Electroretinogram and spike activity in mammalian retina

Distant cancer effects on standardised testing of peripheral vision

BACKGROUND

Profound central-retinal visual losses have been a major presenting factor reported in cancer and melanoma associated retinopathies (CAR, MAR). However, it is well established that standardised tests of peripheral retinal function are often the most sensitive detectors of early eye disease. This is a preliminary investigation of the responsiveness of the peripheral retina to "distant" (non-eye or CNS) cancers using easily obtained standardised tests.

METHODS

The design is a single blind study where test results are compared with published norms and a small age matched control group. Of 120 ambulatory cancer outpatients who were interviewed at routine follow up examinations, 111 volunteered and admitted a range of mild visual changes. 25 cancer patients completed all tests of peripheral vision function and a clinical screening. There were seven control subjects of the same age range.

RESULTS

98% (49 of 50) of eyes from the patient cohort were judged clinically normal following examinations which emphasised the central retina, fundus appearance, and static fields. On testing which emphasised the visual periphery, 46 (92%) eyes showed one or more quantitative abnormalities >2 SD from the age adjusted norm means. These abnormalities clustered mainly about dark adaptation (rod cell) sensitivity (31, 62% of measured sites), the blue sensitive retinal cells (17, 34% of measured eyes), and the oscillatory component (OP) of the electroretinogram (23, 46% of measured eyes). One control eye (7%) showed a significant dark adaptation abnormality and ERG reduction. There was no identifiable interaction between chemotherapy mode and the cancer associated retinal deficits (CARD). Antiretinal antibodies were found in sera from most patients and controls.

CONCLUSION

CARD is common in the retinal periphery of many cancer patients, and is distinct from rare CAR, MAR central-retinal responses. CARD has numerous potential clinical uses which justify expanded research with more defined large samples.

Fast activity and oscillatory potential of carp retina in the frequency domain

There are two kinds of fast activity in the ERG: fast retinal potentials (FRP), an irregular series of spiky wavelets and oscillatory potentials (OP), a rhythmic sequence of events. Corneal ERG from nine intact young carps, evoked by extended pulses of diffuse white light under mesopic adaptation, displayed two different groups of wavelets related to ON and OFF, respectively. Stimulation and recording conditions were established to permit separate Fourier analysis of both groups of wavelets. Power distributions of normalized ON spectra showed both a wide dispersion and a high inter-subject variability. All normalized OFF spectra showed, instead, components within a narrow band from 52 to 56 Hz, most of them maximum relative power peaks. It is concluded that FRP originating in highly labile sources dominate ON fast activity, while the predominant OFF fast activity are OP originating in a stable discrete source.

Effects of aldose reductase inhibition on the retina and health indices of streptozotocin-diabetic rats

Experiments were performed on Streptozotocin-diabetic rats to investigate the preventive effect of an aldose reductase inhibitor, Sorbinil, on the deterioration of electroretinograms and retinal tissue that normally occurs during diabetes, and to determine Sorbinil's effect on the general health indices: food and water intake, urine glucose and ketones, body weight, and blood glucose levels in diabetic rats. Two dosages of Sorbinil were tested, 60 mg/kg/24 hours and 10 mg/kg/24 hours. Electroretinograms and the above health indices were measured before injection of Streptozotocin and again after a three week period of diabetes. Sample eyes were then examined by electron microscope and the thickness of the retinal capillary basement membranes was measured. Statistical evaluation showed that Sorbinil-treated diabetic rats did not experience the same lengthening of latencies and reduction in amplitude of various electroretinogram components that occurred in non-treated diabetic rats. Sorbinil improved the general health of diabetic rats and reduced their mortality rates as well. Preliminary electron microscope studies showed a correlation between thickness of the retinal capillary basement membrane and various electroretinogram parameters.

Electrical activity of anterior parts of the visual system obtained by nasopharyngeal recordings

Electrical responses to single flashes were recorded from the epipharynx and from various intranasal locations by withdrawing a nasopharyngeal electrode through the nose. The anatomical relation between the electrode and the eye ball was checked by taking an X-ray of the subject's skull. In the epipharynx and in the anterior part of the nose the responses were ERGs (a-wave, b-wave, wavelets) which were inverted in polarity. The reversal in polarity occurred when the electrode tip was underneath the posterior pole of the eye. At this electrode position and further back to the epipharynx high-frequency oscillations of increased amplitude and different waveform were recorded. It is concluded that components that change their polarity are generated by retinal dipoles oriented parallel with the electrode path while the increased oscillations recorded in the back of the nose are generated in the optic nerve by dipoles of vertical orientation.

Nasopharyngeal recordings separate retinal from optic nerve potentials

Electrical potentials in response to single flashes and to pattern-reversal stimuli presented in Maxwellian view were recorded from several intranasal locations by withdrawing a nasopharyngeal electrode from the epipharynx through the nose in steps of 1 cm. From the anterior parts of the nose a waveform could be obtained resembling the corneal electroretinogram. In the epipharynx an ERG was recorded which was inverted in polarity. This reversal in polarity was explained by assuming an electrical dipole of the retina oriented parallel to the electrode path. When the electrode was below and behind the posterior pole of the eye ball a high-frequency activity of increased amplitude was observed the origin of which could not be explained by the retinal dipole. It is suggested that these increased oscillations originate from the optic nerve. Their behavior is explained by a hypothetical electrical dipole of the optic nerve oriented vertical to the electrode path.

Distribution coding in the visual pathway

Although a variety of types of spike interval histograms have been reported, little attention has been given to the spike interval distribution as a neural code and to how different distributions are transmitted through neural networks. In this paper we present experimental results showing spike interval histograms recorded from retinal ganglion cells of the cat. These results exhibit a clear correlation between spike interval distribution and stimulus condition at the retinal ganglion cell level. The averaged mean rates of the cells studied were nearly the same in light as in darkness whereas the spike interval histograms were much more regular in light than in darkness. We present theoretical models which illustrate how such a distribution coding at the retinal level could be "interpreted" or recorded at some higher level of the nervous system such as the lateral geniculate nucleus. Interpretation is an essential requirement of a neural code which has often been overlooked in modeling studies. Analytical expressions are derived describing the role of distribution coding in determining the transfer characteristics of a simple interaction model and of a lateral inhibition network. Our work suggests that distribution coding might be interpreted by simply interconnected neural networks such as relay cell networks, in general, and the primary thalamic sensory nuclei in particular.