Biomedical subjects

Marcus Eger

Publications and source records attributed to Marcus Eger.

5 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↗

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↗

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↗