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Previous investigations have shown that, in dim illumination and empty visual fields, accommodation in most observers is for an intermediate distance referred to as the dark focus or resting focus of accommodation. Dynamic properties of the resting focus were examined in the present study with a high-speed infrared optometer. All subjects displayed fluctuations of accommodation in darkness that were primarily characterized by frequency components below 0.5 Hz. These fluctuations were substantially attenuated during cycloplegia or viewing a bright, high-contrast target. Both the mean resting-focus position and the magnitude of fluctuations varied from one subject to another and from day-to-day. A weak association was found between the mean a standard deviation of accommodation responses in the dark. The present findings suggest that the accommodation mechanism in most observers is somewhat unstable under degraded viewing conditions. In addition, the mean resting-focus value for a large sample of subjects was found to be lower for i.r. optometer measurements than for previous investigations employing a laser optometer.

The resting focus of accommodation was measured in the same subjects with both a laser optometer and a high-speed infrared optometer. Although i.r. optometer and laser optometer measures produce similar estimates of the mean resting-focus level in some subjects, others were found to have different levels of resting focus with the two techniques. Control studies demonstrated that these differences were not due to the temporal sampling characteristics of the laser optometer, but resulted instead from making judgments about the direction of speckle motion during the laser optometer procedure. The temporal stability of the resting focus of accommodation was also investigated with i.r. measures obtained several minutes, 1 day, and 1 and 2 weeks apart. Results indicated long-term variability similar to that previously reported with the laser optometer.

We determined the course of myelination of the chorda tympani in rats aged from 4- to 30-days-old, the interval of the most rapid developmental changes in neurophysiological taste responses and behavioral discrimination among chemical stimuli. The overall number of axons in rats aged from 16- to 30-days-old and in mature 120-day-old animals were the same and averaged 1,500. By 30 days, rats had 80% of the total number of myelinated axons observed in adults, but the average thickness of the myelin sheath per neuron and the proportion of the total cross-sectional area that were only about 60% of adult values. Observed increases in myelination closely parallel decreasing response latencies of single chorda tympani fibers to tongue stimulation with salts.

We examined the number and distribution of ganglion cells within a vallate papilla from a healthy human adult. The vast majority of cell bodies were located near the base of the papilla, clustered in the central portion of its core immediately above underlying muscles and von Ebner’s glands. Neurons appeared to be of one morphological type with ovoid cell bodies and spherical nuclei. The possible functional significance of their presence in invaginated or trench-containing gustatory papillae is discussed.

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