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In a normal cat, cells in the visual cortex are orientated to be able to detect horizontal and vertical lines. These cells are arranged in a circular arrangement so that they can pick up edges of items in their visual filed that are of any orientation. Hirsch and Spinelli have previously done work where they have shown that changes to this arrangement is possible by changing what the eyes "see" during their early life. He showed kittens vertical lines into one eye, and horizontal lines into the other eye, keeping the non-looking eye covered. On investigation, they found that the orientation of the nerve cells in the visual cortex had altered according to what visual information was shown.
Author
Development of the
Brain Depends on the Visual Environment
Colin Blakemore and Graham F. Cooper
University of Cambridge
MethodColin Blakemore and Graham F. Cooper
University of Cambridge
This study used an experimental method. The Independent Variable was the ???????? and the Dependent Variable was the ????????
Sample
These were kittens that from the moment of birth were kept in a dark environment, so that they could not see anything. When they were two weeks old they started being used for the study. In the lit environment some kittens saw vertical lines, and some other kittens saw horizontal lines. The kittens were allowed to use both eyes to see the lines. When they were not looking at lines they were returned to the dark environment.
Procedure
The kittens were kept in a dark environment, so they could see nothing, and when they were allowed to see the experimenters put them in a device where they could only see the lines that the experimenters wanted them to see. A picture of this arrangement is below.
The kittens did not seem to be upset at sitting in a tube and sometimes would just sit for a while looking at its environment. The whole procedure stopped when the kittens reached the age of five months, which is when the "critical period" ends for adaptation to their visual system. After the study the cats would be taken to a well illuminated room furnished with chairs and tables and their reactions to their environment was observed.
Results
When they were placed in the well illuminated room with chairs and tables, at first, they were visually inept, and it made no difference if they were exposed to horizontal or vertical lines. Their pupillary reaction was normal but they showed no preparatory movements when they were held above a table and then placed on a table top, and did not respond when an object was brought rapidly close to them. They appeared to guide themselves by touch and showed a fear response when they reached the end of the surface they had been walking on. But they quickly recovered from many of the problems they were experiencing. Within about 10 hours of exposure to this room, the startle response reappeared, visual placing reappeared, and could easily jump from chair to floor. However, some of their defects were permanent. They had very clumsy jerky head movements when following moving objects, and they showed very poor depth perception by trying to touch objects that were out of reach. Perhaps the most important thing was that they would often bump into chair or table legs while they were moving about the room. There were some differences noted between the kittens that had seen vertical lines and those that had seen horizontal lines. They seemed to be "blind" to approaching Plexiglass with the inappropriate orientation of black lines, nor would they show preparatory movements onto the Plexiglass with the inappropriate lines. This difference was most marked when two kittens, one horizontally trained and the other vertically trained. If the experimenter held a long black rod vertically and shook it, one cat would run and play with it, the other would not. When held horizontally the cat that had previously played with it stopped and the other cat took over.
We moved from behavioural studies to neurophysiology when the cats were 7.5 months old. They were anaesthetised and paralysed while recordings were taken from single neurons in the primary visual cortex, using sodium chloride filled micropipettes. To increase the accuracy of any readings they took they had to correct the refractive state of the eyes by using various lenses.
To make sure that they were recording from the correct area they waved a card in front of the kitten that had black and white lines on that were clearly visible. Our initial hope was to take readings from an area that was known to be associated with the kitten's visual system. They estimated that they had taken readings from enough nerve cells so that they could map the nerve cells around the central area of the retina. The diagram below shows readings taken from two kittens, one horizontally trained and one vertically trained. About 75% of the nerve cells, in both kittens were clearly binocular and in almost every way the response were like those of a normal kitten. The distribution of preferred orientation was completely abnormal (see diagram below).
Not one nerve cell had its optimal orientation with 20 degrees of the inappropriate axis and there were, in total twelve within 45 degrees of it. This arrangement of nerve cells is highly significant (p<0.00001 as measured by a chi squared test). Obviously, the kittens early experience has modified their brains and there were profound perceptual consequences. We do not think that the process that produced this arrangement of nerve cells was merely passive, and that unused nerve cells degenerated due to under-activity. The researchers did not find vast areas of "silent" cortex, corresponding to the missing columns. It seems instead that the visual cortex may adjust itself during maturation due to the nature of its visual experience. Cells may even change their preferred orientation towards that of the commonest type of stimulus, so perhaps the nervous system adapts to match the probability of occurrence of features in its visual input - neuroplasticity!