Perception

For 35 to 39 days, four observers wore continuously left–right reversing spectacles which pseudoscopically reverse the order of binocular disparity and direction of convergence. In three tests, we investigated how the visual system copes with the transformation of depth and distance information due to the reversing spectacles. In stereogram observation, after a few days of wearing the spectacles, the observers sometimes perceived a depth order which was opposite to the depth order that they had perceived in the pre-spectacle-wearing period. Monocular depth cues contributed more to depth perception in the spectacle-wearing period than they did in the pre-spectacle-wearing period. While the perceived distance significantly decreased during the spectacle-wearing period, we found no evidence of adaptive change in distance perception. The results indicate that the visual system adapts itself to the transformed situation by not only changing the processing of disparity but also by changing the relative efficiency of each cue in determining apparent depth.

When dissimilar visual scenes are viewed dichoptically, the observer perceives several different representations of the scene over time. To reveal that a distributed intercortical network mediates this phenomenon of binocular rivalry, we used a Kanizsa square-like display consisting of four pairs of color-rivalry-inducing elements. We found that when all four dominant elements had the same color, regardless of whether they were from the same or different eyes, the visual system ably integrated them into a larger subjective surface. Once formed, the same-colored subjective surface enjoyed a relatively longer predominance than mixed-colored patterns. During rivalry alternation, this same-colored surface was more likely to be replaced by a complementary same-colored surface, rather than by mixed-colored patterns (cohesive effect). Further, surface integration, which is mainly an extrastriate cortical function, was stronger when the same eye viewed the same-colored rivalry stimuli. Since the eye-of-origin signature is explicitly represented in V1, these findings together suggest that rivalry is processed along a distributed network including V1 and the extrastriate cortices.

Investigations of the ways in which the eyes move came to prominence in the 19th century, but techniques for measuring them more precisely emerged in the 20th century. When scanning a scene or text the eyes engage in periods of relative stability (fixations) interspersed with ballistic rotations (saccades). The saccade-and-fixate strategy, associated with voluntary eye movements, was first uncovered in the context of involuntary eye movements following body rotation. This pattern of eye movements is now referred to as nystagmus, and involves periods of slow eye movements, during which objects are visible, and rapid returns, when they are not; it is based on a vestibular reflex which attempts to achieve image stabilisation. Post-rotational nystagmus was reported in the late 18th century (by Wells), with afterimages used as a means of retinal stabilisation to distinguish between movement of the eyes and of the environment. Nystagmus was linked to vestibular stimulation in the 19th century, and Mach, Breuer, and Crum Brown all described its fast and slow phases. Wells and Breuer proposed that there was no visual awareness during the ballistic phase (saccadic suppression). The saccade-and-fixate strategy highlighted by studies of nystagmus was shown to apply to tasks like reading by Dodge, who used more sophisticated photographic techniques to examine oculomotor kinematics. The relationship between eye movements and perception, following earlier intuitions by Wells and Breuer, was explored by Dodge, and has been of fundamental importance in the direction of vision research over the last century.

Jacopo Chimenti (c 1551–1640), an artist from Empoli, made two sketches of a young man holding a compass and a plumb line. When these were seen, mounted next to one another, by Alexander Crum Brown in 1859, he combined them by overconvergence and described the stereoscopic depth he saw. Brown's informal observation was conveyed to David Brewster, who suggested that the drawings were produced for a stereoscope, possibly made by Giovanni Battista della Porta. There followed a bitter debate about the supposed stereoscopic effects that could be seen when the pictures combined. Brewster's claims were finally dispelled when precise measurements were made of the drawings: some parts were stereoscopic and others were pseudoscopic. Brewster's attempts to wrest the invention of the stereoscope from Wheatstone were unsuccessful.

Both face recognition and biological-motion perception are strongly orientation-dependent. Recognition performance decreases if the stimuli are rotated with respect to their normal upright orientation. Here, the question whether this effect operates in egocentric coordinates or in environmental coordinates is examined. In addition to the use of rotated stimuli the observers were also rotated and tested both with a same–different face-recognition task and with a biological-motion detection task. A strong orientation effect was found that depended only on the stimulus orientation relative to the observer. This result clearly indicates that orientation effects in both stimulus domains operate in an egocentric frame of reference. This finding is discussed in terms of the particular requirement of extracting sophisticated information for social recognition and communication from faces and biological motion.

Texture can be an effective source of information for perception of slant and curvature. A computational assumption required for some texture cues is that texture must be flat along a surface. There are many textures which violate this assumption, and have some sort of texture relief: variations perpendicular to the surface. Some examples include grass, which has vertical elements, or scattered rocks, which are volumetric elements with 3-D shapes. Previous studies of perception of slant from texture have not addressed the case of textures with relief. The experiments reported here test judgments of slant for textures with various types of relief, including textures composed of bumps, columns, and oriented elements. The presence of texture relief was found to affect judgments, indicating that perception of slant from texture is not robust to violations of the flat-texture assumption. For bumps and oriented elements, slant was underestimated relative to matching flat textures, while for columns textures, which had visible flat top faces, perceived slant was equal or greater than for flat textures. The differences can be explained by the way different types of texture relief affect the amount of optical compression in the projected image, which would be consistent with results from previous experiments using cue conflicts in flat textures. These results provide further evidence that compression contributes to perception of slant from texture.

In this study we investigated the perception and production of line orientations in a vertical plane. Previous studies have shown that systematic errors are made when participants have to match oblique orientations visually and haptically. Differences in the setup for visual and haptic matching did not allow for a quantitative comparison of the errors. To investigate whether matching errors are the same for different modalities, we asked participants to match a visually presented orientation visually, haptically with visual feedback, and haptically without visual feedback. The matching errors were the same in all three matching conditions. Horizontal and vertical orientations were matched correctly, but systematic errors were made for the oblique orientations. The errors depended on the viewing position from which the stimuli were seen, and on the distance of the stimulus from the observer.