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Zhong-Lin Lu

Publications and source records attributed to Zhong-Lin Lu.

At least 19 recordsLinked to original sources
PubMed2005

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.

Adolescent↗
PubMed2005

An ortho-palladated dimethylbenzylamine complex as a highly efficient turnover catalyst for the decomposition of P=S insecticides. Mechanistic studies of the methanolysis of some P=S-containing phosphorothioate triesters.

An ortho-palladated complex Pd(dmba)(py)(OTf) (9), or Pd(N,N-dimethylbenzylamine)(pyridine)(trifluoromethanesulfonate), was synthesized and its solution properties in methanol studied as a function of pH. In neutral solution the triflate dissociates from the complex to give a dominant form Pd(dmba)(py)(HOCH3), and in acid the pyridine dissociates to give Pyr-H+ and Pd(dmba)(HOCH3)(HOCH3). Under basic conditions, Pd(dmba)(py)(HOCH3) ionizes to give Pd(dmba)(py)(-OCH3) from which the pyridine can dissociate to yield a mixture of a bis-methoxy-bridged dimer (Pd(dmba)(-OCH3))2 (15-dimer), and its monomer Pd(dmba)(HOCH3)(-OCH3). Kinetic studies under buffered conditions reveal that 9 is an effective catalyst for the methanolysis of fenitrothion and other P=S pesticides. The active form of the catalyst is a basic one having one associated methoxide generated with an apparent (s)(s)pK(a) of 10.8. Analysis of the change in the UV/vis spectrum as a function of pH generates a spectrophotometric (s)(s)pK(a) of 10.8 +/- 0.1. This catalytic system is shown to promote the methanolysis of fenitrothion (3), diazinon (4), quinalphos (5), coumaphos (10) and dichlofenthion (11) at 0.05 mol dm(-3) triethyl amine buffer, (s)(s)pH 10.8, 25 degrees C, under turnover conditions where the [phosphorothioate]/[9] ratio is 48.6, 13.4, 13.4, 18.6, and 48.6 respectively. In all cases, the products were derived from displacement of the leaving group by methoxide, the second-order turnover rate constants being 36.9, 0.45, 0.12, >146.7 and 44.3 dm3 mol(-1) s(-1) respectively. An associative mechanism for the catalyzed methanolysis of the P=S pesticides is proposed where a transiently coordinated S=P substrate is intramolecularly attacked by the Pd(II)-coordinated methoxide.

Journal Article↗
PubMed2005

Independent perceptual learning in monocular and binocular motion systems.

Eye-transfer tests, external noise manipulations, and observer models were used to systematically characterize learning mechanisms in judging motion direction of moving objects in visual periphery (Experiment 1) and fovea (Experiment 2) and to investigate the degree of transfer of the learning mechanisms from trained to untrained eyes. Perceptual learning in one eye was measured over 10 practice sessions. Subsequent learning in the untrained eye was assessed in five transfer sessions. We characterized the magnitude of transfer of each learning mechanism to the untrained eye by separately analyzing the magnitude of subsequent learning in low and high external noise conditions. In both experiments, we found that learning in the trained eye reduced contrast thresholds uniformly across all of the external noise levels: 47 +/- 10% and 62 +/- 8% in experiments 1 and 2, respectively. Two mechanisms, stimulus enhancement and template retuning, accounted for the observed performance improvements. The degree of transfer to the untrained eye depended on the amount of external noise added to the signal stimuli: In high external noise conditions, learning transferred completely to the untrained eye in both experiments. In low external noise conditions, there was only partial transfer of learning: 63% in experiment 1 and 54% in experiment 2. The results suggest that template retuning, which is effective in high external noise conditions, is mostly binocular, whereas stimulus enhancement, which is effective in low external noise displays, is largely monocular. The two independent mechanisms underlie perceptual learning of motion direction identification in monocular and binocular motion systems.

Fixation, Ocular↗
PubMed2005

Perceptual learning in clear displays optimizes perceptual expertise: learning the limiting process.

Human operators develop expertise in perceptual tasks by practice or perceptual learning. For noisy displays, practice improves performance by learned external-noise filtering. For clear displays, practice improves performance by improved amplification or enhancement of the stimulus. Can these two mechanisms of perceptual improvement be trained separately? In an orientation task, we found that training with clear displays generalized to performance in noisy displays, but we did not find the reverse to be true. In noisy displays, the noise in the stimulus limits performance. In clear displays, performance is limited by noisiness of internal representations and processes. Our results suggest that training in one display condition optimizes the limiting factor(s) in performance in that condition and that noise filtering is also improved by exposure to the stimulus in clear displays. The asymmetric pattern of transfer implies the existence of two independent mechanisms of perceptual learning, which may reflect channel re-weighting in adult visual system. These results also suggest that training operators with clear stimuli may suffice to improve performance in a range of clear and noisy environments by simultaneous learning by two mechanisms.

Humans↗
PubMed2005

Fast decay of iconic memory in observers with mild cognitive impairments.

In a previous clinical report, unusually fast decay of iconic memory was obtained from a subject who later developed Alzheimer's disease. By using the partial-report paradigm, iconic memory (a form of visual sensory memory) in a group of observers with mild cognitive impairments (MCI) was characterized and compared with that of young college-age adults and older controls. Relatively long stimulus exposures were used for all three groups to ensure that older observers could perceive the stimuli. A set of conventional neuropsychological tests assessed cognitive functions of the MCI and older control groups. We found that iconic memory decayed much faster for observers with MCI than for normal controls, old or young, although the two groups of older observers performed at equivalent levels in precue tests (assay of visibility) and tests cued at long delays (assay of short-term memory). The result suggests that fast decay of iconic memory might be a general characteristic of observers with MCI who are at much higher than average risk of developing Alzheimer's disease later in life.

Aged↗
PubMed2005

Perceptual learning of Gabor orientation identification in visual periphery: complete inter-ocular transfer of learning mechanisms.

We combined the external noise paradigm, the Perceptual Template Model approach, and transfer tests to investigate the mechanisms and eye-specificity of perceptual learning of Gabor orientation in visual periphery. Coupled with a fixation task, discriminating a 5 from an S in a rapid small character string at fixation, contrast thresholds were estimated for each of eight external noise levels at two performance criteria using 3/1 and 2/1 staircases. Perceptual learning in one eye was measured over 10 practice sessions, followed by five sessions of practice in the new eye to assess transfer. We found that monocular learning improved performance (reduced contrast thresholds) with virtually equal magnitude across a wide range of external noise levels with no significant change in central task performance. Based on measurements of learning effects at two performance criterion levels, we identified a mixture of stimulus enhancement and external noise exclusion as the mechanism of perceptual learning underlying the observed improvements. Perceptual learning in the trained eye generalized completely to the untrained eye. We related the transfer patterns to known physiology and psychophysics on orientation direction coding.

Contrast Sensitivity↗
PubMed2005

The dynamics of perceptual learning: an incremental reweighting model.

The mechanisms of perceptual learning are analyzed theoretically, probed in an orientation-discrimination experiment involving a novel nonstationary context manipulation, and instantiated in a detailed computational model. Two hypotheses are examined: modification of early cortical representations versus task-specific selective reweighting. Representation modification seems neither functionally necessary nor implied by the available psychophysical and physiological evidence. Computer simulations and mathematical analyses demonstrate the functional and empirical adequacy of selective reweighting as a perceptual learning mechanism. The stimulus images are processed by standard orientation- and frequency-tuned representational units, divisively normalized. Learning occurs only in the "read-out" connections to a decision unit; the stimulus representations never change. An incremental Hebbian rule tracks the task-dependent predictive value of each unit, thereby improving the signal-to-noise ratio of their weighted combination. Each abrupt change in the environmental statistics induces a switch cost in the learning curves as the system temporarily works with suboptimal weights.

Brain↗
PubMed2005

Deficits in perceptual noise exclusion in developmental dyslexia.

We evaluated signal-noise discrimination in children with and without dyslexia, using magnocellular and parvocellular visual stimuli presented either with or without high noise. Dyslexic children had elevated contrast thresholds when stimuli of either type were presented in high noise, but performed as well as non-dyslexic children when either type was displayed without noise. Our findings suggest that deficits in noise exclusion, not magnocellular processing, contribute to the etiology of dyslexia.

Artifacts↗
PubMed2004

Spatial attention excludes external noise without changing the spatial frequency tuning of the perceptual template.

In this study, we investigated the functional mechanism by which spatial attention excludes unwanted information, a consequence of attention that has been consistently demonstrated at the neuronal level, the neural population level, and the overall behavioral level. The effect of spatial attention was measured using a temporal cuing paradigm. External noise, whose spatial frequency characteristics were systematically manipulated, was added to the signal stimulus. Contrast thresholds were measured as functions of the pass-band of the external noise to reveal the spatial frequency characteristics of the perceptual template in both the attended and unattended conditions. We found that spatial attention excludes external noise uniformly across all the spatial frequencies without changing the spatial frequency selectivity of the perceptual template.

Attention↗
PubMed2004

Perceptual learning retunes the perceptual template in foveal orientation identification.

What is learned during perceptual learning? We address this question by analyzing how perceptual inefficiencies improve over the course of perceptual learning (Dosher & Lu, 1998). Systematic measurements of human performance as a function of both the amount of external noise added to the signal stimulus and the length of training received by the observers enable us to track changes of the characteristics of the perceptual system (e.g., internal noise[s] and efficiency of the perceptual template) as perceptual learning progresses, and, therefore, identifies the mechanism(s) underlying the observed performance improvements. Two different observer models, the linear amplifier model (LAM) and the perceptual template model (PTM), however, have led to two very different theories of learning mechanisms. Here we demonstrate the failure of an LAM-based prediction - that the magnitude of learning-induced threshold reduction in high external noise must be less or equal to that in low external noise. In Experiment 1, perceptual learning of Gabor orientation identification in fovea showed substantial performance improvements only in high external noise but not in zero or low noise. The LAM-based model was "forced" to account for the data with a combination of improved calculation efficiency and (paradoxical) compensatory increases of the equivalent internal noise. Based on the PTM framework, we conclude that perceptual learning in this task involved learning how to better exclude external noise, reflecting retuning of the perceptual template. The data provide the first empirical demonstration of an isolable mechanism of perceptual learning. This learning completely transferred to a different visual scale in a second experiment.

Adult↗
PubMed2004

Slower implicit categorical learning in adult poor readers.

We investigated the relationship between reading and explicit and implicit categorical learning by comparing university students with poor reading to students with normal reading abilities on two categorical learning tasks. One categorical learning task involved sorting simple geometric shapes into two groups according to a unidimensional rule. The sorting rule was easily stated by the participants, consistent with explicit learning, and all participants attained criterion levels of performance. The second task involved the integration of features on different dimensions with a more complex rule that could not be described by participants, even though most could attain criterion levels of performance consistent with implicit learning. Poor readers performed as well as those without reading problems in explicit learning but not in implicit learning. Implicit learning was correlated with word reading, phonological decoding, and orthographic skill, independent of verbal ability. We consider the role of implicit learning in reading, and how a deficit could impair phonological and orthographic representation and processing.

Adult↗
PubMed2004

Temporal tuning characteristics of the perceptual template and endogenous cuing of spatial attention.

External noise presented in temporal contiguity with a target impairs perceptual performance, reflecting the temporal tuning of the perceptual template. Deriving the temporal characteristics of the perceptual template, however, requires an observer model that segregates the impact of non-linearities and intrinsic inefficiencies of the observer in order to account for the impact of external noise in various temporal configurations. In Experiment 1, we showed that the perceptual template model successfully accounts for temporal masking functions in foveal Gabor orientation identification masked by external noise with a wide range of temporal configurations, and estimates the temporal characteristics of the perceptual template. In Experiment 2, we extended the paradigm and the model to compare the temporal tuning characteristics of the perceptual template in central pre- and simultaneous cuing of endogenous spatial attention in a Gabor orientation identification task in visual periphery. We found that endogenous spatial attention excludes external noise by both sharpening the temporal window of the perceptual template and (mostly) reducing the impact of external noise uniformly across the entire temporal window.

Attention↗
PubMed2004

The spatial window of the perceptual template and endogenous attention.

One primary function of spatial attention is to exclude external noise [e.g., Psychol. Sci. 11(2) (2000) 139], especially in the region of the target stimulus [J. Vis. 2(4) (2000) 312]. What is not known is the spatial profile of external noise exclusion in the vicinity of the target and how this depends upon attention. The spatial region around an oriented Gabor target was segmented into four concentric rings (R1-R4). Psychometric functions were measured for orientation discrimination with external random Gaussian noise in all combinations of rings (e.g., R1 alone; R1+R2; etc.). Regions with larger impact on performance are weighted more heavily in the perceptual template. In an orientation discrimination task in periphery the effective noise regions aligned closely with the high contrast regions of the target Gabor, with attention reducing the effective noise across the spatial template. The combined effects of external noise regions were well-modeled by a (non-linear) perceptual template model (PTM) [Vis. Res. 38(9) (1998) 1183]. In another experiment in attended fovea, the results were similar to those in periphery, but exhibited additional ability to selectively weight clear spatial regions.

Attention↗
PubMed2004

Parallel processing in visual search asymmetry.

The difficulty of visual search may depend on assignment of the same visual elements as targets and distractors-search asymmetry. Easy C-in-O searches and difficult O-in-C searches are often associated with parallel and serial search, respectively. Here, the time course of visual search was measured for both tasks with speed-accuracy methods. The time courses of the 2 tasks were similar and independent of display size. New probabilistic parallel and serial search models and sophisticated-guessing variants made predictions about time course and accuracy of visual search. The probabilistic parallel model provided an excellent account of the data, but the serial model did not. Asymptotic search accuracies and display size effects were consistent with a signal-detection analysis, with lower variance encoding of Cs than Os. In the absence of eye movements, asymmetric visual search, long considered an example of serial deployment of covert attention, is qualitatively and quantitatively consistent with parallel search processes.

Cognition↗
PubMed2003

Generating high gray-level resolution monochrome displays with conventional computer graphics cards and color monitors.

Display systems based on conventional computer graphics cards are capable of generating images with about 8-bit luminance resolution. However, most vision experiments require more than 12 bits of luminance resolution. Pelli and Zhang [Spatial Vis. 10 (1997) 443] described a video attenuator for generating high luminance resolution displays on a monochrome monitor, or for driving just the green gun of a color monitor. Here we show how to achieve a white display by adding video amplifiers to duplicate the monochrome signal to drive all three guns of any color monitor. Because of the lack of the availability of high quality monochrome monitors, our method provides an inexpensive way to achieve high-resolution monochromatic displays using conventional, easy-to-get equipment. We describe the design principles, test results, and a few additional functionalities.

Algorithms↗
PubMed2003

Object attention revisited: identifying mechanisms and boundary conditions.

Multiple attributes of a visual array are often more efficiently processed when they are attributes of a single object than when they are attributes of different objects-a pattern reflecting the limitations of object attention. This study used psychophysical methods to evaluate the object attention limitations in the report of attributes (orientation and phase) computed early in visual analysis for spatially separated objects. These limitations had large effects on dual-object report thresholds when different judgments were required for the two objects (orientation for one object and phase for the other), but the effects were small or nonexistent when the same judgment was made about both objects. Judgment consistency reduced or eliminated the expression of object attention deficits. Thus, the deficits in dual-object report reflect both division of attention over objects and the calculation of independent reference or judgment operations. Dual-object deficits, when they occurred, were substantial in displays with external noise masks. Smaller effects were observed in clear displays, even when difficulty was equated by stimulus contrast. Thus, the primary consequence of object attention is the exclusion of external noise, or mask suppression, and enhancement of the stimulus in clear displays is a secondary consequence.

Attention↗
PubMed2002

Modification of sudden onset auditory ERP by involuntary attention to visual stimuli.

To investigate the cross-modal nature of the exogenous attention system, we studied how involuntary attention in the visual modality affects ERPs elicited by sudden onset of events in the auditory modality. Relatively loud auditory white noise bursts were presented to subjects with random and long inter-trial intervals. The noise bursts were either presented alone, or paired with a visual stimulus with a visual to auditory onset asynchrony of 120 ms. In a third condition, the visual stimuli were shown alone. All three conditions, auditory alone, visual alone, and paired visual/auditory, were randomly inter-mixed and presented with equal probabilities. Subjects were instructed to fixate on a point in front of them without task instructions concerning either the auditory or visual stimuli. ERPs were recorded from 28 scalp sites throughout every experimental session. Compared to ERPs in the auditory alone condition, pairing the auditory noise bursts with the visual stimulus reduced the amplitude of the auditory N100 component at Cz by 40% and the auditory P200/P300 component at Cz by 25%. No significant topographical change was observed in the scalp distributions of the N100 and P200/P300. Our results suggest that involuntary attention to visual stimuli suppresses early sensory (N100) as well as late cognitive (P200/P300) processing of sudden auditory events. The activation of the exogenous attention system by sudden auditory onset can be modified by involuntary visual attention in a cross-model, passive prepulse inhibition paradigm.

Acoustic Stimulation↗