Effects of aging on biological motion discrimination

Previous studies have shown that older subjects have difficulties discriminating the walking direction of point-light walkers. In two experiments, we investigated the underlying cause in further detail. In Experiment 1, subjects had to discriminate the walking direction of upright and inverted point-light walkers in a cloud of randomly moving dots. In general, older subjects performed less accurately and showed an increased inversion effect. Nevertheless, they were as accurate as young subjects for upright walkers during training, in which no noise was added to the display. These results indicate that older subjects are less able to extract relevant information from noisy displays. In Experiment 2, subjects discriminated the walking direction of scrambled walkers that primarily contained local motion information, random-position walkers that primarily contained global form information, and normal point-light walkers that contained both kinds of information. Both age groups performed at chance when no global form information was present in the display but were equally accurate for walkers that only contained global form information. However, when both local motion and global form information were present in the display, older subjects were less accurate then younger subjects. Older subjects again exhibited an increased inversion effect. These results indicate that both older and younger subjects rely more on global form than local motion to discriminate the direction of point-light walkers. Also, older subjects seem to have difficulties integrating global form and local motion information as efficiently as younger subjects.     

Independence in the processing of first- and second-order motion signals at the local-motion-pooling level

The interaction of first- and second-order motion signals at the local-motion-pooling level were investigated using locally-paired dots that moved orthogonally to each other. Dots were either luminance-defined, which could, potentially drive both first- and second-order local-motion units, or contrast-defined, which only drive second-order local-motion units. The response measure used was the nature of the motion percept: either unidirectional or transparent motion. The likelihood of perceiving transparent motion was varied by adjusting the trajectory length of the dots. Increasing the trajectory length increased the likelihood that observers would perceive transparency. The results, taken as a whole, support the notion of independent first-order and second-order local-motion-pooling units, with the spatial extent of the second-order units being larger than that of the first-order units.     

Selective attention modulates electrical responses to reversals of optic-flow direction

Attended stimuli typically evoke larger event-related potentials (ERPs) than unattended stimuli. We previously reported an exception when an optic-flow pattern is interleaved with stationary dots. Reversals of motion direction evoked a larger N200 peak when attention was directed to the stationary dots. We replicated and further characterized this result: the N200 enhancement was eliminated when the dots moved randomly rather than in optic flow. The effect was also attenuated with isoluminant stimuli. Electrical source analysis suggested the attentional modulation of a configuration of dorsal extrastriate generators. The ERP evoked by reversals of optic flow may reflect the operation of independently configurable attentional filters within visual cortex.     

Relating spatial and temporal orientation pooling to population decoding solutions in human vision

Spatial pooling is often considered synonymous with averaging (or other statistical combinations) of local information contained within a complex visual image. We have recently shown, however, that spatial pooling of motion signals is better characterized in terms of optimal decoding of neuronal populations rather than image statistics (Webb et al., 2007). Here we ask which computations guide the spatial and temporal pooling of local orientation signals in human vision. The observers’ task was to discriminate which of two texture patterns had a more clockwise global orientation. Standard textures had a common orientation; comparison textures were chosen independently from a skewed (asymmetrical) probability distribution with distinct spatial or temporal statistics. We simulated observers’ performance using different estimators (vector average, winner-takes-all and maximum likelihood) to decode the orientation-tuned activity of a population of model neurons. Our results revealed that the perceived global orientation of texture patterns coincided with the mean (or vector average read-out) of orientation signals accumulated over both space and time. To reconcile these results with our previous work on direction pooling, we varied stimulus duration. Perceived global orientation was accurately predicted by a vector average read-out of orientation signals at relatively short stimulus durations and maximum likelihood read-out at longer durations. Moreover, decreasing the luminance contrast of texture patterns increased the duration of the transition from a vector average to maximum likelihood read-out. Our results suggest that direction and orientation pooling use similar probabilistic read-out strategies when sufficient time is available.     

A comparison of monkey and human motion processing mechanisms

Single-cell recording studies have provided vision scientists with a detailed understanding of motion processing at the neuronal level in non-human primates. However, despite the development of brain imaging techniques, it is not known to what extent the response characteristics of motion-sensitive neurons in monkey brain mirror those of human motion-sensitive neurons. Using a motion adaptation paradigm, the direction aftereffect, we recently provided evidence of a strong resemblance in the response functions of motion-sensitive neurons in monkey and human to moving dot patterns differing in dot density. Here we describe a series of experiments in which measurements of the direction aftereffect are used to infer the response characteristics of human motion-sensitive neurons when viewing transparent motion and moving patterns that differ in their signal-to-noise ratio (motion coherence). In the case of transparent motion stimuli, our data suggest suppressed activity of motion-sensitive neurons similar to that reported for macaque monkey. In the case of motion coherence, our results are indicative of a linear relationship between signal intensity (coherence) and neural activity; a pattern of activity which also bears a striking similarity to macaque neural activity. These findings strongly suggest that monkey and human motion-sensitive neurons exhibit similar response and inhibitory characteristics.     

Effects of age and optical blur on real depth stereoacuity

Stereoscopic depth perception utilizes the disparity cues between the images that fall on the retinae of the two eyes. The purpose of this study was to determine what role aging and optical blur play in stereoscopic disparity sensitivity for real depth stimuli. Forty‐six volunteers were tested ranging in age from 15 to 60 years. Crossed and uncrossed disparity thresholds were measured using white light under conditions of best optical correction. The uncrossed disparity thresholds were also measured with optical blur (from +1.0D to +5.0D added to the best correction). Stereothresholds were measured using the Frisby Stereo Test, which utilizes a four‐alternative forced‐choice staircase procedure. The threshold disparities measured for young adults were frequently lower than 10 arcsec, a value considerably lower than the clinical estimates commonly obtained using Random Dot Stereograms (20 arcsec) or Titmus Fly Test (40 arcsec) tests. Contrary to previous reports, disparity thresholds increased between the ages of 31 and 45 years. This finding should be taken into account in clinical evaluation of visual function of older patients. Optical blur degrades visual acuity and stereoacuity similarly under white‐light conditions, indicating that both functions are affected proportionally by optical defocus.     

一种改进一维投影模型运动参数估计算法

一维投影模型运动参数估计是图像序列运动分析的关键技术,本文针对经典最小平方估计算法计算量大的缺点,提出了一种改进的一维投影模型运动参数估计算法。该方法通过对非线性最小平方误差函数的线性化,重新定义了目标函数,采用Newton最优化算法求解参数矢量,得到只与已知特征点对应关系有关的海森矩阵后,利用线性算法求解图像序列相邻两帧间一维投影模型的运动参数。实验结果表明,该算法在保证精确度的条件下,提高了计算效率。     

胰腺占位性病变的运动规律及其影响因素初探

目的 探讨胰腺占位性病变的运动规律及其影响因素,为其在精确放疗时确定PTV的安全边界提供参考价值.方法 16例胰腺癌患者采用B超引导下穿刺的方法将1-2个纯金标记(fiducial)植入胰腺肿瘤体内,并在模拟定位机下对标记的运动幅度进行测量,得出其在患者身体左右(x轴)、前后(y轴)、头脚(z轴)方向的移动数据.用多元线性回归模型分析影响因素.结果 测量结果显示16例患者在x轴方向的移动距离为0.1～0.3 cm[(0.16±0.06)cm],z轴方向为0.5-1.6 cm[(0.88±0.24)cm],y轴方向为0.1-O.4 cm[(0.25±0.12)cm].其移动距离与患者年龄、身高、体重、肿瘤位置和大小无关.结论 胰腺占位性病变的移动主要受呼吸运动影响,且在x轴方向的移动距离最大,在确定PTV安全边界时应主要考虑其在:轴方向的移动.     

基于相似性测度的4D-CT重建实验研究

目的 基于相似性测度实现周期运动靶区的4D-CT重建。方法 自制能模拟呼吸运动的体模,用16排螺旋CT机对体模进行螺旋扫描和电影(Cine)模式扫描;用VC++程序设计语言和VTK工具包,开发了CT图像分相位排序的4D-CT重建软件系统;根据相邻图像互信息量的计算(即相似性测度),把Cine模式扫描的CT图像按相位进行了排序并分别对不同相位的CT图像序列进行三维重建。结果 不同相位CT序列的3D重建明显地消除了运动伪影,与静态扫描重建结果相近。随时间变化的多个相位的3D-CT构成了4D-CT序列。结论 基于相似性测度能在普通CT上实现4D-CT重建,其重建过程不受CT机本身的软硬件限制具有普遍适用性。     

Loss of positional information when tracking multiple moving dots: the role of visual memory

Pylyshyn, Z.W. and Storm, R.W. (1988) (Tracking multiple independent targets: Evidence for a parallel tracking mechanism. Spatial Vision, 3(3), 179-197) proposed that human observers could simultaneously track up to five dots when presented with an array of dots moving in a random manner. In contrast, Tripathy, S.P., and Barrett, B.T. (2004) (Severe loss of positional information when detecting deviations in multiple trajectories. Journal of Vision, 4(12):4, 1020-1043, http://journalofvision.org/4/14/4/, doi: 10.1167/4.12.4) showed that when a threshold paradigm was employed, observers’ ability to track deviations in straight-line trajectories is severely compromised when attending to two or more dots. In this study we present a series of four experiments that investigates the role of attention and visual memory while tracking deviations in multiple trajectories using a threshold paradigm. Our stimuli consisted of several linear, non-parallel, left-to-right trajectories, each moving at the same speed. At the trajectory mid-point (reached simultaneously by all dots), one of the dots (target) deviated clockwise or counter-clockwise. The observers’ task was to identify the direction of deviation. The target trajectory was cued in the second half of the trial either by disappearance of distractors at the monitor’s mid-line (Experiment 1) or by means of a change in colour of the target (Experiment 2); in both cases deviation thresholds rose steeply when the number of distractor trajectories was increased from 0 (typical threshold ∼ 2°) to 3 (typical threshold > 20°). When all the trajectories were presented statically in a single frame (Experiment 3), thresholds for identifying the orientation change of the target trajectory remained relatively unchanged as the number of distractor trajectories was increased. When a temporal delay of a few hundred milliseconds was introduced between the first and second halves of trajectories (Experiment 4), deviation thresholds increased steeply. These results suggest that the persistence of trajectory-traces in visual sensory memory may play an important part in determining thresholds for detecting deviations in trajectories.