Tilt Illusion Research Articles

Effects of V1 surround modulation tuning on visual saliency and the tilt illusion.

Neurons in the primary visual cortex respond to oriented stimuli placed in the center of their receptive field, yet their response is modulated by stimuli outside the receptive field (the surround). Classically, this surround modulation is assumed to be strongest if the orientation of the surround stimulus aligns with the neuron's preferred orientation, irrespective of the actual center stimulus. This neuron-dependent surround modulation has been used to explain a wide range of psychophysical phenomena, such as biased tilt perception and saliency of stimuli with contrasting orientation. However, several neurophysiological studies have shown that for most neurons surround modulation is instead center dependent: it is strongest if the surround orientation aligns with the center stimulus. As the impact of such center-dependent modulation on the population level is unknown, we examine this using computational models. We find that with neuron-dependent modulation the biases in orientation coding, commonly used to explain the tilt illusion, are larger than psychophysically reported, but disappear with center-dependent modulation. Therefore we suggest that a mixture of the two modulation types is necessary to quantitatively explain the psychophysically observed biases. Next, we find that under center-dependent modulation average population responses are more sensitive to orientation differences between stimuli, which in theory could improve saliency detection. However, this effect depends on the specific saliency model. Overall, our results thus show that center-dependent modulation reduces coding bias, while possibly increasing the sensitivity to salient features. NEW & NOTEWORTHY Neural responses in the primary visual cortex are modulated by stimuli surrounding the receptive field. Most earlier studies assume this modulation depends on the neuron's tuning properties, but experiments have shown that instead it depends mostly on the stimulus characteristics. We show that this simple change leads to neural coding that is less biased and under some conditions more sensitive to salient features.

Intra-hemispheric integration underlies perception of tilt illusion

The integration of inputs across the entire visual field into a single conscious experience is fundamental to human visual perception. This integrated nature of visual experience is illustrated by contextual illusions such as the tilt illusion, in which the perceived orientation of a central grating appears tilted away from its physical orientation, due to the modulation by a surrounding grating with a different orientation. Here we investigated the relative contribution of local, intra-hemispheric and global, inter-hemispheric integration mechanisms to perception of the tilt illusion. We used Dynamic Causal Modelling of fMRI signals to estimate effective connectivity in human early visual cortices (V1, V2, V3) during bilateral presentation of a tilt illusion stimulus. Our analysis revealed that neural responses associated with the tilt illusion were modulated by intra- rather than inter-hemispheric connectivity. Crucially, across participants, intra-hemispheric connectivity in V1 correlated with the magnitude of the tilt illusion, while no such correlation was observed for V1 inter-hemispheric connectivity, or V2, V3 connectivity. Moreover, when the illusion stimulus was presented unilaterally rather than bilaterally, the illusion magnitude did not change. Together our findings suggest that perception of the tilt illusion reflects an intra-hemispheric integration mechanism. This is in contrast to the existing literature, which suggests inter-hemispheric modulation of neural activity as early as V1. This discrepancy with our findings may reflect the diversity and complexity of integration mechanisms involved in visual processing and visual perception.

Cultural effects on computational metrics of spatial and temporal context

The concept of “prediction error” - the difference between what occurred and was expected - is key to understanding the cognitive processes of human decision making. Expectations have to be learned so the concept of prediction error critically depends on context, specifically the temporal context of probabilistically related events and their changes across time (i.e. volatility). While past research suggests context differently affects some cognitive processes in East Asian and Western individuals, it is currently unknown whether this extends to computationally-grounded measures of learning and prediction error. Here we compared Chinese and British nationals in an associative learning task that quantifies behavioural effects of prediction error, and—through a hierarchical Bayesian learning model—also captures how individuals learn about probabilistic relationships and their volatility. For comparison, we also administered a psychophysical task, the tilt illusion, to assess cultural differences in susceptibility to spatial context. We found no cultural differences in the effect of spatial context on perception. In the domain of temporal context there was no effect of culture on sensitivity to prediction error, or learning about volatility, but some suggestion that Chinese individuals may learn more readily about probabilistic relationships.

Neck Vibration Proprioceptive Postural Response Intact in Progressive Supranuclear Palsy unlike Idiopathic Parkinson's Disease.

Progressive supranuclear palsy (PSP) and late-stage idiopathic Parkinson’s disease (IPD) are neurodegenerative movement disorders resulting in different postural instability and falling symptoms. IPD falls occur usually forward in late stage, whereas PSP falls happen in early stages, mostly backward, unprovoked, and with high morbidity. Postural responses to sensory anteroposterior tilt illusion by bilateral dorsal neck vibration were probed in both groups versus healthy controls on a static recording posture platform. Three distinct anteroposterior body mass excursion peaks (P1–P3) were observed. 18 IPD subjects exhibited well-known excessive response amplitudes, whereas 21 PSP subjects’ responses remained unaltered to 22 control subjects. Neither IPD nor PSP showed response latency deficits, despite brainstem degeneration especially in PSP. The observed response patterns suggest that PSP brainstem pathology might spare the involved proprioceptive pathways and implies viability of neck vibration for possible biofeedback and augmentation therapy in PSP postural instability.

Preferential processing of cardinal over oblique orientations in human vision.

The oblique effect-poorer performance when contours are in oblique meridians-is here extended from the discrimination of line-orientation to the tilt illusion and to the detection and contextual induction of curvature. The distinction is made between a contour's susceptibility to contextual perturbation and its capacity to induce such perturbation, for which the oblique effect is only about one half. That the cardinal/oblique superiority is retained for the orientation of illusory borders and for the implicit orientation of shapes lacking explicit rectilinear delineation has implications for its neural substrate. To the extent that a geometrical-visual illusion, such as Poggendorff's or Hering's, depends on interaction in the domain of contour orientation, it manifests a corresponding orientational anisotropy. On the other hand, visual functions that govern whether and how well a boundary is visible are invariant with orientation.

Dynamic tilt illusion induced by continuous contextual orientation alternations

In the classic tilt illusion, the perceived orientation of a center patch is shifted away from its oriented context. Additionally, a stronger illusion effect is yielded when the center patch is simultaneously rather than asynchronously presented with a constant context for a shorter duration. However, little is known about the temporal characteristic of the tilt illusion in a reverse situation in which a constant center patch is presented throughout while the contexts change. Therefore, we continuously alternated two opposite-oriented contexts and manipulated alternate speeds to examine how the tilt illusion would build up as a function of dynamic contextual alternation. Our results revealed that dynamic alternations between leftward- and rightward-oriented contexts caused a static vertical grating at the center to apparently sway from side to side. More importantly, the apparent sway illusion was modulated by the alternate speed of the oriented contexts (up to 8-10 Hz); the quicker the alternation is, the faster and weaker the apparent sway is. Intriguingly, the temporal characteristic of the "dynamic tilt illusion" suggests that, under a varying environment, the suppressions from temporally adjacent surrounds would be chunked into discrete epochs before affecting our percept.

The neural basis of spatial vision losses in the dysfunctional visual system

Human vision relies on correct information processing from the eye to various visual areas. Disturbances in the visual perception of simple features are believed to come from low-level network (e.g., V1) disruptions. In the present study, we modelled monocular losses in spatial vision through plausible multiple network modifications in early visual coding. We investigated perceptual deficits in anisometropic amblyopia and used the monocular tilt illusion as a probe of primary visual cortex orientation coding and inhibitory interactions. The psychophysical results showed that orientation misperception was higher in amblyopic eyes (AE) than in the fellow and neurotypical eyes and was correlated with the subject’s AE peak contrast sensitivity. The model fitted to the experimental results allowed to split these observations between different network characteristics by showing that these observations were explained by broader orientation tuning widths in AEs and stronger lateral inhibition in abnormal amblyopic system that had strong contrast sensitivity losses. Through psychophysics measures and computational modelling of V1, our study links multiple perceptual changes with localized modifications in the primary visual cortex.

Bioplausible multiscale filtering in retino-cortical processing as a mechanism in perceptual grouping

Why does our visual system fail to reconstruct reality, when we look at certain patterns? Where do Geometrical illusions start to emerge in the visual pathway? How far should we take computational models of vision with the same visual ability to detect illusions as we do? This study addresses these questions, by focusing on a specific underlying neural mechanism involved in our visual experiences that affects our final perception. Among many types of visual illusion, ‘Geometrical’ and, in particular, ‘Tilt Illusions’ are rather important, being characterized by misperception of geometric patterns involving lines and tiles in combination with contrasting orientation, size or position. Over the last decade, many new neurophysiological experiments have led to new insights as to how, when and where retinal processing takes place, and the encoding nature of the retinal representation that is sent to the cortex for further processing. Based on these neurobiological discoveries, we provide computer simulation evidence from modelling retinal ganglion cells responses to some complex Tilt Illusions, suggesting that the emergence of tilt in these illusions is partially related to the interaction of multiscale visual processing performed in the retina. The output of our low-level filtering model is presented for several types of Tilt Illusion, predicting that the final tilt percept arises from multiple-scale processing of the Differences of Gaussians and the perceptual interaction of foreground and background elements. The model is a variation of classical receptive field implementation for simple cells in early stages of vision with the scales tuned to the object/texture sizes in the pattern. Our results suggest that this model has a high potential in revealing the underlying mechanism connecting low-level filtering approaches to mid- and high-level explanations such as ‘Anchoring theory’ and ‘Perceptual grouping’.

Effect of temporal modulations of dynamic inducer on tilt illusion

Effect of temporal modulations of dynamic inducer on tilt illusion

Brief presentation enhances various simultaneous contrast effects.

Kaneko and Murakami (2012) demonstrated that simultaneous contrast for brightness and color (chromatic saturation) were enhanced by flashing the stimulus very briefly (10 ms). Here we examined whether this effect of duration generalized to other visual features. Tilt illusion and simultaneous hue contrast were both shown to be much stronger with a stimulus duration of 10 ms compared with 500 ms. The similar temporal dynamics for simultaneous contrast across visual features suggest common underlying principles.

The dizzy patient: don't forget disorders of the central vestibular system.

Vertigo and dizziness are among the most common complaints in neurology clinics, and they account for about 13% of the patients entering emergency units. In this Review, we focus on central vestibular disorders, which are mostly attributable to acute unilateral lesions of the bilateral vestibular circuitry in the brain. In a tertiary interdisciplinary outpatient dizziness unit, central vestibular disorders, including vestibular migraine, comprise about 25% of the established diagnoses. The signs and symptoms of these disorders can mimic those of peripheral vestibular disorders with sustained rotational vertigo. Bedside examinations, such as the head impulse test and ocular motor testing to determine spontaneous and gaze-evoked nystagmus or skew deviation, reliably differentiate central from peripheral syndromes. We also consider disorders of 'higher vestibular functions', which involve more than one sensory modality as well as cognitive domains (for example, orientation, spatial memory and navigation). These disorders include hemispatial neglect, the room tilt illusion, pusher syndrome, and impairment of spatial memory and navigation associated with hippocampal atrophy in cases of peripheral bilateral vestibular loss.

Metavestibular disorders and disorders of higher vestibular function

Vertigo, instability, oscillopsia and concomitant autonomic disorders are classical and well-known symptoms of vestibular disorders. At the same time, recent studies suggest that there are more complicated vestibular dysfunctions caused by the cortical projections of the vestibular system. The central vestibular system includes parietal temporal cortex and insular, anterior intraparietal sulcus, posterior parietal and medial parts of the superior temporal gyrus, singular gyrus retrosplenial cortex, hippocampus and parahippocampal area. The central part of the vestibular system closely interacts with other afferent systems forming a multisensory structure of higher brain functions. Dysfunctions of higher vestibular function play an important role in the development of clinical syndromes including pusher syndrome, room tilt illusion, unilateral spatial neglect syndrome, impairment of spatial memory and navigation. These syndromes can develop due to the direct damage of the cortical vestibular system or as a result of disconnection between the vestibular cortex and other parts of the sensory cortex.

Projective Geometry in the Moon Tilt Illusion

Projective Geometry in the Moon Tilt Illusion

Brief presentation enhances various simultaneous contrast effects

Kaneko & Murakami (2012) showed that simultaneous contrast for brightness and for color (saturation-shift type) are both enhanced by briefly presenting the stimulus. Simultaneous contrast is a universal phenomenon seen across many visual features. So how universal is the inverse relationship between the strength of simultaneous contrast and the stimulus duration? In the first study, we examined the relationship between the simultaneous tilt contrast (a.k.a. tilt illusion) and stimulus duration. A vertical test grating (2 cpd sinusoidal) and a tilted surrounding grating (inducer) appeared repeatedly and subjects adjusted the orientation of the comparison grating to make it parallel to the test. Test and inducer appeared simultaneously and their duration was either 10 ms or 500 ms. When the inducer was tilted 15 deg away from the test, the tilt contrast effect was more than doubled for 10 ms condition compared with 500 ms condition. When the inducer was tilted 3 deg, neither duration gave any effect. For tilts greater than 9 deg, the 10 ms condition always yielded greater effect than the 500 ms condition. However, tilts of less than 9 deg gave no significant difference between durations. In the second study, we looked for similar effects of duration in hue-shift type simultaneous color contrast (cf. Klauke & Wachtler, 2015). We chose all our colors from the equiluminant cone-contrast space whose origin was the equal-energy white (20 cd/m2). Results from experiments using unique yellow as a target hue suggest that the unique-yellow shift was strongly enhanced by briefly flashing the stimulus. Results suggest the existence of universal mechanisms that enhance the difference across visual features between neighboring regions, which work only for briefly presented targets. Meeting abstract presented at VSS 2016

Room tilt illusion: a symptom of both peripheral and central vestibular disorders

Room tilt illusion: a symptom of both peripheral and central vestibular disorders

A bias-free measure of the tilt illusion.

The perceived orientation of a central test grating is influenced by the orientation structure of the surrounding image. Measurement of this "tilt illusion" traditionally requires subjects to report the orientation of the test relative to a cardinal reference (e.g., clockwise or anticlockwise of vertical). Given that the test is presented within a surround that is itself oriented clockwise or anticlockwise from vertical, there is obvious potential for the orientation of the surround to bias the subject's response irrespective of any perceptual effects. To avoid this bias, we ran a two temporal interval forced-choice experiment. The two intervals contained opposite surround orientations (±15°), and we manipulated the orientation of the center gratings. Participants were asked to judge which of the test gratings was closer to vertical. We found no significant difference between measurements of the tilt illusion using the traditional and two-interval procedures. We then examined interindividual differences in a larger sample and found a significant correlation between the magnitudes of the tilt illusion measured using the two procedures. Our experiments demonstrate a method free from response bias in measuring the tilt illusion although our results indicate that response biases were unlikely to factor significantly in prior tilt illusion experiments.

A unified account of tilt illusions, association fields, and contour detection based on elastica

As expressed in the Gestalt law of good continuation, human perception tends to associate stimuli that form smooth continuations. Contextual modulation in primary visual cortex, in the form of association fields, is believed to play an important role in this process. Yet a unified and principled account of the good continuation law on the neural level is lacking. In this study we introduce a population model of primary visual cortex. Its contextual interactions depend on the elastica curvature energy of the smoothest contour connecting oriented bars. As expected, this model leads to association fields consistent with data. However, in addition the model displays tilt-illusions for stimulus configurations with grating and single bars that closely match psychophysics. Furthermore, the model explains not only pop-out of contours amid a variety of backgrounds, but also pop-out of single targets amid a uniform background. We thus propose that elastica is a unifying principle of the visual cortical network.

Contextual influence on the tilt after-effect in foveal and para-foveal vision

A sensory stimulus can only be properly interpreted in light of the stimuli that surround it in space and time. The tilt illusion (TI) and tilt after-effect (TAE) provide good evidence that the perception of a target depends strongly on both its spatial and temporal context. In previous studies, the TI and TAE have typically been investigated separately, so little is known about their co-effects on visual perception and information processing mechanisms. Here, we considered the influence of the spatial context and the temporal effect together and asked how center-surround context affects the TAE in foveal and para-foveal vision. Our results showed that different center-surround spatial patterns significantly affected the TAE for both foveal and para-foveal vision. In the fovea, the TAE was mainly produced by central adaptive gratings. Cross-oriented surroundings significantly inhibited the TAE, and iso-oriented surroundings slightly facilitated it; surround inhibition was much stronger than surround facilitation. In the para-fovea, the TAE was mainly decided by the surrounding patches. Likewise, a cross-oriented central patch inhibited the TAE, and an iso-oriented one facilitated it, but there was no significant difference between inhibition and facilitation. Our findings demonstrated, at the perceptual level, that our visual system adopts different mechanisms to process consistent or inconsistent central-surround orientation information and that the unequal magnitude of surround inhibition and facilitation is vitally important for the visual system to improve the detectability or discriminability of novel or incongruent stimuli.

Which way is down? Positional distortion in the tilt illusion.

Contextual information can have a huge impact on our sensory experience. The tilt illusion is a classic example of contextual influence exerted by an oriented surround on a target's perceived orientation. Traditionally, the tilt illusion has been described as the outcome of inhibition between cortical neurons with adjacent receptive fields and a similar preference for orientation. An alternative explanation is that tilted contexts could produce a re-calibration of the subjective frame of reference. Although the distinction is subtle, only the latter model makes clear predictions for unoriented stimuli. In the present study, we tested one such prediction by asking four naive subjects to estimate three positions (4, 6, and 8 o'clock) on an imaginary clock face within a tilted surround. To indicate their estimates, they used either an unoriented dot or a line segment, with one endpoint at fixation in the middle of the surround. The surround's tilt was randomly chosen from a set of orientations (±75°, ±65°, ±55°, ±45°, ±35°, ±25°, ±15°, ±5° with respect to vertical) across trials. Our results showed systematic biases consistent with the tilt illusion in both conditions. Biases were largest when observers attempted to estimate the 4 and 8 o'clock positions, but there was no significant difference between data gathered with the dot and data gathered with the line segment. A control experiment confirmed that biases were better accounted for by a local coordinate shift than to torsional eye movements induced by the tilted context. This finding supports the idea that tilted contexts distort perceived positions as well as perceived orientations and cannot be readily explained by lateral interactions between orientation selective cells in V1.

Awareness is the key to attraction: dissociating the tilt illusions via conscious perception.

The tilt illusion is a compelling example of contextual influence exerted by an oriented surround on a target's perceived orientation. A vertical target appears to be tilted away from a 15° oriented surround but appears to be tilted toward a 75° tilted surround. We tested the claim that these biases result from distinct sensory processes: a low-level repulsive process and a higher-level attractive process. If this claim were correct, then surround visibility would be a requirement for attraction, but it would not necessarily be a requirement for repulsion. Indeed, Motoyoshi and Hayakawa (2010) have demonstrated that repulsion can survive removal of the surround from phenomenal awareness using adaptation-induced blindness. Here we sought to test this prediction by measuring the orientation biases in a parafoveally presented Gabor patch surrounded by tilted gratings after 20-s adaptation. The adapting stimulus was an annularly windowed plaid composed of vertical and horizontal jittering gratings. Observers were instructed to maintain fixation throughout the trial and report whether the Gabor appeared to be tilted clockwise or anticlockwise of vertical. They also had to indicate whether the surround was visible after adaptation. Postadaptation biases were then compared with those obtained in a control experiment without dynamic adaptation. We found large repulsive biases induced by 15° oriented surrounds, but no attractive biases were induced by 75° tilted surrounds. This result shows that attractive effects do require visual awareness and thereby provides robust evidence for the existence of two separate mechanisms mediating the phenomenology of the tilt illusions.