S. J. Crews

Serial examination of the normal visual field using Octopus automated projection perimetry Evidence for a learning effect

Abstract. The influence of prior perimetric experience on the magnitude of both differential sensitivity and the short and long term fluctuations remains unclear, and confounds accurate interpretation of visual field data obtained by computer‐assisted perimetry. The purpose of the experiment was to identify and quantify any influence of training on the automated perimetric response. The full field of the right eye of 10 clinically normal, naive subjects was examined on 8 occasions with Octopus Program 21 (target size 3) on days 1–5 inclusive, 15, 16 and 44. Sensitivity increased with serial examination in 8 subjects. By dividing the field into zones, it was demonstrated that the learning effect was greatest in the superior field and for eccentricities beyond 30°.

Perimetric profiles and cortical representation

The nature of the interaction between spatial summation and M-scaling was investigated by means of the Octopus automated perimeter. It was found that M-scaling of spatial stimulus parameters, alone, did not result in the expected isosensitivity profile. A clinically flat profile was, however, obtained within the central 30 degrees along the horizontal meridian using a large uniform size target (size 3; projected diameter 0.431 degrees). The eccentricities at which presentation of the standard Goldmann targets would result in an isosensitivity profile across the full extent of the visual field were calculated to further illustrate the over-compensation inherent in the current M-scaling equations.

Induced intraocular light scatter and the sensitivity gradient of the normal visual field

The influence of intraocular light scatter on the perimetric sensitivity profile of the normal eye was investigated using a series of light-scattering cells containing 0.01%, 0.02% and 0.025% concentrations of 500 nm diameter latex beads. The degree of induced intraocular light scatter was quantified by measuring contrast sensitivity using the Nicolet CS2000 system in the presence and absence of both wide- and narrow-angle glare light. Perimetric sensitivity out to an eccentricity of 30° was assessed, using the Octopus 201 and the Dicon AP3000 automated perimeters, with the three light-scatter cells and in the cell-free control condition. The results for both functions were expressed as the difference between the control response and that recorded under the particular experimental condition. Perimetric attenuation increased with increase in intraocular light scatter; the extent of the attenuation varied with stimulus type, bowl luminance and eccentricity

The role of intraocular light scatter in the attenuation of the perimetric response

The relationship between perimetric attenuation and intraocular light scatter was investigated in normal subjects using latex bead cells and in patients with uniocular media opacities. The Octopus 201 and Dicon 3000 automated perimeters were used to measure perimetric sensitivity and the Nicolet CS2000 system employed to measure contrast sensitivity with and without glare light in order to calculate intraocular light scatter. A high correlation for both samples was found between intraocular light scatter and perimetric attenuation as measured by both instruments. Attenuation was greatest at the 10 asb bowl luminance for the Dicon, and was greater at fixation than at 27.5°. For the Octopus attenuation was lowest at fixation.

Some concepts on the use of three-dimensional isometric plots for the representation of differential sensitivity

An increasing use is being made of three-dimensional isometric plotting to represent the topography of the visual field. The format of such plots, however, can be manipulated depending upon the parameters of the plotting routine. Problems associated with generating these plots such as resolution of the plotting grid, scaling of sensitivity and orientation of the plot are discussed with reference to both normal and abnormal fields derived by the Octopus 201 automated perimeter (stimulus size III). Recommendations for this type of perimetric representation are suggested.

The qualitative comparative analysis of the visual field using computer assisted, semi-automated and manual instrumentation. I: Scoring system

Previous methods for the qualitative evaluation of visual field instruments are subject to certain limitations. A system is proposed to overcome these deficiencies. It has been developed from experiences of a clinical study involving 5 different visual field instruments. The method uses 4 levels of analysis and permits separate appraisals of diagnostic potential and detailed inter-instrument comparative evaluation.

Peripheral refractive correction and automated perimetric profiles

Abstract. The effect of peripheral refractive error correction on the automated perimetric sensitivity profile was investigated on a sample of 10 clinically normal, experienced observers. Peripheral refractive error was determined at eccentricities of 0°, 20° and 40° along the temporal meridian of the right eye using the Canon Autoref R‐1, an infra‐red automated refractor, under the parametric conditions of the Octopus automated perimeter. Perimetric sensitivity was then undertaken at these eccentricities (stimulus sizes 0 and III) with and without the appropriate peripheral refractive correction using the Octopus 201 automated perimeter. Within the measurement limits of the experimental procedures employed, perimetric sensitivity was not influenced by peripheral refractive correction.

The quantification of the visual field in computer-assisted threshold perimetry

Computer simulations were carried out to investigate the suitability of the Monte Carlo technique as a means for deriving the integral of sensitivity as measured over the three-dimensional surface area and thus providing a quantitative expression of the visual field. The simulations were based upon the standard normal data of Octopus programs 21 and 31 and upon age matched data for the central 30° threshold program of the Dicon AP3000. In addition, the sensitivity values at each eccentricity derived by the Octopus were also weighted for the product of retinal ganglion cell receptive field density and spatial summation and a further integration carried out. The indices for various types of field loss are illustrated.

The interpretation of the differential threshold in the central visual field.

Computer assisted perimetry has revolutionised the investigation of the visual field. Experience of central field assessment with the Octopus Automated Perimeter shows that sensitivity recorded with target size 3 across all age groups can frequently be greater than the published normative values. Use of the latter values can therefore provide a serious underestimation of field loss. Inter-individual variation in sensitivity is found within and between age groups. The limitations associated with the use of the measurement error to define abnormality and the additional problems of hypernormal thresholds and resolution of the blind spot are discussed. It is suggested that methods should be developed to evaluate sensitivity on an intra-individual basis.