Time

Question 1

Periodic

Answer 1

learning to respond at a particular time of day

Question 2

Interval

Answer 2

learning to respond after a particular

interval of time

Question 3

PERIODIC TIMING

Answer 3

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):

Question 4

In constant dim light when no light cues are available

they maintain behaviour on an approximately

Answer 4

25 hour cycle

Question 5

Cockroaches (Roberts, 1965).

Answer 5

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.

Question 6

Bolles & Stokes (1965)

Answer 6

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

Question 7

Is there any evidence for a physiological system that could

provide this 24-hour clock?

Answer 7

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.

Question 8

More recent evidence suggests every cell in the body has a circadian rhythm, which are all under the control of the SCN.

Answer 8

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.

Question 9

Interval timing

Answer 9

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

Question 10

Church & Gibbon, 1982

Answer 10

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).

Question 11

Weber’s Law

Answer 11

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

Question 12

Weber’s Law

Answer 12

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

Question 13

Weber’s Law

Answer 13

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

Question 14

Pacemaker pulses per second - working memory - reference memory - comparator - response

Answer 14

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).

Question 15

5-second stimulus:

Answer 15

successive pulses stored in working memory

Question 16

Storing duration of a stimulus in

Reference memory

Answer 16

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.

Question 17

Using stored value in reference memory to decide whether or not to respond on the next trial

Answer 17

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

Question 18

Potential problems with scalar timing theory

Answer 18

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

Question 19

Potential problems with scalar timing theory

(ii) Another solution that has been proposed is the
Behavioural theory of timing (e.g., Killeen & Fetterman, 1988).

Answer 19

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.

Question 20

Potential problems with scalar timing theory

(ii) Another solution that has been proposed is the
Behavioural theory of timing (e.g., Killeen & Fetterman, 1988).

Answer 20

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.

Question 21

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.

Answer 21

Some theories of timing try and explain conditioning; e.g.,

Gibbon & Balsam (1977).

Question 22

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.

Answer 22

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

Question 23

This theory cannot explain basic phenomena, like blocking.

Answer 23

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