Interocular Transfer in a Marsupial: The Brush-Tailed Possum <i>(Trichosurus vulpecula)</i>

Abstract

The corpus callosum has been implicated as the major commissure mediating the interhemispheric transfer of visual information in placental mammals. As marsupials lack a corpus callosum, they may be incapable of such transfer. This study was undertaken to test this hypothesis. Three brush-tailed possums (Trichosurus vulpecula) underwent midsaggital transection of the optic chiasm to ensure that retinal input was restricted to the ipsilateral side of the brain. Three other possums had sham operations to control for visual or learning impairments that might be caused by any other aspects of the surgery. All six animals were monocularly occluded, and conditioned to respond to the positive of two visual stimuli. When a criterion of ≥ 18/20 correct was attained for two consecutive blocks of 20 trials, the trained eye was occluded, and the stimuli were presented to the untrained eye. Conditioning was continued until the criterion of learning was achieved with this eye. The stimulus contingencies were then reversed, so that the previously negative stimulus became the positive one, and the conditioning procedure was repeated. Black/white and horizontal/vertical visual discriminations were used. Both the experimental and control groups exhibited high levels of interocular transfer on all tasks. It is concluded that T. vulpecula possesses a pathway capable of mediating the interhemispheric transfer of visual information.

Failure of interocular transfer in two types of learning in the crab Chasmagnathus

“…lOT is an important phenomenon relating to mechanistic aspects of acquisition and retention of long-term memory. The occurrence of IOT G. Hermitte 9 J. Aggio 9 H. Maldonado (1~) Laboratorio de Fisiologla del Comportamiento Animal, Facultad de Ciencias Exactas y Naturales, Depto Biologia, Buenos Aires University, (1428) Beunos Aires, Argentina is shown to depend on the following factors: a) the degree of difficulty in a task to be learned (Muntz 1961;Schulte 1957;Menkhaus 1957;Ingle 1968), b) the amount of training given on a particular discrimination problem (Catania 1963), c) the type of response required from the subject (McClearly 1960), d) the specific stimuli employed (Ingle 1965), e) the experimental paradigm used (Green et al 1978), f) the amount of crossed and uncrossed retinal projections in vertebrates (Robinson 1982;Cowie and Parkinson 1973;Ingle 1965;Van Hof 1970;Cowan et al 1961;Sharma 1972;Mohn and Steele Russell 1983) or g) the position of the discriminated stimuli on the retina (Levine 1945;Graves and Goodale 1977;Mallin and Delius 1983).…”

Conserved and divergent mechanisms of cortical development across mammalian brain evolution

“…Despite this conservation of the organisational principles of the mature commissural tracts across mammals, it is possible that the emergence of the corpus callosum in eutherians might result in a difference in the initial axonal elongation of commissural axons. In fact, in eutherians, commissural neurons extend their axons medially to form the corpus callosum as they migrate through the intermediate zone of the developing cortex, while in noneutherian mammals, the adult pattern of commissural projections suggests that commissural axons project laterally, to reach the anterior commissure (Heath and Jones 1971, Robinson 1982, Richards et al 1997, Lickiss et al 2012. In addition to this, a sequential order of axonal elongation, during cortical development has been described in neurons located in the deeper layers of the eutherian neocortex, with laterally projecting axons that reach subcortical targets being extended before medially projecting axons that target the contralateral hemisphere (Richards et al 1997).…”

“…In monotremes and marsupials, the anterior commissure is the main interhemispheric connection (Heath et al 1971, Robinson 1982, which transfers information from both olfactory allocortical and neocortical parts of the pallium (Putnam et al 1968, Heath and Jones 1971, Robinson 1982. In addition to this, in diprotodont marsupials, the anterior commissure also receives neocortical axons from the internal capsule via an additional tract, the fasciculus aberrans, which functions as a shortcut for axons arising from the neocortex (Smith 1902, Ashwell 2010.…”

“…This evolutionary innovation involved a medio-dorsal projection of the axons extending from commissural neurons, whereas in non-eutherians these axons turn latero-ventral to cross the midline at the anterior commissure. While in eutherians the anterior commissure connects the anterior olfactory nucleus and the anterior piriform cortex (anterior limb, pars olfactoria), the perirhinal and rostral cortex (posterior limb, pars interhemispherica) and regions of the amygdala (stria terminalis) (Pires-Neto and Lent 1993, Cummings et al 1996, Pires-Neto et al 1998, in marsupials the anterior commissure is the major commissure connecting all those targets, plus most neocortical areas and cingulate cortices (Putnam et al 1968, Heath and Jones 1971, Robinson 1982. Studies in hamsters and rats have shown that, in these species, the pioneering fibres of the anterior commissure, which extend from neurons located in the olfactory (piriform) cortex, cross the midline from embryonic day (E) 14 and E16, respectively (Santacana et al 1992, Pires-Neto and Lent 1993, Silver 1993, Pires-Neto et al 1998, using chemical and physical cues from surrounding guidepost glial cells (Silver et al 1982, Silver 1993, Cummings et al 1996, Pires-Neto et al 1998.…”

“…In addition to this, neurogenesis in the dunnart piriform cortex has already started at birth and is more advanced than in the neocortex, such that there is a This knowledge is an important initial step to be able to manipulate gene expression specifically in these neurons, in order to explore their pattern of axonal elongation, as well as the timeline and the mechanism of cortical circuits formation, as described in the following sections. project through the corpus callosum in eutherian mammals and through the anterior commissure in marsupials (Putnam et al 1968, Heath and Jones 1971, Robinson 1982. Taking First, the fluorophore eYFP (Niwa et al 1991, Matsuda et al 2004 91.5%, n = 11 litters, 50 total joeys, Figure 3.3B).…”