Biomedical subjects

Nava Rubin

Publications and source records attributed to Nava Rubin.

9 recordsLinked to original sources

PubMed2006

Dynamical characteristics common to neuronal competition models.

Models implementing neuronal competition by reciprocally inhibitory populations are widely used to characterize bistable phenomena such as binocular rivalry. We find common dynamical behavior in several models of this general type, which differ in their architecture in the form of their gain functions, and in how they implement the slow process that underlies alternating dominance. We focus on examining the effect of the input strength on the rate (and existence) of oscillations. In spite of their differences, all considered models possess similar qualitative features, some of which we report here for the first time. Experimentally, dominance durations have been reported to decrease monotonically with increasing stimulus strength (such as Levelt's "Proposition IV"). The models predict this behavior; however, they also predict that at a lower range of input strength dominance durations increase with increasing stimulus strength. The nonmonotonic dependency of duration on stimulus strength is common to both deterministic and stochastic models. We conclude that additional experimental tests of Levelt's Proposition IV are needed to reconcile models and perception.

Action Potentials↗

PubMed2006

Cluster-based analysis of FMRI data.

We propose a method for the statistical analysis of fMRI data that tests cluster units rather than voxel units for activation. The advantages of this analysis over previous ones are both conceptual and statistical. Recognizing that the fundamental units of interest are the spatially contiguous clusters of voxels that are activated together, we set out to approximate these cluster units from the data by a clustering algorithm especially tailored for fMRI data. Testing the cluster units has a two-fold statistical advantage over testing each voxel separately: the signal to noise ratio within the unit tested is higher, and the number of hypotheses tests compared is smaller. We suggest controlling FDR on clusters, i.e., the proportion of clusters rejected erroneously out of all clusters rejected and explain the meaning of controlling this error rate. We introduce the powerful adaptive procedure to control the FDR on clusters. We apply our cluster-based analysis (CBA) to both an event-related and a block design fMRI vision experiment and demonstrate its increased power over voxel-by-voxel analysis in these examples as well as in simulations.

Algorithms↗

PubMed2005

Rapid detection of salient regions: evidence from apparent motion.

Most studies that have used Kanizsa-type illusory figures to investigate perceptual completion have treated the crisp bounding illusory contours (ICs) and the enclosed region as nondissociable stimulus attributes. However, there is evidence that enclosed "salient regions" (SRs; Stanley & Rubin, 2003) are detected even in cases when bounding ICs are not perceptually completed. Here we used apparent motion (AM) to test whether SRs are detected in the absence of crisp bounding ICs. Kanizsa-type stimuli were modified in ways that eliminated the bounding ICs, but the clear impression of an enclosed region remained. SR stimuli were embedded in an array of like inducers. On successive frames, the inducers in the array rotated in a way that resulted in translation of the enclosed region. Four speeds of translation were tested. Observers performed a two-alternative forced-choice task on the direction of translation. Perceptually completed SRs produced robust AM whether they were bound by crisp ICs or not-observer performance was as good and, in certain cases, even better for SRs with no bounding ICs. We interpret these findings within a theoretical framework that makes a distinction between region-based and contour-based segmentation processes that operate in concert to achieve segmentation of the visual scene.

Adult↗

PubMed2005

Differences in real and illusory shape perception revealed by backward masking.

Illusory contours (ICs) are thought to be a result of processes involved in the perceptual recovery of occluded surfaces. Here, we investigate the relationship between real and illusory contour perception using a shape discrimination task and backward masking paradigm. ICs can mask other ICs when times between mask onset and stimulus onset, or SOAs, are very long ( approximately 300 ms), but real contours (RCs) are not similarly effective. Masking is absent for RC masks at perceptually salient contrasts, as well as for those with contrast lowered to match the perceived brightness of the illusory surface. We also find that RCs are not masked at long SOAs, either by ICs or by other RCs. Finally, the masking seen between ICs can occur for different sizes of target and mask. The cross-size masking would not be expected if the masking were at a level sensitive to retinal contour location. The late masking therefore may be related to a higher level of processing of shape categories and surfaces, the level at which shapes defined by ICs and RCs are differentially represented.

Form Perception↗

PubMed2004

The oblique plaid effect.

Plaids are ambiguous stimuli that can be perceived either as a coherent pattern moving rigidly or as two gratings sliding over each other. Here we report a new factor that affects the relative strength of coherency versus transparency: the global direction of motion of the plaid. Plaids moving in oblique directions are perceived as sliding more frequently than plaids moving in cardinal directions. We term this the oblique plaid effect. There is also a difference between the two cardinal directions: for most observers, plaids moving in horizontal directions cohere more than plaids moving in vertical directions. Two measures were used to quantify the relative strength of coherency vs. transparency: C/[C+T] and RTtransp. Those measures were derived from dynamics data obtained in long-duration trials (>1 min) where observers continually indicated their percept. The perception of plaids is bi-stable: over time it alternates between coherency and transparency, and the dynamics data reveal the relative strength of the two interpretations [Vision Research 43 (2003) 531]. C/[C+T] is the relative cumulative time spent perceiving coherency; RTtransp is the time between stimulus onset and the first report of transparency. The dynamics-based measures quantify the relative strength of coherency over a wider range of parameters than brief-presentation 2AFC methods, and exposed an oblique plaid effect in the entire range tested. There was no interaction between the effect of the global direction of motion and the effect of gratings' orientations. Thus, the oblique plaid effect is due to anisotropies inherent to motion mechanisms, not a bi-product of orientation anisotropies. The strong effect of a plaid's global direction on its tendency to cohere imposes new and important constraints on models of motion integration and transparency. Models that rely solely on relative differences in directions and/or orientations in the stimulus cannot predict our results. Instead, models should take into account anisotropies in the neuronal populations that represent the coherent percept (integrated motion) and those that represent the transparent percept (segmented motion). Furthermore, the oblique plaid effect could be used to test whether neuronal populations supposed to be involved in plaid perception display tuning biases in favor of cardinal directions.

Humans↗

PubMed2003

fMRI activation in response to illusory contours and salient regions in the human lateral occipital complex.

Regions in the human Lateral Occipital Complex (LOC) show fMRI responses to illusory surfaces. We show that the LOC activation is due to the globally completed region and occurs even when the region is not bounded by illusory contours (ICs). Kanizsa-type stimuli were modified by rounding the corners of the "pacmen" inducers and misaligning them slightly. The impression of an enclosed, salient region (SR) remained, although ICs were no longer perceived (psychophysical data). fMRI activity was elevated for both the IC and SR stimuli, compared to their control stimuli. The LOC response to salient regions may be the result of fast but crude region-based segmentation processes, which are useful for selecting parts of cluttered images for more detailed, computationally intensive processing.

Adult↗

PubMed2003

The dynamics of bi-stable alternation in ambiguous motion displays: a fresh look at plaids.

Prolonged observations of moving plaids lead to bi-stable alternations between coherency and transparency. However, most studies of plaids used brief presentations and a 2AFC between the two interpretations, thus overlooking the dynamical aspect of plaid perception. In other domains, most notably binocular rivalry, it was shown that the dynamics of the bi-stable alternations reveal important insights about the underlying mechanisms. Here we develop methods to study the dynamics of plaid perception. Observers continually indicated their percept (coherency or transparency) during presentations that lasted 1-5 min. Two measures of the relative strength of the coherency percept were derived from those data: C/[C+T], the relative time spent seeing coherency, and RTtransp, the response time to report transparency. Those measures are independent of each other yet tightly correlated, and both show systematic relations to manipulations of plaid parameters. Furthermore, the two measures are sensitive to manipulations in wide parametric regimes, including ranges where brief-presentation methods suffer from "ceiling" and "floor" effects. We conclude that studying the dynamics of bi-stability in plaids can provide new and unsuspected findings about motion integration and segmentation.

Adaptation, Psychological↗

PubMed2003

Binocular rivalry and perceptual multi-stability.

When different images are presented to the two eyes, binocular rivalry can occur - that is, perceptual alternations between the two monocular images (bi-stability) or between more than two percepts comprised of complementary portions of the two images (multi-stability). A recent paper reports important findings about the dynamics of multi-stable alternations in binocular rivalry.

Adaptation, Physiological↗

PubMed2002

Perceptual completion across the vertical meridian and the role of early visual cortex.

Perceptual completion can link widely separated contour fragments and interpolate illusory contours (ICs) between them. The mechanisms underlying such long-range linking are not well understood. Here we report that completion is much poorer when ICs cross the vertical meridian than when they reside entirely within the left or right visual hemifield. This deficit reflects limitations in cross-hemispheric integration. We also show that the sensitivity to the interhemispheric divide is unique to perceptual completion: a comparable task which did not require completion showed no across-meridian impairment. We propose that these findings support the existence of specialized completion mechanisms in early visual cortical areas (V1/V2), since those areas are likely to be more sensitive to the interhemispheric divide.

Animals↗