Independent perceptual reversals for simultaneously presented ambiguous figures

In two successive experiments using the divided visual field paradigm with vertical or horizontal division, two ambiguous figures, the Rubin’s vase-face or the Necker cube, were projected to the right and left or to the upper and lower visual hemifields of 108 healthy volunteers. Stimulation time was 120 s. The main hypotheses were (a) that different percepts of the same ambiguous figure may be simultaneously experienced in the two hemifields and (b) that the type (vertical vs. horizontal) of visual field division influences the reversal frequency and the temporal interdependence of the percepts.Results from the first experiment showed that the temporal interdependence of reversals was very low for both ambiguous figures, suggesting that during part of the stimulation time the subjects could experience different percepts of the same figure (e.g. a vase in the right and face profiles in the left visual hemifields). The second experiment showed that this perceptual dissociation occurred on average during one third of the stimulation time. In both experiments the type of visual field division did not influence either frequency or temporal interdependence of the reversals. When one single ambiguous figure was presented in the centre of the screen, the number of reversals was approximately the sum of the reversals observed with two figures presented simultaneously each in one hemifield.     

Differences in top-down influences on the reversal rate of different categories of reversible figures

Understanding the mechanisms underlying the multistability of reversible figures may provide valuable insights into the normal functioning of our visual system. The proposed factors that control the perceptual alternations of reversible figures can be classified into bottom-up and top-down processes. In the present study, we report differences in top-down effects on the reversal rate depending on whether a structural perspective (Necker cube, Schr?der staircase) or a meaningful content (duck/rabbit figure, chef/dog figure) is subject to the reversal phenomenon. In order to activate top-down mechanisms explicitly the subjects had the instruction to bring the reversal rate under voluntary control. The results indicated that both slowing down and speeding up the rate of alternations was more effective for the content-reversal figures (duck/rabbit, chef/dog) than for the rather abstract perspective-reversal figures (Necker cube, Schr?der staircase). In order to investigate the effect of meaningfulness in figure/ground reversals, the effect of the same instructional variable was also determined for Rubin's vase/faces and the Maltese cross. The results showed a similar tendency as in the case of the comparison between perspective reversals and content reversals. Possible cognitive processes that may play a role in top-down influences on figure reversal and theoretical implications of these findings for the interaction of bottom-up and top-down processes are discussed.     

“Enacted” auditory images are ambiguous; “pure” auditory images are not

Previous research indicates that visual images are inherently unambiguous. The present study extends this argument to auditory imagery. In Experiment 1, subjects were able to reinterpret an imaged ambiguous auditory figure, but covert subvocalization apparently aided this reinterpretation. When subvocalization was blocked, reinterpretations were eliminated. Experiments 2 and 3 generalize this finding to different procedures and stimuli. Experiment 4 explores further the role of subvocalization, by showing that the likelihood of reinterpreting an imaged stimulus is directly proportional to the degree of enactment allowed. We argue that subvocalization or enactment provides an internal stimulus that is subject to reinterpretation. Without enactment, the “pure” auditory image is as unambiguous as a visual image. Thus, in both visual and auditory modalities, images come into being as representations and so are inherently meaningful.     

Tactile discrimination of shape by octopus

Experiments are described showing that Octopus can be trained to distinguish by touch between a cube and a sphere, a surprising result since previous work has always indicated that octopuses cannot take proprioceptive information into account in learning. It can be shown, however, that the cube/sphere discrimination is made by detecting the distortion imposed on the rims of individual suckers in contact with the corners of the cube. Thus it is shown (1) that a cube with the corners even slightly rounded is less readily distinguishable from a sphere than a cube with sharp corners and, more important, (2) that a narrow rod is readily accepted in place of the cube by trained animals. Indeed, the rod seems to be a “better” cube than the original, and errors of discrimination decrease when it replaces the cube, presumably because it lacks flat surfaces liable to be confused with the surface of the sphere by a system depending on information from sense organs arranged in circles on the rims of the suckers.     

EEG gamma-band response during the perception of Necker cube reversals

In two former studies our research group reported frontal gamma-band enhancement during multistable visual perception and reversal rate dependent differences in the gamma-band. In these studies, a dynamic reversible figure was used which was based on the phenomenon of apparent motion. The aim of this study was to examine whether the results obtained with a dynamic motion paradigm can be replicated with the static Necker cube. The results demonstrate a general frontal gamma-band enhancement and higher induced gamma activity for subjects with a relatively high reversal rate in comparison to subjects with a relatively low reversal rate. This pattern of results fits well to the findings obtained with the dynamic motion paradigm. Therefore, the important role of frontal gamma activity for figure reversals has received further evidence. The results support the involvement of attentional top-down processing of figure reversal that is not directly related to binding processes.     

A further investigation of apparent size and retinal size as determinants of the figural after-effect

A figure which is more distant from the eye than another but subtends the same visual angle is usually seen as the larger. This phenomenon is called perceptual size constancy. Experiments have been reported which show that sometimes the apparent and sometimes the retinal sizes of the stimulus figures determine the direction of the figural after-effect. These experiments are reviewed and two further experiments are reported. In the first experiment two types of stimulus figure (discs and annuli) which subtended a visual angle of 1° 36' resulted in figural after-effects, the direction of which would have been predicted by the retinal sizes of the figures. In the second experiment larger figures were used which subtended a visual angle of about 4° and which were the same as those used by Sutherland (1954). In this experiment the after-effects were generally in a direction which would have been expected if apparent size determined the direction of the after-effect. In the interpretation of the results attention is drawn to the “Central Tendency Effect” which has been observed in size judgements of stimulus figures of varying size. Two hypotheses are suggested; one is that the true figural aftereffect has been obscured by the “Central Tendency Effect” in successive judgements, and the other is that time error in successive judgements of size is essentially a figural aftereffect. Some supportive evidence for the latter hypothesis is reported. It is concluded that apparent size is not a critical determinant of the figural after-effect.     

Selective adaptation with reversible figures: Don’t change that channel

An adaptation-test paradigm was used in two experiments examining processes underlying the perceived reversals of a rotating Necker cube. Adaptation and test cubes were either the same or different with respect to their visual fields of presentation (Experiment 1) or their sizes (Experiment 2). Results of both experiments indicated that, following subjects’ adaptation to a different cube, reversal rate of the test cube did not differ from that obtained without prior adaptation experience. In contrast, reversal rate of the test cube was elevated following adaptation to the same cube. Additional findings of Experiment 1 were that a test cube presented to the same visual field as the adaptation cube yielded a higher reversal rate than did a simultaneously presented cube in the opposite visual field. Also, the reversal rate of one cube was not influenced by the simultaneous presentation of a second cube. Results of both experiments were interpreted in terms of the fatigue and recovery of multiple, largely independent, localized neural channels. Thus, the results tie reversible-figure illusions to other visual phenomena thought to involve similar fatigue processes within localized visual channels (e.g., tilt, motion, and size aftereffects).     

Subjective organization and the visibility of illusory contours

The effects of the differential organization of a Necker cube on the perceived salience of an embedded illusory triangle were examined. Overall figural salience was greater when the triangle appeared to be localized on the front rather than the back cube face. Illusory contour salience also increased with increasing inducing area contrast and was greater when the figure was oriented on cardinal as compared to oblique coordinates. However, the latter effects were independent of perceived location within the cube. The finding that subjective organizational and structural factors influenced the perceived salience of the illusory figure but did not interact is consistent with van Tuijl and Leeuwenberg's (1982) suggestion that top-down and bottom-up determinants can operate independently in illusory contour perception.     

Neuropsychological assessment in illiterates: visuospatial and memory abilities

A basic neuropsychological battery of visuospatial and memory abilities was administered to extreme educational groups (illiterates and professionals). Subjects were matched according to sex and age. The following visuospatial tasks were included: figure copy (cube, house, and Rey-Osterrieth complex figure), telling time, recognition of superimposed figures, recognition of a map, and drawing of the plan of the room. The following memory tasks were used: basic information, digit retention (forward and backward), memory curve, delayed verbal recall, sentence repetition, logical memory, delayed logical memory, immediate recall of the Rey-Osterrieth complex figure, immediate reproduction of a cube, visuospatial memory, and sequential memory. In visuospatial tasks all differences between the two groups were statistically significant. Five of the seven visuospatial tasks (all but telling time and recognition of superimposed figures) showed differences between age groups with a better performance found in the younger groups and four of the tasks (cube, house, Rey-Osterrith complex figure copying, and telling time) were significant between sexes with a better performance in men. In memory tasks, with the exception of the immediate memory of sentences, all tasks showed statistically significant differences between educational groups. Eight of the 13 memory tasks (digits forward and backward, delayed memory of words, immediate and delayed logical memory, Rey-Osterrieth immediate memory, cube immediate memory, and sequential memory) showed significant differences for age while 4 of the tasks (digits backward, memory curve, Rey-Osterrieth immediate memory, and cube immediate memory) were significant for sex. Results are analyzed with regard to current theories in cognitive psychology and anthropology. Emphasis is placed on the finding that cognitive skills usually examined by neuropsychological tests represent learned and highly trained abilities.     

Hierarchical structure in perceptual representation

A theoretical framework for perceptual representation is presented which proposes that information is coded in hierarchical networks of nonverbal propositions. The hierarchical structure of the representations implies selective organization: Some subsets of a figure will be encoded as integral, structural units of that figure, while others will not. A context-sensitive metric for the “goodness” of a part within a figure is developed, corresponding to the probability that the subset will be encoded as a structural unit. Converging evidence supporting this position is presented from four different tasks using simple, straight-line figures. The tasks studied are (a) dividing figures into “natural” parts, (b) rating the “goodness” of parts within figures, (c) timed verification of parts within figures, and (d) timed mental synthesis of spatially separated parts into unitary figures. The results are discussed in terms of the proposed theory of representation, the processes that operate on those representations, and the general implications of the data for perceptual theories.     

Object-centred orienting of attention

Abstract

In the present study two experiments are reported in which the subjects were presented on a computer screen with a two-dimensional line drawing that is perceived as a three-dimensionalobject (i.e. a cube). The cube could be seen as stationary, as rotating about the y-axis (Experiment 1A), or as rotating about the x-axis (Experiment 1B). The subject's attention was directed by a visual precue to a vertex of the cube. As the cube rotated, the precued location moved in viewer-centred co-ordinatesm, but the local feature of the cube that had been precued (i.e. a given vertex) did not move in object-centred co-ordinates. The imperative stimulus was presented at the precued location (valid trials) or at an uncued location (invalid trials). Precued and uncued locations were determined in object-centred coordinates. The subjects were required to signal detection of the imperative stimulus by pressing the space bar on the computer keyboard. There were also control conditions in which the procedure was identical, but the cube was not visible. When the cube was visible, valid trials were faster than invalid trials, regardless of whether it was seen as stationary or rotating. It was concluded that the subjects could allocate spatial attention in object-centred co-ordinates. There wer also indications that responses for invalid trials were faster when the imperative stimulus was presented on the same face as the precue than when it was presented on the opposite face.     

Heart rate variability and respiratory concomitants of visual and nonvisual “imagery” and cognitive style

Attention to visual and nonvisual imagery, elicited by an imagery questionnaire, was studied using both within and between subjects analyses of cardiac and respiratory parameters. Visual imagery was accompanied by more regular interbeat heart rate (HR) and shorter, more stable respiratory cycles than nonvisual imagery. “Visually-oriented” thinkers (visualizers), identified by a word association test, manifested less overall variability in HR than “verbally-oriented” thinkers (verbalizers), as well as less variable HR and respiratory period during visual imagery. Visual and nonvisual imagery differed in HR variability for verbalizers and in respiration period for visualizers. The results are discussed in terms of the concepts of attention deployment, “mental load”, cerebral asymmetry, and stylistic personality differences in cognitive functioning.     

When do auditory/visual differences in duration judgements occur?

Four experiments examined judgements of the duration of auditory and visual stimuli. Two used a bisection method, and two used verbal estimation. Auditory/visual differences were found when durations of auditory and visual stimuli were explicitly compared and when durations from both modalities were mixed in partition bisection. Differences in verbal estimation were also found both when people received a single modality and when they received both. In all cases, the auditory stimuli appeared longer than the visual stimuli, and the effect was greater at longer stimulus durations, consistent with a “pacemaker speed” interpretation of the effect. Results suggested that Penney, Gibbon, and Meck's (2000) “memory mixing” account of auditory/visual differences in duration judgements, while correct in some circumstances, was incomplete, and that in some cases people were basing their judgements on some preexisting temporal standard.     

Selective interference during the retrieval of visual images

Twenty-six subjects memorized lists of (low I and high I) noun pairs under imagery or verbal mediation instructions. At recall the subjects were presented a digit (“1” or “2”) either auditorily or visually as an interfering stimulus.

Visual interference was found to selectively affect the retrieval of high I response terms. Also, the retrieval of nouns studied by imagery mediation was found to be selectively disrupted by visual interference.

These results suggest that the qualities of a visual image are retained all the way from image acquisition to retrieval, and that the visual components of images generated at the acquisition stage are probably not lost by subsequent coding processes.     

Influence of head orientation on visually and memory-guided arm movements

In the absence of visual supervision, tilting the head sideways gives rise to deviations in spatially defined arm movements. The purpose of this study was to determine whether these deviations are restricted to situations with impoverished visual information. Two experiments were conducted in which participants were positioned supine and reproduced with their unseen index finger a 2 dimensional figure either under visual supervision or from memory (eyes closed). In the former condition, the figure remained visible (using a mirror). In the latter condition, the figure was first observed and then reproduced from memory. Participants' head was either aligned with the trunk or tilted 30° towards the left or right shoulder. In experiment 1, participants observed first the figure with the head straight and then reproduced it with the head either aligned or tilted sideways. In Experiment 2, participants observed the figure with the head in the position in which the figure was later reproduced. Results of Experiment 1 and 2 showed deviations of the motor reproduction in the direction opposite to the head in both the memory and visually-guided conditions. However, the deviations decreased significantly under visual supervision when the head was tilted left. In Experiment 1, the perceptual visual bias induced by head tilt was evaluated. Participants were required to align the figure parallel to their median trunk axis. Results revealed that the figure was perceived as parallel with the trunk when it was actually tilted in the direction of the head. Perceptual and motor responses did not correlate. Therefore, as long as visual feedback of the arm is prevented, an internal bias, likely originating from head/trunk representation, alters hand-motor production irrespectively of whether visual feedback of the figure is available or not.     

Evidence for color-contingent organizational aftereffects

An effort was made to produce negative organizational aftereffects in response to a perceptually ambiguous stimulus that can be seen as a cube in either of two orientations. 6 subjects were adapted to alternative disambiguated versions of the ambiguous cube, shown either in green or magenta light. When tested with the ambiguous figure shown in either green or magenta light, 5 subjects showed negative aftereffects while one subject showed a positive aftereffect. The aftereffects showed an average change in preferred organization of 25%. The results are interpreted in terms of both cognitive and hard-wired processes.     

The perceptual-mnemonic/feature conjunction model of perirhinal cortex function

The perirhinal cortex was once thought to be “silent cortex”, virtually ignored by researchers interested in the neurobiology of learning and memory. Following studies of brain damage associated with cases of amnesia, perirhinal cortex is now widely regarded as part of a “medial temporal lobe (MTL) memory system”. This system is thought to be more or less functionally homogeneous, having a special role in declarative memory, and making little or no contribution to other functions such as perception. In the present article, we summarize an alternative view. First, we propose that components of the putative MTL system such as the hippocampus and perirhinal cortex have distinct and dissociable functions. Second, we provide evidence that the perirhinal cortex has a role in visual discrimination. In addition, we propose a specific role for perirhinal cortex in visual discrimination: the contribution of complex conjunctive representations to the solution of visual discrimination problems with a high degree of “feature ambiguity”. These proposals constitute a new view of perirhinal cortex function, one that does not assume strict modularity of function in the occipito-temporal visual stream, but replaces this idea with the notion of a hierarchical representational continuum.     

An experimental contribution to the problem of apparent reality

Certain two-dimensional figures acquire a character of relief and reality when oriented obliquely to the observer and viewed monocularly. The experiments were made with two types of figure—a simple straight line, and more complex patterns of lines. The results indicate that the character of reality gradually increases as the experimental conditions become more favourable. The conditions found to be most favourable are dissimilarities of size, brightness, microstructure, shape, perspective clues, between figure and ground.

When the segregating factors are operating, the figure becomes independent of the paper and takes up its own orientation in space. In the case of the simple line, this orientation may be frontal, but more usually the orientation of the background is taken into account, the resulting structural organization being that of a “needle” projecting from the background. The reality is greatest when the orientation of the background is such that the “needle” appears to be perpendicular to the ground. The complex figures used in the experiments were perspective designs of parallelepipeds. Again, the oblique position of the design associated Tvith the greatest degree of reality of the tridimensional object is that which permits the figure to be seen as projecting perpendicularly from its background.     

A study of the effect of structural information and familiarity in form perception

Two experiments were carried out to investigate the influence of structural information and familiarity on the processing of visual forms. Pairs of “well” structured and nameable and “poorly” structured and non-nameable fragmented forms were employed as stimuli. The effects of structure and familiarity were assessed by manipulating the visual hemifield of presentation and the task. In Experiment 1 stimuli were judged as being either in the same orientation or mirror-reversed, a task that does not require high-level semantic information to be processed. Experiment 2 required physically identical forms to be matched, which may use either physical or name information. In Experiment 1 “same” judgements were equivalent for both types of stimuli, and “different' judgements were longer for the “poorly” structured (non-nameable) forms. In Experiment 2 there was little overall difference between “well” and “poorly” structured forms, though response times to “well” structured (nameable) forms were slowed for right-visual-field presentations. It is suggested that familiarity may not be sufficient to provide a perceptual advantage for nameable forms, as the advantage for nameable stimuli was confined to “same” judgements in Experiment 1 and response times were shorter for non-nameable stimuli in Experiment 2. Rather, performance depends upon factors such as the computation of global shape (due to structural properties of collinearity and closure) and on the use of different kinds or representations (physical versus name) in matching.