Visuospatial working memory mediates inhibitory and facilitatory guidance in preview search.

Visual search is faster and more accurate when a subset of distractors is presented before the display containing the target. This "preview benefit" has been attributed to separate inhibitory and facilitatory guidance mechanisms during search. In the preview task the temporal cues thought to elicit inhibition and facilitation provide complementary sources of information about the likely location of the target. In this study, we use a Bayesian observer model to compare sensitivity when the temporal cues eliciting inhibition and facilitation produce complementary, and competing, sources of information. Observers searched for T-shaped targets among L-shaped distractors in 2 standard and 2 preview conditions. In the standard conditions, all the objects in the display appeared at the same time. In the preview conditions, the initial subset of distractors either stayed on the screen or disappeared before the onset of the search display, which contained the target when present. In the latter, the synchronous onset of old and new objects negates the predictive utility of stimulus-driven capture during search. The results indicate observers combine memory-driven inhibition and sensory-driven capture to reduce spatial uncertainty about the target's likely location during search. In the absence of spatially predictive onsets, memory-driven inhibition at old locations persists despite irrelevant sensory change at previewed locations. This result is consistent with a bias toward unattended objects during search via the active suppression of irrelevant capture at previously attended locations. (PsycINFO Database Record

Aging effects in cueing tasks as assessed by the ideal observer: peripheral cues.

Previous aging and cueing studies suggest that automatic orienting driven by peripheral cues is preserved with aging; however, inconsistencies can be found. One issue might be the use of response times (RT) to assess cueing effects (invalid RT--valid RT), which, in many cases, may not have clear quantitative predictions. We propose an ideal observer (IO) analysis of accuracy estimating participants' internal value of cue validity, or weight, which should equal the actual cue validity. The weight measures the use of information provided by the cue and is insensitive to variations in set size and difficulty, thus potentially providing advantages to RT. Older (n = 54) and younger (n = 58) participants performed a yes/no detection task of a two-dimensional (2-D) Gaussian (60 ms). Square peripheral precues (150 ms) indicated likely target locations (70% valid) across two or six locations (set sizes). For cueing effects, (valid--invalid hit rates), younger participants had set-size effects (larger cueing effects for set size 6), while older participants did not. The opposite pattern was found for weights (younger: no set-size effects, older: set-size effects) due to the IO predicting larger cueing effects for larger set sizes. Comparisons to the ideal weight (cue validity) suggested that older participants used the cue information effectively with set size 2 (as or more so than younger participants), but not with set size 6. These results suggest that attentional deficits from aging in peripheral cueing tasks may only arise as difficulty increases, such as larger set sizes.

Visual discrimination in noise: behavioural correlates of age-related cortical decline.

Neurophysiological evidence has consistently shown that, compared to younger animals, single neurons in aged mammalian visual cortex exhibit reduced selectivity to stimulus direction and orientation. It has been suggested that this may be due, in part, to increased internal noise in the aged visual system. This study measured contrast thresholds for judging the motion direction (left vs. right) and spatial orientation (vertical vs. horizontal) of sinusoidal gratings presented alone and with additive noise in young (20-29 years) and older (65-79 years) adults. Compared to their young counterparts, older adults demonstrated reduced sensitivity to direction and orientation when no noise was present. However, when gratings were presented in conjunction with additive noise, older adults required a greater increase in external noise to elicit a corresponding reduction in sensitivity. Subsequent analysis assessing equivalent noise and sampling efficiency attributed performance differences to an increase in equivalent noise with age.

A unified bayesian observer analysis for set size and cueing effects on perceptual decisions and saccades.

Visual search and cueing tasks have been employed extensively in attentional research, with each having a standard effect (visual search: set size effects, cueing: cue validity). Generally these effects have been treated with different (but often similar) attentional theories. The present study aims to consolidate cueing and set size effects within an ideal observer approach. Four observers performed a yes/no contrast discrimination of a gaussian signal in a task combining cueing with visual search. The signal appeared in half the trials, and effective set size (M, 2 to 8) was determined by one primary precue (having 50% validity in signal present trials) and M-1 secondary precues. There were two stimulus durations: 1 second (eye movements allowed), and the first-saccade latency (in the 1 second duration condition) minus 80 milliseconds. Simulations found that an ideal observer for the perceptual yes/no decisions and the first saccadic localization decisions predicted both set size and cueing effects with a single weighting mechanism, providing a unifying account. For the human observer results, a modified ideal observer (with performance matched to human performance) fit the yes/no perceptual decisions well. For the first saccadic decisions, there was evidence of use of the primary cue, but the modified ideal observer was not a good fit, indicating a suboptimal use of the cue. We discuss possible underlying assumptions about the task that might explain the apparent suboptimal nature of saccadic decisions and the overall utility of the ideal observer for cueing and visual search studies in visual attention and saccades.

Uncued and cued dynamics measured by response classification.

Attention as a serial contiguous spotlight has met challenges from parallel-noisy models and evidence that attention can split across multiple locations. To assess this question, this study compared the dynamics at cued and uncued locations during a cueing task through response classification. Four observers performed a yes/no contrast discrimination of Gabors appearing at two locations, with simultaneous cues appearing at one location with 80% validity. Stimuli were presented for 272 ms in 'contrast noise'; the peak contrasts of the Gabors varied randomly across 12 intervals of the stimulus duration. Response classification yielded a 'classification number' for each location (cued and uncued) and interval, giving two classification number functions (cued and uncued) across the stimulus duration. Serial models predict delays in the uncued functions, whereas parallel models do not. No evidence for delays at uncued locations was found, with no significant differences for the amplitude-matched cued and uncued functions, or for the functions' peak times after smoothing with third-degree polynomials. Also, the relative integrals between the cued and uncued functions were fit well to a parallel-noisy weighted likelihood model assuming a linear summation of responses across intervals. Thus, a parallel description of attention seemed to best explain the results in this study.

Contrast and stimulus information effects in rapid learning of a visual task.

We have previously described a psychophysical paradigm for investigating rapid learning of relevant visual information in detection tasks (M. P. Eckstein, C. K. Abbey, B. T. Pham, & S. S. Shimozaki, 2004). This paradigm uses blocked trials with a set of possible target profiles, and it has demonstrated learning effects after a single trial. When targets are masked by Gaussian luminance noise, there exists a Bayesian ideal observer that also exhibits learning effects over the trials within a block. In this work, we investigate the effect of target contrast and the effect of the information to be learned in the target profile set. Absolute efficiency tracks target contrast closely and ranges from approximately 10% to 25% in these experiments. To disambiguate learning from other effects contributing to absolute efficiency, we define a measure of learning efficiency that measures the observed improvement over a block of trials against the total improvement expected in the ideal observer. We find significant positive trends in learning efficiency both over contrast and the within-block trial number. We find that a two-feature profile set containing orientation and polarity differences leads to a greater within-block gain in performance than a one-feature profile set that contains only orientation differences. However, this apparent difference disappears when efficiency is compared. Lastly, we show that the disparity between task performance and accumulated knowledge of the target profile can be largely explained by a model that only allows learning to occur in trials the observer performs correctly.

The temporal dynamics of selective attention of the visual periphery as measured by classification images.

This study estimates the temporal dynamics of selective attention with classification images, a technique assessing observer information use by tracking how responses are correlated with external noise added to the stimulus. Three observers performed a yes/no discrimination of a Gaussian signal that could appear at one of eight locations (eccentricity-4.6 degrees ). During the stimulus duration (300 ms), a peripheral cue indicated the potential signal location with 100% validity, and stimuli were presented in frames (37.5 ms/frame) of independently sampled Gaussian luminance image noise. Stimuli were presented either with or without a succeeding masking display (100 ms) of high-contrast image noise, with mask presence having little effect. The results from the classification images suggest that observers were able to use information at the cued location selectively (relative to the uncued locations), starting within the first (0-37.5 ms) or second (37.5-75 ms) frame. This suggests a selective attention effect earlier than those found in previous behavioral and event-related potential (ERP) studies, which generally have estimated the latency for selective attention effects to be 75-100 ms. We present a deconvolution method using the known temporal impulse response of early vision that indicates how the classification image results might relate to previous behavioral and ERP results. Applying the model to the classification images suggests that accounting for the known temporal dynamics could explain at least part of the difference in results between classification images and the previous studies.

Similar neural representations of the target for saccades and perception during search.

Are the body's actions and the mind's perceptions the result of shared neural processing, or are they performed largely independently? The brain has two major processing streams, and some have proposed that this division segregates visual processing for action and perception. The ventral pathway is claimed to support conscious experience (perception), whereas the dorsal pathway is claimed to support the control of movement (action). Others have argued that perception and action share much of their visual processing within the primate cortex. During visual search, the brain performs a sophisticated deployment of eye movements (saccadic actions) to gather information to subserve perceptual judgments. The relationship between the neural mechanisms mediating perception and action in visual search remains unexplored. Here, we investigate the visual representation of target information in the human brain, both for perceptual decisions and for saccadic actions during visual search. We use classification image analysis, a form of reverse correlation, to estimate the behavioral receptive fields of the visual mechanisms responsible for saccadic and perceptual responses during the same visual search task. Results show that the behavioral receptive fields mediating the perceptual decisions are indistinguishable from those driving the oculomotor decisions, suggesting that similar neural mechanisms are responsible for both perception and oculomotor action during search. Diverging target representations would result in an inefficient coupling between eye movement planning and perceptual judgments. Thus, a common target representation would be more optimal and might be expected to have evolved through natural selection in the neural systems responsible for visual search.

Attentional cues in real scenes, saccadic targeting, and Bayesian priors.

Performance finding a target improves when artificial cues direct covert attention to the target's probable location or locations, but how do predictive cues help observers search for objects in real scenes? Controlling for target detectability and retinal eccentricity, we recorded observers' first saccades during search for objects that appeared in expected and unexpected locations within real scenes. As has been found with synthetic images and cues, accuracy of first saccades was significantly higher when the target appeared at an expected location rather than an unexpected location. Observers' saccades with target-absent images make it possible to distinguish two mechanisms that might mediate this effect: limited attentional resources versus differential weighting of information (Bayesian priors). Endpoints of first saccades in target-absent images were significantly closer to the expected than the unexpected locations, a result consistent with the differential-weighting model and inconsistent with limited resources being the sole mechanism underlying the effect.

Spatial profiles of local and nonlocal effects upon contrast detection/discrimination from classification images.

We used classification images (A. J. Ahumada, Jr., & J. Lovell, 1971) to estimate the perceptual filter in a task designed to assess both local and nonlocal effects upon contrast detection/discrimination. Three observers performed a yes/no detection or discrimination task of a uniform circular decrement (radius = 0.68 deg) near threshold presented for 100 to 400 ms. Stimuli were presented in ring image noise that either covered the signal and an annular surrounding area (out to 1.36 deg), or only the surrounding annular area (out to 1.36 deg). Both the signal and the annular surround appeared on a uniform background. With ring noise over both the signal and surround, the amplitudes of the classification images in the signal area decreased as radial distance increased from the signal/surround border, and no effect of the surround was found. With ring noise only in the surround, classification images indicated noncontiguous effects at both the signal/surround border (local) and the surround/background border (nonlocal). The spatial extents of the nonlocal effects (< 0.07 deg) were smaller than local effects (0.25 deg), whereas the peak amplitudes of the local and nonlocal effects were comparable. These results suggest that the nonlocal effects were smaller than the local effects, and that the smaller effects would be due to smaller effective areas, as opposed to smaller amplitudes over the same area. Little or no change was found in the classification images across stimulus duration, suggesting that both the local and nonlocal processes found in this study were completed within 100 ms.

The footprints of visual attention during search with 100% valid and 100% invalid cues.

Human performance during visual search typically improves when spatial cues indicate the possible target locations. In many instances, the performance improvement is quantitatively predicted by a Bayesian or quasi-Bayesian observer in which visual attention simply selects the information at the cued locations without changing the quality of processing or sensitivity and ignores the information at the uncued locations. Aside from the general good agreement between the effect of the cue on model and human performance, there has been little independent confirmation that humans are effectively selecting the relevant information. In this study, we used the classification image technique to assess the effectiveness of spatial cues in the attentional selection of relevant locations and suppression of irrelevant locations indicated by spatial cues. Observers searched for a bright target among dimmer distractors that might appear (with 50% probability) in one of eight locations in visual white noise. The possible target location was indicated using a 100% valid box cue or seven 100% invalid box cues in which the only potential target locations was uncued. For both conditions, we found statistically significant perceptual templates shaped as differences of Gaussians at the relevant locations with no perceptual templates at the irrelevant locations. We did not find statistical significant differences between the shapes of the inferred perceptual templates for the 100% valid and 100% invalid cues conditions. The results confirm the idea that during search visual attention allows the observer to effectively select relevant information and ignore irrelevant information. The results for the 100% invalid cues condition suggests that the selection process is not drawn automatically to the cue but can be under the observers' voluntary control.

Perceptual learning through optimization of attentional weighting: human versus optimal Bayesian learner.

Human performance in visual detection, discrimination, identification, and search tasks typically improves with practice. Psychophysical studies suggest that perceptual learning is mediated by an enhancement in the coding of the signal, and physiological studies suggest that it might be related to the plasticity in the weighting or selection of sensory units coding task relevant information (learning through attention optimization). We propose an experimental paradigm (optimal perceptual learning paradigm) to systematically study the dynamics of perceptual learning in humans by allowing comparisons to that of an optimal Bayesian algorithm and a number of suboptimal learning models. We measured improvement in human localization (eight-alternative forced-choice with feedback) performance of a target randomly sampled from four elongated Gaussian targets with different orientations and polarities and kept as a target for a block of four trials. The results suggest that the human perceptual learning can occur within a lapse of four trials (<1 min) but that human learning is slower and incomplete with respect to the optimal algorithm (23.3% reduction in human efficiency from the 1st-to-4th learning trials). The greatest improvement in human performance, occurring from the 1st-to-2nd learning trial, was also present in the optimal observer, and, thus reflects a property inherent to the visual task and not a property particular to the human perceptual learning mechanism. One notable source of human inefficiency is that, unlike the ideal observer, human learning relies more heavily on previous decisions than on the provided feedback, resulting in no human learning on trials following a previous incorrect localization decision. Finally, the proposed theory and paradigm provide a flexible framework for future studies to evaluate the optimality of human learning of other visual cues and/or sensory modalities.

An ideal observer with channels versus feature-independent processing of spatial frequency and orientation in visual search performance.

An influential assumption for the front end of models in vision, visual search, and object recognition is an analysis of independent features that correspond to basic image properties, such as motion, shape, and color. Empirically, one common test of independent features (a cue-summation study) measures performance with increasing available cues or features, with improving performance leading to conclusions of summation across independent features. In a study by Shimozaki et al. [J. Vision 2, 354-370 (2002)], both ideal and human observers showed no summation with large stimulus differences, in contrast to independent-feature models and suggesting that stimulus information (as assessed by an ideal observer) might affect cue-summation studies. Extending the previous summation study, observers performed a visual search of four Gabors differing in only orientation, only spatial frequency, or both orientation and spatial frequency, across a range of target-distractor differences. An ideal observer underpredicted human summation for small differences, whereas the independent-orientation and spatial-frequency feature models overpredicted human summation for large differences. An ideal observer with channels jointly tuned to spatial frequency and orientation predicted human performance across both small and large target-distractor differences.

Comparison of two weighted integration models for the cueing task: linear and likelihood.

In a task in which the observer must detect a signal at two locations, presenting a precue that predicts the location of a signal leads to improved performance with a valid cue (signal location matches the cue), compared to an invalid cue (signal location does not match the cue). The cue validity effect has often been explained with a limited capacity attentional mechanism improving the perceptual quality at the cued location. Alternatively, the cueing effect can also be explained by unlimited capacity models that assume a weighted combination of noisy responses across the two locations. We compare two weighted integration models, a linear model and a sum of weighted likelihoods model based on a Bayesian observer. While qualitatively these models are similar, quantitatively they predict different cue validity effects as the signal-to-noise ratios (SNR) increase. To test these models, 3 observers performed in a cued discrimination task of Gaussian targets with an 80% valid precue across a broad range of SNR's. Analysis of a limited capacity attentional switching model was also included and rejected. The sum of weighted likelihoods model best described the psychophysical results, suggesting that human observers approximate a weighted combination of likelihoods, and not a weighted linear combination.

Effect of parietal lobe lesions on saccade targeting and spatial memory in a naturalistic visual search task.

The eye movements of two patients with parietal lobe lesions and four normal observers were measured while they performed a visual search task with naturalistic objects. Patients were slower to perform the task than the normal observers, and the patients had more fixations per trial, longer latencies for the first saccade during the visual search, and less accurate first and second saccades to the target locations during the visual search. The increases in response times for the patients compared to the normal observers were best predicted by increases in the number of fixations. In order to investigate the effects of spatial memory on search performance, in some trials observers saw a preview of the search display. The patients appeared to have difficulty using previously viewed information, unlike normal observers who benefit from the preview. This suggests a spatial memory deficit. The patients' deficits are consistent with the hypothesis that the parietal cortex has a role in the selection of targets for saccades, in memory for target location.

The footprints of visual attention in the Posner cueing paradigm revealed by classification images.

In the Posner cueing paradigm, observers' performance in detecting a target is typically better in trials in which the target is present at the cued location than in trials in which the target appears at the uncued location. This effect can be explained in terms of a Bayesian observer where visual attention simply weights the information differently at the cued (attended) and uncued (unattended) locations without a change in the quality of processing at each location. Alternatively, it could also be explained in terms of visual attention changing the shape of the perceptual filter at the cued location. In this study, we use the classification image technique to compare the human perceptual filters at the cued and uncued locations in a contrast discrimination task. We did not find statistically significant differences between the shapes of the inferred perceptual filters across the two locations, nor did the observed differences account for the measured cueing effects in human observers. Instead, we found a difference in the magnitude of the classification images, supporting the idea that visual attention changes the weighting of information at the cued and uncued location, but does not change the quality of processing at each individual location.

Stimulus information contaminates summation tests of independent neural representations of features.

Many models of visual processing assume that visual information is analyzed into separable and independent neural codes, or features. A common psychophysical test of independent features is known as a summation study, which measures performance in a detection, discrimination, or visual search task as the number of proposed features increases. Improvement in human performance with increasing number of available features is typically attributed to the summation, or combination, of information across independent neural coding of the features. In many instances, however, increasing the number of available features also increases the stimulus information in the task, as assessed by an optimal observer that does not include the independent neural codes. In a visual search task with spatial frequency and orientation as the component features, a particular set of stimuli were chosen so that all searches had equivalent stimulus information, regardless of the number of features. In this case, human performance did not improve with increasing number of features, implying that the improvement observed with additional features may be due to stimulus information and not the combination across independent features.