Discrimination of spatial phase changes: contrast and position codes

Changing the relative phase of the frequency components of a stimulus usually also produces local contrast variations. Using stimuli composed of the product of a sinusoid (carrier) and a spatial envelope, an attempt was made to distinguish between the visual system's ability to code spatial phase on the one hand and local contrast and position cues on the other. The experiments assess the ability of observers to detect which of two stimuli is farther to the left. In the main experiments a large, easily detectable, envelope shift is presented on every trial and performance is measured as a function of the size of a carrier shift in the same direction. Increasing the size of the carrier shift gradually increases the size of the phase difference between the two stimuli in a trial but simultaneously reduces the contrast change in the bars of the stimulus. If the visual system can code phase directly the ability of observers to detect a change in location should improve as the size of the carrier shift increases but if local contrast is coded performance should be poorer over a small range of carrier shifts than that obtained without a carrier shift. It is shown that a region of poorer performance is obtained and therefore it is concluded that the visual system does not code spatial phase explicitly.     

Aposematic coloration, luminance contrast, and the benefits of conspicuousness

Many organisms use warning, or aposematic, coloration to signaltheir unprofitability to potential predators. Aposematicallycolored prey are highly visually conspicuous. There is considerableempirical support that conspicuousness promotes the effectivenessof the aposematic signal. From these experiments, it is welldocumented that conspicuous, unprofitable prey are detectedsooner and aversion learned faster by the predator as comparedwith cryptic, unprofitable prey. Predators also retain memoryof the aversion longer when prey is conspicuous. The presentstudy focused on the elements of conspicuousness that conferthese benefits of aposematic coloration. Drawing on currentunderstanding of animal vision, we distinguish 2 features ofwarning coloration: high chromatic contrast and high brightness,or luminance, contrast. Previous investigations on aposematicsignal efficacy have focused mainly on the role of high chromaticcontrast between prey and background, whereas little researchhas investigated the role of high luminance contrast. Usingthe Chinese mantid as a model predator and gray-painted milkweedbugs as model prey, we found that increased prey luminance contrastincreased detection of prey, facilitated predator aversion learning,and increased predator memory retention of the aversive response.Our results suggest that the luminance contrast component ofaposematic coloration can be an effective warning signal betweenthe prey and predator. Thus, warning coloration can even evolveas an effective signal to color blind predators.     

Conjunctions of colour, luminance and orientation: the role of colour and luminance contrast on saliency and proximity grouping in texture segregation

To examine whether perceptual grouping on the basis of orientation can be performed simultaneously with or only subsequently to grouping according to colour or luminance, we tested whether subjects are able to segregate arrays of texture elements that differ from surrounding elements by conjunctions of either (i) colour and orientation, or (ii) luminance contrast and orientation, or (iii) luminance contrast polarity and orientation. Subjects were able to use conjunctions between luminance and orientation for segregation but not conjunctions between colour or contrast polarity and orientation. Our results suggest that (i) in agreement with earlier findings, there seem to exist no specific conjunction detectors for colour and orientation or contrast polarity and orientation, and (ii) when orientation defined textures are to be distinguished by virtue of differences in luminance, colour, or contrast polarity, luminance provides a much stronger cue than colour or contrast polarity for saliency-based orientation grouping.     

Scale dependence of spatial patterns and cartography on the detection of landscape change: relationships with species' perception

This paper analyses how landscape pattern detection changes when different spatial and temporal scales and several levels of detail of the cartography are used to describe a landscape affected by land abandonment in northern Spain. In order to integrate landscape composition and structure at different temporal and spatial scales in the same framework, a multiple correspondence factorial analysis was ran for each typology of landscape units. Annual rates of change and scale dependencies were calculated for each typology from the Euclidean distances in the factorial space. Finally, the potential assessment of habitat utilisation by species with different landscape perception and movement capacity was modelled for the range of typologies.
The amount of variance explained by the factorial analysis decreased with the complexity of the typology. Annual rates of change appeared different according to the time span and the detail of the landscape unit typology used. For all typologies, changes were faster during 1983–95, a period characterised by massive land abandonment. However, when the whole period (1956–95) was considered, annual changes were much lower, showing differences between typologies. As a general trend, the variance of the mean annual change decreased with the size of the analysis units. In response to land abandonment, different scale dependencies were found for different levels of detail of the cartography. Coarser typologies are suitable when analysing highly mobile species. However, species with small movement capacity or with a preference for homogeneous habitats perceive more detail in landscape. In this case, a detailed typology is more appropriate.     

pH dependence, substrate specificity and inhibition of human kynurenine aminotransferase I.

Human kynurenine aminotransferase I/glutamine transaminase K (hKAT-I) is an important multifunctional enzyme. This study systematically studies the substrates of hKAT-I and reassesses the effects of pH, Tris, amino acids and alpha-keto acids on the activity of the enzyme. The experiments were comprised of functional expression of the hKAT-I in an insect cell/baculovirus expression system, purification of its recombinant protein, and functional characterization of the purified enzyme. This study demonstrates that hKAT-I can catalyze kynurenine to kynurenic acid under physiological pH conditions, indicates indo-3-pyruvate and cysteine as efficient inhibitors for hKAT-I, and also provides biochemical information about the substrate specificity and cosubstrate inhibition of the enzyme. hKAT-I is inhibited by Tris under physiological pH conditions, which explains why it has been concluded that the enzyme could not efficiently catalyze kynurenine transamination. Our findings provide a biochemical basis towards understanding the overall physiological role of hKAT-I in vivo and insight into controlling the levels of endogenous kynurenic acid through modulation of the enzyme in the human brain.     

Colour and luminance interactions in the visual perception of motion

We sought to determine the extent to which red-green, colour-opponent mechanisms in the human visual system play a role in the perception of drifting luminance-modulated targets. Contrast sensitivity for the directional discrimination of drifting luminance-modulated (yellow-black) test sinusoids was measured following adaptation to isoluminant red-green sinusoids drifting in either the same or opposite direction. When the test and adapt stimuli drifted in the same direction, large sensitivity losses were evident at all test temporal frequencies employed (1-16 Hz). The magnitude of the loss was independent of temporal frequency. When adapt and test stimuli drifted in opposing directions, large sensitivity losses were evident at lower temporal frequencies (1-4 Hz) and declined with increasing temporal frequency. Control studies showed that this temporal-frequency-dependent effect could not reflect the activity of achromatic units. Our results provide evidence that chromatic mechanisms contribute to the perception of luminance-modulated motion targets drifting at speeds of up to at least 32 degrees s(-1). We argue that such mechanisms most probably lie within a parvocellular-dominated cortical visual pathway, sensitive to both chromatic and luminance modulation, but only weakly selective for the direction of stimulus motion.     

The detection of spatial discontinuities: interactions between contrast and spatial contiguity

Thresholds were measured for the detection of spatial discontinuities (notches and bumps) along luminance boundaries. At high contrasts of the boundary, thresholds expressed in terms of the spatial notch/bump height fell well inside the hyperacuity range. Expressed as luminance increment thresholds between adjacent photoreceptors, the same thresholds were similar to those previously reported by Hartridge and by Hecht and Mintz for the detection of a single line. The ability of observers to detect differences in the height of a boundary on either side of a mean luminance gap was also investigated, and the effect of the gap was found to depend upon stimulus contrast. At high contrasts the introduction of a gap increased thresholds, but at the lowest contrasts, thresholds were unaffected by a gap. The role of different spatial frequency and orientational mechanisms in vernier acuity is discussed.     

A model of human pattern perception: association fields for adaptive spatial filters.

Visual neurons in the primary visual cortex 'look' at the retinal image through a four-dimensional array of spatial receptive fields (filter-elements): two spatial dimensions and, at each spatial location, two Fourier dimensions of spatial frequency and orientation. In general, visual objects activate filter-elements along each of these dimensions, suggesting a need for some kind of linking mechanism that determines whether two or more filter-elements are responding to the same or different contours or objects. In the spatial domain, a (spatial) association field between filter-elements, arranged to form first-order curves, has been inferred as a flexible method by which different parts of extended (luminance) contours become associated (Field et al., 1993). Linking has also been explored between filters selective for different regions in Fourier space (e.g. Georgeson and Meese, 1997). Perceived structure of stationary plaids suggests that spatial filtering is adaptive: synthetic filters can be created by the linear summation of basis-filters across orientation or spatial frequency in a stimulus-dependent way. For example, a plaid with a pair of sine-wave components at +/-45 deg looks like a blurred checkerboard; a structure that can be understood if features are derived after linear summation of spatial filters at different orientations. However, the addition of an oblique third-harmonic component causes the plaid to perceptually segment into overlapping oblique contours. This result can be understood if filters are summed across spatial frequency, but, in this case, treated independently across orientation. In the present paper, the architecture of an association field is proposed to permit linking and segmentation of filter-elements across spatial frequency and orientation. Three types of link are proposed: (1) A chain of constructive links around sites of common spatial frequency but different orientation, to promote binding of filters across orientation; (2) Constructive links between sites with common orientation but different spatial frequency, to promote binding of filters across spatial frequency; (3) Long-range links between sites of common spatial frequency but different orientation, whose activation and role are determined by activity in a higher spatial frequency band. A model employing the proposed network of links is consistent with at least six previously reported effects on the perception of briefly presented stationary plaids.     

Effects of sample size on fish parasite prevalence, mean abundance and mean intensity estimates

This study considers the effects of sample size on estimates of three parasitological indices (prevalence, mean abundance and mean intensity) in four different host–parasite systems, each showing a different pattern of infection. Monte Carlo simulation procedures were used in order to obtain an estimation of the parasitological indices, as well as their variance and bias, based on samples of different size. Although results showed that mean values of all indices were similar irrespective of sample size, estimates of prevalence were not significantly affected by sample size whereas mean abundance and mean intensity were affected in at least one sample. Underestimation of values was more perceptible in small (<40) sample sizes. Distribution of the estimated values revealed a different arrangement according to the host–parasite system and to the parasitological parameter. Monte Carlo simulation procedures are, therefore, suggested to be included in studies concerning estimation of parasitological parameters.     

Encoding of luminance and contrast by linear and nonlinear synapses in the retina

Understanding how neural circuits transmit information is technically challenging because the neural code is contained in the activity of large numbers of neurons and synapses. Here, we use genetically encoded reporters to image synaptic transmission across a population of sensory neurons-bipolar cells in the retina of live zebrafish. We demonstrate that the luminance sensitivities of these synapses varies over 10(4) with a log-normal distribution. About half the synapses made by ON and OFF cells alter their polarity of transmission as a function of luminance to generate a triphasic tuning curve with distinct maxima and minima. These nonlinear synapses signal temporal contrast with greater sensitivity than linear ones. Triphasic tuning curves increase the dynamic range over which bipolar cells signal light and improve the efficiency with which luminance information is transmitted. The most efficient synapses signaled luminance using just 1 synaptic vesicle per second per distinguishable gray level.     

Effects of social and spatial density changes inCercopithecus aethiops sabaeus

Findings from two experiments usingCercopithecus aethiops sabaeus (the St. Kitts vervet) are reported. In the first experiment, group number was systematically altered in two groups of animals similar in age and sex but dissimilar in terms of their prior histories. These changes resulted in significant increases in selected affiliative behaviors, decreases in nonsocial behaviors, and no changes in agonistic behaviors. In the second experiment the living space of a well established group of animals was decreased. This change also resulted in a significant increase in the frequency of affiliative behaviors but in no change in agonistic behaviors. Results of the experiments are discussed. This research was in part supported by FY-1976 (Veterans Administration) and The H. F. Guggenheim Foundation through a grant to the Behavioral Sciences Foundation.     

Effects of zinc on creatine kinase: activity changes, conformational changes, and aggregation

The effects of zinc on creatine kinase (CK) are very distinctive compared with other bivalent metal ions. Zinc up to 0.1 mM induced increases in CK activity, accompanied by significant hydrophobic surface exposure and increase in a-helix content of CK. Zinc over 0.1 mM denatured and inactived CK. In the presence of 0.1 mM zinc, the CK activity was very close to that of the native CK, but its conformation changed greatly. The kinetic courses of CK inactivation and conformational change in the presence of 1 mM zinc were measured to determine apparent rate constants of inactivation and conformational change. Zinc over 0.05 mM induced CK aggregation at 37°C, and the aggregation was dependent on zinc concentration, CK concentration, and temperature. The inactivation and aggregation can be reversed by EDTA. An explanation for CK aggregation induced by zinc is proposed, as well as a mechanism for CK abnormality in Alzheimer's disease.To whom correspondence should be addressed.     

Haptic object perception: spatial dimensionality and relation to vision

Enabled by the remarkable dexterity of the human hand, specialized haptic exploration is a hallmark of object perception by touch. Haptic exploration normally takes place in a spatial world that is three-dimensional; nevertheless, stimuli of reduced spatial dimensionality are also used to display spatial information. This paper examines the consequences of full (three-dimensional) versus reduced (two-dimensional) spatial dimensionality for object processing by touch, particularly in comparison with vision. We begin with perceptual recognition of common human-made artefacts, then extend our discussion of spatial dimensionality in touch and vision to include faces, drawing from research on haptic recognition of facial identity and emotional expressions. Faces have often been characterized as constituting a specialized input for human perception. We find that contrary to vision, haptic processing of common objects is impaired by reduced spatial dimensionality, whereas haptic face processing is not. We interpret these results in terms of fundamental differences in object perception across the modalities, particularly the special role of manual exploration in extracting a three-dimensional structure.     

Effects of cholinergic enhancement on visual stimulation, spatial attention, and spatial working memory

We compared behavioral and neural effects of cholinergic enhancement between spatial attention, spatial working memory (WM), and visual control tasks, using fMRI and the anticholinesterase physostigmine. Physostigmine speeded responses nonselectively but increased accuracy selectively for attention. Physostigmine also decreased activations to visual stimulation across all tasks within primary visual cortex, increased extrastriate occipital cortex activation selectively during maintained attention and WM encoding, and decreased parietal activation selectively during maintained attention. Finally, lateralization of occipital activation as a function of the visual hemifield toward which attention or memory was directed was decreased under physostigmine. In the case of attention, this effect correlated strongly with a decrease in a behavioral measure of selective spatial processing. Our results suggest that, while cholinergic enhancement facilitates visual attention by increasing activity in extrastriate cortex generally, it accomplishes this in a manner that reduces expectation-driven selective biasing of extrastriate cortex.     

Phospholipase A2 activity in resting and activated human neutrophils. Substrate specificity, pH dependence, and subcellular localization

We have studied the phospholipase A2 activity in fractionated human neutrophils, employing labeled phosphatidylinositol, phosphatidylcholine, and phosphatidylethanolamine as exogenous substrates. We used these phospholipid substrates labeled in the sn-1 position and measured the resulting labeled lysophospholipid forms in order to ascertain the phospholipase A2 specificity. In postnuclear supernatants from resting and A23187-activated cells, the phospholipase A2 activity showed a similar pH dependence curve with two pH optima at 5.5 and 7.5. Extracts from activated cells showed a 3-6-fold increase in enzyme activity. The subcellular distribution of phospholipase A2 activity in resting and A23187-treated human neutrophils was investigated by fractionation of postnuclear supernatants on continuous sucrose gradients. The neutral phospholipase A2 behaved as a membrane-bound enzyme and was mainly localized in the plasma membrane, the azurophilic granule, and in an ill-defined region of the gradient between the specific granules and mitochondria. The phospholipase A2 located in this undefined region showed a higher degree of activation than that located in other subcellular particulates in A23187-treated cells. This specific activation of an intracellular phospholipase A2 activity during cell stimulation indicates that cell compartmentalization may play a role in the formation of cell-activating and/or signal-transducing agents through the generation of arachidonate metabolites. Phosphatidylinositol was a better substrate for the plasma membrane enzyme, whereas phosphatidylcholine and phosphatidylethanolamine behaved as better substrates for intracellular organelle phospholipase A2 activities. The phospholipase A2 with maximal activity at pH 5.5 behaved as a soluble enzyme, and was almost completely localized in the azurophilic granules. Upon cell activation this acid enzyme activity was released in a similar way to beta-glucuronidase, a marker of azurophilic granules. These results demonstrate the different molecular properties of the phospholipase A2 activity, on the basis of its cellular location.     

Density dependence,spatial scale and patterning in sessile biota

Sessile biota can compete with or facilitate each other, and the interaction of facilitation and competition at different spatial scales is key to developing spatial patchiness and patterning. We examined density and scale dependence in a patterned, soft sediment mussel bed. We followed mussel growth and density at two spatial scales separated by four orders of magnitude. In summer, competition was important at both scales. In winter, there was net facilitation at the small scale with no evidence of density dependence at the large scale. The mechanism for facilitation is probably density dependent protection from wave dislodgement. Intraspecific interactions in soft sediment mussel beds thus vary both temporally and spatially. Our data support the idea that pattern formation in ecological systems arises from competition at large scales and facilitation at smaller scales, so far only shown in vegetation systems. The data, and a simple, heuristic model, also suggest that facilitative interactions in sessile biota are mediated by physical stress, and that interactions change in strength and sign along a spatial or temporal gradient of physical stress.