Perception

The theories of labelled lines and local signs are commonly invoked to explain numerous perceptual phenomena. These theories postulate that perceptual systems use information about which nerve cells or which information channels are activated by the stimulus. The origins of this idea in nineteenth century German psychophysics are traced. From Descartes's idea of a dualistic mind, Kant's idea of a mental ability to conceive space, Da Vinci's ideas of pictorial ‘signs’, and Müller's idea of ‘specific nerve energies’ to explain perceptual qualities, Steinbuch, Lotze, and others derived the conclusion that neural-level signs exist that signal stimulus location to the mind. Helmholtz, Hering, and others soon suggested variations on this basic idea. By the time of James the theory had changed yet again. It was revived elsewhere in the 1920s and again in the 1970s, although used implicitly by many workers in between and since. Against a modern metaphysical background, however, a distinction between labels (hardware) and signs (symbols) is the minimum step needed towards an appropriate and comprehensive explanation of perceptual behaviour and experience.

Much evidence shows that early vision employs an array of spatial filters tuned for different spatial frequencies and orientations. We suggest that for moderately low spatial frequencies these preliminary filters are not treated independently, but are used to perform grouping and segmentation in the patchwise Fourier domain. For example, consider a stationary plaid made from two superimposed sinusoidal gratings of the same contrast and spatial frequency oriented ±45° from vertical. Most of the energy in a wavelet-like (eg simple-cell) transform of this stimulus is in the oblique orientations, but typically it looks like a compound structure containing blurred vertical and horizontal edges. This checkerboard structure corresponds with the locations of zero crossings in the output of an isotropic (circular) filter, synthesised from the linear sum of a set of oriented basis-filters (Georgeson, 1992 Proceedings of the Royal Society of London, Series B 249 235–245). However, the addition of a third harmonic in square-wave phase causes almost complete perceptual segmentation of the plaid into two overlapping oblique gratings. Here we confirm this result psychophysically using a feature-marking technique, and argue that this perceptual segmentation cannot be understood in terms of the zero crossings marked in the output of any static linear filter that is sensitive to all of the plaid's components. If it is assumed that zero crossings or similar are an appropriate feature-primitive in human vision, our results require a flexible process that combines and segments early basis-filters according to prevailing image conditions. Thus, we suggest that combination and segmentation of spatial filters in the patchwise Fourier domain underpins the perceptual segmentation observed in our experiments. Under this kind of image-processing scheme, registration across spatial scales occurs at the level of spatial filters, before features are extracted. This contrasts with many previous schemes where feature correspondence is required between spatial edge-maps at different spatial scales.

To compare transparent motion and kinetic boundaries with unidirectional motion, in many studies the relative motion is generated by superimposing or adjoining unidirectional motions oriented in opposite directions. The presumption, tacitly underlying this comparison, is that the two oppositely directed velocities are independent of one another as far as their speed is concerned, ie the speed of the relative motion is presumed to be equivalent to the speed of the unidirectional components. Here we report that the relative motion between dots moving in opposite directions augments perceived speed. A constant-stimuli procedure was used to pair transparent-motion or kinetic-boundary displays with unidirectional motion, and human observers were asked to match the speed of the relative and unidirectional motions. The results show that transparency and kinetic boundaries increase the perceived visual speed by about 50%, compared with the speed of the individual components.

In four experiments we investigated whether human observers are able to use certain regularities in polygons when they have to indicate which polygon they perceive as the more regular in a paired comparison task. From our results we conclude that regularities that are restricted to the contour do not play a role in judgments of regularity. For instance, equilateral polygons are not considered to be more regular than entirely random polygons. Only bilaterally symmetric polygons are consistently judged as regular. However, we show that this is caused by regularities across the polygon. These results are at odds with approaches that assume that regularities along the contour play an important part in visual perception.

Object recognition was studied in human subjects to determine whether the storage of the visual objects was in a two-dimensional or a three-dimensional representation. Novel motion-based and disparity-based stimuli were generated in which three-dimensional and two-dimensional form cues could be manipulated independently. Subjects were required to generate internal representations from motion stimuli that lacked explicit two-dimensional cues. These stored internal representations were then matched against internal three-dimensional representations constructed from disparity stimuli. These new stimuli were used to confirm prior studies that indicated the primacy of two-dimensional cues for view-based object storage. However, under tightly controlled conditions for which only three-dimensional cues were available, human subjects were also able to match an internal representation derived from motion to that of disparity. This last finding suggests that there is an internal storage of an object's representations in three dimensions, a tenet that has been rejected by view-based theories. Thus, any complete theory of object recognition that is based on primate vision must incorporate three-dimensional stored representations.

Effects of similarity in colour, luminance, size, and shape on apparent motion and perceptual grouping were examined in part 1 in two parallel experiments on the same seven subjects. In both experiments, the effect of similarity was compared with that of proximity in competitive, bistable stimulus situations. A combination of a larger horizontal separation between the homogeneous stimulus elements and a smaller constant vertical separation between heterogeneous stimulus elements produced two kinds of apparent motion (or perceptual grouping) with equal probabilities. Such matched separations between homogeneous stimulus elements were obtained by the double staircase method in various stimulus conditions. In both experiments on apparent motion and perceptual grouping matched separation was found to increase as the difference between the heterogeneous stimulus elements increased. High correlations (0.71 to 0.94) of matched separations were found between apparent motion and perceptual grouping in four stimulus series: colour, luminance, size, and shape. Six of the seven subjects were also tested in part 2. Here, the effects of differences were found to work additively across different perceptual attributes in both phenomena, when multiple differences were combined in heterogeneous elements. The experimental results are discussed from the point of view that apparent motion is an example of perceptual constancy.

Three experiments were conducted to determine whether human observers could identify the gender of 40 domestic cats (20 female, 20 male) depicted in individual color photographs. In experiment la, observers performed at chance for photographs depicting whole cats, cat heads (bodies occluded), and cat bodies (heads occluded). Experiment lb showed that chance performance was also obtained when the photographs were full-face close-ups of the cats. Experiment 2a revealed that even with gender-identification training on 30 (15 female, 15 male) of the 40 face close-ups, observers were unable to generalize their training to reliably identify the gender of the 10 remaining test faces (5 female, 5 male). However, experiment 2b showed that gender-identification training with the 14 most accurately identified faces from experiment 1b (7 female, 7 male) was successful in raising gender identification of the 10 test faces above chance. Experiments 3a and 3b extended this facilitative effect of gender-identification training to a population of animal-care workers. The findings indicate that, with appropriate training, human observers can identify the gender of cat faces at an above-chance level. A perceptual category learning account emphasizing the on-line formation of differentiated male versus female prototypes during training is offered as an explanation of the findings.

Effects of sound and posture cues on veering from the straight-ahead were tested with young blind children in an unfamiliar space that lacked orienting cues. In a pre-test with a previously heard target sound, all subjects walked straight to the target. A recording device, which sampled the locomotor trajectories automatically, showed that, without prior cues from target locations, subjects tended to veer more to the side from which they heard a brief, irrelevant noise. Carrying a load on one side produced more veering to the opposite side. The detailed samples showed that, underlying the main trajectories, were alternating concave and convex (left and right) movements, suggesting stepwise changes in body position. It is argued that the same external and body-centred cues that contribute to reference-frame orientation for locomotion when they converge and concur, influence the direction of veering when the cues occur in isolation in environments that lack converging reference information.

There are some indications that haptic space like visual space is not Euclidean (eg Blumenfeld, 1937 Acta Psychologica 2 125–174). In a series of experiments, we investigated the haptic perception of spatial relations in a systematic way. We restricted ourselves to a horizontal plane at waist height. Blindfolded subjects were asked to perform three tasks with their right hand: (i) a reference bar was presented under four different orientations and subjects were asked to rotate a test bar such that it felt to be parallel to the reference bar; (ii) subjects had to rotate two test bars in such a way that they felt collinear; (iii) subjects had to point a test bar in the direction of a marker. Bars and marker could appear at nine different locations. In all experiments large systematic deviations (up to 40°) were made. The deviations strongly correlated with horizontal (right-left) but not with vertical (forward-backward) distance. Subjects showed qualitatively identical trends but the size of the deviations was strongly subject-dependent. In addition, a significant haptic oblique effect was found. These results provide strong evidence that haptic space in non-Euclidean.