Biomedical subjects

A Frien

Publications and source records attributed to A Frien.

4 recordsLinked to original sources

PubMed2000

Fast oscillations display sharper orientation tuning than slower components of the same recordings in striate cortex of the awake monkey.

We wanted to know whether fast oscillations ( approximately 30-80 Hz) in striate cortex of awake monkeys show sharper orientation selectivity than (i) slower components, including spike rate modulations, and (ii) broad-band signals of the same recordings. As fast oscillations are probably of cortical origin this may further clarify whether cortical network mechanisms are substantially involved in generating orientation selectivity. We recorded multi unit activity (MUA) and local field potentials (LFP, 1-140 Hz) by the same microelectrodes from upper layers of macaque striate cortex during visual stimulation with grating textures of different orientations. An orientation index (OI) was derived from the cortical responses in three frequency ranges (low, 0-11.7 Hz; medium, 11.7-31.3 Hz; and fast oscillations, 31.3-62.5 Hz) and for the broad-band LFP and MUA power. (i) Both LFP and MUA fast oscillations reveal a higher orientation index than signal components in the low and medium frequency ranges. (ii) For MUA the orientation index was significantly higher with fast oscillations than for the lower frequency ranges and the initial broad-band transient responses. (iii) LFPs show a significantly higher orientation index only for the fast oscillations during sustained activation compared with their broad-band power during the transient responses. Thus, our main result is the sharper orientation tuning of fast oscillations in spike activities of local populations compared with slower components of the same broad-band recordings. As fast oscillations occur synchronized in the awake monkey's striate cortex we assume that they have enhanced probability of activating successive stages of visual processing and hence contribute to the perception of orientation.

Animals↗

PubMed2000

Functional coupling shows stronger stimulus dependency for fast oscillations than for low-frequency components in striate cortex of awake monkey.

It has been argued that coupling among the neural signals activated by a visual object supports binding of local features into a coherent object perception. During visual stimulation by a grating texture we studied functional coupling by calculating spectral coherence among pairs of signals recorded in the striate cortex of awake monkeys. Multiple unit activity (MUA) and local field potentials (LFP, 1-140 Hz) were extracted from seven parallel broad band recordings. Spectral coherence was dominated by high-frequency oscillations in the range 35-50 Hz and often by additional low-frequency components (0-12 Hz). Functional coupling among separate cortical sites was more stimulus specific for MUA than for LFP: MUA coherence at high and low frequencies depended highly significantly on: (i) the similarity of the preferred orientations at the two sites - the more similar the higher the coherence; (ii) the orientation of the stimulus grating - with highest coherence at half angle between the preferred orientations at the two sites; (iii) cortical distance - coherence decreases to noise levels at approximately 3 mm (MUA) and 6 mm (LFP). Coherence of fast oscillations did not depend on the degree of coaxiality of the orientation-sensitive receptive fields, whereas low frequencies showed significant dependency. This indicates that different frequency components can engage different coupling networks in the striate cortex which probably support different coding tasks. Changes in average oscillation frequency with stimulus orientation were highly significant for fast oscillations while there was no dependency for low frequencies. Finally, stimulus-related spectral power and coherence of fast oscillations were considerably higher than of low frequency components. Fast oscillations may therefore contribute more to feature binding and coding of object continuity than low-frequency components, at least for texture surfaces as analysed here.

Animals↗

PubMed1994

Stimulus-specific fast oscillations at zero phase between visual areas V1 and V2 of awake monkey.

Synchronization of fast cortical oscillations (35-90 Hz) has been proposed as a basis of sensory integration. This hypothesis requires stimulus specific oscillations that occur synchronously in different cortical areas of awake animals. Here, we demonstrate the presence of, and phase-locking between, high amplitude stimulus specific oscillations (50-90 Hz) in striate (V1) and extra striate (V2) visual cortex of an awake monkey. Oscillations of multiple unit spikes and local field potentials occurred with an average V1-V2 phase difference near zero. This finding was unexpected because V1 and V2 are thought to be serially arranged in the primate's visual processing stream. However, near zero-phase synchronization among cortical areas might enable fast and effective communication via the many reciprocal cortico-cortical connections for processes such as sensory integration.

Animals↗

PubMed1993

High frequency (60-90 Hz) oscillations in primary visual cortex of awake monkey.

It has been proposed that synchronized oscillations play a key role in perceptual feature linking and sensory integration. This idea was supported by the discovery of strongly synchronized stimulus-specific oscillations in the visual cortex of anaesthetized cats. The 'synchronization hypothesis' was controversial because in the visual cortex of awake monkeys either only weak or no oscillations were found. We have now recorded high amplitude synchronized oscillation at the level of spike activity and local field potential from the primary visual cortex of an awake monkey. The dominant frequencies (70-80 Hz) were considerably higher than those observed previously in cats and monkeys (30-50 Hz). However, stimulus specificities of the oscillations were comparable to and amplitudes even higher than those in cats.

Animals↗