Rob van Lier

Integrating Global and Local Aspects of Visual Occlusion

The phenomenon of visual occlusion has frequently been studied by means of two-dimensional line drawings. These drawings may elicit various interpretations. Sometimes a mosaic of shapes is seen, sometimes a shape that partly occludes another shape. In the latter case, observers often have a clear idea about the form of the partly occluded shape. Local and global pattern aspects both seem to be decisive with respect to the preferred interpretation. An attempt is made to integrate these aspects by applying the global-minimum principle to the perceptual complexity of three distinct components of those pattern interpretations: (i) The internal structure, dealing with each of the shapes separately, (ii) the external structure, dealing with the positional relation between these shapes, and (iii) the virtual structure, dealing with the occluded parts of the shapes. The perceptual complexity of each of these three components can be expressed in terms of structural information. The hypothesis that the perceptually preferred interpretation is the one for which the total information load is minimal is tested on many patterns stemming from different studies on pattern completion.

Mental rotation task of hands: differential influence number of rotational axes.

Various studies on the hand laterality judgment task, using complex sets of stimuli, have shown that the judgments during this task are dependent on bodily constraints. More specific, these studies showed that reaction times are dependent on the participant’s posture or differ for hand pictures rotated away or toward the mid-sagittal plane (i.e., lateral or medial rotation, respectively). These findings point to the use of a cognitive embodied process referred to as motor imagery. We hypothesize that the number of axes of rotation of the displayed stimuli during the task is a critical factor for showing engagement in a mental rotation task, with an increased number of rotational axes leading to a facilitation of motor imagery. To test this hypothesis, we used a hand laterality judgment paradigm in which we manipulated the difficulty of the task via the manipulation of the number of rotational axes of the shown stimuli. Our results showed increased influence of bodily constraints for increasing number of axes of rotation. More specifically, for the stimulus set containing stimuli rotated over a single axis, no influence of biomechanical constraints was present. The stimulus sets containing stimuli rotated over more than one axes of rotation did induce the use of motor imagery, as a clear influence of bodily constraints on the reaction times was found. These findings extend and refine previous findings on motor imagery as our results show that engagement in motor imagery critically depends on the used number of axes of rotation of the stimulus set.

Multiple completions primed by occlusion patterns

There is a strong tendency to complete a partly occluded shape. Two types of pattern completion, global and local, are frequently reported. By means of the primed-matching paradigm, it has previously been shown that global completions are prevalent for stimuli in which regularity is abundantly present. In our study the primed-matching paradigm is applied to such stimuli in order to find out whether the rival local completion is generated as well. Therefore anomalous completions are added to the experimental design. Priming effects both on global and on local completions are compared with priming effects on those anomalous completions. The data indeed suggest that the occlusion patterns evoked not only a global but also a local completion.

Global Visual Completion of Quasi-Regular Shapes

The topic of amodal completion has often been investigated by using partly occluded shapes that are regular. In research that has typically been done with displays such as these regular shapes, it has been shown that global aspects of a shape can determine completion. To see how robust these global influences in the completion process are, we investigated quasi-regular shapes, ie shapes with a certain overall regularity but not based on metrical identities. First, in experiment 1 participants had to complete quasi-regular shapes in a drawing task. Then, in experiment 2 the primed-matching paradigm was used. Results from both experiments provided evidence for global completions. In experiment 3 we found that multiple global completions can be primed, which, as a control experiment showed, cannot be explained by some inability of the visual system to see the difference between the different completions. These data support the notion that global influences on visual occlusion are apparent even when the partly occluded stimulus is outside the domain of regular shapes. Implications for a global approach are provided.

From geons to structure. A note on object representation

Two models of object perception are compared: recognition by components (RBC), proposed by Biederman, and structural information theory (SIT), initially proposed by Leeuwenberg. According to RBC a complex object is decomposed into predefined elementary objects, called geons. According to SIT, the decomposition is guided by regularities in the object. It is assumed that the simplest of all possible interpretations of any object is perceptually preferred. The comparison deals with two aspects of the models. One is the representation of simple objects—various definitions of object axes are considered. It is shown that the more these definitions account for object regularity and thus the more they agree with SIT, the better the object representations predict object classification. Another topic concerns assumptions underlying the models: the identification of geons is mediated by cues which are supposed to be invariant under varying viewpoints of objects. It is argued that such cues are not based on this invariance but on the regularity of actual objects. The latter conclusion is in line with SIT. An advantage of RBC, however, is that it deals with the perceptual process from stimulus to interpretation, whereas SIT merely concerns the outcome of the process, not the process itself.

Liking versus Complexity: Decomposing the Inverted U-curve

The relationship between liking and stimulus complexity is commonly reported to follow an inverted U-curve. However, large individual differences among complexity preferences of participants have frequently been observed since the earliest studies on the topic. The common use of across-participant analysis methods that ignore these large individual differences in aesthetic preferences gives an impression of high agreement between individuals. In this study, we collected ratings of liking and perceived complexity from 30 participants for a set of digitally generated grayscale images. In addition, we calculated an objective measure of complexity for each image. Our results reveal that the inverted U-curve relationship between liking and stimulus complexity comes about as the combination of different individual liking functions. Specifically, after automatically clustering the participants based on their liking ratings, we determined that one group of participants in our sample had increasingly lower liking ratings for increasingly more complex stimuli, while a second group of participants had increasingly higher liking ratings for increasingly more complex stimuli. Based on our findings, we call for a focus on the individual differences in aesthetic preferences, adoption of alternative analysis methods that would account for these differences and a re-evaluation of established rules of human aesthetic preferences.

Different mental rotation strategies reflected in the rotation related negativity

In a mental rotation task of objects, typically, reaction time (RT) increases and the rotation related negativity (RRN) increases in amplitude with increasing angles of rotation. However, in a mental rotation task of hands, different RT profiles can be observed for outward and inward rotated hands. In the present study, we examined the neurophysiological correlates of these asymmetries in the RT profiles. We used a mental rotation task with stimuli of left and right hands. In line with previous studies, the behavioral results showed a linear increase in RT for outward rotations, but not for inward rotations as a function of angular disparity. Importantly, the ERP results revealed an RRN for outward rotated stimuli, but not for inward rotated stimuli. This is the first study to show that the behaviorally observed differences in a mental rotation task of hands is also reflected at the neurophysiological level.

The effect of chronic deafferentation on mental imagery : A case study

Visual- and motor imagery rely primarily on perceptual and motor processes, respectively. In healthy controls, the type of imagery used to solve a task depends on personal preference, task instruction, and task properties. But how does the chronic loss of proprioceptive and tactile sensory inputs from the body periphery influence mental imagery? In a unique case study, we investigated the imagery capabilities of the chronically deafferented patient IW when he was performing a mental rotation task. We found that IWs motor imagery processes were impaired and that visual imagery processes were enhanced compared to controls. These results suggest that kinaesthetic afferent signals from the body periphery play a crucial role in enabling and maintaining central sensorimotor representations and hence the ability to incorporate kinaesthetic information into the imagery processes.

Bricks, Butter, and Slices of Cucumber: Investigating semantic influences in amodal completion

Objects in our world are partly occluded by other objects or sometimes even partly by themselves. Amodal completion is a visual process that enables us to perceive these objects as complete and is influenced by both local object information, present at contour intersections, and overall (global) object shape. In contrast, object semantics have been demonstrated to play no role in amodal completion but do so only by means of subjective methods. In the present study, object semantics were operationalised by material hardness of familiar objects which was varied to test whether it leaves amodal completion unaffected. Specifically, we investigated the perceived form of joined naturalistic objects that differ in perceived material hardness, employing the primed matching paradigm. In experiments 1 and 2, probing three different prime durations, amodal completion of a notched circular object changes systematically with the hardness of the object it was joined to. These results are in line with the view that amodal completion is inseparable from general object interpretation, during which object semantics may dominate.

Perceptual grouping measured by color assimilation: Regularity versus proximity

Two powerful grouping principles in visual perception have been studied rather frequently, namely, regularity and proximity. In many cases, however, the impact of either regularity or proximity was examined. The goal of the present research is to provide and test an experimental paradigm on the basis of which the relative strengths of these grouping factors can be studied. The proposed paradigm is based on early Gestalt observations that colors of local elements within a perceptual whole tend to look more like each other than they actually are. Three experiments were set up to test whether this phenomenon of color assimilation can provide a means to compare perceptual groupings induced by regularity and proximity. In all experiments, the stimuli were linear configurations of colored elements with variable distances between the elements. The task of the participants was to compare the color of a single test element with the colors of two different groups of elements. In Experiment 1, for all colors, the color of the test element was judged to be more similar to the color of the elements with which it can be grouped by means of regularity. In Experiment 2, for specific colors, the color of the test element tended to be more similar to the elements with which it can be grouped by means of proximity. Finally, in Experiment 3, configurations with a greater variety of positions were tested, including those yielding conflicting groupings, that is, on the basis of regularity and proximity different groupings were to be expected. The color judgements for these stimuli clearly reflected the competition between the two factors.