Search PubMed⌕ Search

Biomedical subjects

Zhong-Lin Lu

Publications and source records attributed to Zhong-Lin Lu.

12 recordsLinked to original sources
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

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↗
PubMed2002

Spatial attention excludes external noise at the target location.

To investigate the nature of external noise exclusion, we compared central spatial precuing effects in 16 conditions that varied the amount of external noise, the number of signal stimuli, the number of locations masked by external noise, and the number and style of frames surrounding potential target locations. In the absence of external noise, precuing produced only marginal performance improvements in a small number of display conditions. In the presence of high external noise, precuing improved task performance in all the display conditions. The magnitude of these spatial attention effects, as gauged by contrast threshold reduction, is nearly constant across all the display conditions. This suggests that spatial attention mostly excludes external noise at the target location; the presence of external noise and/or signal stimuli in non-target regions has little effect on spatial performance when location uncertainty is eliminated by report cues. However, the presence of other potential locations for the target is critical, because if target location is known in advance, attention can be focused on that location with or without a cue.

Attention↗
PubMed1998

3D Extended Supramolecular Structures via H-Bonded Linkages of 2D Sheetlike or 1D Zigzag Coordination Polymers.

Two 3D supramolecular complexes [Cu(2)(trans-oxpn)(phth)](n)().2nH(2)O, 1, and [Cu(2)(trans-oxpn)(&mgr;-OH)(&mgr;-H(2)O)](n)()Br(n)().2nH(2)O, 2 [H(2)oxpn = N,N'-bis(3-aminopropyl)oxamide, phth = phthalate], were synthesized and characterized by single-crystal X-ray crystallography, thermal analyses, and magnetic measurements. 1 crystallizes in the monoclinic space group P2(1)/c with a = 8.578(2) Å, b = 15.596(2) Å, c = 15.657(2) Å, beta = 101.29(2) degrees, Z = 4; 2 crystallizes in the trigonal space group R&thremacr;c with a = 15.266(2) Å, b = 15.266(2) Å, c = 34.953(5) Å, and Z= 18. The neutral 2D sheetlike coordination polymer [Cu(2)(trans-oxpn)(phth)](n)() in 1 contains alternate trans-oxamidate and phthalate bridges and then is joined together by H(2)O molecules via the hydrogen bond linkage Cu(oxpn).H(2)O.H(2)O.(oxpn)Cu to produce a 3D supramolecular structure. The cationic chains [Cu(2)(trans-oxpn)(&mgr;-OH)(&mgr;-H(2)O)](n)()(n)()(+) in 2 contain alternate trans-oxamidate, &mgr;-OH, and &mgr;-H(2)O bridges and are arranged in three directions, intersecting with one another to yield an extended network via the interchain hydrogen bonds formed by metal-coordinated OH(-) with the linkages Cu(oxpn)-OH.OH-Cu(oxpn). Magnetic calculations showed the presence of antiferromagnetic exchange interactions for both complexes. The 2D magnetic coupling system of 1 was simplified into interacting dimers Cu(trans-oxpn)Cu, with the coupling via phth as an interdimer interaction and the data were interpreted with a modified Bleaney-Bowers equation leading to J = -456.7 cm(-)(1) and theta = 22.0 K. 2 was analyzed qualitatively by an alternating chain model taking into account the interchain interaction in a molecular model with J(1) = -309.6 cm(-)(1), J(2) = -63.5 cm(-)(1), and theta = -12.9 K, where J(1) and J(2) denote the coupling constants of the bridges oxamidate and &mgr;-OH, respectively.

Journal Article↗