Howard S. Hock

Bistability and hysteresis in the organization of apparent motion patterns

In a paradigm for which 2 distinct patterns are perceived for the same stimulus, perceptual hysteresis (persistence of a percept despite parameter change to values favoring the alternative pattern) and temporal stability (persistence despite intrinsic propensities toward spontaneous change) are interdependent. Greater persistence during parameter change reduces temporal stability, slowing the rate of parameter change reduces hysteresis by increasing opportunity for spontaneous change, and increasing temporal stability (by enlarging the stimulus) increases hysteresis. Hysteresis results in the perception of parametrically disfavored patterns; a parameter can influence a percept without specifying it. The visual system thus exhibits time-dependent behavior analogous to dynamical behavior observed in other systems, both physical and biological, for which there is competition among alternative patterns that vary in relative stability. Language: en

Real-world schemata and scene recognition in adults and children

Recognition memory for previously seen multiobject scenes was examined for different types of contextual arrangements between objects in the scenes. It was found that organized scenes with novel but possible interobject relations were recognized more accurately than either organized scenes with familiar interobject relations or unorganized scenes with impossible interobject relations. This finding was obtained for adults, 8- to 10-year-old children, and 5- to 8-year-old children who indicated concrete-operational ability in Piaget’s conservation-of-liquid quantity task. The results were interpreted in conjunction with a two-stage model of scene processing involving the formation of a schema to represent a scene (Stage 1), and the operation of the schema in governing the further processing of detailed information in the scene (Stage 2). It was concluded that preoperational children can form schemata to represent organized scenes (Stage 1), but it is not until the emergence of concrete operations that these schemata become operational with respect to guiding the further processing of information in the scene (Stage 2).

The Effect of Attentional Spread on Spatial Resolution

The effects of attentional spread were studied by having subjects detect a luminance increment along a row of evenly spaced dots. The increment could occur for the central, fixated dot (Narrow Attention) or for either the fixation dot or one of the four dots to its left or right (Broad Attention). Narrow Attention enhanced the detection of luminance increments for the fixated dot, and also enhanced spatial resolution near the fixation dot for judgments of vernier alignment and separation. This indicated that the sensitivity of small spatial filters in the fovea was increased more by narrowly focused than broadly spread attention. Effects of attentional spread on spatial resolution were not obtained for judgments of the separation between two peripherally located targets, perhaps because of their dependence on eccentricity (position) rather than separation.

The effects of stimulus structure and familiarity on same-different comparison

It was hypothesized that performance in the same-different comparison task is based on two modes of processing: (1) structural processes that organize the detailed parts of a stimulus into a well-formed whole, and (2)analytic processes that decompose the stimulus into features. This hypothesis was supported, but with the unexpected finding of individual differences in the mode of processing underlying “same” responses. Those Ss in the “same” condition whose reaction times were faster for symmetrical than for asymmetrical patterns supported the hypothesis for structural processes. The remaining Ss in the “same” condition, as well as all the Ss in the “different” condition, were unaffected by symmetry. These Ss supported the hypothesis for analytic processes. Although familiarity effects were obtained for both structural and analytic Ss, the rotation of the familiar patterns into an unfamiliar orientation virtually eliminated familiarity effects for the structural Ss, but left them intact for analytic Ss.

Dynamic, state-dependent thresholds for the perception of single-element apparent motion: bistability from local cooperativity.

Previous studies have indicated that the formation of coherent patterns for multielement motion displays depends onglobal cooperative interactions among large ensembles of spatially distributed motion detectors. These interactions enhance certain motion directions and suppress others. It is reported here that perceiving one element moving between two nearby locations likewise is subject to cooperative influences (possibly facilitating and inhibiting interactions within alocal ensemble of overlapping detectors). Thresholds depending on luminance contrast were measured for a generalized singleelement apparent-motion stimulus, and evidence for spontaneous switching and hysteresis effects indicated that motion perception near the 50% threshold was bistable. That is, for conditions in which motion and nonmotion were perceived half the time, the two percepts were distinct; when one was perceived, it clearly was discriminable from the other. These results indicated that (1) single-element apparent-motion thresholds depended on the immediately preceding state of the ensemble of motion detectors responding to the stimulus, and (2) the stimulus activation of individual motion detectors always might be influenced by recurrent, cooperative interactions resulting from the detectors’ being embedded within interconnected ensembles.

Mental rotation and perceptual uprightness

Performance in Cooper and Shepard’s (1973) mental rotation task was examined in the context of a model that defined the extent to which alphabet letters could be tilted from their normal orientation and still be perceptually upright. For letters with a broad range of orientations for which they remain perceptually upright, a nonlinear effect of orientation on reaction time was obtained (as in Cooper and Shepard). However, for letters with a narrow range of orientations for which they remain perceptually upright, reaction time was linearly related to orientation. The results supported the hypothesis that subjects in the Cooper and Shepard task would mentally rotate alphabet letters only when they were presented in orientations for which they were not perceptually upright.

The dynamical foundations of motion pattern formation: stability, selective adaptation, and perceptual continuity.

A dynamical model is used to show that global motion pattern formation for several different apparent motion stimuli can be embodied in the stable distribution of activation over a population of concurrently activated, directionally selective motion detectors. The model, which is based on motion detectors being interactive, noisy, and self-stabilizing, accounts for such phenomena as bistability, spontaneous switching, hysteresis, and selective adaptation. Simulations show that dynamical solutions to the motion correspondence problem for a bistable stimulus (two qualitatively different patterns are formed) apply as well to the solution for a monostable stimulus (only one pattern is formed) and highlight the role of interactions among sequentially stimulated detectors in establishing the state dependence and, thereby, the temporal persistence of percepts.

Age differences in the temporal locus of memory organization in children's recall☆

Abstract Age differences in when (i.e., at input or output) children organize information for recall were investigated in terms of a model specifying that information organized at input is more resistant to forgetting over time than information not organized at input. In Experiment 1, recall of items from categorically related and unrelated lists was assessed either immediately or after a 4-min delay. For 9-year-olds, the effect of delay was comparable for the related and unrelated lists, indicative of spontaneous organization at time of output. In contrast, 13-year-olds showed a significantly smaller delay effect with related than with unrelated lists, indicative of spontaneous organization at time of input. Experiments 2 and 3 demonstrated that, for 9-year-olds, high levels of clustering in and of themselves do not eliminate effects of delayed testing characteristic of output organization, and that when 9-year-olds are biased to organize information at input, delay effects are reduced only when measures are taken to ensure that all the category labels are retrieved.

Alternative spatial reference systems: Intentional vs. incidental learning

Subjects were initially shown unfamiliar polygons while their heads were tilted and were then tested for their ability to recognize the polygons when their heads were upright. The objective (gravitational) orientation of the polygons during acquisition affected recognition accuracy only when the polygons were acquired under intentional learning instructions. When acquisition was incidental, evidence for the retention of retinally referenced memory representations was obtained. These results were consistent with a review of previous literature indicating that the use of a task requiring either the storage or retrieval of information from memory is a necessary (although not sufficient) condition for obtaining an effect of objective orientation on performance.

Illusory motion perception in blindsight

Motion detection is typically spared in blindsight, which results from damage to the striate cortex (area V1) of the brain that is sufficient to eliminate conscious visual awareness and severely reduce sensitivity to luminance contrast, especially for high spatial and low temporal frequencies. Here we show that the discrimination of motion direction within cortically blind fields is not attributable to feature tracking (the detection of changes in position or shape), but is due instead to the detection of first-order motion energy (spatiotemporal changes in luminance). The key to this finding was a version of the line motion illusion entailing reverse-phi motion in which opposing motion directions are simultaneously cued by motion energy and changes in stimulus shape. In forced-choice tests, a blindsighted test subject selected the direction cued by shape change when the stimulus was presented in his intact field, but reliably selected the direction cued by motion energy when the same stimulus was presented in his blind field, where relevant position information was either inaccessible or invalid. Motion energy has been characterized as objectless, so reliance on motion energy detection is consistent with impaired access to shape information in blindsight. The dissociation of motion direction by visual field (cortically blind vs. intact) provides evidence that two pathways from the retina to MT/V5 (the cortical area specialized for motion perception) are functionally distinct: the retinogeniculate pathway through V1 is specialized for feature-based motion perception, whereas the retinocollicular pathway, which bypasses V1, is specialized for detecting motion energy.