Biomedical subjects

Reinhard Eckhorn

Publications and source records attributed to Reinhard Eckhorn.

14 recordsLinked to original sources

PubMed2006

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.

Action Potentials↗

PubMed2005

Spatiotemporal receptive field properties of epiretinally recorded spikes and local electroretinograms in cats.

BACKGROUND: Receptive fields of retinal neural signals of different origin can be determined from extracellular microelectrode recordings at the inner retinal surface. However, locations and types of neural processes generating the different signal components are difficult to separate and identify. We here report epiretinal receptive fields (RFs) from simultaneously recorded spikes and local electroretinograms (LERGs) using a semi-chronic multi-electrode in vivo recording technique in cats. Broadband recordings were filtered to yield LERG and multi unit as well as single unit spike signals. RFs were calculated from responses to multifocal pseudo-random spatiotemporal visual stimuli registered at the retinal surface by a 7-electrode array. RESULTS: LERGs exhibit spatially unimodal RFs always centered at the location of the electrode tip. Spike-RFs are either congruent with LERG-RFs (N = 26/61) or shifted distally (N = 35/61) but never proximally with respect to the optic disk. LERG-RFs appear at shorter latencies (11.9 ms +/- 0.5 ms, N = 18) than those of spikes (18.6 ms +/- 0.4 ms, N = 53). Furthermore, OFF-center spike-RFs precede and have shorter response rise times than ON-center spike-RFs. Our results indicate that displaced spike-RFs result from action potentials of ganglion cell axons passing the recording electrode en route to the optic disk while LERG-RFs are related to superimposed postsynaptic potentials of cells near the electrode tip. CONCLUSION: Besides contributing to the understanding of retinal function we demonstrate the caveats that come with recordings from the retinal surface, i.e., the likelihood of recordings from mixed sets of retinal neurons. Implications for the design of an epiretinal visual implant are discussed.

Action Potentials↗

PubMed2004

Task-related coupling from high- to low-frequency signals among visual cortical areas in human subdural recordings.

Cortical cooperativity during cognitive demands includes high- and low-frequency activities, which raises the question whether there are interdependencies between fast and slow processes and how they are reflected in electrical brain signals. We had the opportunity to record signals intracranially from occipital visual areas in an epileptic patient and quantified inter-areal signal coupling while the patient performed a visual delayed-match-to-sample task. We computed coherence, phase consistency and amplitude envelope correlation and we also determined inter-frequency coupling through correlation between low-frequency signal components and amplitude envelopes of high-frequency components. There was a pronounced task-related increase of correlation between gamma-band (28-70 Hz) signal envelopes from a superior (occipital) and low-frequency (0-3.5 Hz) signals from an inferior (occipital) visual area, lasting for approximately 1 s and possibly reflecting a short-term memory encoding process. The correlational delay between envelopes and low-frequency components was 40 ms. In contrast, coherence, phase consistency and envelope correlation showed event-, but no task-related changes of intra-areal and no changes of inter-areal coupling. Our data suggest a specific effect of gamma-activity in the superior onto low-frequency activity in the inferior area. We argue that temporal dispersion of conduction delays might prevent coherent transmission of high-frequency signals and thus account for the absence of gamma-coherence. As such dispersion is a general property of long-range projections, envelope-to-signal correlation possibly reflects a general neuronal mechanism. Hence, our method provides a powerful tool for detecting such inter-areal interactions not visible with conventional linear coupling measures.

Brain Mapping↗

PubMed2004

Perception-related modulations of local field potential power and coherence in primary visual cortex of awake monkey during binocular rivalry.

Cortical synchronization at gamma-frequencies (35-90 Hz) has been proposed to define the connectedness among the local parts of a perceived visual object. This hypothesis is still under debate. We tested it under conditions of binocular rivalry (BR), where a monkey perceived alternations among conflicting gratings presented singly to each eye at orthogonal orientations. We made multi-channel microelectrode recordings of multi-unit activity (MUA) and local field potentials (LFP) from striate cortex (V1) during BR while the monkey indicated his perception by pushing a lever. We analyzed spectral power and coherence of MUA and LFP over 4-90 Hz. As in previous work, coherence of gamma-signals in most pairs of recording locations strongly depended on grating orientation when stimuli were presented congruently in both eyes. With incongruent (rivalrous) stimulation LFP power was often consistently modulated in consonance with the perceptual state. This was not visible in MUA. These perception-related modulations of LFP occurred at low and medium frequencies (< 30 Hz), but not at gamma-frequencies. Perception-related modulations of LFP coherence were also restricted to the low-medium range. In conclusion, our results do not support the expectation that gamma-synchronization in V1 is related to the perceptual state during BR, but instead suggest a perception-related role of synchrony at low and medium frequencies.

Algorithms↗

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↗

PubMed2003

Simultaneous mapping of binocular and monocular receptive fields in awake monkeys for calibrating eye alignment in a dichoptical setup.

We developed a modified Wheatstone stereoscope for simultaneous dichoptical and binocular stimulation in awake monkey. We, therefore, extended the conventional two-screen Wheatstone stereoscope to a setup with an additional third screen viewed binocularly via semi-transparent mirrors. With a sparse noise stimulation we mapped classical receptive field (CRF) positions via each screen independently but simultaneously. This was done for multiple recording positions (16 electrodes) at once in primary visual cortex based on multiple unit spike activity (MUA) and local field potentials (LFP), respectively. The technique can be used to (1) quickly and simultaneously determine binocular as well as left and right eye CRFs, including ocular dominance characteristics (net recording time for the given examples: approximately 2 min), (2) precisely adjust dichoptical stimulation by evaluating offsets between monocular and binocular CRF positions (average spatial incongruency between binocular and left/right eye stimulation after calibration: approximately 0.025 degrees visual angle), and (3) investigate left and right eye interaction in forming binocular CRFs. Due to the precise adjustment of the dichoptical and the simultaneous binocular stimulation investigations on the basis of stereo vision can be done with appropriate eye vergence alignment matching normal binocular viewing conditions in awake animals.

Animals↗

PubMed2002

Activation zones in cat visual cortex evoked by electrical retina stimulation.

BACKGROUND: 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. METHODS: 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 100-400 micro s duration and amplitudes ranging from 1 to 100 micro A. 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. RESULTS: 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 degrees 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 degrees visual angle. CONCLUSIONS: Coarse visuomotor coordination and navigation seems possible with retina implants.

Animals↗

PubMed2002

Perceptual grouping correlates with short synchronization in monkey prestriate cortex.

Synchronization in the visual cortex at 35-80 Hz is assumed to support perceptual grouping. We tested this hypothesis in a figure-ground task in which a trained monkey indicated by a key whether he perceived a figure that was composed of the same blobs as the background distractors. The task was sufficiently difficult such that about 25% of responses were incorrect. We recorded population activity with 7 microelectrodes in prestriate cortex (V2). During a short period before the monkey's perceptual response, locations of figure-activated neurons showed increased synchronization (50-80 Hz) in correct compared to incorrect responses, while other signal measures were unrelated to perception. These are first indications that a short synchronous burst in V2 may support perceptual grouping.

Action Potentials↗

PubMed2002

Classification of neural signals by a generalized correlation classifier based on radial basis functions.

A common problem in neuroscience is to identify the features by which a set of measurements can be segregated into different classes, for example into different responses to sensory stimuli. A main difficulty is that the derived distributions are often high-dimensional and complex. Many multivariate analysis techniques, therefore, aim to find a simpler low-dimensional representation. Most of them either involve huge efforts in implementation and data handling or ignore important structures and relationships within the original data. We developed a dimension reduction method by means of radial basis functions (RBF), where only a system of linear equations has to be solved. We show that this approach can be regarded as an extension of a linear correlation-based classifier. The validity and reliability of this technique is demonstrated on artificial data sets. Its practical relevance is further confirmed by discriminating recordings from monkey visual cortex evoked by different stimuli.

Animals↗

PubMedDate not supplied

Retino-cortical information transmission achievable with a retina implant.

Blind subjects with photoreceptor degeneration perceive phosphenes when their intact retinal ganglion cells are stimulated electrically. Is this approach suitable for transmitting enough information to the visual cortex for partially restoring vision? We stimulated the retina of anesthetized cats electrically and visually while recording the responses in the visual cortex. Transmission of retino-cortical information T was quantified by information theory. T was 20-160 bit/s (per stimulation and recording site) with random electrical or visual impulse stimulation at rates between 20 and 40 s-1. While increasing spatial density of independent electrical stimulation channels T did not saturate with 7 electrodes/mm2 retina. With seven electrodes up to 500 bit/s was transmitted to 15 cortical recording sites. Electrical stimulation basically employs temporal stimulus patterns. They are intimately linked with intensity/contrast information coded by the spike density of retinal ganglion cells. From the cortical information spread we estimated the spatial resolution as 0.5mm cortex corresponding to 0.5-1.0 degrees visual angle. If the human cortex can receive and decode the information transmitted by a retina implant, our quantitative results measured in cats suggest that visuo-motor coordination and object recognition in many in- and out-door situations will be possible.

Action Potentials↗

PubMedDate not supplied

Quantification of sensory information transmission using timeseries decorrelation techniques.

To estimate the information transmitted across a neuronal sensory system one has to deal with serial dependence among consecutive samples of the stimulus and the response signal. Common methods usually require a huge amount of data, or are restricted to Gaussian stimuli. Here, we describe stimulus and response as stochastic processes, i.e. as sequences of random variables, in the same coordinate system. Stimulus-response pairs of these random variables must not be considered independently because otherwise the transinformation is overestimated. To account for the linear fraction of the serial dependence, we present two decorrelation techniques based on coordinate transformation. They provide a representation of the processes with uncorrelated random variables and yield a more precise estimate of the transinformation.

Animals↗

PubMedDate not supplied

Visual resolution with epi-retinal electrical stimulation estimated from activation profiles in cat visual cortex.

Blinds with receptor degeneration can perceive localized phosphenes in response to focal electrical epi-retinal stimuli. To avoid extensive basic stimulation tests in human patients, we developed techniques for estimating visual spatial resolution in anesthetized cats. Electrical epi-retinal and visual stimulation was combined with multiple-site retinal and cortical microelectrode recordings of local field potentials (LFPs) from visual areas 17 and 18. Classical visual receptive fields were characterized for retinal and cortical recording sites using multifocal visual stimulation combined with stimulus-response cross-correlation. We estimated visual spatial resolution from the size of the cortical activation profiles in response to single focal stimuli. For comparison, we determined activation profiles in response to visual stimuli at the same retinal location. Activation profiles were single peaked or multipeaked. In multipeaked profiles, the peak locations coincided with discontinuities in cortical retinotopy. Location and width of cortical activation profiles were distinct for retinal stimulation sites. On average, the activation profiles had a size of 1.28 +/- 0.03 mm cortex. Projected to visual space this corresponds to a spatial resolution of 1.49 deg +/- 0.04 deg visual angle. Best resolutions were 0.5 deg at low and medium stimulation currents corresponding to a visus of 1/30. Higher stimulation currents caused lower spatial, but higher temporal resolution (up to 70 stimuli/s). In analogy to the receptive-field concept in visual space, we defined and characterized electrical receptive fields. As our estimates of visual resolutions are conservative, we assume that a visual prosthesis will induce phosphenes at least at this resolution. This would enable visuomotor coordinations and object recognition in many indoor and outdoor situations of daily life.

Animals↗