Michael S. Pratte

An assessment of fixed-capacity models of visual working memory

Visual working memory is often modeled as having a fixed number of slots. We test this model by assessing the receiver operating characteristics (ROC) of participants in a visual-working-memory change-detection task. ROC plots yielded straight lines with a slope of 1.0, a tell-tale characteristic of all-or-none mnemonic representations. Formal model assessment yielded evidence highly consistent with a discrete fixed-capacity model of working memory for this task.

Exploring the differences in distributional properties between Stroop and Simon effects using delta plots

Stroop and Simon tasks are logically similar and are often used to investigate cognitive control and inhibition processes. We compare the distributional properties of Stroop and Simon effects with delta plots and find different although stable patterns. Stroop effects across a variety of conditions are smallest for fast responses and increase as responses slow. Simon effects across a variety of conditions, however, are largest for fast responses but decrease, and even reverse, as responses slow. We show in three experiments that these diverging patterns hold within participants and even when the stimulus materials are identical across the tasks. These stable differences in time course serve as bedrock phenomena for building and testing theories of cognitive control and inhibition. The results of two additional experiments suggest that the determinant of time course is not simply whether the distracting information is location.

A Task-Difficulty Artifact in Subliminal Priming

Subliminal priming is said to occur when a subliminal prime influences the classification of a subsequent target. Most subliminal-priming claims are based on separate target- and prime-classification tasks. Because primes are intended to be subliminal, the prime-classification task is difficult, and the target-classification task is easy. To assess whether this task-difficulty difference accounts for previous claims of subliminal priming, we manipulated the ease of the prime-classification task by intermixing long-duration (visible) primes with short-duration (near liminal) ones. In Experiment 1, this strategy of intermixing long-duration primes raised classification of the shortduration ones. In Experiments 2 and 3, prime duration was lowered in such a way that prime classification was at chance in intermixed presentations. Under these conditions, we failed to observe any priming effects; hence, previous demonstrations of subliminal priming may simply have reflected a task-difficulty artifact.

Detecting chance: a solution to the null sensitivity problem in subliminal priming.

In many paradigms, the persuasiveness of subliminal priming relies on establishing that stimuli are undetectable. The standard significance test approach is ill-suited as null results may reflect either truly undetectable stimuli or a lack of power to resolve weakly detectable stimuli. We present a novel statistical model as an alternative. The model provides for estimates of the probability that each individual is truly at chance. Researchers may select individuals for whom there are sufficiently high probabilities of true undetectability. The model is hierarchical, and estimation is done within the Bayesian framework.

Spatial specificity of working memory representations in the early visual cortex

Recent fMRI decoding studies have demonstrated that early retinotopic visual areas exhibit similar patterns of activity during the perception of a stimulus and during the maintenance of that stimulus in working memory. These findings provide support for the sensory recruitment hypothesis that the mechanisms underlying perception serve as a foundation for visual working memory. However, a recent study by Ester, Serences, and Awh (2009) found that the orientation of a peripheral grating maintained in working memory could be classified from both the contralateral and ipsilateral regions of the primary visual cortex (V1), implying that, unlike perception, feature-specific information was maintained in a nonretinotopic manner. Here, we evaluated the hypothesis that early visual areas can maintain information in a spatially specific manner and will do so if the task encourages the binding of feature information to a specific location. To encourage reliance on spatially specific memory, our experiment required observers to retain the orientations of two laterally presented gratings. Multivariate pattern analysis revealed that the orientation of each remembered grating was classified more accurately based on activity patterns in the contralateral than in the ipsilateral regions of V1 and V2. In contrast, higher extrastriate areas exhibited similar levels of performance across the two hemispheres. A time-resolved analysis further indicated that the retinotopic specificity of the working memory representation in V1 and V2 was maintained throughout the retention interval. Our results suggest that early visual areas provide a cortical basis for actively maintaining information about the features and locations of stimuli in visual working memory.

Separating Mnemonic Process From Participant and Item Effects in the Assessment of ROC Asymmetries

One of the most influential findings in the study of recognition memory is that receiver operating characteristic (ROC) curves are asymmetric about the negative diagonal. This result has led to the rejection of the equal-variance signal detection model of recognition memory and has provided motivation for more complex models, such as the unequal-variance signal detection and dual-process models. Here, the authors test the possibility that previous demonstrations of ROC asymmetry do not reflect mnemonic process but rather reflect distortions due to averaging data over items. Application of a hierarchical unequal-variance signal detection model reveals that asymmetries are in fact a real phenomenon and do not reflect distortions from averaging data. (PsycINFO Database Record (c) 2009 APA, all rights reserved).

Attention alters orientation processing in the human lateral geniculate nucleus

Orientation selectivity is a cornerstone property of vision, commonly believed to emerge in the primary visual cortex. We found that reliable orientation information could be detected even earlier, in the human lateral geniculate nucleus, and that attentional feedback selectively altered these orientation responses. This attentional modulation may allow the visual system to modify incoming feature-specific signals at the earliest possible processing site.

Sensory uncertainty decoded from visual cortex predicts behavior

Bayesian theories of neural coding propose that sensory uncertainty is represented by a probability distribution encoded in neural population activity, but direct neural evidence supporting this hypothesis is currently lacking. Using fMRI in combination with a generative model-based analysis, we found that probability distributions reflecting sensory uncertainty could reliably be estimated from human visual cortex and, moreover, that observers appeared to use knowledge of this uncertainty in their perceptual decisions.

Delta Plots and Coherent Distribution Ordering

Social scientists often compare subclasses of populations or manipulatons. For example, in comparing task-completion times across two levels of a manipulations, if one group has faster overall mean response, it is natural to ask if the fastest 10%% of the first group has a faster mean than the fastest 10%% of the second group, and so on. Delta plots, a type of quantile-quantile residual plot used by psychologists, shed light on these comparisons and motivate new notions of stochastic ordering. If all percentile classes have faster mean in one group than in the other, we say that there is coherent mean ordering and that one group stochastically dominates the other in mean. A related notion of coherent variance ordering can be defined similarly. Violations of coherent orderings of means or variances are diagnostic signatures of complex effects and suggest further avenues of study. In this note, we derive necessary and sufficient conditions for stochastic dominance in mean and variance. We show that stochastic mean dominance is exactly equivalent to the usual stochastic dominance and stochastic variance dominance is equivalent to ordering of the first derivative of the quantile functions.

Accounting for stimulus-specific variation in precision reveals a discrete capacity limit in visual working memory.

If we view a visual scene that contains many objects, then momentarily close our eyes, some details persist while others seem to fade. Discrete models of visual working memory (VWM) assume that only a few items can be actively maintained in memory, beyond which pure guessing will emerge. Alternatively, continuous resource models assume that all items in a visual scene can be stored with some precision. Distinguishing between these competing models is challenging, however, as resource models that allow for stochastically variable precision (across items and trials) can produce error distributions that resemble random guessing behavior. Here, we evaluated the hypothesis that a major source of variability in VWM performance arises from systematic variation in precision across the stimuli themselves; such stimulus-specific variability can be incorporated into both discrete-capacity and variable-precision resource models. Participants viewed multiple oriented gratings, and then reported the orientation of a cued grating from memory. When modeling the overall distribution of VWM errors, we found that the variable-precision resource model outperformed the discrete model. However, VWM errors revealed a pronounced “oblique effect,” with larger errors for oblique than cardinal orientations. After this source of variability was incorporated into both models, we found that the discrete model provided a better account of VWM errors. Our results demonstrate that variable precision across the stimulus space can lead to an unwarranted advantage for resource models that assume stochastically variable precision. When these deterministic sources are adequately modeled, human working memory performance reveals evidence of a discrete capacity limit.