Differential activation solution to the motion correspondence problem

Flicker-Defined Forms in the Ternus Display

Odic and Pratt (2008, Perception, 37, 1790-1804) proposed that the type of movement seen in the bistable Ternus display depends on the elements' temporal summation of contrast relative to the background. To test this theory, participants viewed a flicker-defined Ternus display where the elements had no temporal summation of contrast. Participants also viewed a luminance-defined control condition. Five interstimulus intervals (ISIs) (0, 20, 40, 60, and 80 ms) and two stimulus durations (SDs) (200 and 400 ms) were used in each condition. If temporal summation of contrast does not influence perceived group and end-to-end movement in flicker-defined forms, it was expected that the frequency of their reports would be equal to those in the luminance- defined control condition at the same ISIs and SDs. As predicted, the main effect of condition was not significant and participants reported both percepts at expected rates in both conditions, contrary to the predictions of Odic and Pratt (2008).

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. In the current study, counterchange was isolated for stimuli composed of translating subjective (Kanizsa) squares, surfaces created by the visual system. Motion energy was isolated for stimuli composed of sequences of luminance increments that spread across perceptually stationary, literal surfaces. Counterchange-specified motion was perceived over a wide range of frame durations, and preferentially for short motion paths. Motion specified by motion energy was diminished for relatively long frame durations, and was unaffected by the length of the motion path. Finally, it was found that blank inter-frame intervals can restore counterchange-specified motion perception for frame durations that are otherwise too brief for motion to be perceived. The results of these and earlier experiments suggest that 1st-order motion energy mechanisms, dedicated to the detection of changes in neural activation initiated by spatiotemporal changes in luminance, provide the basis for objectless motion perception (Wertheimer's phi motion). In contrast, counterchanging neural activation initiated by spatiotemporal changes in any feature, including features created by the visual system, provides a flexible basis for the perception of object motion (Wertheimer's beta motion).

Ternus effect: two processes or differential activation? Comments on Odic and Pratt's 2008 paper

Using a bistable apparent-motion display, Odic and Pratt (2008, Perception 37 1790-1804) have recently presented data that they interpret as being inconsistent with what they call "the two-process theory". Instead, they argue, their data can be explained by the differential-activation theory along with a process they identify as "temporal summation of contrast". It is argued here that Odic and Pratt misinterpreted the two-process distinction and used a display that was too unusual to be adequately addressed by it. Further, their use of the differential-activation theory and, in particular, the temporal summation of contrast, seems problematic. It is concluded that there is little in their data and theoretical interpretation to justify rejection of the two-process approach.

Differential-Activation Theory Can Account for the Ternus Display: Rejoinder to Petersik

The Ternus display is a bistable apparent-motion display that has captivated researchers for decades. Recently, Odic and Pratt (2008, Perception37 1790–1804) provided evidence against the well-known two-process theory of Braddick and Adlard [1978, Visual Psychophysics and Physiology (New York: Academic Press) pp 417–426] and provided an explanatory framework using the differential-activation theory of Gilroy et al (2001, Perception & Psychophysics63 847–861). A comment by Petersik (this issue) challenges the methodology and theoretical implications of Odic and Pratt, and claims that the two-process distinction still has a role to play in the Ternus display. In this rejoinder, we examine the main points made by Petersik and expand on the differential-activation theory and its applicability to the Ternus display.

Solving the correspondence problem within the Ternus display: the differential-activation theory

The Ternus display produces a bistable illusion of motion: at very short interstimulus intervals (ISIs; < 30 ms) observers perceive element motion while at longer ISIs (> 30 ms) observers perceive group motion. In experiment 1, however, we find that, when the Ternus display's ISI contains an occluding box, group motion is mostly eliminated. These results do not fit the predictions made by the short-range/long-range two-process theory [Braddick and Adlard, 1978, in Visual Psychophysics and Psychology (New York: Academic Press)]. We propose that the differential-activation theory (Gilroy et al, 2001 Perception & Psychophysics 63 847-861) accounts for our results. We then extend the differential-activation theory as an explanatory mechanism for the Ternus display in experiment 2 by selectively placing an occluder over the first, second, or third Ternus display element. As predicted by the differential-activation theory, the occlusion of the far-left element produced a normal distribution of group motion increasing with ISI, while the occlusion of the other two elements produced an illusion of occluded elements remaining stationary throughout the display. Furthermore, as predicted by the differential-activation theory, each moving element was assigned to its nearest neighbour, producing, in the case of second and third element occlusion, a novel Ternus display motion illusion where only two out of three elements are perceived as moving.

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. Such interactions maintain currently perceived motion patterns by inhibiting the soon-to-be-stimulated motion detectors that otherwise would be the basis for the perception of an alternative pattern.

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. Partially reducing luminance contrast resulted in bistability during the perturbation; pattern-level hysteresis was maintained when motion was perceived, and eliminated when nonmotion was perceived. The results were attributed to local motion/nonmotion perceptual decisions in area V1 affecting the magnitude of the activation feeding forward to motion detectors in area MT, where the stability of pattern-level perceptual decisions is determined by activation-dependent, future-shaping interactions that inhibit soon-to-be-stimulated detectors responsive to competing motion directions.

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. 34 (1994) 1843] is an ideal stimulus for examining the relationship between the activational/energizing effect of the stimulus and interaction among the activated detectors. This is because the formation of the alternative unidirectional and oscillatory motion patterns for this stimulus requires the stimulation of local motion detectors, but there is no information in the stimulus that specifies either of the patterns. Their formation depends instead on the relative contributions of excitatory and inhibitory interactions to detector activation; the temporal patterns are self-organized. Broadly spread attention affects motion integration by changing the balance of excitatory versus inhibitory interactions, increasing the perception of unidirectional compared with oscillatory motion. (It likewise increases the perception of group compared with element motion for the Ternus stimulus.) There is, however, little if any effect of attentional spread on the luminance contrast required for the perception of single-element motion. The results indicate that the balance of integrative excitatory and/or inhibitory detector interactions can be modified by the perceiver's spread of attention, and further, that such changes need not be mediated by changes in the local, stimulus activation of the detectors.