Tactile perception demonstration: The “comb illusion”

At first, I was underwhelmed by this one.

comb, finger, and stick

Run the stick along the comb. With your eyes closed, notice the resulting percept: something moving down your finger.

Running an object (like a stick, or a pen) along the fine teeth of a comb, causes successive teeth of the comb to deflect laterally. There is no appreciable up-and-down component to the teeth’s movement, just the side-to-side movement. 

If you place your finger along the top of the comb’s teeth and close your eyes while you stroke the teeth with the stick, you’ll feel something running along your finger. I wouldn’t call this much of an illusion, so I didn’t think much of it at first. But Hayward & Cruz-Hernández (2000) pointed out that from this phenomenon, we can conclude your finger is able to sense lateral force, essentially skin stretch, not just indentation of the skin. The brain interprets the successive stretch signals as an object running down the finger. That stretch alone can result in these percepts is interesting.

There are several sorts of touch receptors in the skin. Most detect vibration or indentation. Stretch is detected by a rather distinct class, the Ruffini corpuscles, which are the specialized endings of nerves that carry the corpuscles’ signal to the spinal cord.

Now run the stick along the comb without your finger on top. Look carefully at the comb’s teeth. If you drag the stick along with some force, you’ll be able to see the teeth move a bit. But try running the stick along with very little force, at an acute angle so the side of the stick barely touches the comb, and runs smoothly. Make it nearly parallel to the plane of the comb, so it’s smooth enough that you don’t get the clicking sound and bump-bump vibration from successive teeth that occurs if the stick protrudes between the teeth. I noticed that when doing this, the deflection of the teeth is so small that looking directly at the ends of the teeth, I can’t see the deflection.

Yet if you put your finger again atop the comb, you should still have the sensation of something running along your finger. Hayward & Cruz-Hernández (2000) suggest that here the teeth move only by microns, and I believe them, as we ought to be able to see even a few microns of deflection when we look closely at the comb. Indeed, I suspect that the teeth may be moving less than a micron.

This shows how extraordinarily sensitive our sense of touch is- more spatially acute than vision. But note that the exquisite sensitivity of touch here comes from an ability that has no direct analogy in vision. In vision, when we think about spatial sensitivity, we think of acuity,

Spatial resolution in vision is limited by receptor spacing. Image: Gerald Westheimer.

Spatial resolution in vision is limited by receptor spacing. Image: Gerald Westheimer.

which is limited by receptor spacing (although hyperacuity makes it a bit more complicated than that). Vision has nothing like touch’s stretch receptors, which here are not limited by spatial spacing, but rather by the minimum force the Ruffini corpuscles can register.

Touch can also best visual acuity in another way. Receptors sensitive to ultra-high-frequency vibration allow us to discriminate a rough surface from a smooth one even when the roughness is created by features much smaller than a micrometer. For that phenomenon, I created a picture to put the spatial scales involved into perspective. I might explain it more another time. 


V. Hayward, M. Cruz-Hernandez, Tactile display device using distributed lateral skin stretch, in: Proceedings of the Haptic Interfaces for Virtual Environment and Teleoperator Systems Symposium, Vol. DSC-69-2, ASME, 2000, pp. 1309–1314. 

CHAST public science talks at University of Sydney

3 CHAST (www.chast.org) lectures:

Why we cannot make life

Professor Bert Meijer, Molecular Sciences, Organic Chemistry, Eindhoven University of Technology and 2011 Cornforth Foundation Lecturer, University of Sydney

Where: Old Geology Lecture Theatre, Edgeworth David Bldg, University of Sydney, map: http://bit.ly/hmm5U8

When: Wednesday 31 August, 6:30-7:30pm.   Free admission, no bookings.  All welcome.

Abstract: “The origin of life on earth” is without doubt one of the most intriguing scientific topics, while the wish to create life in a laboratory is amongst its most difficult challenges. The enormous progress in science and technology over the past decades has provided many deep insights into the miraculous composition and functioning of living systems. Today, on the one hand, we can clone sheep, grow organs from stem cells, while cells, plants, animals and bacteria have been genetically modified. On the other hand, the synthesis of small and large molecules has become so sophisticated that almost every molecule that exists on earth can now be made in a laboratory, including long strands of DNA, proteins and complex drugs that can cure diseases. These many insights, however, also show the complexity of the molecular biology of living cells. As a result, the astonishment about how life could ever have originated has further increased. The lecture will illustrate the greatest challenges that are encountered while seeking to understand the origin of life, including an explanation of why it will take a very, very long time before a living cell can be made in a laboratory out of its individual components, if it is possible at all. Special attention will be paid to the self-organization of complex molecular systems as a critical step in the building process.

E.W. “Bert” Meijer is Distinguished University Professor in the Molecular Sciences, Professor of Organic Chemistry at the Eindhoven University of Technology and scientific director of the Institute for Complex Molecular Systems. After receiving his PhD degree at the University of Groningen, he worked for 10 years in industry (Philips and DSM). In 1991 he was appointed in Eindhoven, while in the meantime he has held part-time positions in Nijmegen and Santa Barbara, CA. Bert Meijer is a member of many editorial advisory boards, including Chemical Communications, Angewandte Chemie, and the Journal of the American Chemical Society. Bert Meijer has received numerous awards, including the 1999 Silver Medal of the Macro UK group, the Spinoza Award in 2001, the ACS Award for Polymer Chemistry in 2006, the AkzoNobel Science Award 2010. He is a member of the Royal Netherlands Academy of Science.


Announcing a CHAST (www.chast.org) Lecture:

Numbers: Their Human Aspects. Perspective from Indigenous Cultures

Dr. Helen Verran, History and Philosophy of Science, University of Melbourne

Where: Old Geology Lecture Theatre, Edgeworth David Bldg, University of Sydney, map: http://bit.ly/hmm5U8

When: Tuesday 8 November, 5:30-6:30pm.   Free admission, no bookings.  All welcome.

Abstract: Many people spend a lot of time looking at numbers, or more to the point, looking through numbers at something else.  In this talk I take a look at numbers as such.  How can we ‘see’ numbers? And why would we want to? I will tell of the experience of working with teachers in primary school classrooms in Nigeria.  This had me recognizing that if we are going to understand how science might come to life as a significant cultural element in places like Nigeria we need a way to see the cultural lives that things like numbers have.  Having done some preliminary thinking with the help of Nigerian primary school children I turn to my experiences of working with Yolngu Aboriginal Australians who own lands in northeast Arnhem Land.  I will make a rather surprising analogy which I suggest can help us better understand the sorts of things numbers are.

Helen Verran is a Reader in History and Philosophy of Science at the University of Melbourne. She has a PhD in metabolic biochemistry. For most of the 1980s she worked as a science lecturer in the Institute for Education at Obafemi Awolowo University in Nigeria.  Her book Science and an African Logic (2001) was published out of this experience. Since she returned to Australia she has worked with Yolngu Aboriginal communities in northeast Arnhem Land an early product of this work was the small book Singing the Land Signing the Land now available on-line. http://singing.indigenousknowledge.org/ which provides background for her CHAST Lecture.


The 2011 Templeton Lecture (www.chast.org):

The Emotional Brain

Professor Joseph LeDoux, Center for Neural Science, New York University

Where: Eastern Avenue Auditorium, University of Sydney, map: http://bit.ly/qHUKUd

When: Monday 17 October, 6:00-7:30pm.   Free admission, no bookings.  All welcome.

Abstract: The study of emotion has been hampered by a fixation on feelings.  Feelings are important, but not all important.  Problems arise when we use feelings, and their semantic labels, as guides to studying brain function in other animals.  Rather than imposing concepts based on human introspective experience to the brains of other creatures, we should attempt to understand how the human brain is similar to the brains of other animals. This then becomes a foundation for understanding differences between humans and other animals. I propose that much of what is called emotion in studies of other animals is accounted for by the operation survival circuits, circuits involved in defense, energy/nutrition supplies, fluid balance, thermoregulation, and procreation. These circuits are highly conserved in mammals, including humans. While the behavioral expression of survival circuits can be species-specific, the circuits are species-general. Some other approaches also emphasize the adaptive function of emotions, but typically define emotions in terms of feelings. Survival functions are the real topic in most animal studies of emotion. By focusing on the adaptive function itself (rather than the behavioral expression or the conscious consequences) of survival circuits we have a way of characterizing phenomena that fall under the rubric of “emotion” in all mammals (perhaps all animals) without recourse to feelings. Feelings are what happens when consciousness witnesses the overall outcome (in the brain and body) of survival circuit activation. Feelings, which cannot be studied scientifically in non-human organisms, are neither necessary nor sufficient to understand survival circuits and their functions. By reorienting the comparative study of emotions around survival circuit functions, we have the opportunity to understand similarities and differences in emotional functions between humans and other animals.

“Visual Attention On the Go” seminar on Friday

My talk for the Sydney University psychology department will be at 4pm in the Education building, room 424. Below, the abstract:

Localizing a single object relative to oneself is fairly easy—ever seen a plant reaching towards the sun? It’s a no-brainer. A less trivial task is determining the position of two objects relative to each other. Humans evolved brains that can do it, probably because it’s important for survival, but we don’t know how we do it.  I asked some people to look at a stable scene and report which objects are adjacent. They said, “the red disc is next to the green disc” or “the red disc is next to the yellow disc”. Performance was essentially perfect. When the display started spinning, however, queer things started happening. These things suggest that apprehending the spatial relationship among objects requires a shift of attention from one object to the other. They also suggest that to perceive the spatial relationships among moving objects, the ability to follow an object with attention is critical[1]. The ability to follow an object with attention was tested by many previous investigators, who found that people can keep track of about four objects at once. But previous investigators never moved their objects as fast as we move ours. Our findings with speedy objects dispel previous theories of tracking and suggest that the faster an object moves, the more attentional resource it consumes[2]. Until nothing is left.

1. Holcombe, A., Linares, D., & Vaziri-Pashkam, M. (2011). Perceiving Spatial Relations via Attentional Tracking and Shifting. Current Biology, 21, 1-5. DOI: 10.1016/j.cub.2011.05.031

2. Holcombe, A.O., Chen, W.Y. (2011, submitted). Tracking a single fast-moving object exhausts attentional resources. (See the associated poster)

[Updated post with the time (4pm). Thanks Mat!]

Robert S. McNamara on nuclear weapons

“Is it right and proper that today there are 7500 offensive nuclear warheads of which 2500 are on fifteen-minute alert to be launched by the decision of one human being?””

“The lesson of the Cuban missile crisis is this: the indefinite combination of human fallibility and nuclear weapons will destroy nations.”

“Rational individuals came that close to total destruction of their societies. And that danger exists today.”

-Robert S. McNamara, former U.S. Secretary of Defense, The Fog of War (2003)

The surviving leader on the other side of the Cuban missile crisis, Fidel Castro, has also warned of the continued danger of nuclear weapons.

lecture on the alternative energy challenge

The information so beautifully presented in Saul Griffith’s  lecture is as important as that presented in any talk I’ve seen. Saul has calculated what it’ll take for the world to switch from fossil fuels to renewable energy. And what it takes for an individual to consume an environmentally responsible amount of energy.


Through CHAST, I had the privilege of organizing this lecture. Drop me a line if you want to be added to our email list for notification of future CHAST lectures at Sydney University.

Saul lecturing at Sydney University on 31 March 2010

optimizing your coffee consumption

We live in an era where students, shift workers, and scientists increasingly consume drugs that modify brain activity in order to enhance cognition. Ethicists are right to fret about this as the number of addictive substances with some ill effects proliferates (DeJong et al. 2008). People will use these things regardless whether or not some condemn the phenomenon, so it is important that information is out there about how best to use them.

Caffeine is probably the most widely-used drug for enhancing cognition and productivity. However despite its long history, I have not been able to find a good manual or user’s guide! By a manual, I just mean a description of on what kind of schedule it is best used, given caffeine’s tolerance profile, acute effects, withdrawal symptoms, etc. Here I’ll report a few things I found in the scientific literature, in relation to my own experience.

When I first drank coffee, the effects were perhaps too strong to help me much, because I got some ‘jitters’ and had trouble focussing. But as I gained a bit of tolerance to caffeine’s effects, the jitters faded and the arousal effect became milder but more conducive to productivity. This tendency has in fact been reported in the scientific literature, as a rapid tolerance selective to some negative effects even while positive effects can continue (Evans & Griffiths 1992; Schuh & Griffiths 1997). However after many months of judicious usage during which an afternoon coffee was effective in heightening and prolonging my workday productivity, I gradually became a daily user. After approximately a year of this, my tolerance to the arousal effects became great enough that I needed a daily coffee simply to feel normal. It still provided a boost, but only to what a year ago I would have considered baseline. In contrast to this important slow rise in tolerance, the academic literature focuses on the very rapid increase in tolerance during the first several days of caffeine consumption. Usually this is measured by decrease in effect of caffeine on blood pressure elevation. Not very useful for understanding how to best enhance cognition.

My situation, in which caffeine no longer had its productivity-boosting effects, must be a very common problem. To solve it, one might either increase one’s dosage, or try to regain the original effects by going off caffeine for a while. I decided to try the latter.

Arvind says that science indicates one can restore complete sensitivity to caffeine after only 5 days of abstinence (or 10 days of gradual abstinence), however I haven’t been able to find a study that documents this. A blood pressure study estimates that only 20 hours of abstinence (Shi et al. 1993) will restore total sensitivity to caffeine on the blood pressure response. But the subjective withdrawal effects don’t peak until nearly 48 hours of absence! Apparently, for different caffeine effects, different amounts of time are required to restore sensitivity. So what about the positive subjective and arousal effects the average person is most interested in?

I decided to go nearly cold turkey for 7 days, with only one or two decafs in that interval to blunt my withdrawal-effect blues. Fortunately, I had only mild headaches, but did have significant lethargy and loss of mental focus. After seven days, I think I have regained most of my caffeine sensitivity. But I’m only on day one of using again, so not certain how close I am to the sensitivity I had 6 months or a year ago. I hope to share Arvind’s experience of increased productivity for a long period before needing to abstain again to restore sensitivity.

Are there any scientific papers on the topic, or lacking that, further personal reports to certify that this works? I worry about chronic tolerance effects that might not dissipate even after prolonged abstinence, but haven’t seen a shred of relevant science. To bring us closer to having a real user’s manual for both caffeine and other cognitive enhancers, those already using should report the results of their self-experimentation!

DEJONGH, R., BOLT, I., SCHERMER, M., & OLIVIER, B. (2008). Botox for the brain: enhancement of cognition, mood and pro-social behavior and blunting of unwanted memories Neuroscience & Biobehavioral Reviews, 32 (4), 760-776 DOI: 10.1016/j.neubiorev.2007.12.001

Shi J, Benowitz NL, Denaro CP, & Sheiner LB (1993). Pharmacokinetic-pharmacodynamic modeling of caffeine: tolerance to pressor effects. Clinical pharmacology and therapeutics, 53 (1), 6-14 PMID: 8422743

Update: this post provides related info in the same spirit.