Saccadic oscillations facilitate ocular perfusion from the avian pecten

THE evolution of the eye is constrained by two conflicting requirements--good vascular perfusion of the retina, and an optical path through the retina that is unobstructed by blood vessels. Birds are interesting in that they have higher metabolic rates and thicker retinas than mammals, but have no retinal blood vessels. Nutrients and oxygen must thus reach the neurons of the inner retina either from the choroid through 300 micron of metabolically very active retina, or from the pecten, a pleated vascular structure protruding from the head of the optic nerve into the vitreous chamber, and more than a centimetre away from some retinal neurons. Despite the diffusional distance involved, several lines of evidence indicate that the pecten is the primary source of nutrients for the inner retina: the presence of an oxygen gradient from pecten to retina, the large surface area produced by macroscopic folds and by microscopic infoldings of the luminal and external surfaces of the capillary endothelium, extrusion of circulating fluorescein, high content of carbonic anhydrase and alkaline phosphatase, and retinal impairments after pecten ablation. Another peculiarity of birds, their saccadic oscillations, occur with a large cyclotor-sional component during every saccadic eye movement. In different species, saccades, which occur at intervals of 0.5-40 s, have up to 13 oscillations with frequencies of 15-30 Hz and ampliá-tudes of about 10 degrees. Therefore, as much as 12% of some birds' total viewing time may be subject to the image instability caused by the oscillations. Using fluorescein angiography, we show here that during every saccade, the pecten acts as an agitator which propels perfusate towards the central retina much more effectively than is observed during intersaccadic intervals.     

Neurophysiological basis of directional hearing in amphibia

Discrete regions of neural tissue along the anterior-posterior axis of the torus semicircularis act as resonators, each one broadly tuned to a different range of frequencies, and each one responding optimally to the incident sound from a different direction. In this way, a map of auditory space is represented in the midbrain.     

Localization of the X inactivation centre on the human X chromosome in Xq13

X-chromosome inactivation results in the strictly cis-limited inactivation of many but not all genes on one of the two X chromosomes during early development in somatic cells of mammalian females. One feature of virtually all models of X inactivation is the existence of an X-inactivation centre (XIC) required in cis for inactivation to occur. This concept predicts that all structurally abnormal X chromosomes capable of being inactivated have in common a defineable region of the X chromosome. Here we report an analysis of several such rearranged human X chromosomes and define a minimal region of overlap. The results are consistent with models invoking a single XIC and provide a molecular foothold for cloning and analysing the XIC region. One of the markers that defines this region is the XIST gene, which is expressed specifically from inactive, but not active, X chromosomes. The localization of the XIST gene to the XIC region on the human X chromosome implicates XIST in some aspect of X inactivation.     

A variant of the mammalian somatotopic map in a bat

Two ordered representations of the body surface, S-I and S-II, have been described on the cortical surface of the brains of a variety of mammals; additional separate topographical maps have been found in the somatosensory cortex of the cat and monkey. Except for minor variations in the placement of the body parts, the basic somatotopy of the maps is remarkably consistent across species. As the reasons for this consistency and the minor variations are unclear, we examined the somatotopy of the bat, whose body plan has been modified extensively so that the forelimb can be used for flight. We report here that in both S-I and S-II of the grey-headed flying fox, not only is the representation of the distal forelimb displaced from its usual position on the map, but the digits are directed caudally instead of rostrally as they are in all other mammals studied. The variant somatotopy appears to reflect the postural differences between flying and walking mammals, supporting the notion that topographical maps may have functional significance apart from their point-to-point connections with the sensory periphery.     

Dynamics of the amphibian middle ear.

Using laser speckle interferometry, we show that the directional information of acoustic signals is encoded in the motion of eardrum. Moreover, the frequency sensitivity of the auditory system is determined by the mechanical properties of the middle ear, which acts like a damped resonator.     

Power,Conflicts, and Resolutions: A Change Agent's Perspective on Conducting Action Research Within a Multiorganizational Partnership

Partnership has become the watchword of government policy over the past 10 years. It is a politically expedient solution to a wide range of complex public policy issues. Although examined in the literature from the viewpoint of an external observer, there has been relatively little examination of what it means to be working in a partnership from the inside. Based on experience as a change agent and action researcher working within a multiorganizational partnership, this paper considers the dilemmas and resolutions experienced in this role. The paper provides a first person reflective account of the inherent ethical, political, and personal turmoils and issues around multilevel dynamics, role ambiguity, power games, political entrepreneurship, and shadows of the past. The paper exposes the key learning points to emerge and concludes that change agents need good support mechanisms and bestowed power in order to challenge vested and powerful interests.