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Color vision requires the activity of cone photoreceptors to be compared in post-receptoral circuitry. Decades of psychophysical measurements have quantified the nature of these comparative interactions on a coarse scale. How such findings generalize to a cellular scale remains unclear. To answer that question, we quantified the influence of surrounding light on the appearance of spots targeted to individual cones. The eye's aberrations were corrected with adaptive optics and retinal position was precisely tracked in real-time to compensate for natural movement. Subjects reported the color appearance of each spot. A majority of L-and M-cones consistently gave rise to the sensation of white, while a smaller group repeatedly elicited hue sensations. When blue sensations were reported they were more likely mediated by M- than L-cones. Blue sensations were elicited from M-cones against a short-wavelength light that preferentially elevated the quantal catch in surrounding S-cones, while stimulation of the same cones against a white background elicited green sensations. In one of two subjects, proximity to S-cones increased the probability of blue reports when M-cones were probed. We propose that M-cone increments excited both green and blue opponent pathways, but the relative activity of neighboring cones favored one pathway over the other.

The retina is the most accessible element of the central nervous system for linking behavior to the activity of isolated neurons. We unraveled behavior at the elementary level of single input units-the visual sensation generated by stimulating individual long (L), middle (M), and short (S) wavelength-sensitive cones with light. Spectrally identified cones near the fovea of human observers were targeted with small spots of light, and the type, proportion, and repeatability of the elicited sensations were recorded. Two distinct populations of cones were observed: a smaller group predominantly associated with signaling chromatic sensations and a second, more numerous population linked to achromatic percepts. Red and green sensations were mainly driven by L- and M-cones, respectively, although both cone types elicited achromatic percepts. Sensations generated by cones were rarely stochastic; rather, they were consistent over many months and were dominated by one specific perceptual category. Cones lying in the midst of a pure spectrally opponent neighborhood, an arrangement purported to be most efficient in producing chromatic signals in downstream neurons, were no more likely to signal chromatic percepts. Overall, the results are consistent with the idea that the nervous system encodes high-resolution achromatic information and lower-resolution color signals in separate pathways that emerge as early as the first synapse. The lower proportion of cones eliciting color sensations may reflect a lack of evolutionary pressure for the chromatic system to be as fine-grained as the high-acuity achromatic system.

Current flow is an important biological stimulus for larval anuran amphibians, but little is known about how it is perceived. We quantified behavioral responses to controlled water flow in the bullfrog tadpole (Rana catesbeiana) at developmental stages prior to metamorphic climax, and examined the contribution of a functioning lateral line system to these behaviors. Tadpoles at these developmental stages show a significant preference for the sides and bottom of a flow tank. In response to water flow at three different rates, they exhibit a significant, time-dependent tendency to move downstream, away from the source of the flow, and to remain in areas where flow is minimized. The consistency of these behaviors at all tested flow rates suggests that the animals are not simply passively pushed by the current; instead, they actively swim away from the current source. Tadpoles do not exhibit positive rheotaxis towards the source of the flow at any flow rate but as a group are randomly oriented. Treatment with cobalt chloride, a known blocker of superficial neuromast function, significantly reduces the tendency to move downstream, but does not alter the preference for the sides and bottom of the tank. Tadpoles' movements under flow are consistent with a model of locomotion based on a directed random walk.

The purpose of this study was to test whether transient increases in homocysteine would promote changes in markers of endothelial injury, cellular fibronectin (cFN), and soluble vascular cell adhesion molecule 1 (sVCAM-1). Homocysteine, cFN, and sVCAM-1 concentrations increased significantly in response to a methionine load by 6 hours in human subjects. However, no correlation was observed between homocysteine and cFN or sVCAM-1. To directly test whether homocysteine can injure endothelial cells, human umbilical vein endothelial cells (HUVEC) were incubated with increasing concentrations of homocysteine, plasma, or serum from hyperhomocysteinemic mice or from the methionine-loaded test subjects. cFN release was increased from endothelial cells cultured with plasma (but not serum) of hyperhomocysteinemic transgenic mice or from methionine-loaded human subjects. These data suggest that very high homocysteine concentrations can promote endothelial injury; however, this effect is likely mediated by secondary effects that include a factor(s) present in plasma that affects endothelial cells., (Copyright 2003 Elsevier, Inc. All rights reserved.)