Traveling gamma-Waves: New Insights into Coupling Processes in Visual Texture Coding
Recent experiments in awake rhesus monkeys supported the binding-by-synchronization hypothesis for neural object coding in visual cortex. They demonstrated local synchrony and perceptual modulation of rhythmic or stochastic gamma-activities (30-90 Hz), according to the rules of figure-ground seg...
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Format: | Doctoral Thesis Dataset |
Language: | English |
Published: |
Philipps-Universität Marburg
2004
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Online Access: | PDF Full Text View Record |
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Summary: | Recent experiments in awake rhesus monkeys supported the
binding-by-synchronization hypothesis for neural object coding in
visual cortex. They demonstrated local synchrony and perceptual
modulation of rhythmic or stochastic gamma-activities (30-90 Hz), according
to the rules of figure-ground segregation. But they have also shown
that the spatial extent of gamma-synchrony is restricted to few millimeters
of cortical surface. Hence, the range of gamma-synchronization in
retinotopically organized visual cortex can only cover small regions
of the visual field, challenging the synchronization hypothesis for
larger cortical object representations.
Apart from synchrony, traveling waves of neural activity were found in
a variety of neural structures, frequency ranges and animal
species. Observations in this area are mostly described
qualitatively. Suitable analysis methods were probably not known and
therefore, quantitative examinations and statistical analysis of
traveling waves in conjunction with perceptual binding were not
successfully carried out. The lack of appropriate analysis methods is
remarkable because traveling waves do not just represent another
interesting type of signal coupling. They also include the state of
synchrony as a special case, commonly called standing wave. In this
sense, traveling waves represent a generalized form of spatio-temporal
coupling. Since the preconditions for traveling waves are less
restrictive than for synchrony, they may cover larger cortical regions
and, thereby, help to overcome the spatial limitations of synchrony
with respect to the binding hypothesis. From this point of view, it is
of interest to clarify whether neural signals of gamma-activity in
monkey primary visual cortex (V1) also contain traveling
gamma-waves.
In order to test this hypothesis, a new single-trial analysis method
for multi-channel recordings was developed, called spatio-temporal
correlation (STC). It is capable of detecting and quantifying
traveling waves. The reliability and validity of the STC-method as
well as the significance of its results was tested on artificial and
experimental data in comparison with conventional pairwise correlation
methods that are typically used for synchrony detection
(cross-correlation, coherence). While conventional methods fail to
uncover phase coupling of waves that vary their propagation direction
over time, the STC-method reveals a high accuracy of traveling wave
detection. Fast changes and a high variability of the waves'
propagation direction lead to a destructive superposition in the
temporal and trial averaging process of the conventional methods.
The reinvestigation of data from previous and new experiments of our
group using the STC-method demonstrated that traveling waves often
occur in the gamma-frequency band of local field potential (LFP) in
monkey V1. Moreover, they show strong stimulus-dependency. Their
velocities (peaking at 0.3 m/s) are similar to those of spike
conduction of horizontal connections measured in V1. In particular, it
can be shown that gamma-waves are most likely carried by neurons
representing local features of the object surface, and that their
coupling range (half-height decline about 9.5mm) exceeds the coupling
range of gamma-synchronization (half-height decline about 3mm) by
far. These results demonstrate that gamma-synchrony in V1 is a local
manifestation of large-scale traveling waves.
The coupling dynamics of traveling waves depend strongly on the
continuity of the object s surface. This means that phase continuity
of gamma-waves exists either inside or outside of the cortical
representation of an object, but cannot cross its boundaries. This
decoupling effect of gamma-waves was approximately stronger by a
factor of two than that of gamma-synchrony across the object
boundaries. Thus, all of the above observations indicate that
traveling gamma-waves reflect the functional coupling of neurons
representing the local features of an object's surface, according to
the Gestalt laws of perceptual binding.
In conclusion, these findings strongly suggest that phase continuity
of traveling gamma-waves supports the coding of object continuity as
an extension to the previously mentioned binding-by-synchronization
hypothesis. |
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Physical Description: | 121 Pages |
DOI: | 10.17192/z2004.0645 |