Abstract
Hardly anything appears to be known about the effect of excessive sound on the structure of the embryonic brain. In this experiment white leghorn eggs were examinated at day 10, the position of the head being noted through a window cut at day 3, a pencil mark being made on the shell at a spot as near as possible to the head. A crystal ear-plug transducer was applied at this spot and held firmly against the shell from day 10 to day 17 of incubation. Embryos were treated with sounds having as frequencies: 11 with 124 Hz, 8 with 480 Hz, and 8 with 714 Hz. The sound pressure varied between 90 and 105 dB. At day 20 the chick was removed and decapitated. The brain was fixed for a week, cut at 15 μm, and all sections of the midbrain, pons and medulla were mounted in serial order and stained with 0.1% aqueous thionin.
In 18 (66%) of 27 animals there was a small area of deeply stained shrunken neurons in about the same region in the caudal part of one or both laminar nuclei. In 7 (26%) there were less striking changes and in 2 (8%) there were none, although in 6 (30%) there were some small groups of slightly darker cells resembling those in the 7 experimental animals, and in 14 (70%) there were no changes at all. In many of the shrunken neurons the nucleus could not be seen or was distorted and the appearances contrasted markedly with normal neurons in contiguous regions and, when the effects were unilateral, with neurons in the corresponding part of the contralateral nucleus and with those in control animals. Normal neurons were usually pale staining and contained obvious clear round nuclei. It is apparent that the laminar nucleus is strongly influenced by incoming auditory information. There is insufficient information to determine the nature and possible specificity of these cell changes since only one stain and plane of section have been used, and rather less severe changes do sometimes occur in normal animals.