Neural analysis of visual information during locomotion.

For most vertebrate species, vision is critical during locomotion. In this review, we survey what is known about neural mechanisms that might be involved in this visual analysis. Problems that such mechanisms are likely to solve include: (1) determination of heading (the discrepancy between an observer's direction of motion and direction of gaze); (2) detection of course changes; (3) setting of courses using goals and landmarks; (4) obstacle avoidance; (5) accurate foot placement. To understand how the visual system handles these tasks, we must first consider what neurons 'see' during locomotion: this is determined both by an observer's motion and by his gaze. We then review response properties of neurons in the cortical 'motion pathways' of the monkey and cat as they relate to problems encountered during locomotion. The most studied areas are the medial superior temporal area in the monkey, which has been linked to heading determination, and the lateral suprasylvian area in the cat, where many cells are sensitive to motion in depth, and some are selective for optic flow patterns generated during locomotion. A few subcortical populations have also been linked to visual analysis during locomotion. Most notable are cells in the pigeon's nucleus rotundus: these respond selectively to looming stimuli, some firing at a specific time before the stimulus collides with the bird. Another intriguing population is in the cat's visual pontine nucleus, where cells respond to large displays suggestive of optic flow during locomotion.

Prolonged survival of axons terminating within lesions of cat visual cortex.

It is well known that brain lesions made by the injection of ibotenic acid destroy neuronal cell bodies but do not kill passing axons. We have found that axons terminating within such lesions in visual cortex also survive for at least 2 months, despite the absence of available synaptic sites. We made tracer injections in area 17, and observed dense patches of anterograde label within lesions in other visual cortical areas. Furthermore, because the retinotopic site of the tracer injection was known, we could conclude that the retinotopic site was encompassed within the lesion.

Visual analysis and image motion in locomoting cats.

During locomotion, observers see a characteristic pattern of motion referred to as an optic flow field. To investigate how they make use of this pattern, we have developed a paradigm for testing visual function during locomotion. Foot placement was recorded while cats walked down an alley cluttered with a high density of small objects; the task was to avoid stepping on any object. In the experiments reported here, motion cues were eliminated by the use of low-frequency strobe lighting. In bright continuous light cats performed with great accuracy, and likewise at scotopic light levels. However, in strobe lighting their error rates increased more than threefold. This deterioration could not be attributed to lower acuity, since the cats' performance remained excellent when the light level was reduced well below that afforded by the strobe light. When very dim continuous light was combined with low-frequency strobe lighting, performance was substantially better than under strobe light alone. We conclude that motion-sensitive neurons make a major contribution to visual guidance of foot placement during locomotion. When strobe lighting is combined with very dim continuous light, even the minimal motion information available in the intervals between bright strobe flashes improves performance significantly. Cats were also trained to discriminate between complex patterns, and this discrimination was not affected by strobe lighting, suggesting that motion-sensitive neurons are not critical for this analysis.

Rods affect S-cone discrimination on the Farnsworth-Munsell 100-hue test.

Rod influence on hue discrimination was assessed by the Farnsworth-Munsell 100-hue test. Rod influence was taken as the difference in error scores obtained after complete dark adaptation and during the cone plateau at three mesopic (23, 9, 3 td) and one standard (158 td) light level. On the FM 100, rods produced a differential discrimination loss along a tritan axis as compared with a red-green axis without any bias toward a rod confusion axis. Rods appear to impair discrimination mediated by S-cone pathways, which at moderate levels of illumination can differentially elevate tritan errors on the FM 100.

When discrimination supersedes confidentiality.

Recent decisions have made it clear that the courts are now less inclined to apply the doctrine of academic abstention in cases regarding discrimination in promotion and tenure. Colleges and universities with closed rank and tenure proceedings can now expect to be challenged by unsuccessful candidates suspecting discrimination.

Marine dolomite of unusual isotopic composition.

A piston core taken off of the coast of Oregon in 358 meters of water contained an indurated calcareous layer composed partly of dolomite with a composition Ca(58.7)Mg(41.3). Dolomites of this chemical composition are typical of the supratidal environment. However, the dolomite has isotopic composition delta0(18) = 5.8 per mille, deltaC(13) = 35.1 per mille relative to the Chicago PDB-I standard. The unusual carbon isotope ratio is similar to that of calcites produced as a byproduct of bacterial breakdown of hydrocarbons.