Time Flashcards
Periodic
learning to respond at a particular time of day
Interval
learning to respond after a particular
interval of time
PERIODIC TIMING
e.g. Circadian rhythms.
Question: is the cyclical behaviour really controlled by time per
se? Or is it controlled by stimuli that are always present at that
particular time?
Wheel running in the rat (described in Carlson):
In constant dim light when no light cues are available
they maintain behaviour on an approximately
25 hour cycle
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.
Bolles & Stokes (1965)
Subjects born and reared under either 19, 24 or 29 hour
light/dark cycles. Then fed at a regular point in their own
particular cycle….
animals on the 24-hour cycle learned to anticipate food….but the others didn’t
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; some optic nerve cells are sensitive to light and hence to dawn/dusk.
More recent evidence suggests every cell in the body has a circadian rhythm, which are all under the control of the SCN.
This can dictate e.g. circadian variation in sensitivity of tumours to chemotherapy.
Visually impaired people need to ensure they have enough exposure to light so their circadian rhythm can be entrained.
Disruption in circadian rhythms can be responsible for physical illness (e.g. in shift workers more susceptible to heart disease, diabetes, infections and even cancer).
Sleep and circadian rhythm disruption is also associated with several types of mental illness, such as depression, schizophrenia, bipolar illness.
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 0.8, 4.0
or 7.2 sec (the CS). When the lights went on again a lever was
presented for five seconds. If the rat pressed the lever after a
4-sec CS it got food, otherwise it did not. Then tested with a
range of stimulus durations (0.8 - 7.2 secs).
Weber’s Law
The generalisation that the just noticeable difference is
proportional to the initial intensity/magnitude of the changed
stimulus.
Hence in absolute terms small amounts judged more accurately
than large amounts
RELATIVE change critical
ABSOLUTE change not
Weber’s Law
The generalisation that the just noticeable difference is
proportional to the initial intensity/magnitude of the changed
stimulus.
Hence in absolute terms small amounts judged more accurately
than large amounts
Weber’s Law
This may be called the scalar property of timing (it applies
to other judgements too).
I / I = k
I = Just discriminable change in intensity
I = original intensity (of stimulus being changed)
k = constant
Pacemaker pulses per second - working memory - reference memory - comparator - response
Pacemaker emits pulses at a
roughly constant rate t (there
is random variation).
When a stimulus is presented, a switch is operated, and the pulses are allowed to accumulate in working memory. This will equal t multiplied by the number of seconds that have passed (N).
5-second stimulus:
successive pulses stored in working memory
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 completely accurate -- there is some memory distortion. This is represented by K, a number that is close to 1:
If K=1 the memory is accurate;
If K<1 a smaller number of pulses is stored;
If K>1 a greater number is stored.
After several trials there will be several
numbers stored in reference memory
Nm1, Nm2, Nm3, etc – each equal to the
K * N * t for that particular trial.
Using stored value in reference memory to decide whether or not to respond on the next trial
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.
Another stimulus occurs, and the successive number of pulses is stored in working memory
The animal uses ONE of the values in reference memory to decide when to respond
Potential problems with scalar timing theory
1) There is as yet no physiological evidence for a pacemaker
Alternatives have been proposed:
(i) Instead of a pacemaker, it has been proposed that 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):
let’s say on = red, off = green
Potential problems with scalar timing theory
(ii) Another solution that has been proposed is the
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 will change the behaviour from one class to
another. The behaviour that is occurring when the next
reinforcer occurs becomes a signal for that reinforcer.
Potential problems with scalar timing theory
(ii) Another solution that has been proposed is the
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 will change the behaviour from one class to
another. The behaviour that is occurring when the next
reinforcer occurs becomes a signal for that reinforcer.
Potential problems with scalar timing theory
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).
Potential problems with scalar timing theory
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).
Calculate rate of reinforcement during stimulus, and rate of
reinforcement during background. If first is higher than second,
get conditioning.
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
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.
Stimulus trace
