A new microelectrode positioner for intraretinal recording from the intact mammalian eye

A micro-advancer device for vitreal injection and retinal recording and stimulation

A micro-advancer device that positions a narrow-gauge needle within the vitreous humor of the rat eye is described. The device is compact, simple and inexpensive to manufacture. It consists of an outer guard needle and an inner injection needle that is advanced through the guard needle. With the rat held in a stereotaxic holder and the globe fixed to a stabilizing ring, the outer 25-gauge guard needle is advanced through the sclera using a standard micromanipulator. The inner 31-gauge injection needle is then advanced through the guard needle with a manually controlled leadscrew and carriage mechanism. The inner injection needle is attached to a Hamilton syringe and can be positioned to within microns of the retinal surface under visual observation through a microscope. The injection needle is fixed to the device by a quick-release clamp on the carriage and can be rapidly exchanged while the guard needle remains in place in the vitreous. This permits different solutions to be injected sequentially into the vitreous humor. Recording electrodes, stimulating electrodes, and optical fibers can also be advanced through the guard needle and positioned accurately near the retinal surface or within the retina.

New ocular micromanipulator for measurements of retinal and vitreous physiologic parameters in the mammalian eye

A new ocular micromanipulator for introducing a microelectrode into the mammalian eye is described. Included is a technical description of the device, incorporating arched tracks and DC motors to provide for angular motion around a scleral entrance point. Applications for in vivo measurements of local PO2 in the retina and vitreous of miniature pigs and cats are shown. The new design features of this micromanipulator allow for smooth microelectrode movements over large retinal areas, facilitating the rapid collection of data in many different areas of the retina and vitreous.

Mathematical models of the spatial distribution of retinal oxygen tension and consumption, including changes upon illumination

To better understand oxygen utilization by the retina, a mathematical model of oxygen diffusion and consumption in the cat outer, avascular retina was developed by analyzing previously recorded profiles of oxygen tension (PO2) as a function of retinal depth. Simple diffusion modelling of the oxygen distribution through the outer retina is possible because the PO2 depends only on diffusion from the choroidal and retinal circulations and on consumption within the tissue. Several different models were evaluated in order to determine the best one from the standpoints of their ability to represent the data and to agree with physiological reality. For the steady state one-dimensional diffusion model adopted (the special three-layer diffusion model), oxygen consumption was constant through the middle layer and zero in the layers near the choroid and near the inner retina. On the average, the oxygen consuming layer, as found by nonlinear regression for each profile, extended from about 75% to 85% of the retinal depth from the vitreous. This is a narrow band through the mid-region of the photoreceptors. Oxygen consumption of the entire avascular retina, determined from fitting eight PO2 profiles measured in light-adapted retinas, averaged 2.7 ml O2(STP)/(100 g tissue.min), while the value determined from fitting thirty-two PO2 profiles measured in dark-adapted retinas averaged 4.4 ml O2(STP)/(100 g tissue.min). Consumption in the light was thus only 60% of that in the dark. This suggests that the outer retina is at greater risk of hypoxic injury in the dark than in the light, a finding of considerable clinical significance.

Effects of 2-amino-4-phosphonobutyric acid on responsivity and spatial summation of X cells in the cat retina

1. We studied the effect of locally applied 2-amino-4-phosphonobutyric acid (APB) on the response of X-type cat retinal ganglion cells to stimulation by sinusoidal gratings of photopic mean luminance. 2. Application of APB produced a decrease in the mean firing rate of on-centre X cells, but an increase in the mean firing rate of off-centre X cells. 3. The response and responsivity of both on- and off-centre X cells to drifting sinusoidal gratings were suppressed by APB. Under control conditions, the response amplitude was linear with grating contrast for contrasts less than 10%. During APB application, the slope of the contrast response curve was reduced, and linearity was maintained for a larger range of contrast. 4. Assuming that APB acts selectively on the cone to depolarizing bipolar cell synapse, the suppression of the response of on- and off-centre X cells suggests a functional link between depolarizing bipolar cells and both types of X cell. 5. Because APB reduces the centre response of both on- and off-centre X cells, caution should be observed in interpreting the effect of APB on higher visual centres in terms of the blockage of 'on' signals leaving the retina. 6. By analysing the spatial frequency tuning curve before and during APB application in terms of the difference-of-Gaussians model of receptive field structure, it was found that the balance between the integrated strengths of the centre and surround was not modified by APB. APB, however, had a differential effect on the summating area of the centre and surround. While the centre radius was unaffected by APB, the surround radius was reduced. This suggests that the peak sensitivity of the surround mechanism was reduced less severely than the peak sensitivity of the centre mechanism. 7. Strychnine also suppressed the response of the centre mechanism of both on- and off-centre X cells to drifting gratings.

A new microelectrode positioner for electrophysiology in the intact mammalian eye

This report describes a new microelectrode positioner for intraocular electrophysiology. This positioner provides fine movement along the axis of electrode advance and in two planes at right angles to the axis of advance. Rotation occurs around the scleral insertion point. In addition, this new positioner incorporates a closed chamber system that allows maintenance of constant intraocular pressure (IOP) as well as the ability to set the IOP at any desired level.

Origin and sensitivity of the light peak in the intact cat eye

1. The light peak is a large light-induced change in the DC potential across the eye (standing potential) that reaches its maximum in 5-13 min in mammals. The light peak of the intact cat eye was studied in order to define its cellular origin and stimulus-response characteristics. Direct-coupled recordings were made with a vitreal electrode and also with intraretinal and intracellular micro-electrodes. Light peaks were generally evoked with 300 sec periods of diffuse white illumination.2. Micro-electrode recordings made in the subretinal space just outside the apical membrane of the retinal pigment epithelium (r.p.e.) showed that the light peak was a change in trans-epithelial potential. No component was generated in the neural retina.3. Intracellular recordings from r.p.e. cells showed that the change in trans-epithelial potential resulted from a depolarization of the basal membrane (facing the choroid). This depolarization came after the hyperpolarization of the apical membrane that gave rise to the r.p.e. component of the c-wave of the e.r.g.4. The light peak amplitude at a constant retinal illumination was nearly linear with stimulus duration over the range 15-180 sec, and saturated at about 300 sec. The time-to-peak remained nearly constant at about 300 sec over this range. Large light peaks could be evoked with flashes as short as 10 sec if the retinal illumination was several log units above rod saturation.5. When stimulus duration was held constant at 300 sec, light peak amplitude was graded with illumination over a wide range, from 3 log units below to 2 log units above rod saturation. The threshold of the light peak was below that of the e.r.g. and only about 1.5-2.5 log units above the absolute threshold of the most sensitive ganglion cells. The increase of light peak amplitude above rod saturation was not due primarily to cones.6. The trans-epithelial light peak had an unusual dependence on stimulus area, being at least twice as large in response to diffuse light as it was in response to a large spot (10 deg diameter) of the same retinal illumination.7. These findings indicate that the light peak represents a normal physiological interaction between the retina and the r.p.e. They also suggest that the interaction involves a change in the concentration of a diffusible substance in the retina, which then either enters the r.p.e. itself, or triggers an internal messenger to cause the basal depolarization.

Cross-correlation analysis of the maintained discharge of rabbit retinal ganglion cells

1. Simultaneous recordings were made from pairs of rabbit retinal ganglion cells. Physiological tests were used to classify the receptive field properties of each cell and the receptive field locations were mapped. 2. The statistical dependence between simultaneously recorded retinal ganglion cells was assessed by cross-correlating the maintained discharge of the simultaneously recorded cells. Cross-correlations from cell pairs in which the constituent cells had non-overlapping receptive field centres were statistically flat, reflecting no statistical dependence. 3. Most cell pairs consisting of transient and sustained concentric cells and having overlapping receptive field centres exhibited a correlated maintained discharge indicative of statistical dependence. The strength of the statistical dependence varied approximately inversely with the degree of overlap between the two cells comprising the cell pair. 4. Cell pairs consisting of two ON-centre cells or two OFF-centre cells and having overlapping receptive field centres possessed incremental cross-correlations which were characterized by a peak centred near zero. Cell pairs consisting of an ON-centre cell with an OFF-centre and having overlapping receptive field centres possessed decremental cross-correlations which were characterized by a valley centred near zero. 5. The results are consistent with the hypothesis that a noise source provides shared input to two or more retinal ganglion cells. Bipolar and photoreceptors are the most likely sources of noise responsible for the statistical dependency between retinal ganglion cells.

A stereotaxic method of recording from single neurons in the intact in vivo eye of the cat

A method is described for recording stereotaxically from single retinal neurons in the optically intact in vivo eye of the cat. The method is implemented with the help of a new type of stereotaxic instrument and a specially developed stereotaxic atlas of the cat's eye and retina. The instrument is extremely stable and facilitates intracellular recording from retinal neurons. The microelectrode can be rotated about two mutually perpendicular axes, which intersect in the freely positionable pivot point of the electrode manipulation system. When the pivot point is made to coincide with a small electrode-entrance hole in the sclera of the eye, a large retinal region can be reached through this fixed hole in the immobilized eye. The stereotaxic method makes it possible to choose a target point on the presented eye atlas and predict the settings of the instrument necessary to reach this target. This method also includes the prediction of the corresponding light stimulus position on a tangent screen and the calculation of the projection of the recording electrode on this screen. The sources of error in the method were studied experimentally and a numerical perturbation analysis was carried out to study the influence of each of the sources of error on the final result. The overall accuracy of the method is of the order of 5 degrees of visual angle, which will be sufficient for most purposes.