人眼的阈上调制传递函数

视觉系统的阈上调制传递函数与阈值对比敏感度函数具有明显的区别.但由于阈上测试方法复杂,因而迄今为止阈上调制传递函数的测试数据比阈值对比敏感度函数要少得多.我们应用传统的极限调节法和对比度比配法分别测试了人眼的阈值对比敏感度函数和三种对比度级别的阈上调制传递函数.测试结果表明,一个以DOG(Difference of two Gaussians)函数为基础的函数是一种合理的模型,可由现有阈值对比敏感度函数预测不同对比度和象场亮度级别下的人眼阈上调制传递函激.这个模型也可做为一种有效的人眼生理光学经验公式,用于目视光电成象系统的设计和象质评价.     

The modelling of avian visual perception predicts behavioural rejection responses to foreign egg colours

How do birds tell the colours of their own and foreign eggs apart? We demonstrate that perceptual modelling of avian visual discrimination can predict behavioural rejection responses to foreign eggs in the nest of wild birds. We use a photoreceptor noise-limited colour opponent model of visual perception to evaluate its accuracy as a predictor of behavioural rates of experimental egg discrimination in the song thrush Turdus philomelos. The visual modelling of experimental and natural eggshell colours suggests that photon capture from the ultraviolet and short wavelength-sensitive cones elicits egg rejection decisions in song thrushes, while inter-clutch variation of egg coloration provides sufficient contrasts for detecting conspecific parasitism in this species. Biologically realistic sensory models provide an important tool for relating variability of behavioural responses to perceived phenotypic variation.     

朝向和对比度同时快速变化条件下的分辨能力

在自然的视觉中,投射到视网膜上的视觉图像总是在不停地变化,而人类的感知系统依然可以准确高效地识别物体.因此,人类的感知系统有相应的快速处理机制以应对这种动态变化．然而,前人的实验都是在相对稳定的刺激条件下研究人类被试的感知系统对一个刺激参数的反应,比如在固定对比度下测试朝向分辨能力,或在固定朝向测定对比度分辨能力,而朝向和对比度同时变化时,人类对这两个参数的分辨能力仍然缺乏研究．因此,在本实验中,我们使用朝向和对比度同时变化的刺激,研究了人类被试对朝向和对比度的分辨能力．结果表明,在这种动态变化的条件下,被试对朝向和对比度的分辨阈值都有显著性的降低．而且,朝向分辨阈值降低的幅度与在固定对比度参数条件下的分辨阈值成负相关,即在固定对比度条件下朝向分辨阈值较高的被试,在朝向和对比度同时变化条件下,其朝向分辨阈值降低的幅度相对要大,朝向分辨能力也就相对地提高更大．对比度分辨能力也呈现同样的规律．这些结果说明,朝向和对比度的同时变化提高了被试对朝向和对比度的分辨能力,一个参数变化时其分辨能力越低的被试,两个参数变化时其分辨能力提高的幅度就越大．揭示了视觉系统处理这种多刺激参量信息变化的能力和机制,对人类视觉系统在真实的视觉过程中如何处理朝向和对比度信息提供了认识．     

Magno- and Parvocellular Contrast Responses in Varying Degrees of Autistic Trait

Autistic tendency has been associated with altered visual perception, especially impaired visual motion sensitivity and global/local integration, as well as enhanced visual search and local shape recognition. However, the neurophysiological mechanisms underlying these abnormalities remain poorly defined. The current study recruited 29 young adults displaying low, middle or high autistic trait as measured by Baron-Cohen''s Autism spectrum Quotient (AQ), and measured motion coherence thresholds psychophysically, with manipulation of dot lifetime and stimulus contrast, as well as nonlinear cortical visual evoked potentials (VEPs) over a range of temporal luminance contrast levels from 10% to 95%. Contrast response functions extracted from the major first order and second order Wiener kernel peaks of the VEPs showed consistent variation with AQ group, and Naka-Rushton fits enabled contrast gain and semi-saturation contrasts to be elicited for each peak. A short latency second order response (previously associated with magnocellular processing) with high contrast gain and a saturating contrast response function showed higher amplitude for the High AQ (compared with Mid and Low groups) indicating poorer neural recovery after rapid stimulation. A non-linearity evoked at longer interaction times (previously associated with parvocellular processing) with no evidence of contrast saturation and lower contrast gain showed no difference between autism quotient groups across the full range of stimulus contrasts. In addition, the short latency first order response and a small, early second order second slice response showed gain and semi-saturation parameters indicative of magnocellular origin, while the longer latency first order response probably reflects a mixture of inputs (including feedback from higher cortical areas). Significant motion coherence (AQ group) * (dot lifetime) interactions with higher coherence threshold for limited dot lifetime stimuli is consistent with atypical magnocellular functioning, however psychophysical performance for those with High AQ is not explained fully, suggesting that other factors may be involved.     

Bi-partitioning and boundary detection in natural scenes

We propose a strategy for early vision which tailors visual channels to the object-oriented characteristics of natural scenes. This strategy involves essentially two types of channel, one for encoding the locally dominant edges which form the boundaries of 'objects', and another for 'filling in' the regions within them. The selection of contrasts which characterize object boundaries rather than textural detail can be enhanced by making an estimate local of contrast, and setting a threshold accordingly. This procedure and other aspects of the model were first suggested by observations of insect visual cells.     

Space-variant visual processing: spatially limited visual channels

A multichannel model incorporating visual inhomogeneity is presented in this paper. The parameters that describe inhomogeneity have been experimentally obtained both at threshold and in several suprathreshold conditions. At threshold, probability summation is taken into account in order to determine the spatial extent of visual channels from experimental data showing an asymptotic increase in sensitivity with increasing grating area. At suprathreshold contrast, the region where luminance variations at several scales are visible has also been found. The results support a spatially limited multichannel model of early visual processing and set out a basis for studying perceptual phenomena from the viewpoint of linear space-variant visual processing.     

The effect of interocular phase difference on perceived contrast

Binocular vision is traditionally treated as two processes: the fusion of similar images, and the interocular suppression of dissimilar images (e.g. binocular rivalry). Recent work has demonstrated that interocular suppression is phase-insensitive, whereas binocular summation occurs only when stimuli are in phase. But how do these processes affect our perception of binocular contrast? We measured perceived contrast using a matching paradigm for a wide range of interocular phase offsets (0–180°) and matching contrasts (2–32%). Our results revealed a complex interaction between contrast and interocular phase. At low contrasts, perceived contrast reduced monotonically with increasing phase offset, by up to a factor of 1.6. At higher contrasts the pattern was non-monotonic: perceived contrast was veridical for in-phase and antiphase conditions, and monocular presentation, but increased a little at intermediate phase angles. These findings challenge a recent model in which contrast perception is phase-invariant. The results were predicted by a binocular contrast gain control model. The model involves monocular gain controls with interocular suppression from positive and negative phase channels, followed by summation across eyes and then across space. Importantly, this model—applied to conditions with vertical disparity—has only a single (zero) disparity channel and embodies both fusion and suppression processes within a single framework.     

Visual contributions to human self-motion perception during horizontal body rotation

It is still an enigma how human subjects combine visual and vestibular inputs for their self-motion perception. Visual cues have the benefit of high spatial resolution but entail the danger of self motion illusions. We performed psychophysical experiments (verbal estimates as well as pointer indications of perceived self-motion in space) in normal subjects (Ns) and patients with loss of vestibular function (Ps). Subjects were presented with horizontal sinusoidal rotations of an optokinetic pattern (OKP) alone (visual stimulus; 0.025-3.2 Hz; displacement amplitude, 8 degrees) or in combinations with rotations of a Bárány chair (vestibular stimulus; 0.025-0.4 Hz; +/- 8 degrees). We found that specific instructions to the subjects created different perceptual states in which their self-motion perception essentially reflected three processing steps during pure visual stimulation: i) When Ns were primed by a procedure based on induced motion and then they estimated perceived self-rotation upon pure optokinetic stimulation (circular vection, CV), the CV has a gain close to unity up to frequencies of almost 0.8 Hz, followed by a sharp decrease at higher frequencies (i.e., characteristics resembling those of the optokinetic reflex, OKR, and of smooth pursuit, SP). ii) When Ns were instructed to "stare through" the optokinetic pattern, CV was absent at high frequency, but increasingly developed as frequency was decreased below 0.1 Hz. iii) When Ns "looked at" the optokinetic pattern (accurately tracked it with their eyes) CV was usually absent, even at low frequency. CV in Ps showed similar dynamics as in Ns in condition i), independently of the instruction. During vestibular stimulation, self-motion perception in Ns fell from a maximum at 0.4 Hz to zero at 0.025 Hz. When vestibular stimulation was combined with visual stimulation while Ns "stared through" OKP, perception at low frequencies became modulated in magnitude. When Ns "looked" at OKP, this modulation was reduced, apart from the synergistic stimulus combination (OKP stationary) where magnitude was similar as during "staring". The obtained gain and phase curves of the perception were incompatible with linear systems prediction. We therefore describe the present findings by a non-linear dynamic model in which the visual input is processed in three steps: i) It shows dynamics similar to those of OKR and SP; ii) it is shaped to complement the vestibular dynamics and is fused with a vestibular signal by linear summation; and iii) it can be suppressed by a visual-vestibular conflict mechanism when the visual scene is moving in space. Finally, an important element of the model is a velocity threshold of about 1.2 degrees/s which is instrumental in maintaining perceptual stability and in explaining the observed dynamics of perception. We conclude from the experimental and theoretical evidence that self-motion perception normally is related to the visual scene as a reference, while the vestibular input is used to check the kinematic state of the scene; if the scene appears to move, the visual signal becomes suppressed and perception is based on the vestibular cue.     

The relation between visual acuity and brightness discrimination

1. Visual acuity depends on the brightness contrast between test object and background; and conversely, brightness discrimination depends on the target size. Both functions vary with the brightness of the background. Measurements with rectangular targets of length-width ratio 2 were made over a range of sizes, contrasts, and brightnesses sufficient to determine the relations among these three variables. The rectangles were from 2' to 50' wide; the contrast fraction, DeltaI/I, ranged from 0.01 to 40; the background brightness varied from 0.0001 to 2500 millilamberts. 2. When DeltaI/I or visual acuity is plotted as a function of brightness the data do, in general, follow Hecht's equation. The departure from a simple photochemical theory which the larger targets show is probably due to changes in the functional retinal mosaic with changing brightness. 3. In general also, the relation between visual acuity and brightness, at selected contrasts, fits Hecht's derivation. At low contrasts, as the brightness is reduced a point is reached at which the test object becomes invisible at any size. 4. No simple relation emerges from the data relating visual acuity to contrast, at set levels of illumination. Over only a very short range are visual acuity and contrast directly related. At high contrasts, visual acuity reaches a maximum, whereas at low visual acuity, DeltaI/I reaches a minimum which cannot be passed regardless of size. 5. The shape of the curves relating DeltaI/I to brightness is not significantly altered by changing the exposure time. There is some evidence to show that a 3 second exposure of the target is equivalent to two looks of 0.2 second each. 6. In all these studies the thresholds were determined by a frequency of seeing method, and the data have been considered in terms of a quantum theory of threshold seeing. It was found that a threshold response involves between four and eight independent critical events, which are largely independent of size, brightness, and criterion of seeing.     

Motion perception and motion estimation by total-least squares

A computational model of motion perception is proposed. The model, which is gradient-based, adheres to the neural constraint that transmitted signals are positive-valued functions by posing the estimation of image motion as a quadratic programming problem combined with total-least squares: a model that assumes that image signals are contaminated by noise in both the spatial and temporal dimensions. By shrinking motion estimates with a regularizer whose subtractive effect introduces a contrast dependent speed threshold into motion computations, it is shown that the total-least squares model when posed as a quadratic programming problem, is capable of explaining both increases and decreases in perceived speed as these effects were reported by Thompson (1982) to vary as a function of image contrast and temporal frequency. The correlation that exists between the model's contrast speed response and results reported from visual psychophysics is consistent with the view that the visual system assumes that image signals may be contaminated by noise in both the spatial and the temporal domain, and that visual motion is influenced by the consequence of these assumptions.     

A universal model of esthetic perception based on the sensory coding of natural stimuli

Philosophers have pointed out that there is a close relation between the esthetics of art and the beauty of natural scenes. Supporting this similarity at the experimental level, we have recently shown that visual art and natural scenes share fractal-like, scale-invariant statistical properties. Moreover, evidence from neurophysiological experiments shows that the visual system uses an efficient (sparse) code to process optimally the statistical properties of natural stimuli. In the present work, a hypothetical model of esthetic perception is described that combines both lines of evidence. Specifically, it is proposed that an artist creates a work of art so that it induces a specific resonant state in the visual system. This resonant state is thought to be based on the adaptation of the visual system to natural scenes. The proposed model is universal and predicts that all human beings share the same general concept of esthetic judgment. The model implies that esthetic perception, like the coding of natural stimuli, depends on stimulus form rather than content, depends on higher-order statistics of the stimuli, and is non-intuitive to cognitive introspection. The model accommodates the central tenet of neuroesthetic theory that esthetic perception reflects fundamental functional properties of the nervous system.     

Filling-in and suppression of visual perception from context: a Bayesian account of perceptual biases by contextual influences

Visual object recognition and sensitivity to image features are largely influenced by contextual inputs. We study influences by contextual bars on the bias to perceive or infer the presence of a target bar, rather than on the sensitivity to image features. Human observers judged from a briefly presented stimulus whether a target bar of a known orientation and shape is present at the center of a display, given a weak or missing input contrast at the target location with or without a context of other bars. Observers are more likely to perceive a target when the context has a weaker rather than stronger contrast. When the context can perceptually group well with the would-be target, weak contrast contextual bars bias the observers to perceive a target relative to the condition without contexts, as if to fill in the target. Meanwhile, high-contrast contextual bars, regardless of whether they group well with the target, bias the observers to perceive no target. A Bayesian model of visual inference is shown to account for the data well, illustrating that the context influences the perception in two ways: (1) biasing observers' prior belief that a target should be present according to visual grouping principles, and (2) biasing observers' internal model of the likely input contrasts caused by a target bar. According to this model, our data suggest that the context does not influence the perceived target contrast despite its influence on the bias to perceive the target's presence, thereby suggesting that cortical areas beyond the primary visual cortex are responsible for the visual inferences.     

Conditioned feedback and inhibitory reorganization of the receptive fields of the cortical neurons as a basis for the subconscious change in the thresholds of visual recognition and detection

In the paper three groups of facts are compared: significant adaptative and adaptational modification of receptive fields of cat's visual cortex neurones, conditioned selective subsensory change of the threshold of perception (detection and recognition) of a letter by man in relation to two control ones and the role of spatially specialized cortical inhibition in formation and adaptative modifications of receptive fields and detector properties of the visual cortex neurones. Interconnection is discussed of the phenomena described as well as community of their mechanisms.     

Temporal mechanisms of multimodal binding

The simultaneity of signals from different senses—such as vision and audition—is a useful cue for determining whether those signals arose from one environmental source or from more than one. To understand better the sensory mechanisms for assessing simultaneity, we measured the discrimination thresholds for time intervals marked by auditory, visual or auditory–visual stimuli, as a function of the base interval. For all conditions, both unimodal and cross-modal, the thresholds followed a characteristic ‘dipper function’ in which the lowest thresholds occurred when discriminating against a non-zero interval. The base interval yielding the lowest threshold was roughly equal to the threshold for discriminating asynchronous from synchronous presentations. Those lowest thresholds occurred at approximately 5, 15 and 75 ms for auditory, visual and auditory–visual stimuli, respectively. Thus, the mechanisms mediating performance with cross-modal stimuli are considerably slower than the mechanisms mediating performance within a particular sense. We developed a simple model with temporal filters of different time constants and showed that the model produces discrimination functions similar to the ones we observed in humans. Both for processing within a single sense, and for processing across senses, temporal perception is affected by the properties of temporal filters, the outputs of which are used to estimate time offsets, correlations between signals, and more.     

Lateral sensitivity modulation explains the flanker effect in contrast discrimination

We used a dual-masking paradigm to study how contrast discrimination can be influenced by the presence of adjacent stimuli. The task of the observer was to detect a target superimposed on a pedestal in the presence of flankers. The flankers (i) reduce the target threshold at zero pedestal contrast, (ii) shift the target threshold versus pedestal contrast (TvC) function horizontally to the left on a log-log plot at high pedestal contrasts, and (iii) reduce the size of pedestal facilitation at low pedestal contrasts. The horizontal shift at high pedestal contrasts suggests that the flanker effect is a multiplicative factor that cannot be explained by previous models of contrast discrimination. We extend the divisive inhibition model of contrast discrimination by implementing the flanker effect as a lateral multiplicative sensitivity modulation. This extended model provides a good account of the data.     

Influence of Visual Motion,Suggestion, and Illusory Motion on Self-Motion Perception in the Horizontal Plane

A moving visual field can induce the feeling of self-motion or vection. Illusory motion from static repeated asymmetric patterns creates a compelling visual motion stimulus, but it is unclear if such illusory motion can induce a feeling of self-motion or alter self-motion perception. In these experiments, human subjects reported the perceived direction of self-motion for sway translation and yaw rotation at the end of a period of viewing set visual stimuli coordinated with varying inertial stimuli. This tested the hypothesis that illusory visual motion would influence self-motion perception in the horizontal plane. Trials were arranged into 5 blocks based on stimulus type: moving star field with yaw rotation, moving star field with sway translation, illusory motion with yaw, illusory motion with sway, and static arrows with sway. Static arrows were used to evaluate the effect of cognitive suggestion on self-motion perception. Each trial had a control condition; the illusory motion controls were altered versions of the experimental image, which removed the illusory motion effect. For the moving visual stimulus, controls were carried out in a dark room. With the arrow visual stimulus, controls were a gray screen. In blocks containing a visual stimulus there was an 8s viewing interval with the inertial stimulus occurring over the final 1s. This allowed measurement of the visual illusion perception using objective methods. When no visual stimulus was present, only the 1s motion stimulus was presented. Eight women and five men (mean age 37) participated. To assess for a shift in self-motion perception, the effect of each visual stimulus on the self-motion stimulus (cm/s) at which subjects were equally likely to report motion in either direction was measured. Significant effects were seen for moving star fields for both translation (p = 0.001) and rotation (p<0.001), and arrows (p = 0.02). For the visual motion stimuli, inertial motion perception was shifted in the direction consistent with the visual stimulus. Arrows had a small effect on self-motion perception driven by a minority of subjects. There was no significant effect of illusory motion on self-motion perception for either translation or rotation (p>0.1 for both). Thus, although a true moving visual field can induce self-motion, results of this study show that illusory motion does not.     

Changes in perceived contrast of suprathreshold gratings as a function of orientation and spatial frequency

Orientation anisotropy for suprathreshold gratings of different spatial frequencies was measured using a contrast matching procedure. Observers matched the contrast of sine-wave gratings of various orientations to a vertical reference grating set at different reference contrasts. At threshold, the size of the anisotropy increased with spatial frequency, confirming previous results. When the reference grating contrast was set above threshold, the anisotropy declined, and eventually disappeared for gratings of medium spatial frequencies. At higher spatial frequencies, although the relative anisotropy became smaller, it did not disappear within the range of contrasts used in this study. For medium, but not for high spatial frequencies, the data are consistent with Kulikowski's (1976) model of effective contrast constancy.     

Attention Increases the Temporal Precision of Conscious Perception: Verifying the Neural-ST2 Model

What role does attention play in ensuring the temporal precision of visual perception? Behavioural studies have investigated feature selection and binding in time using fleeting sequences of stimuli in the Rapid Serial Visual Presentation (RSVP) paradigm, and found that temporal accuracy is reduced when attentional control is diminished. To reduce the efficacy of attentional deployment, these studies have employed the Attentional Blink (AB) phenomenon. In this article, we use electroencephalography (EEG) to directly investigate the temporal dynamics of conscious perception. Specifically, employing a combination of experimental analysis and neural network modelling, we test the hypothesis that the availability of attention reduces temporal jitter in the latency between a target''s visual onset and its consolidation into working memory. We perform time-frequency analysis on data from an AB study to compare the EEG trials underlying the P3 ERPs (Event-related Potential) evoked by targets seen outside vs. inside the AB time window. We find visual differences in phase-sorted ERPimages and statistical differences in the variance of the P3 phase distributions. These results argue for increased variation in the latency of conscious perception during the AB. This experimental analysis is complemented by a theoretical exploration of temporal attention and target processing. Using activation traces from the Neural-ST² model, we generate virtual ERPs and virtual ERPimages. These are compared to their human counterparts to propose an explanation of how target consolidation in the context of the AB influences the temporal variability of selective attention. The AB provides us with a suitable phenomenon with which to investigate the interplay between attention and perception. The combination of experimental and theoretical elucidation in this article contributes to converging evidence for the notion that the AB reflects a reduction in the temporal acuity of selective attention and the timeliness of perception.