Do alignment thresholds define a critical boundary in long-range detection facilitation with co-linear lines?

Thresholds for line contrast detection (experiment 2) were measured with a two-alternative temporal forced-choice procedure as a function of the spatial position of a vertical target line with regard to two co-linear context lines. The different spatial positions of the target line corresponded to values near the position discrimination threshold (experiment 1) reflecting the just detectable lateral offset, or non-co-linearity, between the context lines which were vertically separated by about 100 minutes of visual arc. Target and context lines were vertically separated by about 30 minutes of arc, had equal contrast polarity in one case, and opposite contrast polarity in the other. Strong line contrast detection facilitation is found at perceptually co-linear target locations. This facilitation decreases noticeably at a horizontal target offset that corresponds to the alignment threshold measured with the context lines. The effects are independent of the relative contrast polarity of target and context and, as shown in a third experiment, also independent of both the relative length or number of lines, and the magnitude of their absolute co-axial separation. This independence seems to hold, provided individual line length and co-axial distance between lines are larger than what appears to be the lower limit of the long-range spatial domain for orientation or contour integration (i.e. 20 minutes of arc), as determined by previous studies. The findings reported here suggest that alignment thresholds are likely to define a critical lateral boundary in long-range detection facilitation with co-linear lines. They support models of contour integration based on interactions between neural mechanisms that integrate local signals of contrast, orientation, and relative position or end-to-end alignment. Such mechanisms may help to explain the formation of representations of virtual contours and object contours in human perception.     

Simulation of Neuronal Responses Defining Depth Order and Contrast Polarity at Illusory Contours in Monkey Area V2

Neurophysiological, brain imaging, and perceptual studies in animals and humans suggest that illusory (occluding) contours are represented at an early level of visual cortical processing. Comparatively little is known about the mechanisms defining the depth order and the brightness illusion associated with such contours. Baumann et al. (1997) found neurons in area V2 of the alert monkey that signaled not only illusory contours but also the figure-ground direction that human observers perceive at such contours. The majority of these neurons showed this property independent stimulus contrast; a small minority preferred a certain combination of figure-ground direction and contrast polarity at these contours. In this article, we simulate the responses of these neurons by means of a grouping mechanism that uses occlusion cues (line-ends, corners) to define figure-ground direction and contrast polarity at such contours.     

错觉图形组成成分间的亮度对比和颜色对比对错觉程度的影响

用定量的心理物理测量方法,研究了错觉图形组成成分间的亮度对比和颜色对比方位错觉、长度觉及面积错觉幅度的影响。测试结果表明：与通常的错觉效应相比,当错觉图形组成成分间存在亮度对比或颜色对比（等亮度）时,受试者的错觉程度明显降低;其中,当存在颜色对比时,方位错觉的下降幅度更为显著,达到６９．３％。此外还观察到,在单纯亮度对比条件下,只需１．８％和５．３％的低对比度即可分别产生轮廓和边缘错觉;但在等亮度     

The cutaneous rabbit illusion affects human primary sensory cortex somatotopically

We used functional magnetic resonance imaging (fMRI) to study neural correlates of a robust somatosensory illusion that can dissociate tactile perception from physical stimulation. Repeated rapid stimulation at the wrist, then near the elbow, can create the illusion of touches at intervening locations along the arm, as if a rabbit hopped along it. We examined brain activity in humans using fMRI, with improved spatial resolution, during this version of the classic cutaneous rabbit illusion. As compared with control stimulation at the same skin sites (but in a different order that did not induce the illusion), illusory sequences activated contralateral primary somatosensory cortex, at a somatotopic location corresponding to the filled-in illusory perception on the forearm. Moreover, the amplitude of this somatosensory activation was comparable to that for veridical stimulation including the intervening position on the arm. The illusion additionally activated areas of premotor and prefrontal cortex. These results provide direct evidence that illusory somatosensory percepts can affect primary somatosensory cortex in a manner that corresponds somatotopically to the illusory percept.     

Contrast sensitivity and appearance in briefly presented illusory figures.

We examined the contributions of brightness enhancement, illusory figure formation and figural completion to changes in contrast sensitivity in contour gaps. The brightness on the border of a Kanizsa-square and an outline square was measured as the point of subjective equality with the background (PSE) for small line targets. Increment and decrement thresholds were measured at the same location. We found that contrast thresholds were lower than in a control condition without inducers, and that the threshold reduction was independent of the contrast polarity of the inducers. This reduction cannot be explained by a simple summation of stimulus contrast and induced brightness. In a second experiment the inducers that define the contour of the Kanizsa and the outline square were changed so that the figure was no longer closed, keeping the local stimulus surround constant. Thresholds were equally reduced for all conditions, independently of whether the figure was completed or not, or whether an illusory contour was perceived or not. The results suggest that the reduction of contrast threshold in contour gaps is independent of the brightness perceived in these gaps and of the formation of an illusory figure. Processes that cause contrast threshold reduction in contour gaps also seem to operate independently of figural completion.     

Grey matter volume in early human visual cortex predicts proneness to the sound-induced flash illusion

Visual perception can be modulated by sounds. A drastic example of this is the sound-induced flash illusion: when a single flash is accompanied by two bleeps, it is sometimes perceived in an illusory fashion as two consecutive flashes. However, there are strong individual differences in proneness to this illusion. Some participants experience the illusion on almost every trial, whereas others almost never do. We investigated whether such individual differences in proneness to the sound-induced flash illusion were reflected in structural differences in brain regions whose activity is modulated by the illusion. We found that individual differences in proneness to the illusion were strongly and significantly correlated with local grey matter volume in early retinotopic visual cortex. Participants with smaller early visual cortices were more prone to the illusion. We propose that strength of auditory influences on visual perception is determined by individual differences in recurrent connections, cross-modal attention and/or optimal weighting of sensory channels.     

Physiological host specificity: a model using the European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) and microsporidia of row crop and other stalk-boring hosts

We investigated vertical and horizontal transmission as means by which entomopathogenic microsporidia may be isolated in their hosts. Ostrinia nubilalis larvae were challenged with microsporidia isolated from other stalk-boring and row crop Lepidoptera and were susceptible to seven species. Two species were horizontally transmitted. A Nosema sp. from Eoreuma loftini was transmitted among O. nubilalis larvae but not among larvae of the E. loftini host. This species was also vertically transmitted to the offspring of infected O. nubilalis females. An rDNA sequence showed the E. loftini isolate to be Nosema pyrausta, a naturally occurring species in O. nubilalis. Our results suggest that both horizontal and vertical transmission provide physiological barriers to host switching in the microsporidia, thus restricting the natural host range.     

Pattern vision of the honeybee (Apis mellifera). What is an oriented edge?

Pairs of black patterns on a white background, one rewarded the other not, were presented vertically each in one arm of a Y-maze. During training the locations of the black areas were changed every 5 min to prevent the bees using them as cues, but cues from edges were kept consistent. Bees detect orientation even in a gradient that subtends 36° from black to white (normal to the edge). Orientation cues in short lengths of edge are detected and summed on each side of the fixation point, irrespective of the lay-out of the pattern. Edges at right angles reduce the total orientation cue. The polarity of edges in a sawtooth grating is weakly discriminated, but not the orientation of a fault line where two gratings meet. Edge quality can be discriminated, but is not recognised in unfamiliar orientations. When spot location is excluded as a cue, the orientation of a row of spots or squares which individually provide no net orientation cue is not discriminated. In conclusion, when locations of black areas are shuffled, the bees remember the sum of local orientation cues but not the global pattern, and there is no re-assembly of a pattern based on differently oriented edges. A neuronal model consistent with these results is presented. Accepted: 5 March 2000     

Dissociation of color and figure-ground effects in the watercolor illusion

Two phenomena can be observed in the watercolor illusion: illusory color spreading and figure-ground organization. We performed experiments to determine whether the figure-ground effect is a consequence of the color illusion or due to an independent mechanism. Subjects were tested with displays consisting of six adjacent compartments--three that generated the illusion alternating with three that served for comparison. In a first set of experiments, the illusory color was measured by finding the matching physical color in the alternate compartments. Figureness (probability of 'figure' responses, 2AFC) of the watercolor compartments was then determined with and without the matching color in the alternate compartments. The color match reduced figureness, but did not abolish it. There was a range of colors in which the watercolor compartments dominated as figures over the alternate compartments although the latter appeared more saturated in color. In another experiment, the effect of tinting alternate compartments was measured in displays without watercolor illusion. Figureness increased with color contrast, but its value at the equivalent contrast fell short of the figureness value obtained for the watercolor pattern. Thus, in both experiments, figureness produced by the watercolor pattern was stronger than expected from the color effect, suggesting independent mechanisms. Considering the neurophysiology, we propose that the color illusion follows from the principles of representation of surface color in the visual cortex, while the figure-ground effect results from two mechanisms of border ownership assignment, one that is sensitive to asymmetric shape of edge profile, the other to consistency of color borders.     

Crossing the Hands Increases Illusory Self-Touch

Manipulation of hand posture, such as crossing the hands, has been frequently used to study how the body and its immediately surrounding space are represented in the brain. Abundant data show that crossed arms posture impairs remapping of tactile stimuli from somatotopic to external space reference frame and deteriorates performance on several tactile processing tasks. Here we investigated how impaired tactile remapping affects the illusory self-touch, induced by the non-visual variant of the rubber hand illusion (RHI) paradigm. In this paradigm blindfolded participants (Experiment 1) had their hands either uncrossed or crossed over the body midline. The strength of illusory self-touch was measured with questionnaire ratings and proprioceptive drift. Our results showed that, during synchronous tactile stimulation, the strength of illusory self-touch increased when hands were crossed compared to the uncrossed posture. Follow-up experiments showed that the increase in illusion strength was not related to unfamiliar hand position (Experiment 2) and that it was equally strengthened regardless of where in the peripersonal space the hands were crossed (Experiment 3). However, while the boosting effect of crossing the hands was evident from subjective ratings, the proprioceptive drift was not modulated by crossed posture. Finally, in contrast to the illusion increase in the non-visual RHI, the crossed hand postures did not alter illusory ownership or proprioceptive drift in the classical, visuo-tactile version of RHI (Experiment 4). We argue that the increase in illusory self-touch is related to misalignment of somatotopic and external reference frames and consequently inadequate tactile-proprioceptive integration, leading to re-weighting of the tactile and proprioceptive signals.The present study not only shows that illusory self-touch can be induced by crossing the hands, but importantly, that this posture is associated with a stronger illusion.     

Potential versus actual contribution of vertical transmission to pathogen fitness

Theory predicts that virulent parasites cannot be maintained at high prevalence if they are only vertically transmitted. However, parasites with high rates of vertical transmission that cause severe reduction in host fitness have been reported. Atkinsonella hypoxylon is a fungal pathogen capable of both vertical and horizontal transmission that drastically reduces its host''s fitness. In contrast with theoretical predictions, field and laboratory observations suggested that the primary mechanism of transmission was vertical. Using randomly amplified polymorphic DNA markers, we investigated the effective contribution of vertical and horizontal transmission to the genetic structure of three natural populations of A. hypoxylon. We found high genotypic diversity and low linkage disequilibrium, indicating that most established genotypes are derived from horizontally transmitted, sexual spores. The low contribution of vertical transmission to the parasite''s fitness despite its high potential might be due to lower establishment of cleistogamous seeds (through which vertical transmission occurs) or lower vigour of vertically transmitted fungal genotypes. Low establishment of vertically infected hosts might explain the persistence of virulent parasites with high apparent vertical transmission. Our results suggest that caution must be taken when using the potential for vertical transmission to make predictions about the evolution of parasite virulence.     

Am I seeing my hand? Visual appearance and knowledge of controllability both contribute to the visual capture of a person's own body

When confronted with complex visual scenes in daily life, how do we know which visual information represents our own hand? We investigated the cues used to assign visual information to one''s own hand. Wrist tendon vibration elicits an illusory sensation of wrist movement. The intensity of this illusion attenuates when the actual motionless hand is visually presented. Testing what kind of visual stimuli attenuate this illusion will elucidate factors contributing to visual detection of one''s own hand. The illusion was reduced when a stationary object was shown, but only when participants knew it was controllable with their hands. In contrast, the visual image of their own hand attenuated the illusion even when participants knew that it was not controllable. We suggest that long-term knowledge about the appearance of the body and short-term knowledge about controllability of a visual object are combined to robustly extract our own body from a visual scene.     

Fooling the Eyes: The Influence of a Sound-Induced Visual Motion Illusion on Eye Movements

The question of whether perceptual illusions influence eye movements is critical for the long-standing debate regarding the separation between action and perception. To test the role of auditory context on a visual illusion and on eye movements, we took advantage of the fact that the presence of an auditory cue can successfully modulate illusory motion perception of an otherwise static flickering object (sound-induced visual motion effect). We found that illusory motion perception modulated by an auditory context consistently affected saccadic eye movements. Specifically, the landing positions of saccades performed towards flickering static bars in the periphery were biased in the direction of illusory motion. Moreover, the magnitude of this bias was strongly correlated with the effect size of the perceptual illusion. These results show that both an audio-visual and a purely visual illusion can significantly affect visuo-motor behavior. Our findings are consistent with arguments for a tight link between perception and action in localization tasks.     

Species turnover in ant assemblages is greater horizontally than vertically in the world's tallest tropical forest

Abiotic and biotic factors structure species assembly in ecosystems both horizontally and vertically. However, the way community composition changes along comparable horizontal and vertical distances in complex three‐dimensional habitats, and the factors driving these patterns, remains poorly understood. By sampling ant assemblages at comparable vertical and horizontal spatial scales in a tropical rainforest, we tested hypotheses that predicted differences in vertical and horizontal turnover explained by different drivers in vertical and horizontal space. These drivers included environmental filtering, such as microclimate (temperature, humidity, and photosynthetic photon flux density) and microhabitat connectivity (leaf area), which are structured differently across vertical and horizontal space. We found that both ant abundance and richness decreased significantly with increasing vertical height. Although the dissimilarity between ant assemblages increased with vertical distance, indicating a clear distance‐decay pattern, the dissimilarity was higher horizontally where it appeared independent of distance. The pronounced horizontal and vertical structuring of ant assemblages across short distances is likely explained by a combination of microclimate and microhabitat connectivity. Our results demonstrate the importance of considering three‐dimensional spatial variation in local assemblages and reveal how highly diverse communities can be supported by complex habitats.     

Virulence and transmission modes in metapopulations: when group selection increases virulence

Individuals that are infected by a pathogen can transmit it to unrelated conspecifics (horizontal transmission) or to their progeny when they reproduce (vertical transmission). The mechanisms of these two routes of transmission are different and this difference impacts the way virulence evolves in pathogens. More precisely, horizontal transmission depends on the probability that an infected host contacts susceptible conspecifics, and therefore on its lifespan. Vertical transmission additionally depends on the host's fecundity. This additional dependence in vertically transmitted pathogens results in a decrease in their evolutionarily stable (ES) virulence.Spatial structure is another factor that is often supposed to decrease pathogens’ ES virulence, mostly because it impedes competition for transmission in local populations of hosts. In this paper, using the adaptive dynamics framework, we show that spatial structure can increase ES virulence when pathogens are mostly vertically transmitted. This is due to the difference in how pathogens compete for transmission in local population of hosts, depending on how they are transmitted. We also show that symbionts that are horizontally transmitted should respond more to a change in spatial structure than symbionts that are vertically transmitted.     

THE EVOLUTION OF VIRULENCE IN PATHOGENS WITH VERTICAL AND HORIZONTAL TRANSMISSION

The idea that vertical transmission of parasites selects for lower virulence is widely accepted. However, little theoretical work has considered the evolution of virulence for parasites with mixed horizontal plus vertical transmission. Many human, animal, and plant parasites are transmitted both vertically and horizontally, and some horizontal transmission is generally necessary to maintain parasites at all. We present a population-dynamical model for the evolution of virulence when both vertical and horizontal transmission are present. In the simplest such model, up to two infectious strains can coexist within one host population. Virulent, vertically transmitted pathogens can persist in a population when they provide protection against more virulent, horizontally transmitted strains. When virulence is maintained by a correlation with horizontal transmission rates, increased levels of vertical transmission always lower the evolutionarily stable (ESS) level of virulence. Contrary to existing theory, however, increases in opportunities for horizontal transmission also lower the ESS level of virulence. We explain these findings in light of earlier work and confirm them in simulations including imperfect vertical transmission. We describe further simulations, in which both vertical and horizontal transmission rates are allowed to evolve. The outcome of these simulations depends on whether high levels of vertical transmission are possible with low virulence. Finally, we argue against the notion of a virulence-avirulence continuum between horizontal and vertical transmission, and discuss our results in relation to empirical studies of transmission and virulence.     

Sleep Dissolves Illusion: Sleep Withstands Learning of Visuo-Tactile-Proprioceptive Integration Induced by Repeated Days of Rubber Hand Illusion Training

Multisensory integration is a key factor in establishing bodily self-consciousness and in adapting humans to novel environments. The rubber hand illusion paradigm, in which humans can immediately perceive illusory ownership to an artificial hand, is a traditional technique for investigating multisensory integration and the feeling of illusory ownership. However, the long-term learning properties of the rubber hand illusion have not been previously investigated. Moreover, although sleep contributes to various aspects of cognition, including learning and memory, its influence on illusory learning of the artificial hand has not yet been assessed. We determined the effects of daily repetitive training and sleep on learning visuo-tactile-proprioceptive sensory integration and illusory ownership in healthy adult participants by using the traditional rubber hand illusion paradigm. Subjective ownership of the rubber hand, proprioceptive drift, and galvanic skin response were measured to assess learning indexes. Subjective ownership was maintained and proprioceptive drift increased with daily training. Proprioceptive drift, but not subjective ownership, was significantly attenuated after sleep. A significantly greater reduction in galvanic skin response was observed after wakefulness compared to after sleep. Our results suggest that although repetitive rubber hand illusion training facilitates multisensory integration and physiological habituation of a multisensory incongruent environment, sleep corrects illusional integration and habituation based on experiences in a multisensory incongruent environment. These findings may increase our understanding of adaptive neural processes to novel environments, specifically, bodily self-consciousness and sleep-dependent neuroplasticity.     

Individual differences in sound-in-noise perception are related to the strength of short-latency neural responses to noise

Important sounds can be easily missed or misidentified in the presence of extraneous noise. We describe an auditory illusion in which a continuous ongoing tone becomes inaudible during a brief, non-masking noise burst more than one octave away, which is unexpected given the frequency resolution of human hearing. Participants strongly susceptible to this illusory discontinuity did not perceive illusory auditory continuity (in which a sound subjectively continues during a burst of masking noise) when the noises were short, yet did so at longer noise durations. Participants who were not prone to illusory discontinuity showed robust early electroencephalographic responses at 40-66 ms after noise burst onset, whereas those prone to the illusion lacked these early responses. These data suggest that short-latency neural responses to auditory scene components reflect subsequent individual differences in the parsing of auditory scenes.     

Neurogenesis in the vomeronasal epithelium of adult rats: evidence for different mechanisms for growth and neuronal turnover

The pattern of cell migration during neuronal turnover in the vomeronasal sensory epithelium (VN-SE) is controversial. In mice, proliferating cells were detected at the edges and were described as migrating to the center of the VN-SE. In rats, in addition to proliferating cells at the margins of the epithelium, dividing cells are also present along the entire basal lamina of the VN-SE. In marsupials, dividing cells have also been observed in the margins and in the center of the VN-SE, the latter of which migrate vertically and become neurons. To investigate whether the process of neuronal turnover in placental mammals consists of horizontal and/or vertical migration, and whether or not this process is common to mammals, adult rats were injected with bromodeoxyuridine (BrdU) and allowed to survive for different periods of time. The distribution of BrdU-labeled cells in the horizontal and vertical dimension of the VN-SE was analyzed as a function of time. Both horizontal and vertical migrations of BrdU-labeled cells were detected. Since cells in the center of the VN-SE migrate vertically, and, as demonstrated by coexpression of markers of neuronal maturity and BrdU, become mature one day after undergoing mitosis, it is very likely that these cells participate in neuronal turnover. Conversely, because cells in the margins of the VN-SE stop migrating horizontally on day 14 before they have reached the center of the VN-SE, and since the VN-SE continues to grow during adulthood, it is likely that most of these latter cells constitute pools for growth.