Lateral inhibition between orientation detectors in the human visual system

Short intracortical and surround inhibition are selectively reduced during movement initiation in focal hand dystonia

In patients with focal hand dystonia (FHD), pathological overflow activation occurs in muscles not involved in the movement. Surround inhibition is a neural mechanism that can sharpen desired movement by inhibiting unwanted movement in adjacent muscles. To further establish the phenomenon of surround inhibition and to determine whether short intracortical inhibition (SICI) reflecting inhibition from the local interneurons in primary motor cortex (M1), might play a role in its genesis, single- and paired-pulse transcranial magnetic stimulation (TMS), and Hoffmann reflex testing were applied to evaluate the excitability of the relaxed abductor pollicis brevis muscle (APB) at various intervals during a movement of the index finger in 16 patients with FHD and 20 controls. Whereas controls showed inhibition of APB motor-evoked potential (MEP) size during movement initiation and facilitation of APB MEP size during the maintenance phase, FHD patients did not modulate APB MEP size. In contrast, SICI remained constant in controls, but FHD patients showed reduced SICI during movement initiation. The H(max)/M(max) ratio in control subjects increased during movement initiation. The results provide additional evidence for the presence of surround inhibition in M1, where it occurs only during movement initiation, indicating that different mechanisms underlie movement initiation and maintenance. Thus, surround inhibition is sculpted both in time and space and may be an important neural mechanism during movement initiation to counteract increased spinal excitability. SICI may contribute to its generation, because in patients with FHD, the lack of depression of APB MEP size is accompanied by a reduction in SICI.

Effect of asymmetry in a binary state on the collective behavior of a system with spatially modulated interaction and quenched randomness

The properties of solid states, biophysical materials, neuronal circuits, and equilibrium states of a many-body system can be studied by using techniques in statistical physics. It has been common practice to represent a system composed of binary state units by using an Ising spin network where each unit has symmetric {-1,1} states. However, the asymmetry or symmetry of the binary states of the units can affect the property and ergodicity of the system, but better understanding of the quantitative difference is still needed. We compare systems of binary units with symmetric or asymmetric states. The network has spatially modulated interaction with quenched randomness. We can bridge the Ising spin network and McCulloch-Pitts neuron network and analyze the stability of the system via replica method by introducing an interpolating parameter. The effects of the asymmetry states affect the multistability of the system and the stability of replica-symmetry solutions.

Impaired intracortical inhibition in the primary somatosensory cortex in focal hand dystonia

Somesthetic temporal discrimination (STD) is impaired in focal hand dystonia (FHD). We explored the electrophysiological correlate of the STD deficit to assess whether this is due to dysfunction of temporal inhibition in the somatosensory inhibitory pathway or due to dysfunction in structures responsible for nonmodality-specific timing integration. Eleven FHD patients and 11 healthy volunteers were studied. STD threshold was investigated as the time interval required for perceiving a pair of stimuli as two separate stimuli in time. We also examined the somatosensory-evoked potential (SEP) in a paired-pulse paradigm. We compared STD threshold and recovery function of SEP between the groups. STD thresholds were significantly greater in FHD than in healthy volunteers. The amount of P27 suppression in the 5 ms-ISI condition was significantly less in FHD. It was also found that the STD threshold and P27 suppression were significantly correlated: the greater the STD threshold, the less the P27 suppression. Significantly less suppression of P27 with a lack of significant change in N20 indicates that the impairment of somatosensory information processing in the time domain is due to dysfunction within the primary somatosensory cortex, suggesting that that the STD deficit in FHD is more attributable to dysfunction in the somatosensory pathway.

Perceived size and spatial coding

Images of the same physical dimensions on the retina can appear to represent different-sized objects. One reason for this is that the human visual system can take viewing distance into account when judging apparent size. Sequentially presented images can also prompt spatial coding interactions. Here we show, using a spatial coding phenomenon (the tilt aftereffect) in tandem with viewing distance cues, that the tuning of such interactions is not simply determined by the physical dimensions of retinal input. Rather, we find that they are contingent on apparent size. Our data therefore reveal that spatial coding interactions in human vision are modulated by processes involved in the determination of apparent size.

Physiology-based modeling of cortical auditory evoked potentials

Evoked potentials are the transient electrical responses caused by changes in the brain following stimuli. This work uses a physiology-based continuum model of neuronal activity in the human brain to calculate theoretical cortical auditory evoked potentials (CAEPs) from the model's linearized response. These are fitted to experimental data, allowing the fitted parameters to be related to brain physiology. This approach yields excellent fits to CAEP data, which can then be compared to fits of EEG spectra. It is shown that the differences between resting eyes-open EEG and standard CAEPs can be explained by changes in the physiology of populations of neurons in corticothalamic pathways, with notable similarities to certain aspects of slow-wave sleep. This pilot study demonstrates the ability of our model-based fitting method to provide information on the underlying physiology of the brain that is not available using standard methods.

Neural basis of 3-D shape aftereffects

We used selective adaptation to identify the neural mechanisms responsible for 3-D shape perception from orientation flows in retinal images [Li, A., & Zaidi, Q. (2000). Perception of three-dimensional shape from texture is based on patterns of oriented energy. Vision Research 40 (2), 217-242)]. Three-dimensional shape adaptation could involve stages from photoreceptors to non-oriented retinal cells, oriented cells in striate cortex, and extra-striate cells that respond to 3-D slants. To psychophysically isolate the relevant stage, we used 3-D adapting stimuli created from real and illusory orientations, and test stimuli different from the adapting stimuli in phases and frequencies. The results showed that mechanisms that adapt to 3-D shapes combine real and illusory 2-D orientation information over a range of spatial frequencies.

Biases and sensitivities in the Poggendorff effect when driven by subjective contours

PURPOSE

A consensus in the existing literature suggests that the Poggendorff effect (a perceptual misalignment of two collinear transversal segments when separated by a pair of parallel contours) persists when the parallels are defined by Kanizsa-like subjective contours. However, previous studies have often been complicated by a lack of quantitative measures of effect size, statistical tests of significance, appropriate measures of baseline and control biases, or stringent definition of subjective contours. The aim of this study was thus to determine whether subjective contours are capable of driving the Poggendorff effect once other factors are accounted for.

METHODS

Twenty participants were tested on a number of test and control figures incorporating first-order (luminance-defined) and subjective parallels using the method of adjustment. All figures were tested at two different orientations, and observer sensitivities and observer biases were assessed.

RESULTS

A systematic response bias (in the direction of the classical effect) was found for Poggendorff figures that incorporated subjective parallels. The effect was highly significant and greater than for control figures. There was no concomitant change in judgment sensitivity (positional certainty). Finally, there was a positive correlation between the effect size for figures incorporating first-order and subjective parallels.

CONCLUSIONS

The findings reported demonstrate conclusively that true Kanizsa-like subjective contours are capable of driving the Poggendorff effect. Further, the data are consistent with a growing body of evidence that suggests both first-order and subjective contours are processed at early loci in the visual pathways when position is encoded.

Neural model of disinhibitory interactions in the modified Poggendorff illusion

Visual illusions can be strengthened or weakened with the addition of extra visual elements. For example, in the Poggendorff illusion, with an additional bar added, the illusory skew in the perceived angle can be enlarged or reduced. In this paper, we show that a nontrivial interaction between lateral inhibitory processes in the early visual system (i.e., disinhibition) can explain such an enhancement or degradation of the illusory effect. The computational model we derived successfully predicted the perceived angle in the Poggendorff illusion task that was modified to include an extra thick bar. The concept of disinhibition employed in the model is general enough that we expect it can be further extended to account for other classes of geometric illusions.

Fundamental failures of shape constancy resulting from cortical anisotropy

Contrary to the conventional assumption that humans perceive shapes of rigid objects as constant despite retinal-image variations caused by changes in orientation and position, we show that the depths of three-dimensional (3-D) textured shapes appear to vary when the image is rotated. In paired comparisons of static stimuli, depth amplitude was perceived to be greater at vertical than at oblique orientations. A similar oblique bias was found for simple two-dimensional (2-D) obtuse angles. Using projective geometry to link angle magnitude to the orientation flows that convey 3-D shape from texture cues, we show quantitatively that the 2-D bias predicts the 3-D bias. Our finding that perception of angular magnitude is dependent on orientation has broad implications for shape constancy because orientation flows have also been implicated in 3-D perception from reflections and shading, and contour curvature is fundamental to uncovering depth and part-structure of shapes. We examined the roles played in the observed biases by anisotropies in numbers and tuning widths of orientation-tuned cells in striate cortex as well as the distribution of oriented energy in natural scenes. An optimal stimulus decoding model for 2-D angles revealed that the narrower tuning of cells for horizontal orientations combined with cross-orientation inhibition explains the orientation-dependent angle distortion and hence the 3-D shape inconstancy. Variations in properties within neural populations can thus have direct effects on visual perceptions and need to be included in neural decoding models.

Feature-based attention modulates orientation-selective responses in human visual cortex

How does feature-based attention modulate neural responses? We used adaptation to quantify the effect of feature-based attention on orientation-selective responses in human visual cortex. Observers were adapted to two superimposed oblique gratings while attending to one grating only. We measured the magnitude of attention-induced orientation-selective adaptation both psychophysically, by the behavioral tilt aftereffect, and physiologically, using fMRI response adaptation. We found evidence for orientation-selective attentional modulation of neuronal responses-a lower fMRI response for the attended than the unattended orientation-in multiple visual areas, and a significant correlation between the magnitude of the tilt aftereffect and that of fMRI response adaptation in V1, the earliest site of orientation coding. These results show that feature-based attention can selectively increase the response of neuronal subpopulations that prefer the attended feature, even when the attended and unattended features are coded in the same visual areas and share the same retinotopic location.

Orientation repulsion and attraction in alignment perception

Orientation masking induces changes of discrimination thresholds and perceived orientation. Studies on alignment discrimination of Vernier stimuli concentrated on masking induced changes of discrimination thresholds, without considering possible changes of perceived orientation and/or alignment of the two-line segments. Measuring both parameters in an orientation discrimination task, we confirmed a standard repulsion effect between a single line target and a mask grating that co-varied with elevated orientation discrimination thresholds. Masking a Vernier stimulus in an alignment discrimination task, we observed a strong misperception of alignment that was accompanied with elevated alignment discrimination thresholds. Orientation masking on perceived orientation and alignment of a Vernier stimulus revealed orientation repulsion and attraction that depended on the spatio-orientation configuration of the superimposed stimuli. Control of task-dependent effects confirmed that our observed pattern of results was independent of attentional or cognitive demands.

Structural network heterogeneities and network dynamics: a possible dynamical mechanism for hippocampal memory reactivation

The hippocampus has the capacity for reactivating recently acquired memories and it is hypothesized that one of the functions of sleep reactivation is the facilitation of consolidation of novel memory traces. The dynamic and network processes underlying such a reactivation remain, however, unknown. We show that such a reactivation characterized by local, self-sustained activity of a network region may be an inherent property of the recurrent excitatory-inhibitory network with a heterogeneous structure. The entry into the reactivation phase is mediated through a physiologically feasible regulation of global excitability and external input sources, while the reactivated component of the network is formed through induced network heterogeneities during learning. We show that structural changes needed for robust reactivation of a given network region are well within known physiological parameters.

Stochastic re-calibration: contextual effects on perceived tilt

The human visual system exaggerates the difference between the tilts of adjacent lines or grating patches. In addition to this tilt illusion, we found that oblique flanks reduced acuity for small changes of tilt in the centre of the visual field. However, no flanks--regardless of their tilts--decreased sensitivity to contrast. Thus, the foveal tilt illusion should not be attributed to orientation-selective lateral inhibition. Nor is it similar to conventional crowding, which typically does not impair letter recognition in the fovea. Our observers behaved as though the reference orientation (horizontal) had a small tilt in the direction of the flanks. We suggest that the extent of this re-calibration varies randomly over trials, and we demonstrate that this stochastic re-calibration can explain flank-induced acuity loss in the fovea.

Analytic solution of neural network with disordered lateral inhibition

The replica method has played a key role in analyzing systems with disorder, e.g., the Sherrington-Kirkpatrick (SK) model, and associative neural networks. Here we study the influence of disorder in the lateral inhibition type interactions on the cooperative and uncooperative behavior of recurrent neural networks by using the replica method. Although the interaction between neurons has a dependency on distance, our model can be solved analytically. Bifurcation analysis identifies the boundaries between paramagnetic, ferromagnetic, spin-glass, and localized phases. In the localized phase, the network shows a bump like activity, which is often used as a model of spatial working memory or columnar activity in the visual cortex. Simulation results show that disordered interactions can stabilize the drift the of bump position, which is commonly observed in conventional lateral inhibition type neural networks.

A Simple Hebbian/Anti-Hebbian Network Learns the Sparse, Independent Components of Natural Images

Slightly modified versions of an early Hebbian/anti-Hebbian neural network are shown to be capable of extracting the sparse, independent linear components of a prefiltered natural image set. An explanation for this capability in terms of a coupling between two hypothetical networks is presented. The simple networks presented here provide alternative, biologically plausible mechanisms for sparse, factorial coding in early primate vision.

Orientation discrimination in human vision: Psychophysics and modeling

We evaluated orientation discrimination thresholds using an external noise paradigm. Stimuli were spatiotemporal Gaussian patches of 2D orientation noise band-pass filtered in Fourier domain. Orientation acuity was measured for various combinations of stimulus spatial bandwidth, spatial frequency, and size as a function of orientation bandwidths of the stimuli. Stimulus contrast was matched in multiples of detection threshold. Consistent with the idea that stimulus orientation bandwidth acts as a source of external noise, orientation discrimination thresholds increased monotonically in all conditions with stimulus bandwidth. To interpret these results quantitatively, we first fitted a variance summation model to the data and derived the internal orientation noise, relative sampling efficiency, and orientation tuning of the mechanism underlying orientation discrimination. Due to the equivocal biological nature of these parameters for orientation discrimination, we investigated, with a modeling approach, how neural detectors characterized by a broad orientation tuning may support orientation discrimination. We demonstrated using the ideal-observer theory that while linear models, based on either unimodal filtering or center-surround opponency, predict the monotonic relationship between orientation discrimination threshold and orientation noise, a nonlinear model incorporating a broadband divisive inhibition in the orientation domain is a better candidate due to its contrast invariance. This model, using broad and similar orientation tuning for its excitatory and inhibitory inputs, accounts for the acute orientation acuity of human vision and proves to be robust despite the variance found in natural stimuli.

Geometric visual illusions in microgravity during parabolic flight

This investigation explores whether the absence of gravitational information in a microgravity environment affects the perception of several classical visual illusions based on the arrangement of horizontal and vertical lines. Because the perception of horizontal and vertical orientation changes in microgravity, our prediction was that the strength of visual illusions based on the arrangement of horizontal and vertical lines would be altered when study participants were free-floating during parabolic flight. The frequency of appearance of reversed-T, Müller-Lyer, Ponzo, and Hering illusions substantially decreased when observers were free-floating, whereas the Zöllner and the Poggendorff illusions were not affected. Because the former illusions rely more heavily on perspective cues for generating inaccurate judgments of depth and size, these results suggest an alteration in the role of linear perspective for three-dimensional vision in microgravity. They also confirm that the visual system normally relies on otolith and somatosensory information for providing accurate judgments about the size and distance of objects when presented with planar presentations of geometric figures.

Correlated firing in a feedforward network with Mexican-hat-type connectivity

We report on deterministic and stochastic evolutions of firing states through a feedforward neural network with Mexican-hat-type connectivity. The prevalence of columnar structures in a cortex implies spatially localized connectivity between neural pools. Although feedforward neural network models with homogeneous connectivity have been intensively studied within the context of the synfire chain, the effect of local connectivity has not yet been studied so thoroughly. When a neuron fires independently, the dynamics of macroscopic state variables (a firing rate and spatial eccentricity of a firing pattern) is deterministic from the law of large numbers. Possible stable firing states, which are derived from deterministic evolution equations, are uniform, localized, and nonfiring. The multistability of these three states is obtained where the excitatory and inhibitory interactions among neurons are balanced. When the presynapse-dependent variance in connection efficacies is incorporated into the network, the variance generates common noise. Then the evolution of the macroscopic state variables becomes stochastic, and neurons begin to fire in a correlated manner due to the common noise. The correlation structure that is generated by common noise exhibits a nontrivial bimodal distribution. The development of a firing state through neural layers does not converge to a certain fixed point but keeps on fluctuating.

Look little, look often: The influence of gaze frequency on drawing accuracy

The present article attempts to determine what those who draw accurately do differently than those who do not. Four experiments explore the relation between drawing accuracy and the rate at which artists glance between their drawing and the stimulus (termed gaze frequency). Experiment 1 revealed a positive relation between gaze frequency and drawing accuracy (r2 = .33). Experiments 2, 3, and 4 demonstrated that gaze frequency directly influences drawing accuracy. High gaze frequencies may facilitate drawing accuracy by (1) allowing the artist to hold less information in working memory, (2) reducing memory distortion, and (3) facilitating the reduction of context effects through inattentional blindness.

Stochastic resonance of localized activity driven by common noise

We study the influence of spatially correlated noise on the transient dynamics of a recurrent network with Mexican-Hat-type connectivity. We derive the closed form of the order parameter functional in the thermodynamical limit of neuron number N. Our analysis shows that network dynamics is qualitatively changed by the presence of common noise. Network dynamics driven by common noise obtains the global level of fluctuation, which is not observed in a network driven by independent noise only. We show that the optimal level of global fluctuation enhances the transition from non-localized firing states to spatially localized firing states, and also enhances the rotation speed of localized activity.

The Poggendorff illusion explained by natural scene geometry

One of the most intriguing of the many discrepancies between perceived spatial relationships and the physical structure of visual stimuli is the Poggendorff illusion, when an obliquely oriented line that is interrupted no longer appears collinear. Although many different theories have been proposed to explain this effect, there has been no consensus about its cause. Here, we use a database of range images (i.e., images that include the distance from the image plane of every pixel in the scene) to show that the probability distribution of the possible locations of line segments across an interval in natural environments can fully account for all of the behavior of this otherwise puzzling phenomenon.

Influence of the gravitational vertical on geometric visual illusions

The occurrence of geometric orientation illusions and the perception of ambiguous figures were analyzed in 24 subjects during static body tilt relative to gravity on Earth. Results showed that illusions such as the Rock's diamond/square, the Ponzo illusion, and orientation contrast illusions occurred less frequently, and that depth reversal of ambiguous figures took more time when subjects were lying on their side or supine compared to upright, thus suggesting that the gravitational reference plays a significant role in these "visual" illusions. The structure of images, our representation of the environment, and orientation relative to gravity are all integral parts in interpreting visual images. In a weightless environment where no gravitational reference can be used, it is expected that similar alterations in visual perception will occur.

Motion repulsion arises from stimulus statistics when analyzed with a clustering algorithm

Motion repulsion is the perceived enlargement of the angle between the directions of motion of two transparently moving patterns. An explanation of this illusion has long been sought for in the neural circuitry of the brain. We show that motion repulsion already arises from the statistical properties of the motion transparency problem when analyzed with a clustering algorithm.

Natural-scene geometry predicts the perception of angles and line orientation

Visual stimuli that entail the intersection of two or more straight lines elicit a variety of well known perceptual anomalies. Preeminent among these anomalies are the systematic overestimation of acute angles, the underestimation of obtuse angles, and the misperceptions of line orientation exemplified in the classical tilt, Zollner, and Hering illusions. Here we show that the probability distributions of the possible real-world sources of projected lines and angles derived from a range-image database of natural scenes accurately predict each of these perceptual peculiarities. These findings imply that the perception of angles and oriented lines is determined by the statistical relationship between geometrical stimuli and their physical sources in typical visual environments.

Attentional states and the degree of visual adaptation to gratings

We studied top-down attentional effects on adaptation to two aspects of sinusoidal gratings: contrast (CTE: contrast threshold elevation for detection) and orientation (TAE: tilt aftereffect, bias in perceived orientation). Adaptation was examined under five different behavioral conditions designed to assess the effect of alertness, spatial attention and the dimension attended. Alertness increased CTE, but had no effect on TAE. Spatial attention increased TAE, but had no effect on CTE. TAE (but not CTE) was also sensitive to the attended dimension. It was greater when gratings' contrast rather than orientation was attended. The different patterns of top-down effects on CTE compared with TAE are consistent with these two types of adaptation taking place at different levels along the visual hierarchy: CTE occurs at very low-levels, where activity is affected by alertness but not by spatial attention, whereas TAE occurs at subsequent stages, which are modulated by selective attention.

Spatial frequency selective masking of first-order and second-order motion in the absence of off-frequency 'looking'

Converging evidence suggests that, at least initially, first-order (luminance defined) and second-order (e.g. contrast defined) motion are processed independently in human vision. However, adaptation studies suggest that second-order motion, like first-order motion, may be encoded by spatial frequency selective mechanisms each operating over a limited range of scales. Nonetheless, the precise properties of these mechanisms are indeterminate since the spatial frequency selectivity of adaptation aftereffects may not necessarily represent the frequency tuning of the underlying units [Vision Research 37 (1997) 2685]. To address this issue we used visual masking to investigate the spatial-frequency tuning of the mechanisms that encode motion. A dual-masking paradigm was employed to derive estimates of the spatial tuning of motion sensors, in the absence of off-frequency 'looking'. Modulation-depth thresholds for identifying the direction of a sinusoidal test pattern were measured over a 4-octave range (0.125-2 c/deg) in both the absence and presence of two counterphasing masks, simultaneously positioned above and below the test frequency. For second-order motion, the resulting masking functions were spatially bandpass in character and remained relatively invariant with changes in test spatial frequency, masking pattern modulation depth and the temporal properties of the noise carrier. As expected, bandpass spatial frequency tuning was also found for first-order motion. This provides compelling evidence that the mechanisms responsible for encoding each variety of motion exhibit spatial frequency selectivity. Thus, although first-order and second-order motion may be encoded independently, they must utilise similar computational principles.

Investigating local network interactions underlying first- and second-order processing

We compared the spatial lateral interactions for first-order cues to those for second-order cues, and investigated spatial interactions between these two types of cues. We measured the apparent modulation depth of a target Gabor at fixation, in the presence and the absence of horizontally flanking Gabors. The Gabors' gratings were either added to (first-order) or multiplied with (second-order) binary 2-D noise. Apparent "contrast" or modulation depth (i.e., the perceived difference between the high and low luminance regions for the first-order stimulus, or between the high and low contrast regions for the second-order stimulus) was measured with a modulation depth-matching paradigm. For each observer, the first- and second-order Gabors were equated for apparent modulation depth without the flankers. Our results indicate that at the smallest inter-element spacing, the perceived reduction in modulation depth is significantly smaller for the second-order than for the first-order stimuli. Further, lateral interactions operate over shorter distances and the spatial frequency and orientation tuning of the suppression effect are broader for second- than first-order stimuli. Finally, first- and second-order information interact in an asymmetrical fashion; second-order flankers do not reduce the apparent modulation depth of the first-order target, whilst first-order flankers reduce the apparent modulation depth of the second-order target.

Colour and luminance selectivity of spatial and temporal interactions in orientation perception

Previous studies have commented upon the similar phenomenology of simultaneous and successive interactions in the perception of orientation. These similarities have been taken as evidence of common mechanisms underlying the simultaneous tilt illusion (TI) and the successive tilt aftereffect (TAE). We measured the TI and TAE for four subjects for combinations of test and inducing stimuli modulated along either the same or orthogonal axes of colour space within the L+M+S, L-M colour-luminance plane. The largest TI and TAE were found when test and inducer were modulated along the same axis of colour space. The TI consistently showed greater selectivity for colour/luminance than the TAE. The results are discussed in relation to the known chromatic properties of the primate visual pathways. Specifically, we suggest that both the TI and TAE involve colour- and luminance-specific neurons in primary visual cortex as well as cue-invariant mechanisms in extrastriate cortex.

Orientation opponency in human vision revealed by energy-frequency analysis

Studies of second-order visual processing have primarily been concerned with understanding the mechanisms for detecting spatiotemporal variations in such attributes as contrast, orientation, spatial frequency, etc. Here, we have examined the orientation characteristics of second-order processes using bandpass noise whose Fourier energy is sinusoidally modulated across orientation, rather than across space or time. Sensitivity for detecting orientation-energy modulations was measured as a function of modulation frequency. The sensitivity function was bandpass, with a pronounced peak at an orientation frequency of 4 cycles/pi. An inverse Fourier transform of the sensitivity function revealed a filter profile displaying a centre-surround antagonism across orientation, with an excitatory centre within 6-9 deg and inhibitory lobes at 15-20 deg from the filter's centre. The degree of centre-surround antagonism increased with stimulus size far beyond the spatial range of the first-order filters (more than 64 times the dominant spatial wavelength of the noise carrier). These results suggest that second-order processing involves 'orientation-opponent' channels that extract differences in first-order outputs across orientation over a wide area of the visual field.

Visual orientation illusions: global mechanisms involved in hierarchical effects and frames of reference

Five experiments were conducted in order to determine which of two hypotheses, initially proposed by Rock (1990), accounts for interactions between oriented elements in a visual scene. We also explored the suggestion that two hypothetical processes--namely, frame of reference and hierarchical organization--describe phenomena arising from distinct mechanisms (Spinelli, Antonucci, Daini, Martelli, & Zoccolotti, 1999). Double inducing stimulus versions of one-dimensional and two-dimensional tilt illusions, the rod-and-frame illusion, and combinations of these were used. Our data suggest that both hypotheses can predict orientation interactions in conditions in which only one mechanism--namely, the global visual mechanism of symmetry axes extraction (Wenderoth & Beh, 1977)--is activated. Which hypothesis is appropriate to predict the perceived orientation depends on some physical features of the objects.

Contour detection based on nonclassical receptive field inhibition

We propose a biologically motivated method, called nonclassical receptive field (non-CRF) inhibition (more generally, surround inhibition or suppression), to improve contour detection in machine vision. Non-CRF inhibition is exhibited by 80% of the orientation-selective neurons in the primary visual cortex of monkeys and has been shown to influence human visual perception as well. Essentially, the response of an edge detector at a certain point is suppressed by the responses of the operator in the region outside the supported area. We combine classical edge detection with isotropic and anisotropic inhibition, both of which have counterparts in biology. We also use a biologically motivated method (the Gabor energy operator) for edge detection. The resulting operator responds strongly to isolated lines, edges, and contours, but exhibits weak or no response to edges that are part of texture. We use natural images with associated ground truth contour maps to assess the performance of the proposed operator for detecting contours while suppressing texture edges. Our method enhances contour detection in cluttered visual scenes more effectively than classical edge detectors used in machine vision (Canny edge detector). Therefore, the proposed operator is more useful for contour-based object recognition tasks, such as shape comparison, than traditional edge detectors, which do not distinguish between contour and texture edges. Traditional edge detection algorithms can, however, also be extended with surround suppression. This study contributes also to the understanding of inhibitory mechanisms in biology.

Adaptation in the corticothalamic loop: computational prospects of tuning the senses

The present article discusses computational hypotheses on corticothalamic feedback and modulation of cortical response properties. We have recently proposed that the two phenomena are related, hypothesizing that neuronal velocity preference in the visual cortex is altered by feedback to the lateral geniculate nucleus. We now contrast the common view that response adaptation to stimuli subserves a function of redundancy reduction with the idea that it may enhance cortical representation of objects. Our arguments lead to the concept that the corticothalamic loop is involved in reducing sensory input to behaviourally relevant aspects, a pre-attentive gating.

The importance of spatial scale in determining illusions of orientation

The twisted-cord illusion is a powerful demonstration of interaction between 1st-order (luminance-defined) and 2nd-order (contrast-defined) orientation processing. The perceived orientation of contrast-defined objects is pulled towards their 1st-order orientation content when the difference in orientation is small (Fraser effect), yet is pushed away from the 1st-order content at large orientation differences (Zöllner effect). Here we show that the relative spatial scale of carrier and envelope represents a decisive factor in determining the magnitude and direction of such interactions. We conclude that the perceived 2nd-order structure of a stimulus is biased by the properties of the 1st-order structure in a manner that depends on relative, rather than absolute spatial scale.

Neural grouping and geometric effect in the determination of apparent orientation

We propose that neural grouping of retinotopically distributed responses in the primary visual cortex (V1) is essential for the determination of apparent tilt, including the tilt illusion. Our psychophysical study shows that apparent tilt is independent of stereo disparity, hue, or contrast of bars, which determine the ownership of their intersection. This leads us to suspect that the neuronal responses within the intersection are excluded from the computation of apparent tilt. To investigate the underlying cortical mechanisms, we developed and examined a V1 network model including the collinear connections observed in the superficial layers. The model shows good agreement with the results of psychophysical experiments, including segmentation independence, contrast dependence, and apparent tilt for various stimuli. The results suggest that collinear connections underlie the neural grouping that excludes the intersection region and establishes the independence of segmentation.

Sex differences in susceptibility to the Poggendorff illusion

This study presents experimental results indicating that there are sex differences in the susceptibility to a geometric optical illusion. Participants (57 male and 39 female undergraduate students) performed 3 trials on a test involving the Poggendorff illusion. Analysis indicated that the magnitude of the illusion diminished significantly with each trial and that the percent perceived error was significantly larger for women than for men. This finding is consistent with the numerous studies which have indicated better visuospatial abilities for men than for women.

Comparison of texture features based on Gabor filters

Texture features that are based on the local power spectrum obtained by a bank of Gabor filters are compared. The features differ in the type of nonlinear post-processing which is applied to the local power spectrum. The following features are considered: Gabor energy, complex moments, and grating cell operator features. The capability of the corresponding operators to produce distinct feature vector clusters for different textures is compared using two methods: the Fisher (1923) criterion and the classification result comparison. Both methods give consistent results. The grating cell operator gives the best discrimination and segmentation results. The texture detection capabilities of the operators and their robustness to nontexture features are also compared. The grating cell operator is the only one that selectively responds only to texture and does not give false response to nontexture features such as object contours.

Modification of response functions of cat visual cortical cells by spatially congruent perturbing stimuli

Responses of cat striate cortical cells to a drifting sinusoidal grating were modified by the superimposition of a second, perturbing grating (PG) that did not excite the cell when presented alone. One consequence of the presence of a PG was a shift in the tuning curves. The orientation tuning of all 41 cells exposed to a PG and the spatial frequency tuning of 83% of the 23 cells exposed to a PG showed statistically significant dislocations of both the response function peak and center of mass from their single grating values. As found in earlier reports, the presence of PGs suppressed responsiveness. However, reductions measured at the single grating optimum orientation or spatial frequency were on average 1.3 times greater than the suppression found at the peak of the response function modified by the presence of the PG. Much of the loss in response seen at the single grating optimum is thus a result of a shift in the tuning function rather than outright suppression. On average orientation shifts were repulsive and proportional (approximately 0.10 deg/deg) to the angle between the perturbing stimulus and the optimum single grating orientation. Shifts in the spatial frequency response function were both attractive and repulsive, resulting in an overall average of zero. For both simple and complex cells, PGs generally broadened orientation response function bandwidths. Similarly, complex cell spatial frequency response function bandwidths broadened. Simple cell spatial frequency response functions usually did not change, and those that did broadened only 4% on average. These data support the hypothesis that additional sinusoidal components in compound stimuli retune cells' response functions for orientation and spatial frequency.

Interaction between first- and second-order orientation channels revealed by the tilt illusion: psychophysics and computational modelling

This paper examines the interaction between first- and second-order contours in the orientation domain. Using the simultaneous tilt illusion (TI), we show that the apparent rotation of a vertical test grating away from that of a surrounding inducing grating (repulsion effect) occurs when both the inducing and test grating are either first- or second-order. Furthermore, a significant repulsion effect is obtained when a first-order inducing grating surrounds a second-order test. If lateral inhibitory interactions between populations of orientation selective neurons provides a plausible explanation for orientation repulsion effects [Blakemore, C. B. Carpenter, R. H. S. & Georgeson, M. A. (1970) Nature, 228, 37-39], it is likely that the cue-invariant mechanisms that encodes the orientation of first- and second-order contours also exhibit inhibitory interactions. A two-channel computational model of orientation encoding is presented where one channel encodes only first-order stimuli while the second channel encodes both first- and second-order contours. In addition to predicting the orientation repulsion effects we observed, the model also provides a functional account of orientation attraction effects in terms of the responses of populations of orientation-tuned neurons.

The tilt-constancy theory of visual illusions

The authors argue that changes in the perception of vertical and horizontal caused by local visual cues can account for many classical visual illusions. Because the perception of orientation is influenced more by visual cues than gravity-based cues when the observer is tilted (e.g., S. E. Asch & H. A. Witkin, 1948), the authors predicted that the strength of many visual illusions would increase when observers were tilted 30 degrees. The magnitude of Zöllner, Poggendorff, and Ponzo illusions and the tilt-induction effect substantially increased when observers were tilted. In contrast, the Müller-Lyer illusion and a size constancy illusion, which are not related to orientation perception, were not affected by body orientation. Other theoretical approaches do not predict the obtained pattern of results.

Orientation processing mechanisms revealed by the plaid tilt illusion

The tilt after-effect (TAE) and tilt illusion (TI) have revealed a great deal about the nature of orientation coding of 1-dimensional (1D) lines and gratings. Comparatively little research however has addressed the mechanisms responsible for encoding the orientation of 2-dimensional (2D) plaid stimuli. A multi-stage model of edge detection has recently been proposed [Georgeson, M. A. (1998) Image & Vision Computing, 16(6-7), 389-405] to account for the perceived structure of a plaid stimulus that incorporates extraction of the zero-crossings (ZCs) of the plaid. Data is presented showing that the ZCs of a plaid inducing stimulus can interact with vertical grating test stimulus to induce a standard tilt illusion. However, by considering the second-order structure of a plaid rather than ZCs, it was shown that the perceived orientation of the vertical test grating results from the combination of orientation illusions due to the first- and second-order components of an inducing plaid. The data suggest that the mechanisms encoding the orientation of second-order contours are similar to, and interact directly with, those that encode first-order contours.

The Ponzo illusion and the perception of orientation

A new theory, called the tilt constancy theory, claims that the Ponzo illusion is caused by the misperception of orientation induced by local visual cues. The theory relates the Ponzo illusion-along with the Zöllner, Poggendorff, Wündt-Hering, and cafe wall illusions-to the mechanisms that enable us to perceive stable orientations despite changes in retinal orientation or body orientation. In Experiment 1, the magnitude of the misperception of orientation was compared with the magnitude of the Ponzo illusion. In Experiment 2, predictions of the tilt constancy theory were compared with accounts based on (1) low spatial frequencies in the image, (2) memory comparisons (pool-and-store model), and (3) relative sizejudgments. In Experiment 3, predictions of the tilt constancy theory were tested against predictions of the assimilation theory of Pressey and his colleagues. In the final experiment, the orientation account was compared with theories based on linear perspective and inappropriate size constancy. The results support the tilt constancy theory.

Selective visual attention modulates the direct tilt aftereffect

One's being able to allocate attention to particular regions or properties of the visual field is fundamental to visual information processing. Visual attention determines what input is carefully analyzed and what input is more or less ignored. But at what stage of the visual system is this process evident? We describe three experiments that demonstrate an effect of voluntary spatial attention and voluntary object-based attention on an orientation illusion (the tilt aftereffect) that is believed to take place in primary visual cortex. This finding, in which selective visual attention influences adaptation to visual orientation information, contributes to mounting evidence for a view of visual perception in which mutual interaction takes place between high-level and low-level subsystems.

A physiologically-based explanation of disparity attraction and repulsion

Westheimer and Levi [(1987) Vision Research, 27, 1361-1368] found that when a few isolated features are viewed foveally, the perceived depth of a feature depends not only on its own disparity but also on those of its neighbors. The nature of this interaction is a function of the lateral separation between the features: When the distance is small the features appear to attract each other in depth but the interaction becomes repulsive at larger distances. Here we introduce a two-dimensional extension of our recent stereo model based on the physiological studies of Ohzawa, DeAngelis and Freeman [(1990) Science, 249, 1037-1041] and demonstrate through analyses and simulations that these observations can be naturally explained without introducing ad hoc assumptions about the connectivity between disparity-tuned units. In particular, our model can explain the distance-dependent attraction/repulsion phenomena in both the vertical-line configuration used by Westheimer [(1986) Journal for Neurophysiology, 370, 619-629], and the horizontal-line-and-point configuration used by Westheimer and Levi. Thus, the psychophysically observed disparity interaction may be viewed as a direct consequence of the known physiological organization of the binocular receptive fields. We also find that the transition distance at which the disparity interaction between features changes from attraction to repulsion is largely determined by the preferred spatial frequency and orientation distributions of the cells used in the disparity computation. This result may explain the observed variations of the transition distance among different subjects in the psychophysical experiments. Finally, our model can also reproduce the observed effect on the perceived disparity when the disparity magnitude of the neighboring features is changed.

Inter-attribute tilt effects and orientation analysis in the visual brain

A test grating appears to be tilted away from an inducing grating for small angular separations (repulsion), but towards the inducing grating for larger angular separations (attraction). Previous research on luminance gratings suggests that repulsion is caused by local inhibition in cortical areas V1 and/or V2, and that the attraction involves global interactions beyond V1, in extrastriate areas. Experiments reported here demonstrate attribute invariant attraction and repulsion effects for gratings specified by luminance, motion, and disparity contrasts. A frame surrounding the inducing grating abolishes only the attraction effect, but a spatial frequency difference, or a small gap between the inducer and test gratings, abolishes only the repulsion effect, irrespective of the attributes that specify the gratings. It is proposed that detectors selectively sensitive to attribute invariant orientation and size exist in early cortical sites such as V1 and/or V2.

Attention and short-term memory in contrast detection

Low-contrast visual stimuli have been found to produce a memory trace, enhancing subsequent target detection for as much as 16 s. Here we show that the memory trace depends on dynamic interactions between low-level stimulus properties and a higher-level gating process. Detection of vertical targets (Gabor signals) was enhanced by preceding vertical Gabor primes, but suppressed by preceding tilted primes--pointing to a competitive process of dynamic resource allocation. The priming effect was also dependent on a temporal cue, activating a sensory gating process with maximal effect at 300-500 ms delay. The results suggest a two-step process in which attention affects transition between perception and memory: a non-selective gating process followed by competition between overlapping representations.