PubMed · 4543966
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Z Bochenek. 1973. [Vertigo].. https://pubmed.ncbi.nlm.nih.gov/4543966/
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Most naturally occurring sounds are modulated in amplitude or frequency; important examples include animal vocalizations and species-specific communication signals in mammals, insects, reptiles, birds and amphibians. Deciphering the information from amplitude-modulated (AM) sounds is a well-understood process, requiring a phase locking of primary auditory afferents to the modulation envelopes. The mechanism for decoding frequency modulation (FM) is not as clear because the FM envelope is flat (Fig. 1). One biological solution is to monitor amplitude fluctuations in frequency-tuned cochlear filters as the instantaneous frequency of the FM sweeps through the passband of these filters. This view postulates an FM-to-AM transduction whereby a change in frequency is transmitted as a change in amplitude. This is an appealing idea because, if such transduction occurs early in the auditory pathway, it provides a neurally economical solution to how the auditory system encodes these important sounds. Here we illustrate that an FM and AM sound must be transformed into a common neural code in the brain stem. Observers can accurately determine if the phase of an FM presented to one ear is leading or lagging, by only a fraction of a millisecond, the phase of an AM presented to the other ear. A single intracranial image is perceived, the spatial position of which is a function of this phase difference.
The early progress of myelination was studied, by means of myelin basic protein (MBP) immunohistochemistry and luxol-fast-blue (LFB) staining, in the spinal cord, spinal nerve roots and brainstem of 66 fetuses and neonates. The degree of myelination was classified from 1 (slight) to 4 (mature). MBP immunoreactivity exhibited slight LFB positivity. Myelination first occurred in the medial longitudinal fasciculus at 20 weeks of age, reaching degree 4 at 34 weeks, but began at 23-24 weeks in the other sites. Myelination of spinal nerve roots progressed with gestation and attained degree 4 at 35-36 weeks. The cuneate fasciculus also reached degree 4 at 34-36 weeks, but corticospinal tracts and solitary tracts, which exhibited long myelinating phases, were slow and incomplete at 40 weeks. This normal development of MBP and LFB myelination can be used for the assessment of delayed myelination in fetal and neonatal diseases.
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