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Biomedical subjects

Ariella V Popple

Publications and source records attributed to Ariella V Popple.

9 recordsLinked to original sources
PubMed2005

Two sources of error in pop-out localization.

An odd-one-out stimulus, such as a vertical bar among horizontals, pops out from the background and is easily detected, but its location may be slightly ambiguous. Four observers were asked to pinpoint these stimuli on thousands of trials, in 5 x 5 and 9 x 9 arrays of Gabor patches. We found they made frequent errors toward neighbors of the target. Over a range of performance from 41% to 96% correct, the frequency of neighbor errors was well described by a linear function of the total error frequency, a function that might result from mixing together two spatial distributions--one broad, the other narrow. We suggest that these represent two sources of error in pop-out localization; one might correspond to a higher visual area with imprecise retinotopic mapping, and the other to a more fine-grained localization process in primary visual cortex.

Depth Perception↗
PubMed2005

Location coding by the human visual system: multiple topological adaptations in a case of strabismic amblyopia.

Amblyopia, a major cause of vision loss, is a developmental disorder of visual perception commonly associated with strabismus (squint). Although defined by a reduction in visual acuity, severe distortions of perceived visual location are common in strabismic amblyopia. These distortions can help us understand the cortical coding of visual location and its development in normal vision, as well as in amblyopia. The history of retinotopic mapping in the visual cortex highlights the potential impact of amblyopia. Theories of amblyopia include topological disarray of receptors in primary visual cortex, undersampling from the amblyopic eye compared with normal eyes, and the presence of anomalous retinal correspondence or multiple cortical representations of the strabismic fovea. We examined the distortions in a strabismic amblyope, using a pop-out localization task, in which normal observers made errors dependent on the visual context of the stimulus. The localization errors of the strabismic amblyope were abnormal. We found that none of the available theories could fully explain this one patient's localization performance. Instead, the observed behavior suggests that multiple adaptations of the underlying cortical topology are possible simultaneously in different parts of the visual field.

Adult↗
PubMed2004

The perception of spatial order at a glance.

Spatial order is the organizing principle of the visual areas in the brain. But to what extent does this spatial mapping help us see where things are? Observers trained to perfectly recall the spatial order of seven items presented simultaneously for 5 s were asked to report their order when flashed for only 150 ms. We found that the capacity for perceiving the order of these brief stimuli was limited by their spacing. Five or six widely-spaced stimuli were seen in the correct order, but only four crowded stimuli. Regardless of spacing and set-size, confusions between neighbors were unexpectedly frequent, suggesting there is positional as well as object uncertainty.

Attention↗
PubMed2004

Combining cues in contour orientation discrimination.

The perceived orientation of a Gabor-patch contour is determined, in part, by shifts in carrier phase between the patches [Popple, A. V. & Sagi, D., 2000. A Fraser illusion without local cues? Vision Research, 40, 873-878; Popple, A. V. & Levi, D. M., 2000a. A new illusion demonstrates long-range processing. Vision Research, 40, 2545-2549; Popple, A. V. & Levi, D. M., 2000b. Amblyopes see true alignment where normal observers see illusory tilt. Proceedings of the National Academy of Sciences of the United States of America, 97, 11667-11672]. Here we show that perceived orientation results from the combination of at least three stimulus cues: (1) patch orientation, (2) contour envelope orientation, and (3) between-patches orientation, which is a function of phase-shifts. In a series of three experiments, we investigated how these three cues were combined. The data are consistent with weighted cue combination.

Bayes Theorem↗
PubMed2003

Context effects on texture border localization bias.

Observers are able to locate precisely a border defined by changes in texture orientation. The prevailing theory is that such localization takes place using a hierarchical, filter-rectify-filter mechanism. An alternative theory is that contextual modulation causes the border elements to stand out. Here we show that perceived border location is inconsistent with contextual modulation from iso-oriented elements. The perceived location of a vertical border defined by vertical texture on one side, and horizontal texture on the other side, is biased towards the vertical texture. We found the same bias in a single row of texture. Therefore, the bias is not due to contextual influences from surrounding iso-oriented elements. Contextual influences between cross-oriented elements can explain the data.

Field Dependence-Independence↗
PubMed2002

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.

Form Perception↗
PubMed2002

Set-size effects for spatial frequency change and discrimination in multiple targets.

In visual search tasks with a near-threshold target amongst distracters, log detection thresholds rise in proportion to the log of the number of stimuli. Previous research has shown a very steep slope for this set-size effect where the target is a change in spatial frequency (SF) across an ISI, suggesting a low-level explanation for 'change blindness (Wright et al., 2000). Here, we analyse stimulus and task variables in order to determine the contributions of stimulus detection and attention processes. Stimuli consisted of two 150 ms frames each containing 1 to 4 Gabor targets, with an ISI of 250 ms. In a 2AFC detection task with uniform distracters, slopes of 0.23-0.52 were found, in line with visual search results. 2AFC SF discrimination tasks gave slopes of 0.68, 0.69 with homogeneous distracters and 0.76-0.96 with inhomogeneous distracters, consistent with averaging of stimuli within a frame. If the distracters were also made to change across ISI, averaging was impossible, and focal attention was required to solve the discrimination. This always gave set-size slopes > 1. It is concluded that, under conditions where a stimulus array can be analysed globally, change detection performance is limited by signal detection mechanisms, rather than limited capacity attention or memory mechanisms. However, where this is prevented, for example by changing more than one item, limitations due to attention or memory produce an even steeper set-size effect.

Attention↗
PubMed2002

Effects of negative afterimages in visual illusions.

We show that a broad class of visual illusions, including illusory motion, can be explained by the effects of negative afterimages. Two new illusions, illusory shading and illusory tilting, are devised on the basis of the proposed explanation. The general feature of these illusions is an alternation between a high-contrast (white or black) and a low-contrast (gray) local input signal, which can be caused either by eye motion over patterns of varied luminance or by a change in such patterns over time. A simple model of the local signal dynamics qualitatively reproduces the illusory effects by adding the negative afterimage to the original visual stimulus.

Afterimage↗