The stabilization of visibility for sequentially presented, low-contrast objects: Experiments and neural field model.

In any environment, events transpire in temporal sequences. The general principle governing such sequences is that each instance of the event is influenced by its predecessors. It is shown here that this principle is true for a fundamental aspect of visual perception: visibility.

Measuring the perceptual grouping of non-adjacent surfaces that are invisibly (amodally) or visibly connected.

Classic Gestalt examples of perceptual grouping entail arrays of disconnected surfaces that are grouped on the basis of the surfaces' relative similarity or proximity. However, most natural environments contain multiple objects, each with multiple, connected surfaces. Moreover, an object in a scene is likely to partially occlude other objects in the 2-dimensional retinal projection of the scene.

Temporal Asymmetry in Dark-Bright Processing Initiates Propagating Activity across Primary Visual Cortex.

Unlabelled: Differences between visual pathways representing darks and lights have been shown to affect spatial resolution and detection timing. Both psychophysical and physiological studies suggest an underlying retinal origin with amplification in primary visual cortex (V1). Here we show that temporal asymmetries in the processing of darks and lights create motion in terms of propagating activity across V1.

Nonlinear dynamics in the perceptual grouping of connected surfaces.

Evidence obtained using the dynamic grouping method has shown that the grouping of an object's connected surfaces has properties characteristic of a nonlinear dynamical system. When a surface's luminance changes, one of its boundaries is perceived moving across the surface. The direction of this dynamic grouping (DG) motion indicates which of two flanking surfaces has been grouped with the changing surface.

The coupling of vision with locomotion in cortical blindness.

Maintaining or modifying the speed and direction of locomotion requires the coupling of the locomotion with the retinal optic flow that it generates. It is shown that this essential behavioral capability, which requires on-line neural control, is preserved in the cortically blind hemifield of a hemianope. In experiments, optic flow stimuli were presented to either the normal or blind hemifield while the patient was walking on a treadmill.

The independent detection of motion energy and counterchange: flexibility in motion detection.

Motion perception is determined by changing patterns of neural activation initiated by spatiotemporal changes in stimulus features. Motion specified by 1st-order motion energy entails neural patterns that are initiated by spatiotemporal changes in luminance, whereas motion specified by counterchange entails oppositely signed changes in neural activation that can be initiated by spatiotemporal changes in any feature. A constraint in furthering this distinction is that motion energy and counterchange are co-specified by most visual stimuli.

The perception of object versus objectless motion.

Wertheimer, M. (Zeitschrift für Psychologie und Physiologie der Sinnesorgane, 61:161-265, 1912) classical distinction between beta (object) and phi (objectless) motion is elaborated here in a series of experiments concerning competition between two qualitatively different motion percepts, induced by sequential changes in luminance for two-dimensional geometric objects composed of rectangular surfaces. One of these percepts is of spreading-luminance motion that continuously sweeps across the entire object; it exhibits shape invariance and is perceived most strongly for fast speeds.

Motion perception induced by dynamic grouping: a probe for the compositional structure of objects.

A new method is described for determining how the visual system resolves ambiguities in the compositional structure of multi-surface objects; i.e., how the surfaces of objects are grouped together to form a hierarchical structure.

Changes in pupil diameter entrained by cortically initiated changes in attention.

The diameter of the pupil is affected by changes in ambient illumination, color, spatial structure, movement, and mental effort. It has now been found that pupil diameter can be affected by cognitive processes. That is, it can be entrained by alternations between broadly spread and narrowly focused attention that are cued exogenously (attention is "summoned" by the cue) or endogenously (attention changes under the perceiver's intentional control).

The temporal dynamics of global-to-local feedback in the formation of hierarchical motion patterns: psychophysics and computational simulations.

Four motion quartets, each ambiguous with respect to the perception of parallel-path horizontal or vertical motion, were arranged in a diamond configuration. Both global parallel-path motion (the same motion axis for all the quartets), which is typical for multiquartet stimuli, and global rotational rocking are perceived. Experiment 1 indicated that rotational rocking is established at different levels of processing.

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.

The line motion illusion: the detection of counterchanging edge and surface contrast.

A version of the line motion illusion (LMI) occurs when one of two adjacent surfaces changes in luminance; a new surface is perceived sliding in front of the initially presented surface. Previous research has implicated high-level mechanisms that can create or modulate LMI motion via feedback to lower-level motion detectors. It is shown here that there also is a non-motion-energy, feedforward basis for LMI motion entailing the detection of counterchange, a spatial pattern of motion-specifying stimulus information that combines changes in edge contrast with oppositely signed changes in background-relative surface contrast.

Measuring perceptual hysteresis with the modified method of limits: dynamics at the threshold.

This article describes modifications to the psychophysical method of limits that eliminate artifacts associated with the classical method, and thereby indicate whether or not there is perceptual hysteresis. Such hysteresis effects, which are characteristic of dynamical systems, would provide evidence that the near-threshold perception of an attribute is affected by stabilization mechanisms intrinsic to individual neural detectors, and by nonlinear interactions that functionally integrate the detectors when there is sufficient stimulus-initiated activation, thereby stabilizing activation at suprathreshold levels. The article begins with a review of research employing the modified method of limits.

Simultaneity and sequence in the perception of apparent motion.

Motion perception usually is accompanied by the phenomenological impression of sequence as objects move through successions of locations. Nonetheless, there is accumulating evidence that sequential information is neither necessary nor sufficient for perceiving motion. It is shown here that apparent motion is specified by counterchange rather than sequence-that is, by co-occurring toward- and away-from-background changes at two spatial locations, regardless of whether the changes are simultaneous or sequential.

A counterchange mechanism for the perception of motion.

A computational model for the perception of counterchange-specified motion is examined in detail and compared with various versions of the Reichardt motion detection model [Reichardt, W. (1961). Autocorrelation, a principle for the evaluation of sensory information by the central nervous system.

Linking dynamical perceptual decisions at different levels of description in motion pattern formation: computational simulations.

A two-level dynamical model of motion pattern formation is developed in which local motion/ nonmotion perceptual decisions are based on inhibitory competition between area V1 detectors responsive to motion-specifying versus motion-independent stimulus information, and pattern-level perceptual decisions are based on inhibitory competition between area MT motion detectors with orthogonal directional selectivity. The model accounts for the effects of luminance perturbations on the relative size of the pattern-level hysteresis effects reported by Hock and Ploeger (2006) and also accounts for related experimental results reported by Hock, Kelso, and Schöner (1993). Single-trial simulations demonstrated the crucial role of local motion/nonmotion bistability and activation-dependent future-shaping interactions in stabilizing perceived global motion patterns.

Linking dynamical perceptual decisions at different levels of description in motion pattern formation: psychophysics.

The relationship between local-level motion detection and higher level pattern-forming mechanisms was investigated with the motion quartet, a bistable stimulus for which either horizontal or vertical motion patterns are perceived. Local-level perturbations in luminance contrast affected the stability of the perceived patterns and, thereby, the size of the pattern-level hysteresis obtained by gradually changing the motion quartet's aspect ratio. Briefly eliminating luminance contrast (so nonmotion was perceived during the perturbation) eliminated pattern-level hysteresis, and briefly increasing luminance contrast (so motion was perceived during the perturbation) increased pattern-level hysteresis.

Dynamical vs. judgmental comparison: hysteresis effects in motion perception.

Perceptual comparison was investigated by gradually varying the relative length of two apparent motion paths, and independently determining when an initial percept was lost during the course of attribute change and when an alternative percept emerged. Dynamical comparison was indicated by a range of attribute values for which perception was bistable. Within this range, a percept that lost stability was immediately replaced by an alternative percept.

A common mechanism for the perception of first-order and second-order apparent motion.

A common mechanism for perceiving first-order, luminance-defined, and second-order, texture-contrast defined apparent motion between two element locations is indicated by: (1) transitivity--whether or not motion is perceived is inter-changeably affected by activationally equivalent luminance and contrast changes at each location, (2) local integration--whether or not motion is perceived depends on the net activation change resulting from simultaneous background-relative luminance and background-relative contrast changes at the same element location, and (3) inseparability--apparent motion is not perceived through independent first- or second-order mechanisms when luminance and contrast co-vary at the same location. These results, which are predicted by the response characteristics of directionally selective cells in areas V1, MT, and MST, are not instead attributable to changes in the location of the most salient element (third-order motion), attentive feature tracking, or artifactual first-order motion. Their inconsistency with Lu and Sperling's [Lu, Z.

When motion is not perceived: evidence from adaptation and dynamical stability.

Adaptation was used to probe the perceiver's activation state when either motion or nonmotion percepts are formed for bistable, single-element apparent motion stimuli. Although adaptation was not observed in every instance, when it was observed its effect was to increase the probability of both motion-to-nonmotion and nonmotion-to-motion switches, the time scale of adaptation corresponding to neurophysiological observations for directionally selective cortical cells (Giaschi et al. 1993).

Multiplicative nonlinearity in the perception of apparent motion.

Evidence is reported indicating that the perception of apparent motion is better predicted by the multiplicative combination of luminance changes at two element locations than by the sum or squared-sum of the luminance changes, or by the motion energy in the stimulus. Because the results were obtained with a stimulus for which motion was specified by simultaneous luminance changes, they support a Reichardt-style motion detector model, but without the asymmetrical delay specified by current versions. Motion direction in the modified model relies on asymmetrical stimulus information rather than asymmetrical delay.

Detection of counter-changing contrast: second-order apparent motion without postrectification motion-energy analysis or salience mapping/feature tracking.

The perception of 2nd-order, texture-contrast-defined motion was studied for apparent-motion stimuli composed of a pair of spatially displaced, simultaneously visible checkerboards. It was found that background-relative, counter-changing contrast provided the informational basis for the perception of 2nd-order apparent motion; motion began where contrast changed toward the contrast value of the background checkerboard and ended where contrast changed away from the background value. The perceived apparent motion was not attributable to either postrectification motion-energy analysis or salience-mapping/feature-tracking mechanisms.

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.

Self-organized pattern formation: experimental dissection of motion detection and motion integration by variation of attentional spread.

The formation of global motion patterns depends on the stimulus activation of local motion detectors as well as integrative excitatory and/or inhibitory interactions among the activated detectors. The counterphase row-of-elements [Vis. Res.