OPI on a Simulated Machine

There are now five simulators written for the OPI and included in the R OPI package. Three use the following formula based on a Cummulative Gaussian distribution to model the psychometric function (frequency-of-seeing) curve of a simulated responder.

probability of seeing x = fpr + (1 - fpr - fpn) * (1 - pnorm(x, true threshold, sd))

where fpr is the false positive rate; fnr is the false negative rate; and sd is the standard deviation of the Gaussian controlling the slope/spread of the psychometric function.

The first, "SimHenson", varies sd as a function of true threshold, according to the values in Table 1 of Henson et al. [1]. This necessitates using the Humphrey dB scale for your simulations. Response times are not simulated in this mode. As of July 2017, this mode also includes simulation of kinetic stimuli.

The second, "SimHensonRT", is the same as "SimHenson", but also simulates response times according to data from the paper of McKendrick et al.[2]

The third simply allows a fixed sd to be used, and so is unit independant.

The fourth simulator is "SimYes", where the simulated patient always responds "Seen".

The fifth simulator is "SimNo", where the simulated patient always responds "Not Seen".

How to obtain OPI for the Simulated Machine

Both implementations of the OPI are included in the OPI R package. See Downloads for instructions on how to install the package, and then experiment with the chooseOPI() function.

Manual

See help files in pakage and on CRAN

References

1. David B. Henson, Shaila Chaudry, Paul H. Artes, E. Brian Faragher, and Alec Ansons. Response Variability in the Visual Field: Comparison of Optic Neuritis, Glaucoma, Ocular Hypertension, and Normal Eyes. Investigative Ophthalmology & Visual Science, February 2000, Vol. 41, No. 2.
2. A.M. McKendrick, J. Denniss and A. Turpin Response times across the visual field: empirical observations and application to threshold determination. Vision Research 101 (2014) pp 1-10.