Extent-of-motion thresholds under subject-relative and object-relative conditions

The experiment was designed to discover the threshold extent of motion at medium speeds amounting to 41, 82, and 164 min./sec., and to compare the perception of motion arising from subject-relative displacement with the perception of motion arising from object-relative displacement. Extent thresholds were found while velocity was kept constant. Different groups of ten Ss were used for each displacement velocity, and for each S the extent threshold was twice obtained by the method of constant stimuli, once under subjectrelative and once under object-relative displacement conditions. Sensitivity to brief displacements of a continuously visible target was high; average thresholds ranged from 1.0 to 4.4 min. under the various conditions employed. The thresholds were higher for subject-relative conditions and the slower displacement velocities and lower for objectrelative conditions and faster displacements.     

There is no induced motion at near-threshold velocities

Duncker's classic experiments on induced motion at near-threshold velocities were based on the assumption that movement of one stimulus relative to the other (object-relative) is perfectly detected while it remains completely impossible to identify which of the two stimuli is moving (subject-relative). In the present experiments it is shown that the threshold areas of object-relative and subject-relative movement are largely overlapping. Consequently Dunker's assumption cannot be fulfilled: whenever object-relative movement is well detectable accuracy of detecting subject-relative motion will also be over 50%. In four additional experiments it is shown that the well-established effects of fixation, stimulus size and enclosure on induced motion are to be interpreted as effects of these variables on the detection of subject-relative movement: the so-called induction phenomena occured only when accuracy was over 50%.     

Three stimuli for visual motion perception compared

Two arrangements yielding induced motion were used to explore the relative effectiveness of three stimulus conditions known to produce perception of motion—namely, image displacement, ocular pursuit, and object-relative displacement. In these arrangements, object-relative displacement, which resulted in induced motion, was in conflict either with ocular pursuit or with image displacement. The outcomes of these conflicts were determined by measuring the extent of induced motion. Image displacement proved more effective in competing with object-relative displacement than did ocular pursuit, which in one arrangement yielded to object-relative displacement entirely. The same pattern of results was obtained both with the usual arrangement of the moving-center type and with a stationary-center display.     

The effect of perceived distance on induced movement

The magnitude of induced movement was measured as a function of the perceived depth between the test object and the plane of the induction object, with this perceived depth produced by stereoscopic cues. Three experiments were conducted. In each experiment, the induction object (a frame of constant physical size) was positioned at one of three distances with the test object (a point of light) placed successively at each of the three distances. Predictions of the magnitude of induction as a function of the depth separation of the test and induction object were made from the subject-relative and object-relative hypotheses of induced motion. It was expected, however, that neither of these hypotheses would predict the results independently of a factor described in the adjacency principle. This principle states that the effectiveness of whatever cues or processes determine the induced movement will decrease with increased depth between the test and induction object. The data indicate that the adjacency principle must be considered in explaining the results. The subject-relative rather than object-relative hypothesis as modified by the adjacency principle was most successful in predicting the results. Control conditions in which the frame was stationary and the point of light was physically moving were also used. Despite the fact that the relative displacement of the objects on the eye in the experimental and control conditions were the same, the results indicate that O could distinguish between these two kinds of conditions. Although the apparent movement was greater in the control conditions than in the experimental conditions, the reverse is true if the total perceived movement of the test and induction object are considered together.     

Frames of reference in preschoolers’ perception of motion

Experiments 1 and 2 established children’s (mean age 3 years, 7 months) subject-relative and object-relative motion thresholds at 1°31.37′/sec and 1°9.33′/sec, respectively, speeds well above those found for adults. Experiment 3 established that preschoolers, like adults, attribute object-relative motion to the smaller of two objects, with the inducing properties of the larger stimulus greatest when it is surrounding rather than adjacent to a smaller stimulus. The inducing advantage of surroundedness was equivalent for a single-element square frame and a multielement six-dot frame.     

Induced motion and oculomotor capture

Three experiments investigating the basis of induced motion are reported. The proposition that induced motion is based on the visual capture of eye-position information and is therefore a subject-relative, rather than object-relative, motion was explored in the first experiment. Observers made saccades to an invisible auditory stimulus following fixation on a stationary stimulus in which motion was induced. In the remaining two experiments, the question of whether perceived induced motion produces a straight ahead shift was explored. The critical eye movement was directed to apparent straight ahead. Because these saccades partially compensated for the apparent displacement of the induction stimulus, and saccades to the auditory stimulus did not, we conclude that induced motion is not based on oculomotor visual capture. Rather, it is accompanied by a shift in the judged direction of straight ahead, an instance of the straight ahead shift. The results support an object-relative theory of induced motion.     

Chinese subject-relative clauses are more difficult to process than the object-relative clauses

This paper presents an experiment that compared high and low working memory span readers' abilities to process Chinese subject-relative and object-relative clause structures in a self-paced reading paradigm. Comprehension performance results indicated that the object-relative structure was easier to understand than the subject-relative structure. Reading time results showed that participants with low working memory span read the subject-relative structures more slowly than the object-relative structures, but there was no reading time difference for the high working memory span participants. The experiment provides further evidence that the Chinese subject-relative clause structure is more difficult to process than the Chinese object-relative clause structure, especially for low working memory span individuals. Furthermore, these results support a syntactic storage account of the observed complexity difference.     

Induced visual movement as nonveridical resolution of displacement ambiguity

Following the proposals made by Kinchla, it is argued that induced movement is a nonveridical resolution of stimulus ambiguity. The ambiguity derives from an identity between displacement of one element relative to another and displacement of the second relative to the first in a featureless field at velocities below the threshold for subject-relative movement. In such conditions, which element actually moves is perceptually unresolvable and veridical judgments therefore accord with chance. When a stationary field is introduced, perception is veridical, but when the field moves with the moving element, perception is nonveridical, i.e., induced movement of the stationary element and induced stationariness of the moving element occurs. The results from three experiments supported this interpretation and showed also that movement velocities above the subject-relative threshold contribute to the resolution of ambiguity.     

How does speed change affect induced motion?

The nature of a recently reported effect of speed on induced motion was investigated. Wallach and Becklen (1983) had found that the extent of induced motion decreases linearly with increases in presentation speed. We found that this speed effect occurred only in induced motion and not under conditions in which configurational change, which is responsible for induced motion, operated veridically. It took place whether configurational change was in conflict with ocular pursuit or with image displacement, and seemed to consist in diminished effectiveness of configurational change when it competed with one of these subject-relative stimuli.     

Velocity not acceleration of self-motion mediates vestibular-visual interaction

We investigated the influence of vestibular stimulation with different angular accelerations and velocities on the perception of visual motion direction. Constant accelerations resulting in different angular velocities and constant angular velocities obtained at different accelerations were combined in twenty healthy subjects. Random-dot kinematograms with coherently moving pixels and randomly moving pixels were used as visual stimuli during whole-body rotations. The smallest percentage of coherently moving pixels leading to a clear perception of motion direction was taken as the perception threshold. Perception thresholds significantly increased with increasing angular velocity. Increased acceleration, however, had no significant effect on the perception thresholds. We conclude that the achieved angular velocity, and not acceleration, is the predominant factor in the processing of vestibular-visual interaction.     

The role of the adjacency between background cues and objects in visual localization during ocular pursuit

Subjects used eye movements to pursue a light target that moved from left to right with a velocity of 15 deg s-1. The stimulus was a sudden five-fold decrease in target intensity during the movement. The subject's task was to localize the stimulus relative to either a single stationary background point or the midpoint between two points (28 deg apart) placed 0.5 deg above the target path. The stimulus was usually mislocated in the direction of eye movement; the mislocation was affected by the spatial adjacency between background and stimulus. When an auditory, rather than a visual, stimulus was presented during tracking, target position at the time of stimulus presentation was visually mislocated in the direction opposite to that of eye movement. The effect of adjacency between background and target remained the same. The involvement of processes of subject-relative and object-relative visual perception is discussed.     

The effect of refractive error on central and peripheral motion sensitivity at various exposure durations

Pennsylvania State University, University Park, Pennsylvania 16802 The influence of refractive error on movement sensitivity was determined for a range of stimulus durations in the fovea and periphery. At short durations, threshold is determined by a constant displacement of the stimulus. At longer durations, a constant velocity is required. The correction of peripheral refractive error increases movement sensitivity in the displacement (short duration) component, but does not influence sensitivity in the velocity (long duration) component of the motion threshold. Implications concerning the mechanisms underlying motion perception are discussed.     

Vector analysis and process combination in motion perception

Experiments are reported supporting an altered explanation of the vector analysis that occurs in certain motion displays discovered by Johansson (1950). What seemed the result of a perceptual vector analysis is ascribed to the outcome of two different, independent stimulus conditions to which such displays can give rise because of external vector analysis. The different stimulus conditions are configurational change on the one hand and one of the subject-relative stimulus conditions on the other. In two of Johansson's displays, conditions for configurational change were altered by adding stationary reference points in the surround of the displays. Veridical perception of the displays resulted in a majority of instances. We also found that the different motions that result from configurational change and from subject-relative stimulation may combine to form unitary perceived motions and that this happens quite frequently under some conditions.     

Attentional resource and processing speed limitations during sentence processing in Parkinson's disease

Several studies have suggested that patients with Parkinson's disease (PD) have sentence comprehension difficulty in part because of their limited executive resources. However, these assessments confound the executive resources contributing to sentence comprehension with the resources needed for task performance. In the present study, we used a word detection technique that minimizes task demands in order to evaluate attentional and processing speed resources during the comprehension of simple sentences without subordinate clauses and sentences containing subject-relative and object-relative center-embedded subordinate clauses. We found that PD patients have poor sensitivity to phonetic errors embedded in unbound grammatical morphemes, regardless of the clausal structure of the sentence, suggesting difficulty attending to grammatical morphemes. We also found that PD patients are significantly slowed in their sensitivity to phonetic errors in content words embedded in object-relative center-embedded sentences. Slowed sensitivity to content words in object-relative sentences was correlated with timed executive measures of planning. On a traditional measure of comprehension, these PD patients were impaired for sentences containing object-relative center-embedded clauses compared to sentences with subject-relative center-embedded clauses, and comprehension of object-relative sentences was correlated with executive measures. Our findings are consistent with the claim that limited executive resources for strategic attention and processing speed contribute to the sentence comprehension difficulties of PD patients.     

Speech Rate and Syntactic Complexity as Multiplicative Factors in Speech Comprehension by Young and Older Adults

An experiment is reported in which young and older adults heard short English sentences that differed in syntactic complexity and speech rate. The syntactic contrast pitted center-embedded sentences with a subject-relative clause against sentences with center-embedded object-relative clauses. Speech rate was varied using computer time-compression of the speech signal. Both young and older adults showed poorer comprehension accuracy for the more complex object-relative clause sentences than subject-relative sentences, with an age difference appearing only when sentences were presented at a very rapid rate. By contrast to accuracy scores, older adults took longer than the young adults to give their comprehension responses at all speech rates tested, with this age difference amplified by both speech rate and syntactic complexity.     

Observations on stroboscopic induced motion

Subject-relative explanations of motion induction state that induced motion is the result of a misperceived shift of the median plane of the visual field of the subject. This theory does not require relative motion of the spot and frame, in the classical spot-and-frame condition, only asymmetrical stimulation. Three experiments are reported in which stroboscopic induced motion was investigated. The experimental arrangement was unconventional in that the induced object (spot) was presented only during the interstimulus interval between the exposures of the inducing object (frame). This allowed differentiation of the duration of the induced movement and that of the inducing one. In the first experiment it was demonstrated that perception of induced motion depends upon the duration of the interstimulus interval between the presentations of the inducing frame. In the second experiment it was shown that the perceived velocity of the induced movement can be different from that of the inducing one and depends on the duration of exposure of the induced object. In the third experiment a stimulus display was created in which the apparent displacement of an object and its induced motion are incongruous. The results are incompatible with subject-relative displacement as the sole determining factor of motion induction and they present some difficulties for the hypothesis that induced motion is the result of the apportionment of the objective displacement of the frame.     

An analysis of induced motion

The motion induced in a stationary object by a displacing surround invariably was accompanied by an apparent change in the erocentric location of that object. The degree of this apparent spatial displacement significantly correlated with the amount of induced motion seen. Induced movement was, therefore, interpreted in terms of a subjective change in the location of an object in space, as opposed to the object-relative displacement hypothesis proposed by Duncker in 1929. These findings were extended to test critically such other concepts in motion perception as separation of systems.     

Sensitivity to relative and absolute motion.

The threshold of sensitivity to movement could be governed by mechanisms that are sensitive either to change in spatial position, or directly to the movement itself. The use of spatially complex patterns (random-dot patterns) has been suggested to eliminate the former strategy allowing examination of the movement detecting mechanisms in isolation. By means of such a technique, thresholds for directional judgements were determined for patterns which underwent either a simple displacement or a shearing displacement. Thresholds for shearing motion were found to be around one half of those for simple motion, suggesting that relative, rather than absolute, motion governs performance for small displacements. This contrasts with previous experiments which showed that absolute motion governs performance for much larger displacements.     

Discourse structure and relative clause processing

Studies in several languages have shown that subject-relative clauses are easier to process than object-relative clauses. Mak, Vonk, and Schriefers (2006) have proposed the topichood hypothesis to account for the preference for subject-relative clauses. This hypothesis claims that the entity in the relative clause that is most topicworthy will be chosen as the subject. By default, the antecedent of the relative clause will be chosen as the subject of the relative clause, because it is the topic of the relative clause. However, when the noun phrase (NP) in the relative clause is also topicworthy, the preference for the antecedent to be the subject will disappear. This was confirmed in two experiments. In Experiment 1, we tested relative clauses with a personal pronoun in the relative clause. We obtained a preference for object-relative clauses, in line with the assumption that personal pronouns refer to a discourse topic and are thus topicworthy. In Experiment 2, the discourse status of the NP in the relative clause was manipulated; either it was not present in the preceding context, or it was the discourse topic. The experiment showed that when the NP in the relative clause refers to the discourse topic, the difficulty of object-relative clauses is reduced, in comparison with relative clauses with an NP that is new in the discourse, even in the absence of any explicit cue in the relative clause itself. The experiments show that discourse factors guide processing at the sentence level.     

Detection of stimulus motion in 5-month-old infants

Five-month-old infants were tested by the method of preferential looking for discrimination between a pattern undergoing oscillating apparent motion and an identical static pattern. Sensitivity to small spatial displacements was evident at temporal frequencies of 8 and 16 Hz. Preference for the moving display was related independently to the temporal frequency of oscillation and the magnitude of the spatial displacement. Preferences for the moving display increased asymptotically across spatial displacements from 11 to 89 arc min. Preferences peaked between temporal oscillation frequencies of 8 and 16 Hz. Preference was not related to the ratio of these two variables--velocity. The minimum displacement threshold of 7.36 arc min was found to depend on the size of the elements in the pattern and on the temporal frequency of oscillation. The results demonstrate that motion-sensitive mechanisms responsive to small spatial displacements are present at 5 months of age.