Human Brain Regions Involved in Direction Discrimination

L. Cornette1, P. Dupont1, 2, A. Rosier1, S. Sunaert3, P. Van Hecke3, J. Michiels4, L. Mortelmans2, and G. A. Orban1

1  Laboratorium voor Neuro- en Psychofysiologie, KU Leuven, Medical School; 2  Departement Nucleaire Geneeskunde, Centrum voor Positron Emissie Tomografie; 3  Departement Radiologie, Afdeling Röntgendiagnose; and 4  ESAT Radiologie, Laboratorium voor Medische Beeldverwerking, KU Leuven, B-3000 Leuven, Belgium

    ABSTRACT
Abstract
Introduction
Methods
Results
Discussion
References

Cornette, L., P. Dupont, A. Rosier, S. Sunaert, P. Van Hecke, J. Michiels, L. Mortelmans, and G. A. Orban. Human brain regions involved in direction discrimination. J. Neurophysiol. 79: 2749-2765, 1998. To obtain further evidence for the functional specialization and task-dependent processing in the human visual system, we used positron emission tomography to compare regional cerebral blood flow in two direction discrimination tasks and four control tasks. The stimulus configuration, which was identical in all tasks, included the motion of a random dot pattern, dimming of a fixation point, and a tone burst. The discrimination tasks comprised the identification of motion direction and successive direction discrimination. The control tasks were motion detection, dimming detection, tone detection, and passive viewing. There was little difference in the activation patterns evoked by the three detection tasks except for decreased activity in the parietal cortex during the detection of a tone. Thus attention to a nonvisual stimulus modulated different visual cortical regions nonuniformly. Comparison of successive discrimination with motion detection yielded significant activation in the right fusiform gyrus, right lingual gyrus, right frontal operculum, left inferior frontal gyrus, and right thalamus. The fusiform and opercular activation sites persisted even after subtracting direction identification from successive discrimination, indicating their involvement in temporal comparison. Functional magnetic resonance imaging (fMRI) experiments confirmed the weak nature of the activation of human MT/V5 by successive direction discrimination but also indicated the involvement of an inferior satellite of human MT/V5. The fMRI experiments moreover confirmed the involvement of human V3A, lingual, and parietal regions in successive discrimination. Our results provide further evidence for the functional specialization of the human visual system because the cortical regions involved in direction discrimination partially differ from those involved in orientation discrimination. They also support the principle of task-dependent visual processing and indicate that the right fusiform gyrus participates in temporal comparison, irrespective of the stimulus attribute.

    INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References

Simple discrimination tasks are powerful tools for investigating the relationship between neuronal activity and perceptual behavior. Recently we have applied this strategy to chart human brain activity related to orientation discrimination, both with central visual stimuli (Dupont et al. 1993; Orban et al. 1997a) and with stimuli located in the peripheral visual field (Vandenberghe et al. 1996, 1997). In the study involving central grating stimuli (Orban et al. 1997a), we compared regional cerebral blood flow (rCBF) measured while subjects passively viewed gratings, detected their presence, identified their orientation, or made same-different judgments about the orientation of two successive gratings. Although there was relatively little difference among the first three conditions, which activated striate and near extrastriate cortex extending into the posterior fusiform gyrus, the successive discrimination task evoked significantly more activity in the middle fusiform gyrus than did the identification task. This latter difference indicates that information processing in the human visual system depends not only on the stimulus attribute (Corbetta et al. 1991; Zeki et al. 1991) but also on the nature of the task (Orban et al. 1996).

The present experiments used tasks that were similar in nature but employed a moving random dot pattern rather than a static grating (i.e., a different stimulus). They also were applied to direction of motion rather than orientation (i.e., a different attribute). By extending the original experiment to the attribute "direction of motion," we hoped to find additional evidence for two important principles of sensory processing: functional specialization, and task dependency. Functional specialization predicts that different cortical regions will be involved in the processing of orientation and motion direction. Monkey lesion studies strongly suggest that the regions processing direction of motion should belong to the dorsal, occipito-parietal stream (Lauwers et al. 1995; Newsome and Paré 1988; Pasternak and Merigan 1994). In humans, several regions have been shown to be activated by retinal motion when subjects passively viewed stimuli (Cheng et al. 1995; Dupont et al. 1994, 1997b; Sunaert et al. 1996; Tootell et al. 1995, 1997; Watson et al. 1993; Zeki et al. 1991). Because the analysis of retinal motion can serve many behavioral purposes, including perception of motion (for review see Nakayama 1985), one wonders which ofthese human motion areas might be activated in direction discrimination. In this regard, a recent lesion study predicts that human MT/V5 (hMT/V5) may be involved (Barton et al. 1995). Task dependency, on the other hand, predicts that the successive discrimination task will involve additional areas compared with (direction) identification. Single-unit recordings in monkeys (Ferrera et al. 1994) suggest that these additional areas should be localized in the occipito-temporal stream. To test these two principles, we used the same four basic tasks as used by Orban et al. (1997a): successive discrimination, identification, detection, and passive viewing.

It has been proposed that selective attention to one sensory modality is associated with decreased activity in cortical areas dedicated to processing input from other sensory modalities (Haxby et al. 1994; Shulman et al. 1997). Therefore, in addition to the four tasks already described, we added two more control tasks, in which subjects selectively attended stimuli other than the moving random dot pattern. First, by using the detection of a tone as a control task, it might prove possible to increase the differential activation in the experimental visual task. Second, we introduced the detection of a dimming fixation point, a task frequently used in monkey experiments (Wurtz 1969). This task recently was used in a study of human motion perception by Beauchamp et al. (1997) that demonstrated that attention to a different visual stimulus serves as a powerful modulator of activation in visual areas. Targeting attention in the control condition to a visual stimulus different from that used in the experimental visual task also might enhance the differential activation.

For ethical reasons, the number of conditions that could be tested in this positron emission tomography (PET) experiment was limited to six. Thus the activation sites observed in the direction discrimination tasks could not be directly compared with the human motion regions, which classically are defined by the subtraction passive viewing of a moving random dot pattern minus passive viewing of the same pattern but stationary (Dupont et al. 1994; Sunaert et al. 1996; Watson et al. 1993; Zeki et al. 1991). Hence we performed an additional functional magnetic resonance imaging (fMRI) experiment in which we both activated the motion regions and compared the successive direction discrimination, direction identification, and dimming detection with one another.

    METHODS
Abstract
Introduction
Methods
Results
Discussion
References

PET experiment

SUBJECTS. Twelve male subjects [mean age 22.4 ± 3.2 (SD) yr] with normal or corrected-to-normal vision participated in this PET study. None had any history of neurological pathology, and all showed a normal brain structure as visualized with MRI. All subjects were strictly right-handed (Edinburgh inventory). They gave their written informed consent, and the study was approved by the Ethical Committee of the Medical School, Katholieke Universiteit Leuven. Before the scanning session, subjects were trained in two 1.5-h sessions to perform both direction discrimination tasks at a level of 85% correct for a response window of 600 ms and a direction difference of <= 14°.

STIMULUS CHARACTERISTICS. The stimuli were displayed on a high-resolution color screen (Philips Brilliance 2120; width × height: 380 × 285 mm; resolution 800 × 600 pixels; refresh rate 78 Hz noninterlaced), hosted by a 486 TIGA workstation. The monitor was mounted above the scanner bed at an angle of 52° relative to the horizontal. Subjects viewed the stimuli binocularly in a dimly lit room (0.07 Cd/m2) at a distance of 114 cm. Two visual stimuli and one auditory stimulus were presented to the subjects during an 1,800-ms interval. A random dot pattern was present throughout but moved only twice during the 1,800-s period, in a direction close to horizontal rightward. This random dot pattern (diameter 3.35°, density 28.3 dots/deg2) consisted of white dots (20.7 Cd/m2) on a black background (0.01 Cd/m2) and moved uniformly at a speed of 4°/s. These characteristics were chosen in accordance with the study of Dupont et al. (1994), which used a similar diameter (3°) and speed (4°/s). A red central fixation point (diameter 0.26°) dimmed (from 6.7 to 0.7 Cd/m2) once during each 1,800-ms period. A tone (frequency 390 Hz) was presented once during the 1,800-ms period through earphones. Motion of the random dot pattern, dimming of the fixation point and tone burst all lasted 200 ms.

CONDITIONS. The stimulus configuration was identical in all six conditions (Fig. 1). The onsets of the random dot pattern motion occurred randomly within two 300-ms intervals, one starting at the beginning of an 1,800-ms time period and another starting exactly halfway into this 1,800-ms period. The dimming of the fixation point and the tone burst occurred randomly either in a 300-ms interval starting at the beginning of an 1,800-ms time period or within the 800-ms interval beginning at 600 ms into the 1,800-ms period. The only deviation from this scheme was a fixed 300-ms time interval between two motion epochs in the successive discrimination task. The presentation rate of the moving random dot pattern (66.6 motion epochs/min), however, exactly equaled the presentation rate in the five other tasks. In all random dot pattern motion epochs, direction of motion was assigned randomly one of two values placed symmetrically around horizontal rightward. The difference (Delta  angle) between these two directions was adapted individually as a function of the just noticeable difference in direction reached in the second training session (85% correct). The mean (n = 12) difference in direction (Delta  angle) was 12.75 ± 1.65°.


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FIG. 1. Schematic representation of the 6 different experimental conditions. Intervals of random dot pattern (RDP) motion, fixation point dimming, and tone delivery are plotted as a function of time as well as the response windows in the 5 active conditions in which key responses were required. Two intervals of 1,800 ms are shown, corresponding to 2 trials in successive discrimination (SD), dimming detection (DD), and tone detection (TD) and to 4 trials in direction identification (ID) and motion detection (MD). Directions of motion of the RDP are indicated (top). Horizontal arrows indicate the range of randomization of stimuli onsets. Stimulus configuration was completely identical in all conditions except in successive discrimination for which the interval between 2 successive motion epochs was fixed at 300 ms (stippled line). Correct responses are indicated in the response windows: L, left key; R, right key; L and R, both keys.

In all conditions, subjects were instructed to fixate the central fixation point. In the passive viewing (PV) condition, subjects made no response. In the direction identification (ID) task, subjects pressed the key in the left hand for a motion direction rotated counterclockwise from horizontal, and the key in the right hand for a direction rotated clockwise (Fig. 1). In the successive discrimination (SD) or temporal same-different task, subjects pressed the left key for two different directions and the right key for two identical directions. For direction identification, subjects had to press within 600 ms after motion onset, and for successive discrimination, within 600 ms after the onset of every second motion epoch. In the three detection tasks, including detection of the random dot pattern motion in motion detection (MD), dimming of the fixation point in dimming detection (DD), and the tone burst in tone detection (TD), subjects had to press both keys within 400 ms after stimulus onset. To maximize subjects' attention to the dimming fixation point and to the tone burst, the stimulus presentation was more highly randomized for dimming detection and tone detection (stimulus presentation 33.3/min) than for MD (66.6/min). Consequently, the number of responses made in motion detection was double the number of responses made in dimming detection and in tone detection. The six conditions were scanned in a random sequence with each condition lasting 2.25 min.

DATA ACQUISITION. Brain activity was monitored as the relative change in regional cerebral blood flow (rCBF) using the H215O method (Fox et al. 1986). All measurements were performed in two-dimensional mode with an eight-ring PET camera of the type Siemens-CTI 931-08-12 [transaxial spatial resolution 8.5 mm full width at half maximum (FWHM)] (Spinks et al. 1988). Subjects were not allowed to speak during the procedure and had been instructed not to think of anything in particular, apart from concentrating on the stimulus. The room was kept as quiet as possible. The head was immobilized with a foam head holder (Smither medical products, Akron, Ohio) and positioned parallel to the infero-orbito-meatal line using laser alignment beams. Each subject had a catheter inserted into the right brachial vein for tracer administration. Accuracy of fixation was monitored with electro-oculographical recordings (EOG). Before each experiment, this EOG was calibrated for fixation and for horizontally visually guided saccades of 2 and 4° amplitude. A rectilinear scan was taken for positioning, followed by a transmission scan (68Ge-Ga ring source) to correct for attenuation. The start of each task coincided with a 40 mCi (1.48 Gbq) 15O-labeled H2O (half-life = 123 s) intravenously injection during 12 s. Each subject underwent six emission scans, separated by an interval of >= 15 min between two successive injections. Data acquisition began as soon as the intracranial radioactivity count rate rose sharply, i.e., usually ~30 s after injection. The first 40 s of image acquisition then were used for further analysis. The brain tissue radiation count rate was used as a measure of rCBF. The attenuation corrected data were reconstructed as 15 planes (slice thickness 6.75 mm; orientation parallel to the infero-orbito-meatal line) using filtered back projection with a Hanning filter (cutoff frequency 0.5 cycles/pixel).

DATA ANALYSIS. Realignment and spatial normalization. The scans from each subject were realigned using the first scan as a reference. The six parameters of this rigid body transformation were estimated using a least-square approach. Images then were transformed stereotactically to a standard template in the Talairach space (Talairach and Tournoux 1988). This normalizing spatial transformation matches each scan (in a least-square sense) to a reference or template image that already conforms to the standard space. The procedure involves a 12-parameter affine (linear) and quadratic (nonlinear) three-dimensional transformation, followed by a two-dimensional piece-wise (transverse slices) nonlinear matching, using a set of smooth basic functions that allows for normalization at a finer anatomic scale (Friston et al. 1995a). Finally, images were smoothed with a Gaussian kernel (20 × 20 × 12 mm FWHM). The final image smoothness estimates (FWHM) were x = 22.0 mm, y = 24.4 mm, z = 18.8 mm.

Statistical analysis. The statistical analysis was carried out on Sun SPARC computers (Sun Microsystems, Mountain View, CA) with the statistical parametric mapping software (Wellcome Department of Cognitive Neurology, London, UK), version SPM95, implemented in MATLAB (Mathworks, Sherborn, MA). Statistical parametric maps (SPMs) are extended spatially statistical processes used to characterize regionally specific effects in imaging data, combining the general linear model (to create the statistical map of SPM) and the theory of Gaussian fields (to make statistical inferences about regional effects) (Friston et al. 1991, 1994; Worsley et al. 1992). The statistical analysis can be regarded as an analysis of covariance (ANCOVA), as the design matrix includes global brain activity as covariate of no interest fixed at 50 ml·dl-1·min-1 (Friston et al. 1995b). The condition, subject, and covariate effects were estimated according to the general linear model at each voxel. To test hypotheses about regionally specific condition effects, the estimates were compared using linear contrasts. The resulting set of voxel values for each contrast constitutes a statistical parametric map of the t-statistic SPM(t). The SPM(t) was transformed to the unit normal distribution [SPM(Z)] and thresholded at 3.09 (P < 0.001 uncorrected for multiple comparisons) and at 4.06 (P < 0.05 corrected for multiple comparisons, as used in SPM95). In principle, only activation sites significant after correction for multiple comparisons should be considered valid. The activation sites reaching only the P < 0.001 uncorrected level are added for descriptive purpose. These levels of significance were used to detect the presence of a differentially active region. To test the significance of differences between conditions in regions already defined by an orthogonal subtraction, a more liberal threshold of Z = 1.64 (P < 0.05 uncorrected for multiple comparisons) was used.

MRI template. Each subject also underwent a high-resolution MRI scan of the cranium, using a three-dimensional magnetization prepared rapid gradient echo sequence (Mugler and Brookeman 1990). Acquisition parameters were: repetition time, 10 ms; echo time, 4 ms; flip angle, 8°; field of view, 256 mm; and acquisition matrix, 256 × 256. The 3-D volume had a thickness of 160 mm, partitioned into 128 sagittal slices. MRI images of each subject were registered to the corresponding PET images with multimodality image registration using information theory (MIRIT) (Maes et al. 1997). Then the same transformations into the standard space as those that were used for the PET images were applied to the resampled (and registered) MRI images. An average (n = 12) MRI image in the standard space was constructed, and the thresholded parametric maps were projected onto these images for visualization.

PLANNED SUBTRACTIONS. Table 1 lists the major cognitive components of the six tasks performed by the subjects. Preattentive processing refers to the processing of a sensory input without operation of any spatial or featural selection mechanisms (Treisman 1982). Other cognitive components relate to the motor responses, attentional mechanisms, and temporal comparison. Motion detection was first compared with passive viewing (subtractions 1 and 2) to characterize the control condition both in terms of increased and decreased rCBF. Direction identification and successive discrimination then were compared with motion detection as control (subtractions 3 and 4). These subtractions isolate attention to the attribute direction and either comparison with an internal standard (subtraction 3) or temporal comparison (subtraction 4). The two discrimination tasks then were compared with each other (subtractions 5 and 6) to isolate temporal comparison and comparison with an internal standard. Successive discrimination also was compared with dimming detection and tone detection (subtractions 7 and 8) to isolate attention to a visual stimulus, in addition to featural attention and temporal comparison (Table 1). The three detection tasks share identical decisions (presence of stimulus) and motor selections (press both keys) but differ in the stimulus attended and in the number of responses. Three subtractions (9-11) allowed us to compare these three tasks with one another.

 
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TABLE 1. Components involved in the different tasks

fMRI experiment

DATA ACQUISITION. Five subjects, two of whom also participated in the PET experiment, took part in the fMRI experiment. All functional time series consisted of 60 gradient echo-planar imaging (GE-EPI) scans (Siemens Vision 1.5 Tesla) acquired every 4.5 s (repetition time/echo time, 4,500/66 ms; field of view/ 200 mm; 64 × 64 matrix; slice thickness, 4 mm; gap, 1 mm; number of slices, 32). During a time series, two experimental conditions were alternated every 10 scans. The first functional series was a control series in which the subject remained passive and viewed either a moving randomly textured pattern or a stationary pattern. This is our standard test used to localize human motion regions, particularly hMT/V5. The stimulus in this test was the same as that used in our earlier fMRI experiments (Sunaert et al. 1996; Van Oostende et al. 1997) and consisted of a 7° patch of a randomly textured pattern (50% density) centered on a red fixation point (mean luminance, 79.40 Cd/m2; contrast, 0.97). The stimulus moved at 4°/s in one of eight directions for 427 ms, reversed for 427 ms and then changed to another randomly selected axis. We refer to this test as the passive motion paradigm. Three experimental functional series then were acquired in which the stimulus configuration was identical to that used in the PET experiment except that the tone-burst was not delivered. In each functional series, the five subjects alternately performed two tasks with an identical stimulus display: they alternated between dimming detection and direction identification in one series, between dimming detection and successive discrimination in a second one, and between successive discrimination and direction identification in a third series. Dimming detection was used as the control condition rather than motion detection because subjects alternated more easily between tasks when these involved attention to different stimuli. Each of the four types of time series was repeated four times, yielding 120 images per condition.

DATA ANALYSIS. Data analysis was performed with SPM95 software (Friston et al. 1994). The functional images were corrected for head movements and transformed into Talairach space. Single subject SPMs were thresholded at Z = 1.64 (P < 0.05 uncorrected for multiple comparisons). For the group analysis, SPMs were thresholded at Z = 4.5 (P < 0.05 corrected for multiple comparisons).

    RESULTS
Abstract
Introduction
Methods
Results
Discussion
References

Task performance during PET scanning

All 12 subjects maintained fixation well during the different conditions as witnessed by the EOG. The average performances for the five active conditions are shown in Table 2. Subjects performed all tasks at 84% correct or better. Performance in the two discrimination tasks was indistinguishable. There were, however, small but significant [repeated-measures 1-way ANOVA, F(4,44) = 4.705, P < 0.005] differences in performance among the five tasks. Post hoc testing indicated that only the difference between tone detection and the two discriminations was significant (Scheffé, P < 0.05).

 
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TABLE 2. Psychophysical performance during scanning

Motion detection compared with passive viewing

Comparison of motion detection with passive viewing yielded significant activations in four motor regions: right and left central sulcus, midline frontal cortex, and cerebellar vermis (Fig. 2). The activity profiles of these four regions show that they were active in all five conditions requiring motor responses (Fig. 3). In the subtraction motion detection-passive viewing, there was a weak activation (P < 0.001 uncorrected) in left lateral cerebellum (Table 3).


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FIG. 2. Statistical parametric maps showing regions differentially activated in the subtractions: motion detection minus passive viewing (MD - PV), identification minus motion detection (ID - MD), successive discrimination minus motion detection (SD - MD), and successive discrimination minus identification (SD - ID). Yellow and green pixels indicate increased and decreased regional cerebral blood flow (rCBF), respectively, in the experimental condition compared with the control condition. Yellow and green pixels correspond to rCBF differences significant at the P < 0.001 uncorrected (Z > 3.09), black pixels at the P < 0.05 corrected (Z > 4.06), and white pixels at the P < 0.01 corrected (Z > 4.44) level respectively. These regions are superimposed on 9 horizontal sections [from -16 mm below to 48 mm above the anterior commissure (AC)-posterior commissure (PC) line in steps of 8 mm] through the standard average magnetic resonance image of all 12 subjects to show the anatomic brain features. L, left side of the brain; R, right side.


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FIG. 3. Activity profiles of 10 significant (P < 0.05 corrected) nonvisual regions. Adjusted rCBF is plotted for the 6 different conditions (from left to right: SD, ID, MD, DD, TD, and PV). Hatching indicates experimental and control conditions of the subtraction generating the significant differential activation. Vertical lines indicate standard error of the mean. Numbers of the regions correspond to the numbers in Tables 3-5.

 
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TABLE 3. PET experiment: subtraction motion detection-passive viewing

Although hardly any visual region was activated in motion detection compared with passive viewing, a substantial number of visual areas were deactivated in this subtraction. These regions consisted chiefly of left parieto-occipital regions (Fig. 2). Significant deactivation in motion detection was observed in left middle occipital gyrus and left parieto-occipital sulcus (Table 4). Weak deactivation was observed in right middle occipital gyrus as well as in right and left middle occipital cortex, in positions close to those of the human homologue of MT/V5 observed in our earlier work (Dupont et al. 1994). Although the left occipital and parietal regions were deactivated uniformly by all five active conditions,the right middle occipital region was less deactivated bythe discrimination tasks than in the detection conditions(Fig. 4).

 
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TABLE 4. PET experiment: subtraction passive viewing---motion detection


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FIG. 4. Activity profiles of 10 visual regions. Activity profiles are plotted using the same conventions as in Fig. 3. Locations of these regions are indicated on coronal sections (from -96 to -38 mm). Different colors indicate the subtraction yielding the significant pixels: magenta, SD - TD (P < 0.001 uncorrected); green, PV - MD (P < 0.001 uncorrected); red, SD - MD (P < 0.001 uncorrected); black, P < 0.05 corrected; white, P < 0.01 corrected. To complete the pattern of regional activation, outlines of all significant (P < 0.001 uncorrected) regions (same color convention as above) are given on each coronal section. In the activity profiles, the colored bars indicate the conditions defining the differential activation.

In addition to occipito-parietal regions, significant deactivation in motion detection was observed in frontal cortex: in left inferior frontal gyrus and midline superior frontal gyrus (Fig. 2, Table 4). The left inferior frontal cortex was deactivated in all five active conditions (Fig. 3). The midline superior frontal region was less deactivated in tone detection than in the other four active tasks (Fig. 3). Weak deactivation was observed in left superior and inferior frontal gyri and in right superior frontal cortex as well as in left and right middle temporal gyri and left inferior temporal gyrus (Table 4).

Direction identification compared with detection

Direction identification compared with motion detection yielded no significant activation sites (Fig. 2), a result very reminiscent of that observed for orientation discrimination (Orban et al. 1997a). Weak activation was observed in left brain stem [-10, -22, -8, Z = 3.73] and in cerebellar vermis [-8, -62, -8, Z = 3.69]. Comparison of direction identification with dimming detection and tone detection yielded no significant activation sites either. It is highly unlikely that subjects covertly performed direction identification during the detection tasks, given the significant difference in reaction times among these tasks. These findings might simply reflect the fact that the identification task is computationally a little demanding task (Devos and Orban 1990).

Successive discrimination compared with detection

Successive discrimination of direction compared with motion detection significantly activated right middle fusiform gyrus, right lingual gyrus, right frontal operculum, left inferior frontal gyrus, and right thalamus (Table 5). This table, as well as Fig. 2 (cf. 2nd and 3rd rows), indicate that the successive discrimination causes a more widespread activation pattern than identification, as was the case for orientation discrimination (Orban et al. 1997a). Weak activation was observed in three regions of the left hemisphere: left posterior fusiform gyrus, left anterior cingulate and left middle frontal gyrus (Table 5). The activity profiles of the right fusiform and lingual regions clearly show that the successive discrimination evoked much more activity than the other five conditions, although there is some suggestion of an activation by direction identification (Fig. 4). In fact, the difference between direction identification and motion detection was significant at P < 0.05 uncorrected in both regions (Table 6). The activity profiles of the frontal regions involved in successive discrimination are rather different (Fig. 3): while activity in the right frontal operculum is much larger than that in any other condition, the activity of left inferior frontal cortex is lower in motion detection than in any other condition. The profile of the right thalamus is reminiscent of that of right fusiform and frontal cortices.

 
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TABLE 5. PET experiment: successive discrimination compared with the detection tasks

 
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TABLE 6. PET experiment: Z scores of different subtractions in the 10 visual regions of Fig. 4

Differential activity in hMT/V5 was not even weakly significant in either comparison of direction discrimination to motion detection. This is all the more remarkable considering that lesion studies in monkeys and in humans implicate area MT/V5 in direction discrimination (Barton et al. 1995; Lauwers et al. 1995; Newsome and Paré 1988). Because differential activation in hMT/V5 regions reached weak significance in passive viewing minus motion detection (Table 4), we computed their activity profiles. These profiles (Fig. 4) show that while the left hMT/V5 is equally active in the discriminations and motion detection, the right hMT/V5 shows a slightly elevated activity in the discriminations, especially the successive one, compared with motion detection. The difference between successive discrimination and motion detection was significant at P < 0.05 uncorrected in the right hMT/V5 (Table 6).

Comparing successive discrimination to dimming detection yielded results similar to the comparison with motion detection (Table 5). All regions described above were activated again, except for left posterior fusiform gyrus. Additional activation was observed in right occipital gyrus, a number of parietal regions and left precentral cortex. The activation in right occipital cortex, which may correspond to hV3A (Tootell et al. 1997; Van Oostende et al. 1997), reached statistical significance (Table 5).

Comparing successive discrimination to tone detection yielded a greater number of significant activation sites than the subtraction successive discrimination minus motion detection (Table 5). Significant activation sites also were observed in left posterior fusiform gyrus, which was only weakly significant in successive discrimination-motion detection, and in the right occipital gyrus (presumed hV3A), which also displayed activation in the subtraction successive discrimination-dimming detection. Other activation sites included the left anterior intraparietal sulcus, which was weakly significant in successive discrimination-dimming detection, the left parieto-occipital sulcus, and the left precentral gyrus. Weak additional activation in successive discrimination-tone detection was observed in left lingual gyrus and right middle frontal gyrus. The activity profiles of the significantly active visual regions are shown in Fig. 4. In all regions, the activity evoked by tone detection is lower than that evoked by motion detection. The left occipito-parietal regions are deactivated by the detection tasks compared with passive viewing, in agreement with the description given above.

Comparison between identification and successive discrimination

Subtraction of direction identification from successive discrimination revealed two significant activation sites (Table 7): right middle fusiform gyrus and right frontal operculum, two regions also significantly activated in successive discrimination compared with motion detection (Table 5). Their activity profile indeed shows that rCBF in these two regions is clearly higher in successive discrimination than in any of the other conditions (Fig. 3 and 4). Figure 2 shows the topography of the right fusiform activation in the comparisons of successive discrimination to motion detection and to direction identification. In the latter subtraction the activation is more anterior and more restricted than in the former. Hence the most anterior activation is related to the temporal comparison, a situation very similar to that observed in orientation discrimination (Orban et al. 1997a). Four left hemisphere regions were activated weakly in successive discrimination compared with direction identification (Table 7): left inferior parietal cortex, in a position slightly lateral to the region active in successive discrimination-tone detection (Table 5), left insula, left middle frontal gyrus, and left anterior cingulate, a region also active in successive discrimination compared with each of the detection tasks.

 
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TABLE 7. PET experiment: subtraction successive discrimination-identification

The opposite subtraction (direction identification - successive discrimination) revealed only a single significant activation site in right anterior cingulate [14, 40, 16, Z = 5.04, Fig. 2]. The activity profile of this region (not shown) indicates that this activation was due to decreased rCBF in successive discrimination, which was lower than in any of the other conditions. Thus this activation is unrelated to the use of an internal standard.

Attention to different stimuli

The direct comparison of the three detection tasks revealed few significant differences. Comparing dimming detection to tone detection yielded a significant activation site in left posterior fusiform gyrus [-36, -76, -16, Z = 5.04]. The opposite subtraction revealed a significant activation site in left superior frontal gyrus [-4, 50, 32, Z = 4.07]. Finally comparing motion detection with tone detection yielded a significant activation site in postcentral gyrus [54, -24, 36, Z = 4.20]. Although the number of motor responses required in dimming and tone detection was half of that in motion detection, adjusted rCBF in motor regions was not significantly different across all active tasks (Fig. 3).

Table 5 suggests that drawing attention in the control tasks away from the stimulus attended to in the experimental task enhances the differential activation. Indeed, more significant activation sites were observed when using tone detection as the control condition rather than motion detection. Thus it seems that these three detection tasks induce slightly different activity levels in a number of visual regions. We tested for statistical significance of the differences between motion detection and either dimming detection or tone detection in the 10 visual regions significantly activated in the different subtractions (Tables 4 and 5) and which are shown in Fig. 4. Because these regions were defined by orthogonal contrasts, a more liberal threshold of P < 0.05 uncorrected then could be used that would detect small differences. No significant difference was found between motion detection and dimming detection. The activity in tone detection was significantly (Table 5, *) lower than in motion detection only in the two left parietal regions, in right occipital gyrus, and in left posterior fusiform gyrus. The effect was strongest in the two left parietal regions, where it reached a level of significance withstanding correction for multiple comparisons (Table 5). In the other six visual regions, there was no significant difference between tone detection and motion detection.

Functional MRI experiments

In five subjects, the human homologue of MT/V5 and other motion processing regions were visualized using the passive motion paradigm. These passive motion activation sites were compared with the sites obtained when the subjects alternated between dimming detection and successive discrimination or direction identification and between direction identification and successive discrimination. Although performance of the subjects in successive discrimination and dimming detection was similar to that during the PET scanning, their performance was unexpectedly poor in the direction identification despite attempts to increase the difference in direction (Table 2). Debriefing suggested that this might be due to the absence of any visual references in the display. Such surround cues might be used by relatively inexperienced subjects to build up their internal reference (Vogels and Orban 1986), especially in experiments in which no feedback was provided. Consequently we used only the fMRI data related to the comparison successive discrimination-dimming detection. The results are shown for a single subject in Fig. 5. Right and left hMT/V5 were localized with the comparison moving-stationary randomly textured pattern. The time courses of this activation are shown in row A: while the signal is stronger in left than in right hMT/V5, the MR signal in both regions clearly is modulated in phase with the alternations between moving and stationary pattern. However, there is basically no change in MR signal in right and left hMT/V5 when the subject alternated between successive discrimination and dimming detection. In contrast, the right lingual and fusiform gyri, and left superior parietal lobe (SPL) displayed a distinctly higher MR signal in successive discrimination compared with dimming detection, in agreement with the PET results. In the lingual gyrus there was also a modulation in the MR signal accompanying the alternations between moving and stationary patterns (Fig. 5B). This was not the case in the right fusiform gyrus (Fig. 5C). Neither were there any changes in the MR signal in left SPL, measured in the most significant voxel for successive discrimination-dimming detection, when alternating between moving and stationary pattern. There was, however, an activation in the passive motion paradigm just lateral to that observed in successive discrimination-dimming detection.


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FIG. 5. Functional magnetic resonance imaging (fMRI):single subject analysis. Percent change in fMRI-signal in the most significant voxel of left and right area hMT/V5 (A), right posterior collateral sulcus (lingual gyrus) (B), right anterior collateral sulcus (fusiform gyrus) (C), and left superior parietal lobe (D) as a function of time (sequence of 60 images) for the subtractions moving-stationary randomly textured pattern (left) and successive discrimination-dimming detection (middle). Thick lines at the bottom correspond to the functional images of the experimental condition. Locations of the areas are indicated in sagittal, coronal and transaxial sections (right: red and blue arrows, hMT/V5; green arrows, lingual gyrus; yellow arrows, fusiform gyrus; orange arrows, superior parietal lobe). Color scale of significance is indicated. L, left side of the brain; R, right side. Data from subject 2 (Table 9).

This pattern of results also was observed in the group analysis of the five subjects. Figure 6 contrasts the result of the passive motion paradigm with the subtraction successive discrimination minus dimming detection. The latter subtraction yielded an activation in the right and left lingual gyrus as well as in the right fusiform gyrus (Table 8). This pattern is similar to the PET result, although its distribution was more bilaterally symmetrical in the fMRI than in the PET. Only the lingual regions were also active in the passive motion paradigm in agreement with earlier observations (Sunaert et al. 1996). This passive motion paradigm yielded a very clear bilateral activation of hMT/V5, which reached a maximum at z = +4 mm (Fig. 6). At this level, there was very little differential activation in the subtraction successive discrimination-dimming detection. However, 4-8 mm inferior to the maximum of the hMT/V5 activation, there was a significant activation in the subtraction successive discrimination-dimming detection. This activation was stronger on the right side, which is again reminiscent of the PET results (Table 8). There was a significant activation in the subtraction successive discrimination-dimming detection in the right occipital gyrus (Table 8), which appears to correspond to hV3A, according to the passive motion paradigm. In the subtraction successive discrimination-dimming detection, a number of differentially active regions were observed bilaterally in the parietal cortex (Table 8), again in agreement with the PET results (Table 5). Most of these regions were located close to the intraparietal sulcus, in positions activated in the passive motion paradigm. Although these latter differential activation sites were very clear in single subjects, they tended to become less distinct in the group average, possibly due to the intersubject variability in the location of the intraparietal sulcus (Michio et al. 1990). Indeed, probing the voxels that had proven most significant in the group average in SPMs of individual subjects gave significant results in only two or three subjects for most parietal regions. For the occipital regions this was the case in four or five subjects (Table 9). Finally, a number of frontal regions were differentially active in the subtraction successive discrimination minus dimming detection (Table 8). It is noteworthy that the three regions displaying the most significant MR signal modulations also were observed in the PET study: right frontal operculum, left precentral cortex, and left middle frontal cortex.


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FIG. 6. fMRI: group analysis. Statistical parametric maps showing regions differentially activated in the subtractions moving minus stationary randomly textured pattern (A) and successive discrimination-dimming detection (B). These regions are superimposed on horizontal sections (from -20 mm below to +8 mm above the AC-PC line in steps of 4 mm) through the standard average magnetic resonance image of all 5 subjects. Yellow arrow, fusiform gyrus; green arrows, lingual gyrus; purple arrows, hMT/V5 satellite region; magenta arrows, hV3A; red and blue arrows, hMT/V5. Color scale of significance is indicated. R, right side of the brain.

 
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TABLE 8. fMRI experiment: group data for subtraction successive discrimination-dimming detection

 
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TABLE 9. fMRI experiment: individual subject data for subtraction successive discrimination---dimming detection

    DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References

Our results show that direction discrimination involves chiefly ventral occipital regions and to a lesser extent middle and dorsal occipital and parietal regions but that when a temporal comparison is required, the right fusiform gyrus and right frontal operculum are recruited. They also show that drawing attention to a nonvisual stimulus decreases activity in visual regions but in a nonuniform manner. Because the number of visual activation sites related to successive discrimination was largest in the subtraction successive discrimination minus tone detection, two conditions requiring equal numbers of responses, it is highly unlikely that these differential activations were due to differences in motor responses. In fact, the activity profiles of Fig. 3 and 4 show that in none of the major activation sites did the activity simply reflect the number of motor responses. Although the regions of the human brain involved in processing retinal motion have been investigated extensively with passive viewing paradigms, very little has been done using tasks involving moving stimuli, with the exception of the speed discrimination studies of Corbetta et al. (1991) and Beauchamp et al. (1997). Therefore we will discuss each of our main findings in turn.

Involvement of ventral occipital areas in direction discrimination

Both the PET and the fMRI experiments concur insofar as the strongest attentional modulation by direction discrimination is observed in the ventral occipital regions: the right and left lingual gyrus and the right fusiform gyrus. The overall agreement between the two techniques is in fact, excellent. The only minor discrepancy is that the left activation in the fMRI is more nearly equal to that on the right side and located somewhat more posteriorly than that obtained in the PET.

Activation of the lingual gyrus by a central visual stimulus has been observed repeatedly in our PET studies (Table 10). Comparison of a centrally presented grating with one presented peripherally yielded a bilateral lingual activation in the study of Vandenberghe et al. (1996). Orban et al. (1997a) observed a right lingual activation in comparing rapid presentations of a central grating with a slow presentation rate. In the present experiment, the lingual gyral region was activated by a moving random dot pattern rather than a static grating (Table 10). A similar moving stimulus was used by Dupont et al. (1997a), who described differential activation of right lingual gyrus in successive speed discrimination compared with detection. It is noteworthy that a left lingual activation (-28, -80, -8) was observed by de Jong et al. (1994) in comparing an optic flow stimulus to incoherent motion as was a right lingual activation (34, -80, -14) by Bonda et al. (1996) when comparing biological motion to incoherent motion. The present fMRI experiment shows that this lingual region is also weakly active in the passive motion paradigm used to define hMT/V5 and hV3A, in agreement with results obtained in a larger sample by Sunaert et al. (1996). Because the lingual region in our studies is activated by a small central visual stimulus, it is unlikely to correspond to hV2 or hV3 because the peripheral visual field representation of these regions is located in the lingual gyrus (DeYoe et al. 1996; Engel et al. 1997; Sereno et al. 1995). However, these three fMRI studies describe a region anterior to ventral V3, which they have designated human V4v. We suggest that our lingual region corresponds to the representation of central vision of either human V4v or a more anterior area. This is not necessarily contradictory to the above-mentioned fMRI studies because central vision representations largely are missed by the sweeping stimulus techniques used in these studies. This identification is supported by the observation that V4 neurons respond in a direction matching task in the monkey (Ferrera et al. 1994) and by the fact that the PET study revealed the lingual region to be more active in successive discrimination than direction identification (Table 6). Motion responses also have been observed in monkey V4 with metabolic mapping (Orban et al. 1997b). It is worth noting that the lingual activation described here is 15-20 mm posterior to the region involved in color processing, as identified by Zeki et al. (1991).

 
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TABLE 10. Localization of lingual activation by a central visual stimulus

The right middle fusiform activation was observed in both the comparison of the successive discrimination with the detection tasks and in the subtraction successive discrimination-direction identification. Thus this activation reflects the temporal comparison component present in the successive direction discrimination. This result is in strikingly parallel to that of the orientation discrimination reported by Orban et al. (1997a). According to these authors, right middle fusiform cortex is also differentially active when comparing successive discrimination for orientation with detection of a grating or to orientation identification. Exactly as in the orientation study, the region of differential activation in successive discrimination-identification lies slightly more anterior than that obtained in successive discrimination-detection. Coordinates were (38, -50, -12) and (40, -62, -12) for orientation compared with (34, -58, -8) and (32, -62, -8) for direction. These results indicate that the right fusiform gyrus is involved in temporal comparison of orientation as well as direction of motion. Related studies suggest that this cortex is also involved in the temporal comparison of speed of motion (Dupont et al. 1997a).

Involvement of classical motion areas in direction discrimination

According to recent evidence, the two main motion areas in humans are the homologues of MT/V5 (Cheng et al. 1995; Dupont et al. 1994; Sunaert et al. 1996; Tootell et al. 1995; Watson et al. 1993; Zeki et al. 1991) and of V3A (Sunaert et al. 1996; Tootell et al. 1997; Van Oostende et al. 1997). Although the region tentatively identified as hV3A did not reach the strict criterion of P < 0.05 corrected in successive discrimination-motion detection, it did so when successive discrimination was compared with dimming detection or to tone detection. Furthermore a probe of this region showed that it was differentially active in successive discrimination-motion detection, and even in successive discrimination-direction identification or direction identification-motion detection (Table 6). In both the PET and the fMRI experiments, the activation was right-sided. The coordinates of the region in the present study (18, -96, 0) were very close to those of a region (20, -98, 0) tentatively identified as hV3 by Orban et al. (1997a). This is not surprising because the central vision representations of hV3 and hV3A are located adjacent to one another (Tootell et al. 1996, 1997). The present fMRI experiments strongly suggest that the posterior occipital region differentially active in successive discrimination-dimming detection is hV3A (Fig. 6).

Although the involvement of hV3A, at least on the right side, in direction discrimination is relatively clear, that of hMT/V5 is less evident. This is unexpected because a series of lesion studies in nonhuman primates (Lauwers et al. 1995; Newsome and Paré 1988; Pasternak and Merigan 1994) and in humans (Barton et al. 1995) implicate MT/V5 in direction discrimination. Although Newsome and Paré (1988) have provided evidence that restricted MT/V5 lesions produce deficits in direction judgments, the study of Lauwers et al. (1995) is most relevant because the monkeys' task was similar to the direction identification task used here. The PET study demonstrated little involvement of hMT/V5 in either direction discrimination task. This is particularly surprising where the computationally more demanding successive discrimination task is concerned. The absence of differential hMT/V5 activity was not due to saturation because its activity in discrimination and detection was lower than that during passive viewing (Fig. 4). Furthermore, attention to the fixation point was required in all six tasks (see Table 1), making it unlikely that the paucity of hMT/V5 activation can be accounted for by the restriction of attention to the vicinity of the fixation point. Neither could the apparent lack of hMT/V5 involvement in direction discrimination be attributed to the use of the rather small stimulus size. Indeed, in both PET and fMRI functional imaging studies (Dupont et al. 1994; Sunaert et al. 1996), many regions of the occipito-parietal pathway, including hMT/V5, are significantly activated in comparisons of a moving random dot pattern to a stationary pattern of a similarly small size.

There was, however, a small differential activation of right hMT/V5 in the subtraction successive discrimination-motion detection (Table 6). The fMRI revealed that there was in fact a region involved that was located just ventral to the main hMT/V5 activation in the passive motion paradigm. It has been stressed repeatedly that the activation of hMT/V5 in the passive paradigm reflects not only the activation of hMT/V5 but also that of its satellites (de Jong et al. 1994; Tootell et al. 1996). Activation of the lower edge of the hMT/V5 complex has been observed by de Jong et al. (1994) in the subtraction optic flow stimulus minus incoherent motion and by Bonda et al. (1996) in the subtraction body movement minus random motion. It is unclear to which of the satellites of monkey MT/V5 this human satellite may correspond. The homologue of medial superior temporal visual area, dorsal part (MSTd) (Desimone and Ungerleider 1986) has been tentatively localized more anteriorly in the complex (Tootell et al. 1996), and de Jong et al. (1994) also dismiss the possibility that the inferior satellite corresponds to MSTd. An alternative possibility is V4t (Desimone and Ungerleider 1986), which was included in the MT/V5 lesions of Lauwers et al. (1995). Whatever the exact nature of this satellite, it was likely to have been included in the lesions of Barton et al. (1995).

Involvement of parietal regions in direction discrimination

Both the PET and fMRI experiments show that there is some involvement of the parietal regions in direction discrimination. The fMRI results suggest that most of these regions lie along the intraparietal sulcus. In the passive motion paradigm, a number of motion areas have been observed along this sulcus in both the present study and in our earlier one (Sunaert et al. 1996).

The comparison of the detection task with passive viewing reveals a number of occipito-parietal regions, which are less active in direction discrimination and detection than in passive viewing. Activation of one of these regions, the one high in the parieto-occipital sulcus, has been observed in an earlier study using the passive motion paradigm (Dupont et al. 1994). To a certain degree, a deactivation during the active tasks also is present in hMT/V5 (Fig. 4). A possible interpretation is that the moving random dot pattern automatically engages a number of motion selective areas and that the resulting activity is down-modulated during the performance of a number of tasks where the contributions of these areas is not required.

Involvement of frontal regions in direction discrimination

The subtraction successive discrimination-direction identification, which reveals areas involved in the temporal comparison of motion direction, yielded not only activation in the right middle fusiform cortex but in the right frontal operculum as well. This latter region is clearly posterior to those involved in object working memory (Baker et al. 1996; Courtney et al. 1996; McCarthy et al. 1996). The interpretation of this activation is presently unclear. Both the PET and the fMRI suggest an involvement of the left middle frontal gyrus in successive discrimination. This location is relatively close to the region involved in shape working memory according to McCarthy et al. (1996).

Both the PET and the fMRI experiments also indicate involvement of the precentral sulcus, corresponding to the human frontal eye field (FEF) (Sweeney et al. 1996), in direction discrimination. The left precentral region was more active than the right in both experiments. The FEF is also active in the passive motion paradigm (present experiment and Sunaert et al. 1996), which is not surprising because, in the monkey, there is a strong connection between MT/V5 and its satellites and FEF (Schall et al. 1995).

Functional specialization

When the present results are compared with those obtained in a previous experiment with similar design but involving orientation discrimination (Orban et al. 1997a), they provide evidence for a functional specialization of human visual cortex. Indeed, different regions are involved in identical discrimination tasks of the two attributes. First, comparison of successive discrimination to its corresponding detection task (compare Table 5 of the present study with Table 6 in Orban et al. 1997a) reveals that orientation successive discrimination engages posterior fusiform cortex and direction successive discrimination lingual cortex, located more posteriorly. Second, at a more sensitive level, both studies yielded functional profiles for the right lingual region and for the hV3/V3A region (compare Fig. 4 of the present study with Fig. 5 in Orban et al. 1997a). Differences between profiles are relatively minor for the lingual region: direction identification produced significantly (P < 0.05 uncorrected) more activity than detection, unlike orientation identification. Differences are much more evident at the level of the hV3/V3A region: direction successive discrimination yielded more activation than either detection or direction identification, and direction identification itself yielded more activity than detection, while none of the three tasks resulted in a different rCBF for orientation.

The direction discrimination tasks involved parietal regions, which was not the case for the orientation discrimination. It could be argued that these parietal activation sites were observed only in successive discrimination-dimming detection, which was not tested in the orientation study of Orban et al. (1997a). However, an earlier study of orientation identification, using both central and peripheral stimuli (Vandenberghe et al. 1996), did yield parietal activation in regions close to those involved in direction discrimination in the present study. These parietal regions were active only for orientation identification using a peripheral target but not using a central target, as we have done in the present study.

There is some overlap, however, between the orientation and direction discrimination networks, especially at the occipital level. This is in agreement with earlier studies showing that the occipito-parietal and -temporal pathways (Haxby et al. 1994) as well as motion and shape processing regions (Dupont et al. 1997b) overlap in near-extrastriate regions and diverge only in more anterior, higher order regions.

Task-dependent processing

The present data strongly support the view that processing in the human visual system depends not only on the attribute to be processed but also on the nature of the task. The same visual input and the same attribute (direction of motion) produce different activation sites depending on whether or not a temporal comparison is required. The temporal comparison of direction involves the right middle fusiform cortex and the right frontal operculum. This is in excellent agreement with our earlier orientation discrimination data, in which successive discrimination also activated the right middle fusiform gyrus (Orban et al. 1997a) when compared with identification. The presence of significant activation in the fusiform cortex in the temporal comparison of direction corresponds with the monkey study of Ferrera et al. (1994), who showed that direction matching involved a ventral stream cortical area. Thus task-dependent processing is a general sensory-processing principle, which applies to several attributes in the human visual system as well as to the monkey auditory (Colombo et al. 1990) and visual modality (Vogels et al. 1997). It is noteworthy that, just as in the orientation discrimination studies, the successive discrimination task, which is computationally more demanding, involved more cortical regions than the identification task (Table 6). Also, in both our orientation and direction discrimination experiments, temporal comparison involved the right fusiform gyrus, as did the short-delay working memory task of Haxby et al. (1995).

The present study shows that the task dependency of visual processing arises from both positive and negative modulations of the areas passively engaged by the moving stimuli. Human MT/V5 exemplified negative modulation, especially in the left hemisphere. On the other hand, the lingual region, hV3A, and the parietal regions displayed a positive modulation. This sort of bidirectional modulation of areas passively engaged by the stimulus also was observed in the earlier orientation discrimination study (Orban et al. 1997a). In that study, the right middle fusiform cortex was in fact deactivated by identification and detection compared with passive viewing. Thus although middle fusiform cortex was engaged by the temporal comparison in both orientation and direction discrimination, the mechanism of its recruitment is different (compare activity profiles in Fig. 4 of present study with that in Fig. 5 of Orban et al. 1997a) and involves relatively more positive modulation in the present experiment.

Attention to nonvisual stimuli

Our results provide only modest support for the view that attention to a given modality reduces activity in other sensory systems (Haxby et al. 1994). The effect of attention to a tone strongly depends on the visual region involved, insofar as the modulation of activation was much stronger in the two left parietal regions than in most other visual cortical regions (Table 5). Furthermore, our results are difficult to reconcile with the observation of Haxby et al. (1994) that attention to complex visual stimuli reduces activity in primary and association auditory cortex. The recent observations of Shulman et al. (1997), however, are in good agreement with our results. Further experiments are required to characterize cross-modal suppression of neural activity in the visual cortex because this might depend on the nature of the nonvisual stimulus used (Shulman et al. 1997). At any rate, the present results suggest that it is not advisable to employ a task involving attention to a different sensory modality as a control condition. Differential activation might be ascribed wrongly to the experimental condition when visual regions are unequally active in a control condition because it depends as much on the control as on the experimental condition.

The present experiments indicate the need for caution and further exploration of the issue of cross-modal attention, but, more importantly, they clearly extend the evidence for the task dependency of processing in the human visual system to its motion processing component. In addition, they underscore the importance of using tasks to explore the human visual system because the present results show a far greater involvement of the lingual visual region and hV3A combined with less involvement of hMT/V5 in direction discrimination than would have been anticipated from the literature.

    ACKNOWLEDGEMENTS

  We thank S. Ballet, G. Bormans, W. Costermans, D. Crombez, T. De Groot, S. Stroobants, and S. Vleugels for assistance during PET scanning; P. Kayenbergh and G. Meulemans for technical help; M. De Paep and P. Falleyn for software programming; and S. Raiguel and R. Vogels for comments on the manuscript. We are indebted to Prof. R. Frackowiak for making available the SPM software and to Prof. Suetens for providing us with the MIRIT software.

  This work was supported by a grant from the Queen Elisabeth Medical Foundation to L. Mortelmans and Grants 9.0007.88, 3.0043.89 and 3.0227.95 from the Fund for Scientific Research, Flanders (Belgium). L. Cornette and S. Sunaert are research assistants of the Fund for Scientific Research, Flanders (Belgium). P. Dupont and A. Rosier are postdoctoral fellows of the Fund for Scientific Research, Flanders (Belgium).

    FOOTNOTES

  Address for reprint requests: G. A. Orban, Laboratorium voor Neuro- en Psychofysiologie, KU Leuven, Medical School, Campus Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium.

  Received 28 July 1997; accepted in final form 8 January 1998.

    REFERENCES
Abstract
Introduction
Methods
Results
Discussion
References

0022-3077/98 $5.00 Copyright ©1998 The American Physiological Society