Salvatore Maugeri Foundation, IRCCS, Center of Telese, Loc. S. Stefano in Lanterria, I-82037 Telese Terme (BN), , 1 Departments of Neurological Sciences, , 4 Electronic Engineering and , 6 Radiological Sciences, Federico II University, Nuovo Policlinico, Via S. Pansini 5, I-80131 Naples, Italy, , 2 Max-Planck-Institut für Hirnforschung, Deutschordenstrasse 46, D-60528 Frankfurt am Main and , 3 Departments of Neurology and , 5 Neuroradiology, Klinikum der Johann Wolfgang Goethe-Universität, Schleusenweg 216, D-60528 Frankfurt am Main, Germany
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Abstract |
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Introduction |
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One major issue in the search for the neural basis of mental imagery is whether perception and imagery share a common neural substrate. On the basis of studies suggesting that there are two visual systems, one for identifying objects and one for locating them (Ungerleider and Mishkin, 1982), it has been proposed that visual object and spatial imagery are functionally independent processes, which must rely on different underlying neural systems (Levine et al., 1985
). Patients with selective deficits in the performance of visual or spatial imagery tasks have been reported in the literature (Farah et al., 1988
; Luzzatti et al., 1998
). Several brain mapping studies (Mellet et al., 1998
) tried to identify the neural basis of spatial mental imagery in the absence of visual stimulation, but none of them verified actual execution of the imagery task on-line.
In the present study we used functional magnetic resonance imaging (fMRI) to explore the neural correlates of a behaviourally controlled spatial imagery task. The experimental task was derived from the mental clock test (Paivio,1978; Grossi et al., 1989
, 1993
), a paradigm particularly suitable for the fMRI investigation of mental imagery because it involves a behavioural control that can be performed on-line during scanning. Subjects are asked to imagine pairs of times that are presented acoustically and to judge at which of the two times the clock hands form the greater angle.
The spatial imagery task was used in two experiments. In the first experiment it was alternated with a control condition requiring only a verbalsemantic judgement, which did not rely on spatial or imagery processes.
In the second experiment, we compared the activity during the mental clock test (imagery) with that evoked by the same operation performed on visually presented material (perception) and by a different non-spatial control task (syllable counting), the attentional load of which was comparable to that of the imagery task.
From the lesion-based evidence on the cortical substrate of spatial mental imagery we expected to see an increase of neuronal activity, as reflected by the blood oxygen level-dependent (BOLD) signal, in the parietal lobes bilaterally during the mental clock test. In addition, our analysis included the entire cortex in order to assess the coactivation of other brain areas.
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Materials and Methods |
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We recruited seven right-handed post-graduate students (four male, three female; mean age 27 years; range 2332), who gave their informed consent to participate in the study. None of the subjects was taking any medication or was affected by neurologic or psychiatric conditions. All of them were unaware of the purposes and predictions of the experiment at the time of testing. All seven subjects participated in experiment 1; four of them also participated in experiment 2.
Magnetic Resonance (MR) Hardware and Sequences
The MR scanner used for imaging was a 1.5 T whole-body superconducting system (MAGNETOM Vision, Siemens Medical Systems, Erlangen, Germany) equipped with a standard head coil, an active shielded gradient coil (25 mT/m) and Echo Planar (EPI) sequences for ultra-fast MR imaging.
For functional imaging, we used a BOLD sensitive single shot EPI sequence [echo time (TE) = 66 ms; flip angle (FA) = 90°; matrix size = 128 x 128, voxel dimensions = 1.4 x 1.4 x 4 mm] with an interscan temporal spacing of 5 s. Each functional time series consisted of 5 resting volumes (25 s, discarded from further analysis) followed either by 100 (experiment 1) or 72 (experiment 2) acquisitions during which the imagery condition was periodically alternated with a control condition every ten (experiment 1) or eight scans (experiment 2).
After the functional acquisitions, a high-resolution three-dimensional data set covering the whole brain (referred to as 3-D anatomical image) was collected for each subject with a 3-D magnetization-prepared rapid acquisition gradient echo (MP-RAGE) sequence (TR = 9.7 ms, TE = 4 ms, FA = 12°, matrix = 256 x 256, thickness = 1 mm, number of partitions = 170180, Voxel dimensions = 1mm x 1mm x 1mm). For the four subjects participating in experiment 2, we furthermore performed a 3-D T1 weighted fast low-angle shot measurement at the same resolution for subsequent surface reconstruction and surface-based statistical analysis (see Data Analysis).
Experiment 1
Experimental Paradigm
Subjects were asked to imagine two analogue clock faces based on the times that were presented verbally by the examiner [e.g. 9.30 and 10.00; inter-stimulus interval (ISI) = 1 s] and to judge at which of the two times the clock hands form the greater angle (imagery). We selected 50 pairs of times, involving only half-hours (i.e. 7.30) or hours (i.e. 9.00); in half of the conditions, the correct answers corresponded to numerically greater times (i.e. 3.00 versus 1.00), and in the other half to numerically smaller times (i.e. 8.30 versus 11.00) in order to avoid subjects considering only the numerically greater pairs. The clock faces were balanced for the side on which the hands had to be imagined and presented in pseudo-random order. Subjects had to push a button with their right index finger if the hands of the first clock formed the greater angle, or their left index finger for the second. Subjects' responses were registered by an optic fibre answer box and analysed for accuracy.
As a control task, we asked subjects to judge which of two times was numerically greater. Materials, presentation and response modality were the same as in the imagery task, but the control condition required only a verbalsemantic judgement, which did not rely on the activation of imagery processes.
The two tasks were alternated in blocks of ten trials (total number of trials = 100); every trial block was preceded by the appropriate instruction. Trials were presented every 5 s (1 trial/volume).
In a pre-experimental session all subjects completed an angle comparison task on visually presented pairs of analogue clocks and several practice trials of both the imagery and the control task, to familiarize them with materials, tasks and response modality.
Following the common procedure for imagery paradigms, subjects were asked to keep their eyes closed during the scanning session. To exclude the possible confounding effect of eye movements, subjects were also instructed to keep their eye position steady. Control scans with open eyes and infrared eye-tracking were performed on two subjects (see Results).
Data Analysis
Data analysis, registration and visualization were performed with the fMRI software package BrainVoyager 2.0 (Goebel et al., 1998a,b
).
Prior to statistical analysis, the time series of functional images were aligned for each slice in order to minimize the signal changes related to small motions of the subject during the acquisition. The realigned time series were at first spatially filtered by convolving each EPI image with a bidimensional Gaussian smoothing kernel with full width at half maximum (FWHM) = 2 pixels, and then temporally filtered by convolving each pixel's time course with a smoothing Gaussian kernel with FWHM = 2 samples. Furthermore, the linear drifts of the signal with respect to time were removed from each pixel's time course.
After these pre-processing steps, the cerebral regions responding to the stimulus were identified by means of a cross-correlation analysis (Bandettini et al., 1993). The reference vector used in our analysis was a box-car ideal vector which assumed a value of 0 during the control period and 1 during the imagery period. In order to take into account the variations in the haemodynamic delay of the activation signal throughout the cortex, the cross-correlation coefficient was evaluated for each pixel using the vector obtained by shifting the original reference vector by one sample. The maximum of the two cross-correlation coefficients was then considered in the activation map. Activated brain areas were selected in the cross-correlation maps by imposing a conservative intensity threshold for the cross-correlation coefficients (r > 0.5, corresponding to P < 107; uncorrected).
Two-dimensional statistical maps were converted into polychromatic images and incorporated into the 3-D MP-RAGE data sets through interpolation to the same resolution (voxel size: 1.0 x 1.0 x 1.0 mm3). This made it possible to superimpose 3-D statistical maps onto the 3-D anatomical data sets. Since the 2-D functional and 3-D structural measurements were performed within the same recording session, co-registration of the respective data sets could be computed directly based on the Siemens slice position parameters of the T2*-weighted measurement (number of slices, slice thickness, distance factor, Tra-Cor angle, FOV, shift mean, off-centre read, off-centre phase, in plane resolution) and on parameters of the T1-weighted 3-D MP-RAGE measurement (number of sagittal partitions, shift mean, off-centre read, off-centre phase, resolution) with respect to the initial overview measurement (scout).
For each subject the structural 3-D data sets were transformed into Talairach space. The Talairach transformation was performed in two steps. The first step consisted in rotating the 3-D data set of each subject to be aligned with the stereotaxic axes. For this step the location of the anterior commissure (AC), the posterior commissure (PC) and two rotation parameters for midsagittal alignment had to be specified manually in the 3-D MP-RAGE data set. In the second step the extreme points of the cerebrum were specified. These points together with the AC and PC coordinates were then used to scale the 3-D data sets into the dimensions of the standard brain of the Talairach and Tournaux atlas (Talairach and Tournaux, 1988) using a piecewise affine and continuous transformation for each of the 12 defined subvolumes.
The individual Talairach 3-D maps were averaged across subjects and superimposed on a normalized anatomical 3-D data set. Prior to averaging, the functional 3-D maps were smoothed with a Gaussian kernel of 5 mm FWHM. Activated brain areas were selected in the average map by imposing lower overall correlation values (r > 0.25) to take into account the interindividual spatial variability of activated clusters.
The high-resolution T1-weighted 3-D recording of one subject was used for a surface reconstruction of both hemispheres. The white/grey matter border was segmented using a region-growing algorithm. The discrimination between white and grey matter was improved by several manual interactions (e.g. labelling subcortical structures as white matter). The white/grey matter border was finally tesselated in a single step using two triangles for each side of a voxel located at the margin of white matter. The tesselation of a single hemisphere typically consists of roughly 240 000 triangles. The reconstructed surface is subjected to iterative corrective smoothing (100200 iterations). An interactive morphing algorithm (Goebel et al., 1998a) was used to let the surface grow smoothly into the grey matter. Through visual inspection, this process was halted when the surface reached the middle of the grey matter (approximately layer 4 of the cortex). The resulting surface was used as the reference mesh for the visualization of functional data. The iterative morphing algorithm was further used to inflate each hemisphere. An inflated hemisphere possesses a link to the folded reference mesh so that functional data may be shown at the correct position of the inflated representation. This link was also used to keep geometric distortions during inflation to a minimum with a morphing force that keeps the area of each triangle of the inflated hemisphere as close as possible to the value of the folded reference mesh. This display of functional maps on an inflated hemisphere allows the topographic representation of the 3-D pattern of cortical activation without loss of the lobular structure of the telencephalon (Goebel et al., 1998a
; Linden et al., 1999
).
Experiment 2
Experimental Paradigm
In this experiment, we compared the activity during the imagery condition with that evoked by the same cognitive operation on visually presented material (perception) and by a different non-spatial control task (syllable counting).
The imagery condition was the same as in experiment 1, and employed 48 pairs of times different from those of the previous experiment but counterbalanced in the same way.
In the perception condition we used 48 pairs of analogic clock faces; the clock faces of each pair were generated on a computer screen and projected one at a time (ISI = 1 s) in the central visual field. The subjects had to decide at which of the two times the clock hands formed the greater angle.
During the syllable counting condition, we asked subjects to count the syllables of each of 48 auditorily presented pairs of times and to report whether the total syllable number was odd or even.
During the imagery and perception condition, materials and response modality were the same. In the latter case, however, the pairs of times were presented visually to the subject while in the imagery condition they had to be visualized mentally.
In the syllable counting condition, material, presentation and response modality were the same as in the imagery task, but here a verbalphonological judgement was required that was matched for response time with the imagery conditions.
Three sessions of 72 measurements were conducted for each subject. Within each session, the three conditions were alternated in blocks of eight trials (1 trial/measurement, 8 measurements/block) and separated by blocks of four resting measurements. The sequence of stimulation conditions consisted of: imagery, perception, syllable counting. This sequence was repeated twice per session. In all the conditions, the inter-trial interval was 5 s (1 trial/scan), and every trial block was preceded by the appropriate instruction.
In this experiment subjects had to press a button with their right index or middle finger; responses were registered by an optic fibre answer box and analysed for accuracy and response times. Mean correct reaction times for each subject were used for the speed of performance analysis. Because of the low number of subjects, statistical analysis was performed by nonparametric methods (MannWhitney test for two-group comparisons). Subjects were asked to keep their eyes open during the scanning session and foveate a fixation cross in order to avoid eye movements.
Data Analysis
As this experiment comprised three conditions, the statistical analysis was based on the application of the multiple regression analysis to time series of task-related functional activation (Friston et al., 1995). Furthermore, a method based on the reconstruction of a cortical sheet was used for adjusting the significance values for multiple comparisons (Goebel and Singer, 1999
). These extended analytical tools were implemented in BrainVoyager 3.0 (Goebel et al., 1998a
,b
; Dierks et al., 1999
).
Talairach transformation (see experiment 1) was performed for the complete set of functional data of each subject, yielding a 4-D data representation (volume time course: 3 x space, 1 x time). Prior to statistical analysis, the time series of functional images were aligned in order to minimize the effects of head movements. The central volume of the time series was used as a reference volume to which all other volumes were registered, using a 3-D motion correction that estimates the three translation and three rotation parameters of rigid body transformation. Spatial and temporal filtering of the image time series was the same as in experiment 1.
For each subject, the high-resolution T1-weighted 3-D recording was used for the surface reconstruction of the grey/white matter boundary of the brain. Since the surface reconstruction and the 4-D functional data set were co-registered, it was possible to tag those voxels of the functional data which were within 0 and 2 mm with respect to the reconstructed cortical sheet and to limit the statistical analysis to these voxels. The general linear model (GLM) of the experiment was then computed from the 12 (four subjects, three sessions per subject) tagged and z-normalized volume time courses. The signal values during the imagery, perception and syllable counting conditions were considered the effects of interest. The corresponding predictors, obtained by convolution of an ideal box-car response (assuming the value 1 for the time points of task presentation and the value 0 for the remaining time points) with a linear model of the haemodynamic response (Boynton et al., 1996), were used to build the design matrix of the experiment. The global level of the signal time courses in each session was considered to be a confounding effect. To analyse the effects of conditions compared with baseline and contrasts between conditions, 3-D individual and group statistical maps were generated by associating each tagged voxel with the F value corresponding to the specified set of predictors and calculated on the basis of the least mean squares solution of the GLM. Effects were only accepted as significant when, considering an F distribution with n1 and n2 degrees of freedom (n1 = number of orthogonal predictors and n2 = number of time samples n1 1), the associated P value yielded P' = P/N 103, where N represented the number of independent statistical tests. In single-subject analysis, N was chosen as the total number of voxels indexed from the surface mesh (28 118, 27 267, 28 528 and 26 771 respectively for the four subjects) and the uncorrected P threshold was adequately adjusted in order to keep the corrected P' threshold at 103. In multi-subject analysis, N was chosen as the number of voxels resulting from the logic OR of the individual surface meshes. This procedure provided an effective means of nonparametrically adjusting for multiple comparisons while preserving from an excessive loss of statistical power of the conventional Bonferroni approach (Goebel and Singer, 1999
).
Statistical results were then visualized through projecting 3-D statistical maps on folded and inflated surface reconstructions of the cortical sheet. For significantly activated voxels, the relative contribution RC between two selected sets of conditions in explaining the variance of a voxel time course were computed as RC = (bs1 bs2)/(bs1 + bs2) where bsi is the sum of the estimates of the standardized regression coefficients of all conditions included in set si. The RC index was visualized with a redgreen pseudo-colour scale. An RC value of 1 (red) indicates that a voxel time course is solely explained with predictor set s1 whereas an RC value of 1 (green) indicates that a voxel time course is explained solely with predictor set s2. An RC value of 0 indicates that a voxel time course is explained with an equal contribution of both predictor sets. In the contrast maps, only |RC| values >0.7 were visualized.
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Results |
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Behavioural Results
All subjects performed the imagery task without difficulty, and none of them made more than three errors (mean correct responses: 47.6 ± 1.3). The semantic task proved to be slightly easier than the imagery task (mean correct responses: 48.9 ± 1.1). The difference in accuracy between the two tasks was not significant (t = 2).
FMRI Results
The contrast between imagery and control conditions yielded an increase of the BOLD signal in several parietal and frontal regions.
The individual correlation maps (r > 0.5) showed a bilateral activation in the posterior parietal cortex (PPC) [primarily Brodmann area (BA) 7], in the fronto-basal region centred on the ascending ramus of the lateral sulcus and on parts of the inferior frontal gyrus (BA 45), and in the anterior insula in all subjects. The middle frontal gyrus including the dorsolateral prefrontal cortex (DLPFC, BAs 9, 46) was bilaterally activated in five of seven subjects, the superior frontal gyrus including the supplementary motor area in four volunteers, and the frontal eye fields (FEF) in 3 subjects. The inferior occipito-temporal region (BA 37) was also activated (4/7 left, 3/7 right).
The analysis of the averaged correlation map (n = 7, r > 0.25) confirmed the main results of the individual analysis. The largest clusters of activation were located bilaterally in the PPC, mainly along the intraparietal sulcus IPS (Fig. 1). Smaller foci of activity were also visible in the inferior frontal and perisylvian region of both sides (Table 1
). These areas of group activation corresponded to those that were found consistently in the individual correlation maps: each centre of mass of a cluster in the average map lay within two standard deviations of the distribution of the centres of mass of the respective clusters in the individual maps (Table 1
). The DLPFC, FEF and occipito-temporal cortex, which were activated only in some volunteers, did not appear in the averaged map at the selected rigorous threshold.
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The experiment was repeated with two subjects, who were requested to keep their eyes open while their eye movements were monitored by the fMRI-compatible Ober2 (Permobil Meditech, Timra, Sweden) infrared eye tracking system (Aisenberg, 1996), sampling the horizontal and vertical positions of both eyes at 120 Hz. This system has already been used for fixation control in a recent study of attentional effects on fMRI activity in human MT/MST (O'Craven et al., 1997
). We could thus confirm that subjects maintained fixation during the entire experiment.
The magnetic artefacts produced by the eye monitoring device affected the quality of the EPI signal, but only in areas anterior to the clusters of activation described above. The BOLD activation pattern during eye monitoring did not differ significantly from that obtained without the eye tracking system.
Experiment 2
Behavioural Results
The syllable counting task proved to be the most difficult condition, with higher reaction times (2940 ± 1039 ms) and error rates (39.5 ± 4.9 correct responses/48 trials) than either the imagery (2517 ± 860 ms; 45.5 ± 6.4 correct responses) or the perception conditions (1433 ± 618 ms; 47.1 ± 0.8 correct responses). However, only the comparison between the perception and syllable counting conditions yielded significant differences both in accuracy (MannWhitney test: z = 2.39; P = 0.02) and in reaction time (MannWhitney test: z = 2.30; P = 0.02), whereas the behavioural differences between the syllable counting and imagery conditions were not significant.
FMRI Results
All three tasks, compared with baseline, were accompanied by an increased activation of left sensorimotor cortex (contralateral to the hand of the button press response). The two tasks that involved auditory stimulation (imagery, syllable counting) were accompanied by activation of auditory cortex in the temporal lobes bilaterally (Table 2).
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The comparison between the perception and baseline conditions showed an activation of the posterior and inferior parietal cortex very similar to that observed during the imagery condition. Increased activity was also found in the basal and lateral surfaces of the occipital lobe. More consistently observed in the left hemisphere, this activity extended anteriorly in the basal surface of the temporal lobe. An activation was also seen in the superior mesial frontal region bilaterally (Table 2).
In the syllable versus baseline contrast increased activation was recognizable bilaterally in the inferior parietal lobule, in the inferior frontal and perisylvian region, in the superior mesial frontal region and in the right prefrontal cortex (Table 2). The contrast between the imagery and syllable counting conditions yielded, in all four subjects, a bilateral activation of the PPC, mainly along the IPS (Table 3
), closely corresponding to the largest cluster of experiment 1 (Figs 1, 2 and 3a
). During the syllable counting task, activity increased bilaterally in the inferior parietal lobules and right prefrontal cortex (Fig. 3a
).
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Discussion |
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The most striking results of our two experiments demonstrate that cortical activation (as measured by an increase of the fMRI BOLD signal) during the mental clock test was most prominent in the posterior parietal lobes of both hemispheres.
The absence of imagery-related activation in early visual areas in both experiments provides new data bearing on the debate regarding the cortical representations of mental imagery, especially because the present study employed an on-line behavioural control to ensure the actual generation of mental images. Our results therefore confirm the recent findings that certain imagery conditions, particularly those which rely on abstract patterns and schematic figures, produce increases in activity in primary visual areas to only a small extent (Goebel et al., 1998a) or not at all (Mellet et al., 1996
).
The activation of the prefrontal cortex, which was observed in a number of our subjects in the first experiment, has been associated with the attentional demand involved in different kinds of mental imagery (Mellet et al., 1996; Goebel et al., 1998a
) and with tasks of working memory (Ungerleider et al., 1998
). Accordingly, increased prefrontal activity was evident in the second experiment only when a task was compared with the baseline or when a more difficult task was compared with an easier one. In the second experiment, activations of roughly equal strength during the imagery and syllable counting conditions were observed in superior mesial frontal cortex. Activation of this area, unlike that of the PPC, was thus related to the attentional demand of the task and not to its specific nature. Likewise, the activation of perisylvian areas in experiment 1 should be considered not to be specific for mental imagery because a similar activation pattern was found for syllable counting versus imagery in experiment 2. The activation of the inferior parietal lobule during the syllable counting task can be explained by the requirement of phonetic segmentation and short-term memory (Wildgruber et al., 1999
).
Increased activity in the posterior parietal lobes, particularly along the IPS, was observed in experiment 1 and was particularly evident when we compared the imagery condition of experiment 2 (execution of the mental clock test) with the baseline. This activation maintained the contrast with a nonspatial control task that was performed on the same auditorily presented material and matched for difficulty (imagery versus syllable counting, Fig. 3a) and was consistently observed between individual subjects (Fig. 2
). This differential activation of the posterior parietal lobes by the spatial task confirms the specific role of the parietal cortex in spatial processing, rather than an unspecific activation related to attentional demand.
Activation of posterior parietal areas has been observed in conjunction with the spatial transformation of visually presented stimuli (Cohen et al., 1996; Alivisatos and Petrides, 1997
) and the orientation discrimination of tactile stimuli (Sathian et al., 1997
). In a recent PET study of mental rotation (Alivisatos and Petrides, 1997
), the right PPC was seen to participate in the processing of mirror images of letters or digits. Activation of the superior parietal lobule and of the IPS in both hemispheres has been described in a recent fMRI study of the analysis of spatially transformed words and phrases, which also distinguished this spatial transformation-related activation from general attentional effects (Goebel et al., 1998b
). The construction of 3-D mental images from auditory instructions, as studied by PET (Mellet et al., 1996
), involved a distributed system of frontal, occipital and parietal areas. The parietal activation, however, was most prominent in the right precuneus and supramarginal gyrus and did not involve the IPS region. This absence of IPS activation might reflect the specific nature of the task of Mellet et al. (Mellet et al., 1996
), which did not require the spatial comparison between objects and was not performed through mental rotation.
The majority of earlier studies of mental rotation and related tasks suffered from the possible confound of saccade-related activation in the parietal lobe (Milner and Goodale, 1995). The spatial transformation-related activation of the IPS region that was observed by Goebel et al. (Goebel et al., 1998b
) could be separated from the activity related to overt (saccades) and covert attention shift by additional control tasks and correlation analysis of the BOLD signal time course. In the present study, we excluded the contribution of saccadic activity to the task-related BOLD signal changes by controlling eye movements during the scanning session.
Goebel et al. (Goebel et al., 1998b) identified a spatial transformation area in the parietal lobe. Their spatial transformation task, like those of Alivisatos and Petrides (Alivisatos and Petrides, 1997
) and Cohen et al. (Cohen et al., 1996
), was performed on visually presented material. The present study shows that the area in the superior IPS is active when the spatial task is performed on mental images, in the absence of any visual stimulation. This similarity of activation patterns can be explained in two ways. First, the superior IPS might be instrumental in the computation of spatial transformations, regardless of whether the material is present in the visual field or merely as a mental image. Alternatively, any spatial transformation task, whether it involves visually perceived or imagined material, or indeed tactile stimulation (Sathian et al., 1997
), might require the implicit generation of mental visual representations (Kosslyn and Sussmann, 1995
).
In the second experiment the perception condition (spatial matching of visually presented clocks), compared with the fixation baseline, yielded only weak activation of primary visual cortex (probably because of the low contrast of the visual stimulus, and the TR of 5 s, which was relatively long compared with the short presentation of the stimulus), but very prominent bilateral activation of the inferior temporal and the inferior and lateral occipital lobes. These areas, which include the fusiform gyri, have been shown to be involved in the cortical processing of visual objects (Ungerleider and Mishkin, 1982; Malach et al., 1995
). In the posterior parietal lobes, a strong activation of the IPS was observed during this task. The direct contrast between the perception and imagery conditions (i.e. the clock test performed on visually presented and mentally imagined material, respectively) led to the disappearance of this activity. The comparison of the present results with the data on transformations of visually presented material from our (Goebel et al., 1998a
) and other groups (Cohen et al., 1996
; Alivisatos and Petrides, 1997
) suggested a close spatial overlap between the two sets of paradigms (Table 4
).The new evidence from visuospatial perceptual and mental imagery tasks performed by the same subjects in the same experimental session confirmed that IPS activation is common to both conditions. Our results reveal a striking similarity between brain activation during spatial comparisons of visually perceived and imagined material.
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Conclusion |
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Notes |
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Address correspondence to Dr Luigi Trojano, Salvatore Maugeri Foundation, IRCCS, Center of Telese, Loc. S. Stefano in Lanterria, I82037 Telese Terme (BN), Italy. Email: lutroj{at}tin.it.
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References |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Alivisatos B, Petrides M (1997) Functional activation of the human brain during mental rotation. Neuropsychologia 35:111118.[ISI][Medline]
Bandettini PA, Jesmanowicz A, Wong EC, Hyde JS (1993) Processing strategies for time-course data sets in functional MRI of the human brain. Magn Reson Med 30:161173.[ISI][Medline]
Boynton GM, Engel SA, Glover GH, Heeger DJ (1996) Linear systems analysis of functional magnetic resonance imaging in human V1. J Neurosci 16:42074241.
Cohen MS, Kosslyn SM, Breiter HC, DiGirolamo GJ, Thompson WL, Anderson AK, Brookheimer SY, Rosen BR, Belliveau JW (1996) Changes in cortical activity during mental rotation. A mapping study using functional MRI. Brain 119:89100.[Abstract]
Dierks T, Linden DE, Jandl M, Formisano E, Goebel R, Lanfermann H, Singer W (1999) Activation of Heschl's gyrus during auditory hallucinations. Neuron 22:615621.[ISI][Medline]
Farah MJ (1984) The neurological basis of mental imagery: a componential analysis. Cognition 18:24572.[ISI][Medline]
Farah MJ, Hammond KM, Levine DN, Calvanio R (1988) Visual and spatial imagery: dissociable systems of representation. Cogn Psychol 20: 439462.[ISI][Medline]
Friston KJ, Holmes AP, Worsley KJ, Poline JB, Frith CD, Frackowiak RSJ (1995) Statistical parametric maps in functional imaging: a general linear approach. Hum Brain Map 2:189210.
Goebel R, Khorram-Sefat D, Muckli L, Hacker H, Singer W (1998a) The constructive nature of vision: direct evidence from functional magnetic resonance imaging studies of apparent motion and motion imagery. Eur J Neurosci 10:15631573.[ISI][Medline]
Goebel R, Linden DEJ, Lanfermann H, Zanella FE, Singer W (1998b) Functional imaging of mirror and inverse reading reveals separate coactivated networks for oculomotion and spatial transformations. Neuroreport 9:713719.[ISI][Medline]
Goebel R, Singer W (1999) Cortical surface-based statistical analysis of functional magnetic resonance imaging data. Neuroimage 9:S64.
Grossi D, Modafferi A, Pelosi L, Trojano L (1989) On the different roles of the two cerebral hemispheres in mental imagery: the O'Clock test' in two clinical cases. Brain Cogn 10:1827.[ISI][Medline]
Grossi D, Angelini R, Pecchinenda A, Pizzamiglio L (1993) Left imaginal neglect in heminattention: experimental study with the oclock test. Behav Neurol 6:155158.[ISI]
Kosslyn SM, Sussmann AL (1995) Roles of imagery in perception: or, there is no such thing as immaculate perception. In: The cognitive neurosciences (Gazzaniga M, ed.), pp. 10351042. Cambridge, MA: MIT Press.
Levine DN, Warach J, Farah M (1985) Two visual systems in mental imagery: dissociation of what and where in imagery disorders due to bilateral posterior cerebral lesions. Neurology 35:10101018.[Abstract]
Linden DEJ, Prvulovic D, Formisano E, Völlinger M, Zanella FE, Goebel R, Dierks T (1999) The functional neuroanatomy of target detection: an fMRI study of visual and auditory oddball tasks. Cereb Cortex 9: 515523.
Luzzatti C, Vecchi T, Agazzi D, Cesa-Bianchi M, Vergani C (1998) A neurological dissociation between preserved visual and impaired spatial processing in mental imagery. Cortex 34:461469.[ISI][Medline]
Malach R, Reppas JB, Benson RR, Kwong KK, Jiang H, Kennedy WA, Ledden PJ, Brady TJ, Rosen BR, Tootell RB (1995) Object-related activity revealed by functional magnetic resonance imaging in human occipital cortex. Proc Natl Acad Sci USA 92:81358139.[Abstract]
Mellet E, Tzourio N, Crivello F, Joliot M, Denis M, Mazoyer B (1996) Functional anatomy of spatial mental imagery generated from verbal instructions. J Neurosci 16:65046512.
Mellet E, Petit L, Mazoyer B, Denis M, Tzourio N (1998) Reopening the mental imagery debate: lessons from functional anatomy. Neuroimage 8:129139.[ISI][Medline]
Milner AD, Goodale MA (1995) The visual brain in action. Oxford: Oxford University Press.
O'Craven KM, Rosen BR, Kwong KK, Treisman A, Savoy RL (1997) Voluntary attention modulates fMRI activity in human MT-MST. Neuron 18:591598.[ISI][Medline]
Paivio A (1978) Comparisons of mental clocks. J Exp Psychol (Hum Percept) 4:6171.[Medline]
Sathian K, Zangaladze A, Hoffmann JM, Grafton ST (1997) Feeling with the mind's eye. Neuroreport 8:38773881.[ISI][Medline]
Talairach J, Tournoux P (1988) Co-planar stereotaxic atlas of the human brain: 3-dimensional proportional system: an approach to cerebral imaging. Stuttgart: Thieme.
Trojano L, Grossi D (1994) A critical review of mental imagery defects. Brain Cogn 24:213243.[ISI][Medline]
Ungerleider LG, Mishkin M (1982) Two cortical visual systems. In: Analysis of visual behavior (Ingle DJ, Goodale MA, Mansfield RJW, eds), pp. 549586. Cambridge, MA: MIT Press.
Ungerleider LG, Courtney SM, Haxby JV (1998) A neural system for human visual working memory. Proc Natl Acad Sci USA 95:883890.
Wildgruber D, Kischka U, Ackermann H, Klose U, Grodd W (1999) Dynamic pattern of brain activation during sequencing of word strings evaluated by fMRI. Cogn Brain Res 7:28594.[ISI][Medline]