Visual adaptation and face perception

The appearance of faces can be strongly affected by the characteristics of faces viewed previously. These perceptual after-effects reflect processes of sensory adaptation that are found throughout the visual system, but which have been considered only relatively recently in the context of higher level perceptual judgements. In this review, we explore the consequences of adaptation for human face perception, and the implications of adaptation for understanding the neural-coding schemes underlying the visual representation of faces. The properties of face after-effects suggest that they, in part, reflect response changes at high and possibly face-specific levels of visual processing. Yet, the form of the after-effects and the norm-based codes that they point to show many parallels with the adaptations and functional organization that are thought to underlie the encoding of perceptual attributes like colour. The nature and basis for human colour vision have been studied extensively, and we draw on ideas and principles that have been developed to account for norms and normalization in colour vision to consider potential similarities and differences in the representation and adaptation of faces.     

From single cells to whole organisms

A report on the European Science Foundation Conference 'Functional Genomics and Disease', Oslo, Norway, 6-10 September 2005.     

From single cells to deep phenotypes in cancer

In recent years, major advances in single-cell measurement systems have included the introduction of high-throughput versions of traditional flow cytometry that are now capable of measuring intracellular network activity, the emergence of isotope labels that can enable the tracking of a greater variety of cell markers and the development of super-resolution microscopy techniques that allow measurement of RNA expression in single living cells. These technologies will facilitate our capacity to catalog and bring order to the inherent diversity present in cancer cell populations. Alongside these developments, new computational approaches that mine deep data sets are facilitating the visualization of the shape of the data and enabling the extraction of meaningful outputs. These applications have the potential to reveal new insights into cancer biology at the intersections of stem cell function, tumor-initiating cells and multilineage tumor development. In the clinic, they may also prove important not only in the development of new diagnostic modalities but also in understanding how the emergence of tumor cell clones harboring different sets of mutations predispose patients to relapse or disease progression.     

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.     

Adaptation improves discrimination of face identity

Whether face adaptation confers any advantages to perceptual processing remains an open question. We investigated whether face adaptation can enhance the ability to make fine discriminations in the vicinity of the adapted face. We compared face discrimination thresholds in three adapting conditions: (i) same-face: where adapting and test faces were the same, (ii) different-face: where adapting and test faces differed, and (iii) baseline: where the adapting stimulus was a blank. Discrimination thresholds for morphed identity changes involving the adapted face (same-face) improved compared with those from both the baseline (no-adaptation) and different-face conditions. Since adapting to a face did not alter discrimination performance for other faces, this effect is selective for the facial identity that is adapted. These results indicate a form of gain control to heighten perceptual sensitivity in the vicinity of a currently viewed face, analogous to forms of adaptive gain control at lower levels of the visual system.     

Sensory adaptation for timing perception

Recent sensory experience modifies subjective timing perception. For example, when visual events repeatedly lead auditory events, such as when the sound and video tracks of a movie are out of sync, subsequent vision-leads-audio presentations are reported as more simultaneous. This phenomenon could provide insights into the fundamental problem of how timing is represented in the brain, but the underlying mechanisms are poorly understood. Here, we show that the effect of recent experience on timing perception is not just subjective; recent sensory experience also modifies relative timing discrimination. This result indicates that recent sensory history alters the encoding of relative timing in sensory areas, excluding explanations of the subjective phenomenon based only on decision-level changes. The pattern of changes in timing discrimination suggests the existence of two sensory components, similar to those previously reported for visual spatial attributes: a lateral shift in the nonlinear transducer that maps relative timing into perceptual relative timing and an increase in transducer slope around the exposed timing. The existence of these components would suggest that previous explanations of how recent experience may change the sensory encoding of timing, such as changes in sensory latencies or simple implementations of neural population codes, cannot account for the effect of sensory adaptation on timing perception.     

From optics to attention: visual perception in barn owls

Barn owls are nocturnal predators which have evolved specific sensory and morphological adaptations to a life in dim light. Here, some of the most fundamental properties of spatial vision in barn owls are reviewed. The eye with its tubular shape is rigidly integrated in the skull so that eye movements are very much restricted. The eyes are oriented frontally, allowing for a large binocular overlap. Accommodation, but not pupil dilation, is coupled between the two eyes. The retina is rod dominated and lacks a visible fovea. Retinal ganglion cells form a marked region of highest density that extends to a horizontally oriented visual streak. Behavioural visual acuity and contrast sensitivity are poor, although the optical quality of the ocular media is excellent. A low f-number allows high image quality at low light levels. Vernier acuity was found to be a hyperacute percept. Owls have global stereopsis with hyperacute stereo acuity thresholds. Neurons of the visual Wulst are sensitive to binocular disparities. Orientation based saliency was demonstrated in a visual-search experiment, and higher cognitive abilities were shown when the owl’s were able to use illusory contours for object discrimination.     

From local to global dilemmas in social networks

Social networks affect in such a fundamental way the dynamics of the population they support that the global, population-wide behavior that one observes often bears no relation to the individual processes it stems from. Up to now, linking the global networked dynamics to such individual mechanisms has remained elusive. Here we study the evolution of cooperation in networked populations and let individuals interact via a 2-person Prisoner's Dilemma--a characteristic defection dominant social dilemma of cooperation. We show how homogeneous networks transform a Prisoner's Dilemma into a population-wide evolutionary dynamics that promotes the coexistence between cooperators and defectors, while heterogeneous networks promote their coordination. To this end, we define a dynamic variable that allows us to track the self-organization of cooperators when co-evolving with defectors in networked populations. Using the same variable, we show how the global dynamics--and effective dilemma--co-evolves with the motifs of cooperators in the population, the overall emergence of cooperation depending sensitively on this co-evolution.     

From social to genetic structures in central Asia

Pastoral and farmer populations, who have coexisted in Central Asia since the fourth millennium B.C., present not only different lifestyles and means of subsistence but also various types of social organization. Pastoral populations are organized into so-called descent groups (tribes, clans, and lineages) and practice exogamous marriages (a man chooses a bride in a different lineage or clan). In Central Asia, these descent groups are patrilineal: The children are systematically affiliated with the descent groups of the father. By contrast, farmer populations are organized into families (extended or nuclear) and often establish endogamous marriages with cousins. This study aims at better understanding the impact of these differences in lifestyle and social organization on the shaping of genetic diversity. We show that pastoral populations exhibit a substantial loss of Y chromosome diversity in comparison to farmers but that no such a difference is observed at the mitochondrial-DNA level. Our analyses indicate that the dynamics of patrilineal descent groups, which implies different male and female sociodemographic histories, is responsible for these sexually-asymmetric genetic patterns. This molecular signature of the pastoral social organization disappears over a few centuries only after conversion to an agricultural way of life.     

Cortical networks for face perception in two-month-old infants

Newborns have an innate system for preferentially looking at an upright human face. This face preference behaviour disappears at approximately one month of age and reappears a few months later. However, the neural mechanisms underlying this U-shaped behavioural change remain unclear. Here, we isolate the functional development of the cortical visual pathway for face processing using S-cone-isolating stimulation, which blinds the subcortical visual pathway. Using luminance stimuli, which are conveyed by both the subcortical and cortical visual pathways, the preference for upright faces was not observed in two-month-old infants, but it was observed in four- and six-month-old infants, confirming the recovery phase of the U-shaped development. By contrast, using S-cone stimuli, two-month-old infants already showed a preference for upright faces, as did four- and six-month-old infants, demonstrating that the cortical visual pathway for face processing is already functioning at the bottom of the U-shape at two months of age. The present results suggest that the transient functional deterioration stems from a conflict between the subcortical and cortical functional pathways, and that the recovery thereafter involves establishing a level of coordination between the two pathways.     

Duration channels mediate human time perception

The task of deciding how long sensory events seem to last is one that the human nervous system appears to perform rapidly and, for sub-second intervals, seemingly without conscious effort. That these estimates can be performed within and between multiple sensory and motor domains suggest time perception forms one of the core, fundamental processes of our perception of the world around us. Given this significance, the current paucity in our understanding of how this process operates is surprising. One candidate mechanism for duration perception posits that duration may be mediated via a system of duration-selective 'channels', which are differentially activated depending on the match between afferent duration information and the channels' 'preferred' duration. However, this model awaits experimental validation. In the current study, we use the technique of sensory adaptation, and we present data that are well described by banks of duration channels that are limited in their bandwidth, sensory-specific, and appear to operate at a relatively early stage of visual and auditory sensory processing. Our results suggest that many of the computational principles the nervous system applies to coding visual spatial and auditory spectral information are common to its processing of temporal extent.     

Role of facial expressions in social interactions

The expressions we see in the faces of others engage a number of different cognitive processes. Emotional expressions elicit rapid responses, which often imitate the emotion in the observed face. These effects can even occur for faces presented in such a way that the observer is not aware of them. We are also very good at explicitly recognizing and describing the emotion being expressed. A recent study, contrasting human and humanoid robot facial expressions, suggests that people can recognize the expressions made by the robot explicitly, but may not show the automatic, implicit response. The emotional expressions presented by faces are not simply reflexive, but also have a communicative component. For example, empathic expressions of pain are not simply a reflexive response to the sight of pain in another, since they are exaggerated when the empathizer knows he or she is being observed. It seems that we want people to know that we are empathic. Of especial importance among facial expressions are ostensive gestures such as the eyebrow flash, which indicate the intention to communicate. These gestures indicate, first, that the sender is to be trusted and, second, that any following signals are of importance to the receiver.     

From single biobanks to international networks: developing e-governance

The future holds the possibility to link and network biobanks, existing biorepositories and reference databases for research purposes in ways that have not been possible before. There is the potential to develop 'research portals' that will enable researchers to access these research resources that are located around the globe with the click of a mouse. In this paper, I will argue that our current governance system for research is unable to provide all of the oversight and accountability mechanisms that are required for this new way of doing research that is based upon flows of data across international borders. For example, our current governance framework for research is nationally based, with a complex system of laws, policies and practice that can be unique to a jurisdiction. It is also evident that many of the nationally based governance bodies in this field do not have the legal powers or expertise to adjudicate on the complex issues, such as privacy and disclosure risks that are raised by cross-border data sharing. In addition, the conceptual underpinning of this research governance structure is based on the "one researcher, one project, one jurisdiction" model. In the conclusion of this paper, I lay out some preliminary ideas as to how this system has to change to accommodate research that is based on networks. I suggest that a move to digital governance mechanisms might be a start to making research governance systems more appropriate for the 21st century.     

Adaptation of trigeminal ganglion cells to periodic whisker deflections

Trigeminal ganglion neurons in adult rats adapt to periodic whisker deflections in the range of 1–40?Hz, manifested as a reduction in spike counts to progressively later stimuli in a train of pulsatile or sinusoidal deflections. For high velocity, pulsatile deflections, adaptation is time- and frequency-dependent; as in the case of thalamic and cortical neurons, adaptation is greater at higher stimulus frequencies. With slower velocity, sinusoidal movements, trigeminal ganglion cells differ from central neurons, however, by exhibiting strong adaptation even at low frequencies. For both types of stimuli, effects in trigeminal ganglion neurons were more pronounced in rats maintained during the recording session under neuromuscular blockade than in non-paralysed animals. Results are consistent with previous findings in other systems that frequency-dependent adaptation of cutaneous primary afferent neurons is affected by mechanical properties of the skin. Such effects are likely to vary depending on the nature of the whisker stimuli and physiological states that affect skin viscoelasticity.     

从胚胎干细胞到视网膜

视网膜退行性病变影响着全世界数百万人。然而,视网膜是人体再生能力很差的一类组织,成年机体无法自我更新那些病变中丢失的视网膜细胞,导致视网膜退行性病变的不可逆性。因此,恢复患者视觉将依赖于引入外源细胞替代丢失的视网膜神经元。胚胎干细胞（ES细胞）具有无限的自我更新能力和形成机体所有类型细胞的巨大分化潜力。这两个特性使得ES细胞成为细胞替代疗法的理想供体细胞。近年来,人们在探索将ES和诱导多能干细胞（iPS细胞）体外定向诱导分化为视网膜神经元,甚至整个视网膜方面已取得多项进展,并且体外形成的视网膜细胞可以与宿主视网膜整合。在此篇综述中,首先简要概括哺乳动物视网膜的组织结构、发育过程和调控机制,然后,重点阐述近年来科研工作者探索ES/iPS细胞体外诱导分化为视网膜细胞和组织的研究进展。     

From stem cells to prion signalling

A strategy based upon the introduction of an adenovirus-SV40 plasmid into multipotential cells was designed to immortalize clones displaying properties of lineage stem cells. The murine 1C11 cell line behaves as a neuroepithelial progenitor. Upon appropriate induction, almost 100% of 1C11 precursor cells develop neurite extensions and convert into either serotonergic or noradrenergic neurons. The two mutually exclusive neuronal programs are autoregulated by serotonergic or adrenergic receptors. PrPc is constitutively expressed by 1C11 cells. Antibody-mediated cross-linking of PrPc promotes the dephosphorylation of the tyrosine kinase Fyn associated to a Fyn kinase activation. The coupling of PrPc to Fyn is dependent on caveolin-1. It is restricted to the fully differentiated serotonergic or noradrenergic cells and occurs mainly at neurites. Thus, PrPc may represent a signal transduction protein which may fine-tune neuronal functions. Since the 1C11 stem cell supports prion replication, it may provide a tool to investigate whether PrPSc accumulation interferes with PrPc signalling activity.