Hermann Grid Illusion Research Articles

The Hermann grid illusion fails to fool patients with schizophrenia: Experimental support for a reduced lateral inhibition hypothesis

A large body of evidence indicated that patients with schizophrenia spectrum disorders show deficit in perceptual and visual processing, but little is known about the origin of this defect. One of the strategies used to reveal the origin of such defects is to use tests that have a strong neuropsychological basis in normal individuals. In this study, we used the Hermann grid task to reveal the source of visual deficit in patients with schizophrenia. Several findings have shown that the Hermann grid illusion results from lateral inhibition of neuro-visual pathway cells. We compared 26 patients with schizophrenia to 26 participants with high schizotypy traits and 26 normal controls in a computer test we designed for the Hermann grid illusion. Chi-square analysis revealed that the patients with schizophrenia and the participants with high schizotypy traits reported significantly less the Hermann grid illusion than healthy controls. Therefore, the results support the hypothesis of a defect in lateral inhibition of patients with schizophrenia. This might suggest that the visual pathway cells in schizophrenic patients failed to properly combine the data from the visual receptors as they do in healthy people. • The Hermann grid illusion has wealthy neuropsychological background. • Visual perception difficulties in schizophrenia have been always reported. • Patients with schizophrenia do not report the Hermann grid illusion. • Evidence show that lateral inhibition contributes in the Hermann grid illusion. • Lack of Hermann grid illusion support a lateral inhibition defect in schizophrenia.

The Mechanisms of the Visual System Behind Visual Illusions: From Eye to Brain

Visual illusion is a psychological phenomenon characterized by perception that appears to differ from physical reality. Illusory perception persists even though the sufferers are aware of the physical properties of what they are observing. Thus, studying visual illusions has led to an improved understanding of the neural mechanism underlying visual information processing. Visual illusions are important tools in neuroscience. Some brightness illusions, such as the Hermann grid illusion or Chevreul illusion, can be explained by the function of the center-surround antagonistic receptive field of retinal ganglion cells. Additionally, color aftereffects were found to be produced in the retina. In addition, neurons in the primary visual cortex are responsible for the orientation-contingent color aftereffect, known as the McCollough effect. ISI reversal, a visual motion illusion, is known to be caused by a biphasic temporal mechanism located in the retinal or lower visual areas. Higher visual areas are responsible for the production of visual illusions such as the Ponzo illusion, size constancy illusion, or tabletop illusion. These illusions are perceived through the process of achieving size constancy from a two-dimensional retinal image. Thus, both low-level and high-level vision are involved in the perception of visual illusions.

How comparatively widespread are visual illusions?

Researchers and artists have long been interested in visual illusions because they illustrate the interesting, complicated, and constrained ways in which we perceive the world. Although we may not be familiar with the names of the many different visual illusions that exist (e.g., the Necker cube, the Müller-Lyer illusion, and the Hermann grid illusion, to name a few), people with typically functioning will certainly have seen many of these. We have known for centuries that humans perceive these illusions. We have known for the past few decades that nonhuman mammals can perceive these illusions, and very recent work has revealed that birds and fish perceive some of these illusions, though sometimes in a way opposite to how our own species does. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Computational Mechanisms Responsible for the Hermann Grid Illusion

Computational Mechanisms Responsible for the Hermann Grid Illusion

Alcohol and Lateral Inhibitory Interactions in Human Vision

Acute alcohol consumption detrimentally affects many aspects of visual function, but few studies have addressed the neural mechanisms underlying such changes. One candidate mechanism that may be responsible for some alcohol-induced changes in visual function is lateral inhibition. Alcohol has been shown to abolish lateral inhibitory interactions in experimental preparations in which it is applied directly to the retina, but few studies have attempted to link alcohol-induced reductions in lateral inhibitory interactions with psychophysical performance in assessments of visual function dependent on this mechanism. In the present series of studies we addressed this by investigating the effects of alcohol consumption on a perceptual phenomenon mediated in part by lateral inhibition, the Hermann grid illusion. Participants estimated the contrast of the illusory blobs present at the grid intersections using a matching procedure after consumption of a drink containing alcohol or a nonalcoholic drink. The magnitude of the illusion was diminished in the alcohol condition, and this effect was consistent when we parametrically varied the contrast of the grid squares and widths of the grid bars. These data suggest that alcohol reduces lateral inhibitory interactions in human vision.

The Tilted Hermann Grid Illusion: ‘Illusory Spots’ versus ‘Phantom Bands’

The Hermann grid illusion became a cause célèbre, when it was reported that small figural changes from straight to curved bars abolish the dark illusory spots. We demonstrate that this is not an all-or-none effect; rather, the visual system tolerates some tilt/curviness. We transformed straight and curved Hermann grids to rhombic Motokawa grids by gradually tilting the horizontal bars. Initially, we observed only dark illusory spots, then dark spots combined with phantom bands traversing the rhomb along the minor axis, and finally dark phantom bands only. This shows that two kinds of illusions can coexist in the same grid pattern.

A direct estimate of the size of the illusory spots in the Hermann Grid Illusion

A direct estimate of the size of the illusory spots in the Hermann Grid Illusion

The chromatic Hermann grid illusion for stimuli equated in chroma

The chromatic Hermann grid illusion for stimuli equated in chroma

Capturing Lightness between Contours

Homogeneously coloured bars may exhibit lightness differences at the intersections. A well-known example is the Hermann grid illusion, where crossing white bars on a black background show dark patches at the crossings. Jung (1973, Handbook of Sensory Physiology volume VII/3, pp 1-152) found that the dark patches persist when thin outlines are drawn at the intersections, and are even visible in foveal vision. Recently, it has been shown that making distortions to the contours of a Hermann grid-like configuration results in the disappearance of the illusory dark spots (Geier et al, 2008 Perception 37 651 665). We show that thin outlines at the crossings of the distorted Hermann grid induce lightness differences in the same direction as in the original Hermann grid illusion, even in foveal vision and in displays consisting of two crossing bars. Our experiments reveal that the induced lightness differences are independent of the luminance polarity and shape of the contours at the intersection. We suggest that the effect results from lateral inhibition and an additional spreading and capturing of these differences between luminance contours. A similar capturing between collinear contours may play a role in peripheral vision in the original Hermann grid.

Die Hermann-Gitter-Täuschung: Lehrbucherklärung widerlegt

The Hermann grid is an optical illusion in which the crossings of white grid lines appear darker than the grid lines outside the crossings. The illusion disappears when one fixates the crossings. The discoverer, Ludimar Hermann (1838-1914), interpreted the illusion as evidence for lateral connections in the retina. In most textbooks on sensory physiology and ophthalmology, the Hermann grid illusion serves to illustrate "lateral inhibition." This paper summarises new findings that show that the classic explanation is incomplete. In 2004, a seemingly subtle modification, a small undulation of the grid lines, was shown to demolish the illusion. In 2007, a more convincing explanation appeared: An artificial neural network was trained for "lightness constancy"- the ability of our visual system to interpret luminance in the interest of object recognition, independent of illumination. After having learned lightness constancy, the network was subjected to a number of lightness illusions, among them the Hermann grid illusion. An analysis of the coupling constants of this neural network promises to further our understanding of the Hermann grid illusion.

Straightness as the Main Factor of the Hermann Grid Illusion

The generally accepted explanation of the Hermann grid illusion is Baumgartner's hypothesis that the illusory effect is generated by the response of retinal ganglion cells with concentric ON-OFF or OFF-ON receptive fields. To challenge this explanation, we have introduced some simple distortions to the grid lines which make the illusion disappear totally, while all preconditions of Baumgartner's hypothesis remain unchanged. To analyse the behaviour of the new versions of the grid, we carried out psychophysical experiments, in which we measured the distortion tolerance: the level of distortion at which the illusion disappears at a given type of distortion for a given subject. Statistical analysis has shown that the distortion tolerance is independent of grid-line width within a wide range, and of the type of distortion, except when one side of each line remains straight. We conclude that the main cause of the Hermann grid illusion is the straightness of the edges of the grid lines, and we propose a theory which explains why the illusory spots occur in the original Hermann grid and why they disappear in curved grids.

The Use of the Cancellation Technique to Quantify the Hermann Grid Illusion

When observers view a grid of mid-gray lines superimposed on a black background, they report seeing illusory dark gray smudges at the grid intersections, an effect known as the Hermann grid illusion. The strength of the illusion is often measured using the cancellation technique: A white disk is placed over one of these intersections and the luminance of the disk is reduced until the disk disappears. Its luminance at this point, i.e., the disk's detection threshold, is taken to be a measure of the strength of the illusion. Our experiments showed that some distortions of the Hermann grid, which were sufficient to completely disrupt the illusion, did not reduce the disk's detection threshold. This showed that the cancellation technique is not a valid method for measuring the strength of the Hermann grid illusion. Those studies that attempted to use this technique inadvertently studied a different effect known as the blanking phenomenon. We conclude by presenting an explanation for the latter effect.

Modelling the spatial tuning of the Hermann grid illusion

Does a physiologically plausible model of the retinal ganglion cell (RGC) receptive field (RF) predict the spatial tuning properties of the Hermann Grid Illusion (HGI)? The spatial tuning of a single intersection HGI was measured psychophysically in normal observers using a nulling technique at different vertical grid line luminances. We used a model based upon a standard RGC RF, balanced to produce zero response under uniform illumination, to predict the response of the model cell to the equivalent range of stimulus conditions when placed in either the 'street' or the 'intersection' of a single element of a Hermann grid. We determined the equivalent of the nulling luminance required to balance these responses and minimise the HGI. The model and the psychophysical data demonstrated broad spatial tuning with similarly shaped tuning profiles and similar strengths of illusion. The line width at the peak of the model tuning function was around twice the model RGC RF centre size. Modelling the psychophysical functions gave RF centre sizes smaller than expected from human anatomical evidence but similar to that suggested by primate physiological evidence. In the model and psychophysically the strength of the illusion varied with the luminance of the vertical grid line when HGI strength was expressed as a Michelson nulling contrast, but this effect was smaller when HGI strength was expressed as a nulling luminance. The shape, width, height and position of the spatial tuning function of the HGI can be well modelled by a RGC RF based model. The broad tuning of these functions does not appear to require a broad range of different cell sizes either in the retina or later in the visual pathway.

The Hermann Grid Illusion Revisited

The Hermann grid illusion consists of smudges perceived at the intersections of a white grid presented on a black background. In 1960 the effect was first explained by a theory advanced by Baumgartner suggesting the illusory effect is due to differences in the discharge characteristics of retinal ganglion cells when their receptive fields fall along the intersections versus when they fall along non-intersecting regions of the grid. Since then, others have claimed that this theory might not be adequate, suggesting that a model based on cortical mechanisms is necessary [Lingelbach et al, 1985 Perception 14(1) A7; Spillmann, 1994 Perception 23 691 708; Geier et al, 2004 Perception 33 Supplement, 53; Westheimer, 2004 Vision Research 44 2457 2465]. We present in this paper the following evidence to show that the retinal ganglion cell theory is untenable: (i) varying the makeup of the grid in a manner that does not materially affect the putative differential responses of the ganglion cells can reduce or eliminate the illusory effect; (ii) varying the grid such as to affect the putative differential responses of the ganglion cells does not eliminate the illusory effect; and (iii) the actual spatial layout of the retinal ganglion cell receptive fields is other than that assumed by the theory. To account for the Hermann grid illusion we propose an alternative theory according to which the illusory effect is brought about by the manner in which S1 type simple cells (as defined by Schiller et al, 1976 Journal of Neurophysiology 39 1320-1333) in primary visual cortex respond to the grid. This theory adequately handles many of the facts delineated in this paper.

Use of the Hermann grid illusion in the measurement of contrast perception in dyslexia

We measured contrast thresholds for perception of the Hermann grid illusion, using different contrast polarities and mean luminances, in dyslexics and non-dyslexics. Both groups of subjects gave significantly lower thresholds with grids having dark squares and light paths, but there was no significant threshold difference between groups. Perceived strength of illusion was also measured in grids at suprathreshold contrast levels. Dyslexics perceived the illusion to be significantly stronger than non-dyslexics when the grid had light paths and low luminance.

Chromatic and achromatic processing in the Hermann Grid illusion

Chromatic and achromatic processing in the Hermann Grid illusion

The orientation dependence of the Hermann grid illusion.

The Hermann grid illusion (HGI), elicited by a grid displayed as either horizontal-vertical (HV) or oblique (45 degrees ) configuration, was measured as the luminance necessary to cancel the illusory spots at the grid intersections. Overall, the HGI produced by the oblique grid was about one-third of that produced by the HV grid. The observers exhibited different sensitivities to the HGI orientation, and seemed to perceive the illusion in two manners: with moderate anisotropy (reduction of about 20%, three subjects) or large anisotropy (90% reduction, four subjects). The quantitative reduction of the HGI elicited by the oblique pattern tested and its reduction to almost zero in some subjects, constitute a benchmark for any model aimed at explaining the HGI on psychophysical grounds.

A global factor in the Hermann grid illusion or an artifact?

In the first of the present experiments, subjects were required to estimate the strength of the Hermann grid illusion in grids containing various numbers of intersections even though those grids were not actually presented. The positive relationship found by Wolfe (1984) for real grids was, nevertheless, replicated. It is argued that this suggests that a response bias might have been the source of his effect (although other possibilities are also noted). In addition, in a second experiment, subjects who were not aware of the fact that grid size was being manipulated (i.e., between subjects) showed no consistent effect of that factor, thus supporting the same suggestion.

An Analogy between Colour and Spatial Coding

The early stages of colour coding are well established in that the trichromatic receptor stage is followed by a set of opponent colour channels. One interpretation of the sequence is that opponent channels carry unrelated aspects of the colour stimulus, unlike the cone channels. The overlap of the cone channels can be removed by decorrelating their spectral-sensitivity functions, and this procedure has been found to give opponent colour channels which match those found psychophysically. Since the known spatial-frequency channels also show considerable overlap, the question arises which aspects of the spatial stimulus are captured by decorrelating the spatial-frequency channels. The results of decorrelating the spatial-frequency channels are that the first decorrelated spatial filter acts as a broad bandpass filter which has a peak sensitivity at 7.9 cycles deg-1, and that the second decorrelated spatial filter acts as an opponent spatial-frequency channel, with a minimum output at a low (4.1 cycles deg-1) spatial frequency and a maximum output at a high (15.1 cycles deg-1) spatial frequency. The characteristics of the first decorrelated filter closely resemble the properties of the foveal perceptive field which have been used to explain the Hermann grid illusion. Thus, the decorrelation analysis produces a model for the functional organisation of the channel implementation at the neural and psychophysical levels, but which directly relates to the subjective appearance of the visual stimuli.

Spatial Parameters Affecting the Hermann Grid and the Scintillating-Grid Illusions

The Hermann grid and the scintillating grid are examples of brightness-contrast illusions. The scintillating grid consists of a Hermann grid with bright disks about 1.4 times the diameter of the grid elements superimposed upon the intersections. The resulting scintillation effect can be described as striking illusory spots, darker than the background, perceived as flashing within the bright disks with each flick of the eye. Although the scintillation effect is different from the classical Hermann grid illusion, one prerequisite for its occurrence is the presence of a grid eliciting the Hermann grid illusion (Schrauf et al, Vision Research in press). At the 12th ECVP we showed that the Hermann grid illusion can be reduced by bright diagonals within the squares of the grid which have the effect of suppressing the 45°/135° components of the Fourier spectrum (Lingelbach et al, 1989 Perception18 536 – 537). In a pilot study we successively reduced the length of the diagonals, and found that this suppression works well even if the diagonals are reduced to small tips near the grid intersections. According to Baumgartner's receptive-field account (Baumgartner, 1960 Pflügers Archiv272 21 – 22), such elements should cause additional inhibition, thus decreasing the neuronal discharge rate and corresponding illusory percept. This is not the case: if any illusion is perceived, it is considerably weaker. The strength of the scintillation effect is similarly reduced by this manipulation. Thus the present results cannot easily be accounted for by Baumgartner's model.