Paul D. Wilson

Development of the optic nerve of the opossum (Didelphis virginiana)

The development of the optic nerve of a marsupial, the North American opossum, was examined in 24 animals from postnatal days 5 to 78 (P5-P78): gestation is 13 days. The estimated number of axons increased from 24,000 at P5, to 267,000 at P27, approximately 2.7 times the mean number in the adult. Following P27, axon numbers decreased rapidly to 140,000 at P40, then decreased more slowly, attaining adult values between P50 and P59. Thus, the opossum is similar to placental mammals examined in evidencing an overproduction and later attenuation to adult values in the number of axons in the optic nerve during development. Monocular enucleation of 3 animals at P17, 10 days before peak axon counts, resulted in a mean population increase of 24,000 (range 19,000-30,000) above the normal adult mean. Additionally, a 4th animal monocularly enucleated on P7, 3 days prior to the arrival of migrating fibers to central target sites, had a similar value of 26,500 supernumerary axons. Our findings in the opposum, when coupled with previous reports in other mammals, suggest that binocular interactions during development account only for optic nerve axon loss approximately equal in magnitude to the ipsilateral projection from one eye.

Reticular modulation of evoked potentials at the cortex and lateral geniculate nucleus of the unanesthetized squirrel monkey

Abstract The brain stem of the unanesthetized squirrel monkey was explored with electrical stimulation in order to determine which brain stem loci most effectively modulate potentials evoked at the lateral geniculate nucleus and striate cortex. Potentials were evoked at the lateral geniculate nucleus by optic tract stimulation and at the cortex by optic radiation stimulation. Conditioning stimulation in the pretectum, the rostral superior colliculus, or the mesencephalic reticular formation was equally effective and strongly enhanced (50–150%) the amplitude of responses at the lateral geniculate nucleus, but only slightly enhanced (15–45%) cortical responses. Stimulation in caudal superior colliculus and in pontine regions was less effective, and stimulation in dorsal thalamus was ineffective, except when in the ventral anterior nucleus. Stimulation here slightly enhanced cortical potentials, but had no effect on potentials at the lateral geniculate nucleus. Injecting small amounts of barbiturate severely reduced or eliminated the effectiveness of pretectal and tectal stimulation, but attenuated much less the effectiveness of stimulation in the mesencephalic tegmentum.

Beta-like ganglion cells in the retina of the North American opossum

Injection of horseradish peroxidase into the severed optic tract or nerve of the opossum retrogradely filled retinal ganglion cells. An abundance of well-labeled ganglion cells had dendritic morphology closely resembling that of beta-type ganglion cells in the cat retina. This finding suggests that an X-like functional class of ganglion cells is prominent in the retina of the opossum.

The effect of monocular deprivation on cells in the C-laminae of the cat lateral geniculate nucleus

Five cats monocularly deprived by lid suture between 2 and 3 weeks of age were used experimentally between the ages of 16 and 52 weeks. An intraocular injection of horseradish peroxidase was used to reveal the individual C laminae (C, C1, C2) in the dorsal lateral geniculate nucleus. Soma size measurements of cells in the C laminae showed significant differences between cells in the deprived layers C and C1 and the corresponding nondeprived layers, but not between cells in deprived and nondeprived layer C2.

The development of neurons in the cat perigeniculate nucleus and reticular nucleus of the thalamus.

The postnatal development of soma size and cytochrome oxidase activity was examined in the perigeniculate nucleus (PGN) and reticular nucleus of the thalamus (RNT). Neurons in the PGN and RNT exhibited a rapid increase in soma size between 2 and 4 weeks of age. During this period of cell growth there is an increase in the intensity of cytochrome oxidase staining within the cell body. Cells in both the PGN and RNT decrease in size after 4 weeks of age, and become very fusiform in shape. During this postnatal period, there is also a shift in cytochrome oxidase staining from the cell body to the dendrites.

Delayed response deficits in squirrel monkeys with unilateral lesions in the midbrain reticular formation

Abstract Electrophysiological experiments suggest that the midbrain reticular formation plays a role in visual attention. However, behavioral tests of this hypothesis have been few and their results inconsistent. Squirrel monkeys with unilateral lesions in the midbrain reticular formation were compared to control animals in their performance on a delayed-response task and a visual pattern discrimination. Nystagmus, adduction of limbs, and lack of visual responsivity were noted a few days after the lesion was made, but these symptoms were transient and were not seen during delayed response testing. The monkeys with lesions had delayed response deficits when the baited food well was contralateral to the lesion and responses were delayed 10 sec or longer. They did not differ significantly from controls on the delayed response task when the baited food well was ipsilateral to the lesion or in learning the visual pattern discrimination. The delayed response task presumably measures the ability to maintain orienting responses, overtly or covertly, during the delay interval. This experiment suggests that the midbrain reticular formation facilitates orienting responses over long delays.

The development of soma size changes in the C-laminae of the cat lateral geniculate nucleus following monocular deprivation

This study examined the pattern of soma size changes in the cat dorsal lateral geniculate nucleus (dLGN) from 4 weeks of age to adulthood following monocular lid suture at two weeks of age. Different patterns of soma size changes were found between the A-laminae and C-laminae. In layers A, A1, and C significant soma size differences were found between the deprived and non-deprived laminae by 4 weeks of age. However, the magnocellular portion of layer C was affected more by deprivation than the parvocellular portion. Layer C1 did not reveal significant soma size changes until 20 weeks of age. Layer C2 did not exhibit any soma size changes at any age. These differential responses to monocular deprivation suggest different time courses of development among the dLGN laminae.