Visual discovery in mind and on paper

The effectiveness of mental imagery with and without drawing support (perceptual assistance) in the visual synthesis of novel patterns was studied in three experiments. When the task was to create one recognizable pattern from three simple shapes in a 2-min assembly period, subjects were as likely to produce a recognizable or creative pattern per trial whether mental imagery was augmented by external drawing support or not. When the task was to create as many patterns as possible in a 3-min assembly period, more patterns were produced per trial with external drawing support than without; however, neither the recognizability nor the creativity of the patterns differed. Differences in performance in the visual synthesis task with and without external drawing support are interpreted in terms of limited cognitive resources.     

Some consequences of visualization in pattern identification and detection

The goal of this study was to establish some of the conditions under which mental imagery facilitates or interferes with the identification and detection of visual patterns. In Experiment 1, subjects identified simple bar patterns presented at orientations 90 degrees apart under normal viewing conditions. Their reaction times were shorter when they had imagined seeing the patterns in advance at the same orientation, but were longer when they had imagined seeing the patterns at orientations that were in-between those of the actual presented patterns, relative to baseline conditions in which they were instructed not to imagine the patterns. In Experiment 2, where the subjects had only to detect the target patterns without identifying them, there was no effect of image formation or image-target alignment. In Experiment 3, where the detection task was repeated but where the target exposure duration was reduced, imagery significantly interfered with detection. In contrast to the results of Experiment 1, reaction time and error rate in this case were greatest when the imagined patterns were perfectly aligned with the target patterns. These findings demonstrate that whether imagery facilitates or interferes with performance on a visual task depends on the nature and difficulty of the task and on how closely the imagined and presented patterns correspond.     

Area and contrast effects upon perceptual and imagery acuity

We measured the size of visual fields within which actual and imagined circular patterns could be resolved, as the patterns varied in both area and relative contrast. As pattern area increased, imagery fields increased in size at the same rate as perceptual fields. However, as the relative contrast between parts of the patterns was reduced, perceptual fields diminished in size, while imagery fields did not. for all variations of pattern area and contrast, fields in imagery were roughly the same shape as fields in perception. A control experiment provided no support for the possible criticism that these results might be explained on the basis of subjects' expectations for the size and shape of the imagery fields, especially since control subjects falsely believed that reductions in contrast would reduce considerably the size of these fields. Although these findings suggest that images and physical objects are functionally equivalent at pattern-processing levels of the visual system, they demonstrate that images and objects are not functionally equivalent at certain levels of the visual system where information about relative contrast is processed.     

A computational analysis of mental image generation: evidence from functional dissociations in split-brain patients

Recent efforts to build computer simulation models of mental imagery have suggested that imagery is not a unitary phenomenon. Rather, such efforts have led to a modular analysis of the image-generation process, with separate modules that can activate visual memories, inspect parts of imaged patterns, and arrange separate parts into a composite image. This idea was supported by the finding of functional dissociations between the kinds of imagery tasks that could be performed in the left and right cerebral hemispheres of two patients who had previously undergone surgical transection of their corpus callosa. The left hemisphere in both subjects could inspect imaged patterns and could generate single and multipart images. In contrast, although the right hemisphere could inspect imaged patterns and could generate images of overall shape, it had difficulty in generating multipart images. The results suggest a deficit in the module that arranges parts into a composite. The observed pattern of deficits and abilities implied that this module is not used in language, visual perception, or drawing. Furthermore, the results suggest that the basis for this deficit is not a difficulty in simply remembering visual details or engaging in sequential processing.     

Evaluating the mind's eye: the metacognition of visual imagery

Can people evaluate phenomenal qualities of internally generated experiences, such as whether a mental image is vivid or detailed? This question exemplifies a problem of metacognition: How well do people know their own thoughts? In the study reported here, participants were instructed to imagine a specific visual pattern and rate its vividness, after which they were presented with an ambiguous rivalry display that consisted of the previously imagined pattern plus an orthogonal pattern. On individual trials, higher ratings of vividness predicted a greater likelihood that the imagined pattern would appear dominant when the participant was subsequently presented with the binocular rivalry display. Off-line self-report questionnaires measuring imagery vividness also predicted individual differences in the strength of imagery bias over the entire study. Perceptual bias due to mental imagery could not be attributed to demand characteristics, as no bias was observed on catch-trial presentations of mock rivalry displays. Our findings provide novel evidence that people have a good metacognitive understanding of their own mental imagery and can reliably evaluate the vividness of single episodes of imagination.     

Sensory features of mental images in the framework of human actions

What determines the sensory impression of a self-generated motor image? Motor imagery is a process in which subjects imagine executing a body movement with a strong kinesthetic and/or visual component from a first-person perspective. Both sensory modalities can be combined flexibly to form a motor image. 90 participants of varying ages had to freely generate motor images from a large set of movements. They were asked to rate their kinesthetic as well as their visual impression, the perceived vividness, and their personal experience with the imagined movement. Data were subjected to correlational analyses, linear regressions, and representation similarity analyses. Results showed that both action characteristics and experience drove the sensory impression of motor images with a strong individual component. We conclude that imagining actions that impose varying demands can be considered as reexperiencing actions by using one’s own sensorimotor representations that represent not only individual experience but also action demands.     

The mind's nose: Effects of odor and visual imagery on odor detection

Abstract— We examined odor imagery by looking for its effects on detection of weak odors. Seventy-two healthy subjects performed a forced-choice odor detection task in one of three conditions: after being told to imagine an odor (odor imagery), after being told to imagine an object (visual imagery), or without having received imagery instructions (no-imagery control). For the two imagery conditions, the presented and imagined stimuli were either the same (matched) or different (mismatched). There was a significant difference between detection in the matched and mismatched conditions for odor imagery, but not for visual imagery. We conclude that our paradigm does measure odor imagery and that the effect of imagery on detection is both content- and modality-specific. Further, the difference between conditions was due to lower detection with mismatched odor imagery than without imagery, indicating that interference underlies the effect.     

Abilities and strategies of blind and sighted subjects in visuo-spatial imagery

An active visuo-spatial memory task was used in order to determine the characteristics of mental imagery in subjects with and without visual experience. Subjects were instructed to generate a mental representation of verbally presented 2D patterns that were placed in a grid and to indicate how many pattern elements were in corresponding positions in the two halves of the grid according to a specific grid axis (vertical or horizontal). Unexpectedly, results showed a similar performance in early blind, late blind and sighted subjects. However, subjects' debriefing showed that the three groups used different strategies. The sighted and the late blind subjects took advantage of a visuo-spatial strategy. They generated a mental image of the matrix and they simplified this image to maintain only the relevant information in memory. In contrast, the early blind subjects encoded each pattern element by its location in a (X,Y) coordinate system without visual representation. This indicates that both early and late blind subjects are able to perform an active visuo-spatial imagery task as well as sighted subjects although they use different strategies.     

Eye scanpaths during visual imagery reenact those of perception of the same visual scene

Eye movements during mental imagery are not epiphenomenal but assist the process of image generation. Commands to the eyes for each fixation are stored along with the visual representation and are used as spatial index in a motor‐based coordinate system for the proper arrangement of parts of an image. In two experiments, subjects viewed an irregular checkerboard or color pictures of fish and were subsequently asked to form mental images of these stimuli while keeping their eyes open. During the perceptual phase, a group of subjects was requested to maintain fixation onto the screen's center, whereas another group was free to inspect the stimuli. During the imagery phase, all of these subjects were free to move their eyes. A third group of subjects (in Experiment 2) was free to explore the pattern but was requested to maintain central fixation during imagery. For subjects free to explore the pattern, the percentage of time spent fixating a specific location during perception was highly correlated with the time spent on the same (empty) locations during imagery. The order of scanning of these locations during imagery was correlated to the original order during perception. The strength of relatedness of these scanpaths and the vividness of each image predicted performance accuracy. Subjects who fixed their gaze centrally during perception did the same spontaneously during imagery. Subjects free to explore during perception, but maintaining central fixation during imagery, showed decreased ability to recall the pattern. We conclude that the eye scanpaths during visual imagery reenact those of perception of the same visual scene and that they play a functional role.     

Einstein’s jacket: Evidence for long-term perceptual specificity in mental imagery

To what extent are visual fantasies constrained by our perceptual experience of the real world? Our study exploits the fact that people’s knowledge of the appearance of individuals from the early 20th Century (e.g., Albert Einstein, Winston Churchill) derives predominantly from viewing black-and-white media images. An initial experiment shows that mental imagery for individuals from this period are experienced as significantly less colourful than imagery for individuals from the era of colour media. A second experiment manipulated whether participants were instructed to explicitly imagine using colour or not (i.e., “imagine Albert Einstein wearing a green jacket” vs. “imagine Albert Einstein wearing a jacket”). Results show that colour manipulation only influences imagery for black-and-white era individuals, with no comparable effect on imagery for colour era individuals. This finding is replicated in a third experiment that includes an additional control condition of imagining generic characters (i.e., “Imagine a knight wearing a cloak” vs. “imagine a knight wearing a red cloak”). Again, only imagery for black-and-white era individuals is affected by the colour manipulation. Overall these results provide evidence for long-term perceptual specificity effects in mental imagery. We argue that visual fantasies can be constrained by surface features of underlying representations in memory, even when imagining something we have never directly perceived.     

Imagined body orientation and perception of the visual vertical

The existence of body orientation mental imagery was tested by examining whether self roll tilt imagery affects the subjective visual vertical (SVV). Twenty healthy subjects judged the orientation of a dim luminous bar with respect to gravitational vertical, while normally seated in complete darkness with their head firmly restrained earth vertically. SVV was measured in three conditions: a reference condition with no imagery, and a left and a right imagery condition, during which the bar orientation was to be judged while the subjects imagine themselves roll-tilted towards left or right, respectively. The imagined roll tilts were of the same magnitude as roll tilts which generally induce an E- effect, i.e., an SVV lean toward the side opposite to those of body tilt. If imagery and perception of self roll tilt share common processes, self roll tilt imagery should induce an E-like effect. Results show an imagery- induced E-like effect, which strongly supports the idea that humans can perform mental imagery of body orientation about gravity. Received: 4 April 2000 / Accepted: 1 September 2000     

Mental image reversal and verbal recoding: When ducks become rabbits

A recent debate has concerned whether classical bistable configurations (e.g., duck/rabbit) can be reinterpreted using mental imagery. Research in this field indicates that image reversal is possible only when subjects change their specification of orientation. In a series of four experiments, we demonstrate that mental reversal of classical bistable configurations( CBCs) is impeded by verbally recoding the visual pattern at the time of input. When subjects were prevented from verbally recoding visual stimuli in short-term memory, they fared systematically better in mentally reversing the CBC, even when they received no instructions to change their reference frame or specification of orientation. On this basis, we suggest a model of image reversal that takes into account the interaction between memory codes and provides a new perspective on verbal recoding, verbal overshadowing, and mental discoveries.     

Individual differences in the capacity limitations of visuospatial short-term memory: Research on sighted and totally congenitally blind people

The study of visuospatial imagery processes in totally congenitally blind people makes it possible to understand the specific contribution of visual experience for imagery processes. We argue that blind people may have visuospatial imagery processes, but they suffer from some capacity limitations. Similar, although smaller, limitations and individual differences may be found in sighted people. Visuospatial imagery capacity was explored by asking people to follow an imaginary pathway through either two- or three-dimensional matrices of different complexity. The blind appear to use specific visuospatial processes in this task (Experiments 2 and 3), but they have difficulty with three-dimensional matrices; sighted people have no such difficulty with three-dimensional matrices (Experiment 1). On the other hand, when a three-dimensional pattern exceeded sighted capacity, the blind and sighted showed similar patterns of errors. Subsequent analyses suggested that both visuospatial processes and verbal mediation were used.     

Manipulation of visual mental images in children and adults.

Recently, there has been a debate on whether visual patterns can be transformed and reinterpreted in mental imagery. In the present study, task demands and age of subjects were manipulated to see whether children and adults were able to discover a novel visual pattern after transforming a mental image. Two tasks, called combination and subtraction, were devised. They consist of either compounding or taking away parts of images to discover a new construal. Results indicate that not only adults, but also children aged 6 and 10, are able to transform a mental image so as to yield another image with a different interpretation. Task demands had a greater effect on children than adults, consistent with the suggestion that the ease of manipulating mental images is a function of the efficiency of control processes.     

The quantitative measure of pattern representation in images using orientation-specific color aftereffects

A quantitative method is developed for assessing the quality of pattern information in imagery, using the magnitude of color aftereffects as an objective index. Subjects were given instructions to project imagined bar patterns of particular width and orientation onto adapting color fields, in such a manner as to simulate standard conditions for establishing the McCollough effect. Our control procedures indicate that the resulting orientation-specific complementary color aftereffects cannot be attributed to the conditioning of particular directions of eye scanning movements to color processing during adaptation, or to other possible sources of experimental bias. Furthermore, subjects who rated themselves prior to the adaptation procedure as having relatively vivid imagery showed significantly larger aftereffects than those who reported having relatively low imagery. These results not only provide an important confirmation of our earlier finding that imagination can replace physical pattern information in the formation of basic color-feature associations in the human visual system, but also demonstrate that these aftereffects can provide a practical measure of the fidelity of pattern representation in visual images.     

Squinting with the mind’s eye: Effects of stimulus resolution on imaginal and perceptual comparisons

Subjects either viewed or visualized arrays that were divided into four quadrants, with each quadrant containing a set of stripes. In two experiments, one array contained only relatively narrow (high-resolution) stripes, and one contained only relatively thick (low-resolution) stripes. The subjects compared sets of stripes in different quadrants according to their length, spacing, orientation, or width. When the subjects visualized the arrays, they required much more time to compare high-resolution patterns than low-resolution patterns; when the subjects saw the arrays, they evaluated both types of arrays equally easily. In addition, the results from the third experiment provide strong evidence that people use imagery in this task; in one condition, the subjects evaluated oblique sets of stripes, and in another condition, they evaluated vertical and horizontal stripes. In both imagery and perception, the subjects made more errors when evaluating oblique stimuli; in imagery, they also required more time to evaluate oblique stimuli. The results suggest that additional effort is required in imagery to represent visual patterns with high resolution. This finding demonstrates that, although imagery and perception may activate common brain regions, it is more difficult to represent high-resolution information in imagery than in perception.     

Visual mental imagery influences attentional guidance in a visual-search task

Visual mental imagery resembles visual working memory (VWM). Because both visual mental imagery and VWM involve the representation and manipulation of visual information, it was hypothesized that they would exert similar effects on visual attention. Several previous studies have demonstrated that working-memory representations guide attention toward a memory-matching task-irrelevant stimulus during visual-search tasks. Therefore, mental imagery may also guide attention toward imagery-matching stimuli. In the present study, five experiments were conducted to investigate the effects of visual mental imagery on visual attention during a visual-search task. Participants were instructed to visualize a color or an object clearly associated with a specific color, after which they were asked to detect a colored target in the visual-search task. Reaction times for target detection were shorter when the color of the target matched the imagined color, and when the color of the target was similar to that strongly associated with the imagined object, than when the color of the target did not match that of the mental representation. This effect was not observed when participants were not instructed to imagine a color. These results suggest that similar to VWM, visual mental imagery guides attention toward imagery-matching stimuli.     

ERP evidence of visualization at early stages of visual processing

Recent neuroimaging research suggests that early visual processing circuits are activated similarly during visualization and perception but have not demonstrated that the cortical activity is similar in character. We found functional equivalency in cortical activity by recording evoked potentials while color and luminance patterns were viewed and while they were visualized with the eyes closed. Cortical responses were found to be different when imagining a color pattern vs. imagining a checkerboard luminance pattern, but the same when imagining a color pattern (or checkerboard pattern) vs. seeing the same pattern. This suggests that early visual processing stages may play a dynamic role in internal image generation, and further implies that visual imagery may modulate perception.