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Biomedical subjects

M Gur

Publications and source records attributed to M Gur.

36 records · Page 2Linked to original sources
PubMed1987

Intensity coding and luxotonic activity in the ground squirrel lateral geniculate nucleus.

The responses of contrast-sensitive cells in the ground squirrel LGN were studied. In most cells the response to an on-off stimulus was comprised of two components: a sustained on and a transient on-off. The sustained component amplitude was a power function of the light intensity and disappeared altogether at low temperature while the transient component was insensitive to the light intensity and to a drop in temperature. These cells were also luxotonic; they increased their average firing rate when the intensity of a steady stimulus was increased. The possible relation of the luxotonic activity to the diurnal nature of the ground squirrel is discussed.

Action Potentials↗
PubMed1987

Studying striate cortex neurons in behaving monkeys: benefits of image stabilization.

Responses of single cells in the striate cortex of a behaving monkey were studied while the eye movements of fixation were monitored with high precision. Receptive fields of cortical neurons moved in space with the eye. When the eye position signal was used to stabilize the image on the retina, response rates were more vigorous and more reliable. When the image was not stabilized, the estimates of receptive field activating areas were influenced (usually inflated) in unpredictable ways. With stabilization, small receptive fields can be studied and powerful surround interactions become apparent.

Action Potentials↗
PubMed1987

Changes in the oscillatory potentials of the electroretinogram in glaucoma.

Dark adapted and progressively light adapted electroretinograms (ERGs) were recorded from 34 normals and from 45 glaucoma patients. To enhance the oscillatory potentials (OP) the ERGs were highpass-filtered. The OP were characterized by two indices: their root-mean-square value and the inter-flash interval for which the maximal amplitude was obtained. In most of the glaucomatous OP, 50 of 81 eyes, both indices were abnormal, in 22 one parameter was abnormal and only in 9 were both parameters normal. In 13 of the 14 OP recorded from the opposite ('normal') eye of patients with unilateral glaucoma either one or both of the indices were abnormal. These findings indicate that the damage to the retina in glaucoma may extend more distally than the ganglion cell layer and that subclinical changes might be revealed by OP changes.

Adaptation, Physiological↗
PubMed1986

The physiological basis of wavelength discrimination: evidence from dichoptic and Ganzfeld viewing.

It has been suggested that human wavelength discrimination depends on the peripheral visual system outputting a signal which already indicates the difference between the two test wavelengths. In the present paper an alternative mechanism whereby wavelength discrimination is carried out by the visual cortex and not by the retina or the LGN is suggested. Wavelength discrimination was first tested under dichoptic viewing so that discrimination could, most likely, be performed by the visual cortex. Since performance was normal it is concluded that the visual cortex is capable of fine, normal level of wavelength discrimination. When the task was performed under ganzfeld viewing (alternating wavelengths) so that the LGN would output only the difference between the two wavelengths, discrimination was poor and sometimes, absent. This indicates that it is unlikely that wavelength discrimination is complete at the LGN level.

Color Perception↗
PubMed1986

Time domain analysis of VEPs. Detection of waveform abnormalities in multiple sclerosis.

Visual evoked potentials (VEPs) are a useful tool in the diagnosis of multiple sclerosis. To improve their diagnostic power in the early stages of the disease, the VEP shape, as well as its latency, were considered. The Karhunen-Loeve expansion (KLE) was used because it affords an unique advantage in classifying the VEPs by their shape, and gives a well-defined criterion based on similarity to the normative prototype. Using this criterion, it was possible to quantitatively define normative limits of waveform. We were also able to detect early changes in VEPs recorded from suspected multiple sclerosis patients. KLE promises to enable classification of waveforms in early stages of the pathology, when peak-latency comparisons are the least effective.

Adult↗
PubMed1983

Stereotaxic head restraint for chronic and semi-chronic marmoset and macaque monkeys.

A device allowing head holding and stereotaxic manipulations in both small-sized Marmoset and in relatively large Macaca monkeys is described. Its design permits free access to most targets in the visual system and optimizes presentation of visual stimuli in awake-chronic and anaesthetized semi-chronic preparations. Extra-aural attachments to temporal bone are implanted stereotaxically, under anaesthesia, on both sides of the skull. This provides means for stereotaxic head-holding which minimizes discomfort to the animal. Maintenance of stereotaxic precision over long periods of time, possibility of realignment and use of one device for various monkey species are special features of the method.

Animals↗
PubMed1983

Spherical aberration of the lens of the ground squirrel (Spermophilis tridecemlineatus).

Spherical aberration of the excised ground squirrel lens was measured by photographing the variation in back vertex distance for parallel helium - neon laser beams of various separations. The results indicate high levels of negative spherical aberration in amounts similar to that measured earlier for the rat lens. The effect of the aberration on resolution ability is likely minimized by the small size of the ground squirrel pupil in diurnal conditions and possibly by the existence of positive corneal aberration.

Animals↗
PubMed1980

Frequency-domain analysis of the human electroretinogram.

Normal corneal electroretinograms (ERG) are analyzed in the frequency domain using the fast Fourier transform (FFT) and linear prediction (LP) methods. Four dominant frequencies at 18, 79, 126, and 159 Hz are found in the dark-adapted state. Light adaptation shifts the low frequency to higher frequency and the mid- and the two high-frequency components to lower frequencies. The relative amplitude of the high-frequency component resulting from the oscillatory potentials is quantified. It is shown that frequency-domain features are of a smaller variability than time-domain components, and can be extracted even from a noisy surface ERG.

Adaptation, Ocular↗
PubMed1980

Visual stability and space perception in monocular vision: mathematical model.

A deterministic model for monocular space perception is presented. According to the model, retinal luminance changes due to involuntary eye movements are detected and locally analyzed to yield the angular velocity of each image point. The stable three-dimensional spatial coordinates of viewed objects are then reconstructed using a method of infinitesimal transformations. The extraction of the movement (parallax) field from the optical flow is represented by a set of differential equations, the derivation of which is based on the conservation of energy principle. The relation of the model to retinal neurophysiology and to various aspects of visual space perception is discussed.

Depth Perception↗
PubMed1979

Some temporal output properties of color opponent units in the ground squirrel retina.

Responses of BG-G units to equal energy stimuli of different wavelengths reveal a tonic to phasic pattern of responses in which short wavelengths (blue) evoke tonic activity, medium wavelengths (green to yellow) evoke phasic activity and intermediate wavelengths evoke in-between patterns. The patterns correlate with wavelength but are relatively insensitive to intensity. Output patterns of BG-G cells to white and to monochromatic light at the animal's neutral point are almost identical, and the pattern identities may be a neural correlate to the behavioral phenomenon of the neutral point.

Animals↗
PubMed1979

Time and frequency analysis of simultaneously recorded corneal and non-corneal electroretinogram.

Electroretinograms (ERGs) were simultaneously recorded with corneal and non-corneal (lower or upper eye lid) electrodes in four healthy subjects. 64 traces of the non-corneal ERG were averaged. The two signals were compared in both time and frequency domains. The power spectrum of both signals was computed by the fast Fourier (FFT) and linear prediction (LP) algorithms. Except for amplitude differences, the two signals had almost identical time domain (a- and b-wave latencies) and frequency domain (dominant power spectrum peaks) parameters. It is concluded that non-corneal ERG does not differ significantly from corneal ERG, and that its clinical interpretation can be based on existing corneal ERG data.

Adult↗
PubMed1979

Refractive state of the eye of a small diurnal mammal: the ground squirrel.

Previous work dealing with the question of whether or not the eyes of small mammals are hyperopic has been restricted largely to the study of nocturnal animals such as the rat. In this study, refractive state of the eye of a small diurnal mammal, the ground squirrel, was measured by retinoscopy and from the electrophysiological response of retinal ganglion cells to grating targets. Both measurements show the squirrel to be emmetropic. We conclude that the refractive state of the eye is not a function of eye size per se, but rather a function of whether the animal is nocturnal or diurnal.

Activity Cycles↗
PubMed1976

Blood pressure responses as an indicator of the anesthetic level in ground squirrels.

In many experiments in visual physiology which employ halothane anesthesia, it is difficult to judge accurately an animal's anesthetic state, particularly so because of the necessity for paralysis and artificial respiration. For the ground squirrel (Citellus tridecemlineatus and C. mexicanus), we find that measures of arterial pressure response to a noxious stimulus combined with mean arterial pressure allow for an accurate division of halothane anesthetic state into five stages. All five stages are identified and are the same regardless of whether the animal is unparalyzed or is paralyzed and artificially respired. Use of these measures in long-term experiments allows for recognition of and adjustment for small perturbations of anesthetic state that may occur even though inhaled halothane concentration remains constant.

Anesthesia↗
PubMed1972

Ultrastructure within the lateral plexus of the Limulus eye.

The ultrastructure of the lateral plexus in the compound eye of Limulus is investigated by serial section technique. "Cores" of tissue containing the axons, lateral plexus, and neuropile associated with one sensory ommatidium show the following features: (a) collateral branches from retinular cells do not contribute to the lateral plexus proper, but do form retinular neuropile by contacting collaterals of a self-contained cluster of retinular axons; (b) collateral branches from eccentric cell axons always branch repeatedly upon leaving the parent axon, and compose the bulk of the lateral plexus; (c) the most distal collateral branches from an eccentric cell axon appear to form neuropile and synaptic contacts with each other, whereas more proximal branches form synaptic contacts with collaterals from eccentric cell axons of neighboring ommatidia. We conclude that the ribbon synapses and associated transmitter substance in eccentric cell collaterals must be inhibitory, and that two pathways for self-inhibition may exist. We suggest, as a working hypothesis for the structure of the lateral plexus, a branching pattern with depth that mirrors the horizontal spread of lateral inhibition measured physiologically.

Animals↗
PubMedDate not supplied

Selective activation of visual cortex neurons by fixational eye movements: implications for neural coding.

During normal vision, when subjects attempt to fix their gaze on a small stimulus feature, small fixational eye movements persist. We have recorded the impulse activity of single neurons in primary visual cortex (V1) of macaque monkeys while their fixational eye movements moved the receptive-field activating region (AR) over and around a stationary stimulus. Three types of eye movement activation were found. (1) Saccade cells discharged when a fixational saccade moved the AR onto the stimulus, off the stimulus, or across the stimulus. (2) Position/drift cells discharged during the intersaccadic (drift) intervals and were not activated by saccades that swept the AR across the stimulus without remaining on it. To activate these neurons, it was essential that the AR be placed on the stimulus and many of these cells were selective for the sign of contrast. They had smaller ARs than the other cell types. (3) Mixed cells fired bursts of activity immediately following saccades and continued to fire at a lower rate during intersaccadic intervals. The tendency of each neuron to fire transient bursts or sustained trains of impulses following saccades was strongly correlated with the transiency of its response to stationary flashed stimuli. For one monkey, an extraretinal influence accompanying fixational saccades was identified. During natural viewing, the different eye movement classes probably make different contributions to visual processing. Position/drift neurons are well suited for coding spatial details of the visual scene because of their small AR size and their selectivity for sign of contrast and retinal position. However, saccade neurons transmit information that is ambiguous with respect to the spatial details of the retinal image because they are activated whether the AR lands on a stimulus contour, or the AR leaves or crosses the contour and lands in another location. Saccade neurons may be involved in constructing a stable world in spite of incessant retinal image motion, as well as in suppressing potentially confusing input associated with saccades.

Action Potentials↗