Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, FIN-02015 HUT Espoo, Finland
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ABSTRACT |
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Vanni, Simo and Kimmo Uutela. Foveal Attention Modulates Responses to Peripheral Stimuli. J. Neurophysiol. 83: 2443-2452, 2000. When attending to a visual object, peripheral stimuli must be monitored for appropriate redirection of attention and gaze. Earlier work has revealed precentral and posterior parietal activation when attention has been directed to peripheral vision. We wanted to find out whether similar cortical areas are active when stimuli are presented in nonattended regions of the visual field. The timing and distribution of neuromagnetic responses to a peripheral luminance stimulus were studied in human subjects with and without attention to fixation. Cortical current distribution was analyzed with a minimum L1-norm estimate. Attention enhanced responses 100-160 ms after the stimulus onset in the right precentral cortex, close to the known location of the right frontal eye field. In subjects whose right precentral region was not distinctly active before 160 ms, focused attention commonly enhanced right inferior parietal responses between 180 and 240 ms, whereas in the subjects with clear earlier precentral response no parietal enhancement was detected. In control studies both attended and nonattended stimuli in the peripheral visual field evoked the right precentral response, whereas during auditory attention the visual stimuli failed to evoke such response. These results show that during focused visual attention the right precentral cortex is sensitive to stimuli in all parts of the visual field. A rapid response suggests bypassing of elaborate analysis of stimulus features, possibly to encode target location for a saccade or redirection of attention. In addition, load for frontal and parietal nodi of the attentional network seem to vary between individuals.
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INTRODUCTION |
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During attention to one visual target, a
parietofrontal attentional network is essential for search and
selection of new targets (Corbetta et al. 1991;
LaBerge 1995
; Posner and Petersen 1990
; Vanni et al. 1997
). This search requires shifts of
attention between objects or locations and shifts of gaze. In monkeys,
functional areas participating in these functions are rather well
known. The lateral intraparietal area (LIP) in the intraparietal sulcus and the frontal eye field (FEF) anterior to the arcuate sulcus have
been mainly associated with planning and execution of saccades (Andersen 1989
; Schall 1997
). Both areas
are sensitive to sensory stimuli, even without eye movements
(Barash et al. 1991
; Kodaka et al. 1997
).
The 7a in the inferior parietal lobule (IPL) participates in the
spatial representation of the visual environment (Siegel and
Read 1997
). In addition, area 7a contains cells that are most sensitive to peripheral visual stimuli when visual attention is focused
onto foveal targets (Mountcastle et al. 1981
,
1987
). These 7a neurons thus seem to monitor the
nonattended visual field, an operation necessary for deciding where to
attend or move the eyes next.
In humans, attentional and oculomotor tasks activate partially
overlapping cortical areas. Strongest parietal activation can be found
postcentrally and close to the intraparietal sulcus and frontal
activation in the precentral frontal cortex (Corbetta 1998; Nobre et al. 1997
). The monkey's LIP and
7a are most likely the homologues to these human parietal areas,
whereas the frontal locus corresponds to monkey FEF. Although the
locations of the human parietal and frontal areas involved in shifts of
eyes and attention are relatively well characterized, sensitivity of
these areas to nonattended sensory stimuli and the timing of their
activation are still poorly known.
In some previous human studies, activation in the FEF was found when
attention was directed to peripheral stimuli (Kim et al.
1999; Nobre et al. 1997
; Vandenberghe et
al. 1997
). We measured neuromagnetic signals to peripheral
luminance stimuli while attention was directed to a fixation point,
released while maintaining fixation, and during longer periods of
passive fixation. The peripheral luminance stimuli were never targets
in the main experiment. In control experiments, precentral responses to
visual stimuli were studied during an auditory task, and during
attention to visual periphery. The aim of this study was to find out
which areas are sensitive to nonattended peripheral stimuli during
visual attention, and the timing of this enhancement. The main
experiment followed a paradigm used in several monkey studies probing
functions of the IPL (Motter and Mountcastle 1981
;
Mountcastle et al. 1981
, 1987
;
Siegel and Read 1997
). The visuospatial control
experiment followed the paradigm of Steinmetz et al.
(1994)
. A brief report of these results has appeared in an
abstract form (Vanni and Uutela 1999
).
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METHODS |
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We studied 11 right-handed healthy volunteers (5 females, 6 males, mean age 27.7 yr, range 22-36 yr). Figure 1 shows the experimental setup. The subjects fixated a small (0.3°) white (53 cd/m2) square with a dark center projected on a dark background (4 cd/m2) at 63 cm distance. First, the center of the fixation square (half of its width) increased in luminosity (fixation-on), and after a randomized 1- to 4-s interval the center dimmed (fixation-dimming) for 0.5 s before becoming dark again (fixation-off). A 12 × 6° rectangular luminance stimulus (53 cd/m2) was presented 10° off the fixation along the horizontal meridian, either left or right in random order. The stimulus appeared 1.5 s after the fixation onset and again 1.5 s after fixation dimming with a duration of 200 ms. If the fixation-dimming followed fixation-onset within 2 s (20% of the epochs), no stimulus was delivered.
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During the trials, the subjects focused their attention on the fixation square and lifted their right index finger when they detected its dimming. The dimming was adjusted to be barely detectable, and the subjects were encouraged to react as fast as possible. During intertrial intervals (3-s periods between 2 trials) the subjects waited for the fixation square to light up, which marked the beginning of the next trial. During passive viewing the subjects fixated the central square with no task while the peripheral luminance stimuli were presented. The passive viewing was measured in two separate blocks, one before and the other one after the active trial-intertrial measurement.
Signals were recorded with a Neuromag-122 whole-scalp neuromagnetometer
that is equipped with planar gradiometers (Ahonen et al.
1993). Signals were filtered (pass-band 0.03-90 Hz), sampled at 300 Hz, and averaged time locked to the onset of the peripheral luminance stimulus. A ±8-ms jitter of the stimulus onset, due to
asynchrony between the stimulus computer and the data projector, smoothed the averaged signals roughly as a 25-Hz low-pass filter. One
hundred responses were averaged for each distinct file (6 files: trial,
intertrial, and passive viewing for both the left and right visual fields).
To estimate the cerebral currents, the average signals were
preprocessed by applying a 200-ms prestimulus baseline and a 40-Hz low-pass filter. We used the minimum current estimate (MCE)
(Uutela et al. 1999), an implementation of the minimum
l1-norm estimate. MCE explains most of the measured signals with a
current distribution that has the smallest sum of current amplitudes.
The estimate is calculated separately for each time point. The
estimates were studied both as a group average over the subjects, to
increase the signal-to-noise ratio and to easily detect the activation consistent across the subjects, and separately for each subject, to
detect individual variability.
Active areas in the individual subjects were determined from the
average of the three conditions (trials, intertrials, and passive
viewing), separately for the left and right visual field stimuli.
Regions of interest (ROIs) were selected to cover all steadily active
areas within 400 ms from stimulus onset. The mean and extent of the
ROIs were automatically adjusted to fit the estimated activity. The
activity of the region was calculated as a weighted average of the
estimate: the maximum weight was at the center of the ROI and extended
to the neighboring locations with the form of a three-dimensional
generalized normal distribution. The ROI was accepted for further study
if its estimated activity during the response exceeded 2 nanoamperemeters (nAm) and twice the peak activity during prestimulus
baseline, the duration exceeded 10 ms, and the active area seemed to be
stationary. The coordinates of the centers of the ROIs were transformed
to the Talairach and Tournoux (1988) coordinate system.
Group average of the estimates was calculated without taking
orientation of the sources into account. The estimates were aligned based on three anatomic landmarks on the surface of the heads, and the
results were shown on a standard brain volume (Roland et al.
1994). Selection criteria for the ROI were the same as for the
individual subjects, but the three conditions were screened separately.
Evoked electroencephalographic (EEG) responses were recorded simultaneously with the magnetoencephalographic (MEG) recordings from 19 locations of the International 10-20 system and from the right mastoid, referred to left mastoid. For the study of the left hemisphere the signals referred to right mastoid were calculated off-line. Signals were filtered (pass-band 0.03-100 Hz), sampled at 300 Hz, and averaged time locked to stimulus onset.
Eight subjects participated in a separate saccade reaction time experiment. In the Saccade task, subjects made immediate saccades to a 1.5° square-shaped luminance stimuli when they appeared 11.5° left or right from the fixation in random order. The stimuli were presented for 400 ms and interstimulus interval was randomized from 1 to 28 s. In the Attention + Saccade task, subjects had to perform a dual task. First, they had to indicate transient luminance increments of the fixation with immediate finger lifts. In addition, they had to make immediate saccades when the peripheral luminance stimuli appeared. The luminance increments of the fixation appeared randomly at 1- to 4-s intervals, whereas the peripheral saccade targets had the 1- to 28-s intervals as in the Saccade task; the fixation and peripheral stimuli were at least 1 s apart from each other. Twenty saccades to both left and right targets were collected. The saccade onset latencies were calculated off-line from horizontal electrooculogram (EOG) signals.
Subjects 2 and 7 participated in two MEG control experiments. In an auditory attention task, the subjects viewed the same visual stimuli as in the main experiment, but the changes in fixation luminance were replaced by changes in tone frequency. Tones (500 Hz) were presented for 1-4 s binaurally at 40 dB above the hearing threshold, and the subjects had to detect a 200-ms 10-Hz frequency glide upwards at the end of the tone. There were 3-s intervals between the tones. Hence the tone and intertone intervals corresponded to the trial and intertrial intervals in the main experiment.
In a visuospatial task, the two subjects fixated a point while 6° square-shaped luminance stimuli were shown randomly to one of eight possible peripheral locations, 11.5° horizontally and/or vertically from fixation. Thus the distance between fixation and stimulus was either 11.5 or 16°. First, one location was cued by presenting a luminance stimulus for 400 ms. After the cue, one to five stimuli in other locations were delivered before the cued location was stimulated again. Subjects were asked to lift their right index finger after the end of the second stimulus at the cued location. If no response was delivered or it was delivered outside a 500-ms window after stimulus, the epoch was rejected. Subjects received immediate visual feedback about their performance, and the session continued until 100 responses at the cued locations were collected and averaged. Signals for the noncued stimuli were averaged separately.
The control studies were measured 1 yr after the main experiments with a new 306-channel Neuromag Vectorview neuromagnetometer. Signals were filtered (pass-band 0.03-200 Hz), sampled at 600 Hz, and averaged time locked to the onset of the peripheral luminance stimulus. In both the behavioral and MEG control experiments the stimuli were back-projected on a screen, and the viewing distance was 60-64 cm.
In all MEG experiments vertical and horizontal EOGs were recorded, and epochs contaminated with saccades or blinks were rejected on-line from the averaged signals. In addition to artifact rejection, the horizontal EOG signals were quantified off-line to reveal possible differences in eye movements between the experimental conditions. Epochs with horizontal EOG signals exceeding a low threshold were considered to include overt eye movements.
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RESULTS |
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In individual subjects the peripheral luminance stimulus evoked on average 12 distinct neuromagnetic responses. In the occipital lobe, middle, ventral, lateral, and superior (cuneus) parts were all activated in at least one-half of subjects at least by one hemifield stimuli. Similarly, inferior and superior temporal cortices and the temporooccipital border were commonly active. At the parietal lobe, inferior parietal lobulus and precuneus/parietooccipital regions were commonly active as well as the temporoparietal border. In the frontal cortex activation was common in the precentral and premotor cortices. Although both individual and group analysis showed clear signals in parietal lobes, the most systematic difference between the experimental conditions was found, to our surprise, in the right precentral region.
Figure 2A shows minimum current estimates for two subjects projected on triangular meshes covering their brain. Figure 2B shows the estimated locations of the right precentral activation projected on the surface of magnetic resonance images (MRIs). The estimated source location is between the central sulcus and precentral gyrus in both subjects. For subject 2, the center of the estimated location is on the precentral gyrus. For subject 7, the left visual field (LVF) activation is on the precentral gyrus, and the right visual field (RVF) activation is in the precentral sulcus. Figure 2B shows the source strengths as a function of time for the two hemifields during the trials. After the stimulus onset, the activation was strongest between 130 and 160 ms for subject 2 and between 120 and 150 ms for subject 7. Both subjects showed stronger activation for the contralateral stimulation, and subject 7 had 10-20 ms shorter latencies for the contralateral compared with ipsilateral stimuli. An offset response emerged between 300 and 400 ms in subject 7, whereas in subject 2 the possible offset response was close to baseline noise level.
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Figure 3A shows the group average of the current distribution between 100 and 160 ms after the left visual field stimuli. Estimated activities for trial, intertrial, and passive viewing conditions are projected on the surface of the standard brain. The right precentral region shows the strongest activation during trials, clearly above the baseline noise level.
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Figure 3B shows the estimated current in the right hemisphere precentral region as a function of time. The mean strength 100-160 ms after the left visual field stimulation was significantly stronger during the trials (1.4 ± 0.8 nAm, mean ± SD across subjects) than intertrials (0.7 ± 0.4 nAm, P < 0.005, Wilcoxon signed rank test) or during passive viewing (0.6 ± 0.3 nAm, P < 0.005). The corresponding values after the right visual field stimulation were 1.5 ± 0.8, 1.0 ± 0.6 (P < 0.05), and 0.8 ± 0.2 nAm (P < 0.005), respectively. When the right and left visual field responses were averaged, the right precentral region showed the strongest responses during trials in all subjects (P < 0.001 for both trial-intertrial, and trial-passive viewing comparisons, binomial test). The small amplitude values in the group analysis compared with individual analysis are due to inclusion of a time interval usually exceeding the individual active period, possible positional jitter in individual source location, and from the inclusion of subjects whose activity in this region was weaker than the individual criteria for applying a ROI.
Analysis of the individual datasets revealed right precentral or
premotor activity fulfilling the source criteria in 6/11 subjects
(Table 1). In line with the group
average, individual activity was always strongest during the trial
(P < 0.02, binomial test). Its mean (±SD) x,
y, and z Talairach coordinates (Talairach and
Tournoux 1988) were 49 ± 3, 0 ± 9, and 35 ± 8 mm. No systematic amplitude differences between the tasks were found in
the left precentral region either in the group or individual analysis
(source found in 6 subjects, coordinates
37 ± 8,
9 ± 11, and 43 ± 12 mm).
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The parietal lobes contained more sources and showed more individual
variability than the precentral region; no significant differences
between the stimulus conditions were found in the group data. Although
functional areas cannot be distinguished based on the MR image only, we
found some systematic features in the responses. Figure
4 shows estimated location of a
right-hemisphere posterior parietal activation for six subjects. This
source was found in 8/11 subjects and was mainly located inferior to
the intraparietal sulcus and posterior to the supramarginal gyrus (Talairach coordinates 46 ± 6, 50 ± 9, 40 ± 5 mm).
It was strongest on average at 180-220 ms; for timing of individual
activity, see Table 1. In the left hemisphere an IPL source was present
in five observers on average between 180-210 ms (x, y, and
z coordinates
44 ± 6, 53 ± 18, and 31 ± 7 mm). The left IPL sources were located somewhat inferior to the right
IPL sources, closer to the temporoparietal border.
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Individual analysis revealed possible functional differences between the subjects. For four subjects (boldface in Table 1), at least one hemifield stimulus evoked the right precentral cortex before 160 ms, and in two subjects after 210 ms; all six showed stronger responses during trials compared with the less attention demanding conditions. On the other hand, right inferior parietal lobule was active in eight subjects, but showed the strongest responses during trials in only four subjects (boldface in Table 1). Activation latencies in these four subjects were longer than 180 ms, and none of them showed precentral activation before 160 ms. The IPL and precentral responses were modulated in an interesting way in subjects with evoked activity in both of them. When the right precentral region was active later (after 210 ms, subject 1 RVF and subject 9 LVF), the right IPL responses during trials were enhanced, whereas when the right precentral region was active earlier (before 160 ms, both hemifields in subjects 2 and 7), the IPL responses during trials were not enhanced.
Figure 5 presents the dynamics of the right IPL and precentral activation in subject 1. The IPL source is transiently active around 200 ms, producing strongest signals during the trials. After this, the right precentral region reaches its maximum activity, and again the strongest signals are produced during trials. Although more right IPL than right FEF sources were found from individual data, a greater individual variability of IPL source locations might have hampered comparisons within the group average.
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Magnetoencephalography may miss activity if the current is radial to
the skull (Hämäläinen et al. 1993). To
exclude radial activation as an explanation to missing differences
between the experimental conditions, we recorded evoked potentials with
19 EEG channels simultaneously with the MEG recordings. Six subjects showed possible signal enhancement during trials between 100 and 250 ms
in their dorsal right hemisphere or midline electrodes. From these,
only one subject (subject 10, right visual field) had no
simultaneous enhancement in right hemisphere IPL or precentral regions
in his MEG recordings. Comparison of our MEG and EEG results suggests
that the signal enhancement during trials is generated mainly in the
fissural cortex, for which the MEG has the best sensitivity.
With two separate saccade experiments, we evaluated whether individual latency differences in MEG recordings are reflected in behavioral latencies. In the first task (Saccade) the subjects made saccades from fixation to a small peripheral luminance stimulus, appearing randomly in the left or right visual fields. In a second task (Attention + Saccade), subjects both had to indicate with finger lift transient luminance increments of the fixation point and make saccades to the peripheral luminance stimuli. Because the subjects with early right precentral enhancement (Frontal group, n = 4) showed the effect of attention on average ~60 ms earlier than the subjects with right IPL enhancement (Parietal group, n = 4) we presumed that saccade latencies should increase more in the Parietal group when subjects are attending the fixation. Figure 6 shows saccade latencies for the two tasks. During the Saccade-task, the mean (±SE) latencies were 354 ± 37 ms for the Frontal group and 309 ± 33 ms for the Parietal group. During the Attention + Saccade task the latencies were 390 ± 32 and 389 ± 58 ms, for the Frontal and Parietal groups. Thus attention to fixation seemed to extend saccade latencies on average 44 ms more in the parietal group, although this difference was not statistically significant (P = 0.057, 1-sided Wilcoxon rank-sum test).
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To specify the type of attention modulating the right precentral MEG response, we showed two subjects the visual stimuli of the main experiment (Fig. 1) during an auditory task. Subjects 2 and 7 participated in this study because they showed clear responses from both visual fields in the main experiment. Figure 7A shows activation within their right precentral ROI during the main experiment and Fig. 7B during the auditory attention control tasks. Subject 2 showed minor activation during auditory trials, but clearly less than during the original visual trials. During auditory intertrials no activation was present. She had, however, activation 2 cm posteromedially close to right central sulcus during both trials and intertrials. For subject 7, no precentral activation above noise emerged during this experiment.
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With a visuospatial control experiment we wanted to find out whether the precentral response is present during attention to peripheral stimuli. Subjects 2 and 7 fixated a point and attended one of eight possible stimulus locations at a time. When stimuli appeared at nonattended locations (Fig. 7C), minor activation emerged for both subjects. When stimuli were attended, subject 2 showed no activation, whereas subject 7 showed strong activation. To clarify this result we relocalized the precentral ROIs from the control data.
Figure 8A displays the estimate of the cortical current location projected on the surface of brain, separately for nontarget and target stimuli. Both stimuli evoke precentral responses, but for subject 2 the source for targets is 2 cm anterolaterally to the source for nontargets. For subject 7, the difference between the locations is ~1 cm. Figure 8B shows that both attended and nonattended stimuli evoke responses between 145 and 170 ms for subject 2 and between 110 and 140 ms for subject 7. For subject 2, the target ROI captures a signal also between 190 and 240 ms.
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In the main experiment, the proportion of epochs containing overt horizontal eye movements was equal during all experimental conditions. After the left hemifield stimuli, horizontal eye movements were detected in 2% of trial, 2% of intertrial, and 3% of passive viewing epochs. After right hemifield stimuli, the corresponding numbers were 1, 3, and 2%.
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DISCUSSION |
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The results show that focusing attention to a fixation point
enhances responses to nonattended peripheral stimuli in the right precentral cortex. The estimated location of the precentral source, between central sulcus and precentral gyrus, corresponds to the known
location of human frontal eye field (Paus 1996)
(Table 2). Activity of this area has been
detected in imaging studies of visuospatial attention (Kim et
al. 1999
; Nobre et al. 1997
; Vandenberghe et al. 1997
) and during saccades (Anderson et al.
1994
; Sweeney et al. 1996
). Its direct
stimulation evokes eye movements (Godoy et al. 1990
),
and patients with cerebral lesions in this area have defects in eye
movements (Rivaud et al. 1994
). Corbetta
(1998)
has shown that visuospatial attention and eye movements
activate at least partially overlapping cortical areas. Our control
studies support the view that visually evoked right FEF responses
depend on visual attention. During auditory attention the right FEF was rather silent, making motor preparation and general alertness unlikely
causes of its activation. When the visual stimuli were targets, as in
the second control study, they also evoked activation with the same
latency but perhaps somewhat anterior and lateral to nontarget
responses. Our results contrast results of a positron emission
tomography study (Corbetta et al. 1993
) that found
frontal signals close to FEF only to stimuli that were targets for an overt response. The precence of nontarget activation in our study may
be due to a different paradigm but also the fact that MEG is especially
sensitive to transient changes in the neural activation. On the other
hand, Steinmetz et al. (1994)
, whose paradigm we followed in the visuospatial control experiment, found in monkeys strongest IPL responses to nontarget stimuli. The increased sensitivity in the frontal or parietal areas might also reflect different requirements for the gaze control system during the different experimental conditions. However, all conditions contained a similar small number of overt horizontal saccades giving no support for a
simple increased inhibition-of-movement explanation.
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In conclusion, our findings suggest that the right FEF in humans may be
part of a system that during visual attention automatically becomes
sensitive to stimuli in all parts of the visual field. Functionally,
this might reflect scanning for new stimuli for redirection of
attention or eyes, a role Mountcastle et al. (1981, 1987
) suggested for the posterior parietal area 7a/PG in
monkeys. Monkey data support the scanning function of the FEF. Firing
rate in FEF neurons predicts the overt response to a peripheral visual stimulus (Thompson and Schall 1999
), and its lesions
result in neglect of the contralateral hemispace (Rizzolatti et
al. 1983
; van der Steen et al. 1986
). In humans,
right-hemisphere frontal cortex is important for attention to novel
stimuli. Its lesions may cause contralateral neglect (Heilman
and Valenstein 1972
; Maeshima et al. 1994
) and
prolonged attentional blink (Husain et al. 1997
).
The stimulus in this study was not optimal for a detailed study of
primary visual areas, but earlier neuromagnetic studies have shown peak
V1/V2 activation for luminance and pattern stimuli ~60-90 ms after
stimulus onset (Portin et al. 1998,
1999
). Although this study concentrated in the early FEF
activations (100-160 ms), the same area was active also later.
Previously, Nishitani et al. (1999)
have found
neuromagnetic FEF activation around 290 ms during an eye-finger pursuit
task. The beginning of our attention-sensitive FEF activation at 100 ms
is in line with monkey data showing that FEF and other dorsal stream
responses emerge relatively early during cortical activation
(Schmolesky et al. 1998
). Rafal and Robertson
(1995)
suggested that the FEF participates mainly in generation
of voluntary saccades and actually inhibits the superior colliculus,
which primarily guides the reflexive saccades.
We believe that the functional area producing the IPL responses in this
study is the human homologue to monkey 7a. The human IPL, Brodman areas
39 and 40, corresponds most likely to monkey IPL, areas 7a and 7b, also
named PG and PF (Grafton et al. 1996; Hyvärinen 1982
). In previous human visuospatial
attention studies, areas presumably homological to monkey 7a and LIP
were active (Corbetta 1998
; Nobre et al.
1997
). In monkeys 7a is lateral to LIP (Felleman and Van
Essen 1991
), as is our IPL source compared with some of the
earlier imaging data (Table 2). In our study, seven of eight subjects
showing right IPL responses displayed activation to both left and right
visual field stimuli. Whereas some monkey 7a cells respond to both
contra- and ipsilateral visual fields, the LIP cells respond only to
contralateral stimuli (Andersen et al. 1990
;
Blatt et al. 1990
; Motter and Mountcastle
1981
).
The LIP has more hierarchical ascending connections to FEF
(Andersen et al. 1990; Bullier et al.
1996
; Felleman and Van Essen 1991
) and responds
earlier to visual stimuli than the 7a (Barash et al.
1991
). Therefore LIP responses in our data should have preceded
the FEF responses, whereas we found IPL activity after FEF. In
conclusion, both the anatomic location inferior to the intraparietal
sulcus and the functional properties of our right IPL signals
correspond more to monkey 7a than LIP.
Our individual data show more right than left parietal responses to
stimuli in peripheral vision. This is in line with earlier imaging
(Corbetta et al. 1993; Kim et al. 1999
)
and patient studies (Posner et al. 1984
; Rafal
and Robertson 1995
) that show lateralization of parietal
activity during redirection of attention. Similarly, saccade latencies
for external cues are prolonged more after right than left angular
gyrus lesions (Pierrot-Deseilligny et al. 1991
).
In monkeys, focused visual attention enhances responses in 7a neurons
whose receptive fields are at the nonattended parts of the visual field
(Mountcastle et al. 1981, 1987
;
Steinmetz et al. 1994
). However, in our data different
individuals showed differences in their response patterns. A lack or
weakness of preceding right precentral responses seemed to be a
prerequisite for the right IPL sensitivity during visual attention.
Perhaps in humans novel peripheral stimuli during focused attention can alternatively be monitored by the right FEF or the right IPL, the
latter showing activation with longer latencies. Behavioral data
supported this assumption: attention to fixation tended to increase
saccade latencies more in subjects who showed the enhancements during
focused attention in their IPL area. Accordingly,
Pierrot-Deseilligny et al. (1986)
have suggested that
both the IPL and the FEF can control visually guided saccades. Although
individual division of labor between the right IPL and FEF cannot be
established by the present data only, this suggestion provides a
testable hypothesis for future research.
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ACKNOWLEDGMENTS |
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We thank R. Hari, S. Salenius, and T. Tanskanen for valuable comments on the manuscript. The MRIs were obtained at the Department of Radiology, Helsinki University Central Hospital.
This study was supported by the Academy of Finland and the Ministry of Education.
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FOOTNOTES |
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Address for reprint requests: S. Vanni, Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, PO Box 2200, FIN-02015 Espoo, Finland.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 9 July 1999; accepted in final form 5 January 2000.
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REFERENCES |
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