Re: Critical Learning Periods & Project Management?

[Guest guest]

Dickman wrote:

>

> <The best known example is the critical period for imprinting in certain

> species of ducks, which is a 'window' of a few hours following hatching. If

> imprinting does not occur in the critical period it can never be learned, and

> the duck will not develop normally. >

Mel Siff:

> ***I remember reading about this years ago, but cannot recall the exact

> learning that was involved.

Oakley (1983) calls these kinds of learning 'variable epigenesis', as

experience can only influence future behaviour in a limited number of

directions and the effects, which tend to be irreversible, are only

achievable during a brief period.

Mel Siff:

> Do you recall if that work excluded the

> possibility of using some of the classical Pavlovian conditioning methods to

> facilitate later learning? Is it entirely impossible to achieve the learning

> by the use of appropriately designed and sufficiently stimulating 'mistimed'

> imprinting events? Of course, a duck community would not have the

> intelligence or scientific ability to assess the situation and structure

> later learning processes to rectify the situation, but, in the case of

> humans, this is certainly possible.

As I mentioned, ducks (and other various early-maturing bird species), when

newly hatched, rapidly learn to follow any of a range of moving animate and

inanimate objects to which they are exposed. I don't think classical Pav

conditioning would reverse the behaviour. What the early ethologists, best

known being Konrad Lorenz (1937), studying imprinting, who first put forward

the notion of a critical period in learning, found that the attachments once

formed were difficult to reverse, and that after the critical period, the

young were more likely to flee from a moving object, rather than follow it.

I think this would make any attempt at Pav conditioning very difficult or

impossible.

Oakley (1983) calls these kinds of learning 'variable epigenesis', as

experience can only influence future behaviour in a limited number of

directions and the effects, which tend to be irreversible, are only

achievable during a brief period.

Apparently the critical period varies across different species, behaviours,

and environments. From what I have read, it appears that the 'lower' the

species, and the more predictable the environment, the less readily

reversible are the acquired behaviours.

Two other examples of critical period learning that also appear to be

irreversible are seen in cats and lambs. Lambs isolated from their mothers

and raised by humans never fully reintegrate into the flock.

In the case of cats, neurons in their visual cortex become preferentially

sensitive to bars which are orientated either vertically or horizontally,

depending on whether the cats were brought up, during the critical period of

their visual development, in environments with stimuli predominantly

horizontal or vertical. The cats brought up in these selective environments,

are visually insensitive to stimuli orientated in ways which they have not

previously experienced (Blakemore and , 1970) Later selective

experience does not have these effects, nor can it reverse the effects of

early experience.

[Yes, I think the original work in this field was done by Hubel and Wiesel. Mel

Siff]

Has anyone on the list had first hand experience with a squint. From what I

have read this is another example of visual sensitisation taking place

during a critical period. Apparently, in humans who do not have a squint

treated before the age of 7 or8, the previously lazy eye remains partially

blind even though the muscular fault has been corrected. The deficiency

appears to lie in the brain, and cannot be compensated by later experience.

Petigrew (1978) suggests that the critical period plasticity of cortical

neurons in the visual system depends on a complex interaction with

noradrenaline and certain neurons in the locus coeruleus in the brainstem.

Mel Siff:

> So, we really need to talk about 'optimal' rather than critical periods.

> Even then, if we use less than optimal methods to teach something during a

> critical period, the results would not be as good as if optimal teaching or

> conditioning methods were used during optimal periods of learning or

> adaptation.

I think 'optimal period' may replace 'sensitive period', but in the case of

'critical period' as I have defined and discussed it in 'lower' species, it

appears that teaching per se does not enter in to this type of learning. The

organisms appear to be 'hard wired' for a particular behaviour to be learned

during a very specific time frame. This may be neuronal ??? hormonal???

One final example to illustrate the hormonal hypothesis. There appears to be

critical periods for some birds (eg sparrows, chaffinches) to acquire adult

bird song. In some species it seems that the critical period can be extended

for late-born young which have received minimal exposure to adult song

(Marler, 1981). The timing of the period may be controlled by sex hormone

activity. It has been shown that an implant of testosterone into male

chaffinches who have been earlier castrated, enables them to sing the full

adult bird song a year later than normal birds (Thorpe, 1972).

Selected References:

Blakemore, C., and , G.F.(1970). Development of the brain depends on

the visual environment. Nature, 228. p.477-478.

Oakley, D. A. (1983). The varieties of memory: a phylogenetic approach. In

A. R. Mayes, ed., Memory in Humans and Animals. Van Nostrand.

Pettigrew, J. D. (1978). The locus coeruleus and cortical plasticity. Trends

in Neurosciences, 1. p.73-74.

Regards,

Dickman,

Australia.