Aspects of a cognitive neuroscience of mental imagery.

Although objects in visual mental images may seem to appear all of a piece, when the time to form images is measured this introspection is revealed to be incorrect; objects in images are constructed a part at a time. Studies with split-brain patients and normal subjects reveal that two classes of processes are used to form images--ones that activate stored memories of the appearances of parts and ones that arrange parts into the proper configuration. Some of the processes used to arrange parts are more effective in the left cerebral hemisphere and some are more effective in the right cerebral hemisphere; the notion that mental images are the product of right hemisphere activity is an oversimplification.

The role of area 17 in visual imagery: convergent evidence from PET and rTMS.

Visual imagery is used in a wide range of mental activities, ranging from memory to reasoning, and also plays a role in perception proper. The contribution of early visual cortex, specifically Area 17, to visual mental imagery was examined by the use of two convergent techniques. In one, subjects closed their eyes during positron emission tomography (PET) while they visualized and compared properties (for example, relative length) of sets of stripes. The results showed that when people perform this task, Area 17 is activated. In the other, repetitive transcranial magnetic stimulation (rTMS) was applied to medial occipital cortex before presentation of the same task. Performance was impaired after rTMS compared with a sham control condition; similar results were obtained when the subjects performed the task by actually looking at the stimuli. In sum, the PET results showed that when patterns of stripes are visualized, Area 17 is activated, and the rTMS results showed that such activation underlies information processing.

Cerebral lateralization.

Recent findings suggest that cerebral laterality will not be understood in terms of simple dichotomies, such as the idea that the left hemisphere is analytic and the right holistic. Rather, hemisphere differences can be better explained in terms of relatively specific principles that extend over limited domains. For visual perception and mental imagery, the left hemisphere appears to be relatively better than the right at encoding component parts, representing visual categories, and encoding categorical spatial relations; in contrast, the right hemisphere appears to be relatively better at encoding overall patterns, representing specific instances, and encoding coordinate metric spatial relations. Recent neural network models support a hypothesis that ties together these disparate findings.

The status of cognitive neuroscience.

Cognitive neuroscience rests on findings, methods, and theory from three fields: experimental psychology, systems-level neuroscience, and computer science. The strong trend over the past few years has been for a greater integration across these fields. The influence of this interdisciplinary approach on current research on memory, perception, and language will be illustrated.

Visual imagery and perception in posttraumatic stress disorder. A positron emission tomographic investigation.

BACKGROUND: Relative regional cerebral blood flow (rCBF) changes were measured in Vietnam combat veterans with and without posttraumatic stress disorder (PTSD) during exposure to combat-related stimuli. METHODS: Positron emission tomography was used to measure rCBF in 7 combat veterans with PTSD (PTSD group) and 7 healthy combat veterans (control group) who viewed and generated visual mental images of neutral, negative, and combat-related pictures. RESULTS: Unlike control subjects, subjects with PTSD had increased rCBF in ventral anterior cingulate gyrus and right amygdala when generating mental images of combat-related pictures; when viewing combat pictures, subjects with PTSD showed decreased rCBF in Broca's area. CONCLUSIONS: Results suggest that ventral anterior cingulate gyrus and right amygdala play a role in the response of combat veterans with PTSD to mental images of combat-related scenes. Reexperiencing phenomena of PTSD, which often involve emotional visual mental imagery, may be likewise associated with increased rCBF in these regions.

Hypnotic visual illusion alters color processing in the brain.

OBJECTIVE: This study was designed to determine whether hypnosis can modulate color perception. Such evidence would provide insight into the nature of hypnosis and its underlying mechanisms. METHOD: Eight highly hypnotizable subjects were asked to see a color pattern in color, a similar gray-scale pattern in color, the color pattern as gray scale, and the gray-scale pattern as gray scale during positron emission tomography scanning by means of [(15)O]CO(2). The classic color area in the fusiform or lingual region of the brain was first identified by analyzing the results when subjects were asked to perceive color as color versus when they were asked to perceive gray scale as gray scale. RESULTS: When subjects were hypnotized, color areas of the left and right hemispheres were activated when they were asked to perceive color, whether they were actually shown the color or the gray-scale stimulus. These brain regions had decreased activation when subjects were told to see gray scale, whether they were actually shown the color or gray-scale stimuli. These results were obtained only during hypnosis in the left hemisphere, whereas blood flow changes reflected instructions to perceive color versus gray scale in the right hemisphere, whether or not subjects had been hypnotized. CONCLUSIONS: Among highly hypnotizable subjects, observed changes in subjective experience achieved during hypnosis were reflected by changes in brain function similar to those that occur in perception. These findings support the claim that hypnosis is a psychological state with distinct neural correlates and is not just the result of adopting a role.

Regional cerebral blood flow during script-driven imagery in childhood sexual abuse-related PTSD: A PET investigation.

OBJECTIVE: The purpose of this study was to determine whether anterior limbic and paralimbic regions of the brain are differentially activated during the recollection and imagery of traumatic events in trauma-exposed individuals with and without posttraumatic stress disorder (PTSD). METHOD: Positron emission tomography (PET) was used to measure normalized regional cerebral blood flow (CBF) in 16 women with histories of childhood sexual abuse: eight with current PTSD and eight without current PTSD. In separate script-driven imagery conditions, participants recalled and imagined traumatic and neutral autobiographical events. Psychophysiologic responses and subjective ratings of emotional state were measured for each condition. RESULTS: In the traumatic condition versus the neutral control conditions, both groups exhibited regional CBF increases in orbitofrontal cortex and anterior temporal poles; however, these increases were greater in the PTSD group than in the comparison group. The comparison group exhibited regional CBF increases in insular cortex and anterior cingulate gyrus; increases in anterior cingulate gyrus were greater in the comparison group than in the PTSD group. Regional CBF decreases in bilateral anterior frontal regions were greater in the PTSD group than in the comparison group, and only the PTSD group exhibited regional CBF decreases in left inferior frontal gyrus. CONCLUSIONS: The recollection and imagery of traumatic events versus neutral events was accompanied by regional CBF increases in anterior paralimbic regions of the brain in trauma-exposed individuals with and without PTSD. However, the PTSD group had greater increases in orbitofrontal cortex and anterior temporal pole, whereas the comparison group had greater increases in anterior cingulate gyrus.

Discrimination within and between hemifields: a new constraint on theories of attention.

Subjects were presented with two groups of characters and were to decide whether they were the same or different. The stimulus groups differed either by a single feature ("preattentive" trials) or by a conjunction of features ("attentive" trials). The two stimulus groups appeared at the corners of an imaginary square centered about the fixation point, falling either in the same or different hemifields. In two experiments, subjects evaluated both types of stimuli faster when they were presented in different hemifields than in the same hemifield. Subjects also compared pairs of single characters faster when they appeared in different hemifields. Finally, this different-hemifield advantage was eliminated when the characters appeared sequentially. These results indicated that two stimuli that appear simultaneously in the same hemifield engender either a competition for common processing structures or intrahemispheric inhibition.

Mental imagery and sensory experience in congenital blindness.

Imagery of congenitally blind and normally sighted subjects was compared in two experiments. In the first, subjects were asked to estimate how far away objects appeared in images. The results showed that blind subjects imaged objects "within arms' reach", and with only a slight tendency to image larger objects farther from them. In contrast, sighted subjects tended to image larger objects as if they were farther away. In addition, unlike the sighted, blind subjects' images also failed to overflow an "image space" of fixed size. Finally, blind subjects were, with one exception, unable to mimic successfully the responses of a sighted person, when explicitly asked to do so. In the second experiment, subjects pointed to the left and right sides of three objects imaged at three distances. Sighted and blind groups both showed a decrease in pointing span with the size of the object, but only the sighted subjects showed a decrease with increased image distance, in accordance with the laws of perspective.

Gestalt laws of perceptual organization in an embedded figures task: evidence for hemispheric specialization.

An embedded figures task was used to investigate hemispheric specialization for visual parsing. Error data from 40 normal right-handed males revealed a right-hemisphere advantage for parsing governed by the Gestalt laws of organization. In contrast, for parsing that violated those laws, subjects who could perform such parsing at better-than-chance levels showed a left-hemisphere advantage. A simple left hemisphere/analytic, right hemisphere/holistic dichotomy is too ambiguous to account for the results, because it fails to specify the level of visual structure (e.g. whole figures, intermediate components, or elementary segments) that is to be taken as a reference point for defining particular tasks as analytic or holistic; what is analytic from one point of view is holistic from another. We focus instead on principles of perceptual organization that characterize the results of perceptual processing, regardless of whether those results are seen as the outcome of analytic or holistic processing.

The role of the corpus callosum in the representation of lateral orientation.

How do people recognize objects when they face in a novel lateral (left/right) orientation? The results of three experiments with a split-brain patient, who has a totally nonfunctional corpus callosum, demonstrate that the corpus callosum cannot play a critical role in allowing one to recognize mirror-reversed objects. First, both cerebral hemispheres could recognize mirror-reversed versions of pictures as accurately as the original renditions in an incidental memory task. Second, when asked to decide whether pictures faced the same way that they had originally, neither hemisphere performed better than chance in an incidental memory task--suggesting that the shape representations in the hemispheres do not specify lateral orientation. Third, neither hemisphere exhibited "priming" for lateral orientation, as assessed in an "object decision task", and only the left hemisphere exhibited priming for encoding the shape (independent of its lateral orientation).

Two types of image generation: evidence for left and right hemisphere processes.

The results from seven experiments provide evidence that visual mental images can be generated by either the left or right cerebral hemisphere, but in different ways. Subjects were cued to form images within a grid or within a set of four corner brackets; a single X mark was enclosed within each stimulus, and the subjects were to determine whether the X mark was enclosed within each stimulus, and the subjects were to determine whether the X mark would have fallen on an imaged pattern. When subjects memorized descriptions of how parts were arranged, they could later form images of the composite pattern when cued in the right visual field (left hemisphere) more accurately than when they were cued in the left visual field (right hemisphere). In contrast, when subjects memorized individual segments on a screen, and 'mentally glued' them into a single pattern, they later could form images more accurately, at least in some circumstances, when cued in the left visual field. These results were predicted by the theory that images are built up by arranging parts, and that two different processes can be used to arrange them. One process uses stored descriptions to arrange parts, and is more effective in the left cerebral hemisphere; the other process uses stored memories of metric positions to arrange parts, and is more effective in the right cerebral hemisphere. Convergent evidence was obtained by having subjects memorize letters in grids (which are easily encoded using descriptions of the positions of segments) or within a space delineated by four brackets (which require memorizing the precise positions of the segments). Subjects were relatively more accurate when cued in the left visual field with bracket stimuli, but tended to be relatively more accurate when cued in the right visual field with grids stimuli. Control experiments showed that this finding was not due to hemispheric differences in the ease of forming images at different sizes or differences in the ease of perceptually encoding the probes.

Encoding words and pictures: a positron emission tomography study.

Subjects viewed words, pictures, crosshairs, or a large X flanked by two smaller xs on either side while their brain activity was monitored using positron emission tomography (PET). When activation from the pictures, crosshairs, or Xs condition was subtracted from activation in the words condition, the left angular gyrus and Broca's area were found to be activated. In the comparison of words and pictures, additional language areas were activated. These results provide support for the classical neurological model of reading. The results also suggest that a "word form area" is near the margin of the left angular gyrus.

The judgement of absence in neglect.

The usual way of looking at neglect is by investigating how neglect patients fail to detect that something is there. In this study, we look at how neglect patients correctly detect that something is not there. Patients with parietal lesions (11 with and 16 without neglect) and 23 control subjects indicated whether a dot target was or was not present in a geometrical display. While control subjects were consistently (and unexpectedly) faster in the no-dot than in the dot condition, the distinguishing response time pattern of right parietal patients with neglect was not--as one might expect--a relatively longer response time to left vs right targets, but a longer response time to target absence vs presence. This may be due to a serial search or, alternatively, it might result from double-checking for target absence, produced by lowered perceptual confidence. Since this "wariness" about stimulus absence seems to operate in parallel with neglect patients' denial of the deficit, we conclude that the response time pattern observed in this study could be used as a measure of subjective (un)awareness of neglect.

When does "no" really mean "yes"? A case study in unilateral visual neglect.

A patient with unilateral visual neglect indicated whether a dot was or was not present in a display. When present, the dot appeared equally often in the left and right visual fields. Although he typically denied having seen dots in his left visual field, he was able to make this judgment much more quickly than when no dot was in fact present. The mean response times when the dot was present (1135 and 1004 msec, for left and right) were almost twice as fast as the response times when no dot was present (2025 msec). This result suggests that the patient searched the visual fields individually, and in fact generated a "No" response based on detecting the dot in his neglected field. Thus, the mechanisms used to detect stimuli apparently are not rigidly linked to those used to classify them or to produce a response.

A left hemisphere basis for visual mental imagery?

The lateralization of visual mental imagery was investigated by presenting each hemisphere of a commissurotomy patient with a letter classification task known to require imagery and with a pair of control tasks designed to require all of the same processes as the imagery task except for the imagery processing itself. Whereas both hemispheres performed well on the control tasks, only the left hemisphere performed the imagery task.

Deficits in visual cognition and attention following bilateral anterior cingulotomy.

A series of eight tests of visual cognitive abilities was used to examine pre- to post-operative performance changes in a patient receiving bilateral anterior cingulotomy. Compared with a set of eight matched control participants, post-operatively, the patient exhibited deficits in (a) the ability to sequence novel cognitive operations required to generate multipart images or rotate perceptual stimuli; (b) the ability to search for, select, and compare images of objects when the instructions did not specify precisely which objects should be visualized; and, (c) the ability to select a controlled and unpracticed response over an automatic one. Other imagery and cognitive tasks were not affected. Results are consistent with the hypothesis that anterior cingulate cortex is a component of an executive control system. One of the anterior cingulate's roles may be to monitor on-line processing and signal the motivational significance of current actions or cognitions.

Visual imagery and visual-spatial language: enhanced imagery abilities in deaf and hearing ASL signers.

The ability to generate visual mental images, to maintain them, and to rotate them was studied in deaf signers of American Sign Language (ASL), hearing signers who have deaf parents, and hearing non-signers. These abilities are hypothesized to be integral to the production and comprehension of ASL. Results indicate that both deaf and hearing ASL signers have an enhanced ability to generate relatively complex images and to detect mirror image reversals. In contrast, there were no group differences in ability to maintain information in images for brief periods or to imagine objects rotating. Signers' enhanced visual imagery abilities may be tied to specific linguistic requirements of ASL (referent visualization, topological classifiers, perspective shift, and reversals during sign perception).

Detecting high-level and low-level properties in visual images and visual percepts.

In this article we provide further evidence that visual mental imagery and visual perception share modality-specific mechanisms, and we find that representing visual information in a mental image (activating stored information to create a picture-like mental representation) preserves relatively low-level visual detail. Subjects either saw or visualized simple pictures, and evaluated them for the presence or absence of six types of non-accidental properties. These properties varied from very 'low-level' ones, such as T junctions, to very 'high-level' ones, such as global symmetry. The question was whether both sorts of information are equally accessible in percepts and mental images. If mental images are equivalent to descriptions of perceptual units and their organization, as some have argued, then subjects should have greater difficulty accessing low-level properties in a mental image compared to the difficulty they experience when the drawing is visible. The results of two experiments were clearcut: Subjects could evaluate high-level properties more easily than low-level ones, but this difference was the same in imagery and perception. These findings suggest that mental images preserve relatively low-level visual features, and are not simply descriptions of a pattern.

Motor processes in mental rotation.

Much indirect evidence supports the hypothesis that transformations of mental images are at least in part guided by motor processes, even in the case of images of abstract objects rather than of body parts. For example, rotation may be guided by processes that also prime one to see results of a specific motor action. We directly test the hypothesis by means of a dual-task paradigm in which subjects perform the Cooper-Shepard mental rotation task while executing an unseen motor rotation in a given direction and at a previously-learned speed. Four results support the inference that mental rotation relies on motor processes. First, motor rotation that is compatible with mental rotation results in faster times and fewer errors in the imagery task than when the two rotations are incompatible. Second, the angle through which subjects rotate their mental images, and the angle through which they rotate a joystick handle are correlated, but only if the directions of the two rotations are compatible. Third, motor rotation modifies the classical inverted V-shaped mental rotation response time function, favoring the direction of the motor rotation; indeed, in some cases motor rotation even shifts the location of the minimum of this curve in the direction of the motor rotation. Fourth, the preceding effect is sensitive not only to the direction of the motor rotation, but also to the motor speed. A change in the speed of motor rotation can correspondingly slow down or speed up the mental rotation.