Abstract
If the caudal half of the goldfish tectum is removed reorganization may take place so that the representation of the whole visual field is compressed on to the remaining half tectum. We have found that such reorganization only takes place during summer. Fish operated on in the autumn and mapped during winter do not undergo compression. Taking this seasonal effect into account and using fish larger than 6.0cm in body length, we are now able to repeat the previously reported effect of exposure to constant light in preventing compression. In a proportion of smaller fish exposure to constant light can induce compression during winter. Electron microscopy shows that failure of compression of the retinotectal projection in large fish, whether due to season or lighting conditions, is not because of lack of axonal growth. We suggest that a common factor, perhaps hormonal, prevents the development or maturation of synaptic connections in both situations.
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