Illusions Of Direction Research Articles

Visual objects interact differently during encoding and memory maintenance.

The storage mechanisms of working memory are the matter of an ongoing debate. The sensory recruitment hypothesis states that memory maintenance and perceptual encoding rely on the same neural substrate. This suggests that the same cortical mechanisms that shape object perception also apply to maintained memory content. We tested this prediction using the Direction Illusion, i.e., the mutual repulsion of two concurrently visible motion directions. Participants memorized the directions of two random dot patterns for later recall. In Experiments 1 and 2, we varied the temporal separation of spatially distinct stimuli to manipulate perceptual concurrency, while keeping concurrency within working memory constant. We observed mutual motion repulsion only under simultaneous stimulus presentation, but proactive repulsion and retroactive attraction under immediate stimulus succession. At inter-stimulus intervals of 0.5 and 2 s, however, proactive repulsion vanished, while the retroactive attraction remained. In Experiment 3, we presented both stimuli at the same spatial position and observed a reappearance of the repulsion effect. Our results indicate that the repulsive mechanisms that shape object perception across space fade during the transition from a perceptual representation to a consolidated memory content. This suggests differences in the underlying structure of perceptual and mnemonic representations. The persistence of local interactions, however, indicates different mechanisms of spatially global and local feature interactions.

Determinants of the direction illusion: motion speed and dichoptic presentation interact to reveal systematic individual differences in sign.

When two fields of dots with different directions of movement are presented in tandem, the perceived direction of one is biased by the presence of the other. Although this ‘‘direction illusion’’ typically involves repulsion, with an exaggeration of the perceived angular difference in direction between the dot fields, attraction effects, where the perceived difference is reduced, have also been found under certain presentation conditions. Earlier literature has been inconsistent, and there is debate surrounding the nature of the interactions that facilitate the direction illusion, as well as whether they occur at a local or global stage of the motion processing hierarchy. Here we measured the operating characteristics of the direction illusion by parametrically varying inducer contrast and coherence while examining the effects of stimulus speed and dichoptic presentation. It was found that the magnitude and sign of the direction illusion differed substantially from earlier research. Furthermore, there appeared to be significant interindividual variability, with dichoptic presentation producing an attractive rather than repulsive direction illusion in some participants.

Paradox lost: variable colour-pattern geometry is associated with differences in movement in aposematic frogs

Aposematic signal variation is a paradox: predators are better at learning and retaining the association between conspicuousness and unprofitability when signal variation is low. Movement patterns and variable colour patterns are linked in non-aposematic species: striped patterns generate illusions of altered speed and direction when moving linearly, affecting predators' tracking ability; blotched patterns benefit instead from unpredictable pauses and random movement. We tested whether the extensive colour-pattern variation in an aposematic frog is linked to movement, and found that individuals moving directionally and faster have more elongated patterns than individuals moving randomly and slowly. This may help explain the paradox of polymorphic aposematism: variable warning signals may reduce protection, but predator defence might still be effective if specific behaviours are tuned to specific signals. The interacting effects of behavioural and morphological traits may be a key to the evolution of warning signals.

Differential processing: Towards a unified model of direction and speed perception

In two experiments, we demonstrate a misperception of the velocity of a random-dot stimulus moving in the presence of a static line oriented obliquely to the direction of dot motion. As shown in previous studies, the perceived direction of the dots is shifted away from the orientation of the static line, with the size of the shift varying as a function of line orientation relative to dot direction (the statically-induced direction illusion, or 'SDI'). In addition, we report a novel effect - that perceived speed also varies as a function of relative line orientation, decreasing systematically as the angle is reduced from 90° to 0°. We propose that these illusions both stem from the differential processing of object-relative and non-object-relative component velocities, with the latter being perceptually underestimated with respect to the former by a constant ratio. Although previous proposals regarding the SDI have not allowed quantitative accounts, we present a unified formal model of perceived velocity (both direction and speed) with the magnitude of this ratio as the only free parameter. The model was successful in accounting for the angular repulsion of motion direction across line orientations, and in predicting the systematic decrease in perceived velocity as the line's angle was reduced. Although fitting for direction and speed produced different best-fit values of the ratio of underestimation of non-object-relative motion compared to object-relative motion (with the ratio for speed being larger than that for direction) this discrepancy may be due to differences in the psychophysical procedures for measuring direction and speed.

Challenging the distribution shift: Statically-induced direction illusion implicates differential processing of object-relative and non-object-relative motion

The direction illusion is the phenomenal exaggeration of the angle between the drift directions, typically, of two superimposed sets of random dots. The direction illusion is commonly attributed to mutual inhibition between direction-selective cell populations (distribution-shift model). A second explanation attributes the direction illusion to the differential processing of relative and non-relative motion components (differential processing model). Our first experiment demonstrates that, as predicted by the differential processing model, a static line can invoke a misperception of direction in a single set of dots – a phenomenon we refer to as the statically-induced direction illusion. In a second experiment, we find that the orientation of a static line can also influence the size of the conventional direction illusion. A third experiment eliminates the possibility that these results can be explained by the presence of motion streaks. While the results of these experiments are in agreement with the predictions made by the differential processing model, they pose serious problems for the distribution-shift account of shifts in perceived direction.

The Hierarchical Order of Processes Underlying the Direction Illusion and the Direction Aftereffect

Motion perception involves the processing of velocity signals through several hierarchical stages of the visual cortex. To better understand this process, a number of studies have sought to localise the neural substrates of two misperceptions of motion direction, the direction illusion (DI) and the direction aftereffect (DAE). These studies have produced contradictory evidence as to the hierarchical order of the processing stages from which the respective phenomena arise. We have used a simple stimulus configuration to further investigate the sequential order of processes giving rise to the DI and DAE. To this end, we measured the two phenomena invoked in combination, and also manually parsed this combined effect into its two constituents by measuring the two phenomena individually in both possible sequential orders. Comparing the predictions made from each order to the outcome from the combined effect allowed us to test the tenability of two models: the DAE-first model and the DI-first model. Our results indicate that DAE-invoking activity does not occur earlier in the motion processing hierarchy than DI-invoking activity. Although the DI-first model is not inconsistent with our data, the possible involvement of non-sequential processing may be better able to reconcile these results with those of previous studies.

Changing expectations about speed alters perceived motion direction

Summary Our perceptions are fundamentally altered by our knowledge of the world. When cloud-gazing, for example, we tend spontaneously to recognize known objects in the random configurations of evaporated moisture. How our brains acquire such knowledge and how it impacts our perceptions is a matter of heated discussion. A topic of recent debate has concerned the hypothesis that our visual system ‘assumes' that objects are static or move slowly [1] rather than more quickly [1–3]. This hypothesis, or ‘prior on slow speeds', was postulated because it could elegantly explain a number of perceptual biases observed in situations of uncertainty [2]. Interestingly, those biases affect not only the perception of speed, but also the direction of motion. For example, the direction of a line whose endpoints are hidden (as in the ‘aperture problem') or poorly visible (for example, at low contrast or for short presentations) is more often perceived as being perpendicular to the line than it really is — an illusion consistent with expecting that the line moves more slowly than it really does. How this ‘prior on slow speeds' is shaped by experience and whether it remains malleable in adults is unclear. Here, we show that systematic exposure to high-speed stimuli can lead to a reversal of this direction illusion. This suggests that the shaping of the brain's prior expectations of even the most basic properties of the environment is a continuous process.

Dichoptic reduction of the direction illusion is not due to binocular rivalry

Simultaneous direction repulsion (the direction illusion) occurs in bidirectional motion displays, typically transparent motion random dot kinematograms. Several laboratories have reported a greatly reduced illusion with dichoptic presentation of the two coherently translating stimuli as compared to monocular or binocular presentation. Some researchers have argued that those results might be due to a confounding factor, namely binocular rivalry occurring between test and inducing stimuli in the dichoptic condition, and so have attributed decisive weight to the results reported by Kim and Wilson (1997, Vision Research, 37, 991–1005) who used centre–surround grating stimuli and found large monocular as well as large dichoptic effects. Here we use centre–surround dot stimuli – with which no binocular rivalry occurs – to confirm a strong monocular contribution to the direction illusion. In addition, we fail to find evidence of a direction illusion with centre–surround grating stimuli, even when seeking to replicate the methods of Kim and Wilson (1997). In light of other evidence that a global motion-sensitive mechanism can determine the magnitude of the direction illusion, we propose that simultaneous direction repulsion can result from activity at multiple stages of the motion processing hierarchy.

Application of Riesz transforms to the isotropic AM-PM decomposition of geometrical-optical illusion images

The existence of a special second-order mechanism in the human visual system, able to demodulate the envelope of visual stimuli, suggests that spatial information contained in the image envelope may be perceptually relevant. The Riesz transform, a natural isotropic extension of the Hilbert transform to multidimensional signals, was used here to demodulate band-pass filtered images of well-known visual illusions of length, size, direction, and shape. We show that the local amplitude of the monogenic signal or envelope of each illusion image conveys second-order information related to image holistic spatial structure, whereas the local phase component conveys information about the spatial features. Further low-pass filtering of the illusion image envelopes creates physical distortions that correspond to the subjective distortions perceived in the illusory images. Therefore the envelope seems to be the image component that physically carries the spatial information about these illusions. This result contradicts the popular belief that the relevant spatial information to perceive geometrical-optical illusions is conveyed only by the lower spatial frequencies present in their Fourier spectrum.

The different mechanisms of the motion direction illusion and aftereffect

Direction repulsion is the illusory expansion of the angle between two directions of motion, and may occur when the two directions are presented simultaneously (an illusion) or successively (an aftereffect). Here we demonstrate that the motion direction illusion (DI) and aftereffect (DAE) have different mechanisms. Two experiments show that when the two interacting stimuli are presented to different eyes, the DI is greatly reduced but the DAE is obtained at near to full strength. These results suggest that different populations of cells within the visual pathway produce the DI and DAE.

A Spatially Oriented Decision Does Not Induce Consciousness in a Motor Task

Visual information follows at least two branches in the human nervous system, following a common input stage: a cognitive “what” branch governs perception and experience, while a sensorimotor “how” branch handles visually guided behavior though its outputs are unconscious. The sensorimotor system is probed with an isomorphic task, requiring a 1:1 relationship between target position and motor response. The cognitive system, in contrast, is probed with a forced qualitative decision, expressed verbally, about the location of a target. Normally, the cognitive system is influenced by context-induced illusions of visual direction, while the sensorimotor system is not. Here, we inquire whether the process of making a spatially based decision is critical in forcing subjects to use the information in the cognitive system for spatial tasks. Subjects hear a tone that determines whether they jab an “X” or an “O'' with the forefinger. Despite making a decision about which target to contact, the jab is not influenced by the position of a surrounding frame, indicating that choice can be handled within the context-insensitive sensorimotor system.

“Lost Altogether to the Civilised World”: Race and the Cabanagem in Northern Brazil, 1750 to 1850

The colonisation of the tropical lowlands of South America was until fairly recently often portrayed by historians as a straightforward catalogue of genocide, epidemics, and forced catechisation. See, for example, the work of John Hemming: Red Gold (London: Macmillan, 1978), Amazon Frontier (London: Macmillan, 1983), “Indians and the Frontier” in L. Bethell, ed., Colonial Brazil (Cambridge: Cambridge University Press, 1987), 145–89. In its emphasis on the undeniable fact of the violent subjection of much of Amerindia, this orthodoxy obscured the vital detail of the varieties of historical experience in the Amazon and Orinoco basins and the Guianas, projecting an illusion of uniformity and direction to historical processes which were in fact much more complicated and open-ended. N. Whitehead, “Ethnic Transformation and Historical Discontinuity in Native Amazonia and Guayana, 1500–1900,” L'Homme, 126–8, 13:4 (avril–decembre 1993), 285–305. A particularly good example of the complexity of “conquest” from the Indian perspective in the period immediately prior to the Cabanagem is N. Farage, As Muralhas dos Sertoes: Os Povos Indigenas no Rio Branco e a Colonizacao (Rio: Paz e Terra, 1991). Historians, anthropologists, and archaeologists have more recently emphasised the historically contingent nature of ethnic identity and traced out its empirical consequence, the interweaving of identity construction and ethnicity with colonialism, economic expansion and state formation from the sixteenth century onwards. General volumes covering all or most of the tropical lowlands are A. Roosevelt, ed., Amazonian Indians from Prehistory to the Present (Tucson: University of Arizona Press, 1994); E.V. de Castro and M.C. da Cunha, eds., Amazonia: Etnologia e Historia Indigena (Sao Paulo: FAPESP, 1993); M.C. da Cunha, ed., Historia dos Indios no Brasil (Sao Paulo: Companhia das Letras 1992) and G. Urban and J. Sherzer, eds., Nation States and Indians in Latin America (Austin: University of Texas Press, 1991). For the Orinoco and Guianas, see chapters by Whitehead and Arvelo-Jimenez and Biord in Roosevelt, Amazonian Indians ; Dreyfus, in Castro and da Cunha, Amazonia ; and N. Whitehead, Lords of the Tiger Spirit: A History of the Caribs in Colonial Venezuela and Guayana, 1498–1820 (Dordrecht and Providence: Foris Publications, 1988). Price's trilogy on the Saramaka Maroons of Surinam, First Time (Baltimore: Johns Hopkins University Press, 1983), To Slay the Hydra (Ann Arbor: Karoma Publishers, 1983), and Alabi's World (Baltimore: Johns Hopkins University Press 1990) is an example of a similar approach to an Afro-American ethnic formation. As a result, in certain parts of the tropical lowlands—notably the Guianas—it has been possible to produce a much more sophisticated reading of regional history, a “thick description” which can relate the fluidity and specificity of local circumstances to wider historical processes and can thus serve as the basis for a properly comparative mapping of the past.

The Phantom Array: A Perisaccadic Illusion of Visual Direction

Naive observers (N = 75) were asked to saccade in the dark across a point light source blinking on and off at 200 Hz and to describe the resultant phantom array (Hershberger, 1987). The vast majority represented this perisaccadic illusion of visual direction essentially as Hershberger described it. Replicable features of the phantom array imply that the perisaccadic shift of retinal local signs (i.e., spatiotopic coordinates) is discontinuous, comprising two separate parts, a discrete partial shift that occurs before the eyes start to move, and a continuous partial shift that is completed at about the same time as the eye movement. Although these implications are consistent with recent experimental findings, they are inconsistent with the received view that retinal local signs shift sluggishly. The phantom array implies that the faster, presaccadic, partial shift has a time constant of 5 ms or less.

Influence of acoustic context on sound localization: An auditory Roelofs effect

The Roelofs effect is a visual direction illusion: if a large rectangular frame is seen offset from the straight-ahead direction, a small target presented simultaneously is mislocalized in the opposite direction. To investigate whether a similar context illusion might affect auditory localization, we presented a “frame” of 6 speakers driven with a 300-Hz square wave, 30° left or right of center. The “target” was a speaker driven with the same waveform, with the two sources in random phase relationship. The target was mislocalized in a direction opposite the frame, an auditory Roelofs effect. A second experiment, using dissimilar sounds for frame and target, yielded no frame-dependent mislocalizations. The effect appeared both in verbal position estimation, a measure of cognitive localization, and in open-loop pointing, a measure of localization in a sensorimotor system. We conclude that audition possesses only one representation of space, in contrast to the two (cognitive and sensorimotor) of vision. The auditory representation corresponds most closely to vision's cognitive system.

Application of a cellular neural network to facial expression animation and high-level image processing

The cellular neural network (CNN) is used to animate facial expressions of a human being. First, the change in facial expressions is regarded as a smooth 2D transformation which is restricted by a bending energy function and displacements of some key-points. Second, the parameters of the CNN are determined by comparing the bending energy function with the energy function of the CNN. Finally, the CNN is used to realize the transformation by minimizing its energy function. Also, the CNN is used to model some visual illusions which are frequently used in psychological tests. First, the retinal induction field is modelled by using a template. The comparison of this CNN model with the real physiological measurements is presented. Then, based on this template, the CNN universal machine is used to model four types of visual illusions: subjective contour illusion (Kanizsa illusion), size illusion (Mueller-Lyer illusion, Ponzo illusion), direction and location illusion (angular illusion of location, Poggendorff illusion) and contrast illusion (Herring illusion). Computer simulations are provided for animating facial expressions and modelling visual illusions.

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Around 25% of patients with visual hallucinations secondary to eye disease report hallucinations of text. The hallucinated text conveys little if any meaning, typically consisting of individual letters, words, or nonsense letter strings (orthographic hallucinations). A patient is described with textual visual hallucinations of a very different linguistic content following bilateral occipito-temporal infarcts. The hallucinations consisted of grammatically correct, meaningful written sentences or phrases, often in the second person and with a threatening and command-like nature (syntacto-semantic visual hallucinations). A detailed phenomenological interview and visual psychophysical testing were undertaken. The patient showed a classical ventral occipito-temporal syndrome with achromatopsia, prosopagnosia, and associative visual agnosia. Of particular significance was the presence of pure alexia. Illusions of colour induced by monochromatic gratings and a novel motion–direction illusion were also observed, both consistent with the residual capacities of the patient’s spared visual cortex. The content of orthographic visual hallucinations matches the known specialisations of an area in the left posterior fusiform gyrus—the visual word form area (VWFA)—suggesting the two are related. The VWFA is unlikely to be responsible for the syntacto-semantic hallucinations described here as the patient had a pure alexic syndrome, a known consequence of VWFA lesions. Syntacto-semantic visual hallucinations may represent a separate category of textual hallucinations related to the cortical network implicated in the auditory hallucinations of schizophrenia.

Individual differences in visual-geometric illusions: predictions from measures of spatial cognitive abilities.

A sample of 490 observers was tested on 26 illusion variants and five tests of spatial ability. There was some suggestion that overall perceptual accuracy was related to perceptual ability. More importantly, the individual differences in the magnitude of visual illusions scores were significantly predicted by spatial abilities measures. The general relationship suggested that higher levels of spatial ability were associated with reduced illusion magnitude; however, a canonical correlation analysis revealed that the direction of this relationship depended on the type of illusion. Illusions of linear extent showed an inverse relationship between the two sets of measures, with higher levels of spatial abilities associated with lower degrees of illusion susceptibility. High spatial skills scores were related positively to illusion magnitude for illusions of area and direction.

An efferent component in the visual perception of direction and extent.

After outlining the history of motor theories of visual perception, a new theory linking information extraction patterns, specifically adapted for the guidance of eye movements, to the visual perception of direction and extent is presented. Following a brief discussion of comparative and physiological considerations, a research strategy to test for efferent involvement in visual perception in humans is presented. In seven demonstration experiments, predictions from efferent considerations are used to create a new set of illusions of direction and extent and to demonstrate new predictable variations in the magnitude of some classical illusion figures. Another demonstration illustrates that systematic changes in visual perception occur as a function of changes in motoric demands, even in the absence of any configurational changes in the stimulus. A final section shows the relationship between attention and efferent readiness and their interaction in the formation of the conscious visual percept. From a historical perspective, most contemporary theories of visual perception are quite conservative. This conservatism springs from an apparent acceptance of the premise that any proper analysis of visual experience must avoid reference to nonvisual mechanisms, except for labeling and semantic aspects of the perceptual process. It follows that most visual theorists tend to derive virtually every aspect of the conscious percept solely from either the physical characteristics of the visual stimulus array or the operation of readily definable neurological units in the visual system. Characteristic of the former viewpoint is Gibson's (1979) theory of ecological optics, which maintains that virtually all aspects of the final percept are predictable from invariants in the stimulus array. Current attempts to derive the conscious percept from a hypothesized Fourier analysis occurring within the visual system are similar in approach, merely relying on higher

The Poggendorff illusion: an illusion of linear extent?

In the standard version of the Poggendorff figure a transversal intersects two parallel verticals and the segment of the transversal between the two intersection points, A and B, is not shown. The two portions of the transversal outside the parallels then seem to be misaligned. Besides this illusion of direction, there is also an illusion of size, the distance AB being underestimated in the standard figure. The influence of configural components in determining this spatial distortion of the Poggendorff figure was examined by having subjects reproduce the inner oblique (at 45 degrees) extent AB in variations of the figure. This distance was found to be underestimated in Poggendorff variations which contained parallel (vertical) components that formed an acute angle with AB; and the underestimation increased as the number of these components present in the figure increased. The distance AB was found not be significantly distorted in figures which contained only those parallel (vertical) components that formed an obtuse angle with AB, yet their presence in the figure tended to counteract the underestimation. When the transversals were omitted, the underestimation was found to increase. The findings are interpreted in support of an explanation that reduces the Poggendorff effect to those factors which mediate the Müller-Lyer illusion.

Size-analysis of retinal images by orientation detectors.

A model for the determination of retinal-image size is presented. The size-analysis is based upon the range of orientation detectors activated by a stimulus. The model is applied to size aftereffects and is also used to predict changes in perceived size in configurations which may be expected to affect the range of orientation detectors activated. The relevance of the model to illusions of direction and the perceived length of lines forming angles is also discussed.