Visual search for apparent-length targets is modulated by the Müller-Lyer illusion

Is apparent object size represented in pre-attentive vision and can it influence visual search for size-defined targets in a spatially parallel manner? This question was investigated, using the Müller-Lyer illusion. Observers searched for a target line that was longer than the distractor lines. Test lines could be presented without context arrows (control); be adjoined by obtuse-angle context arrows (arrow heads pointing inward), making the lines appear longer; or by acute-angle arrows (heads pointing outward), making the lines appear shorter. These apparent-length modulations were larger for the target than for the distractor lines, thereby increasing and, respectively, decreasing the target-distractor length contrast. In line with these changes in contrast, target detection was found to be expedited by obtuse-angle arrows and impeded by acute-angle arrows, independently of the number of elements in the display. This finding provides further evidence for the pre-attentive processing of apparent object size.     

Features of perception of length of segments under conditions of ponzo and Müller-Lyer illusions in schizophrenia

In order to better appreciate the neurophysiologic mechanisms of perception of length under conditions of geometrical visual illusions, we studied sensitivity of mentally healthy subjects and schizophrenic patients to Ponzo and Müller-Lyer illusion. Patients with schizophrenia estimated length of segments of Müller-Lyer figure less precisely. Accuracy of perception of length of segments in Ponzo figure was ambiguously connected with the duration of the disease. Persons suffering from schizophrenia for a short time were less inclined to Ponzo illusion than mentally healthy subjects. On the contrary, patients who suffered from schizophrenia for a long time were more sensitive to this illusion. Ponzo illusion can be used as a marker of schizophrenia which is found out during the specific period of development of the disease. High sensitivity of patients with schizophrenia to Müller-Lyer and Ponzo illusions supports a hypothesis about the role of the global analysis of an image during processing of its low-frequency component in formation of the illusions under study.     

The Müller-Lyer Illusion in Ant Foraging

The Müller-Lyer illusion is a classical geometric illusion in which the apparent (perceived) length of a line depends on whether the line terminates in an arrow tail or arrowhead. This effect may be caused by economic compensation for the gap between the physical stimulus and visual fields. Here, we show that the Müller-Lyer illusion can also be produced by the foraging patterns of garden ants (Lasius niger) and that the pattern obtained can be explained by a simple, asynchronously updated foraging ant model. Our results suggest that the geometric illusion may be a byproduct of the foraging process, in which local interactions underlying efficient exploitation can also give rise to global exploration, and that visual information processing in human could implement similar modulation between local efficient processing and widespread computation.     

Study of parvocellular and magnocellular visual channels in normal subjects and in patients with psychopathology

We measured susceptibility to the Müller-Lyer illusion in schizophrenic patients and normal observers. The images of the Müller-Lyer figure were digitally filtered in a high-frequency and low-frequency band by wavelet filter. Patients with schizophrenia are more susceptible to Müller-Lyer illusion, than mentally healthy examinees. The images of the Müller-Lyer figure with low spatial frequency were perceived in a similar way by the schizophrenic patients on the initial stage of disease and the control subjects. Patients with schizophrenia were more sensitive to the Müller-Lyer illusion when the image contained only high or medium spatial frequency. Schizophrenic patients in advanced stage were more susceptible to the illusion while presented with all types of images of the Müller-Lyer figure than the control group. It is hypothesized that those differences arise from the mismatch work of the magnocellular and parvocellular systems. It is known that images with the high spatial frequencies are most relevant for the parvocellular visual channels. The magnocellular visual channels are most sensitive to the images with the low spatial frequencies. Thus these findings demonstrate a significant impairment in parvocellular pathway function in patients on initial stage of schizophrenia. The patients on advanced stage of schizophrenia demonstrate impairment of both the parvocellular and magnocellular systems.     

The Muller-Lyer illusion explained and its theoretical importance reconsidered

The Müller-Lyer illusion is the natural consequence of the construction of the vertebrate eye, retina and visual processing system. Due to imperfections in the vertebrate eye and retina and due to the subsequent processing in the system by ever increasing receptive fields, the visual information becomes less and less precise with respect to exact location and size. The consequence of this is that eventually the brain has to calculate a weighted mean value of the information, which is spread out over a population of neurons. In the case of the Müller-Lyer illusion this inevitably leads to extension of one and reduction of the other line. The arguments presented explain several published experimental results concerning the Müller-Lyer illusion and shed new light upon the philosophical neutrality of observation sentences.     

Orientation selectivity and visual acuity in schoolchildren and adults

We continued our study of the mechanisms of visual acuity (VA) in ontogenesis. We measured the VA and sensitivity to lines orientation and determined the minimal length of lines for discrimination of the vertical and the horizontal ones in subjects aged 7–25 years. It was found that the thresholds of discrimination of vertical and horizontal lines in subjects with normal VA decreased with age up to 9–10 years and then remained constant, while the orientation selectivity was improving up to the age of 14–16 years. The average VA almost did not depend on age. Individual thresholds of line lengths and orientation discrimination correlated with the VA of subjects.     

Age-related size invariance in perception of illusory and fragmented contours

We investigated invariant perception to sizes of images. Observers were secondary school students aged 7–17 years and adults. Two types of stimuli were used: fragmented line drawing of common objects and discs with deleted sectors which represented illusory Kanizsa contours, when discs were in particular positions. In experiments with fragmented images, we found an improvement in image recognition with observer’s age, increasing up to 13–14 years. The probability of recognition of fragmented line drawings increased significantly with decreasing stimulus size for children aged 7–12 years, indicating that size invariance at recognition for fragmented line drawings was absent in these children. However, size invariance was found for observers aged 7–12 years and for adults in this task. Upon the Kanizsa illusion appearance, the ratio of the separation between discs and disc diameter was smaller when we used larger disc diameters. This ratio increased with increasing age of observers. The obtained results provide evidence for the absence of size invariance when perceiving the Kanizsa illusion under our experimental conditions.     

Age related size invariance in perception of illusory and fragmented contours

We investigated invariant perception to sizes of images. Observers were schoolmates of 7-17 years and adults. Two types of stimuli were used: fragmented line drawing of common objects and discs with deleted sectors, which represented illusory Kanizsa contours when discs were in particular positions. In experiments with fragmented images, we found an improvement in image recognition with observers' age, increasing up to 13-14 years. The probability of recognition of fragmented line drawings increased significantly with decreasing stimulus size for children of 7-12 years, indicating that size invariance at recognition for fragmented line drawings was absent in these children. However, size invariance was found for observers of 13-17 years and for adults in this task. At Kanizsa illusion appearance, the ratio of the separation between discs and disc diameter was smaller when we used larger disc diameters. This ratio increased with increasing age of observers. Obtained results provide evidence for the absence of size invariance when perceiving the Kanizsa illusion in our experimental conditions.     

Geometrical illusions: study and modelling

The phenomena of geometrical illusions of extent suggest that the metric of a perceived field is different from the metric of a physical stimulus. The present study investigated the Müller-Lyer and Oppel-Kundt illusions as functions of spatial parameters of the figures, and constructed a neurophysiological model. The main idea of the modelling is based on the uncertainty principle, according to which distortions of size relations of certain parts of the stimulus, so-called geometrical illusions, are determined by processes of spatial filtering in the visual system. Qualitative and quantitative agreement was obtained between psychophysical measurement of the strength value of the illusions and the predictions of our model. Received: 18 June 1996 / Accepted in revised form: 24 June 1997     

The Impact of Spatial Incongruence on an Auditory-Visual Illusion

Background

The sound-induced flash illusion is an auditory-visual illusion – when a single flash is presented along with two or more beeps, observers report seeing two or more flashes. Previous research has shown that the illusion gradually disappears as the temporal delay between auditory and visual stimuli increases, suggesting that the illusion is consistent with existing temporal rules of neural activation in the superior colliculus to multisensory stimuli. However little is known about the effect of spatial incongruence, and whether the illusion follows the corresponding spatial rule. If the illusion occurs less strongly when auditory and visual stimuli are separated, then integrative processes supporting the illusion must be strongly dependant on spatial congruence. In this case, the illusion would be consistent with both the spatial and temporal rules describing response properties of multisensory neurons in the superior colliculus.

Methodology/Principal Findings

The main aim of this study was to investigate the importance of spatial congruence in the flash-beep illusion. Selected combinations of one to four short flashes and zero to four short 3.5 KHz tones were presented. Observers were asked to count the number of flashes they saw. After replication of the basic illusion using centrally-presented stimuli, the auditory and visual components of the illusion stimuli were presented either both 10 degrees to the left or right of fixation (spatially congruent) or on opposite (spatially incongruent) sides, for a total separation of 20 degrees.

Conclusions/Significance

The sound-induced flash fission illusion was successfully replicated. However, when the sources of the auditory and visual stimuli were spatially separated, perception of the illusion was unaffected, suggesting that the “spatial rule” does not extend to describing behavioural responses in this illusion. We also find no evidence for an associated “fusion” illusion reportedly occurring when multiple flashes are accompanied by a single beep.     

The dissociation between perception and action in the Ebbinghaus illusion: nonillusory effects of pictorial cues on grasp

According to a recently proposed distinction [1] between vision for perception and vision for action, visually guided movements should be largely immune to the perceptually compelling changes in size produced by pictorial illusions. Tests of this prediction that use the Ebbinghaus illusion have revealed only small effects of the illusion on grasp scaling as compared to its effect on perception [2-4]. Nevertheless, some have argued that the small effect on grasp implies that there is a single representation of size for both perception and action [5]. Recent findings, however, suggest that the 2-D pictorial elements, such as those comprising illusory backgrounds, can sometimes be treated as obstacles and thereby influence the programming of grasp [6]. The arrangement of the 2-D elements commonly used in previous studies examining the Ebbinghaus illusion could therefore give rise to an effect on grasp scaling that is independent of its effect on perceptual judgements, even though the two effects are in the same direction. We present evidence demonstrating that when the gap between the target and the illusion-making elements in the Ebbinghaus illusion is equidistant across different perceptual conditions (Figure 1a), the apparent effect of the illusion on grasp scaling is eliminated.     

Head-centred motion perception in the ageing visual system

Stationary objects appear to move in the opposite direction to a pursuit eye movement (Filehne illusion) and moving objects appear slower when pursued (Aubert-Fleischl phenomenon). Both illusions imply that extra-retinal, eye-velocity signals lead to lower estimates of speed than corresponding retinal motion signals. Intriguingly, the velocity (i.e. speed and direction) of the Filehne illusion depends on the age of the observer, especially for brief display durations (Wertheim and Bekkering, 1992). This suggests relative signal size changes as the visual system matures. To test the signal-size hypothesis, we compared the Filehne illusion and Aubert-Fleischl phenomenon in young and old observers using short and long display durations. The trends in the Filehne data were similar to those reported by Wertheim and Bekkering. However, we found no evidence for an effect of age or duration in the Aubert-Fleischl phenomenon. The differences between the two illusions could not be reconciled on the basis of actual eye movements made. The findings suggest a more complicated explanation of the combined influence of age and duration on head-centred motion perception than that described by the signal-size hypothesis.     

Involvement of the Extrageniculate System in the Perception of Optical Illusions: A Functional Magnetic Resonance Imaging Study

Research on the neural processing of optical illusions can provide clues for understanding the neural mechanisms underlying visual perception. Previous studies have shown that some visual areas contribute to the perception of optical illusions such as the Kanizsa triangle and Müller-Lyer figure; however, the neural mechanisms underlying the processing of these and other optical illusions have not been clearly identified. Using functional magnetic resonance imaging (fMRI), we determined which brain regions are active during the perception of optical illusions. For our study, we enrolled 18 participants. The illusory optical stimuli consisted of many kana letters, which are Japanese phonograms. During the shape task, participants stated aloud whether they perceived the shapes of two optical illusions as being the same or not. During the word task, participants read aloud the kana letters in the stimuli. A direct comparison between the shape and word tasks showed activation of the right inferior frontal gyrus, left medial frontal gyrus, and right pulvinar. It is well known that there are two visual pathways, the geniculate and extrageniculate systems, which belong to the higher-level and primary visual systems, respectively. The pulvinar belongs to the latter system, and the findings of the present study suggest that the extrageniculate system is involved in the cognitive processing of optical illusions.     

Subjective Size Perception Depends on Central Visual Cortical Magnification in Human V1

In the Ebbinghaus illusion, the context surrounding an object modulates its subjectively perceived size. Previous work implicates human primary visual cortex (V1) as the neural substrate mediating this contextual effect. Here we studied in healthy adult humans how two different types of context (large or small inducers) in this illusion affected size perception by comparing each to a reference stimulus without any context. We found that individual differences in the magnitudes of the illusion produced by either type of context were correlated with V1 area defined through retinotopic mapping using functional MRI. However, participants'' objective ability to discriminate the size of objects presented in isolation was unrelated to illusion strength and did not correlate with V1 area. Control analyses showed no correlations between behavioral measures and the overall V1 area estimated probabilistically on the basis of neuroanatomy alone. Therefore, subjective size perception correlated with variability in central cortical magnification rather than the anatomical extent of primary visual cortex. We propose that such changes in subjective perception of size are mediated by mechanisms that scale with the extent to which an individual''s V1 selectively represents the central visual field.     

Limited Energy Supply in Müller Cells Alters Glutamate Uptake

The viability of retinal ganglion cells (RGC) is essential for the maintenance of visual function. RGC homeostasis is maintained by the surrounding retinal glial cells, the Müller cells, which buffer the extracellular concentration of neurotransmitters and provide the RGCs with energy. This study evaluates if glucose-deprivation of Müller cells interferes with their ability to remove glutamate from the extracellular space. The human Müller glial cell line, Moorfields/Institute of Ophthalmology-Müller 1, was used to study changes in glutamate uptake. Excitatory amino acid transporter (EAAT) proteins were up-regulated in glucose-deprived Müller cells and glutamate uptake was significantly increased in the absence of glucose. The present findings revealed an up-regulation of EAAT1 and EAAT2 in glucose-deprived Müller cells as well as an increased ability to take up glutamate. Hence, glucose deprivation may result in an increased ability to protect RGCs from glutamate-induced excitotoxicity, whereas malfunction of glutamate uptake in Müller cells may contribute to retinal neurodegeneration.     

Grey matter volume in early human visual cortex predicts proneness to the sound-induced flash illusion

Visual perception can be modulated by sounds. A drastic example of this is the sound-induced flash illusion: when a single flash is accompanied by two bleeps, it is sometimes perceived in an illusory fashion as two consecutive flashes. However, there are strong individual differences in proneness to this illusion. Some participants experience the illusion on almost every trial, whereas others almost never do. We investigated whether such individual differences in proneness to the sound-induced flash illusion were reflected in structural differences in brain regions whose activity is modulated by the illusion. We found that individual differences in proneness to the illusion were strongly and significantly correlated with local grey matter volume in early retinotopic visual cortex. Participants with smaller early visual cortices were more prone to the illusion. We propose that strength of auditory influences on visual perception is determined by individual differences in recurrent connections, cross-modal attention and/or optimal weighting of sensory channels.     

Factors contributing to depth perception: behavioral studies on the reverse perspective illusion

In three behavioral experiments using depth-inverted visual stimuli, the factors that contribute to the 'reverse perspective' illusion were measured. The density of linear perspective grid lines was found to induce the illusion most strongly, followed by shading/shadows, and texture/color information. The relative contributions of such pictorial cues to depth perception are similar to those that facilitate the normal perception of 3D space in 2D paintings.     

Lifting without Seeing: The Role of Vision in Perceiving and Acting upon the Size Weight Illusion

Background

Our expectations of an object''s heaviness not only drive our fingertip forces, but also our perception of heaviness. This effect is highlighted by the classic size-weight illusion (SWI), where different-sized objects of identical mass feel different weights. Here, we examined whether these expectations are sufficient to induce the SWI in a single wooden cube when lifted without visual feedback, by varying the size of the object seen prior to the lift.

Methodology/Principal Findings

Participants, who believed that they were lifting the same object that they had just seen, reported that the weight of the single, standard-sized cube that they lifted on every trial varied as a function of the size of object they had just seen. Seeing the small object before the lift made the cube feel heavier than it did after seeing the large object. These expectations also affected the fingertip forces that were used to lift the object when vision was not permitted. The expectation-driven errors made in early trials were not corrected with repeated lifting, and participants failed to adapt their grip and load forces from the expected weight to the object''s actual mass in the same way that they could when lifting with vision.

Conclusions/Significance

Vision appears to be crucial for the detection, and subsequent correction, of the ostensibly non-visual grip and load force errors that are a common feature of this type of object interaction. Expectations of heaviness are not only powerful enough to alter the perception of a single object''s weight, but also continually drive the forces we use to lift the object when vision is unavailable.     

A first- and second-order motion energy analysis of peripheral motion illusions leads to further evidence of "feature blur" in peripheral vision

Background

Anatomical and physiological differences between the central and peripheral visual systems are well documented. Recent findings have suggested that vision in the periphery is not just a scaled version of foveal vision, but rather is relatively poor at representing spatial and temporal phase and other visual features. Shapiro, Lu, Huang, Knight, and Ennis (2010) have recently examined a motion stimulus (the “curveball illusion”) in which the shift from foveal to peripheral viewing results in a dramatic spatial/temporal discontinuity. Here, we apply a similar analysis to a range of other spatial/temporal configurations that create perceptual conflict between foveal and peripheral vision.

Methodology/Principal Findings

To elucidate how the differences between foveal and peripheral vision affect super-threshold vision, we created a series of complex visual displays that contain opposing sources of motion information. The displays (referred to as the peripheral escalator illusion, peripheral acceleration and deceleration illusions, rotating reversals illusion, and disappearing squares illusion) create dramatically different perceptions when viewed foveally versus peripherally. We compute the first-order and second-order directional motion energy available in the displays using a three-dimensional Fourier analysis in the (x, y, t) space. The peripheral escalator, acceleration and deceleration illusions and rotating reversals illusion all show a similar trend: in the fovea, the first-order motion energy and second-order motion energy can be perceptually separated from each other; in the periphery, the perception seems to correspond to a combination of the multiple sources of motion information. The disappearing squares illusion shows that the ability to assemble the features of Kanisza squares becomes slower in the periphery.

Conclusions/Significance

The results lead us to hypothesize “feature blur” in the periphery (i.e., the peripheral visual system combines features that the foveal visual system can separate). Feature blur is of general importance because humans are frequently bringing the information in the periphery to the fovea and vice versa.     

ANATOMICAL DEVELOPMENT OF THE LEAF TRICHILIUM AND MÜLLERIAN BODIES OF CECROPIA PELTATA L.

The anatomical development of trichilia and Müllerian bodies of Cecropia peltata was investigated at the light microscope level. The petiole base is initiated in a normal manner and the abaxial surface becomes keel-shaped during later stages of development. At maturity the trichilium is well supplied with vascular tissue but no transfer cells were observed. Before Müllerian body development, trichilium epidermal cells become modified into two types of trichomes. Müllerian bodies are initiated in sub-epidermal tissue and by continued cell division and expansion reach a size of 1 mm wide X 3 mm in length. There is no storage polysaccharide present during early stages of Müllerian body development. Glycogen production and storage begins as the food body reaches three-fourths final size and continues after cell division ceases. The mature Müllerian body is mutlicellular and rich in lipid and glycogen.