A statistical decision model for sensory discrimination which predicts Weber’s law and other sensory laws: Some results of a computer simulation

A model for visual intensity threshold discrimination is described. Simplified assumptions represent the main features of the visual afferent paths. Discriminative responses are selected centrally by a statistical decision procedure, limited by the noise level. Noise arises from the irreducible physical variability of light, from spontaneous firing, and from variations in transmission in the afferent paths. These variations will tend to be positively correlated, The model was simulated on a computer: it correctly predicts the form of the Weber function (the relation between difference threshold and background intensity) and the features of spatial and temporal “summation”, It also shows that the function relating the eentral effeet of a stimulus to its physical intensity is unlikely to be logarithmic or a power function with a small exponent.     

Contextual cues in selective listening

Two messages were presented dichotically and subjects were asked to “shadow” whatever they heard on one ear. Somewhere in the middle the two passages were switched to the opposite ears. Subjects occasionally repeated one or two words, at the break, from the wrong ear, but never transferred to it for longer than this. The higher the transition probabilities in the passage the more likely they were to do this. One explanation might be that the “selective filter” (Broadbent, 1958) acts by selectively raising thresholds for signals from the rejected sources rather than acting as an all-or-none barrier.     

Conjunction search revisited

Search for conjunctions of highly discriminable features can be rapid or even parallel. This article explores three possible accounts based on (a) perceptual segregation, (b) conjunction detectors, and (c) inhibition controlled separately by two or more distractor features. Search rates for conjunctions of color, size, orientation, and direction of motion correlated closely with an independent measure of perceptual segregation. However, they appeared unrelated to the physiology of single-unit responses. Each dimension contributed additively to conjunction search rates, suggesting that each was checked independently of the others. Unknown targets appear to be found only by serial search for each in turn. Searching through 4 sets of distractors was slower than searching through 2. The results suggest a modification of feature integration theory, in which attention is controlled not only by a unitary "window" but also by a form of feature-based inhibition.     

Search asymmetry: a diagnostic for preattentive processing of separable features

The search rate for a target among distractors may vary dramatically depending on which stimulus plays the role of target and which that of distractors. For example, the time required to find a circle distinguished by an intersecting line is independent of the number of regular circles in the display, whereas the time to find a regular circle among circles with lines increases linearly with the number of distractors. The pattern of performance suggests parallel processing when the target has a unique distinguishing feature and serial self-terminating search when the target is distinguished only by the absence of a feature that is present in all the distractors. The results are consistent with feature-integration theory (Treisman & Gelade, 1980), which predicts that a single feature should be detected by the mere presence of activity in the relevant feature map, whereas tasks that require subjects to locate multiple instances of a feature demand focused attention. Search asymmetries may therefore offer a new diagnostic to identify the primitive features of early vision. Several candidate features are examined in this article: Colors, line ends or terminators, and closure (in the sense of a partly or wholly enclosed area) appear to be functional features; connectedness, intactness (absence of an intersecting line), and acute angles do not.     

The midstream order deficit

The relative order of an auditory sequence can be more difficult to apprehend when it is presented repeatedly without pause (i.e., cycling) than when it is presented only once (Warren, Obusek, Farmer, & Warren, 1969). We find that this phenomenon, referred to as themidstream order deficit (MOD), can also occur with visual stimuli. The stimuli need not form separate perceptual “streams,” and the effect can occur with presentation rates as slow as five items per second, even though the identification of individual letters is very accurate at this rate. However, if the first item of the sequence is visually very distinct from the preceding items, relative order reports can be as accurate in the cycling condition as in the single-presentation condition. Our results suggest that the MOD is not due to masking, attentional blink, repetition blindness, Reeves and Sperling’s (1986) order illusion, memory limitations, or decision criteria. The MOD may reflect an attentional cost to the initiation of order encoding, which is distinct from the allocation of attention is required in order to detect and identify individual items. To initiate order encoding successfully, one’s attention must be set for, or captured by, an initial salient event.     

Distractor inhibition is more effective at a central than at a peripheral location

The "distractor eccentricity effect" refers to the finding of reduced interference from an incompatible distractor at a central relative to a peripheral location (Chen, 2008). The present study examines the mechanism that underlies the distractor eccentricity effect, and relates it to the inattentional blindness explored by Mack and Rock (1998), which was also more marked at a foveal than at a parafoveal location. The results suggest that these two visual phenomena may reflect the same underlying mechanism--a gradient of increasing attentional suppression from the periphery to the center.     

Statistical processing: not so implausible after all

Myczek and Simons (2008) have shown that findings attributed to a statistical mode of perceptual processing can, instead, be explained by focused attention to samples of just a few items. Some new findings raise questions about this claim. (1) Participants, given conditions that would require different focused attention strategies, did no worse when the conditions were randomly mixed than when they were blocked. (2) Participants were significantly worse at estimating the mean size when given small samples than when given the whole display. (3) One plausible suggested strategy--comparing the largest item in each display, rather than the mean size--was not, in fact, used. Distributed attention to sets of similar stimuli, enabling a statistical-processing mode, provides a coherent account of these and other phenomena.     

Modularity and attention: Is the binding problem real?

Abstract

Van der Heijden rejects the feature integration theory of visual attention (Treisman, 1988, 1993; Treisman & Gelade, 1980) and proposes instead a theory relating modularity in the visual system to selection for action. His positive proposals about the relations between visual processing, intention, and selection for action are interesting, but I do not believe they are incompatible with my theory. In this paper I will focus on and question his arguments about the binding problem and the role of attention in visual perception. Van der Heijden attacks two claims that are fundamental to my theory: (1) the idea that modularity gives rise to a “binding problem” (the need to specify which of the features present characterize any particular object), and (2) the more general idea that there are limits to capacity at the level of perceptual processing. I will argue that he is wrong to reject the two claims, that the binding problem is a real one, for his model as for others, and that his account is more similar to mine than he appears to realize.     

Binding in short-term visual memory

The integration of complex information in working memory, and its effect on capacity, shape the limits of conscious cognition. The literature conflicts on whether short-term visual memory represents information as integrated objects. A change-detection paradigm using objects defined by color with location or shape was used to investigate binding in short-term visual memory. Results showed that features from the same dimension compete for capacity, whereas features from different dimensions can be stored in parallel. Binding between these features can occur, but focused attention is required to create and maintain the binding over time, and this integrated format is vulnerable to interference. In the proposed model, working memory capacity is limited both by the independent capacity of simple feature stores and by demands on attention networks that integrate this distributed information into complex but unified thought objects.