The role of terminators and occlusion cues in motion integration and segmentation: a neural network model

The perceptual interaction of terminators and occlusion cues with the functional processes of motion integration and segmentation is examined using a computational model. Inte-gration is necessary to overcome noise and the inherent ambiguity in locally measured motion direction (the aperture problem). Segmentation is required to detect the presence of motion discontinuities and to prevent spurious integration of motion signals between objects with different trajectories. Terminators are used for motion disambiguation, while occlusion cues are used to suppress motion noise at points where objects intersect. The model illustrates how competitive and cooperative interactions among cells carrying out these functions can account for a number of perceptual effects, including the chopsticks illusion and the occluded diamond illusion. Possible links to the neurophysiology of the middle temporal visual area (MT) are suggested

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

The Effects of Aging on Orientation Discrimination

Visual perception relies on low-level encoding of local orientation. Recent studies show an age-dependent impairment in orientation discrimination of stimuli embedded in external noise, suggesting that encoding of orientation is inefficient in older adults. In the present study we ask whether aging also reduces decoding, i.e., selecting the neural representations of target orientation while discarding those conflicting with it. We compared younger and older participants capability (mean age 24 and 68 years respectively) in discriminating whether the orientation of a Gabor target was left or right from the vertical. We measured (d0), an index of discrimination sensitivity, for orientation offset ranging from 1 to 12. In the isolated target condition, d0 was reduced by aging and, in the older group, did not increase with orientation offset, thus resulting in a larger group difference at large than small orientation offsets from the vertical. Moreover, oriented elements in the background impaired more discrimination in the older group. However, distractors reduced more d0 when target-background orientation offset was large than when target and flanker had similar orientation, indicating that the effect of the background was not local, i.e., due to target inhibition by similarly oriented flankers. Altogether, these results indicate that aging reduces the efficiency in discarding the response to orientations differing from the target. Our results suggest that neural decision-making mechanisms, involving not only signal enhancement but also non-signal inhibition, become inefficient with age. This suggestion is consistent with the neurophysiological evidence of inefficient visual cortical inhibition in aging

Spatial context in the early visual system

Important visual objects in our everyday life, such as fellow people, passing cars or birds perhaps, are not point-like structures but often occupy considerable amounts of the visual field. However, each photoreceptor in our eyes samples just a tiny portion of the visual field and somehow the visual system should integrate these local signals. This process takes place mainly in the visual cortex and, while higher-order visual areas play an important role in perception of extended structures, it is now well established that visual neurons at the first cortical steps of seeing integrate broad spatial context into their responses. The main purpose of this thesis was to provide detailed information concerning the spatial structure of the mechanisms that underlie integration of spatial context in the early visual system. The opening study of this thesis showed that the antagonistic Gaussians structure that has been used for modeling context integration in single visual neurons provides a relatively accurate description of the process also in the human visual system. The first study introduced a novel method for connecting perceptual and neuroimaging measurements and this method was applied in the second study of this thesis. The second study showed that the human visual system integrates spatial context in terms of its visual field size instead of the size of its cortical representation. The third study showed that context is integrated over an unexpectedly large region of the visual field and that spatially distant context may sometimes increase the contrast response of the visual system. The closing study showed that orientation specificity of the integration of spatial context depends on distance both in single neurons in the macaque primary visual cortex and in human perception. The knowledge acquired in this thesis will be generally useful in applications that require understanding of the human visual system.Arkielämän kannalta tärkeät visuaaliset objektit kuten ihmiset, ohikiitävät autot ja kenties kissat, ovat harvoin pistemäisiä, mutta sen sijaan voivat peittää laajankin alueen näkökentästä. Näköaistinsolut prosessoivat kuvainformaatiota erittäin pieneltä näkökentän alueelta ja näköjärjestelmän tulee jollain tavoin yhdistää nämä paikalliset signaalit. Vaikka näköaivokuoren myöhäisten alueiden merkitys spatiaalisesti laajojen objektien havaitsemisessa onkin merkittävä, nykytietämyksen valossa on kiistatonta että myös varhaisten näköaivokuorten hermosolut integroivat spatiaalista kontekstia laajalta näkökentän alueelta. Tässä väitöskirjassa tutkitaan konteksti-integraation taustalla olevien mekanismien spatiaalista rakennetta varhaisessa näköjärjestelmässä. Väitöskirjan ensimmäisessä osatyössä osoitettiin että konteksti-integraatiota yksittäisissä hermosoluissa kuvaavat kahden antagonistisen Gaussilaisen mallit ovat melko hyviä kuvauksia konteksti-integraatiomekanismien spatiaalisesta rakenteesta myös ihmisen näköjärjestelmässä. Ensimmäisessä osatyössä kehitettiin menetelmä joka mahdollistaa havainto- ja aivokuvantamismittausten uudenlaisen yhdistämisen. Tätä menetelmää sovellettiin toisessa osatyössä, jonka päätulos oli konteksti-integraation riippuvuus ärsykkeen koosta näkökentässä sen sijaan että se olisi sidoksissa ärsykkeen edustuksen kokoon aivokuorella. Kolmannessa osatyössä osoitettiin, että kontekstia integroidaan huomattavan laajalta alueelta ja että spatiaalisesti etäinen konteksti saattaa toisinaan vahvistaa näköjärjestelmän kontrastivastetta. Neljäs tutkimus osoitti, että konteksti-integraation valikoivuus orientaatiolle riippuu etäisyydestä niin ihmisen näköhavainnoissa kuin makaki-apinan ensimmäisen näköaivokuoren soluissakin. Tämän väitöskirjan tuloksia voidaan hyödyntää sovelluksissa joissa tarvitaan tietoa ihmisen näköjärjestelmän toiminnasta

Context-Sensitive Binding by the Laminar Circuits of V1 and V2: A Unified Model of Perceptual Grouping, Attention, and Orientation Contrast

A detailed neural model is presented of how the laminar circuits of visual cortical areas V1 and V2 implement context-sensitive binding processes such as perceptual grouping and attention. The model proposes how specific laminar circuits allow the responses of visual cortical neurons to be determined not only by the stimuli within their classical receptive fields, but also to be strongly influenced by stimuli in the extra-classical surround. This context-sensitive visual processing can greatly enhance the analysis of visual scenes, especially those containing targets that are low contrast, partially occluded, or crowded by distractors. We show how interactions of feedforward, feedback and horizontal circuitry can implement several types of contextual processing simultaneously, using shared laminar circuits. In particular, we present computer simulations which suggest how top-down attention and preattentive perceptual grouping, two processes that are fundamental for visual binding, can interact, with attentional enhancement selectively propagating along groupings of both real and illusory contours, thereby showing how attention can selectively enhance object representations. These simulations also illustrate how attention may have a stronger facilitatory effect on low contrast than on high contrast stimuli, and how pop-out from orientation contrast may occur. The specific functional roles which the model proposes for the cortical layers allow several testable neurophysiological predictions to be made. The results presented here simulate only the boundary grouping system of adult cortical architecture. However we also discuss how this model contributes to a larger neural theory of vision which suggests how intracortical and intercortical feedback help to stabilize development and learning within these cortical circuits. Although feedback plays a key role, fast feedforward processing is possible in response to unambiguous information. Model circuits are capable of synchronizing quickly, but context-sensitive persistence of previous events can influence how synchrony develops. Although these results focus on how the interblob cortical processing stream controls boundary grouping and attention, related modeling of the blob cortical processing stream suggests how visible surfaces are formed, and modeling of the motion stream suggests how transient responses to scenic changes can control long-range apparent motion and also attract spatial attention.Defense Advanced Research Projects agency and the Office of Naval Research (N00014-95-1-0409); National Science Foundation (IRI 94-01659, IRI 97-20333); ONR (N00014-92-J-1309, N00014-95-1-0657

Temporal Characteristics of Monoptic, Dichoptic and Half-Binocular Collinear Lateral Masking of Contrast Detection

Purpose: The temporal characteristics of dichoptic contrast integration across space in primary visual cortex are relatively unknown. This study investigated the effect of varying interstimulus interval (ISI) and flank duration on contrast detection threshold (CDT) of a sinusoid target under monoptic, dichoptic and half-binocular viewing. Methods: Eleven subjects with normal vision participated for a mean of 25 hours each. In the main experiment, target and flanks were 3 cpd vertical sinusoids with 6 lambda (sigma = 1.5 lambda) center-to-center vertical separation. Flank contrast was normalized to 3X flank CDT. Flanks were presented at 4 durations (67-500ms) and ISIs were presented at 8 durations (0-2500ms) resulting in 0-3000ms stimulus onset asynchronies (SOA). Target presentations were 250ms to dominant eye using a mirror haploscope and septum. Flanks were presented to dominant (monoptic and half-binocular) and non-dominant eyes (dichoptic and half-binocular). Forward masking was used throughout with a 1-FC detection paradigm and 7-level MOCS. Each target CDT was the product of approximately 700 trials. Results: As expected, simultaneous presentation resulted in CDT facilitation (monoptic = 19%± 3.86% (SE), dichoptic = 13.9%± 4.00%, half-binocular = 18.0%± 4.20%). For all viewing conditions, relative facilitation decreased as SOA increased up to 1000ms. Unexpectedly, dichoptic flanks produced significant CDT suppression (p \u3c 0.05) at 500-1000ms SOAs that was maximal at the 1000ms SOA (9.9%± 5.1%). All viewing conditions approached no effect at the longest SOAs (1500-3000ms). Flank duration had a significantly greater contribution to the overall effect than ISI for monoptic (p \u3c 0.01) and half-binocular (p \u3c 0.05) viewing. Discussion: The collinear CDT facilitation produced by intra-ocular and inter-ocular flanks at shorter SOAs is consistent with lateral connections in primary visual cortex. The temporal aspects of longer SOA, dichoptic CDT suppression observed in this study are consistent with prior studies of illusory contour perception. Conclusion: I propose the novel hypothesis that the CDT suppression produced by dichoptic collinear flanks at longer SOAs is due to one-way, contrast adaptation from lateral propagation that produced the effect of a collinear, illusory contour. This hypothesis was supported by the results of a supplemental, orthogonal flank experiment

A Push-Pull CORF Model of a Simple Cell with Antiphase Inhibition Improves SNR and Contour Detection

We propose a computational model of a simple cell with push-pull inhibition, a property that is observed in many real simple cells. It is based on an existing model called Combination of Receptive Fields or CORF for brevity. A CORF model uses as afferent inputs the responses of model LGN cells with appropriately aligned center-surround receptive fields, and combines their output with a weighted geometric mean. The output of the proposed model simple cell with push-pull inhibition, which we call push-pull CORF, is computed as the response of a CORF model cell that is selective for a stimulus with preferred orientation and preferred contrast minus a fraction of the response of a CORF model cell that responds to the same stimulus but of opposite contrast. We demonstrate that the proposed push-pull CORF model improves signal-to-noise ratio (SNR) and achieves further properties that are observed in real simple cells, namely separability of spatial frequency and orientation as well as contrast-dependent changes in spatial frequency tuning. We also demonstrate the effectiveness of the proposed push-pull CORF model in contour detection, which is believed to be the primary biological role of simple cells. We use the RuG (40 images) and Berkeley (500 images) benchmark data sets of images with natural scenes and show that the proposed model outperforms, with very high statistical significance, the basic CORF model without inhibition, Gabor-based models with isotropic surround inhibition, and the Canny edge detector. The push-pull CORF model that we propose is a contribution to a better understanding of how visual information is processed in the brain as it provides the ability to reproduce a wider range of properties exhibited by real simple cells. As a result of push-pull inhibition a CORF model exhibits an improved SNR, which is the reason for a more effective contour detection.</p

Components of bottom-up gaze allocation in natural images

Recent research [Parkhurst, D., Law, K., & Niebur, E., 2002. Modeling the role of salience in the allocation of overt visual attention. Vision Research 42 (1) (2002) 107–123] showed that a model of bottom-up visual attention can account in part for the spatial locations fixated by humans while free-viewing complex natural and artificial scenes. That study used a definition of salience based on local detectors with coarse global surround inhibition. Here, we use a similar framework to investigate the roles of several types of non-linear interactions known to exist in visual cortex, and of eccentricity-dependent processing. For each of these, we added a component to the salience model, including richer interactions among orientation-tuned units, both at spatial short range (for clutter reduction) and long range (for contour facilitation), and a detailed model of eccentricity-dependent changes in visual processing. Subjects free-viewed naturalistic and artificial images while their eye movements were recorded, and the resulting fixation locations were compared with the models’ predicted salience maps. We found that the proposed interactions indeed play a significant role in the spatiotemporal deployment of attention in natural scenes; about half of the observed inter-subject variance can be explained by these different models. This suggests that attentional guidance does not depend solely on local visual features, but must also include the effects of interactions among features. As models of these interactions become more accurate in predicting behaviorally-relevant salient locations, they become useful to a range of applications in computer vision and human-machine interface design