on a stop sign are white.”), or the abstract domain (e.g., “The capital of North Dakota is Bismarck.”). As a baseline condition, people were just asked to say “true” or “false” several times.
Why choose such questions? There is both behavioural and brain-imaging evidence that language comprehension involves the simulation of what was said. This set of findings is often summarised as embodied cognition and its take-home message is something like this: in order to understand it, you mentally do it. For example, to answer a motor question, you use your brain areas doing motor control and make them simulate what it would be like to open a jar. Based on the outcome of this simulation you answer the question.
So, will visual or motor questions affect driving differently than abstract questions because the former engage the same brain areas as those needed for driving while the latter don’t? The alternative would be that asking anything distracts because general attention gets pulled away from driving.
The results go both ways. First, one measure was affected by the true/false statements but not by which kinds: quickly breaking when the car in front breaks. The time it took to do so was longer if any sort of question was asked compared to baseline. This suggests that domain general mechanisms were interfered with through language, e.g., attention.
Liza minelli driving

Was she a safe driver? May depend on whether she talked and if so about what.

Second, one measure was affected by what kind of statements had to be judged:generally holding a safe distance to other cars. This distance was greater if visual questions were asked compared to abstract questions and compared to baseline. A similar, albeit not as clear, pattern emerged for motor questions. It looks as if participants were so distracted by these kinds of questions that they fell behind their optimal driving distance. This suggests that a task such as keeping a safe driving distance which requires visual working memory (compare ideal distance to actual distance) and corrective motor responses (bring ideal and actual distances closer together) is influenced by language comprehension through mental simulation.
On the one hand, the scientific implications are quite straight forward. Bergen and colleague’s results suggest that those low level perception and action control areas which are needed for quick reactions are not what embodied cognition is about. Instead it seems like embodied cognition happens in higher perceptual and motor planning areas. Furthermore, the whole embodied cognition idea gets quite a boost from a conceptual replication under relatively realistic conditions.
On the other hand, the practical implications are somewhat controversial. Because talking in general impairs quick reactions by the driver, even hands-free devices pose a risk. This danger is compounded by talking about abstract topics since the driving distance is reduced compared to visual topics.
The authors refrain from saying that any sort of conversation should be prohibited. Passengers share perceptual experiences with the driver and can adjust their conversations to the dangerousness of the situation. Mobile phone contacts can’t do this. But what if you want to be really really safe? Well, cut your own risk of dying and take public transport. There you can chat and cut your death risk by 90% (bus) or even 95% (train or flight) compared to car travel (EU numbers).
London bus

A safe way to travel.


Bergen, B., Medeiros-Ward, N., Wheeler, K., Drews, F., & Strayer, D. (2012). The Crosstalk Hypothesis: Why Language Interferes With Driving. Journal of experimental psychology. General PMID: 22612769



1) By Ed Brown as Edbrown05 (Own work) [CC-BY-SA-2.5 (], via Wikimedia Commons

2) By Alan Light (Flickr) [CC-BY-2.0 (, via Wikimedia Commons

3) By Original author was User:Kameragrl at Wikitravel Shared, transferred to Commons by User:Oxyman ( [CC-BY-SA-1.0 (, via Wikimedia Commons

Psychological principles as guidelines for effective PowerPoint presentations

A presentation using Powerpoint. Corporate pre...

How good can it get?

You probably wouldn’t have much difficulty if I asked you to imagine a bad PowerPoint presentation. Nowadays one sits through so many of them that confusing, boring or annoying slide shows are sometimes perceived as the norm rather than the exception. A research team from the universities of Stanford, Amsterdam and Harvard headed by Stephen Kosslyn explains how to do it better. In order to reap off the benefits and avoid the pitfalls of visual aids, presenters should think about avoiding weaknesses of human information processing and play on the strengths of such processing.

Kosslyn and colleagues see the task of the audience viewing a PowerPoint presentation as composed of three steps: a) information needs to be acquired, b) information needs to be processed, c) information needs to be connected to knowledge. They derive eight principles that a presenter should follow based on this analysis.
a) encoding, i.e. acquiring information and turning it into a usable form
1) Discriminability: make it easy for the audience to discriminate colours, letters, sizes, line orientations etc.
2) Perceptual Organisation: group things effectively in the visual space you’ve got
3) Salience: use large perceptual differences to guide attention to what is IMPORTANT
b) working memory: holding information in mind in order to integrate it online
4) Limited Capacity: understanding breaks down once too much information has to be retained
5) Informative Change: when something perceptual changes, this change has to mean something
c) accessing long term memory: connect the new information with knowledge in order to extract meaning.
6) Appropriate Knowledge: avoid as much novel concepts, jargon or symbols as possible
7) Compatibility: the meaning of a message needs to be compatible with its form
8) Relevance: provide neither too much nor too little information
These principles may look very obvious but they are frequently violated. From an internet sample of slide shows it became clear that on average a PowerPoint presentation violates six principles at least once. Some principles were nearly always ignored: 1) discriminability, 4) limited capacity, 5) informative change.
Now, one may argue that these principles are simply guidelines that lay people are unaware of. No wonder they get violated. However, in a subsequent laboratory experiment participants were 80% correct in choosing a non-violating slide and rejecting a bad one. Moreover, when asked to say why one slide was better, more than 80% of the correct choices were appropriately justified.
So, this study is about what one already knows but still ignores when designing a slide show. The authors use a backdrop of psychological literature to predict what sorts of principles should guide PowerPoint presentations. What they, unfortunately, fail to do is to empirically test each principle’s impact on presentation understanding and memory. As such, this study simply presents a set of guidelines, says that presentations usually violate guidelines and that most people are aware of these violations. How important the guidelines are to begin with remains unclear.
The main take-home message is that the more work a presenter does for his/her audience, the more the audience can tune into the content of the presentation. For my part I am always guided by a more memorable principle:
Look around the room and search for the newbie or the bored one or the least intelligent listener. S/he is your target audience.
For a complete list of useful rules which may help you and especially your audience, see the appendix of Kosslyn and colleagues’ paper.

Kosslyn S.M., Kievit R.A., Russell A.G., & Shephard J.M. (2012). PowerPoint® Presentation Flaws and Failures: A Psychological Analysis Front. Psychology, 3 DOI: 10.3389/fpsyg.2012.00230



1) Photo credit: Wikipedia

Neuroscience is Forgetting

The scientific enterprise, like anything else humans are doing, is subject to fashions, trends and hypes. Old ideas get replaced by new ones; good new ideas spread; progress is made. However, is Neuroscience forgetting where it came from?

This is not that hard to investigate in a quantitative way because every source that is cited by an article is referenced. Below, I plot the ages of all references found in reviews and review-like articles (opinions, perspectives…) published in a well known Neuroscience journal (Nature Reviews Neuroscience). As you can see, the relative majority of articles which a review cites are very very recent, i.e. less than two years old. Twenty year old sources hardly every get mentioned.
Forgetting Rate in Nature Reviews Neuroscience
One could argue that the pattern seen above – the very fast rate of decline which levels off near zero – is simply a reflection of the publication rate in Neuroscience. If more is published, more can get referenced. However, compare the above plot with the one below, which shows the publication rate in the Neurosciences from 2011 all the way back to 1975. An increase in publication rate is there but it is not nearly as quick as the aforementioned graph would have you believe.
Neuroscience Publication Rate
Something else is happening.
Notice the white bars in Figure 2. These show the overall number of reviews in Neuroscience. While in 1975 only 46 Neuroscience reviews got published, in 2011 it was nearly 3000. Given that it is impossible to read anything like 40000 original articles each year, it makes sense to only read the summaries. Perhaps, once summarised, an original article’s gist survives in the form of a brief mention in a review while its details are simply forgotten.
Another possibility is the decline effect I blogged about before. Old articles which proved unreliable should be forgotten in order to edge closer to some scientific truth. They may have suffered from publication bias, sub-optimal techniques and/or sampling bias.
So, Neuroscience is indeed forgetting. However, whether this is all that bad is another question.


1) Data processed with own code. Original data from NRN website.
2) Data from Web of Science.

How are invisible colours possible?


Pharell Williams: ‘I could always visualize what I was hearing… Yeah, it was always like weird colors.’

To some people I am half colour blind even though I can see everything from blue to red like most people. For them it is odd that I can only see colours when they are directly presented to me. More than that, I can only see colours for which there are proper words. These people literally see black and white symbols in colour depending on which symbol it is. And the colours they see are sometimes those which I will never see in my life because they are invisible. Incredible? A journey through the visual brain shows how this feat is possible.

The first report of invisible colour perception came from Ramachandran and Hubbard (2001), then at San Diego. They briefly mentioned a man only known as S.S. who suffers from s-cone weakness, i.e. the cells in his eyes which are sensitive to blue are impaired. Therefore, he cannot see the full colour spectrum the way most people can.
S.S. also happens to be a grapheme-colour synaesthete, i.e. he literally sees a certain colour consistently when presented with, for example, a given number. However, these number-evoked colours were special. He described them as ‘weird’ or ‘extraspectral’. He had only ever seen them in his mind’s eye, never in the outside world.
Rather than being an isolated case, regular synaesthetes can see these so called ‘Martian’ colours as well  (Ramachandran & Hubbard, 2003). Furthermore, Oliver Sacks writes about a musician he calls Michael whose synaesthesia links musical keys with colours. Sacks writes that ‘some keys seem to have a strange hue which he can hardly describe, and which he has almost never seen in the world about him’ (2007, p. 182). Pharell Williams could be yet another case.
How can we make sense of this? To understand how the human brain can give rise to Martian colour perception, a quick tour through the visual brain is necessary. When light hits the eye, it is transformed into the electrochemical information currency of the brain. The information travels from the eyes directly to the back of the brain within about 100 milliseconds (a tenth of a second). Because this area in the back of the head is so well understood to be the primary target for visual information, it is simply called the primary visual cortex or V1 for short.
visual pathway

Note how information travels from the front to the back.

After some low level information processing in V1, information is passed on to other visual areas which are specialized in certain jobs, e.g. V5 processes motion, V4 processes colour, and grapheme areas process numbers and letters.
The brain’s left hemisphere. Grapheme area in green. Colour area in red.
When looking at where these different areas lie, one gets a sense for why motion-colour synaesthesia is not found while word/number-colour synaesthesia is so common. The human brain happens to be organised in such a way that V4 (colour) lies very close to the grapheme area. V5, on the other hand, is a lot farther. Research in the last decade (reviewed in Hubbard et al., 2011) has revealed that in synaesthetes colour and grapheme areas are unusually well connected and they show activation of the colour area just after the grapheme area responds when synaesthetes view graphemes.
Martian colours are thought to be so unusual because information has not taken the usual route (eye -> V1 -> V4) but instead went to a grapheme area first and then entered V4 (eye -> V1 -> grapheme area -> V4). Similarly, musically induced Martian colours may look so weird because auditory information recoded in V4 simply isn’t of the same quality as visual information which comes from V1.
So, colour perception appears not only determined by where information is processed but also by where it originates. A good lesson to remember whenever you try to localise function X in brain area Y: information origin matters.
Some people out there see things which are so unusual that there isn’t even a proper word for these experiences. To them I am not only half-colour blind (even though I do not suffer from s-cone weakness like S.S.). I am also unimaginative – as not just my eyes but also my imagination is limited to the colours of the rainbow, a subset of the colours which can be experienced. If you believe that the rainbow is complete, you may well be as colour blind as I am.

Hubbard, E.M., Brang, D., & Ramachandran, V.S. (2011). The cross-activation theory at 10 Journal of Neuropsychology, 5, 152-177 DOI: 10.1111/j.1748-6653.2011.02014.x
Ramachandran, V.S., & Hubbard, E.M. (2001). Psychophysical investigations into the neural basis of synaesthesia Proceedings of the Royal Society B, 268, 979-983 DOI: 10.1098/rspb.2000.1576

Ramachandran, V.S., Hubbard, E.M. (2003). The phenomenology of synaesthesia. Journal of Consciousness Studies, 10, 49-57.

Sacks, O. (2007). Musicophilia. New York: Vintage.

1) By kallerna (Own work) [CC-BY-SA-3.0 ( or GFDL (], via Wikimedia Commons
2) By Wiley (Wikimedia) [CC-BY-SA-3.0 (, via Wikimedia Commons
3) from Ramachandran, V.S., & Hubbard, E.M. (2001). Psychophysical investigations into the neural basis of synaesthesia. Proceedings of the Royal Society of London B, 268, p. 981.