Kwok-Fai So

Lexical and conceptual representation in beginning and proficient bilinguals

Two hypotheses about the association between the equivalent words in a bilinguals two languages are considered. The word association hypothesis proposes that a direct association is established between words in the two languages. During second-language acquisition, that association is used to understand and produce words in the second language by retrieving a word in the first language. The concept mediation hypothesis proposes that the only connection between the two languages is via an underlying, amodal conceptual system, one to which pictured objects also have access. The hypotheses make different predictions about the time to name pictures in the second language relative to the time to translate first-language words into the second language. Two experiments are reported, one with proficient Chinese—English bilinguals and the second with nonfluent English—French bilinguals (American high school students). Subjects read words aloud, named pictures, and translated words; one Chinese—English group categorized pictures and words. The results were consistent with the concept mediation hypothesis and contradicted predictions of the word association hypothesis. There was no evidence for a direct association between words in the two languages in either bilingual group.

Postnatal development of retinal projections in Syrian hamsters: a study using autoradiographic and anterograde degeneration techniques.

Abstract (1) In the Syrian hamster, there is a delay between the laying down of the trajectory of the optic tract (principally a prenatal event) and the formation by the optic tract of its various dense terminal projections (principally a postnatal event): At birth (Day 0), crossed retinofugal axons already cover the lateral geniculate body and extend into the superior colliculus. Although pioneering retinal efferents enter their target nuclei before Day 3, robust development of retinofugal axon telodendria is apparently delayed until that date; the onset of this arborization occurs simultaneously in the lateral geniculate body and superior colliculus, even though the axons of the optic tract pass over the lateral geniculate body before they arrive in the superior colliculus. (2) In all structures receiving binocular projections except the suprachiasmatic nucleus, the development of uncrossed optic tract axons lags behind the development of the crossed axons. (3) In regions where optic tract axons terminate in precise retinotopic order, the projections of the two eyes are segregated in adult animals, and the definitive patterns of connection develop gradually from less differentiated patterns: In the dorsal nucleus of the lateral geniculate body, crossed optic tract axons initially fill the entire nucleus, overlapping with the uncrossed axons; subsequently they withdraw from the ipsilateral projection zone, as the terminal distributions of uncrossed axons increase in density and volume. In adult hamsters, much of the uncrossed retinal input to the superior colliculus is distributed in multiple discrete clusters. Initially the clusters are not present, but develop gradually from a more diffuse pattern. (4) Since the adult distributions of crossed and uncrossed retinofugal projections are established before the time when the eyes open (Day 14), the factors which determine these distributions appear to be independent of exposure to patterned visual input. (5) In neonatal hamsters, as in adults, different populations of retinofugal axons may be distinguished by their differential rates of degeneration. (6) Some of the developmental changes in the hamsters retinal projections are similar to ontogenetic phenomena reported for other populations of central and peripheral axons.

Postnatal development of retinal projections to the lateral geniculate body in Syrian hamsters

The hamster visual system is a convenient model for studying the factors governing the formation of orderly connections in the developing mammalian brain. The immaturity of the hamster nervous system at parturition makes it possible to study the development of some connections in postnatal animals. In the adult hamster, points in the visual field are represented along lines of projection within the dorsal and ventral nuclei of the lateral geniculate body (LGBd, LGBv) and the superior colliculus (SC). In the areas receiving input from the binocular part of the visual field, the terminal arbors of axons originating from the two eyes are found at opposite ends of the lines of projectionS, ~. As part of an effort to understand how this pattern of connections is established, we have studied the development of contraand ipsilateral retinal projections to LGBd 11. It has recently been demonstrated in the monkey visual systemS,9,10 and rat olfactory cortex s that two populations of axon terminals which are segregated from one another in a common target structure in adult animals, may have an overlapping distribution at early stages of development. We now report evidence that, in the LGBd of the hamster, the axons from the ipsilateral eye arrive after the axons from the contralateral eye have become distributed throughout the entire nucleus. These later-arriving axons arborize medially in the nucleus, overlapping initially with the contralateral input; then, within a few days time the axons of the contralateral projection greatly decrease in density in the area of the ipsilateral innervation. Our experiments were carried out on 37 neonatal Syrian hamsters (Mesocricetus auratus Waterhouse). The time of impregnation was determined to within about half an hour by observation of mating behavior. The gestation period of the hamster is 16 days. We counted the first 24 h of postnatal life as day zero. At various postnatal ages (Table I), the pups were taken away from the nest and prepared for surgery. Before day 4, the only anesthesia was the mild hypothermia produced by removal of the pups from the nest. For animals injected on or after day 4, general anesthesia was induced by

Abnormal recrossing retinotectal projections after early lesions in Syrian hamsters: age-related effects.

If the superficial layers of the right superior colliculus (SC) of a newborn hamster are removed, fibers from the left eye not only terminate in the surviving deeper layers of the right SC, but also cross the tectal midline and terminate in the medial third of the left SC. If the right eye is also removed at birth, the abnormally recrossing fibers from the left eye will spread over the entire surface of the left SC31. In this series of experiments, we ablated the right SC of hamsters at birth, but enucleated the right eye on different days postnatally in order to examine the spreading of the axon terminal pattern as a function of age. When the animals were young adults, the pattern of retinofugal projection of the left eye was traced using the Fink-Heimer technique or with autoradiography. It was found that the projections from the left eye continue to spread over the entire left SC when the right eye was removed up until day 10, though their distribution was more sparse when the eye was removed on days 7-10 than when it was removed on days 0-6. When the removal of the right eye was delayed until day 12, the lateral spreading of recrossing axons was markedly reduced. When the right eye was removed on day 14, the distribution of the projections from the left eye was restricted to the medial third of the left SC, just as if no early eye removal had been performed. It appears that after a critical age is reached, even when terminal space is available the axons and axon terminals will not move, at least not over any appreciable distance.

Lesions of the brachium of the superior colliculus in neonate hamsters: correlation of anatomy with behavior.

Abstract We studied the structural and functional consequences of lesions of the right brachium of the superior colliculus during the early postnatal period (day 0, 3, 4, 5, or 6) in Syrian hamsters. Visually elicited turning behavior was spared only if the transection was made sufficiently early (on or before day 3); furthermore, in some animals with the transection made on the day of birth, misdirected turning developed. The orienting behavior studied was correlated with the optic tract inputs to the superior colliculus; it apparently cannot be mediated by the increased diencephalic projections observed in these animals. Thus, optic axons were observed to enter the superior colliculus across the transection of the right brachium of the superior colliculus made on the day of birth or day 3 but not on day 4 or later. The maladaptive function observed (wrong-way turning) in five animals with this transection on the day of birth was correlated with the formation of an abnormal ipsilateral retinotectal projection.

Altered retinotectal topography in hamsters with neonatal tectal slits

Abstract Alterations in normal retinotectal topography by mechanical disruption of fiber passage during development was studied in Syrian hamsters using both neuroanatomical and electrophysiological techniques. The mechanical block to fiber passage was created with a medial to lateral slit across the superior colliculus on the day of birth. At maturity, topographically aberrant projections were found in areas of residual scar tissue. These aberrant projections were synaptically functional, producing neurons with mutiple, spatially separated visual receptive fields. No evidence was found for an orderly compression of the retinotopic map in the tectum consequent to the transitory blockage of fiber passage.