"Generalization" number 10 introduces the concept of after-effects. You may have encountered such phenomena in a different context, for example in form of visual illusions. Let's do a small experiment that makes the concept of after-effects immediately obvious. Please fixate for a minute or so both eyes on the cross in the center of the yellow square. And now please keep your fixation on the cross when I change to the next slide. Your brain should have made you see the opposite colors in the empty squares with blue in the center and yellow in the frame. The reason for this after-effect are interactions in your retina that involve receptors, neurons, and small neuronal networks, called receptive fields. While you looked at the first picture, for some time, the retinal neuronal circuits established certain interactions that helped to see the contrast between the two squares. Once the contrast was gone, the established interactions didn't immediately cease, and in combination with the new white background, they made you see the opposite colors. Conceptually similar effects can be observed in circadian systems. Let's presume an organism that is exposed to light from 6 am to 6 pm for one week -as indicated by the yellow area- and then is released to constant darkness. In these constant conditions, the free-running rhythm may adopt a period that is slightly longer than 24 hours. Let me explain the details of this very chronobiological plot. The horizontal axis at the top represents the local time, and the days of the experiments are plotted one below each other. The horizontal bars represent some rhythmic output of the circadian clock, locomotor activity, or body temperature above the daily mean, etc. The first week of the experimental protocol shown here involves a 12 hour light and 12 hour dark cycle, also called LD 12:12. Now let's presume a different protocol, where the light comes on half an hour earlier each day and also is turned off 30 minutes earlier each day. The results of such an experiment would look like this. And the zeitgeber-regime applied here would be called LD 11.5:11.5. Although both rhythms are released after one week to constant darkness- that is to identical conditions- the period of the free-run following the shorter LD regime is different. The onsets of the bars fall on average on a vertical line indicating a period of very close to 24 hours. The difference in the free-running rhythm is an aftereffect of the prior zeitgeber regime. The complex network of components that make up the circadian system have arranged themselves to work optimally under the respective light:dark regime. And that arrangement continues in constant darkness. But the difference in the free-running rhythms were already preceded by differences in entrained phase. While the onsets of the daily episodes closely followed dawn under the LD 12:12 regime, they begin much later under the shorter light-dark cycle. You have seen earlier that longer free-running rhythm show later phases of entrainment than shorter free-running rhythms. This systematic relationship can be turned around: Let's presume an individual organism has a certain period in its free-running rhythm. When it is exposed to an LD12:12 zeitgeber-regime, it may reach a maximum every day at noon. If we lengthen the zeitgeber period, the rhythm will reach its maximum systematically earlier, for example at dawn. And if we shorten the zeitgeber-cycle, the rhythm will peak later, for example in the afternoon. "Generalization" number 11 addresses a pivotal quality of all circadian systems, namely temperature compensation. Biochemistry had shown that the rate or speed of most of the observed reactions -be it a single enzyme step or complex biochemical processes, like photosynthesis or respiration- are temperature dependent. The warmer the faster the reaction, the higher the rate. Very early on in chronobiology, the circadian system was called "clock", and this term was chosen with some justification. The system represented the external day -like the clocks we know. The system is used to time-reference to orient in space and time- similarly to how we use clocks. One of the big challenges of early mechanical clocks was to make sure that they don't run slower or faster depending on ambient temperature. The knowledge about this problem and how it was solved reminded chronobiologists about the importance that circadian clocks are temperature independent despite of being built of biochemical reactions. The controversy between Frank Brown and Colin Pittendrigh actually originated in different interpretations of temperature-dependent reactions. Frank Brown was convinced that whatever was responsible for the observed sustained rhythms in "constant" conditions could not serve as a faithful clock because it must be based on biochemical reactions and would therefore be temperature dependent. Pittendrigh -on the other hand- was convinced that a circadian system that could serve as a clock, internally representing its cyclic environment, would be an advantage for organisms because it allowed them to "anticipate" their immediate future. He therefore postulated that the circadian "clock" would show temperature compensation, DESPITE being based on biochemical reactions. It was an easy challenge for Pittendrigh to win the debate because all he needed was to show experimentally that the period length of a free running rhythm does not change with ambient temperature. The documented temperature independence of period was called "temperature compensation". It was clear that the generation of circadian rhythms was based on temperature-dependent biochemical reactions but the end-product was indeed shown to be temperature-independent. One therefore presumed that this independence was achieved by a compensatory processes within the network.
