Is There Any Evidence for a Physiological System That Could Provide This 24-Hour Clock

Learning about Time

We are good at estimating time periods, and making judgements about whether intervals are shorter or longer than each other. We are also sensitive to the day/night time cycle: e.g. jetlag (timing obviously not just to do with the sun), waking up just before your alarm goes off

How do we do it? Can animals do these things? How do they do it?

Distinguish periodic (learning to respond at a particular time of day) and interval timing (learning to respond after a particular interval of time).

PERIODIC TIMING e.g. Circadian rhythms

Cockroaches (Roberts, 1965). Increased activity at dusk. When removed visual cues cycle drifted until increased activity started 15 hours before dusk (cycle slightly less than 24 hours).

Restoring visual cues produced a gradual shift back to correct time. Entrainment : light acts as a zeitgeber synchronising the internal clock.

Is there any evidence for a physiological system that could provide this 24-hour clock? The suprachiasmatic nucleus (SCN) of the hypothalamus may be a candidate.

The metabolic rate in the SCN appears to vary as a function of the day-night cycle.

Lesions of the SCN will abolish the circadian regularity of foraging and sleeping in the rat. It also receives direct and indirect inputs from the visual system, which could keep circadian rhythms entrained with the real day-night cycle.

INTERVAL TIMING

Consider a normal classical conditioning procedure: Tone (20 sec) --> food

.....so what happens if the stimulus keeps on going (and you omit the food)?

The peak procedure

Church & Gibbon, 1982 Rats in lit chamber. Occasionally houselight went off for a period of time 0.8, 4.0 or 7.2 sec; this was the stimulus. When the lights went on again a lever was presented for five seconds. If the rat pressed the lever after a 4-sec stimulus it got food; after the other stimuli it did not. Then the rats were tested with a range of stimuli, with durations between 0.8 and 7.2 seconds.

Weber’s Law The generalisation that the just noticeable difference is proportional to the magnitude of the stimulus. Hence small amounts judged more accurately than large amounts

This may be called the scalar property of timing (it applies to other judgements too).

DI / I = k DI = Just discriminable change in intensity

I = original intensity

k = constant

1. Storing duration of a stimulus in Short term memory

When a stimulus is presented, a switch is operated; the number of pulses that accumulate in working memory will equal t multiplied by the number of seconds that have passed (N).

2. Storing duration of a stimulus in Reference memory

When the reinforcement occurs, pulses stop accumulating; the number of pulses in working memory (N * t) is now stored in reference memory;

This storage is not always completely accurate -- there is some memory distortion. This is represented by K, a number that is close to 1. If K=1 then the memory is accurate; if K<1 then a smaller number of pulses is stored; if K>1 then a greater number is stored.

After several trials several numbers are stored in reference memory Nm1, Nm2, Nm3, etc -- each equal to the K * N * t for that trial. The error on each trial will not be the same.

3. Using stored value in reference memory to decide whether to respond

On each trial the animal compares the number of pulses in working memory (N * t) with a random value drawn from those stored in reference memory Nmx. This is done by the comparator. If the values are close, then the animal responds.

The comparator works out how close the values are using a ratio rule – NOT a difference rule

i.e. NOT N * t - NMx but N * t - NMx / NMx

This is one of the reasons that accuracy is better with short intervals.

Potential problems with scalar timing theory

1) There is as yet no physiological evidence for a pacemaker. Alternatives proposed:

(i) Instead of a pacemaker, timing could be achieved by a series of oscillators, each of which has two states, on or off. If each oscillator switches after a different period of time, then the entire pattern of activation could be used to determine the exact time (e.g., Gallistel, 1990; Church & Broadbent, 1991):

(ii) Another proposed solution: Behavioural theory of timing (e.g., Killeen & Fetterman, 1988).

When the animal gets a reward, this stimulates behaviour. The animal moves across an invariant series of behavioural classes in between reinforcements. A pulse from an internal pacemaker changes the behaviour from one class to another. The behaviour that is occurring when the next reinforcer occurs becomes a signal for that reinforcer.

2) Conditioning and timing supposedly occur at the same time, and yet are controlled by completely different learning mechanisms.

Some theories of timing try and explain conditioning; e.g., Gibbon & Balsam (1977). Calculates rate of reinforcement during stimulus, and rate of reinforcement during background. If first is higher than second, get conditioned responding.

6 reinforcers in 60 minutes of background = 1/10 = 0.1

4 reinforcers in 15 minutes of stimulus = 4/15 = 0.27 0.27 > 0.1

…But this theory cannot explain basic phenomena, like blocking.

Some conditioning models try to explain timing -- e.g. Real time models (e.g., Sutton & Barto, 1981). They work with the Rescorla- Wagner model, just like regular conditioning theories. However, the stimulus is assumed to change over the course of its presentation, and this allows the animal to learn about when a reinforcer occurs.

General references

Bouton, M.E. (2007). Learning and Behavior.

Sinauer Associates.
Carlson, N.R. (2001) Physiology of Behaviour.

Allyn & Bacon. Chapter 9.

Domjan, M. (1988). The principles of learning and behavior. Brooks/Cole

Publishing Company. Chapter 12.

Pearce, J.M. (1997). Animal Learning and Cognition. Lawrence Erlbaum

Associates. Chapter 7.

Shettleworth, S.J. (1998). Cognition, Evolution and Behaviour. Oxford

University Press. Chapter 8.

Wynne, C.D.L. (2000). Animal Cognition. Macmillan. Chapter 5 pp.96-101