Internal noise determines external stochastic resonance in visual perception

We provide the first experimental evidence that the internal noise level determines whether external noise can enhance the detectability of a weak signal. We conduct a visual detection experiment in the absence and presence of visual noise. We define three indices of external stochastic resonance effects, consider the spread of the psychometric function without external noise as an internal noise level index, and find that the indices of external stochastic resonance effects negatively correlate with the internal noise level index. Our results suggest that external stochastic resonance depends not only on the external but also on the internal noise level.     

Spatial contrast sensitivity in dynamic and static additive luminance noise

The purpose of this study was to define the quantitative relationship between the temporal characteristics of additive luminance noise and the properties of the spatial contrast sensitivity function (CSF). CSFs were obtained from two observers using Gabor patch targets of short duration that were added to white luminance noise with a range of root-mean-square contrasts (c_rms). The noise was either dynamic or static and was either of the same duration as the test target (synchronous) or of longer duration (asynchronous). For targets presented in asynchronous dynamic, synchronous dynamic, and synchronous static noise, the CSFs became increasingly band-pass with increasing c_rms, whereas the CSFs were low-pass at all levels of c_rms for targets presented in asynchronous static noise. For all noise types, the properties of the CSFs were well-predicted by the linear amplifier model (LAM), in which the signal energy at threshold (E_t) is related linearly to noise spectral density (N). The fundamentally different characteristics of CSFs obtained in asynchronous static noise can be accounted for by a previous proposal that this noise type biases contrast sensitivity toward transient (inferred magnocellular) mechanisms. The other three modes of noise presentation appear to emphasize detection by sustained (inferred parvocellular) mechanisms.     

Contrast thresholds in additive luminance noise: Effect of noise temporal characteristics

This study investigated the way in which the temporal properties of additive luminance noise influence threshold contrast and affect estimates of equivalent noise and sampling efficiency. Threshold contrast was obtained from four visually normal observers for a 2-cycle-per-degree Gabor patch across a range of target durations in the absence and presence of additive luminance noise that was either static or dynamic. In addition, the temporal relationship between target and noise was either synchronous (simultaneous presentation of both) or asynchronous (noise duration longer than target duration). For both synchronous and asynchronous presentation modes, the extent of temporal integration differed for targets presented in dynamic vs. static noise. Furthermore, for a fixed-duration target, increasing the degree of temporal asynchrony between target and noise monotonically increased threshold contrast in dynamic noise, but had a non-monotonic effect on threshold contrast in static noise. For both dynamic and static noise, estimates of equivalent noise and sampling efficiency were dependent on the degree of temporal asynchrony between target and noise. The observed differences between the effects of dynamic and static noise are consistent with a previous proposal that detection of targets of intermediate spatial frequency in the presence of these two noise types is governed by sustained-like and transient-like visual mechanisms, respectively.     

Spatial frequency selectivity of visual suppression during convergence eye movements

Visual suppression of low-spatial frequency information during eye movements is believed to contribute to a stable perception of our visual environment. While visual perception has been studied extensively during saccades, vergence has been somewhat neglected. Here, we show that convergence eye movements reduce contrast sensitivity to low spatial frequency information around the onset of the eye movements, but do not affect sensitivity to higher spatial frequencies. This suggests that visual suppression elicited by convergence eye movements may have the same temporal and spatial characteristics as saccadic suppression.     

Perceptual learning improves contrast sensitivity and visual acuity in adults with anisometropic amblyopia

To evaluate the effects of perceptual learning on contrast-sensitivity function and visual acuity in adult observers with amblyopia, 23 anisometropic amblyopes with a mean age of 19.3 years were recruited and divided into three groups. Subjects in Group I were trained in grating detection in the amblyopic eye near pre-training cut-off spatial frequency. Group II received a training regimen of repeated contrast-sensitivity function measurements in the amblyopic eye. Group III received no training. We found that training substantially improved visual acuity and contrast-sensitivity functions in the amblyopic eyes of all the observers in Groups I and II, although no significant performance improvement was observed in Group III. For observers in Group I, performance improvements in the amblyopic eyes were broadly tuned in spatial frequency and generalized to the fellow eyes. The latter result was not found in Group II. In a few cases tested, improvements in visual acuity following training showed about 90% retention for at least 1 year. We concluded that the visual system of adult amblyopes might still retain substantial plasticity. Perceptual learning shows potential as a clinical tool for treating child and adult amblyopia.     

Psychophysical channels and ERP population responses in human visual cortex: Area summation across chromatic and achromatic pathways

In the early stages of vision, information is transmitted through distinct physiologically defined pathways. These may be related with three post-receptoral detection mechanisms defined psychophysically in humans. Accordingly, the parvocellular pathway is very sensitive to L-M-cone contrast, processes mainly foveal information and underlies fine discrimination of visual features. The magnocellular pathway is most sensitive to luminance contrast and is important for visuo-spatial and motion processing. The less understood koniocellular pathway responds to S-cone modulation outside the foveola. As such, the three pathways process visual information in a different manner, with the L-M-cone psychophysical channel being more devoted to central vision and the two other channels responding significantly to peripheral information. We measured size response functions of these three processing channels using event related potential (ERP/EEG) recordings and stimuli with various sizes and contrasts with the aim of studying coding of stimulus properties within each of these channels. The effect of stimulus size was significantly smaller for the L-M-cone channel consistent with its dominance in the central visual field. Furthermore, for this pathway, the effect of size was not modulated by stimulus contrast. In contrast, both the S-cone and achromatic channels showed a strong effect of size that was significantly modulated by contrast. Interestingly, both the S-cone and achromatic channels responded proportionally to the area of cortex activated, suggesting that the S-cone channel represents space in a similar manner to the achromatic channel. In conclusion, a fundamental relation exists between previously identified psychophysical mechanisms and population responses in the visual cortex.     

Crowding in a detection task: external noise triggers change in processing strategy

External noise paradigms have been widely used to probe different levels of visual processing (Pelli & Farell, 1999). A basic assumption of this paradigm is that the processing strategy is noise-invariant, remaining the same in low and high external noise. We tested this assumption by examining crowding in a detection task where traditionally crowding has no effect. In the first experiment, we measured detection thresholds for a vertically oriented sine wave grating (target) surrounded by four sine wave gratings (flankers) that were either vertically or horizontally oriented. At low noise levels, the detection threshold for the target was unaffected by the orientation of the flankers - there was no crowding. Surprisingly, however, there was crowding for detection at high noise levels: the threshold increased for the similarly-oriented flankers. This suggests that high noise triggered a change in processing strategy, increasing the range of space or features over which the visual signal was sampled. In a second experiment, we evaluated the impact of the spatial and temporal window of the noise on this crowding effect. Although crowding was observed for detection when the spatial and/or temporal window of the noise was localized (i.e. identical to the signal window), no crowding was observed when the noise was spatially and temporally extended (i.e. continuously displayed, full screen dynamic noise). Our results show that certain spatiotemporal distributions of external noise can elicit a change in processing strategy, invalidating the noise-invariant assumption that underlies external noise paradigms. In contrast, spatiotemporally extended noise maintains the required noise-indifference, perhaps because it matches the characteristics of the internal noise that determines the contrast threshold in low noise.     

Sampling efficiency and internal noise for motion detection,discrimination, and summation

By comparing real observers to an ideal observer, previous studies have found that the detection of static patterns is limited by internal noise and by imperfect sampling efficiency. We developed and applied ideal observer models for the detection, discrimination, and summation of oppositely drifting gratings in Gaussian white noise. The three tasks share a common source of internal noise. The sampling efficiencies were on the order of 1-2% except for much lower efficiency in direction discrimination for faster moving gratings. The efficiency of direction discrimination relative to detection systematically declines as the speed is increased from 1 to 6 Hz. These results suggest that observers use mismatched filters tuned to slow speeds regardless of the signal speed. Human visual motion sensing appears to use distorted representations of the incoming signals, and this distortion is a major limitation to visual performance.     

Spatial summation in dark-adapted human infants

Behavioral measures of spatial summation in 4- and 11-week old human infants were obtained using the forced-choice preferential looking technique. Infants exhibit complete spatial summation over large areas (4 weeks: 8.9 degrees; 11 weeks: 5.5 degrees), much larger than those of adults tested in the same apparatus (2.6 degrees). Thus, infants' summation areas are larger than adults' by factors of 12 at 4 weeks and 4 at 11 weeks. It is argued that at most, one-half of this difference between the infant and adult data may be due to optical factors (e.g. defocus and small eye size). The remainder probably reflects differences in the neural organization of the immature and mature visual systems.     

Temporal properties of the visual responses to luminance and contrast modulated noise

Vision is sensitive to first-order luminance modulations and second-order modulations of carrier contrast. Our knowledge of the temporal properties of second-order vision is insufficient and contradictory. Using temporal summation and reaction time paradigms, we found that the type of visual noise (static or dynamic) determines the temporal properties of the responses to luminance and contrast modulations. In the presence of static noise, the temporal responses to both types of modulation of low and higher spatial frequencies were transient. When dynamic noise was used, the temporal responses to luminance and contrast modulations of higher spatial frequencies were sustained. At low spatial frequency, however, luminance modulations elicited transient responses, while contrast modulated dynamic noise produced sustained responses. The reaction times to near-threshold contrast modulations of low spatial frequency were slower than those to first-order patterns and they did not significantly differ at modulations of higher spatial frequency. The results suggest that the temporal characteristics of first-stage linear filters which feed the second-order pathway may determine the temporal responses to contrast modulated noise.     

Spatial frequency tuning in the visual evoked potential elicited by sine-wave gratings

This investigation examined the onset response of the transient visually evoked potential elicited by the appearance-disappearance of sine-wave gratings at various levels of spatial frequency, suprathreshold contrast, and stimulus duration. The response was most easily characterized by two negative-positive complexes that were differentially tuned to spatial frequency. The earlier complex peaked at high spatial frequencies while the later complex peaked at low spatial frequencies. For both complexes, amplitude showed only slight variations across three-octave ranges of contrast and duration. Latency was curvilinearly related to spatial frequency, decreased with increasing contrast, and showed no apparent change as a function of duration.     

Towards a classification of visual impairment

For the rehabilitation of people with impaired vision, it is essential to have adequate (preferably quantitative) information about their residual visual functions. Special attention is given to the extra information provided by the results of measurement of the contrast sensitivity, especially in combination with the results of other measurements, such as the visual field, the amount of intraocular straylight and the visual acuity. The value of the contrast sensitivity function as a predictor of the extent of dysfunctioning in the visual activities of everyday life, such as outdoor vision, reading and recognition of faces, will be discussed. As far as this is concerned, a comparison is made between the value of visual acuity and contrast sensitivity as measures of the extent of vision and visual dysfunctioning.     

葡萄膜炎的视功能预后

葡萄膜炎是造成视功能损害的重要病因之一。视功能的评价手段包括视力、视野、对比敏感度、色党、视觉电生理等,其中,视力是最直观实用的手段。各种病因所引起的葡萄膜炎视力预后差异较大,炎症的严重程度、炎症部位、并发症不同,对视力的影响程度也不同。视力损害最常见于全葡萄膜炎患者,且年龄较大的全葡萄膜炎患者发生严重视力损害的风险增加,双眼葡萄膜炎患者炎症一般较重且不易控制。在各种病因中,结节病性葡萄膜炎、Behcet病、青少年类风湿性关节炎伴发葡萄膜炎者视力预后较差。葡萄膜炎的并发症也可以直接引起视功能损害,黄斑囊样水肿（CME）是葡萄膜炎最常见的并发症,也是引起葡萄膜炎患者视力损害最常见的原因。葡萄膜炎患者的对比敏感度、视野、视网膜电图检查同样可以遗留异常。就葡萄膜炎的视功能预后及其影响因素进行综述。     

Psychophysical measures of visual acuity in autism spectrum conditions

Previously reported superior visual acuity (VA) in autism spectrum conditions (ASC) may have resulted from methodological settings used (Ashwin, Ashwin, Rhydderch, Howells, & Baron-Cohen, 2009). The current study re-tested whether participants with (N = 20) and without (N = 20) ASC differ on psychophysical measures of VA. Participants’ vision was corrected before acuity measurement, minimising refractive blur. VA was assessed with an ETDRS chart as well as the Freiburg Visual Acuity and Contrast Test (FrACT). FrACT testing was undertaken at 4 m (avoiding limitations of pixel-size), using 36 trials (avoiding fatigue). Best corrected VA was significantly better than the initial habitual acuity in both groups, but adults with and without ASC did not differ on ETDRS or FrACT binocular VA. Future research should examine at which level of visual processing sensory differences emerge.     

7～16岁学生不同对比度视力的观察

目的研究7-16岁学生不同对比度下的视力（CVA）及不同对比度、年龄和性别对CVA的影响,评估在学生体检时引入低对比度视力测量的重要性。方法横断面研究。入选标准：年龄7-16岁学生,单眼裸眼视力≥1．0。共入选299名学生,将学生分为7-10岁、11-13岁和14-16岁3个年龄组。对受检者先行视力及眼部检查,然后使用电子视力测量仪MFVA-200检测对比度分别为100％标高及10％标低下的CVA。结果随着MFVA-200检查时对比度的下降,各年龄组受检者的右眼和左眼的CVA均有下降趋势,差异有统计学意义（t右眼=56．28,t左眼=63．93,P值均〈0．01）;在100％对比度下,3个年龄组间CVA差异无统计学意义（右眼CVA：F100=1．74,P=0．18;左眼CVA：F100=1．73,P=0．18）,在标低10％对比度下,3个年龄组间CVA差异有统计学意义（右眼CYA：F10：4．59,P=0．01;左眼CVA：F10=5．65,P=0．004）,右眼7-10岁、11-13岁和14-16岁3个年龄组CVA分别为（0．53±0．14）、（0．50±0．14）和（0．46±0．15）,左眼分别为（0．53±0．16）、（0．49±0．15）和（0．45±0．15）;男性和女性在标高和标低对比度下,男性CVA值均高于女性,差异有统计学意义（右眼CVA：t100=3．67,P〈0．001;t10=2．54,P〈0．01;左眼CVA：t100=3．64,P〈0．001;t10=3．83,P〈0．001）。结论7-16岁学生随着对比度的下降,视力也出现下降趋势,高对比度视力年龄组间无差异的学生,在低对比度下年龄较大组视力差于年龄较小组,同-对比度下男性CVA好于女性。对于近视高发年龄段的学生,在体检时引入对比度视力的测量是十分重要的。     

超常视力眼波前像差与对比度视力相关性研究

目的 分析超常视力眼波前像差与不同对比度视力的相关关系,探讨与视力相关的像差函数.方法 应用Snellcn视力表检测裸眼视力≥1.5者81例(81只眼),用WASCA波阵面像差仪测眼波前像差,用多功能电子视力测量仪测量、暗环境下对比度分别为100%、25%、10%和5%的视力.用多元线性相关与回归分析波前像差与不同对比度视力的相关关系.结果 总像差、各阶像差及各Zemike函数与亮、暗环境中对比度为100%和25%视力间的直线相关关系均无统计学意义(P＞0.05).总像差、2阶像差及离焦像差与亮、暗环境中对比度为10%和5%视力间直线相关关系均有统计学意义(P＜0.01)相关系数分别为-0.30、-0.35和-0.28、-0.35:-0.33、-0.38和-0.28、-0.39;-0.31、-0.39和-0.28、-0.39(P＜0.01).其余像差与对比度为10%利5%视力间直线相关关系均无统计学意义(P＞0.05).结论 超常视力眼波前像差与高对比度视力无直线相关关系,与低对比度视力有直线相关关系,总像差、2阶像差和离焦像差越小,低对比度视力越好.     

早期皮质性白内障不同对比度下视力的临床观察

目的：分析早期皮质型白内障患者在不同对比度下视力（CVA）的改变,了解测量CVA在评价早期白内障患者视功能中的作用。方法病例对照研究。入选标准：选取早期皮质性白内障患者52例(104只眼),正常对照组52例（104只眼）,共入选104例。性别年龄不限,屈光状态检查球镜度数-3.00～3.00 D,柱镜度数为-1.50～1.50 D,单眼最佳矫正视力≥0.8,将其按照性别相同、年龄相仿(≤5岁)及最佳矫正远视力相近（ETDRS视力表相差≤1行）的原则配对分组,共计52对。对受试者先行眼部及屈光检查,随后在暗室内使用多功能视力测试仪检测对比度（MFVA）为100％、25％、10％、5％CVA。采用配对t检验及F检验分别对检测数据进行分析。结果白内障患者组在对比度为100％、25％、10％、5％下的右眼CVA分别为0.99±0.26、0.59±0.23、0.38±0.19、0.23±0.12,左眼CVA分别为0.98±0.28、0.59±0.18、0.38±0.14、0.23±0.10,双眼CVA分别为1.15±0.26、0.73±0.20、0.49±0.16、0.30±0.10,均较健康对照组显著下降（右眼t=-6.49,-7.25,-7.57,-5.45;左眼t=-5.78,-7.51,-6.41,-7.16;双眼t=-4.33,-7.60,-7.53,-9.75;P均<0.01）。不同晶状体皮质的混浊程度均会造成在对比度为100％、25％、10％、5％下的CVA显著下降（I级：F=50.18,P<0.01;Ⅱ级：F=23.08,P<0.01;Ⅲ级：F=20.17,P<0.01）不同晶状体皮质的混浊程度均会造成在对比度为100％、25％、10％、5％下的CVA较健康对照组显著下降（I级：t=-6.49,-7.25,-7.57,-5.45,P均<0.05;Ⅱ级：t＝-5.78,-7.51,-6.41,-7.16,P均<0.01;Ⅲ级：t=-4.33,-7.60,-7.53,-9.75,P均<0.01）。结论对于早期白内障患者,对比度为100％、25％、10％、5％CVA的测量较最佳矫正视力能更全面、更灵敏的评价视功能。（中华眼科杂志,2015,51：510-514）     

Human cortical responses to contrast modulations of visual noise

We studied visual evoked potentials (VEPs) elicited by second-order contrast modulations of binary dynamic noise and first-order luminance modulations. Using a 3-point Laplacian operator centred on Oz, we found that contrast modulations of both low and higher spatial frequencies elicited a negative component whose latency was about 200 ms. The latency of this component was significantly longer than that of the early Laplacian components to first-order luminance modulations. These findings could be due to slower first-stage linear filters and additional processing stages of the second-order pathway. The topographical analysis of scalp recorded VEPs to central and half-field stimulation has suggested that the responses to second-order patterns are likely to be generated by neuronal structures within the primary visual cortex which may have inputs from extrastriate neurons via feedback connections.