3.3 Plasticity and permanency
The visual system relies on, amongst other things, the exquisitely precise connections between the retina, the lateral geniculate nucleus of the thalamus and the visual cortex. In precocial organisms, i.e. those born or hatched able to see and move about, such as horses and ducks, these connections arise in complete darkness. The information necessary to establish the connections must therefore be in the genome. The question then arises as to whether the environment, visual stimulation in this case, could influence the formation of these connections? To answer this question, Blakemore and Cooper (1970) dramatically altered the visual environment, and hence the visual input their subjects received.
Do you think Blakemore and Cooper used a precocial organism, or one that is relatively immature at birth, an altricial organism?
They reasoned that they would be more likely to influence the development of the visual system in an organism whose visual system was still developing. So they used an altricial organism, the cat Felis catus.
They discovered that kittens raised for the first three months of life in a visual environment consisting solely of vertical (or near vertical) lines were subsequently unable to see horizontal lines and they bumped into horizontal bars. Similarly, kittens raised for the first three months of life in a visual environment consisting solely of horizontal (or near horizontal) lines were subsequently unable to see vertical lines and they bumped into vertical bars. Neurophysiological examination of the visual cortices of cats reared under such strange visual conditions revealed a corresponding change at the cellular level. It is possible to record from individual neurons in the visual cortex whilst presenting visual stimuli to the animal. If the visual stimulus is a line, then it is found that neurons respond differently to different orientations of the line. In cats reared in a normal visual environment, some 50 per cent of neurons give a noticeable response to a vertical line. However, in cats reared in a vertical visual environment, some 87 per cent of neurons give a noticeable response to a vertical line. Thus, there is an increase in the number of neurons that respond to vertical lines. Similarly, in cats reared in stroboscopic light and therefore unable to see continuous movement, only 10 per cent of neurons in the visual cortex responded to a moving line, compared with 66 per cent of such neurons in cats reared in a normal visual environment.
These differences in visual responsiveness reflect underlying differences in the micro-anatomy of the visual system. Axons in the visual pathways have made slightly different connections with their targets, under the influence of the unusual visual stimulation.
The visual environment therefore does indeed influence the development of the visual system. Furthermore, the cats remained visually impaired for the four years during which they were tested after their visual environment had been returned to normal. The effect is normally considered to be permanent.
Having established that the developing nervous system is responsive to environmental factors, i.e. it is plastic, the next question to consider is whether there is a time limit on this plasticity. Does it matter, for example, when the visual input is altered for these changes in the visual cortex to appear? In other words, is there a particular period in development which is sensitive to particular environmental stimuli?