Biomedical subjects

Andreas Gabriel

Publications and source records attributed to Andreas Gabriel.

3 recordsLinked to original sources

PubMed2004

Different types of signal coupling in the visual cortex related to neural mechanisms of associative processing and perception.

The hypothesis of object representation by synchronization in the visual cortex has been supported by our recent experiments in monkeys. They demonstrated local synchrony among gamma activities (30-90 Hz) and their perceptual modulation, according to the rules of figure-ground segregation. However, gamma-synchrony in primary visual cortex is restricted to few mm, challenging the synchronization hypothesis for larger cortical object representations. The restriction is due to randomly changing phase relations among locally synchronized patches which, however, form continuous waves of gamma-activity, traveling across object representations. The phase continuity of these waves may support coding of object continuity. Interactions across still larger distances, measured among cortical areas in human data, involve amplitude envelopes of gamma signals. Based on models with spiking neurons we discuss potentially underlying mechanisms. Most important for gamma synchronization are local facilitatory connections with distance-dependent delays. They also explain the occurrence of gamma waves and the restriction of gamma-synchrony. Fast local feedback inhibition generates gamma oscillations and supports local synchrony, while slow shunting inhibitory feedback supports figure-ground segregation. Finally, dispersion in inter-areal far projections destroys coherence of gamma signals, but preserves their amplitude modulations. In conclusion, we propose that the hypothesis of associative processing by gamma synchronization be extended to more general forms of signal coupling.

Action Potentials↗

PubMed2004

Neural mechanisms of visual associative processing.

This is a review of our work on multiple microelectrode recordings from the visual cortex of monkeys and subdural recordings from humans--related to the potential underlying neural mechanisms. The former hypothesis of object representation by synchronization in visual cortex (or more generally: of flexible associative processing) has been supported by our recent experiments in monkeys. They demonstrated local synchrony among rhythmic or stochastic gamma-activities (30-90 Hz) and perceptual modulation, according to the rules of figure-ground segregation. However, gamma-synchrony in primary visual cortex is restricted to few millimeters, challenging the synchronization hypothesis for larger cortical object representations. We found that the spatial restriction is due to gamma-waves, traveling in random directions across the object representations. It will be argued that phase continuity of these waves can support the coding of object continuity. Based on models with spiking neurons, potentially underlying neural mechanisms are proposed: (i) Fast inhibitory feedback loops can generate locally synchronized gamma-activities; (ii) Hebbian learning of lateral and feed forward connections with distance-dependent delays can explain the stabilization of cortical retinotopy, the limited size of synchronization, the occurrence of gamma-waves, and the larger receptive fields at successive levels; (iii) slow inhibitory feedback can support figure-ground segregation; (iv) temporal dispersion in far projections destroys coherence of fast signals but preserves slow amplitude modulations. In conclusion, it is proposed that the hypothesis of flexible associative processing by gamma-synchronization, including coherent representations of visual objects, has to be extended to more general forms of signal coupling.

Animals↗

PubMed2003

A multi-channel correlation method detects traveling gamma-waves in monkey visual cortex.

Correlations among simultaneously recorded signals are mostly analyzed pairwise and include temporal averaging. However, pairwise methods are not suitable for characterizing relationships among multiple channels for signals which vary temporally in an unpredictable way. Here we develop a time-resolved spatio-temporal correlation (STC) measure among simultaneously recorded signals. We demonstrate the capabilities of the method with artificial data sets and with multiple-channel recordings from striate cortex of awake monkeys. We concentrate on correlations in the gamma-frequency range (gamma: 30-90 Hz) because they were prominent in the analyzed recordings and gained high interest in the recent years due to their assumed role in associative processing, including perceptual binding. Former analyses of gamma-activities in visual cortex, using pairwise correlation methods, mostly revealed zero-delay correlation, indicating synchrony. In cat and monkey visual cortex this gamma-synchrony is restricted to 1.5-3.0 mm (half-height decline). However, our spatio-temporal correlation (STC)-method demonstrates for striate cortex from awake monkeys that gamma-synchrony is a local phenomenon of more global traveling plane waves that appear stimulus-induced at randomly varying orientations. These gamma-waves are coupled over much larger cortical distances (approximately 7 mm half-height decline) than the gamma-synchrony ranges obtained by pairwise correlation analyses from the same data. Our STC-method therefore suggests that the previously reported results of short-range and zero-delay correlations were often due to temporal averaging of traveling gamma-waves.

Animals↗