A computational model of visual marking using an inter-connected network of spiking neurons: the spiking search over time & space model (sSoTS).

In the real world, visual information is selected over time as well as space, when we prioritise new stimuli for attention. Watson and Humphreys [Watson, D., Humphreys, G.W., 1997. Visual marking: prioritizing selection for new objects by top-down attentional inhibition of old objects. Psychological Review 104, 90-122] presented evidence that new information in search tasks is prioritised by (amongst other processes) active ignoring of old items - a process they termed visual marking. In this paper we present, for the first time, an explicit computational model of visual marking using biologically plausible activation functions. The "spiking search over time and space" model (sSoTS) incorporates different synaptic components (NMDA, AMPA, GABA) and a frequency adaptation mechanism based on [Ca(2+)] sensitive K(+) current. This frequency adaptation current can act as a mechanism that suppresses the previously attended items. We show that, when coupled with a process of active inhibition applied to old items, frequency adaptation leads to old items being de-prioritised (and new items prioritised) across time in search. Furthermore, the time course of these processes mimics the time course of the preview effect in human search. The results indicate that the sSoTS model can provide a biologically plausible account of human search over time as well as space.     

Local and relational judgements of surface colour: constancy indices and discrimination performance

Colour constancy is generally assumed to arise from a combination of perceptual constancy mechanisms operating to partially discount illumination changes and relational mechanisms involved in judging the colour relationships between object surfaces. Here we examined the characteristics of these mechanisms using a 'yes/no' task. Subjects judged whether a target colour patch embedded in an array of coloured patches (a) stayed the same across a simulated temporal illuminant change (local colour judgement), or (b) changed in a manner consistent with the illuminant change (relational colour judgement). The colour of the target patch remained constant in one-third of the trials, changed in accord with the illuminant shift in another third, and shifted partially with the illuminant change in the remaining third. We found that perceptual constancy was relatively weak and relational constancy strong, as assessed using a modified colour constancy index. Randomising the spatial positions of coloured patches across the illuminant change did not affect subjects' constancy indices. Application of signal detection analysis revealed some otherwise hidden effects. In the case of relational judgements, subjects adopted more conservative criteria (fewer true and false positives) with randomisation, maintaining a constant level of discrimination performance (d'). For local judgements, randomisation led to small increases in performance but no changes in criteria. We conclude that signal detection theory provides a useful tool to supplement conventional approaches to understanding colour constancy.     

Motion edges and regions guide image segmentation by colour.

Image segmentation is an important early stage in visual processing in which the visual system groups together parts of the image that belong together, prior to or in conjunction with object recognition. Two principal processes may be involved in image segmentation: an edge-based process that uses feature contrasts to mark boundaries of coherent regions, and a region-based process that groups similar features over a larger scale. Earlier, we have shown that motion and colour interact strongly in image segmentation by the human visual system. Here we explore the nature of this interaction in terms of edge- and region-based processes. We measure performance on a region-based colour segmentation task in the presence of distinct types of motion information, in the form of edges and regions which in themselves do not reveal the location of the colour target. The results show that both motion edges and regions may guide the integrative process required for this colour segmentation task. Motion edges appear to act by delimiting areas over which to integrate colour information, whereas motion similarities define primitive surfaces within which colour grouping and segmentation processes are deployed.     

Spatial context and top-down strategies in visual search

Marvin M. Chun and Yuhong Jiang (1998) investigated the role of spatial context on visual search. They used two display conditions. In the Old Display condition, the spatial arrangement of items in the search display was kept constant throughout the experiment. In the New Display condition, the spatial arrangement of items was always novel from trial to trial. The results showed better performance with Old Displays than with New Displays. The authors proposed that repeated spatial context help guiding attention to the target location, thus they termed this effect Contextual Cueing. We present three attempts to reproduce this effect. Experiments 1 and 2 were near exact replications of experiments in Chun and Jiang's report, where we failed to obtain Contextual Cueing. Post-experimental interviews revealed that participants used different search strategies when performing the task: an 'active' strategy (an active effort to find the target), or a 'passive' strategy (intuitive search). In Experiment 3, we manipulated task instructions to bias participants into using active or passive strategies. A robust Contextual Cueing Effect was obtained only in the passive instruction condition.     

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.     

Gaze in Visual Search Is Guided More Efficiently by Positive Cues than by Negative Cues

Visual search can be accelerated when properties of the target are known. Such knowledge allows the searcher to direct attention to items sharing these properties. Recent work indicates that information about properties of non-targets (i.e., negative cues) can also guide search. In the present study, we examine whether negative cues lead to different search behavior compared to positive cues. We asked observers to search for a target defined by a certain shape singleton (broken line among solid lines). Each line was embedded in a colored disk. In “positive cue” blocks, participants were informed about possible colors of the target item. In “negative cue” blocks, the participants were informed about colors that could not contain the target. Search displays were designed such that with both the positive and negative cues, the same number of items could potentially contain the broken line (“relevant items”). Thus, both cues were equally informative. We measured response times and eye movements. Participants exhibited longer response times when provided with negative cues compared to positive cues. Although negative cues did guide the eyes to relevant items, there were marked differences in eye movements. Negative cues resulted in smaller proportions of fixations on relevant items, longer duration of fixations and in higher rates of fixations per item as compared to positive cues. The effectiveness of both cue types, as measured by fixations on relevant items, increased over the course of each search. In sum, a negative color cue can guide attention to relevant items, but it is less efficient than a positive cue of the same informational value.     

The heritable basis and cost of colour plasticity in coastrange sculpins

Both genetic and plastic traits contribute to adaptation in novel environments. Phenotypic plasticity can facilitate adaptation by allowing for existence in a wider range of conditions and a faster response to environmental change than gene‐based selection. Coastrange sculpins (Cottus aleuticus) colonize new and variable streams arising in the wake of receding glaciers in south‐east Alaska, and substrate‐matching plasticity may enhance colonization success by reducing detection by visual predators. As part of a long‐term study of the fitness consequences of colour plasticity and its capacity to respond to both positive and negative selection, we investigated whether it is heritable and costly. We raised full‐sib broods of sculpins in the laboratory: one half of each brood was raised in white containers, the other half in black. After 4 months, we digitally analysed their colour and found significant but weak heritability in both baseline colour and colour plasticity. To investigate the cost of colour plasticity, we compared the growth and mortality rates of juvenile sculpins reared under constant substrate colours to those reared on substrates that changed colour frequently, and compared growth rates among sculpin that differed in their colour change ability. We found evidence of small costs of plasticity, consistent with other studies of natural populations. Evidence of heritable genetic variation for plasticity and small costs to its maintenance and expression contributes to explanations of how plasticity is variable and persistent among wild populations and underscores its ability to respond both positively and negatively to selection in variable habitats.     

Natural levels of colour polymorphism reduce performance of visual predators searching for camouflaged prey

Polymorphism, the coexistence of two or more variants within a population, has served as a classic model system to address questions about the evolution and maintenance of intraspecific variation. It has been hypothesized that a natural level of colour polymorphism may impair the search efficiency of visually orientated predators. To test this polymorphism protects hypothesis, we asked human participants to search for images of natural black, striped or grey Tetrix subulata grasshopper colour morphs presented against photographs of their natural habitat on computer screens. Fewer grasshoppers were detected when morphs were presented in mixed than in uniform sequences. All three morphs benefited to comparable degrees, in terms of reduced detection, from being presented in polymorphic sequences. Our findings demonstrate that natural levels of polymorphic variation can impede the efficiency of visually orientated predators and increase survival of prey. This protective effect supports the limited attention hypothesis, explains why predators develop ‘search images’, may account for the spread and establishment of novel colour variants, and contributes to maintenance of polymorphisms. © 2014 The Authors. Biological Journal of the Linnean Society published by John Wiley & Sons Ltd on behalf of The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112 , 546–555.     

Ontogenetic colour change signals sexual maturity in a non-territorial damselfly

Conspicuous colouration increases male reproductive success through female preferences and/or male–male competition. Despite the advantages of conspicuous colouration, inconspicuous male morphs can exist simultaneously in a population due to genetic diversity, condition dependence or developmental constraints. We are interested in explaining the male dichromatism in Xanthagrion erythroneurum damselflies. We reared these damselflies in outdoor insectaries under natural conditions and showed that this species undergoes ontogenetic colour changes. The younger males are yellow and change colour to red 6–7 days after their emergence. We took red and yellow male reflectance spectra and found that red males are brighter than yellow males. Next, we aimed to determine whether ontogenetic colour change signals sexual maturity with field observations and laboratory experiments. Our field observational data showed that red males are in higher abundance in the breeding territory, and they have a higher mating frequency than yellow males. We confirmed these field observations by enclosing a red and a yellow male with two females and found that yellow males do not mate in presence of red males. To determine whether colour change signals sexual maturity, we measured mating success of males before and after colour changes by enclosing a single male at different age (day 3-day 7) and colour (yellow, intermediate and red) with a single female in a mating cage. Males did not mate when yellow but the same male mated after it changed colour to red, suggesting the ontogenetic colour change signals sexual maturity in this species. Our study shows that male dichromatism can be age-dependent and ontogenetic colour change can signal age and sexual readiness in non-territorial insects.     

Colour pattern as a single trait driving speciation in Hypoplectrus coral reef fishes?

Theory shows that speciation in the presence of gene flow occurs only under narrow conditions. One of the most favourable scenarios for speciation with gene flow is established when a single trait is both under disruptive natural selection and used to cue assortative mating. Here, we demonstrate the potential for a single trait, colour pattern, to drive incipient speciation in the genus Hypoplectrus (Serranidae), coral reef fishes known for their striking colour polymorphism. We provide data demonstrating that sympatric Hypoplectrus colour morphs mate assortatively and are genetically distinct. Furthermore, we identify ecological conditions conducive to disruptive selection on colour pattern by presenting behavioural evidence of aggressive mimicry, whereby predatory Hypoplectrus colour morphs mimic the colour patterns of non-predatory reef fish species to increase their success approaching and attacking prey. We propose that colour-based assortative mating, combined with disruptive selection on colour pattern, is driving speciation in Hypoplectrus coral reef fishes.     

Body size influences differently the detectabilities of colour morphs of cryptic prey

Body size and coloration may contribute to variation in performance and fitness among individuals; for example, by influencing vulnerability to predators. Yet, the combined effect of size and colour pattern on susceptibility to visual predators has received little attention, particularly in camouflaged prey. In the colour polymorphic pygmy grasshopper Tetrix subulata (Linnaeus, 1758), females are larger than males, although there is a size overlap between sexes. In the present study, we investigated how body size and colour morph influenced detection of these grasshoppers, and whether differences in protective value among morphs change with size. We conducted a computer‐based experiment and compared how human ‘predators’ detected images of large, intermediate or small grasshoppers belonging to black, grey or striped colour morphs when embedded in photographs of natural grasshopper habitats. We found that time to detection increased with decreasing size, that differences in time to detection of the black, grey and striped morphs depended differently on body size, and that no single morph provided superior or inferior protection in all three size classes. By comparing morph frequencies in samples of male and female grasshoppers from natural populations, we also examined whether the joint effects of size and colour morph on detection could explain evolutionary dynamics in the wild. Morph frequency differences between sexes were largely in accordance with expectations from the results of the detection experiment. The results of the present study demonstrate that body size and colour morph can interactively influence detection of camouflaged prey. This may contribute to the morph frequency differences between male and female pygmy grasshoppers in the wild. Such interactive effects may also influence the dynamics of colour polymorphisms, and contribute to the evolution of ontogenetic colour change and sexual dichromatism. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 112–122.     

Male bill colour and competition in zebra finches

Several studies have assessed the role of bill colour in sexual selection and especially with respect to sexual preferences. Even though there are indications that bill colour is related to male quality, so far it has not been shown that bill colour influences male-male interaction. We used male zebra finches with artificially coloured bills in a competitive context to measure the effect of bill colour. In these tests the experimental bird could choose between two feeding sites, each near a stimulus bird with a different bill colour. We tested orange against red, no bird against orange/red and orange/red against green respectively. We found no difference in behaviour towards an orange compared to a red billed stimulus. However the birds spent relatively more time eating when alone compared to being close to a potential competitor. In addition, more time was spent eating than on other behaviours when the birds were close to the orange/red billed stimulus compared to the green billed stimulus. So, although no effect was found in the orange against red test, the results suggest that bill colour may play some role in male-male interaction.     

On the Hunt: Searching for Poorly Defined Camouflaged Targets

As camouflaged targets share visual characteristics with the environment within which they are embedded, searchers rarely have access to a perfect visual template of such targets. Instead, they must rely on less specific representations to guide search. Although search for camouflaged and non-specified targets have both received attention in the literature, to date they have not been explored in a combined context. Here we introduce a new paradigm for characterizing behavior during search for camouflaged targets in natural scenes, while also exploring how the fidelity of the target template affects search processes. Search scenes were created from forest images, with targets a distortion (varied size) of that image at a random location. In Experiment 1 a preview of the target was provided; in Experiment 2 there was no preview. No differences were found between experiments on nearly all measures. Generally, reaction times and accuracy improved with familiarity on the task (more so for small targets). Analysis of eye movements indicated that performance benefits were related to improvements in both Search and Target Verification time. Combined, our data suggest that search for camouflaged targets can be improved over a short time-scale, even when targets are poorly defined.     

The uses of colour vision: behavioural and physiological distinctiveness of colour stimuli

Colour and greyscale (black and white) pictures look different to us, but it is not clear whether the difference in appearance is a consequence of the way our visual system uses colour signals or a by-product of our experience. In principle, colour images are qualitatively different from greyscale images because they make it possible to use different processing strategies. Colour signals provide important cues for segmenting the image into areas that represent different objects and for linking together areas that represent the same object. If this property of colour signals is exploited in visual processing we would expect colour stimuli to look different, as a class, from greyscale stimuli. We would also expect that adding colour signals to greyscale signals should change the way that those signals are processed. We have investigated these questions in behavioural and in physiological experiments. We find that male marmosets (all of which are dichromats) rapidly learn to distinguish between colour and greyscale copies of the same images. The discrimination transfers to new image pairs, to new colours and to image pairs in which the colour and greyscale images are spatially different. We find that, in a proportion of neurons recorded in the marmoset visual cortex, colour-shifts in opposite directions produce similar enhancements of the response to a luminance stimulus. We conclude that colour is, both behaviourally and physiologically, a distinctive property of images.     

Retrieval Intention Modulates the Effects of Directed Forgetting Instructions on Recollection

The neurocognitive basis of memory retrieval is often examined by investigating brain potential old/new effects, which are differences in brain activity between successfully remembered repeated stimuli and correctly rejected new stimuli in a recognition test. In this study, we combined analyses of old/new effects for words with an item-method directed-forgetting manipulation in order to isolate differences between the retrieval processes elicited by words that participants were initially instructed to commit to memory and those that participants were initially instructed to forget. We compared old/new effects elicited by to-be-forgotten (TBF) words with those elicited by to-be-remembered (TBR) words in both an explicit-memory test (a recognition test) and an implicit-memory test (a lexical-decision test). Behavioral results showed clear directed forgetting effects in the recognition test, but not in the lexical decision test. Mirroring the behavioral findings, analyses of brain potentials showed evidence of directed forgetting only in the recognition test. In this test, potentials from 450–650 ms (P600 old/new effects) were more positive for TBR relative to TBF words. By contrast, P600 effects evident during the lexical-decision test did not differ in magnitude between TBR and TBF items. When taken in the context of prior studies that have linked similar parietal old/new effects to the recollection of episodic information, these data suggest that directed-forgetting effects manifest primarily in greater episodic retrieval by TBR than TBF items, and that retrieval intention may be important for these directed-forgetting effects to occur.     

Enhanced colour polymorphisms in island populations of the land snail Euhadra peliomphala

Variations in visible genetic polymorphisms are assumed to decrease in populations on small islands because of intense founder effects, genetic drift and inbreeding. However, we have found evidence of a marked enhancement of colour polymorphisms within populations on small oceanic islands that were colonized from the mainland. The source populations on the mainland of the land snail Euhadra peliomphala in four oceanic islands were estimated by phylogenetic analysis of mitochondrial DNA sequences. Diversity of shell colour was higher in the island populations than in the source populations on the mainland. In addition, the shell colour morphs differed not only among populations from different islands but also between the island populations and the source populations on the mainland. By contrast, no mtDNA variations were found in any of the island populations, even though the source populations possessed high mtDNA diversity. Thus, components of colour morphs changed in the island populations after their colonization, and colour polymorphisms are enhanced in these islands despite the loss of genetic variation. The above findings suggest that ecological mechanisms such as morphological release owing to a release from competition may overcome the tendency toward reduced genetic variation in islands to enhance the colour polymorphism.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 417–425.     

Demonstration of a foraging advantage for trichromatic marmosets (Callithrix geoffroyi) dependent on food colour

It has been suggested that the major advantage of trichromatic over dichromatic colour vision in primates is enhanced detection of red/yellow food items such as fruit against the dappled foliage of the forest. This hypothesis was tested by comparing the foraging ability of dichromatic and trichromatic Geoffroy's marmosets (Callithrix geoffroyi) for orange- and green-coloured cereal balls (Kix) in a naturalized captive setting. Trichromatic marmosets found a significantly greater number of orange, but not green, Kix than dichromatic marmosets when the food items were scattered on the floor of the cage (at a potential detection distance of up to 6 m from the animals). Under these conditions, trichromats but not dichromats found significantly more orange than green Kix, an effect that was also evident when separately examining the data from the end of the trials, when the least conspicuous Kix were left. In contrast, no significant differences among trichromats and dichromats were seen when the Kix were placed in trays among green wood shavings (detection distance < 0.5 m). These results support an advantage for trichromats in detecting orange-coloured food items against foliage, and also suggest that this advantage may be less important at shorter distances. If such a foraging advantage for trichromats is present in the wild it might be sufficient to maintain the colour vision polymorphism seen in the majority of New World monkeys.     

Patterns of floral colour neighbourhood and their effects on female reproductive success in an Antirrhinum hybrid zone

The maintenance of genetic integrity of parental populations is often explained by selection against hybrids. However, the selection agents are usually unknown. The role of environmental biotic interactions is often suspected but has rarely been demonstrated. In plants for instance, mutualism with pollinators may be involved. After verification that pollen deposition is a limiting factor for fruit set, we used an individual-based study and a representation of pollinator colour perception to test the effects of local plant density and floral colour neighbourhood on female reproductive success in an Antirrhinum hybrid zone. In addition to flower colour and density effects, the composition of the floral neighbourhood was found to influence fruit set, suggesting that most plants were usually better fertilized when similar to their neighbours. However, the plants of one particular type were sometimes favoured when very different from their neighbours. The implications for hybrid zone dynamics are discussed.     

Mating dominance amongst male Himalayan tahr: Blonds do better

In ungulates, rank order is determined by differences in weight, body size, weapon size and age. In the Caprini tribe (Bovidae: Caprinae), adult male Himalayan tahr are unique to show different coat colours, but no sexual dimorphism in weapons. A highly significant correlation between hair colour and rank order was found during the rut: males with a lighter coloured ruff dominated over darker ruffed ones, in both aggressive interactions and access to oestrus females. We studied colour-based dominance in relation to weight, age and testosterone levels, which establish the social rank in most ungulates. No differences in weight and testosterone concentrations were found between adult male colour classes, but males with paler ruffs were significantly younger than darker adult males. The distribution of physical traumas from fights confirmed that younger, lighter-coloured males had a higher rank than older, darker males, a pattern which is unusual amongst ungulates. Coat colour seems to work as a signal of rank in male-male aggressive interactions and it changes according to age, whereas the relevant physiological determinants deserve further research. Intrasexual male competition has not changed weapon size or shape in the Himalayan tahr, but ruff colours are apparently used to signal rank and dominance. Colour patterns of adult males may then be homologous to ritualised weapons, apparently being a unique feature of male tahr amongst mammals.