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1 University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, United Kingdom; 2 Department of Neurology, Johann Wolfgang Goethe University, Theodor-Stern-Kai 7, D-60590 Frankfurt, Germany; 3 Medical Research Council Spatial Disorientation Group, Academic Department of Neuro-Otology, Division of Neuroscience and Psychological Medicine, Imperial College London, Charing Cross Campus, London W6 8RF, United Kingdom
Submitted 2 October 2002; accepted in final form 29 March 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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;
Dichgans and Brandt 1978). In
particular, the visual and the vestibular sensory systems share various
processing stages in the CNS. Yet, the visual sense alone can underlie
self-motion perception when this self-motion is of a type where the input
provided by other spatial senses, such as the vestibular and proprioceptive
systems, is not informative. Accordingly, uniform and sustained motion of the
visual environment, resembling optic flow patterns produced by locomotion, is
a powerful stimulus to induce the illusory perception of contra-directional
self-motion, or vection (Dichgans and
Brandt 1978). This can frequently be experienced in motion
simulators and when seated in a stationary railway carriage where the sight of
a moving train elicits the perception of self-motion. During sustained
optokinetic stimulation around the sagittal, or x-axis, the
perception of circularvection does not typically continue uninterruptedly but
is spontaneously interspersed by periods of perceived object-motion and
concurrent self-stationarity (Cheung and
Howard 1991; Finke and Held
1978; Thilo et al.
1999).
A number of electrophysiological studies investigating vestibular function
and visual-vestibular interaction by means of evoked potentials have been
conducted in the past using optokinetic stimulation
(Hood 1983;
Mergner et al. 1989),
vestibular stimulation on earth (Hood
1983; Hood and Kayan
1985; Mergner et al.
1989; Probst and Wist
1990; Probst et al.
1995,
1997), and during microgravity
induced by parabolic flight (Probst et al.
1996). These have identified characteristic scalp responses to
optokinetic and rotational vestibular stimulation as well as a modulation of
evoked potential parameters by head orientation relative to the
gravitoinertial vector. More recently, the neural responses to rotatory
optokinetic stimulation (Brandt et al.
1998) and the correlates of visually induced self-motion
perception (Kleinschmidt et al.
2002) have been investigated using functional neuroimaging. In
this latter study, constant optokinetic stimulation around the sagittal axis
intermittently elicited the perception of self-rotation, and we contrasted
activity levels between the perception of object-motion and that of
self-motion during identical sensory input. We found a deactivation in
parieto-insular vestibular cortex (PIVC;
Bottini et al. 2001;
Fasold et al. 2002;
Grüsser et al. 1990;
Guldin and Grüsser 1998)
but also in striate and early extrastriate cortex during the perception of
self-motion as compared with perceived object-motion. In the anterior part of
lateral occipito-temporal cortex (human motion complex) and in dorsomedial
parieto-occipital cortex (putative V6/PO), however, activity levels during
vection remained unchanged compared with the perception of the same sensory
input as motion in the environment.
In the present study we used a VEP paradigm to further characterize the functional behavior of early occipital cortex during the two contrasting perceptual states induced by optokinetic stimulation. In comparison to the electrophysiological and neuroimaging studies mentioned above, the novelty of this approach was that we studied the influence of the percept evoked by large-field peripheral sensory input on the processing of a central stimulus. In other words, we tested whether the response to continuous pattern-reversal in the central visual field would be affected by the perceptual bistability of simultaneous and sustained optokinetic stimulation in the frontal plane, subtending a large portion of the peripheral visual field (Fig. 1A). This peripheral sensory input is perceptually ambiguous and results in spontaneous alternations between the subjective percepts of object-motion or self-motion. The electrophysiological responses to the probe stimulus in the central visual field were grouped according to whether they were obtained during the perceived periods of object-motion or those of self-motion. Based on our earlier findings, we hypothesized that during vection responses to the visual stimulus should be attenuated. Thus this study investigated whether the bistable perceptual interpretation of a moving visual environment forms a context-dependent influence on early cortical processing of simultaneously present, but unrelated, visual input.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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Five women and three men, from 24 to 29 yr of age (mean 26.0 yr), without any history of relevant neurological or sensory disease, participated in the experiment. All had normal or corrected-to-normal vision. Informed consent had been obtained from all subjects prior to the start of the experiment.
Apparatus
The optokinetic stimulus consisted of an alternating pattern of black and white radial sectors, spaced at equal intervals, that was back-projected onto a screen subtending 110° of visual angle horizontally and 110° vertically, refreshing at a rate of 60 Hz. The stimulus rotated around an earth-horizontal axis that was aligned with the subjects' line of sight. The white and black stimulus elements had luminances of 0.11 and 0.08 cd/m2, respectively. The checkerboard-reversal stimulus was presented on an LCD monitor, 287 mm wide and 215 mm high, that was positioned approximately 2 cm in front of the optokinetic stimulus and centered with respect to the subjects' line of sight. The checkerboard pattern consisted of 32 vertical columns and 24 horizontal rows of alternating black and white squares that inverted continuously every 750 ms and subtended approximately 45' of visual angle. At its center was a medium gray circular fixation target with a diameter a quarter of the side length of a square. Figure 1A gives a schematic overview of the experimental setup.
Electrophysiological recording
Following skin preparation, 8 mm silver/silver chloride electrodes were
fixed to the scalp according to the 10–20 system
(Jasper 1958) at positions
OZ (occipital midline), O1 (occipital left),
O2 (occipital right), and FZ (frontal midline) using
adhesive conductive electrode paste. Additional electrodes were attached to
each earlobe and connected together as inactive reference. Impedances between
pairs of electrodes were below 5 k
. Differential recordings were
obtained between the electrode pairs O1-FZ,
OZ-FZ, O2-FZ, and
FZ-earlobes at a gain of 50 µV/V, a high-cut frequency of 30
kHz, and a time constant of 500 ms. As a control of fixation and eye blinks,
cyclopean horizontal and right monocular vertical DC-electrooculogram was
recorded. Sweeps were sampled at a rate of 1 kHz for a duration of 300 ms
commencing 50 ms prior to pattern reversal.
Procedure
Subjects were seated at a distance of 68 cm from the fixation target, and thus approximately 70 cm from the background optokinetic display, with their heads supported by a chin rest. They were instructed to look at the fixation target and to avoid eye blinks and other sources of artifacts. Sessions began with the recording of 200 pattern-reversal sweeps while the optokinetic stimulus was stationary. After a break, 600 sweeps were recorded with the optokinetic stimulus revolving clockwise at a velocity of 45°/s. For the duration of this trial, subjects signaled the perception of object-motion versus that of rollvection by switching a hand-held dial between two predefined settings. See Fig. 1B for an example of the time-course of an experimental session.
Bistable percepts of ambiguous visual stimuli are not always mutually
exclusive and sometimes both perceptual interpretations can coexist
simultaneously, especially during transition periods
(Blake and Logothetis 2001).
During optokinetic stimulation, the perception of self-motion often develops
gradually with a simultaneously perceived slowing of environment- or
object-motion (Wertheim 1994).
Despite this gradual build-up, subjects were required to decide in a binary,
or all-or-nothing, manner whether they perceived the optokinetic stimulation
as originating from environment or self-motion according to the perceived
prevalent dominance. This was done to ensure a simple and intuitive task as
well as to permit statistical analysis of the reported perceptual states as a
categorical variable, consistent with our recent work on bistable percepts
across sensory modalities (e.g., Thilo and
Gresty 2002) as well as with studies on purely visual ambiguities
and binocular rivalry (e.g., Leopold et
al. 2002).
Data analysis
All recordings were analyzed and processed off-line after data acquisition had terminated. Sweeps containing artifacts, mainly caused by eye blinks, were excluded following visual inspection of the raw data. Subsequently, the five sweeps preceding as well as following each transition between perceptual states were excluded to avoid contamination of the signal by components related to transitional states and their reporting, which involved a motor response. Following this, condition-specific evoked potentials were computed by averaging trials for two subsequent levels of analysis. At the first level of analysis, we addressed whether the presence of constant peripheral sensory input per se affected the VEP in response to central pattern reversal. Accordingly, we averaged the sweeps that were recorded while the optokinetic stimulus was stationary (VEP-STAT) and the sweeps sampled during optokinetic stimulation (VEP-OKS), irrespective of the perceptual state reported by the subjects. At the second (embedded) level of analysis, we addressed whether the perceptual state that this peripheral sensory input resulted in would affect the VEP. Accordingly, the entirety of sweeps recorded during optokinetic stimulation was split into two separate groups as a function of the concurrent perceptual states experienced by the subjects. We then averaged the sweeps that were recorded while subjects reported the perception of object-motion (VEP-OM) and compared them to the average of the sweeps during which circularvection was perceived (VEP-SM). In other words, VEP-OKS was split up into VEP-OM and VEP-SM.
Following three-point smoothing, baseline was calculated as the average signal obtained during the 50 ms preceding pattern reversal. The first negative inflection (N70) and the first positive (P100) inflection of the signals were determined automatically. Latencies relative to pattern-reversal and amplitudes relative to baseline were computed for each subject and conditioned as defined above.
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RESULTS |
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Following optokinetic stimulation onset, all subjects reported perceiving rollvection with an average onset latency of 11.0 s (SE, 4.8 s). During the 450 s of optokinetic stimulation, subjects perceived circularvection for, on average, 215.6 s (SE, 16.3 s).
Evoked potentials
After rejections of sweeps containing artifacts and of those that were acquired close to perceptual transitions, the individual averages obtained were computed from 152 to 355 single sweeps. A detailed list containing mean onset latencies of the VEP N70 and P100 components during moving versus stationary peripheral optokinetic stimulation and during the perception of object-motion versus circularvection for the different electrode positions is presented in Table 1. The corresponding baseline-to-peak amplitudes are listed in Table 2. It should also be noted that in the two-way repeated measures analyses of variance (ANOVAs) presented subsequently no interaction reached significance unless stated otherwise. See Fig. 2 for a sample series of evoked potentials obtained in a single subject at different electrode positions, stimulation conditions, and perceptual dominances.
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Peripheral visual field stationarity versus optokinetic stimulation—N70
AMPLITUDE. The baseline-to-peak amplitudes of the first negative inflection of the VEP were not significantly affected by rotation of the optokinetic stimulus as demonstrated by a two-way repeated measures ANOVA. Averaged across occipital electrode positions, mean amplitudes were –3.8 µV during stationarity and –3.4 µV at a rotating background (F1,7 < 1). A significant main effect of electrode position (O1: –3.2 µV; OZ: –5.2 µV; O2: –2.3 µV; F2,14 = 5.7; P < 0.05) was found and demonstrated ex post to be due to a significant quadratic contrast (F1,7 = 7.1; P < 0.05). Figure 3 depicts the average amplitudes of the VEP N70 component across all subjects and electrode positions.
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PEAK LATENCY. A similar, nonsignificant, result was obtained for the corresponding peak latencies with values of 62.0 ms during stationary trials and 61.8 ms during optokinetic stimulation averaged across electrode positions (F1,7 < 1, 2-way repeated measures ANOVA). There was a significant main effect of electrode position (F2,14 = 4.1; P < 0.05) on onset latency with a latency of 58.4 ms at O1, 67.1 ms at OZ, and 60.2 ms at O2. This main effect is explained by the increased latency at the midline electrode position as demonstrated by a significant post-hoc quadratic contrast (F1,7 = 10.3; P < 0.05).
Peripheral visual field stationarity versus optokinetic stimulation—P100
AMPLITUDE. Motion of the visual surround had no significant effect on baseline-to-peak amplitudes which were, on average, 11.6 µV without and 11.0 µV with optokinetic stimulation (F1,7 = <1). There was, however, a significant effect of electrode position on P100 amplitudes, which were 9.6 µV at O1, 12.4 µV at OZ, and 12.0 µV at O2 (F2,14 = 6.3; P < 0.05).
PEAK LATENCY. Mean peak onset latencies for the first positive inflection were, averaged across sites, 97.2 ms without and 95.8 ms with optokinetic stimulation (F1,7 = 1.1; n.s.). Electrode position also had no significant effect on onset latencies (O1: 96.8 ms; OZ: 95.3 ms; O2: 97.3 ms; F2,14 = 1.6; n.s.).
Perception of object-motion versus circularvection—N70
AMPLITUDE. During perception of object-motion, baseline-to peak amplitude of the N70 component was –3.8 µV, whereas during circularvection the inflection was –3.0 µV and therefore significantly lower as demonstrated by ANOVA (F1,7 = 7.6; P < 0.05). A main effect of recording site also was significant (F2,14 = 5.0; P < 0.05) with corresponding values being –3.0 µV (O1), –5.0 µV (OZ), and –2.3 µV (O2). Figure 3 shows the group means of the N70 amplitudes across perceptual states and electrode sites.
PEAK LATENCY. The perceptual state subjects experienced while their brain activity was recorded had a significant effect on the N70 peak latency. During reported object-motion, mean latency was 63.3 ms and during circularvection it was marginally reduced to 62.8 ms (F1,7 = 6.0; P < 0.05). Although statistically significant, there are no conceivable functionally relevant implications of an average latency difference of 0.5 ms across subjects, especially when considering that signals were recorded at a sample rate of 1 kHz. Averaged over perceptual states, mean latencies obtained at the different electrode positions were 63.1 ms at O1, 65.9 ms at OZ, and 60.0 ms at O2. There was also a significant main effect of recording site (F2,14 = 3.84; P < 0.05) with a significant quadratic post-hoc contrast (F1,7 = 12.1; P < 0.05).
Perception of object-motion versus circularvection—P100
AMPLITUDE. During perception of object-motion, average P100 amplitude was 11.0 and 11.2 µV during vection with no main effect of perceptual state detected by ANOVA (F1,7 < 1). Electrode position had a highly significant effect with amplitudes being 9.3 µV at O1, 12.3 µV at OZ, and 11.7 µV at O2 (F2,14 = 7.2; P < 0.01).
PEAK LATENCY. No effect of perceptual state (OM: 95.7 ms; SM: 95.4 ms; F1,7 < 1) nor of electrode position (O1: 95.5 ms; O2: 95.1 ms;O3: 96.0 ms; F2,14 < 1) on peak latency of the first positive inflection was detected.
Contribution of the prefrontal electrode site
Since all results described so far were obtained as differential recording between an occipital electrode site (OZ, O1, O2) and another electrode placed medially over the prefrontal cortex (FZ), any potential recorded is reflecting an electrical dipole between occipital and frontal regions. To determine to what extent the results observed were caused by changes in electrical activity underlying the prefrontal electrode, amplitudes and latencies of the N70 and P100 peaks were analyzed for the differential recording between the prefrontal electrode position and the linked earlobe electrodes, serving as inactive reference. Therefore paired t-tests, comparing stationarity versus optokinetic stimulation and perception of object-motion versus circularvection, were computed for the latencies and amplitudes of the N70 and P100 components separately. No comparison demonstrated a significant effect of optokinetic stimulation or perceptual state on the frontal electrode site (all P > 0.05).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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The neuronal generators of the N70 and P100 components of the
pattern-reversal VEP are likely to be located in primary visual cortex
(Brodmann area 17). In patients with lesions restricted to striate cortex,
these components are generally absent or abnormal
(Aldrich et al. 1987), whereas
in patients with lesions in visual association cortex they are preserved
(Bodis-Wollner et al. 1977).
The two components seem functionally dissociated since patients with a
preserved N70 and an abnormal P100 have been reported (Celesia et al.
1980,
1982). Intracortical
recordings in monkeys have demonstrated that the N70 component reflects
excitatory postsynaptic potentials of stellate cells in primary visual cortex
layer 4C (Schroeder et al.
1991). These cells receive input from primary thalamic afferents,
from other striate cortical layers, and from extrastriate cortex. The ensuing
P100 component presumably reflects inhibitory postsynaptic potentials of
pyramidal neurons in layers 2 and 3
(Schroeder et al. 1991),
possibly mediated by GABAergic transmission
(Halgren 1990;
Zemon et al. 1986). At this
level of specification it is not possible to distinguish the relative
contributions of the magnocellular and parvocellular pathways to the N70
component since geniculofugal fibers of both systems terminate in striate
cortex layer 4C—the magnocellular pathway in layer 4C
and the
parvocellular pathway in layer 4C
(Lamme et al. 1998).
From a physiological perspective, the finding of reduced net excitation in
primary visual cortex during the perception of circularvection fits well with
the observation of a deactivation in calcarine cortex during vection as
demonstrated in our previous experiment using fMRI. It has recently been
established that the BOLD fMRI response of monkey visual cortex is best
accounted for by transient local field potentials, which suggests that the
fMRI signal is a probable marker of input to, and intrinsic synaptic activity
in, a given neuronal population
(Logothetis et al. 2001).
It has been well established that parieto-insular vestibular cortex
undergoes deactivation during visually induced self-motion sensation
(Brandt et al. 1998;
Kleinschmidt et al. 2002). The
functional interpretation of this reduction of vestibular cortical sensitivity
has been that it could protect against irrelevant and presumably detrimental
vestibular input from involuntary head accelerations and thereby improve
computation of self-motion from vision alone
(Brandt et al. 1998;
Dieterich and Brandt 2000).
Our previous results with functional neuroimaging and the results presented
here suggest that this general notion of reduced susceptibility to
"noise" input during self-motion might also apply to processes
within the visual modality. To maintain an uncontaminated perception of
self-motion, the sensorial weight might be shifted away from visual processing
of distracting visual motion signals.
Another functionally important adjustment during optokinetic stimulation is
the generation of eye movements. Rotation of the visual environment in the
frontal plane elicits involuntary and reflexive torsional optokinetic eye
movements (Brecher 1934;
Morrow and Sharpe 1993).
Although our introduction of a stationary reference in the central visual
field would have dampened the gain and amplitude of torsional eye movements to
some degree, the major drive for optokinetic torsion stems from the retinal
periphery and a large proportion of nystagmus would have been preserved
(Suzuki et al. 2000;
Wade et al. 1991). Torsional
optokinetic nystagmus is performed at a gain far too low to enable efficient
slow-phase pursuit during visual field rotation and thus attenuates retinal
motion stimulation only by about 10%. In previous studies, we have
demonstrated that optokinetic nystagmus performs systematic changes when
observers switch between perceptual states. During vection as opposed to
perceived object-motion, mean eye position deviates more in an anticipatory
direction (Thilo et al. 2000,
2002) and, in the case of
torsional nystagmus, slow phase gain is enhanced, which necessarily also
corresponds to an increase in the velocity, amplitude, or frequency of
(saccadic) nystagmus fast phases (Thilo et
al. 1999). Hence, our previous observations raise the question
whether the changes in neural activity observed here might be not directly
related to the differential perception of visual motion but be attributable to
percept-dependent adjustments in oculomotor behavior and the ensuing changes
in retinal image slip. The latter source of artifact, i.e., a change in
retinal stimulation, can be ruled out because optokinetic stimulation was
constant and would not have been reflected in VEPs averaged to the reversals
of the central checkerboard pattern. Furthermore, the fact that pattern
reversals occurred during eye movements would, if anything, result in greater
visual stimulation by virtue of added retinal slip and not be expected to
reduce the sensory cortical response.
Percept-related differences in eye movements are equally unlikely to
account for the neural response changes found, since although there was a
significant reduction of negativity in the evoked potential during
circularvection in comparison to the perception of object-motion, no such
effect was found when the response during optokinetic stimulation was compared
with that obtained during a stationary peripheral visual field. Irrespective
of the resulting percept, rotation of the stimulus induces torsional
optokinetic nystagmus and therefore creates a fundamental change in oculomotor
behavior that may exert a suppressive influence on sensory processing in
visual areas (Gallant et al.
1998). In our experiment, the difference in torsional nystagmus
was much greater when comparing the responses during optokinetic versus
stationary background (nystagmus present vs. absent) than when comparing those
during the two different perceptions of optokinetic stimulation in the
background (nystagmus gain increase of 20–40% during vection). Therefore
the subtle changes of torsional nystagmus that accompany the alternations
between object- and self-motion cannot account for the differences in the VEPs
that were recorded during these two perceptual states. In summary, these
considerations underline the finding that the differential activation patterns
observed here as well as using fMRI
(Kleinschmidt et al. 2002)
cannot be explained by the changes in torsional optokinetic nystagmus, which
is facilitated during circularvection, nor by the resulting changes in retinal
image slip.
The claim that the VEP changes observed in response to central stimulation
are not caused by a change in retinal stimulus does not contradict the fact
that the N70 component is generated by excitation of target neurons of the
optic radiation. A large body of evidence illustrates the influence of
feedback projections onto primary visual cortex neurons that originate in
other layers of striate cortex as well as in extrastriate visual and higher
order areas as parietal cortex (Lamme et
al. 1998) mediating, among others, attentional phenomena
(Ashbridge et al. 1997).
Although likely to be involved in the evoked potential changes observed,
feedback projections from higher cortical areas are not necessarily the only
mechanism involved. Attentional effects have also been shown to be mediated by
feedforward gating through the thalamus, presumably relayed via the thalamic
reticular nucleus (TRN) (Guillery and
Sherman 2002; Guillery et al.
1998). Furthermore, the lateral geniculate nucleus (LGN) of the
thalamus could itself be involved in a possible modulatory influence since it
not only receives input from retinal afferents but also from striate cortex
and subcortical sources such as the TRN and the superior colliculus (SC). All
of these would favor the LGN as a candidate early site for the action of
top-down influences on visual information processing. In support of this
proposition, covert spatial attention has recently been shown to modulate fMRI
responses in the human LGN (O'Connor et
al. 2002) and it has been hypothesized that the LGN is a likely
site of saccadic suppression, the reduction of mainly magnocellular components
of visual sensitivity during rapid gaze shifts
(Burr et al. 1994; Ross et al.
1996,
2001). However, further
specific studies would be needed to elucidate conceivable analogies between
our observation and the phenomenon of saccadic suppression.
Whatever the precise routing of the effect we observed for the N70
component, our experimental result is likely to be evidence for a
context-dependent modulation (self- or object-motion) of early visual cortical
processing of sensory input. Such mechanisms seem to follow a general
principle and are not confined to specific functional settings. For instance,
Rees et al. (1997) have shown
that the attentional demand, or load, of linguistic processing of visual
stimuli presented in the central visual field influences the degree to which
distracting irrelevant motion stimulation in the peripheral visual field
translates into activity in the human motion complex (hMT+) at the
occipito-temporal junction. When cognitive load by the task was high, the same
visual motion input yielded less hMT+ activation than when the load was low.
The roles of central and peripheral visual input were reversed in our
experiment as we studied the modulation of constant central input as a
function of the bistable perceptual interpretation of an equally constant
peripheral input. Yet, the functional interpretation may also be related to
attention in that the processing of visual information for the purpose of
reconstructing our ego-motion presumably enjoys high attentional priority,
thus suppressing the resources otherwise dedicated to processing of items in
the visual scene.
In conclusion, the finding that early excitation in primary visual cortex is reduced during perceived self-motion (vection) provides electrophysiological evidence that is in accordance with our functional neuroimaging experiment in which occipital cortex was less activated during the perception of self-motion than during the perception of object-motion when subjects viewed constant rotary optokinetic stimulation. This may reflect a top-down redistribution of attentional resources, indicating their active recruitment by visual processing of self-motion and the concomitant suppression of processing of other sources of irrelevant visual input in the environment.
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DISCLOSURES |
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FOOTNOTES |
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Address for reprint requests: K. V. Thilo, Univ. Laboratory of Physiology, Univ. of Oxford, Parks Road, Oxford OX1 3PT, UK (E-mail: kai.thilo{at}physiol.ox.ac.uk).
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