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function eccDegrees = RetinalEccentricityMMToDegrees(eccMm,species,method,eyeLengthMm)
% eccMm = DegreesToRetinalEccentricityMM(eccDegrees,[species],[method],[eyeLengthMm])
%
% Convert eccentricity in degrees to retinal eccentricity in mm. By
% default, this takes into account a simple model eye, rather than just
% relying on a linear small angle approximation.
%
% Input:
% eccDegrees -- retinal eccentricity in degrees
% species -- what species
% 'Human' -- Human eye [default]
% 'Rhesus' -- Rhesus monkey
% method -- what method
% 'DaceyPeterson' -- formulae from Dacey & Peterson (1992) [default]
% 'Linear' -- linear, based on small angle approx
% eyeLengthMm -- Eye length to assume for linear calculation, should be
% the posterior nodal distance. Defaults to the default values returned
% by function EyeLength for the chosen species.
%
% The Dacey and Peterson formulae are based on digitizing and fitting
% curves published by
% 1) Drasdo and Fowler, 1974 (British J. Opthth, 58,pp. 709 ff., Figure 2,
% for human.
% 2) Perry and Cowey (1985, Vision Reserch, 25, pp. 1795-1810, Figure 4,
% for rhesus monkey.
% These curves, I think, were produced by ray tracing or otherwise solving
% model eyes.
%
% The default eye length returned by EyeLength for Human is currently the Rodiek value of
% 16.1 mm. Drasdo and Fowler formulae are based on a length of about this,
% so the linear and DaceyPeterson methods are roughly consistent for small
% angles. Similarly with the Rhesus default. Using other EyeLength's will
% make the two methods inconsistent.
%
% The Dacey and Peterson equations don't go through (0,0), but rather
% produce a visual angle of 0.1 degree for an eccentricity of 0. This
% seems bad to me. I modified the formulae so that they use the linear
% approximation for small angles, producing a result that does go through
% (0,0). This may be related to the fact that there is some ambiguity in
% the papers between whether the center should be thought of as the fovea
% or the center of the optical axis. But I think this difference is small
% enough that the same formulae would apply across such a shift in origin.
%
% I digitized Drasdo and Fowler Figure 2 and compared it to what
% DegreesToRetinalEccentricity produces. I'd call agreement so-so, but
% considerably better than what the linear approximation produces. One
% could probably do better, but my intuition is that the deviations are
% small compared to eye to eye differences and differences that would be
% produced by different model eyes, so that juice isn't worth the squeeze.
% I pasted my digitization at the end of DegreesToRetinalEccentricity if
% anyone wants to fuss with this. But probably if you're going to do that,
% you should do the whole ray tracing thing with our best current model
% eye.
%
% I have not checked the fit to the Perrry and Cowey curve for Rhesus
% against a digitization of that figure.
%
% See also: EyeLength, DegreesToRetinalEccentricityMM, DegreesToRetinalMM, RetinalMMToDegrees
%
% 6/30/2015 dhb Wrote it.
%% Set defaults
if (nargin < 2 || isempty(species))
species = 'Human';
end
if (nargin < 3 || isempty(method))
method = 'DaceyPeterson';
end
if (nargin < 4 || isempty(eyeLengthMm))
switch (species)
case 'Human'
eyeLengthMm = EyeLength(species,'Rodieck');
case 'Rhesus'
eyeLengthMm = EyeLength(species,'PerryCowey');
otherwise
error('Unknown species specified');
end
end
%% Checks
if (any(eccMm < 0))
error('Can only convert non-negative eccentricities');
end
%% Do the method dependent thing
switch (method)
case 'DaceyPeterson'
% Out of paranoia, make sure we use the right eye length parameters
% for this method, so that the low angle linear approximation that
% we tag on comes out right.
switch (species)
case 'Human'
eyeLengthMm = EyeLength(species,'Rodieck');
case 'Rhesus'
eyeLengthMm = EyeLength(species,'PerryCowey');
otherwise
error('Unknown species specified');
end
% Set quadratic parameters
switch (species)
case 'Human'
a = 0.035; b = 3.4; c1 = 0.1;
case 'Rhesus'
a = 0.038; b = 4.21; c1 = 0.1;
otherwise
error('Unknown species passed');
end
% Evaulate the quadratic
eccDegrees = c1 + b*eccMm + a*(eccMm.^2);
% Replace small angles by the linear approximation
degreeThreshold = 0.2;
index = find(eccDegrees < degreeThreshold);
if (~isempty(index))
eccDegrees(index) = RetinalMMToDegrees(eccMm(index),eyeLengthMm,false);
end
case 'Linear'
eccDegrees = RetinalMMToDegrees(eccMm,eyeLengthMm,false);
otherwise
error('Unknown method passed')
end
end
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