Reinhard Eckhorn

Coherent oscillations: A mechanism of feature linking in the visual cortex?

Primary visual coding can be characterized by the receptive field (RF) properties of single neurons. Subject of this paper is our search for a global,second coding step beyond the RF-concept that links related features in a visual scene. In recent models of visual coding, oscillatory activities have been proposed to constitute such linking signals. We tested the neurophysiological relevance of this hypothesis for the visual system. Single and multiple spikes as well as local field potentials were recorded simultaneously from several locations in the primary visual cortex (A17 and A18) using 7 or 19 individually advanceable fibermicroelectrodes (250 or 330 μm apart).Stimulusevoked (SE)-resonances of 35–85 Hz were found in these three types of signals throughout the visual cortex when the primary coding channels were activated by their specific stimuli. Stimulus position, orientation, movement direction and velocity, ocularity and stationary flicker caused specific SE-resonances.Coherent SE-resonances were found at distant cortical positions when at least one of the primary coding properties was similar. Coherence was found1) within a vertical cortex column,2) between neighbouring hypercolumns, and3) between two different cortical areas. We assume that the coherence of SE-resonances is mediated by recurrent excitatory intra- and inter-areal connections via phase locking between assemblies that represent the linking features of the actual visual scene. Visually related activities are, thus, transiently labelled by a temporal code that signalizes their momentary association.

Feature linking via synchronization among distributed assemblies: Simulations of results from cat visual cortex

We recently discovered stimulus-specific interactions between cell assemblies in cat primary visual cortex that could constitute a global linking principle for feature associations in sensory and motor systems: stimulus-induced oscillatory activities (35-80 Hz) in remote cell assemblies of the same and of different visual cortex areas mutually synchronize, if common stimulus features drive the assemblies simultaneously. Based on our neurophysiological findings we simulated feature linking via synchronizations in networks of model neurons. The networks consisted of two one-dimensional layers of neurons, coupled in a forward direction via feeding connections and in lateral and backward directions via modulatory linking connections. The models performance is demonstrated in examples of region linking with spatiotemporally varying inputs, where the rhythmic activities in response to an input, that initially are uncorrelated, become phase locked. We propose that synchronization is a general principle for the coding of associations in and among sensory systems and that at least two distinct types of synchronization do exist: stimulus-forced (event-locked) synchronizations support crude instantaneous associations and stimulus-induced (oscillatory) synchronizations support more complex iterative association processes. In order to bring neural linking mechanisms into correspondence with perceptual feature linking, we introduce the concept of the linking field (association field) of a local assembly of visual neurons. The linking field extends the concept of the invariant receptive field (RF) of single neurons to the flexible association of RFs in neural assemblies.

A new method for the insertion of multiple microprobes into neural and muscular tissue, including fiber electrodes, fine wires, needles and microsensors

We developed a new method for the insertion of thin-shaft probes into neural and muscular tissue. Axial forces for driving the probes into tissue and radial forces against buckling are both provided by a stretched elastic rubber tube in which the probe is guided outside the tissue. Various geometric arrangements of arrays with independently advanceable probes are possible. Prototypes with 7 linearly aligned fiber electrodes and computer-controlled positioning motors were successfully used in single- and multiple-unit recordings from the visual system of awake monkeys (Eckhorn et al., 1993). The method is suitable in a wide range of applications, including insertion of fine microprobe fibers and wire electrodes into brain and muscle through the skin or dura, provided that the tips of the probes are sharp and hard enough.

Visual stimulation elicits locked and induced gamma oscillations in monkey intracortical- and EEG-potentials, but not in human EEG

Abstractu2002Stimulus-related fast oscillations in the γ-range (30–100 Hz) were clearly demonstrated with microelectrode recordings in visual cortex of awake monkeys, and they were also reported for recordings of human electroencephalograms (EEG). However, the presence of stimulus-related γ-modulation in human EEG has repeatedly been disputed. To clarify this dispute, we recorded the scalp EEG of man and monkey as well as intracortical field potentials (LFP) from monkey primary visual cortex (V1) during identical visual stimulation (large-field sinusoidal gratings, which proved to induce the largest γ-amplitudes in monkey V1 and V2). We found a strong stimulus-related increase of γ-oscillations in monkey LFP and EEG, but no modulation of γ-activity in human EEG. In contrast to previous results, γ-oscillations in the monkey were strongly phase-locked to stimulus onsets in early response periods (80–160 ms) and became gradually independent in later periods. Our negative result on γ-modulation in human subjects contradicts several published findings. We conclude from our results that visually evoked γ-modulations in humans EEG are not as accessible as in the monkey.

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 (≈u200330–80u2003Hz) 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, 1–140u2003Hz) 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, 0–11.7u2003Hz; medium, 11.7–31.3u2003Hz; and fast oscillations, 31.3–62.5u2003Hz) 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 monkeys striate cortex we assume that they have enhanced probability of activating successive stages of visual processing and hence contribute to the perception of orientation.

Stimulation with a Wireless Intraocular Epiretinal Implant Elicits Visual Percepts in Blind Humans

PURPOSEnElectrical stimulation of retinal neurons has been shown to be a feasible way to elicit visual percepts in patients blind from retinal degenerations. The EPIRET3 retinal implant is the first completely wireless intraocular implant for epiretinal stimulation. Stimulation tests have been performed during a clinical trial that was carried out at the eye clinics of Aachen and Essen to evaluate the safety and the efficacy of the implant.nnnMETHODSnSix legally blind retinitis pigmentosa patients were included in the study. In accordance with the regulations laid down in the study protocol, three 1-hour perceptual tests for each subject were performed within 4 weeks of surgery. Stimuli were charge-balanced square current pulses of various durations and current amplitudes.nnnRESULTSnAll subjects reported visual percepts as a result of electrical stimulation by the implant. Thresholds for eliciting visual percepts varied between them but were below the safety limits of electrical stimulation. Stimulation success depended stronger on pulse duration than on current amplitude or total charge delivered. Subjects were able to discriminate between stimulation patterns of different orientations or at different locations of the electrode array.nnnCONCLUSIONSnThe EPIRET3 system is suitable to elicit visual percepts in blind retinitis pigmentosa patients.

Amplitude envelope correlation detects coupling among incoherent brain signals.

&NA; Cognitive processing involves γ‐activation over broad cortical regions. Phase coupling of these activities has rarely been reported for areas far apart. Other forms of coupling are generally not detected by conventional measures. Here, we use amplitude envelope correlation (AEC), which can detect signal coupling without phase coherence, even among different frequencies. We apply it to subdural recordings from humans performing a visual delayed match‐to‐sample task and systematically compare it with spectral amplitude and coherence. The different measures often show divergent results. In particular, AEC reveals γ‐coupling completely missed by coherence. We argue that coherence and AEC are adapted to different cortical mechanisms of short‐ and long‐range interactions, respectively.

Phase correlation of cortical rhythms at different frequencies: higher-order spectral analysis of multiple-microelectrode recordings from cat and monkey visual cortex

Abstract In classical EEG analysis rhythms with different frequencies occuring at separable regions and states of the brain are analysed. Rhythms in different frequency bands have often been assumed to be independent and their occurrence was interpreted as a sign of different functional operations. Independence has scarcely proved because of conceptual and computational difficulties. It is, on the other hand, probable that different rhythmic brain processes are coupled because of the broad recurrent connectivity among brain structures. We therefore set out to find interactions among rhythmic signals at different frequencies. We were particularly interested in interactions between lower frequency bands and γ-activities (30–90 Hz), because the latter have been analysed in our laboratory in great detail and had properties suggesting their involvement in perceptual feature linking. Fast oscillations occurred synchronized in a stimulus-specific way in the visual cortex of cat and monkey. Their presence was often accompanied by lower frequency components at considerable power. Such multiple spectral peaks are known from many cortical and subcortical structures. Despite their well known occurrence, coupling among different frequencies has not been established, apart from harmonic components. For the present investigation we extended existing analytical tools to detect non-linear correlations among signal pairs at any frequency (including incommensurate ones). These methods were applied to multiple microelectrode recordings from visual cortical areas 17 and 18 of anesthetized cats and V1 of awake monkeys. In particular, we assessed non-linear correlations by means of higher order spectral analysis of multi-unit spike activities (MUA) and local slow wave field potentials (LFP, 1–120 Hz) recorded with microelectrodes. Non-linear correlations among signal components at different frequencies were investigated in the following steps. First, the frequency content of short (≈250 ms) sliding window signal epochs was analyzed for simultaneously occurring rhythms of significant power at different frequencies. This was done by a newly developed method derived from the trispectrum using separate averaging of the products of short-epoch power spectra for any possible combination of frequency pairs. Second, non-linear (quadratic) phase coupling between different frequencies was assessed by the methods of bispectrum and bicoherence . We found phase correlations at different frequencies in the visual cortex of the cat and monkey. These couplings were significant in about 60% of the investigated MUA and LFP recordings, including several cases of coupling among incommensurate (i.e. non-harmonic) frequencies. Significant phase correlations were present: (1) within the γ-frequency range; (2) between γ- and low frequency ranges (1–30 Hz, including α- and β-rhythms); and (3) within the low frequency range. Phase correlations depended, in most cases, on specific visual stimulation. We discuss the possible functional significance of phase correlations among high and low frequencies by including proposals from previous work about potential roles of single-frequency rhythms of the EEG. Our suggestions include: (1) visual feature linking across different temporal and spatial scales provided by coherent oscillations at high and low frequencies; (2) linking of visual cortical representations (high frequencies) to subcortical centers (low frequencies) like the thalamus and hippocampus; and (3) temporal segmentation of the sustained stream of incoming visual information into separate frames at different temporal resolutions in order to prevent perceptual smearing due to shifting retinal images. These proposals are, at present, merely speculative. However, they can, in principle, be proved by microelectrode recordings from trained behaving animals.xa0© 1997 Elsevier Science B.V.

Visual resolution with retinal implants estimated from recordings in cat visual cortex.

We investigated cortical responses to electrical stimulation of the retina using epi- and sub-retinal electrodes of 20-100 microm diameter. Temporal and spatial resolutions were assessed by recordings from the visual cortex with arrays of microelectrodes and optical imaging. The estimated resolutions were approximately 40 ms and approximately 1 degrees of visual angle. This temporal resolution of 25 frames per second and spatial resolution of about 0.8 cm at about 1m and correspondingly 8 cm at 10 m distance seems sufficient for useful object recognition and visuo-motor behavior in many in- and out-door situations of daily life.

Activation zones in cat visual cortex evoked by electrical retina stimulation

AbstractnBackground. A retina implant for restoring simple basic visual perception in patients who are blind due to photoreceptor loss requires optimisation of stimulation parameters for obtaining high spatio-temporal resolution. We developed effective low-power epi-retinal stimulation and intracortical recording in semichronically prepared cats.nMethods. Individually driveable fibre electrodes were inserted through a small scleral incision and positioned at the area centralis. Polyimide–platinum film electrodes were inserted via a corneal incision and fixed by instillation of perfluorocarbon liquid on the internal limiting membrane. For electrical stimulation we used short charge-balanced current impulses of 100–400xa0µs duration and amplitudes ranging from 1 to 100xa0µA. During stimulation we recorded multiple single-cell and population activities from areas 17 and 18. Recordings were stored digitally. Stimulus–response relations including response strength, cortical activation zones, information transmission, and electrical receptive fields were analysed off-line.nResults. We found low-threshold activations with fibre electrodes and polyimide–platinum film electrodes in close mechanical contact to the retina. Retinal stimulation with bipolar charge-balanced impulses resulted in cortical activation zones corresponding to 1–5° visual angle at paracentral locations dependent on the eccentricity of the retinal stimulation point. Retino-cortical transinformation analysis revealed 20–30 bits/s per electrode, corresponding to 10–15 four-level pictures/s. Electrical receptive fields had sizes of 1–3° visual angle.nConclusions. Coarse visuomotor coordination and navigation seems possible with retina implants.