Differential activation solution to the motion correspondence problem

The correspondence problem arises in motion perception when more than one motion path is possible for discontinuously presented visual elements. Ullman's (1979) "minimal mapping" solution to the correspondence problem, for which costs are assigned to competing motion paths on the basis of element affinities (e.g., greater affinity for elements that are closer together), is distinguished from a solution based on the differential activation of directionally selective motion detectors. The differential activation account was supported by evidence that path length affects detector activation in a paradignm for which motion correspondence is not a factor. Effects on detector activation in this paradigm also were the basis for the successful prediction of path luminance effects on solutions to the motion correspondence problem. Finally, the differential activation account was distinguished from minimal mapping theory by an experiment showing that the perception of an element moving simultaneously in two directions does not depend on whether the two motions are matched in path-length determined affinity; it is sufficient that the activation of detectors responding to each of the two motion directions is above the threshold level required for the motions to be perceived. Implications of the differential activation solution are discussed for the stability of perceived motions once they are established, and the adaptation of perceived and unperceived motions.     

Differential activation solution to the

The correspondence problem arises in motion perception when more than one motion path is possible for discontinuously presented visual elements. Ullman’s (1979) “minimal mapping” solution to the correspondence problem, for which costs are assigned to competing motion paths on the basis of element affinities (e.g., greater affinity for elements that are closer together), is distinguished from a solution based on the differential activation of directionally selective motion detectors. The differential activation account was supported by evidence that path length affects detector activation in a paradigm for which motion correspondence is not a factor. Effects on detector activation in this paradigm also were the basis for the successful prediction of path luminance effects on solutions to the motion correspondence problem. Finally, the differential activation account was distinguished from minimal mapping theory by an experiment showing that the perception of an element moving simultaneously in two directions does not depend on whether the two motions are matched in path-length determined affinity; it is sufficient that the activation of detectors responding to each of the two motion directions is above the threshold level required for the motions to be perceived. Implications of the differential activation solution are discussed for the stability of perceived motions once they are established, and the adaptation of perceived andunperceived motions.     

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.     

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.     

What matters in the cued task-switching paradigm: Tasks or cues?

Schneider and Logan (2006) recently showed that cue-switch and task-switch costs are sensitive to the relative probability of cue switches versus task switches. From this they concluded that taskswitch costs reflect priming of cue-cue transitions rather than actual task-switching operations. However, because this design confounded probability of specific cue transitions with probability of task switches, the results could also reflect task-switch-level adjustments. The present experiment (N = 80) pits the critical prediction of the cue-priming account, namely that costs for high-probability cue-cue transitions are smaller than for low-probability cue-cue transitions, against the main prediction of the switch-probability account, namely that switch probability, irrespective of specific cue-cue transitions, determines switch costs. Whereas the cue-priming prediction was rejected, a specific version of the probability account—that subjects are sensitive to the probability of a task switch, given a cue switch—was fully confirmed. Thus, tasks are in fact the critical representational units that determine task-switch cost.     

Visible persistence and form correspondence in Ternus apparent motion

Visual stimuli remain visible for some time after their physical offset (visible persistence) Visible persistence has been hypothesized to play an important role in determining the pattern of correspondence matching in the Ternus apparent-motion display. In this display, one or more elements reappears in overlapping locations at different times, whereas another element appears alternately to the right or the left of these elements. Usually either the elements are perceived to move coherently as a group (group motion), or one element may be perceived to hop over one or more other elements (element motion). According to the visible-persistence account of the perceptual organization of the Ternus display, element motion is seen when the temporal gap between elements in overlapping locations is small enough to be bridged by visible persistence; if it is not, group motion is seen. We conducted four experiments to test this visible-persistence account. In Experiments 1 and 2, a form correspondence cue (line length) was introduced to bias the visual system toward the element-motion interpretation, while visible persistence was either reduced or eliminated. The element-motion percept dominated despite the elimination of visible persistence. In Experiments 3 and 4, we found that Ternus elements presented without interruption, and thus presumably persisting over time, can be perceived in group motion. Together, the results indicate that visible persistence is neither necessary nor sufficient to account for the pattern of correspondence matches in the Ternus display.     

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 Sch?ner (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.     

Visually perceived motion in depth resulting from proximal changes. II

According to a model for motion and form perception proposed by Johansson (1964). every two-dimensional change in a changing proximal stimulation is projected out as a motion in depth The model assumes that the amount of perceivedrelative motion (the fraction between the perceived amount of motion of the object and the perceived initial distance to the object) is determined only by the amount ofrelative change (the fraction between the absolute amount of change and the initial size). The aim of the present study was to test this hypothesis by studying the effect of some other variables on perceived relative motion in depth. As stimuli, continuously shrinking and growing squares were used. No effects were found when varying the absolute amount of change. Neither did the rate of change influence the perceived relative motion in any important way. The only variable that gave rise to strong and systematic effects on perceived relative motion was the initial distance to the perceived object. The greater the initial distance, the less relative motion was perceived.     

Visible persistence and form correspondence in Ternus apparent motion.

Visual stimuli remain visible for some time after their physical offset (visible persistence). Visible persistence has been hypothesized to play an important role in determining the pattern of correspondence matching in the Ternus apparent-motion display. In this display, one or more elements reappears in overlapping locations at different times, whereas another element appears alternately to the right or the left of these elements. Usually either the elements are perceived to move coherently as a group (group motion), or one element may be perceived to hop over one or more other elements (element motion). According to the visible-persistence account of the perceptual organization of the Ternus display, element motion is seen when the temporal gap between elements in overlapping locations is small enough to be bridged by visible persistence; if it is not, group motion is seen. We conducted four experiments to test this visible-persistence account. In Experiments 1 and 2, a form correspondence cue (line length) was introduced to bias the visual system toward the element-motion interpretation, while visible persistence was either reduced or eliminated. The element-motion percept dominated despite the elimination of visible persistence. In Experiments 3 and 4, we found that Ternus elements presented without interruption, and thus presumably persisting over time, can be perceived in group motion. Together, the results indicate that visible persistence is neither necessary nor sufficient to account for the pattern of correspondence matches in the Ternus display.     

The interplay between stereopsis and structure from motion

In a series of psychophysical experiments, an adaptation paradigm was employed to study the influence of stereopsis on perception of rotation in an ambiguous kinetic depth (KD) display. Without prior adaptation or stereopsis, a rotating globe undergoes spontaneous reversals in perceived direction of rotation, with successive durations of perceived rotation being random variables. Following 90 sec of viewing a stereoscopic globe undergoing unambiguous rotation, the KD globe appeared to rotate in a direction opposite that experienced during the stereoscopic adaptation period. This adaptation aftereffect was short-lived, and it occurred only when the adaptation and test figures stimulated the same retinal areas, and only when the adaptation and test figures rotated about the same axis. The aftereffect was just as strong when the test and adaptation figures had different shapes, as long as the adaptation figure contained multiple directions of motion imaged at different retinal disparities. Nonstereoscopic adaptation figures had no effect on the perceived direction of rotation of the ambiguous KD figure. These results imply that stereopsis and motion strongly interact in the specification of structure from motion, a result that complements earlier work on this problem.     

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. In W. A. Rosenblith (Ed.), Sensory communication (pp. 303-317). New York: Wiley]. The counterchange model is composed of a pair of temporally biphasic subunits at two retinal locations, one detecting decreases and the other increases in input activation. Motion is signaled when both subunits are simultaneously excited, as determined by the multiplicative combination of their transient responses. In contrast with the Reichardt detector, which effectively tracks motion energy and accounts solely for results obtained with standard apparent motion stimuli (a surface is visible at one location, then at another), the counterchange model also accounts for the generalized apparent motion perceived between pairs of simultaneously visible surfaces. This indicates that standard apparent motion can be perceived via the same non-sequential, non-motion-energy mechanism as generalized apparent motion. There is no need for either an explicit delay mechanism to account for optimal motion perception at non-zero inter-stimulus intervals, or for inhibitory interaction between subunits to account for the absence of motion in the detector’s null direction (Barlow, H. B., & Levick, W. R., 1965). Both are emergent properties that result from the inhibitory states of the counterchange detector’s biphasic subunits. In addition to apparent motion, the counterchange principle potentially accounts for the perception of motion for drifting gratings, the short range motion perceived for random-dot cinematograms, and the motion perceived for continuously moving objects.     

Aftereffect of high-speed motion.

A visual illusion known as the motion aftereffect is considered to be the perceptual manifestation of motion sensors that are recovering from adaptation. This aftereffect can be obtained for a specific range of adaptation speeds with its magnitude generally peaking for speeds around 3 deg s-1. The classic motion aftereffect is usually measured with a static test pattern. Here, we measured the magnitude of the motion aftereffect for a large range of velocities covering also higher speeds, using both static and dynamic test patterns. The results suggest that at least two (sub)populations of motion-sensitive neurons underlie these motion aftereffects. One population shows itself under static test conditions and is dominant for low adaptation speeds, and the other is prevalent under dynamic test conditions after adaptation to high speeds. The dynamic motion aftereffect can be perceived for adaptation speeds up to three times as fast as the static motion aftereffect. We tested predictions that follow from the hypothesised division in neuronal substrates. We found that for exactly the same adaptation conditions (oppositely directed transparent motion with different speeds), the aftereffect direction differs by 180 degrees depending on the test pattern. The motion aftereffect is opposite to the pattern moving at low speed when the test pattern is static, and opposite to the high-speed pattern for a dynamic test pattern. The determining factor is the combination of adaptation speed and type of test pattern.     

Intentional switching between patterns of bimanual coordination depends on the intrinsic dynamics of the patterns

Two predictions arising from previous theoretical and empirical work which demonstrated that spontaneous changes of bimanual coordination patterns result from a loss of pattern stability (i.e., a nonequilibrium phase transition) were tested: (a) that the time it takes to intentionally switch from one pattern to another depends on the differential stability of the patterns themselves; and (b) that an intention, defined as an intended behavioral pattern, can change the dynamical characteristics, e.g., the overall stability of the behavioral patterns. Subjects moved both index fingers rhythmically at one of six movement frequencies while performing either an in-phase or antiphase pattern of finger coordination. On cue from an auditory signal, subjects switched from the ongoing pattern to the other pattern. The relative phase of movement between the two fingers was used to characterize the ongoing coordinative pattern. The time taken to switch between patterns, or switching time, and relative phase fluctuations were used to evaluate the modified pattern dynamics resulting from a subject's intention to change patterns. Switching from the in-phase to the antiphase pattern was significantly slower than switching in the opposite direction for all subjects. Both the mean and distribution of switching times in each direction were found to be in agreement with model predictions. movement frequency had little effect on switching time, a finding that is also consistent with the model. Relative phase fluctuations were significantly larger when moving in the antiphase pattern at the highest movement frequencies studied. The results show that, although intentional influences act to modify a coordinative pattern's intrinsic dynamics, the influence of these dynamics on the resulting behavior is always present and is particularly strong at high movement frequencies.     

Adaptation to spiral motion in crowding condition

When a single, moving stimulus is presented in the peripheral visual field, its direction of motion can be easily distinguished, but when the same stimulus is flanked by other similar moving stimuli, observers are unable to report its direction of motion. In this condition, known as 'crowding', specific features of visual stimuli do not access conscious perception. The aim of this study was to investigate whether adaptation to spiral motion is preserved in crowding conditions. Logarithmic spirals were used as adapting stimuli. A rotating spiral stimulus (target spiral) was presented, flanked by spirals of the same type, and observers were adapted to its motion. The observers' task was to report the rotational direction of a directionally ambiguous motion (test stimulus) presented afterwards. The directionally ambiguous motion consisted of a pair of spirals flickering in counterphase, which were mirror images of the target spiral. Although observers were not aware of the rotational direction of the target and identified it at chance levels, the direction of rotation reported by the observers during the test phase (motion aftereffect) was contrarotational to the direction of the adapting spiral. Since all contours of the adapting and test stimuli were 90 degrees apart, local motion detectors tuned to the directions of the mirror-image spiral should fail to respond, and therefore not adapt to the adapting spiral. Thus, any motion aftereffect observed should be attributed to adaptation of global motion detectors (ie rotation detectors). Hence, activation of rotation-selective cells is not necessarily correlated with conscious perception.     

Switching in the cocktail party: exploring intentional control of auditory selective attention

Using a novel variant of dichotic selective listening, we examined the control of auditory selective attention. In our task, subjects had to respond selectively to one of two simultaneously presented auditory stimuli (number words), always spoken by a female and a male speaker, by performing a numerical size categorization. The gender of the task-relevant speaker could change, as indicated by a visual cue prior to auditory stimulus onset. Three experiments show clear performance costs with instructed attention switches. Experiment 2 varied the cuing interval to examine advance preparation for an attention switch. Experiment 3 additionally isolated auditory switch costs from visual cue priming by using two cues for each gender, so that gender repetition could be indicated by a changed cue. Experiment 2 showed that switch costs decreased with prolonged cuing intervals, but Experiment 3 revealed that preparation did not affect auditory switch costs but only visual cue priming. Moreover, incongruent numerical categories in competing auditory stimuli produced interference and substantially increased error rates, suggesting continued processing of task-relevant information that often leads to responding to the incorrect auditory source. Together, the data show clear limitations in advance preparation of auditory attention switches and suggest a considerable degree of inertia in intentional control of auditory selection criteria.     

Response inhibition under task switching: its strength depends on the amount of task-irrelevant response activation

Under task switch conditions, response repetitions usually produce benefits if the task also repeats, but costs if the task switches. So far, it is largely undecided how to account for these effects. In the present study, we provide additional evidence in favor of the account that each response is inhibited in order to prevent its accidental re-execution. To test this hypothesis, the risk of an accidental re-execution of a given response was manipulated by modulating the activation of the response in the previous task. In Experiment 1, this was done by means of congruent and incongruent stimuli. As expected, on task switch trials, the repetition costs were larger if a congruent rather than an incongruent stimulus occurred in the previous task. In Experiment 2, the same effect occurred for stimulus-response compatible versus incompatible stimuli in the previous task. In Experiment 3, both manipulations were applied together, which produced almost additive effects. Altogether, the results support the inhibition account for the response repetition effects under task switch conditions.     

Two-year-olds' sensitivity to speakers' intent: an alternative account of Samuelson and Smith

Seventy-two 2-year-olds participated in a study designed to test two competing accounts of the effect of contextual change on children's ability to learn a word for an object. The mechanistic account hypothesizes that any change in context that highlights a target object will lead to word learning; the social-pragmatic account maintains that a change in context must be perceived as relevant to the speaker's communicative intentions. Consistent with the latter account, we found that children learned the word when a change in context was intentional but not when it was accidental, and children failed to learn the word for the highlighted object when a speaker naive to the preceding context named the object.     

The role of awareness in delay and trace fear conditioning in humans

Expression of conditional fear without awareness has been previously demonstrated during delay conditioning, a procedure in which the conditioned stimulus (CS) and unconditioned stimulus (UCS) overlap. However, less is known about the role of awareness in trace fear conditioning, where an interval of time separates the CS and UCS. The present study assessed skin conductance response (SCR) and UCS expectancy during delay and trace conditioning. UCS predictability was varied on a trial-bytrial basis by presenting perithreshold auditory CSs. Differential UCS expectancies were demonstrated only on perceived delay and trace trials. Learning-related SCRs were observed during both perceived and unperceived delay CSs. In contrast, differential SCRs were demonstrated only for perceived trace CSs. These data suggest that awareness is necessary for conditional responding during trace, but not delay, fear conditioning.     

Task selection cost asymmetry without task switching

The switch cost asymmetry (i.e., larger costs when switching from a nondominant into a dominant task than vice versa) has been explained in terms of the trial-to-trial carryover of activation levels required for the dominant versus the nondominant task. However, there is an open question about whether an actual switch in task is in fact necessary to obtain a “selection” cost asymmetry. In Experiments 1 and 2, we modified an alternatingruns paradigm to include either long or short response-to-stimulus intervals (RSIs) after each pair of trials (i.e., AA-AA-BB-BB), thereby inducing selection costs not only at the point of a task switch (i.e., AA-BB), but also between same-task pairs (i.e., AA-AA). Using spatially compatible versus incompatible response rules (Experiment 1) and Stroop word versus color naming (Experiment 2), we found asymmetric effects not only at task-change transitions, but also at task-repeat transitions when the RSI was long (presumably inducing frequent losses of set). In Experiments 3A and 3B, a cost asymmetry for long RSIs was obtained even when competing tasks were separated into alternating single task blocks, but not when the tasks were compared in a betweensubjects design. This general pattern cannot be explained by activation carryover models, but is consistent with the idea that the asymmetry arises as a result of interference from long-term memory traces.