Contrast Adaptation Contributes to Contrast-Invariance of Orientation Tuning of Primate V1 Cells

BACKGROUND: Studies in rodents and carnivores have shown that orientation tuning width of single neurons does not change when stimulus contrast is modified. However, in these studies, stimuli were presented for a relatively long duration (e. g., 4 seconds), making it possible that contrast adaptation contributed to contrast-invariance of orientation tuning. Our first purpose was to determine, in marmoset area V1, whether orientation tuning is still contrast-invariant with the stimulation duration is comparable to that of a visual fixation. METHODOLOGY/PRINCIPAL FINDINGS: We performed extracellular recordings and examined orientation tuning of single-units using static sine-wave gratings that were flashed for 200 msec. Sixteen orientations and three contrast levels, representing low, medium and high values in the range of effective contrasts for each neuron, were randomly intermixed. Contrast adaptation being a slow phenomenon, cells did not have enough time to adapt to each contrast individually. With this stimulation protocol, we found that the tuning width obtained at intermediate contrast was reduced to 89% (median), and that at low contrast to 76%, of that obtained at high contrast. Therefore, when probed with briefly flashed stimuli, orientation tuning is not contrast-invariant in marmoset V1. Our second purpose was to determine whether contrast adaptation contributes to contrast-invariance of orientation tuning. Stationary gratings were presented, as previously, for 200 msec with randomly varying orientations, but the contrast was kept constant within stimulation blocks lasting >20 sec, allowing for adaptation to the single contrast in use. In these conditions, tuning widths obtained at low contrast were still significantly less than at high contrast (median 85%). However, tuning widths obtained with medium and high contrast stimuli no longer differed significantly. CONCLUSIONS/SIGNIFICANCE: Orientation tuning does not appear to be contrast-invariant when briefly flashed stimuli vary in both contrast and orientation, but contrast adaptation partially restores contrast-invariance of orientation tuning

Bigeye Tuna behavior and physiology. Their relevance to stock assessments and fishery biology.

Bigeye tuna (Thunnus obesus) have distinctive depth distributions and vertical movement patterns. They remain in the uniformed temperature surface layer at night and can descend to greater than 500 m depth at dawn. They thus mirror the vertical migrations of the small nektonic organisms of the deep sound scattering layer and extensively exploit these as a food resource. At their maximum depths, bigeye tuna frequently experience prolonged exposure to ambient temperatures (.5 EC) that are up to 20EC colder than surface layer temperature, and oxygen concentrations less than 1.5 ml O2 l-1. In contrast, skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (T. albacares) generally limit their forays to depths where water temperatures are no more than 8EC below surface layer temperatures, and ambient oxygen levels are above 3.5 ml O2 l-1. Understanding the vertical movements and depth distribution of bigeye tuna, as well as the physiological abilities/tolerances and oceanographic conditions controlling them, has been shown key for improving longline catch-per-unit effort analysis and long-term populations assessments in the Pacific. Similar work is needed in the Atlantic

Relative eye size in elasmobranchs

Variation in relative eye size was investigated in a sample of 46 species of elasmobranch, 32 species of sharks and 14 species of batoids (skates and rays). To get a measure of eye size relative to body size, eye axial diameter was scaled with body mass using least-squares linear regression, using both raw species data, where species are treated as independent data points, and phylogenetically independent contrasts. Residual values calculated for each species, using the regression equations describing these scaling relationships, were then used as a measure of relative eye size. Relative and absolute eye size varies considerably in elasmobranchs, although sharks have significantly relatively larger eyes than batoids. The sharks with the relatively largest eyes are oceanic species; either pelagic sharks that move between the epipelagic (0 -200 m) and 'upper' mesopelagic (200-600 m) zones, or benthic and benthopelagic species that live in the mesopelagic (200-1,000 m) and, to a lesser extent, bathypelagic (1,000-4,000 m) zones. The elasmobranchs with the relatively smallest eyes tend to be coastal, often benthic, batoids and sharks. Active benthopelagic and pelagic species, which prey on active, mobile prey also have relatively larger eyes than more sluggish, benthic elasmobranchs that feed on more sedentary prey such as benthic invertebrates. A significant positive correlation was found between absolute eye size and relative eye size, but some very large sharks, such as Carcharodon carcharias have absolutely large eyes, but have relatively small eyes in relation to body mass. Copyright © 2007 S. Karger AG, Base