Stanley A. Klein

Measuring, estimating, and understanding the psychometric function: A commentary.

The psychometric function, relating the subject’s response to the physical stimulus, is fundamental to psychophysics. This paper examines various psychometric function topics, many inspired by this special symposium issue ofPerception & Psychophysics: What are the relative merits of objective yes/no versus forced choice tasks (including threshold variance)? What are the relative merits of adaptive versus constant stimuli methods? What are the relative merits of likelihood versus up-down staircase adaptive methods? Is 2AFC free of substantial bias? Is there no efficient adaptive method for objective yes/no tasks? Should adaptive methods aim for 90% correct? Can adding more responses to forced choice and objective yes/no tasks reduce the threshold variance? What is the best way to deal with lapses? How is the Weibull function intimately related to thed’ function? What causes bias in the likelihood goodness-of-fit? What causes bias in slope estimates from adaptive methods? How good are nonparametric methods for estimating psychometric function parameters? Of what value is the psychometric function slope? How are various psychometric functions related to each other? The resolution of many of these issues is surprising.

Suppressive and facilitatory spatial interactions in peripheral vision: Peripheral crowding is neither size invariant nor simple contrast masking

Peripheral vision is characterized by reduced spatial resolution and inhibitory spatial interactions that extend over long distances. This work had three goals. (1) We considered whether the extensive crowding in peripheral vision is a consequence of a shift in the spatial scale of analysis. To test this, using a large range of target sizes and spatial frequencies, we measured the extent of crowding for targets that were limited in their spatial frequency content. (2) We considered whether crowding in peripheral vision can be explained on the basis of contrast masking by remote flanks. To test this hypothesis, we measured and compared crowding in a direction-identification experiment with masking by remote flanks in a detection experiment. In each of the experiments, our targets and flanks were composed of Gabor features, thus allowing us to control the feature contrast, spatial frequency, and orientation. (3) We examined the relationship between the suppressive and facilitatory interactions in peripheral contrast detection and crowding. Our results show that unlike the normal fovea (Levi, Klein, & Hariharan, 2002), peripheral crowding is not scale invariant nor is it attributable to simple contrast masking. Rather, our results suggest that inhibitory spatial interactions in peripheral crowding extend over larger distances than in the fovea for targets of the same size. In peripheral vision, the critical distance for crowding is approximately 0.1 times the target eccentricity. Observers can easily detect the features that compose our targets (Gabor patches) under conditions where crowding is strong. Thus, we speculate that peripheral crowding occurs because the target and flanks are combined or pooled at a second stage, following the stage of feature extraction. In peripheral vision, this pooling takes place over a large distance.

Rule-Based Learning Explains Visual Perceptual Learning and Its Specificity and Transfer

Visual perceptual learning models, as constrained by orientation and location specificities, propose that learning either reflects changes in V1 neuronal tuning or reweighting specific V1 inputs in either the visual cortex or higher areas. Here we demonstrate that, with a training-plus-exposure procedure, in which observers are trained at one orientation and either simultaneously or subsequently passively exposed to a second transfer orientation, perceptual learning can completely transfer to the second orientation in tasks known to be orientation-specific. However, transfer fails if exposure precedes the training. These results challenge the existing specific perceptual learning models by suggesting a more general perceptual learning process. We propose a rule-based learning model to explain perceptual learning and its specificity and transfer. In this model, a decision unit in high-level brain areas learns the rules of reweighting the V1 inputs through training. However, these rules cannot be applied to a new orientation/location because the decision unit cannot functionally connect to the new V1 inputs that are unattended or even suppressed after training at a different orientation/location, which leads to specificity. Repeated orientation exposure or location training reactivates these inputs to establish the functional connections and enable the transfer of learning.

Undercounting features and missing features: evidence for a high-level deficit in strabismic amblyopia

Abnormal visual development in strabismic amblyopia drastically affects visual perception and properties of neurons in primary visual cortex (V1). To test the notion that amblyopia also has consequences for higher visual areas, we asked humans with amblyopia to count briefly presented features. Using the amblyopic eye, strabismic amblyopes counted inaccurately, markedly underestimating the number of features. This inaccuracy was not due to low-level considerations (blur, visibility, crowding, undersampling or topographical jitter), as they also underestimated the number of features missing from a uniform grid. Rather, counting deficits in strabismic amblyopes reflected a higher-level limitation in the number of features the amblyopic visual system can individuate.

Suppressive and facilitatory spatial interactions in foveal vision: Foveal crowding is simple contrast masking

Spatial interactions are a critical and ubiquitous feature of spatial vision. These interactions may be inhibitory (reducing sensitivity as occurs in crowding) or facilitatory (enhancing sensitivity). In this work, we had four goals. 1. To test the hypothesis that foveal crowding depends on target size by measuring the extent of crowding for novel targets that were limited in their spatial frequency content. We used a large range of target sizes and spatial frequencies. 2. To assess whether the critical spatial frequency model (Hess, Dakin, & Kapoor, 2000) provides a general model for foveal crowding. To test this model, we measured crowding for a direction-identification task that did not require judging the orientation of the gap. 3. To test the hypothesis that foveal crowding is simply contrast masking by remote flanks we measured and compared crowding in a direction-identification experiment with masking by remote flanks in a detection experiment. In each of the experiments, our targets and flanks were composed of Gabor features, thus allowing us to control the feature contrast, spatial frequency, and orientation. 4. To assess the relationship between suppressive and facilitatory spatial interactions in foveal vision. Our results show that (1) foveal crowding is proportional to feature size over the more than 50-fold range of target sizes that we examined. Over this large range, foveal crowding is scale invariant. Our results also show it is the size of the envelope (SD) rather than the carrier (SF) that determines the extent of crowding in the fovea. 2. Crowding that occurs in the direction-identification task is quite similar to crowding where orientation information is available. Thus we conclude that the critical spatial frequency model does not provide a general explanation for foveal crowding. 3. Threshold elevation for crowding is similar to threshold elevation for masking as predicted by our test-pedestal model. Thus we conclude that foveal crowding is simple contrast masking. 4. Based on our comparison of threshold changes in crowding and masking, we conclude that in foveal vision, the suppressive spatial interactions due to nearby flanks are similar in the two tasks. However, the facilitatory interactions are quite different. In the crowding task, we find very little evidence for facilitation by flankers, whereas in the detection task, we find strong facilitation. We suggest that facilitation of detection by remote flanks may be, at least in part, a consequence of uncertainty reduction.

Pupil dilation during visual target detection

It has long been documented that emotional and sensory events elicit a pupillary dilation. Is the pupil response a reliable marker of a visual detection event while viewing complex imagery? In two experiments where viewers were asked to report the presence of a visual target during rapid serial visual presentation (RSVP), pupil dilation was significantly associated with target detection. The amplitude of the dilation depended on the frequency of targets and the time of target presentation relative to the start of the trial. Larger dilations were associated with trials having fewer targets and with targets viewed earlier in the run. We found that dilation was influenced by, but not dependent on, the requirement of a button press. Interestingly, we also found that dilation occurred when viewers fixated a target but did not report seeing it. We will briefly discuss the role of noradrenaline in mediating these pupil behaviors.

An Analysis of Signal Detection and Threshold Models of Source Memory

The authors analyzed source memory performance with an unequal-variance signal detection theory model and compared the findings with extant threshold (multinomial and dual-process) models. In 3 experiments, receiver operating characteristic (ROC) analyses of source discrimination revealed curvilinear functions, supporting the relative superiority of a continuous signal detection model when compared with a threshold model. This result has implications for both multinomial and dual-process models, both of which assume linear ROCs in their description of source memory performance.

Optimal corneal ablation for eyes with arbitrary Hartmann-Shack aberrations.

New technologies for accurately measuring corneal shape and full eye aberrations are now available. An algorithm that uses these technologies to predict the amount of ablation needed to produce a corneal surface that optimally focuses light is developed. It is found that knowledge of the aberrations is far more important than knowledge of corneal shape. Neglect of corneal shape information introduces an error of less than approximately 0.05 micron in the optimal ablation depth. Neglect of the aberrations is a different story. Small changes in the aberration structure, such as going from the optimal ablation to a spherical ablation, introduce ablation changes of greater than 10 microns. It is argued that there are many occasions when less ablation can lead to improved image quality.

Reconstructing curved surfaces from specular reflection patterns using spline surface fitting of normals

Apple Computer, 1 Infinite Loop M/S 301-3J, Cupertino, CA 95014. Computer Science Division, University of California, Berkeley, CA 94720-1776. www.cs.berkeley.edu/projects/optical/ School of Optometry, University of California, Berkeley, CA 94720-2020. We present an algorithm that reconstructs a threedimensional surface model from an image. The image is generated by illuminating a specularly reflective surface with a pattern of light. We discuss the application of this algorithm to an important problem in biomedicine, namely the measurement of the human cornea, although the algorithm is also applicable elsewhere. The distinction between this reconstruction technique and more traditional techniques that use light patterns is that the image is formed by reflection. Therefore, the reconstruction algorithm fits a surface to a set of normals rather than to a set of positions. Furthermore, the normals do not have prescribed surface positions. We show that small surface details can be recovered more accurately using this approach. The results of the algorithm are used in an interactive visualization of the cornea.

Prolonged Perceptual Learning of Positional Acuity in Adult Amblyopia: Perceptual Template Retuning Dynamics

Amblyopia is a developmental abnormality that results in physiological alterations in the visual cortex and impairs form vision. It is often successfully treated by patching the sound eye in infants and young children, but is generally considered to be untreatable in adults. However, a number of recent studies suggest that repetitive practice of a visual task using the amblyopic eye results in improved performance in both children and adults with amblyopia. These perceptual learning studies have used relatively brief periods of practice; however, clinical studies have shown that the time-constant for successful patching is long. The time-constant for perceptual learning in amblyopia is still unknown. Here we show that the time-constant for perceptual learning depends on the degree of amblyopia. Severe amblyopia requires >50 h (≈35,000 trials) to reach plateau, yielding as much as a five-fold improvement in performance at a rate of ≈1.5%/h. There is significant transfer of learning from the amblyopic to the dominant eye, suggesting that the learning reflects alterations in higher decision stages of processing. Using a reverse correlation technique, we document, for the first time, a dynamic retuning of the amblyopic perceptual decision template and a substantial reduction in internal spatial distortion. These results show that the mature amblyopic brain is surprisingly malleable, and point to more intensive treatment methods for amblyopia.