Papers selected by Miho Tanaka

　　　　■The interaction of first- and second-order cues to orientation
　　　　■Interaction between primary and secondary mechanisms in human motion perception
　　　　■Does motion perception follow Weber's law?
　　　　■Effect of exposure duration on visual contrast sensitivity with square-wave gratings

The interaction of first- and second-order cues to orientation

Dakin SC, Williams CB, Hess RF.
Vision Res., 1999, 39(17), 2867-84.

The visual system is sensitive to orientation information defined both by first-order (luminance) and by second-order (texture) cues. We consider how these orientation cues are computed and how they affect one another. We measured the perceived orientation of the first and second-order components of Gabor patches (the carrier and envelope, respectively) and report a dependence of the perceived orientation of each on the orientation of the other, and on the spatial frequency of the carrier. Fixing the carrier orientation near that of the envelope interferes with envelope orientation judgements. This interference is reduced by adding a small (subthreshold) rotation to the carrier indicating that the site of interference is early. When the gross relative orientation of carrier and envelope is varied, the carrier appears systematically tilted towards the envelope. However, provided envelope and carrier are separated by more than approximately 10 degrees, the perceived envelope orientation appears tilted away from the carrier. The size of these effects increases with decreasing carrier spatial frequency, and with increasing exposure duration. When the envelope and carrier are both non parallel and non-perpendicular Fourier energy is distributed asymmetrically across orientation. We demonstrate that, for a channel-based orientation code, this asymmetry induces a shift in mean orientation that is sufficient to explain illusory tilting of carriers. The illusory tilting of the envelope, as a function of carrier orientation and spatial frequency, demonstrates that human ability to demodulate contrast information is far from ideal and cannot be explained by existing two-stage filter-rectify-filter models. We propose that illusory tilting of the envelope is due to selective connectivity between first- and second-stage filters whose purpose is to dissociate the type of image structure producing each class of cue.

Interaction between primary and secondary mechanisms in human motion perception

Zanker JM, Hupgens IS.
Vision Res., 1994, 34(10), 1255-66.

Two layers of information processing can be distinguished as being involved in human motion perception. The primary motion detection stage processes displacements of the luminance distribution across space, such as experienced in natural scenes during the pursuit of moving targets. Primary motion detection is often investigated with artificial motion stimuli realized as random-dot kinematograms (RDKs). Such stimuli belong to the class of "Fourier motion", and their perception can be easily explained by means of elementary motion detectors (EMDs) of the correlation type. Other tasks require the comparison of motion signals from neighbouring areas in the visual field. The perception of the displacement of the motion distribution, for instance, has been accounted for by a secondary motion processing stage. In order to understand the principles of interaction between the motion in neighbouring areas of the visual field, we investigated the sensitivity of the human visual system for moving objects which are defined by moving dots in variable directions. These experiments lead to "secondary tuning curves" of direction discrimination for secondary motion as function of primary motion direction. A base level of sensitivity for all dot motion directions without a velocity component in the same direction of the object movement is enhanced when the object and the dots have a common velocity component. Thus primary motion in any direction can be exploited by the secondary stage, and primary and secondary system both feed into the object motion percept. Furthermore it is suggested from the shape of the secondary tuning curve that the outputs from the two layers of motion processing do not superimpose linearly, but are combined by some sort of veto-like mechanism which increases the directional sensitivity when the two processing layers experience movement along the same direction.

Does motion perception follow Weber's law?

Zanker JM.
Perception, 1995, 24(4), 363-72.

The subjective strength of a percept often depends on the stimulus intensity in a nonlinear way. Such coding is often reflected by the observation that the just-noticeable difference between two stimulus intensities (JND) is proportional to the absolute stimulus intensity. This behaviour, which is usually referred to as Weber's Law, can be interpreted as a compressive nonlinearity extending the operating range of a sensory system. When the noise superimposed on a motion stimulus is increased along a logarithmic scale (in order to provide linear steps in subjective difference) in motion-coherency measurements, observers often report that the subjective differences between the various noise levels increase together with the absolute level. This observation could indicate a deviation from Weber's Law for variation of motion strength as obtained by changing the signal-to-noise ratio in random-dot kinematograms. Thus JNDs were measured for the superposition of uncorrelated random-dot patterns on static random-dot patterns and three types of motion stimuli realised as random-dot kinematograms, namely large-field and object 'Fourier' motion (all or a group of dots move coherently), 'drift-balanced' motion (a travelling region of static dots), and paradoxical 'theta' motion (the dots on the surface of an object move in opposite direction to the object itself). For all classes of stimuli, the JNDs when expressed as differences in signal-to-noise ratio turned out to increase with the signal-to-noise ratio, whereas the JNDs given as percentage of superimposed noise appear to be similar for all tested noise levels. Thus motion perception is in accordance with Weber's Law when the signal-to-noise ratio is regarded as stimulus intensity, which in turn appears to be coded in a nonlinear fashion. In general the Weber fractions are very large, indicating a poor differential sensitivity in signal-to-noise measurements.

Effect of exposure duration on visual contrast sensitivity with square-wave gratings

Nachmias, Jacob
1967, Journal of the Optical Society of America, 57(3), 421-427.

Contrast sensitivity for square-wave gratings of spatial frequencies between .44 and 33.2 cycles/degree was determined for exposure durations between 11 and 500 msec. The space-average luminance of the targets was kept constant at 10 ml., regardless of contrast, and equal to that of the pre- and postexposure fields, which contained a cross-hair reticle to help maintain accommodation and fixation. At the longest exposure duration (500 msec.) the contrast sensitivity function exhibited both the high- and low-frequency declines described previously. At the briefest exposure duration tested (11 msec.), the low-frequency decline of contrast sensitivity was virtually absent. Log contrast sensitivity improves with increasing exposure duration, but more for high-frequency than for low-frequency gratings. These results are compatible with the assumption that there is a time delay in the occurrence of inhibitory interactions in the retina.