Biomedical subjects

D Alais

Publications and source records attributed to D Alais.

17 recordsLinked to original sources

PubMed2001

Form constraints in motion binding.

Visual analyses of form and motion proceed along parallel streams. Unified perception of moving forms requires interactions between these streams, although whether the interactions occur early or late in cortical processing remains unresolved. Using rotating outlined shapes sampled through apertures, we showed that binding local motions into global object motion depends strongly on spatial configuration. Identical local motion components are perceived coherently when they define closed configurations, but usually not when they define open configurations. Our experiments show this influence arises in early cortical levels and operates as a form-based veto of motion integration in the absence of closure.

Form Perception↗

PubMed2000

On binocular alternation.

Diaz-Caneja (1928) made some prescient observations about binocular rivalry. Being in French, however, his paper remained largely unknown to the broader research community. His findings are similar to those reported very recently by contemporary researchers who had independently observed similar phenomena. Using concentric circles and parallel lines as stimuli, Diaz-Caneja presented half of each form to opposite eyes to provoke binocular rivalry. He observed periods in the ensuing binocular alternations in which rivalry occurred between the good Gestalt forms, despite the fact that they were distributed between the eyes. He proposed that each half of a good form generates synchronised oscillations in the visual system, and that this synchronisation enables the dichoptically viewed halves of the one form to be perceived as a whole.

Animals↗

PubMed1999

Grouping visual features during binocular rivalry.

During binocular rivalry, portions of one eye's view may be perceptually dominant while other portions are suppressed; at any given moment, overall dominance often resembles a patchwork mixture of the two eyes' views. This study investigates the potency of two Gestalt grouping cues--good continuation and common fate--to promote synchronous fluctuations in dominance of two, spatially separated rival targets. Two grating patches were presented to the left eye paired dichoptically with random-dot patches presented to corresponding right eye locations. The orientations of the two gratings were either collinear, parallel or orthogonal. Gratings underwent contrast modulations that were either correlated (identical contrast changes) or uncorrelated (independent contrast changes). Over 60 s trials, observers pressed one key when the left grating predominated, another when the right grating predominated and both keys when both were concurrently visible. Correlated contrast modulation promoted joint grating predominance relative to the uncorrelated conditions, an effect strongest for collinear gratings. Joint predominance depended strongly on the angular separation between gratings and the temporal phase-lag in contrast modulations. These findings may reflect neural interactions subserved by lateral connections between cortical hypercolumns.

Gestalt Theory↗

PubMed1999

Neural strength of visual attention gauged by motion adaptation.

Single-cell and neuroimaging studies reveal that attention focused on a visual object markedly amplifies neural activity produced by features of the attended object. In a psychophysical study, we found that visual attention could modulate the strength of weak motion signals to the point that the perceived direction of motion, putatively registered early in visual processing, was powerfully altered. This strong influence of attention on early motion processing, beside complementing neurophysiological evidence for attentional modulation early in the visual pathway, can be measured in terms of equivalent motion energy, and thus provides a useful metric for quantifying attention's effects.

Adaptation, Physiological↗

PubMed1999

Determinants of fusion of dichoptically presented orthogonal gratings.

Liu, Tyler, and Schor (1992 Vision Research 32 1471-1479) reported the surprising finding that dichoptically presented orthogonal sine-wave gratings do not always produce binocular rivalry. Gratings of high spatial frequency, and especially of low contrast, fuse to produce a stable percept of a dichoptic plaid. Using a somewhat different perceptual task, we replicated those findings and extended them. The probability of a plaid percept is higher for square-wave gratings than for sine-wave gratings, and higher still for rectangular-wave gratings with high duty cycles (with very thin light or dark bars). Experiments were conducted to test whether this duty-cycle effect was due to changes in overall luminance, or in the size of the regions of luminance congruity (which may reduce the probability of rivalry), but no such effects could account for the results. The presence of locally conflicting contour information in the two eyes was shown to be an important determinant of rivalry onset, but, since removing such regions did not eliminate rivalry, other factors also have a role to play. The spatial frequency composition of the gratings is one such factor which is consistent with all of the findings we report.

Contrast Sensitivity↗

PubMed1998

Interactions between global motion and local binocular rivalry.

Binocular rivalry is thought to arise from a low-level cortical site. Experiment 1 evaluates this claim with respect to local and global motion processing by using a multiple-aperture motion stimulus and measuring the predominance of global coherence while one of the component gratings is engaged in rivalry. Results show that rivalry suppression of the component grating precludes global coherence. Presumable, suppression prevents the component motion signal from advancing to higher-level global motion areas, suggesting rivalry occurs between local and global motion processing. However, feedback from higher-level mechanisms might exert an influence on binocular rivalry and thus Experiment 2 measures how the predominance of a local target engaged in binocular rivalry with a competing local stimulus is affected when the target forms part of a globally coherent motion stimulus. The augmented level of target predominance during global motion relative to local motion indicates that higher-level motion mechanisms can feedback and influence the binocular rivalry process. Together, these data imply a looping hierarchy of motion processing stages, with rivalry suppression transpiring at an intermediate level and subject to feedback from higher-level motion areas.

Humans↗

PubMed1998

Local and global factors affecting the coherent motion of gratings presented in multiple apertures.

Using stimuli composed of two independent gratings viewed through multiple apertures, we investigate a number of parameters affecting the integration of locally ambiguous motions into globally coherent motion. In four experiments, we varied local factors (grating spatial frequency, speed, contrast, duty cycle, orientation) and global factors (degree of similarity and common fate between the gratings, and symmetry in the configuration of the grating pattern) and examined their effects on global motion coherence. Our results, confirming accounts offered by previous investigators, indicate that local competition between motion signals generated by contours (ambiguous) and their line terminations (unambiguous) is important in determining global motion coherence in multiple-aperture stimuli. Our results also indicate that global factors can affect perceived coherence independently of local motion signals, suggesting the involvement of higher-level motion areas and a role for non-motion processes such as those involved in pattern and form perception. Comparing motion coherence with other two-dimensional (2-D) stimuli (plaids) shows that 2-D multiple-aperture stimuli are not analogous and that coherence models derived from plaid stimuli do not account for the data.

Humans↗

PubMed1998

Visual features that vary together over time group together over space.

The visual system perceives objects as coherent even when some parts are hidden or discontinuous. How this representation is constructed from local features of many nearby objects is termed the 'binding problem.' Here we manipulate contrast in several drifting gratings that can be perceived as either independent objects or parts of a single object. Contrast modulations that are correlated in time enhance perceptual coherence, whereas uncorrelated modulations impair coherence. Presumably, correlated contrast modulations produce correlated responses in cortical neurons. Therefore, our results are consistent with the hypothesis that temporal correlation of neural activity is important for feature binding.

Contrast Sensitivity↗

PubMed1997

The size and number of plaid blobs mediate the misperception of type-II plaid direction.

The misperceived direction of type-II plaids has posed a problem for the intersection of constraints (IOC) model of two-dimensional motion perception. Alais et al. (1994, Vision Research, 34, 1823-1834) examined the perceived direction of type-II plaids and concluded that in addition to the direction signalled by the IOC process, a monocular mechanism signalling the motion of plaid features (blobs) is also involved in plaid perception. It was shown that the prominence of this monocular signal in plaid direction judgements depended on several variables, and the notion of blob "optimality" was introduced. This explained the more veridical direction of "optimal" blob plaids in terms of their more effectively activating the proposed feature-sensitive motion mechanism. One distinction between "optimal" and "non-optimal" blob plaids is their different component spatial frequencies, which necessarily entails a difference in the number and size of the blobs and thus raises potential confounds, since both the nature of the blobs and the components differ, which might affect the postulated blob mechanism and/or the IOC process. In the present paper, by offsetting changes in spatial frequency with changes in aperture size so that blob number is held constant, we examine whether differences in sheer blob number or size can alter perceived type-II plaid direction. The results reveal effects of both blob number and blob size, and their implications for the underlying mechanism are considered. Alternative accounts of the results in terms of the IOC model or revisions of it cannot explain the data. Comparison of monocular and binocular conditions adds further systematic evidence in support of the monocularity of the feature-sensitive motion mechanism.

Analysis of Variance↗

PubMed1996

Further evidence for monocular determinants of perceived plaid direction.

This report adds to existing evidence that a monocular, feature-sensitive motion mechanism is involved in two-dimensional (2-D) motion processing, and also accounts for an earlier, unexplained result [Alais et al.(1994) Vision Research, 34, 1823-1834]. The central finding is that the perceived direction of a monocularly viewed type II plaid changes over a period of continuous exposure such that post-adaptation direction judgements exhibit more of the component-direction bias known to occur with these stimuli than pre-adaptation judgements. These adaptation effects are confined to the adapted eye: when the adapting stimulus is presented to one eye, pre- and post-adaptation direction judgements made with the other, non-adapted eye are identical. These results strongly suggest the involvement of a monocular motion mechanism in two-dimensional motion processing, in addition to the more commonly presumed binocular mechanisms.

Adaptation, Ocular↗

PubMed1995

The perceived direction of textured gratings and their motion aftereffects.

The stimuli in these experiments are square-wave luminance gratings with an array of small random dots covering the high-luminance regions. Owing to the texture, the direction of these gratings, when seen through a circular aperture, is disambiguated because the visual system is provided with an unambiguous motion energy. Thus, the direction of textured gratings can be varied independently of grating orientation. When subjects are required to judge the direction of textured gratings moving obliquely relative to their orientation, they can do so accurately (experiment 1). This is of interest because most studies of one-dimensional motion perception have involved (textureless) luminance-defined since-wave or square-wave gratings, and the perceived direction of these gratings is constrained by the aperture problem to be orthogonal to their orientation. Thus, direction and orientation have often been confounded. Interestingly, when subjects are required to judge the direction of an obliquely moving textured grating during a period of adaptation and then the direction of the motion aftereffect (MAE) immediately following adaptation (experiments 2 and 3), these directions are not directly opposite each other. MAE directions were always more orthogonal to the orientation of the adapting grating than the corresponding direction judgments during adaptation (by as much as 25 degrees). These results are not readily explained by conventional MAE models and possible accounts are considered.

Adaptation, Ocular↗

PubMed1994

A role for a low level mechanism in determining plaid coherence.

A number of recent studies have suggested that the "intersection of constraints" model of two dimensional motion perception, put forward by Adelson and Movshon [(1982) Nature, 300, 523-525], is incomplete. Evidence has been mounting that there is a second two-dimensional motion sensitive mechanism which is monocular and which appears to respond directly to the movement of the intersections (or "blobs") in a two-dimensional image. The current study extends these findings by demonstrating that the perceived coherence of a drifting plaid is largely under the control of a monocular mechanism. Prior exposure to a similarly drifting grating or plaid substantially raises the coherence threshold of a test plaid only if the same eye is adapted and tested. The threshold elevation is much more modest if the test plaid is presented to the unadapted eye, suggesting that coherence judgements are primarily based on the activity level of a monocular process--possibly the "blob tracking mechanism". The results of Expt 2 suggest the possibility that this monocular mechanism is inhibited by binocular exposure.

Adaptation, Ocular↗

PubMed1994

The contribution of one-dimensional motion mechanisms to the perceived direction of drifting plaids and their after effects.

When motion aftereffects (MAEs) are measured by adapting to a drifting plaid (simultaneous adaptation) or by adapting to the plaid's component gratings in alternation (alternating adaptation), it has been shown that the velocity and duration of the MAE are smaller in the latter case [Wenderoth, P., Bray, R. & Johnstone, S. (1988) Perception, 17, 81-91; Burke, D. & Wenderoth, P. (1993) Vision Research, 33, 351-359]. However, Burke and Wenderoth additionally reported that the directions of MAEs induced by simultaneous and alternating adaptation were identical, an apparent inconsistency if the differences in duration and velocity were due to the presence of "blobs" at the component grating intersects in the simultaneous case. Presumably, the direction of the "blobs" should also affect perceived plaid direction during adaptation and, hence, the MAE direction. In five experiments, we have measured both perceived adapting plaid and MAE direction, tested with both alternating and simultaneous adaptation, measured interocular transfer of plaid-induced MAEs and obtained MAE and plaid direction judgments under monocular and binocular viewing conditions. All of the data indicate that there is a blob tracking mechanism which is preferentially stimulated by plaids whose component gratings have high spatial frequency, low temporal frequency and high contrast. Differences between simultaneous and alternating adaptation emerge only when more optimal blobs are used, thus accounting for Burke and Wenderoth's failure to find a difference. The data also support Burke and Wenderoth's claim that the blob tracking mechanism is monocular: alternating and simultaneous adaptation produce identical MAEs under interocular transfer conditions, even using plaids with more optimal blobs. We also report the unexpected finding that plaids with more- and less-optimal blobs appear to drift in directions 20 degrees apart yet their aftereffects differ in direction by only 3-5 degrees. That is, more optimal blob plaids--compared with less optimal blob plaids--change both perceived plaid direction during adaptation and subsequent perceived MAE direction but the latter change is much more modest. Possible explanations of this dissociation are considered.

Adaptation, Ocular↗

PubMed1994

The role of the blobs in determining the perception of drifting plaids and their motion aftereffects.

Motion aftereffects (MAEs) can be induced by adaptation to a pair of differently oriented drifting gratings whether the gratings are presented simultaneously, as a coherent plaid, or in alternation. The fact that the former MAEs were generally larger than the latter led to the suggestion that simultaneous adaptation involved higher-level extrastriate processes not involved in the alternating effects. In the past few years evidence has accumulated that the difference is in fact due to a low-level monocular process which can be termed the 'blob-tracking mechanism'. A review is presented of the evidence on MAEs induced by simultaneous and alternating adaptation, the evidence for the monocularity of the blob-tracking mechanism, the data which implicate the blob mechanism in the determination of MAE magnitude, perceived plaid drift direction, and in perceived plaid coherence.

Humans↗

PubMed1990

Lack of evidence for a tactual Poggendorff illusion.

Lucca, Dellantonio, and Riggio (1986) reported large distortions in a tactual analogue of the visual Poggendorff illusion. They also reported large effects in the direction opposite to the visual illusion, which they termed "inversions." However, their evidence for such effects is questionable; they used what we consider to be inappropriate measurement and analysis procedures. In attempting to replicate their experiment, and in conducting four additional experiments, we found no evidence at all for their alleged tactual analogue of the visual Poggendorff effect. Instead, we demonstrated that "inversions" are likely due to the use of a raised stimulus display that causes artifactual mistracking, which is totally unrelated to normal mechanisms of alignment judgment. We also discuss the possible role of intersensory factors in the generation of tactual illusions.

Attention↗

PubMedDate not supplied

Reduction of a pattern-induced motion aftereffect by binocular rivalry suggests the involvement of extrastriate mechanisms.

Previous research suggests that plaid-induced motion aftereffects (MAEs) involve extrastriate mechanisms (Wenderoth et al., 1988). There is evidence also that binocular rivalry occurs beyond V1 and that it disrupts the processing of MAEs which are believed to be based upon extrastriate mechanisms (e.g. the spiral MAE) but not MAEs, such as linear MAE induced by a drifting grating, which are thought to arise in striate cortex (Wiesenfelder & Blake, 1990). The logical inference is that binocular rivalry during drifting plaid-induced adaptation should reduce the MAEs which result. We report experiments which confirm this prediction.

Adaptation, Ocular↗

PubMedDate not supplied

Monocular mechanisms determine plaid motion coherence.

Although the neural location of the plaid motion coherence process is not precisely known, the middle temporal (MT) cortical area has been proposed as a likely candidate. This claim rests largely on the neurophysiological findings showing that in response to plaid stimuli, a subgroup of cells in area MT responds to the pattern direction, whereas cells in area V1 respond only to the directions of the component gratings. In Experiment 1, we report that the coherent motion of a plaid pattern can be completely abolished following adaptation to a grating which moves in the plaid direction and has the same spatial period as the plaid features (the so-called "blobs"). Interestingly, we find this phenomenon is monocular: monocular adaptation destroys plaid coherence in the exposed eye but leaves it unaffected in the other eye. Experiment 2 demonstrates that adaptation to a purely binocular (dichoptic) grating does not affect perceived plaid coherence. These data suggest several conclusions: (1) that the mechanism determining plaid coherence responds to the motion of plaid features, (2) that the coherence mechanism is monocular, and thus (3), that it is probably located at a relatively low level in the visual system and peripherally to the binocular mechanisms commonly presumed to underlie two-dimensional (2-D) motion perception. Experiment 3 examines the spatial tuning of the monocular coherence mechanism and our results suggest it is broadly tuned with a preference for lower spatial frequencies. In Experiment 4, we examine whether perceived plaid direction is determined by the motion of the grating components or the features. Our data strongly support a feature-based model.

Adaptation, Ocular↗