On Time Perception & Sleep

Courbet’s Sleep

Why is it that our perception of the passage of time changes around and during periods of sleep? While it is known that there are diurnal variations in time perception1, and that insomniacs have irregular perception of duration of sleep2, this basic question remains.

In an article concerning regular and pathological conscious perception of time, Oliver Sacks speculates that “visual perception might in a very real way be analogous to cinematography, taking in the visual environment in brief, instantaneous, static frames, or ‘stills,’ and then, under normal conditions, fusing these to give visual awareness its usual movements and continuity3.

This suggests the possibility that our perception of time is a function of our ability to impose a sense of continuity on our own perceptions. Thus, in the absence of external stimuli for this continuity system to act on, we have no mechanism to calculate the passage of time, and instead estimate this variable in a noisy, post-hoc manner.

In any case, the fact that it’s possible to drastically misestimate how long one a bout of sleep has lasted implies that there is something fundamental about the state of consciousness (wakefullness) and judgement of time perception which remains to be understood.

References:

1. Pöppel E, Giedke H. (1970) Diurnal variation of time perception. Psychol Forsch. 34(2):182-98.
2. Knab B, Engel RR. (1988) Perception of waking and sleeping: possible implications for the evaluation of insomnia. Sleep. 11(3):265-72.
3. Sacks, O. (2004) In the River of Consciousness. The New York Review of Books. 51(1):

On Reading The Unreadable


Phaistos Disc

This is the Phaistos Disc. It is the first piece of printed writing and nobody can read it. It is loosely dated to 1900BC (the Greek Government will not allow it to be subjected to thermoluminescent testing which would clear up its time of origin). It is clearly printed in the sense that each character was stamped onto this piece of clay, not etched or written as all other pieces of writing from this period. Many attempts have been made to decipher it, but the prevailing feeling is that these have been unsuccessful. I find mysteries like this beautiful and exciting because they area great challenge and reminder of how much our knowledge relies on a kind of collective remembrance of things. Without such distributed knowledge, where would we be?

On Motivating the Anecdotal

I was recently asked the question: “Why do things taste better when they’re free?” While this hardly constitutes a rigorous scientifically testable hypothesis, I thought I’d wax speculative about it for a moment.

The explanation of why some may have this experience lies in the fact that your brain has a unified reward system based on the small molecule dopamine. This system is responsible for many types of associative learning (including classical Conditioning a la Pavlov), and signals rewards to be gained from engaging in various behaviors such as eating, having sex, or doing drugs1. Further, it seems likely that the dopamine system is responsible for making us feel happy about having received monetary returns1.

In the free-food-tastes-better case, this system is double activated. There is a reward for having gotten something for free along with a reward for eating something. However, there is sometimes only one action to assign the reward to: eating. Perhaps when there is no immediate action to be applauded for providing the free-food-prize, the brain simply heaps all the praise on the eating itself, thus resulting in the delightful (and dangerous) experience of the free snack being extra desirable.

References:
1. Caplin, D & Dean M (2008) Dopamine, Reward Prediction Error, and Economics. The Quarterly Journal of Economics. 123(2), 663-701

On Dopamine and Ethics?

It has long been known that the hormone dopamine plays a key role in both addictive and learning behaviors. As with all neurotransmitters, “normal” functioning of these small molecules in the body depends on their receptors, of which there can be several types.

Two papers, published last year in the journal Science, both focus on particular dopamine receptor types, and behaviors associated with them. Work from Jeffrey Dalley’s lab1 uses PET to demonstrate that impulsive rats have significantly lower availability of two dopamine receptor types in a certain brain area, and further, that such impulsivity predicts how effective cocaine is in reinforcing behaviors thatt cause its administration. Meanwhile, experiments conducted by Tilmann Klein2 indicate that people with a certain allele of one type of dopamine receptor are significantly less able to learn a task via negative feedback.

The latter work, however, has implications beyond the nature of learning. This is the first example of a certain type of human behavioral trait about which one can make quantitative predictions through genomic analysis. This trait is one that has the potential to stratify people; categorize them before birth according their potential abilities.

This gene presents the possibility for exactly the sort of genetic profiling that many have predicted with dread. The former work mentioned above carries some of the same baggage, a PET scan based screening of individuals could potentially be done quite early, and it is not hard to imagine the development of genetic screens to forecast “dopamine receptor availability.” An interesting conclusion to the presentation of these ideas would be some exploration of the actual value of being able to learn from negative feedback. Are those less able to learn in this way simply more bold? Fearless? Risk-takers? More creative? Could there be some value in attempting to tailor one’s activities to one’s genetic disposition?

Just as the construction of the atomic bomb raised questions about the ethics of conducting certain types of research in the physical sciences, recent work such as that mentioned here does the same for neuroscience. As it gains widespread interest and makes increasingly strong predictions about human beings and their behaviors, the power of great neuroscientific research must be wielded as gingerly as any.

References:
1. Dalley, JW et al, (2007) Nucleus Accumbens D2/3 Receptors Predict Trait Impulsivity and Cocaine Reinforcement, Science, Vol. 315:1267-1270 | 
DOI: 10.1126/science.1137073

2. Klein, TA et al, (2007) Genetically Determined Differences in Learning from Errors, Science, Vol. 318:1642-1645 |
DOI: 10.1126/science.1145044

On the Dynamic and Ever Changing Brain

brain sites stimulated and measured; from Ref. 1

It is well accepted that the activity in our brains is not simply defined by the anatomical connectivity therein. However, one of the great mysteries of free will (if it actually does exist) is how a conscious state (the current pattern of electrical activity) is causally linked to subsequent states (the next pattern of activity). If it is truly deterministic, then free will may truly be an illusion.

A recent paper reporting results from an experiment in which awake human cortices were stimulated and responses measured, makes an implicit comment on these matters. In this experiment, patients going in for brain surgery volunteered to have their cerebral cortices experimented on. This is not as invasive as it sounds since the experiment involved only a reanalysis of data obtained from necessary pre-surgical procedures. The experimental paradigm consisted of simply electrically stimulating the cortex at various points and measuring the evoked activity at other sites. The authors were curious how regular these responses would be, and how they would vary over time.

They found that “The likelihood of an afterdischarge at an individual site after stimulation was predicted by spontaneous electroencephalographic activity at that specific site just prior to stimulation, but not by overall cortical activity” (ref. 1). The intriguing part about this is that the overall activity does not predict the subsequent activity, supporting the notion that there is something else (free will?) that intervenes or contributes to the causation involved in moving to another state of activity. However, it must be noted that the overall activity might not be predictive for another reason. Aggregate brain activity is made up of parts devoted to distinct cognitive functions. Thus if the brain area stimulated was one devoted to motor function and the subject happened to be thinking about a movie prior to stimulation, the activity devoted to the visual experience wouldn’t necessarily be predictive of how the activity might spread through the brain from this motor area. Nonetheless, it may be the case that this sort of variation – the seemingly random, highly dynamic snatches of activity present in the brain at any given moment – contribute to our sense of free will, and the rich landscape of experience that we go through.

References:
1. Lesser RP, Lee HW, Webber WR, Prince B, Crone NE, Miglioretti DL. (2008) Short-term variations in response distribution to cortical stimulation. Brain, 131 :1528-1539

On Science from Art

from Ref. 1

With the exception of certain information which can be obtained from glacial core sampling, climate data goes back only so far. This makes the task of determining the impact of human intervention on the environment quite difficult, since our effects ride on top of natural variations which would be revealed by pre-human climate data.

An ingenious method for estimating one aspect of climatological variations (specifically aerosol density) by observing the effect of volcanic eruptions on sunsets has been developed by a team of atmospheric scientist. In a lovely example of art informing science, they correlated historical reports with paintings of sunsets from the same periods (sunsets being particularly effected by volcanic eruptions) to accurately estimate atmospheric aerosol density outside of the period for which human instrumentation data is available.

References:
1. Zerefos, CS, Gerogiannis, VT, Balis, D, Zerefos, SC & Kazantzidis, A. (2007) Atmospheric effects of volcanic eruptions as seen by famous artists and depicted in their paintings. Atmos. Chem. Phys., 7, 4027–4042

On Recognizing Faces

An example of the constructed faces
used by HR Wilson in his research.
(b) is derived from (a).

Although humans have no problem identifying them from all orientations, faces look extremely different depending on the angle from which one views them. That is to say that despite our facility in dealing with this challenge, it is an extremely difficult problem to solve with computers. Further, because we have some understanding of the way in which vision is hierarchically organized, progressively building up complex forms starting from small line segments, it points out a formidable computational ability that our brains possess.

Hugh R. Wilson spoke on this issue on Monday (6/16/2008), at the SUNY College of Optometry on 42nd Street. He showed that humans compute facial orientation as suggested by the fact that a subject’s sense of this orientation can be adapted by a brief (5 seconds) exposure to a face at a given angle. In other words, before adaptation, humans can fairly accurately identify facial orientations, and this sense is shifted by the adaptation procedure. This basic finding is probably employed to compute what the face would look like from the front, thusly making identification possible.

Of course this psychophysical experiment is only suggestive of such a mechanism, but it is an intriguing possibility.