Identify mechanisms of amblyopia in Gabor orientation identification with external noise

In this study, we applied the external noise method and the PTM model to identify mechanisms underlying performance deficits in amblyopia. Amblyopic and normal observers performed a Gabor orientation identification task in fovea. White external noise was added to the Gabor stimuli. Threshold versus external noise contrast (TvC) functions were measured at two performance criterion levels. For a subset of observers, we also manipulated the center spatial frequency of the Gabor. We found that two independent factors contributed to amblyopic deficits: (1) increased additive internal noise, and (2) deficient perceptual templates. Whereas increased additive noise underlay performance deficits in all spatial frequencies, the degree of perceptual template deterioration increased with the center spatial frequency of the Gabor.     

Chromatic detection and discrimination analyzed by a Bayesian classifier

Detection and threshold-level discrimination of Gabor patches were studied under the conditions of noise masking, in an attempt to isolate 'higher-order' or nonclassical color mechanisms. Detection contours in the equiluminant plane of cone contrast space were measured by varying test chromaticity in the presence of chromatic masking noise. Three equiluminant noise directions were used, in separate experiments. In the discrimination experiment, observers had to discriminate between pairs of stimuli that were fixed at their masked threshold contrasts. A Bayesian color classifier model was used to analyze the discrimination data, with no free parameters. There was no evidence of nonclassical color mechanisms in either the detection or discrimination data.     

Transfer of contrast sensitivity in linear visual networks.

Contrast sensitivity is a useful measure of the ability of an observer to distinguish contrast signals from noise. Although usually applied to human observers, contrast sensitivity can also be defined operationally for individual visual neurons. In a model linear neuron consisting of a filter and noise source, this operational measure is a function of filter gain, noise power spectrum, signal duration, and a performance criterion. This definition allows one to relate the sensitivities of linear neurons at different levels in the visual pathway. Mathematical formulae describing these relationships are derived, and the general model is applied to the specific problem of relating the sensitivities of parvocellular LGN neurons and cortical simple cells in the primate.     

Attentional effects on contrast detection in the presence of surround masks

We studied how attention affects contrast detection performance when the target is surrounded by mask elements. In each display quadrant we presented a hexagon of six vertical Gabor patches (the 'surround'). Only one of the hexagons contained a central Gabor patch (the 'target') and the task was to report that quadrant (spatial four-alternative-forced choice). Attention was manipulated by means of a double-task paradigm: in one condition observers had to perform concurrently a central letter-discrimination task, and the contrast-detection task was then only poorly attended, while attention was fully available in the other condition. We find that under poorly attended conditions targets can be detected only when the target contrast exceeds the surround contrast (contrast popout) or when the target orientation differs from the surround orientation by more than 10-15 degrees (orientation popout). When the target orientation is similar to the surround orientation, attention can reduce the contrast detection thresholds in some cases more than four-fold, demonstrating a very strong attentional effect.     

Suppressive effect of sustained low-contrast adaptation followed by transient high-contrast on peripheral target detection

We observed that presenting a low-contrast Gabor patch (2 cpd, 5 degrees eccentricity, contrast=4%) for 8 s and then flashing a 20-30 ms high-contrast patch over it could elicit the perceptual disappearance of a subsequent low-contrast stimulus, whereas neither low-contrast adaptation nor high-contrast flash alone had any considerable effect (p<0.00001). In other experiments we found (a) suppressive components are phase-insensitive, (b) the effect transfers between eyes, (c) suppression is selective for orientation, and (d) the induction by the transient high-contrast Gabor patch could be transferred to another previously adapted location up to a few degrees. Results indicate synergy between contrast and adaptation through a non-linear interaction between rapid gain adjustment to transient change and adaptation to sustained spatial patterns. Findings are compatible with non-local mechanisms presumably at the cortical level.     

Involuntary attention with uncertainty: peripheral cues improve perception of masked letters, but may impair perception of low-contrast letters

Improvements of perceptual performance following the presentation of peripheral cues have been ascribed to accelerated accrual of information, enhanced contrast perception, and decision bias. We investigated effects of peripheral cues on the perception of Gabor and letter stimuli. Non-predictive, peripheral cues improved perceptual accuracy when the stimuli were masked. In contrast, peripheral cues degraded perception of low-contrast letters and did not affect the perception of low-contrast Gabors. The results suggest that involuntary attention accelerates accrual of information but are not entirely consistent with the idea that involuntary attention enhances subjective contrast. Rather, peripheral cues may cause crowding with single letter targets of low contrast. Further, we investigated the effect of the amount of uncertainty on involuntary attention. Cueing effects were (initially) larger when there were more possible target locations. In addition, cueing effects were larger when error feedback was absent and observers had no knowledge of results. Despite these strategic factors, location uncertainty was not sufficient to produce cueing effects, showing that location uncertainty paired with non-predictive cues reveals perceptual and not (only) decisional processes.     

Factors limiting suprathreshold vision measured by a flash-sound simultaneity paradigm

Internal noise and sampling efficiency are the main factors which limit visual performance. In a previous study [Vis. Res. 43 (2003) 1103] we compared the variance of human reaction time to that of an ideal observer and found that the sampling efficiency to suprathreshold stimuli was much lower than that obtained in detection experiments. In order to bypass the effects of the motor system on visual performance, we used a flash-sound simultaneity paradigm. We found that the sampling efficiency for 0.4- and 4-c/deg near-threshold Gabor patches is higher only by a factor of 2.5 than that to above-threshold patterns. The signal-dependent multiplicative internal noise was similar to the additive internal noise at lower signal contrast levels and exceeded it at higher signal contrast levels. The results show that real observers' performance for detecting suprathreshold stimuli can be accounted for by a model taking into account the non-linear visual-signal transduction and multiplicative components of the internal noise induced by the signal and external noise. In addition, this model assumes that performance depends on the response duration, rather than signal duration. The results imply that the multiplicative internal noise induced by high contrast visual signals determines performance for suprathreshold visual detection.     

Spatial covert attention increases contrast sensitivity across the CSF: support for signal enhancement

This study is the first to report the benefits of spatial covert attention on contrast sensitivity in a wide range of spatial frequencies when a target alone was presented in the absence of a local post-mask. We used a peripheral precue (a small circle indicating the target location) to explore the effects of covert spatial attention on contrast sensitivity as assessed by orientation discrimination (Experiments 1-4), detection (Experiments 2 and 3) and localization (Experiment 3) tasks. In all four experiments the target (a Gabor patch ranging in spatial frequency from 0.5 to 10 cpd) was presented alone in one of eight possible locations equidistant from fixation. Contrast sensitivity was consistently higher for peripherally- than for neutrally-cued trials, even though we eliminated variables (distracters, global masks, local masks, and location uncertainty) that are known to contribute to an external noise reduction explanation of attention. When observers were presented with vertical and horizontal Gabor patches an external noise reduction signal detection model accounted for the cueing benefit in a discrimination task (Experiment 1). However, such a model could not account for this benefit when location uncertainty was reduced, either by: (a) Increasing overall performance level (Experiment 2); (b) increasing stimulus contrast to enable fine discriminations of slightly tilted suprathreshold stimuli (Experiment 3); and (c) presenting a local post-mask (Experiment 4). Given that attentional benefits occurred under conditions that exclude all variables predicted by the external noise reduction model, these results support the signal enhancement model of attention.     

Position jitter and undersampling in pattern perception.

The present paper addresses whether topographical jitter or undersampling might limit pattern perception in foveal, peripheral and strabismic amblyopic vision. In the first experiment, we measured contrast thresholds for detecting and identifying the orientation (up, down, left, right) of E-like patterns comprised of Gabor samples. We found that detection and identification thresholds were both degraded in peripheral and amblyopic vision; however, the orientation identification/detection threshold ratio was approximately the same in foveal, peripheral and amblyopic vision. This result is somewhat surprising, because we anticipated that a high degree of uncalibrated topographical jitter in peripheral and amblyopic vision would have affected orientation identification to a greater extent than detection. In the second experiment, we investigated the tolerance of human and model observers to perturbation of the positions of the samples defining the pattern when its contrast was suprathreshold, by measuring a 'jitter threshold' (the amount of jitter required to reduce performance from near perfect to 62.5% correct). The results and modeling of our jitter experiments suggest that pattern identification is highly robust to positional jitter. The positional tolerance of foveal, peripheral and amblyopic vision is equal to about half the separation of the features and the close similarity between the three visual systems argues against extreme topographical jitter. The effects of jitter on human performance are consistent with the predictions of a 'template' model. In the third experiment we determined what fraction of the 17 Gabor samples are needed to reliably identify the orientation of the E-patterns by measuring a 'sample threshold' (the proportion of samples required for 62.5% correct performance). In foveal vision, human observers are highly efficient requiring only about half the samples for reliable pattern identification. Relative to an ideal observer model, humans perform this task with 85% efficiency. In contrast, in both peripheral vision and strabismic amblyopia more samples are required. The increased number of features required in peripheral vision and strabismic amblyopia suggests that in these visual systems, the stimulus is underrepresented at the stage of feature integration.     

Collinear context (and learning) change the profile of the perceptual filter

The effect of collinear context on the filter mediating the detection of a Gabor stimulus was investigated by using the classification image method. Classification images were estimated for a 1.5 cpd horizontal Gabor target and the same target flanked by two collinear Gabors horizontally 1.7 degrees displaced from the target. The target was masked by a low-contrast white-noise mask. Obtained classification images were fitted by Gabor functions. The results show that collinear flankers increase the length of the classification image profiles along the collinear axis. At the same time, modest facilitory effects were observed in most subjects. The specificity and the amount of context-induced elongation in the classification images makes it hard to be explained by uncertainty reduction alone. In previous studies, collinear facilitation has been reported to abolish due to perceptual learning. We report a possibly related phenomenon: classification image data was re-analyzed in two parts consisting of the early and the late trials. In the latter trials, differences between the classification images in flankers and no-flankers condition are no longer significant.     

Learning letter identification in peripheral vision

Performance for a variety of visual tasks improves with practice. The purpose of this study was to determine the nature of the processes underlying perceptual learning of identifying letters in peripheral vision. To do so, we tracked changes in contrast thresholds for identifying single letters presented at 10 degrees in the inferior visual field, over a period of six consecutive days. The letters (26 lowercase Times-Roman letters, subtending 1.7 degrees) were embedded within static two-dimensional Gaussian luminance noise, with rms contrast ranging from 0% (no noise) to 20%. We also measured the observers' response consistency using a double-pass method on days 1, 3 and 6, by testing two additional blocks on each of these days at luminance noise of 3% and 20%. These additional blocks were the exact replicates of the corresponding block at the same noise contrast that was tested on the same day. We analyzed our results using both the linear amplifier model (LAM) and the perceptual template model (PTM). Our results showed that following six days of training, the overall reduction (improvement across all noise levels) in contrast threshold for our seven observers averaged 21.6% (range: 17.2-31%). Despite fundamental differences between LAM and PTM, both models show that learning leads to an improvement of the perceptual template (filter) such that the template is more capable of extracting the crucial information from the signal. Results from both the PTM analysis and the double-pass experiment imply that the stimulus-dependent component of the internal noise does not change with learning.     

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.     

Global shape discrimination at reduced contrast in enucleated observers

Previous research has shown that observers with early unilateral enucleation have selectively better sensitivity to luminance contrast than monocular viewing controls [González et al., 2002; Vision Research 36 (1) (1996) 175; Vision Research 36 (1996) 3011; Vision Research 37 (17) (1997) 2465]. We asked whether unilateral enucleation specifically enhances all levels of luminance processing. Enucleated observers, as well as binocular and monocular viewing controls, detected global shape in radial frequency (RF) patterns [Vision Research 38 (1998) 2555] at low contrast. Control observers were tested in two monocular conditions in which the stimulus was presented to one eye, while the fellow eye: (1) viewed a luminance-matched grey field or (2) was covered by a dark eye patch. Sensitivity to low-contrast global shape was equivalent in enucleated observers and binocular controls. More importantly, enucleated observers showed superior performance to that of controls in either monocular condition. At low contrast, the dichoptic control group was more sensitive than controls wearing an eye patch, which suggests that dichoptic viewing is a superior method of testing when comparing monocular control performance to that of monocularly deprived populations. The previously reported enhanced sensitivity to luminance-defined form in early enucleated observers also occurs for low-contrast global shape discrimination.     

Synchronous sounds enhance visual sensitivity without reducing target uncertainty

We examined the crossmodal effect of the presentation of a simultaneous sound on visual detection and discrimination sensitivity using the equivalent noise paradigm (Dosher and Lu, 1998). In each trial, a tilted Gabor patch was presented in either the first or second of two intervals embedded in dynamic 2D white noise with one of seven possible contrast levels. The results revealed that the sensitivity of participants' visual detection and discrimination performance were both enhanced by the presentation of a simultaneous sound, though only close to the noise level at which participants' target contrast thresholds started to increase with the increasing noise contrast. A further analysis of the psychometric function at this noise level revealed that the increase in sensitivity could not be explained by the reduction of participants' uncertainty regarding the onset time of the visual target. We suggest that this crossmodal facilitatory effect may be accounted for by perceptual enhancement elicited by a simultaneously-presented sound, and that the crossmodal facilitation was easier to observe when the visual system encountered a level of noise that happened to be close to the level of internal noise embedded within the system.     

The functional role of oriented spatial filters in the perception of mirror symmetry--psychophysics and modeling

We investigated human sensitivity to vertical mirror symmetry in noise patterns filtered for narrow bands of variable orientations. Sensitivity is defined here as the amount of spatial phase randomization corresponding to 75% correct performance in a 2AFC detection task. In Experiment 1, sensitivity was found to be high for tests patterns of all orientations except those parallel to the axis of symmetry. This implies that corresponding mirror-orientations (e.g. -45 and +45 degrees ) are combined prior to symmetry detection. In Experiment 2, observers detected symmetry in tests of variable orientation in the presence of either non-symmetric or symmetric masks filtered for orientations either parallel or perpendicular to the axis. Observers were found to be primarily affected by masks of the same orientation as the test, thus suggesting that symmetry is computed separately in distinct mirror-orientation channels. In Experiment 3, observers detected a symmetric test of variable height and width embedded in random noise. Data revealed that mirror symmetry is computed over a spatial integration region (IR) that remains approximately constant in area but whose height-to-width aspect ratio changes from 20:1 to 2:1 as orientation is varied from parallel to perpendicular to the axis. We compare human data against that of an ideal observer to identify key factors that limit visual performance and discuss the implications for the functional architecture of symmetry perception. We also propose a multi-channel model of symmetry detection that combines the output of oriented spatial filters in a simple and physiologically plausible manner. Particular emphasis is placed on the notion that changes in the shape of the IR with orientation compensate for changes in information density and partially equate performance across orientations.     

Classification images for detection, contrast discrimination, and identification tasks with a common ideal observer

We consider three simple forced-choice visual tasks--detection, contrast discrimination, and identification--in Gaussian white noise. The three tasks are designed so that the difference signal in all three cases is the same difference-of-Gaussians (DOG) profile. The distribution of the image noise implies that the ideal observer uses the same DOG filter to perform all three tasks. But do human observers also use the same visual strategy to perform these tasks? We use classification image analysis to evaluate the visual strategies of human observers. We find significantly different subject classification images across the three tasks. The domain of greatest variability appears to be low spatial frequencies [<5 cycles per degree (cpd)]. In this range, we find frequency enhancement in the detection task, and frequency suppression and reversal in the contrast discrimination task. In the identification task, subject classification images agree reasonably well with the ideal observer filter. We evaluate the effect of nonlinear transducers and intrinsic spatial uncertainty to explain divergence from the ideal observer found in detection and contrast discrimination tasks.     

Adaptation and gain pool summation: alternative models and masking data

Foley [J. Opt. Soc. Am. A 11 (1994) 1710] has proposed an influential psychophysical model of masking in which mask components in a contrast gain pool are raised to an exponent before summation and divisive inhibition. We tested this summation rule in experiments in which contrast detection thresholds were measured for a vertical 1 c/deg (or 2 c/deg) sine-wave component in the presence of a 3 c/deg (or 6 c/deg) mask that had either a single component oriented at -45 degrees or a pair of components oriented at +/-45 degrees. Contrary to the predictions of Foley's model 3, we found that for masks of moderate contrast and above, threshold elevation was predicted by linear summation of the mask components in the inhibitory stage of the contrast gain pool. We built this feature into two new models, referred to as the early adaptation model and the hybrid model. In the early adaptation model, contrast adaptation controls a threshold-like nonlinearity on the output of otherwise linear pathways that provide the excitatory and inhibitory inputs to a gain control stage. The hybrid model involves nonlinear and nonadaptable routes to excitatory and inhibitory stages as well as an adaptable linear route. With only six free parameters, both models provide excellent fits to the masking and adaptation data of Foley and Chen [Vision Res. 37 (1997) 2779] but unlike Foley and Chen's model, are able to do so with only one adaptation parameter. However, only the hybrid model is able to capture the features of Foley's (1994) pedestal plus orthogonal fixed mask data. We conclude that (1) linear summation of inhibitory components is a feature of contrast masking, and (2) that the main aftereffect of spatial adaptation on contrast increment thresholds can be assigned to a single site.     

Adaptation to sine-wave gratings selectively reduces the contrast gain of the adapted stimuli

Adapting to sinusoidal gratings selectively reduces contrast sensitivity to subsequent test stimuli. To investigate the perceptual processes underlying selective adaptation, we developed an external noise plus adaptation paradigm and a theoretical framework based on a noisy observer model (the contrast-gain-control Perceptual Template Model [cgcPTM]). After adapting to a 45 deg, 2-Hz counter-flickering sine grating of 0.8 contrast, observers performed two-interval forced-choice detection of Gabors of matched spatial frequency, tilted at either 45 or 135 deg and embedded in one of six levels of white external noise (Experiment 1) or embedded in orientation band-pass-filtered external noise (Experiment 2). On the basis of the cgcPTM, we found that adaptation selectively reduced the contrast gain of the perceptual template at the adapted spatial frequency and orientation without altering either pre- or post-gain-control (additive and multiplicative) noises or changing transducer nonlinearity. Modeled as notches on the perceptual templates, the estimated full orientation bandwidth of adaptation at half height was about 8.3 deg.     

Dynamics of attentional deployment during saccadic programming

The dynamics of attentional deployment before saccade execution was studied with a dual-task paradigm. Observers made a horizontal saccade whose direction was indicated by a symbolic precue and had to discriminate the orientation of a Gabor patch displayed at different delays after the precue (but before saccade onset). The patch location relative to the saccadic target was indicated to observers before each block. Therefore, on each trial, observers were informed simultaneously about the respective absolute locations of the saccadic and perceptual targets. The main result is that orientational acuity improved over a period of 150-200 ms after the precue onset at the saccadic target location, where overall performance is best, and at distant locations. This effect is due to attentional factors rather than to an alerting effect. It is also dependent on the efficiency of the temporal masks displayed before and after the Gabor patches.