Haptic adaptation to slant: No transfer between exploration modes

Human touch is an inherently active sense: to estimate an object’s shape humans often move their hand across its surface. This way the object is sampled both in a serial (sampling different parts of the object across time) and parallel fashion (sampling using different parts of the hand simultaneously). Both the serial (moving a single finger) and parallel (static contact with the entire hand) exploration modes provide reliable and similar global shape information, suggesting the possibility that this information is shared early in the sensory cortex. In contrast, we here show the opposite. Using an adaptation-and-transfer paradigm, a change in haptic perception was induced by slant-adaptation using either the serial or parallel exploration mode. A unified shape-based coding would predict that this would equally affect perception using other exploration modes. However, we found that adaptation-induced perceptual changes did not transfer between exploration modes. Instead, serial and parallel exploration components adapted simultaneously, but to different kinaesthetic aspects of exploration behaviour rather than object-shape per se. These results indicate that a potential combination of information from different exploration modes can only occur at down-stream cortical processing stages, at which adaptation is no longer effective

Intramanual and intermanual transfer of the curvature aftereffect

The existence and transfer of a haptic curvature aftereffect was investigated to obtain a greater insight into neural representation of shape. The haptic curvature aftereffect is the phenomenon whereby a flat surface is judged concave if the preceding touched stimulus was convex and vice versa. Single fingers were used to touch the subsequently presented stimuli. A substantial aftereffect was found when the adaptation surface and the test surface were touched by the same finger. Furthermore, a partial, but significant transfer of the aftereffect was demonstrated between fingers of the same hand and between fingers of both the hands. These results provide evidence that curvature information is not only represented at a level that is directly connected to the mechanoreceptors of individual fingers but is also represented at a stage in the somatosensory cortex shared by the fingers of both the hands

Using curvature information in haptic shape perception of 3D objects

Are humans able to perceive the circularity of a cylinder that is grasped by the hand? This study presents the findings of an experiment in which cylinders with a circular cross-section had to be distinguished from cylinders with an elliptical cross-section. For comparison, the ability to distinguish a square cuboid from a rectangular cuboid was also investigated. Both elliptical and rectangular shapes can be characterized by the aspect ratio, but elliptical shapes also contain curvature information. We found that an elliptical shape with an aspect ratio of only 1.03 could be distinguished from a circular shape both in static and dynamic touch. However, for a rectangular shape, the aspect ratio needed to be about 1.11 for dynamic touch and 1.15 for static touch in order to be discernible from a square shape. We conclude that curvature information can be employed in a reliable and efficient manner in the perception of 3D shapes by touch

The initial direction and landing position of saccades

We studied the trajectories of self-paced saccades in two experimental conditions. Saccades were made between two visual targets in one condition and between the same two, not visible, positions in the other condition. Target pairs were presented which required oblique saccades of 20 or 40 deg. At least 200 saccades were made between each pair of targets. Horizontal and vertical eye movements were measured of the right eye with a scleral coil technique. We computed the angle between starting and end point of each primary saccade (effective direction). We also computed the angle between starting point and eye position when the saccade had covered a distance of 2.5 deg (initial direction). We found that variability in initial directions was two to seven times larger than variability in the effective directions. This effect was found in both experimental directions for saccades made in all tested directions. We conclude that curvedness of saccades is the result of a purposeful control strategy. The saccadic trajectories show that, initially, the eye is accelerated roughly in the direction of the target and subsequently is guided to the target. This behavior cannot be described by present models of saccade generation. We suggest that the coupling between saccadic pulse and step signals is not as tight as generally is accepted in the literature

Smoothness and flicker perception of temporal color transitions

\u3cp\u3eWe present results from two experiments designed to explore temporal properties of human color vision relevant to dynamic lighting applications. Sensitivity for smoothness perception of linear temporal transitions and flicker visibility was tested. Stimuli in the first experiment were linear color transitions, varying in either lightness, chroma or hue, around a base color represented in CIE LCh. Results show a significantly lower smoothness threshold for lightness changes than for chroma and hue changes. Moreover, the thresholds for lightness change show independence from the chroma and hue of the base color in contrast to thresholds for chroma and hue changes. A difference between the sensitivity for chroma and hue changes was also demonstrated. In the second experiment, the sensitivity for linear transitions is compared to flicker sensitivity for the same base colors. Results show that visibility thresholds for flicker are significantly lower than the thresholds smoothness of linear changes, demonstrating an influence of the type of change to the temporal sensitivity. The results from the flicker experiment show the same tendencies as the linear changes. The results from these experiments show a need for a model of perceived smoothness to control temporal changes in dynamic lighting systems and give the first steps towards building such a model.\u3c/p\u3

The effect of spatial luminance distribution on dark adaptation

Recent studies show that dark adaptation in the visual system depends on local luminance levels surrounding the viewing direction. These studies, however, do not explain to what extent veiling luminance is responsible for the outcome. To address the latter, in this study dark adaptation was measured for three different spatial luminance distributions surrounding a target to be detected, while keeping the veiling luminance at the location of the target equivalent. The results show that a background with bright areas close to the viewing direction yields longer adaptation times than a background with bright areas at a larger visual angle. Therefore, we conclude that dark adaptation is affected to a great extent by local luminance, even when controlling for veiling luminance. Based on our results, a simple but adequate model is proposed to predict the adaptation luminance threshold for backgrounds having a nonuniform luminance distribution

Quantifying the visibility of periodic flicker

\u3cp\u3eThree experiments that measure the visibility of periodic flicker are presented. Temporal light modulations were presented to a large visual field to make the results valid for general lighting applications. In addition, the experiments were designed to control for flicker adaptation. In the first experiment, the sensitivity of human observers to light modulations with a sinusoidal waveform at several temporal frequencies up to 80 Hz was measured. The results showed that the sensitivity to flicker (that is, the inverse of the Michelson contrast) is as high as 500 for frequencies between 10 and 20 Hz, which is more than twice the maximum sensitivity reported in the literature. In the second experiment, the sensitivity to flicker for light modulations with complex waveforms, composed of two or three frequency components, was measured. Sensitivity to flicker was found to be higher than the sum of the sensitivities of the individual frequency components of the complex waveform. Based on these results, we defined the flicker visibility measure (FVM), predicting flicker visibility by a weighted summation of the relative energy of the frequency components of the waveform. In the third experiment, sensitivity to realistic waveforms (that is, waveforms of light emitting diode [LED] light sources available on the market) was measured. The flicker predictions of FVM showed a high correlation with the experimental data, in contrast to some other existing flicker measures, including flicker index and percent flicker, demonstrating the usefulness of the measure to objectively assess the visibility of periodic flicker for lighting applications.\u3c/p\u3

Stroboscopic effect:Contrast threshold function and dependence on illumination level

\u3cp\u3eThe stroboscopic visibility measure (SVM) is a method used to quantify the stroboscopic effect visibility in general illumination application. SVM has been defined previously based on a limited number of frequencies and participants. To validate and extend SVM, five perception experiments are presented, measuring the visibility threshold of light waveforms modulated at several frequencies, conducted in two different labs. A power function is fitted through the aggregated results to develop a stroboscopic effect contrast threshold function for a “standard observer,” which can be used to normalize SVM. An additional experiment shows the dependency on illumination level, extending the validity of SVM to other applications.\u3c/p\u3