by Jalees Rehman
A child drops a chocolate chip cookie on the floor, immediately picks it up, looks quizzically at a parental eye-witness and proceeds to munch on it after receiving an approving nod. This is one of the versions of the “three second rule”, which suggests that food can be safely consumed if it has had less than three seconds contact with the floor. There is really no scientific basis for this legend, because noxious chemicals or microbial flora do not bide their time, counting “One one thousand, two one thousand, three one thousand,…” before they latch on to a chocolate chip cookie. Food will likely accumulate more bacteria, the longer it is in contact with the floor, but I am not aware of any rigorous scientific study that has measured the impact of food-floor intercourse on a second-to-second basis and identified three seconds as a critical temporal threshold. Basketball connoisseurs occasionally argue about a very different version of the “three second rule”, and the Urban Dictionary provides us with yet another set of definitions for the “three second rule”, such as the time after which one loses a vacated seat in a public setting. I was not aware of any of these “three second rule” versions until I moved to the USA, but I had come across the elusive “three seconds” time interval in a rather different context when I worked at the Institute of Medical Psychology in Munich: Stimuli or signals that occur within an interval of up to three seconds are processed and integrated by our brain into a “subjective present”.
I joined the Institute of Medical Psychology at the University of Munich as a research student in 1992 primarily because of my mentor Till Roenneberg. His intellect, charm and infectious enthusiasm were simply irresistible. I scrapped all my plans to work on HIV, cancer or cardiovascular disease and instead began researching the internal clock of marine algae in Till's laboratory – in an Institute of Medical Psychology. Within weeks of working at the institute, I realized how fortunate I was. Ernst Pöppel, one of Germany's leading neuroscientists and the director of the institute, had created a multidisciplinary research heaven. Ernst assembled a team of remarkably diverse researchers who studied neurobiology, psychology, linguistics, mathematics, philosophy, endocrinology, cell physiology, marine biology, computer science, ecology – all on the same floor. Since I left the institute nearly 20 years ago, I have worked in many academic departments at various institutions, each claiming to value multidisciplinary studies, but I have never again encountered any place that has been able to successfully integrate natural sciences, social sciences and the humanities in the same way as the Munich institute.
The central, unifying theme of the institute was time. Not physical time, but biological and psychological time. How does our brain perceive physical time? What is the structure of perceived time? What regulates biological oscillations in humans, animals and even algae? Can environmental cues modify temporal perception? The close proximity of so many disciplines made for fascinating coffee-break discussions, forcing us to re-evaluate our own research findings in the light of the discoveries made in neighboring labs and inspired us to become more creative in our experimental design.
Some of the most interesting discussions I remember revolved around the concept of the subjective present, i.e. the question of what it is that we perceive as the “now“. Our brain continuously receives input from our senses, such as images we see, sounds we hear or sensations of touch. For our brain to process these stimuli appropriately, it creates a temporal structure so that it can tell apart preceding stimuli from subsequent stimuli. But the brain not only assigns a temporal order to the stimuli, it also integrates them and conveys to us a sense of the subjective past and the subjective present. We often use vague phrases such as “living in the moment” and we all have a sense of what is the “now“, but we do not always realize what time intervals we are referring to. If we just saw an image or heard a musical note one second ago, physical time would clearly place them in “the past”. Decades of research performed by Ernst Pöppel and his colleagues at the institute, as well as several other laboratories around the world, suggest that our brain integrates our subjective temporal reality in chunks of approximately three second duration.
Temporal order can be assessed in a rather straightforward experimental manner. Research subjects can be provided sequential auditory clicks, one to each ear. If the clicks are one second apart, nearly all participants can correctly identify whether or not the click in the right ear came before the one in the left ear. It turns out that this holds true even if the clicks are only 100 milliseconds (0.1 seconds) apart. The threshold for being able to correctly assign a temporal order to such brief stimuli lies around 30 milliseconds for young adults (up to 25 years old) and 60 milliseconds for older adults.
Temporal integration of stimuli, on the other hand, cannot be directly measured through experiments. It is not possible to ask research subjects “Are these two stimuli part of your now?” and expect a definitive answer, because everyone has a different concept and definition of what constitutes “now“. Therefore, researchers such as Ernst Pöppel have had to resort to indirect assessments of temporal integration, and ascertain what interval of time is grasped as a perceptual unit by our brain. An excellent summary of the work can be found in the paper “A hierarchical model of temporal perception“. Instead of reviewing the hundreds of experiments that have lead researchers to derive the three-second interval, I will just review two studies which I believe are among the most interesting.
In one of the studies, Pöppel partnered up with the American poet Frederick Turner. Turner and Pöppel recorded and measured hundreds of Latin, Greek, English, Chinese, Japanese, French and German poems, analyzing the length of each LINE. They used the expression LINE to describe a “fundamental unit of metered poetry”. In many cases, a standard verse or line in a poem did indeed fit the Turner-Pöppel definition of a LINE, but they used the more generic LINE for their analysis because not all languages or orthographic traditions write or print a LINE in a separate space as is common in English or German poems. If a long line in a poem was divided by a caesura into two sections, Turner and Pöppel considered this to be two LINES.
The basic idea behind this analysis was that each unit of a poem (LINE) conveys one integrated idea or thought, and that the reader experiences each LINE as a “now” moment while reading the poem. Turner and Pöppel published their results in the classic essay “The Neural Lyre: Poetic Meter, the Brain, and Time” for which they also received the Levinson Prize in 1983. Their findings were quite remarkable. The peak duration of LINES in poems was between 2.5 seconds and 3.5 seconds, independent of what language the poems were written in. For example, 73% of German poems had a LINE duration between 2 and 3 seconds. Here are some their other specific findings:
Epic meter (a seven-syllable line followed by a five-syllable one) (average) 3.25 secs.
Waka (average) 2.75 secs.
Tanka (recited much faster than the epic, as 3 LINES of 5, 12, and 14 syllables) (average) 2.70 secs.
Four-syllable line 2.20 secs.
Five-syllable line 3.00 secs.
Seven-syllable line 3.80 secs.
Pentameter 3.30 secs.
Seven-syllable trochaic line 2.50 secs.
Stanzas using different line lengths 3.00 secs., 3.10 secs.
Ballad meter (octosyllabic) 2.40 secs.
Poets all around the world did not conspire to write three-second LINES. It is more likely that our brain may be attuned to processing poetic information in 3 second chunks and that poets are subconsciously aware of this. This was not a controlled, rigorous scientific study, but the results are nevertheless fascinating, not only because they points towards the three second interval that neuroscientists have established in recent decades for temporal integration in the brain, but also because they suggest that the rules for metered poetry may be universal. I strongly advise everyone to read the now classic essay by Turner and Pöppel, to then try reading aloud their own favorite poems and see if the LINES indeed approximate three seconds.
A second approach to glean into the inner workings of temporal integration process in our brain is the use of perceptual reversal experiments, such as those performed with the Necker cube. This cube is a 2-D line drawing, which our brain perceives as a cube – or actually two distinct cubes. Most people who stare at the drawing for a while will notice that their mind creates two distinct cube representations. Once the mind perceives the two different cubes, it becomes very difficult to cling to just one cube representation. Our brain starts flip-flopping between the two cubes; even when we try our best to just hang on to one of the cube representations in our mind. Interestingly, the average duration that it takes for our mind to automatically shift from one cube representation to the other one approximates three seconds.
Nicole von Steinbüchel, a colleague of Ernst Pöppel at the Institute of Medical Psychology, asked a fascinating question. If the oscillatory perceptual shift between the two cube representations is indeed indicative of the “subjective present” and the temporal integration capacity, would brain injury affect the oscillation? She studied patients who had brain lesions (usually due to a stroke) in either the left or right hemisphere of the brain. She and her team of researchers were able to show that while healthy participants reported a three second interval between the automatic shifting of the cube representations in their brain, the average shift time was four seconds in patients with brain damage in the left brain hemisphere and up to six seconds if the damage had occurred in a certain part of the right brain hemisphere. Nicole von Steinbüchel's research demonstrates the clinical relevance of studying temporal integration, but it also suggests that the brain may have designated areas which specialize in creating a temporal structure.
The analysis of poetry and the Necker cube experiments are just two examples of cognitive studies indicating that our brain uses three second intervals to process information and generate the experience of the “now” or the “subjective present“. Taken alone, none of these studies are a conclusive proof that our brain uses three second intervals, but one cannot help but notice a remarkable convergence of data pointing towards a cognitive three second rule.
Frederick Turner and Ernst Pöppel (1983) “The Neural Lyre: Poetic Meter, the Brain, and Time” Poetry 142(5): 277-309. A reprint also available online here: http://www.cosmoetica.com/B22-FT2.htm
Ernst Pöppel (1997) “A hierarchical model of temporal perception” Trends in Cognitive Sciences 1(2): 56-61.
Nicole von Steinbüchel (1998) “Temporal ranges of central nervous processing: clinical evidence” Experimental Brain Research 123 (1-2): 220-233.