Perception

The aim of this research is to model and simulate the loss of visual resolution as a function of retinal eccentricity in the perception of natural scenes. The model of visual resolution is based on a space-variant low-pass filter, having a variable convolution kernel according to retinal eccentricity. The parameters of the model are computed from psychophysical measures of visual acuity as a function of retinal eccentricity. The implementation of the model allowed us to generate images of scenes with nonhomogeneous space-variant resolution, simulating the filtering executed by the eye. These scenes are used to test and optimise the model by means of experiments in static vision (through tachistoscopic presentations) and in dynamic vision where the resolution of the scene is computed, in real-time, as a function of the location of gaze.

The aesthetic significance of the golden-section rectangle was tested in two studies designed to obviate some of the criticisms of earlier experiments. In the first, employing the method of use, the mean sides-ratios of samples of paintings from five subject-matter categories (landscape, still life, head-and-shoulders portrait, upper-body portrait, full-length portrait) provided no evidence for the significance of the golden section. However, the sides ratio of portraits varied between categories in ways that were consistent with the requirements of the proportions of the subject matter. In the second study, using the method of production, participants produced the most pleasing four-sided shape, under four instruction conditions. Under a ‘portrait painting’ condition and a ‘landscape painting’ condition, the mean sides-ratios differed significantly from the golden section. Under two ‘context free’ geometric shape conditions—horizontal rectangle and vertical rectangle—the mean sides-ratio approximated the golden section. The results are discussed in terms of the methodological requirements for a valid test of the aesthetic significance of the golden section and the possibility that this ratio may indeed have special significance.

During everyday life the brain is continuously integrating multiple perceptual cues in order to allow us to make decisions and to guide our actions. In this study we have used a simulated (virtual reality—VR) visual environment to investigate how cues to speed judgments are integrated. There are two sources that could be used to provide signals for velocity constancy: temporal-frequency or distance cues. However, evidence from most psychophysical studies favours temporal-frequency cues. Here we report that two depth cues that provide a relative object–object distance—disparity and motion parallax—can provide a significant input to velocity–constancy judgments, particularly when combined. This result indicates that the second mechanism can also play a significant role in generating velocity constancy. Furthermore, we show that cognitive factors, such as familiar size, can influence the perception of object speed. The results suggest that both low-level cues to spatiotemporal structure and depth, and high-level cues, such as object familiarity, are integrated by the brain during velocity estimation in real-world viewing.

Diaz-Caneja (1928) made some prescient observations about binocular rivalry. Being in French, however, his paper remained largely unknown to the broader research community. His findings are similar to those reported very recently by contemporary researchers who had independently observed similar phenomena. Using concentric circles and parallel lines as stimuli, Diaz-Caneja presented half of each form to opposite eyes to provoke binocular rivalry. He observed periods in the ensuing binocular alternations in which rivalry occurred between the good Gestalt forms, despite the fact that they were distributed between the eyes. He proposed that each half of a good form generates synchronised oscillations in the visual system, and that this synchronisation enables the dichoptically viewed halves of the one form to be perceived as a whole.

The early stages of colour coding are well established in that the trichromatic receptor stage is followed by a set of opponent colour channels. One interpretation of the sequence is that opponent channels carry unrelated aspects of the colour stimulus, unlike the cone channels. The overlap of the cone channels can be removed by decorrelating their spectral-sensitivity functions, and this procedure has been found to give opponent colour channels which match those found psychophysically. Since the known spatial-frequency channels also show considerable overlap, the question arises which aspects of the spatial stimulus are captured by decorrelating the spatial-frequency channels. The results of decorrelating the spatial-frequency channels are that the first decorrelated spatial filter acts as a broad bandpass filter which has a peak sensitivity at 7.9 cycles deg⁻¹, and that the second decorrelated spatial filter acts as an opponent spatial-frequency channel, with a minimum output at a low (4.1 cycles deg⁻¹) spatial frequency and a maximum output at a high (15.1 cycles deg⁻¹) spatial frequency. The characteristics of the first decorrelated filter closely resemble the properties of the foveal perceptive field which have been used to explain the Hermann grid illusion. Thus, the decorrelation analysis produces a model for the functional organisation of the channel implementation at the neural and psychophysical levels, but which directly relates to the subjective appearance of the visual stimuli.

According to the duplex theory of tactile texture perception, detection of cutaneous vibrations produced when the exploring finger moves across a surface contributes importantly to the perception of fine textures. If this is true, a vibrating surface should feel different from a stationary one. To test this prediction, experiments were conducted in which subjects examined two identical surfaces, one of which was surreptitiously made to vibrate, and judged which of the two was smoother. In experiment 1, the vibrating surface was less and less often judged smoother as the amplitude of (150 Hz) vibration increased. The effect was comparable in subjects who realized the surface was vibrating and those who did not. Experiment 2 showed that different frequencies (150–400 Hz) were equally effective in eliciting the effect when equated in sensation level (dB SL). The results suggest that vibrotaction contributes to texture perception, and that, at least within the Pacinian channel, it does so by means of an intensity code.

The visual environment is distorted with respect to the physical environment. Luneburg [1947, Mathematical Analysis of Binocular Vision (Princeton, NJ: Princeton University Press)] assumed that visual space could be described by a Riemannian space of constant curvature. Such a space is described by a metric which defines the distance between any two points. It is uncertain, however, whether such a metric description is valid. Two experiments are reported in which subjects were asked to set two bars parallel to each other in a horizontal plane. The backdrop consisted of wrinkled black plastic sheeting, and the floor and ceiling were hidden by means of a horizontal aperture restricting the visual field of the subject vertically to 10 deg. We found that large deviations (of up to 40°) occur and that the deviations are proportional to the separation angle: on average, the proportion is 30%. These deviations occur for 30°, 60°, 120°, and 150° reference orientations, but not for 0° and 90° reference orientations; there the deviation is approximately 0° for most subjects. A Riemannian space of constant curvature, therefore, cannot be an adequate description. If it were, then the deviation between the orientation of the test and the reference bar would be independent of the reference orientation. Furthermore, we found that the results are independent of the distance of the bars from the subject, which suggests either that visual space has a zero mean curvature, or that the parallelity task is essentially a monocular task. The fact that the deviations vanish for a 0° and 90° orientation is reminiscent of the oblique effect reported in the literature. However, the ‘oblique effect’ reported here takes place in a horizontal plane at eye height, not in a frontoparallel plane.

Comparative literature provides conflicting findings whether animals experience amodal completion. Five experiments were conducted to verify if baboons perceive partly occluded objects as complete. The first three experiments used a go/no-go procedure and a video monitor for stimulus presentation. These experiments failed to reveal amodal completion, suggesting that the stimuli were processed as 2-D images rather than 3-D objects. In contrast, completion was demonstrated in a fourth experiment with cardboard stimuli in a two-alternative forced-choice (2AFC) discrimination task presented in a Wisconsin General Test Apparatus. Although in experiment 5 the same 2AFC procedure was used as in experiment 4, completion was absent when the stimuli were shown with a computer graphic system. The results suggest that baboons share with humans the ability for amodal completion, but also underline some procedural factors that might affect the elicitation of this capacity.

Perception 29(9) 1087

In the abstract of the paper “Discrimination of spectrally blended natural images: Optimisation of the human visual system for encoding natural images” by David J Tolhurst and Yoav Tadmor the last line has been omitted. The complete abstract is printed below:

Abstract. We have developed a protocol for testing experimentally the hypothesis that the human visual system is optimised for making visual discriminations amongst natural scenes.Visual stimuli were made by gradual blending of the Fourier spectra of digitised photographs of natural scenes. The statistics of the stimuli were made unnatural to varying degrees by changing the overall slopes of the amplitude spectra of the stimuli. Thresholds were measured for discriminating small amounts of spectral blending at different spectral slopes. We found that thresholds were lowest when the spectral slope was natural; thresholds were increased when the slopes were either shallower or steeper than natural. A number of spurious cues were considered, such as differences in mean luminance or overall spectral power or contrast between test and reference stimuli. Control experiments were performed to remove such spurious cues, and the discrimination thresholds were still lowest for stimuli that were most natural. Thus, these experiments do provide experimental support for the idea that human vision and the human visual system are optimised for processing natural visual information.