Biomedical subjects

Ipek Oruç

Publications and source records attributed to Ipek Oruç.

5 recordsLinked to original sources

PubMed2005

Illusory motion from change over time in the response to contrast and luminance.

A striking illusion of motion is generated by static repeated asymmetric patterns (RAPs) such as Kitaoka's (2003) "Rotating Snakes" and Fraser and Wilcox's (1979) peripheral drift illusion. How do RAPs generate spurious motion signals, and what critical difference between RAPs and natural static scenes prevents the latter from appearing to move? Small involuntary eye movements during fixation have been suspected to play a critical role in these illusions, but here we give an account that does not depend on fixation jitter. We propose that these illusions result primarily from fast and slow changes over time in the neuronal representation of contrast ("contrast-driven RAPs") or luminance ("luminance-driven RAPs"). We show that temporal phase advance in the neural response at high contrast can account for the early, fast motion in contrast-driven RAPs (such as "Rotating Snakes") after each fixation change. An essential part of this explanation is that motion detectors fail to compensate for the dynamics of neuronal encoding. We argue that static natural patterns also generate local gain changes, but that these signals do not often trigger illusory motion because they are not usually aligned to drive global motion detectors. Movies in which real luminance changes over time, to mimic the proposed neuronal adaptations to contrast and luminance, evoke qualitatively similar percepts of motion. Experimental data are consistent with the explanation. Color and overall contrast both enhance the illusion.

Contrast Sensitivity↗

PubMed2005

Depth aftereffects mediated by vertical disparities: evidence for vertical disparity driven calibration of extraretinal signals during stereopsis.

Perceptual adaptation often results in a repulsive aftereffect: stimuli are seen as biased away from the adaptation stimulus (). Here we report the absence of a repulsive aftereffect for a vertical gradient of vertical disparity (or vertical size ratio, VSR). We exposed observers to a binocular stimulus consisting of horizontal lines. This stimulus contains vertical, but not horizontal disparities. The visual system was able to measure the VSR of this stimulus: although the lines themselves always appeared unslanted, the VSR carried by the lines had a dramatic effect on the apparent slant of a horizontal row of dots, as predicted by recent accounts of Ogle's (1938) induced effect (e.g., Backus, Banks, van Ee, & Crowell, 1999). Yet we observed no repulsive aftereffect for the VSR signal: after adaptation to horizontal lines that were vertically larger in one eye, we found an attractive aftereffect, the magnitude of which was largest in stimuli that did not contain a VSR signal. We interpret these results as a case of recalibration: disagreement between extra-retinal eye position signals (EP) and VSR causes a recalibration in the use of EP as used in the stereoscopic perception of slant.

Adaptation, Physiological↗

PubMed2005

Noise masking reveals channels for second-order letters.

We investigate the channels underlying identification of second-order letters using a critical-band masking paradigm. We find that observers use a single 1-1.5 octave-wide channel for this task. This channel's best spatial frequency (c/letter) did not change across different noise conditions (indicating the inability of observers to switch channels to improve signal-to-noise ratio) or across different letter sizes (indicating scale invariance), for a fixed carrier frequency (c/letter). However, the channel's best spatial frequency does change with stimulus carrier frequency (both in c/letter); one is proportional to the other. Following Majaj et al. (Majaj, N. J., Pelli, D. G., Kurshan, P., & Palomares, M. (2002). The role of spatial frequency channels in letter identification. Vision Research, 42, 1165-1184), we define "stroke frequency" as the line frequency (strokes/deg) in the luminance image. That is, for luminance-defined letters, stroke frequency is the number of lines (strokes) across each letter divided by letter width. For second-order letters, letter texture stroke frequency is the number of carrier cycles (luminance lines) within the letter ink area divided by the letter width. Unlike the nonlinear dependence found for first-order letters (implying scale-dependent processing), for second-order letters the channel frequency is half the letter texture stroke frequency (suggesting scale-invariant processing).

Contrast Sensitivity↗

PubMed2003

Weighted linear cue combination with possibly correlated error.

We test hypotheses concerning human cue combination in a slant estimation task. Observers repeatedly adjusted the slant of a plane to 75 degrees. Feedback was provided after each setting and the observers trained extensively until their setting error stabilized. The slant of the plane was defined by either linear perspective alone (a grid of lines) or texture gradient alone (diamond-shaped texture elements) or the two cues together. We chose a High and Low variance version of each cue type and measured setting variability in four single-cue conditions (Low, High for each cue) and in the four possible combined-cue conditions (Low-Low, Low-High, etc.). We compared performance in the combined-cue conditions to predictions based on single-cue performance. The results were consistent with a linear combination of estimates from cues. Six out of eight observers did better with combined cues than with either cue alone. For three observers, performance was consistent with optimal combination of uncorrelated cues. Three other observers' results were also consistent with optimal combination, but with the assumption that internal cue estimates were correlated. The remaining two observers were consistent with sub-optimal cue combination.

Cues↗

PubMed2002

Properties of second-order spatial frequency channels.

The segregation of texture patterns may be carried out by a set of linear spatial filters (to enhance one of the constituent textures), a nonlinearity (to convert the higher contrast of response to that constituent to a higher mean response), and finally subsequent ("second-order") linear spatial filters (to provide a strong response to the texture-defined edge itself). In this paper, the properties of such second-order filters are characterized. Observers were required to detect or discriminate textures that were modulated between predominantly horizontally oriented and predominantly vertically oriented noise patterns. Spatial summation for these patterns reached asymptote for a stimulus size of 15 x 15 deg. Modulation contrast sensitivity was nearly flat over a five-octave range of spatial frequency, but was bandpass when stated as efficiency (relative to an idealized observer confronted with the same task). Increment threshold showed the improved performance with a sub-threshold pedestal seen in the "dipper effect", but the typical Weber's law behavior at higher pedestal contrasts was not observed at the highest pedestal modulation contrasts achievable with our stimuli. Sub-threshold summation experiments indicate that second-order filters have a moderate bandwidth.

Contrast Sensitivity↗