Solving a mental rotation task in congenital hemiparesis: motor imagery versus visual imagery

A recent study showed that motor imagery was compromised after right congenital hemiparesis. In that study, posture of the displayed stimuli and the actual posture of the hand making the response were incongruent. Ample evidence exists that such an incongruency may negatively influence laterality judgements in a mental rotation task. In the present study, three participant groups (controls, left hemiparesis, right hemiparesis [all n = 11]) performed a mental rotation task in which posture of the displayed hand and the responding hand were congruent. A small amount of errors were made and linear relations between reaction times and rotation angles of the stimuli were found for all groups, suggesting intact motor imagery. However, reaction times for the participants with hemiparesis were consistently slower compared to controls and no asymmetry in responding between the affected and less-affected hand was found, suggesting a visual imagery strategy. Collectively, these results suggest that the ability to mentally rotate stimuli is still intact in right hemiparesis. The results are discussed in relation to two strategies that may have been used to solve the task: visual imagery and motor imagery.     

Selective deficit of mental visual imagery with intact primary visual cortex and visual perception

There is a vigorous debate as to whether visual perception and imagery share the same neuronal networks, whether the primary visual cortex is necessarily involved in visual imagery, and whether visual imagery functions are lateralized in the brain. Two patients with brain damage from closed head injury were submitted to tests of mental imagery in the visual, tactile, auditory, gustatory, olfactory and motor domains, as well as to an extensive testing of cognitive functions. A computerized mapping procedure was used to localize the site and to assess the extent of the lesions. One patient showed pure visual mental imagery deficits in the absence of imagery deficits in other sensory domains as well as in the motor domain, while the other patient showed both visual and tactile imagery deficits. Perceptual, language, and memory deficits were conspicuously absent. Computerized analysis of the lesions showed a massive involvement of the left temporal lobe in both patients and a bilateral parietal lesion in one patient. In both patients the calcarine cortex with the primary visual area was bilaterally intact. Our study indicates that: (i) visual imagery deficits can occur independently from deficits of visual perception; (ii) visual imagery deficits can occur when the primary visual cortex is intact and (iii) the left temporal lobe plays an important role in visual mental imagery.     

Vivid visual mental imagery in the absence of the primary visual cortex

The role of the primary visual cortex in visual mental imagery has provided significant debate in the imagery literature. Functional neuroimaging studies show considerable variation depending on task and technique. Patient studies can be difficult to interpret due to the diverse nature of cortical damage. The type of cortical damage in patient SBR is exceedingly rare as it is restricted to the gray matter of the calcarine sulcus. In this study, we show that in spite of his near-complete cortical blindness, SBR exhibits vivid visual mental imagery both behaviorally and when measured with functional magnetic resonance imaging. The pattern of cortical activation to visual mental imagery in SBR is indistinguishable from individual sighted subjects, in contrast to the visual perceptual responses, which are greatly attenuated.     

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.     

Aberrant visual circuitry associated with normal spatial match-to-sample accuracy in schizophrenia

A goal of this study was to evaluate the function of the anterior cingulate cortex (ACC) and of the dorsolateral prefrontal cortex (DLPFC) in medicated patients with schizophrenia (SZ), a small group of first-degree relatives, and healthy controls using a visual delayed match-to-sample task in conjunction with functional magnetic resonance imaging (fMRI). To mitigate performance differences between SZ and healthy controls, we used a novel task that allows for individualized adjustment of task difficulty to match ability level. We also trained participants on the task prior to scanning. Using an event-related design, we modeled three components of the match-to-sample trial: visual encoding, delay, and discrimination. We did not find significant differences in ACC/medial frontal cortex activation between the groups. However, compared to healthy controls, SZ showed decreased activation in visual processing areas during the encoding and discrimination phases of the task and in the ventrolateral prefrontal cortex during the delay. These findings emphasize the tendency of schizophrenia subjects to solve perceptual memory problems by engaging diverse regions.     

Extended visual simultaneity thresholds in patients with schizophrenia

Clinical observations suggest that the experience of time phenomenology is disturbed in schizophrenia, possibly originating disorders in dynamic cognitive functions such as language or motor planning. We examined the subjective evaluation of temporal structure using an experimental approach involving judgments of simultaneity of simple, visually presented stimuli. We included a priming procedure, ie, a subthreshold presentation of simultaneous or asynchronous stimuli. This allowed us to evaluate the effects of subthreshold synchrony and to check for bias effects, ie, changes in the criteria used by the subjects to rate the stimuli. Primes were adapted to the responses of the subjects. Bias effects were thus expected to yield a change in the efficiency of the prime and to induce a change in the amplitude of the priming effect. Nineteen outpatients with schizophrenia and their individually matched controls participated in the study. In all tests, patients required longer delays between stimuli to detect that they were asynchronous. In other words, they judged stimuli to be synchronous even when their onset was separated by delays of 100 milliseconds and even more in some cases. These results contrasted with preserved effects of subthreshold synchrony. Our findings argue against the hypothesis that the patients' responses were influenced by biases. We conclude that the subjective evaluation of simultaneity/asynchrony is impaired in schizophrenia, thus leading to impairment in the phenomenology of event-structure coding. The method used in the present study provides a novel approach to the assessment of those disturbances related to time in patients with schizophrenia.     

Brain areas underlying visual mental imagery and visual perception: an fMRI study

We used functional magnetic resonance imaging (fMRI) to assess the maximal degree of shared neural processing in visual mental imagery and visual perception. Participants either visualized or saw faint drawings of simple objects, and then judged specific aspects of the drawings (which could only be evaluated properly if they used the correct stimulus). The results document that visual imagery and visual perception draw on most of the same neural machinery. However, although the vast majority of activated voxels were activated during both conditions, the spatial overlap was neither complete nor uniform; the overlap was much more pronounced in frontal and parietal regions than in temporal and occipital regions. This finding may indicate that cognitive control processes function comparably in both imagery and perception, whereas at least some sensory processes may be engaged differently by visual imagery and perception.     

Brain networks underlying mental imagery of auditory and visual information

Mental imagery is a complex cognitive process that resembles the experience of perceiving an object when this object is not physically present to the senses. It has been shown that, depending on the sensory nature of the object, mental imagery also involves correspondent sensory neural mechanisms. However, it remains unclear which areas of the brain subserve supramodal imagery processes that are independent of the object modality, and which brain areas are involved in modality‐specific imagery processes. Here, we conducted a functional magnetic resonance imaging study to reveal supramodal and modality‐specific networks of mental imagery for auditory and visual information. A common supramodal brain network independent of imagery modality, two separate modality‐specific networks for imagery of auditory and visual information, and a common deactivation network were identified. The supramodal network included brain areas related to attention, memory retrieval, motor preparation and semantic processing, as well as areas considered to be part of the default‐mode network and multisensory integration areas. The modality‐specific networks comprised brain areas involved in processing of respective modality‐specific sensory information. Interestingly, we found that imagery of auditory information led to a relative deactivation within the modality‐specific areas for visual imagery, and vice versa. In addition, mental imagery of both auditory and visual information widely suppressed the activity of primary sensory and motor areas, for example deactivation network. These findings have important implications for understanding the mechanisms that are involved in generation of mental imagery.     

Construct validity of tests of verbal and visual mental imagery

To examine the construct validity of a verbal and visual mental imagery task, 59 male and female neurologically impaired patients were administered the imagery tasks as well as the Wechsler Memory Scale and The Wechsler Adult Intelligence Scale -Revised. Pearson- product correlation coefficients and a factor analysis were conducted between the two imagery tasks and the various subtests and IQ scores from the Wechsler scales. Results showed modality-specific correlations between the verbal and visual imagery tasks and verbal and visual cognitive abilities. The imagery tasks also showed secondary loadings on an attention/concentration factor. The results support the convergent and discriminative validity of these two imagery tasks as measures of verbal and visual functioning after brain injury.     

Four types of visual mental imagery processing in upright and tilted observers

We investigated the role of body position on performance in four distinct types of mental imagery processing. Previous studies used the upright body position as standard procedure and therefore do not address the issue of whether mental imagery tasks are processed in accordance with ego-centered or gravitational coordinates. In the present study, the subjects were brought into one of three different body positions: upright, horizontal, or supine. In each of these body positions, we measured performance in four imagery tasks, which assessed (1) the ability to generate vivid, high-resolution mental images; (2) the ability to compose mental images from separate parts; (3) the ability to inspect patterns in mental images; and, (4) the ability to mentally rotate patterns in images. Not all processes were affected in the same way when subjects performed them in different body positions. Performance in the image composition and detection tasks depended on body position, whereas there was no such effect for the transformation and resolution tasks.     

Effects of peripheral and central visual impairment on mental imagery capacity

This paper reviews a number of behavioral, neuropsychological and neuroimaging studies that bear on the question of whether and how visual disorders of peripheral or central origin lead to disorders of mental imagery capacity. The review of the literature suggests that in cases of blindness of peripheral origin lack of vision can progressively lead to representational disorders. However, in patients suffering from peripheral visual deficits, representational disorders can partially or completely be compensated by other sensory modalities as well as by cortical reorganization. Interestingly, in brain-damaged patients, neurovisual disorders following occipital or parietal lesions are not systematically associated with representational deficits, thus demonstrating that visual perception and visual imagery may not rely on the same cortical structures as previously hypothesized. Impairments seen on mental imagery tasks among brain-damaged patients with visual and/or spatial deficits might be due to an often co-existing attentional deficit. We discuss this possible dissociation between visual perception and visual mental imagery and its implications for theoretical models of mental representation.     

The neural correlates of visual mental imagery: an ongoing debate

The functional mechanisms and the neural correlates of visual mental imagery (the faculty whereby we can use our "mind's eye" to visualize objects in their absence) are at the centre of a lively debate in cognitive neuroscience. Neurocognitive models have proposed a functional equivalence between visual perception and visual mental imagery, which would be subserved by common neural substrates, such as the retinotopic areas in the occipital lobe. However, brain-damaged patients may demonstrate either impaired imagery and preserved perception, such as the classical Charcot and Bernard case and the patients described by Moro et al. (2008, this issue), or the opposite pattern of performance, consisting of preserved imagery and impaired perception. This double dissociation provides a strong challenge to models postulating a functional and anatomical equivalence of perception and imagery, and suggests that these functions have partly distinct neural correlates.     

Cortical activation evoked by visual mental imagery as measured by fMRI

One of the major controversies in cognitive neuroscience is whether the primary visual cortex and nearby areas are involved in visual mental imagery. In an fMRI study we examined the brain activity of 10 healthy subjects under different task conditions: in the perception condition subjects saw complex geometrical shapes and had to decide whether other highlighted stimuli fell inside or outside the figure. In the imagery condition subjects saw only the highlighted stimuli and were instructed to imagine the previously studied geometrical shapes to solve the same task. Although the behavioral data show a distance effect that would be expected based on topographically organized mental images, the functional imaging data do not show increased activity in the primary visual cortex in the imagery condition. In the occipital cortex a slightly increased activity was found only in the visual association cortex (BA 19), whereas the highest activation was observed in the parietal cortex (BA 7 and 40). The results of the study do not support the assumption that the primary visual cortex is involved in visual mental imagery, but rather that a network of spatial subsystems and higher visual areas appears to be involved.     

Deficits in language-mediated mental operations in patients with schizophrenia.

We found previously that a subgroup of schizophrenic patients who passed screening tests of attentional competence showed memory deficits on word memory tasks, but were comparable with controls on tone memory tasks. To better understand the nature of language-specific memory deficits in this subgroup of patients, the present experiment was designed to bypass early perceptual processing of verbal material and determine if patients continue to show impaired performance on verbal memory tasks.Patients who passed the screening tests ('discriminator' patients; DSz) received four serial position tasks. In two, familiar sounds or line drawings were presented and subjects were required to remember the word associated with each stimulus item. In the other two, subjects received hard-to-label auditory and visual stimuli (birdsongs or snowflakes).DSz patients showed large memory deficits compared with controls when required to remember words associated with the familiar sounds or drawings, providing clear evidence of deficits in verbal memory processes independent of sensory processing of verbal stimuli. The interaction between diagnosis and labeling was highly significant, confirming that these patients have particular difficulty with verbal as opposed to non-verbal memory. This was particularly striking on the auditory tests where two patients out-performed all controls on the birdsong test, but were below all controls on the easy-to-label sounds test. The verbal memory tests were easier than the non-verbal memory tests for controls, thus deconfounding task difficulty and deficit specificity.     

Prospective mental imagery in patients with major depressive disorder or anxiety disorders

Prospective negative cognitions are suggested to play an important role in maintaining anxiety disorders and major depressive disorder (MDD). However, little is known about positive prospective mental imagery. This study investigated differences in prospective mental imagery among 27 patients with anxiety disorders, 24 patients with MDD, and 32 control participants. Measures of both deliberately generated and intrusive imagery were completed. Results indicated that both patients with anxiety disorders and those with MDD provided poorer vividness ratings for deliberately generated prospective positive scenarios compared to the control group. Patients with anxiety disorders showed a greater ability to vividly generate imagery for prospective negative scenarios than both patients with MDD and control participants. Finally, both clinical groups reported greater levels of intrusive prospective imagery of personally-relevant events as compared to the control group. The current findings underline the necessity to target prospective positive mental imagery in treating MDD and anxiety disorders.     

Impairment in visual and spatial perception in schizophrenia and delusional disorder

The Judgment of Line Orientation Test, a visuospatial processing task, was administered to normal subjects, to schizophrenic patients and to patients with delusional disorder. Significantly better performance was seen in the normal subjects than in the schizophrenic and delusional patients. Delusional patients, in turn, showed better performance than the schizophrenic patients.     

Attentional guidance during visual search among patients with schizophrenia

The current study investigated visual guidance and saccadic selectivity during visual search among patients with schizophrenia (SCZ). Data from a previous study (Elahipanah, A., Christensen, B.K., & Reingold, E.M., 2008. Visual selective attention among persons with schizophrenia: The distractor ratio effect. Schizophr. Res. 105, 61-67.) suggested that visual guidance for the less frequent distractors in a conjunction search display (i.e., the distractor ratio effect) is intact among SCZ patients. The current study investigated the distractor ratio effect among SCZ patients when: 1) search is more demanding, and 2) search involves motion perception. In addition, eye tracking was employed to directly study saccadic selectivity for the different types of distractors. Twenty-eight SCZ patients receiving a single antipsychotic medication and 26 healthy control participants performed two conjunction search tasks: a within-dimension (i.e., colour × colour) search task; and a cross-dimension (i.e., motion × colour) search task. In each task the relative frequency of distractors was manipulated across 5 levels. Despite slower search times, patients' eye movement data indicated unimpaired visual guidance in both tasks. However, in the motion × colour conjunction search task, patients displayed disproportionate difficulty detecting the moving target when the majority of distractors were also moving. Results demonstrate that bottom-up attentional guidance is unimpaired among patients with SCZ; however, patients' impairment in motion discrimination impedes their ability to detect a moving target against noisy backgrounds.