Spatial frequency content of visual imagery

Three experiments employing the McCollough paradigm were conducted to determine the spatial-frequency content of visual imagery. In Experiment 1, large and reliable pattern-contingent color aftereffects were obtained after adaptation to visual imagery. The direction of the aftereffects indicated that subjects were adapting to higher spatial frequencies in their imagery. These results contrast with the data of Experiment 2, which demonstrate that color aftereffects obtained with adaptation to physically present stimuli are mediated by the fundamental spatial frequency components. The magnitude of the imagery-induced aftereffects in Experiment 1 equaled the magnitude of the externally induced aftereffects obtained in Experiment 2 with the same subjects. By blurring the to-be-imaged patterns (Experiment 3), the fundamental Fourier components became the salient perceptual features of the stimuli, and the direction of the imagery-induced aftereffects was reversed from that of Experiment 1, indicating that the spatial frequency content of the imagery had changed from higher to lower frequencies. Under normal viewing conditions, subjects use the higher spatial frequencies associated with the perceptually salient edges of stimuli to construct their images. The results of Experiments 1 and 3 are discussed in light of a current controversy over the nature of information representation in imagery, and it is concluded that support has been obtained for the analog model of visual imagery.     

Visual perception and visual mental imagery of emotional faces generate similar expression aftereffects

What is the relationship between visual perception and visual mental imagery of emotional faces? We investigated this question using a within-emotion perceptual adaptation paradigm in which adaptation to a strong version of an expression was paired with a test face displaying a weak version of the same emotion category. We predicted that within-emotion adaptation to perception and imagery of expressions would generate similar aftereffects, biasing perception of weak emotional test faces toward a more neutral value. Our findings confirmed this prediction. Adaptation to mental images yielded aftereffects that inhibited emotion recognition of test expressions, as participants were less accurate at recognising these stimuli compared to baseline. While the same inhibitory effect was observed when expressions were visually perceived, the size of the aftereffects was greater for perception than imagery. These findings suggest the existence of expression-selective neural mechanisms that subserve both visual perception and visual mental imagery of emotional faces.     

Stimulus selectivity of figural aftereffects for faces

Viewing a distorted face induces large aftereffects in the appearance of an undistorted face. The authors examined the processes underlying this adaptation by comparing how selective the aftereffects are for different dimensions of the images including size, spatial frequency content, contrast, and color. Face aftereffects had weaker selectivity for changes in the size, contrast, or color of the images and stronger selectivity for changes in contrast polarity or spatial frequency. This pattern could arise if the adaptation is contingent on the perceived similarity of the stimuli as faces. Consistent with this, changing contrast polarity or spatial frequency had larger effects on the perceived identity of a face, and aftereffects were also selective for different individual faces. These results suggest that part of the sensitivity changes underlying the adaptation may arise at visual levels closely associated with the representation of faces.     

The functional equivalence of mental images and errors of movement.

Four experiments are reported demonstrating that mental images are functionally equivalent to physical errors of movement in producing changes in visual-motor coordination, at both central and peripheral levels of the visual-motor system. In the first experiment, subjects in one condition pointed at a target seen through laterally displacing prisms and were instructed to imagine pointing errors identical to those recorded previously for subjects in a separate condition who actually observed their pointing errors. Changes in pointing accuracy during adaptation procedures and visual-motor aftereffects following these procedures for subjects who imagined their errors were proportional to visual-motor shifts and aftereffects for subjects who observed their errors. In the second experiment, these same imagery instructions resulted in identical pointing shifts and aftereffects even in the case where prisms did not displace the target. The third experiment showed that when subjects believe that their mental images of pointing errors do not correspond to their actual pointing errors, pointing aftereffects result that are characteristic of the processing of error information at peripheral, but not central, levels of the visual motor system. The final experiment showed that when subjects do believe that their images of pointing errors correspond to actual pointing errors, but imagine the pointing movement itself in addition to their errors, pointing aftereffects result that are characteristic of the processing of error information at central, but not peripheral, levels of the visual-motor system. Contributions to visual-motor aftereffects from these two levels appear to be additive. Another significant result was that, in the imagery feedback conditions of each experiment, subjects who gave high ratings of vividness to their mental imagery showed the greatest magnitude of pointing aftereffects. These findings establish that mental images for errors of movement can produce stable visual-motor changes that cannot be accounted for simply by subjects' expectations regarding the actual consequences of their actions.     

Perception and imagery of faces generate similar gender aftereffects

Visual adaptation is known to bias perception away from the properties of the adapting stimuli, toward opposite properties, resulting in perceptual aftereffects. For example, prolonged exposure to a face has been shown to produce an identity aftereffect, biasing perception of a subsequent face toward the opposite identity. Such repulsive aftereffects have been observed for both visually perceived and visually imagined faces, suggesting that both perception and imagery yield typical aftereffects. However, recent studies have reported opposite patterns of aftereffects for perception and imagery of face gender. In these studies, visually perceived faces produced typical effects in which perception of androgynous faces was biased away from the gender of the adaptor, whereas imagery of the same stimuli produced atypical aftereffects, biasing the perceived gender of androgynous faces toward the gender of the adaptor. These findings are highly unusual and warrant further research. The present study aimed to gather new evidence on the direction of gender aftereffects following perception and imagery of faces. Experiment 1 had participants view and imagine female and male faces of famous and non-famous individuals. To determine the effect of concomitant visual stimulation on imagery and adaptation, participants visualized faces both in the presence and in the absence of a visual input. In Experiment 2, participants were adapted to perceived and imagined faces of famous and non-famous actors matched on gender typicality. This manipulation allowed us to determine the effect of face familiarity on the magnitude of gender aftereffects. Contrary to evidence from previous studies, our results demonstrated that both perception and imagery produced typical aftereffects, biasing the perceived gender of androgynous faces in the opposite direction to the gender of the adaptor. Famous faces yielded largest adaptation effects across tasks. Experiment 2 confirmed that these effects depended on familiarity rather than on sexual dimorphism. In both experiments, this effect was greater for perception than imagery. Additionally, imagery of famous faces produced strongest aftereffects when it was performed in the absence of visual stimulation. The implications of these findings are discussed.     

Effects of spatial filtering and lack of effects of visual imagery on pattern-contingent color aftereffects

Checkerboards contain fundamental two-dimensional Fourier components oriented 45° from the edges of individual checks. Previous studies have shown that contingent color aftereffects following adaptation to chromatic checkerboard stimuli were associated with the fundamental components rather than the edges, In the present experiments, we measured contingent color aftereffects, using the method of constant stimuli, after subjects adapted to unfiltered checkerboards and checkerboards with the fundamental Fourier components removed. The adaptation stimuli were magenta (or green) squares and green (or magenta) diamonds; the test stimuli were vertical or oblique sine-wave gratings with different saturations, After adaptation to unfiltered checkerboards, aftereffects contingent on the fundamental components were obtained. In contrast, after adaptation to filtered stimuli, aftereffects of smaller magnitude were found to be aligned with the edges. The data support the previous findings of spatial-frequency-contingent color after-effects with checkerboard adaptation stimuli and indicate that the aftereffects can be associated with edges if the fundamental components of adaptation stimuli are removed by spatial filtering. We reexamined the possibility of color aftereffects induced by imagery of checkerboards. Contrary to the previous reports, no significant aftereffects were obtained.     

Internally augmented displays: contingent aftereffects as performance aids

Previous research on visual contingent aftereffects has been concerned with examining the effects of various parameters (e.g., spatial frequency and luminance) on the adaptation to, and decay of, contingent aftereffects. The current study tested the viability of using visual contingent aftereffects in a display context. Using established characteristics of contingent aftereffects, a program of contingent aftereffect adaptation was designed. Studies were conducted to determine if subjects who were adapted to see visual contingent aftereffects invoked by a visual display could achieve more rapid or certain identification of a display under low luminance conditions. The results confirmed (a) that contingent aftereffects can improve performance on a visual discrimination task requiring information from a display and (b) that contingent aftereffects are more enhanced at low levels of illumination.     

数量适应后效的皮层映射特征

本研究探讨了观察者与观察目标存在相对运动时视觉系统对目标数量特征的适应后效的皮层映射特征, 并与对比度适应后效的映射规律进行比较。包括两项实验。其中, 实验一要求被试在适应目标后转换注视点, 考察眼跳后相同和不同视网膜区域以及相同和不同空间区域的适应后效, 发现数量适应后效具有部分空间-皮层映射特性, 而对比度适应后效则表现出完全的视网膜-皮层映射特征。实验二采用固定的注视点, 考察目标运动后目标原位置和新位置区域的适应后效, 发现数量适应后效不完全依赖于视网膜-皮层映射, 它可以“追随”客体运动重新映射到新的位置, 表现出基于客体映射的特征, 而对比度适应后效则完全依赖于视网膜-皮层映射, 不能“追随”客体移动在目标新位置重新形成映射。两项实验结果提示, 相对于对比度等低级表面特征而言, 数量特征对目标的描述涉及更高的加工水平, 它可以与观察目标的相对运动信息进行整合, 且这种整合在眼跳和非眼跳的观察条件下都可发生。     

Gait symmetric adaptation: Comparing effects of implicit visual distortion versus split-belt treadmill on aftereffects of adapted step length symmetry

Understanding gait adaptation is essential for rehabilitation, and visual feedback can be used during gait rehabilitation to develop effective gait training. We have previously shown that subjects can adapt spatial aspects of walking to an implicitly imposed distortion of visual feedback of step length. To further investigate the storage benefit of an implicit process engaged in visual feedback distortion, we compared the robustness of aftereffects acquired by visual feedback distortion, versus split-belt treadmill walking. For the visual distortion trial, we implicitly distorted the visual representation of subjects’ gait symmetry, whereas for the split-belt trial, the speed ratio of the two belts was gradually adjusted without visual feedback. After adaptation, the visual feedback or the split-belt perturbation was removed while subjects continued walking, and aftereffects of preserved asymmetric pattern were assessed. We found that subjects trained with visual distortion trial retained aftereffects longest. In response to the larger speed ratio of split-belt walking, the subjects showed an increase in the size of aftereffects compared to the smaller speed ratio, but it steeply decreased over time in all the speed ratios tested. In contrast, the visual distortion group showed much slower decreasing rate of aftereffects, which was evidence of longer storage of an adapted gait pattern. Visual distortion adaptation may involve the interaction and integration of the change in motor strategy and implicit process in sensorimotor adaptation. Although it should be clarified more clearly through further studies, the findings of this study suggest that gait control employs distinct adaptive processes during the visual distortion and split-belt walking and also the level of reliance of an implicit process may be greater in the visual distortion adaptation than the split-belt walking adaptation.     

Interocular transfer of orientation-contingent color aftereffects with external and internal adaptation

The issue of whether McCollough effects transfer interocularly was examined in two experiments. In Experiment 1, as in previous experiments, no interocular transfer of McCollough effects was obtained when subjects adapted monocularly to externally present patterns with their unused eyes fully occluded. However, when subjects adapted monocularly (with the unused eye fully occluded) to visual images of those patterns superimposed on physically present chromatic backgrounds, McCollough effects transferred interocularly. In Experiment 2, subjects adapted to either physically present patterns or to images of those patterns (as in Experiment 1), but the unused eye was exposed to unpatterned diffuse white light. In contrast to Experiment 1, the externally adapted subjects showed interocular transfer of McCollough effects. In Experiment 2, the magnitude of the interocular transfer effects produced by imagery was significantly larger than the magnitude of the effects produced by external adaptation, but the magnitude of the imagery-induced aftereffects did not differ from Experiment 1 to Experiment 2. These results extend earlier findings by Kunen and May (1980) and show that McCollough effects can be produced through adaptation to imagery, even though the direction of the imagery-induced aftereffects indicates adaptation to higher spatial frequencies whereas externally derived aftereffects indicate adaptation to lower spatial frequencies. It is concluded that failure of previous studies to obtain interocular transfer of McCollough effects may have resulted from complete occlusion of the contralateral eye. These data point up some interesting similarities and some important differences between imagery and actual vision. The implications for analogical models of visual imagery are discussed.     

The influence of imagery ability on color aftereffects produced by physically present and imagined induction stimuli

Two methods of induction were used to produce orientation-contingent color aftereffects for observers assigned to one of three groups (high, medium, and low) on the basis of self-rated imagery ability. In Experiment 1, observers were required to make magnitude estimates of color aftereffects following inspection of stimulus patterns normally used to produce McCollough effects (e.g., red vertical contours, green horizontal contours). Experiment 2 was a partial replication of Experiment 1, with additional induction conditions in which observers were required to imagine the presence of appropriately oriented contours when particular homogeneous color patches were presented. The results indicated that self-rated imagery ability was not a significant factor in differentiating between observers’ performance when orthodox induction procedures were used (Experiment 1). In addition, there were no reliable indications (Experiment 2) that imagined stimulus attributes can be effectively substituted for real stimulus attributes in order to produce orientation-contingent color aftereffects. The results are discussed in terms of their implications for the use of imagery-induced perceptual phenomena as a paradigm for investigating the possibility of common neural mechanisms in perception and imagination; in addition, the general implications of the results for understanding the functional significance of self-reported imagery ability are examined.     

The effect of familiarity on face adaptation

Face aftereffects can provide information on how faces are stored by the human visual system (eg Leopold et al, 2001 Nature Neuroscience 4 89-94), but few studies have used robustly represented (highly familiar) faces. In this study we investigated the influence of facial familiarity on adaptation effects. Participants were adapted to a series of distorted faces (their own face, a famous face, or an unfamiliar face). In experiment 1, figural aftereffects were significantly smaller when participants were adapted to their own face than when they were adapted to the other faces (ie their own face appeared significantly less distorted than a famous or unfamiliar face). Experiment 2 showed that this 'own-face' effect did not occur when the same faces were used as adaptation stimuli for participants who were unfamiliar with them. Experiment 3 replicated experiment 1, but included a pre-adaptation baseline. The results highlight the importance of considering facial familiarity when conducting research on face aftereffects.     

Storage of spatially specific threshold elevation

The decay of several visual aftereffects may be prolonged by interposing a period of light-free or pattern-free viewing between adaptation and testing. We demonstrate that this storage phenomenon can be observed using the threshold elevation aftereffect that follows inspection of a high-contrast grating pattern. Control experiments comparing thresholds for vertical and horizontal gratings after adaptation to a vertical grating reveal that the stored aftereffect, like its unstored counterpart, is pattern-selective. Storage is equally pronounced with stimuli that are detected by pattern-analyzing or movement-analyzing visual channels. Unlike other aftereffects, the threshold-elevation aftereffect requires that the storage period be light-free; no storage is seen if a blank field is inspected between adaptation and testing. The results are discussed with respect to the nature of visual aftereffects, and possible cognitive or physiological models of storage.     

Absence of binocular interaction between spatial and color attributes of visual stimuli

The hypothesis that induction of the McCollough effect (spatially selective color aftereffects) entails adaptation of monocularly driven detectors tuned to both spatial and color attributes of the visual stimulus was examined in four experiments. The McCollough effect could not be generated by displaying contour information to one eye and color information to the other eye during inspection, even in the absence of binocular rivalry. Nor was it possible to induce depth-specific color aftereffects following an inspection period during which random-dot stereograms were viewed, with crossed and uncrossed disparity seen in different colored light. Masking and aftereffect in the perception of stereoscopic depth were also nonselective to color; in both cases, perceptual distortion was controlled by stereospatial variables but not by the color relationship between the inspection and test stimuli. The results suggest that binocularly driven spatial detectors in human vision are insensitive to wavelength.     

Orientation-specific aftereffects to mentally generated lines

After staring at a pattern of tilted lines, subsequent lines appear to be tilted in the opposite direction (direct tilt aftereffect, TAE). In a previous fMRI study we have demonstrated a direct TAE solely induced by the mental imagination accompanied by adaptation of orientation-selective neurons located in the extrastriate cortex, supporting the assumption of a perception-like coding of mental images. In this study we enlarge and specify the evidence for a perception-like coding of orientation-imagination. First, we replicated the previously detected direct TAE induced by line imagination with altered design-variations to control possible perceptual task confounds. Second, we tried to induce two other orientation-specific aftereffects: indirect TAE and contrast-threshold elevation aftereffect by mental imagery. The results replicate a robust direct TAE by mental imagery and by visual stimulation, with no influence of attentional resource allocation or perceptual task confounds. We could not induce an indirect TAE, but observed a perception bias in the opposite direction of the indirect TAE. The mental imagery of lines induced no orientation-selective contrast-threshold elevation aftereffect. In general, mental imagery seems to influence visual perception, indicating that perceptual resources are used by mental imagery. However, the utilisation of visual resources seems to be somewhat different from utilisation by perception.     

Lecturers' faces fatigue their students: Face identity aftereffects for dynamic and static faces

Face adaptation has been used as a tool to probe our representations for facial identity. It has also been claimed to play a functional role in face processing, perhaps calibrating the visual system towards encountered faces. However, for this to be so, face aftereffects must be observable following adaptation to ecologically valid moving stimuli, not just after prolonged viewing of static images. We adapted our participants to videos, static image sequences or single images of the faces of lecturers who were personally familiar to them. All three stimulus types produced significant, and equivalent, face identity aftereffects, demonstrating that aftereffects are not confined to static images but can occur after exposure to more naturalistic stimuli. It is also further evidence against explanations of face adaptation effects solely in terms of low-level visual processing.     

Curvature-specific color aftereffects

Adaptation to convex and concave arcs in different colored light results in curvature-specific color aftereffects when arcs are later viewed in white light. In three experiments, it was shown that these color aftereffects often are partial (restricted to limited regions of the test arcs) rather than uniform, and in addition that aftereffects induced by exposure to arcs transfer to straight-line displays of particular orientation, and vice versa. These data were interpreted as evidence that arcs are processed in the visual system in relation to the orientations of local straight line approximations instead of on a global basis. In these terms, curvature-specific color aftereffects are merely complex forms of the orientation-specific color aftereffects first described by McCollough (1965).     

A reevaluation of angle-contingent color aftereffects

It has been argued that adaptation to a series of angles with vertices pointing up and illuminated in one color, and to angles with vertices pointing down and illuminated in the opponent color, results in color aftereffects that are contingent on angle direction. In the present paper, using a number of test figures, we demonstrate that adaptation to these ascending/descending angles results in color aftereffects that can be accounted for in terms of spatially localized, orientation-color pairings. In the light of our results, we suggest that previous inferences concerning angle-contingent color aftereffects should be reconsidered.     

Color aftereffects contingent on perceptual organization

The present experiment demonstrated that (a) cognitive organizational factors influence an orientation-contingent color (McCollugh) aftereffect, and (b) adaptation to colored gratings affected the perceptual organization of a reversible figure. Following adaptation to colored gratings, color aftereffects were reported in only one of two organizations of a reversible figure, and the colors conformed to the subjective contours of the test pattern rather than exclusively to the regions defined by horizontal and vertical pattern elements. In addition, an organization of the reversible figure that segregated vertical and horizontal pattern elements was dominant subsequent to adaptation.     

Awareness becomes necessary between adaptive pattern coding of open and closed curvatures

Visual pattern processing becomes increasingly complex along the ventral pathway, from the low-level coding of local orientation in the primary visual cortex to the high-level coding of face identity in temporal visual areas. Previous research using pattern aftereffects as a psychophysical tool to measure activation of adaptive feature coding has suggested that awareness is relatively unimportant for the coding of orientation, but awareness is crucial for the coding of face identity. We investigated where along the ventral visual pathway awareness becomes crucial for pattern coding. Monoptic masking, which interferes with neural spiking activity in low-level processing while preserving awareness of the adaptor, eliminated open-curvature aftereffects but preserved closed-curvature aftereffects. In contrast, dichoptic masking, which spares spiking activity in low-level processing while wiping out awareness, preserved open-curvature aftereffects but eliminated closed-curvature aftereffects. This double dissociation suggests that adaptive coding of open and closed curvatures straddles the divide between weakly and strongly awareness-dependent pattern coding.