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Zusammenfassung:
When dissimilar images are presented to the two eyes, perception starts alternating spontaneously
between each monocular view, a phenomenon called binocular rivalry (Leopold and
Logothetis, 1999). Several imaging studies in humans have shown the involvement of a frontoparietal
network of cortical areas in perceptual transitions during binocular rivalry (Lumer
et al., 1998). Here we investigate the possible role of parietal visual areas in perceptual alternations
during rivalry in the rhesus macaque. Neural activity in the lateral intraparietal area
(LIP) was recorded extracellularly while the subject was presented dichoptically and asynchronously
with two rivalrous patterns, resulting in flash suppression (Keliris et al., 2010).
The paradigm ensures excellent control over the subjectâs perceptual state. Preliminary results
confirm the transient change of brain activity around perceptual reversals at the single
cell level. The recorded cells typically showed an initial burst of activity after the onset of a
stimulus as well as at stimulus/perceptual changes, followed by a sustained response (Bisley,
2004). The transient response of recorded neurons has a short latency, lasts a few hundred
milliseconds and is always positive while the sustained response is suppressive in some cells
and excitatory in others. We speculate that these responses may reflect two separate underlying
processes. The short latency response may reflect a fast sensory signal conveying
the information in a bottom-up manner, while the sustained activity may represent top-down
influences originating from higher areas in the prefrontal cortex. The functional magnetic resonance
imaging (fMRI) studies performed previously could not dissociate these two tightly
overlapping signals because of the poor temporal resolution of the technique. Analysis of the
firing rates of single and multi-units indicate that the transient part of the response predicts
well the change in perception while the sustained activity does not show a significant correlation
with perceptual state. This might be explained by the little selectivity of the sustained
response of parietal neurons towards particular stimuli (Lehky and Sereno, 2007). It is believed
that LIP neurons provide a representational map of saliency, integrating bottom-up
and top-down information to guide the allocation of spatial attention (Bisley et al., 2011). We
argue that the transient response of LIP neurons after perceptual switches is an indication
for a role of this region in providing a change signal to higher areas. It is possible, that the
intraparietal activation observed in humans around perceptual transitions may simply reflect
the elevation of neural activity as a result of a novel percept rather than a causal role of the
region in driving the switches. We are therefore planning to extend the binocular flash suppression
paradigm to normal binocular rivalry and monitor the activity around spontaneous
perceptual alternations in order to delineate what happens without any concomitant physical
change in the stimulus. Furthermore, local field potentials, temporal dynamics of single
unit activity and synchronization between neurons might provide a better understanding of
the top-down influences of prefrontal cortex especially during the sustained response. This
analysis is currently in progress.