The late onset of visual texture segmentation in kittens

Texture segmentation was studied developmentally in kittens using a two-alternative forced choice jumping stand paradigm. None of 15 kittens tested solved a texture segmentation task based on orientation contrast prior to 83 days of age, despite their rapid acquisition age, despite their rapid acquisition of an analogous luminance-based image segmentation problem. However, three kittens showed rapid acquisition of the texture segmentation task when the textures were composed of non-oriented elements (dots and annuli), reaching criterion performance by 52-59 days of age. A control experiment demonstrated that kittens can discriminate between vertical and horizontal gratings comparable in line width to the oriented texture elements as early as 53 days of age. The surprisingly late appearance of orientation-based texture segmentation is considered in the context of current models of texture segmentation and is compared to recent reports of a similar finding in human infants [3,54].

Development of an in vitro system for the analysis of ultraviolet radiation-induced suppression of natural killer cell activity

Previous studies have shown that natural killer (NK) cell activity was suppressed in volunteer subjects exposed to ultraviolet radiation (UVR) from solarium lamps. The present studies were carried out to determine the spectrum of UVR responsible for suppression of NK activity and to develop in vitro methods to analyze the effectiveness of sunscreen agents in prevention of UVR-mediated suppression of NK activity and other aspects of immune function. UVR from a xenon arc lamp source was used to irradiate peripheral blood lymphocytes (PBL) in wells of tissue culture flasks, and transmission interference filters were used to eliminate UVR of particular wavelengths. The results indicated that UVR from this source inhibited NK activity of PBL in a dose-dependent manner with a 50% inhibitory dose of 5.5 mJ/cm2 when unfiltered and 29.6 mJ/cm2 when diluted through cellulose acetate, which gave a UV spectrum similar to that in solar radiation. Equivalent suppression of NK activity was mediated by UV-A (UVR > 315 nm) at dose levels of 4.2 J/cm2, which was approximately 140 times greater than the amount of UV-B (UVR < 315 nm) needed to suppress NK activity. Similar dose-response curves were seen for inhibition of mitogenic responses to phytohemagglutinin except that the latter appeared less sensitive than NK to inhibition by UV-A. These studies suggest that when the greater proportion of UV-A in solar radiation and its greater penetration into skin is taken into account, UV-A may have equivalent or greater direct immunosuppressive effects than UV-B.(ABSTRACT TRUNCATED AT 250 WORDS)

Distractor ratio and grouping processes in visual conjunction search

According to feature integration theory, conjunction search is conducted via a serial self-terminating search. However, effects attributed to search processes operating on the entire display may actually reflect search restricted to elements defined by a single feature. In experiment 1 this question is addressed in a reaction-time (RT) paradigm by varying distractor ratios within an array of fixed size. For trials in which the target was present in the array, RT functions were roughly symmetric, the shortest RTs being for extreme distractor ratios, and the longest RTs being for arrays in which there were an equal number of each distractor type. This result is superficially consistent with Zohary and Hochstein's interpretation that subjects search for only one distractor type and are able to switch search strategy from trial to trial. However, negative-trial data from experiment 1 case doubt on this interpretation. In experiment 2 the possible role of 'pop out' and of distractor grouping in visual conjunction search is investigated. Results of experiment 2 suggest that grouping may play a more important role than does distractor ratio, and point to the importance of the spatial layout of the target and of the distractor elements in visual conjunction search. Results of experiment 2 also provide clear evidence that groups of spatially adjacent homogeneous elements may be processed as a unit.

The role of the corpus callosum and extra striate visual areas in stereoacuity in macaque monkeys

Stereoacuity was measured in six normal male rhesus monkeys by requiring them to indicate the closer of two vertical line targets. Their stereoacuity ranged from 13-23 arc sec and was as good as that of human observers tested in the same way. Sectioning the splenium of the corpus callosum, either before or after removing the foveal representation in visual area 2, had no effect on stereoacuity thresholds, indicating that the splenium is not essential for the detection of small retinal disparities at the visual mid-line. In contrast, removal of the foveal representation in V2 permanently and markedly elevated stereoacuity thresholds, almost to the level observed in two monkeys after removal of the representation in striate cortex of the central 5 degrees of the retina. Posterior infero-temporal ablation also impaired stereoacuity, though less strikingly. In these two monkeys, the effect of subsequent damage to the rostral superior colliculi was examined. One animal was unimpaired. In the other, in which the pre-tectum was damaged, the stereoacuity threshold was substantially raised, most plausibly as a consequence of imperfect binocular fixation.

The development of vernier acuity in the cat

The development of vernier acuity was assessed in nine kittens using offsets in a series of square-wave gratings as stimulus targets. The kittens were tested on a jumping stand in a two-choice simultaneous discrimination paradigm, and daily thresholds were tracked with a modified staircase procedure. Vernier acuity improved gradually from initial values of 13-57' arc and reached asymptote at 1.5-5.4' arc by 70-90 days of age. The developmental function for vernier acuity is discussed in relation to physiologic development of the kitten visual system and is related to published data on the development of stereoacuity and spatial resolution in the same species.

Intrazeolite photochemistry. VI. Diffuse reflectance laser flash photolysis and product studies of diphenylmethyl radicals on solid supports

Diphenylmethyl radicals have been generated by 266 nm laser excitation of 1,1,3,3-tetraphenylacetone adsorbed on silica gel and included in NaX and Silicalite zeolites and have been studied using diffuse reflectance laser flash photolysis techniques. The spectrum for the radical shows λmax at ~335 nm in all three supports and is similar to that in solution. The radicals decay over time scales that vary from hundreds of nanoseconds to minutes and there are indications that some radicals may be decaying on shorter time scales than we can monitor. The efficiency of oxygen quenching increases in going from Silicalite to NaX to silica gel, consistent with the greater accessibility of oxygen to silica gel pores as compared to the narrow channels in Silicalite. Laser dose and ketone loading effects were also examined for the various supports. Potential applications of a kinetic treatment of the data based on dispersive reaction kinetics are also discussed as a means of dealing with the problem of decay kinetics that occur over a wide range of time scales. Keywords: zeolites, kinetics, diphenylmethyl radicals, laser photolysis.