How echogenic is echogenic? Quantitative acoustics of the renal cortex.

The echogenicity of the cortex is an important parameter in interpreting renal sonograms that suggest changes in cortical structure. Echogenicity is currently measured qualitatively, and no attempts have been made at quantification. We developed a method to quantify renal cortical echogenicity in reference to the liver and evaluated its reproducibility, dependence on scanning variables, and potential utility. Sonograms of the right kidney were digitized, and the mean pixel density of regions of the renal cortex and liver was measured and normalized to the gray scale. Echogenicity was expressed as the ratio of the brightness (inverse of mean pixel density) of the cortex to that of the liver. The mean coefficient of variation among measurements performed on multiple sonograms from the same study was 2.8%, and the coefficient of variation among multiple measurements performed on the same kidney over 1 year was 1.8%. The correlation between measurements obtained by two different individuals on identical images was 0.92, with a mean variation of 3.0%. Echogenicity was not significantly affected by type of scanner or probe frequency, but varied inversely with gain. However, the effect of gain was very small within the useful range. Water loading after an overnight fast increased echogenicity in all cases, with a mean increase of 6.4%. Echogenicity of normal kidneys was significantly less than that of the liver (range, 0.810 to 0.987), and in clinical sonograms analyzed retrospectively but blindly, echogenicity correlated with the qualitative gradations of echogenicity originally assigned. The most echogenic kidneys were 62% brighter than normal kidneys, many times greater than the variability of the measurement. We conclude that quantification of renal cortical echogenicity is feasible and reproducible and may be useful in detecting and following renal disease. Echogenicity of the renal cortex is less than that of the liver in healthy subjects and is influenced by the state of diuresis.

Response variability in the mammalian auditory cortex: an objection to feature detection?

Research strategy in the auditory system has tended to parallel that in the visual system, where neurons have been shown to respond selectively to specific stimulus parameters. Auditory neurons have been shown to be sensitive to changes in acoustic parameters, but only rarely have neurons been reported that respond exclusively to only one biologically significant sound. Even at higher levels of the auditory system very few cells have been found that could be described as "vocalization detectors." In addition, variability in responses to artificial sounds have been reported for auditory cortical neurons similar to the response variability that has been reported in the visual system. Recent evidence indicates that the responses of auditory cortical neurons to species-specific vocalizations can also be labile, varying in both strength and selectivity. This is especially true of the secondary auditory cortex. This variability, coupled with the lack of extreme specificity in the secondary auditory cortex, suggests that secondary cortical neurons are not well suited for the role of "vocalization detectors."

Response variability of auditory cortex cells in the squirrel monkey to constant acoustic stimuli.

Sixty-three cells in the superior temporal gyrus of awake squirrel monkeys were tested with 8 species-specific vocalizations plus noise, clicks and tones. Identical series of stimuli were repeatedly presented over 1-5 hour intervals. The responses elicited by both vocalizations and artificial stimuli in primary and secondary cortical neurons often varied over time. In several cases the selectivity of a cell to specific vocalizations appeared to change, i.e., a vocalization which was effective in eliciting a response at one point in the experiment, later became ineffective. In the primary cortex 50% of the cells gave variable responses to one or more of the vocalizations. Twenty percent of the primary cortical cells appeared to change the selectivity of their responses to specific vocalizations. In the secondary cortex 62% of the cells varied in their responses to vocalizations; 42% showing apparent changes in selectivity.