Dark (version 0.9.4)

P7c: Seven parameter model

Description

The seven parameter model. An exponential decay followed by two linear phases.

Usage

P7c(a, X)

Arguments

a

An array of parameters; lcc{ a: parameter Units a[1]: cone threshold (log lum) a[2]: cone coefficient (log lum) a[3]: cone time constant (minutes) a[4]: S2 (log lum/ minute) a[5]: alpha point (minutes) a[6]: -(S2 + S3

X

The times when the model predicts thresholds.

Value

• The output depends upon the input. If the input is an array of length 1L or a boolean then a list is returned
• Pn
  number of parameters
• Mod
  name of the model
• If a parameter array is passed then the sum of residuals squared is calculated. This is used by optim to optimise the parameter estimates. Passing a parameter array and a series of putative times causes the function to return predicted thresholds.

Details

This function has three roles, to calculate the thresholds for given parameters a and times X. If missing X, then the function calls the values x and y from the .Globalenv and calculates the sum of residuals squared error (SSE) for a. If a is an array of length 1L or boolean then a description of the model is returned.

The use of the function H rather than an impulse function gives a three-fold increase is speed for the optim function.

References

O. Mahroo and T. Lamb. Recovery of the human photopic electroretinogram after bleaching exposures: estimation of pigment regeneration kinetics. The Journal of Physiology, 554(2):417, 2004.

T. Lamb and E. Pugh. Dark adaptation and the retinoid cycle of vision. Progress in Retinal and Eye Research, 23(3):307-380, 2004.

See Also

P3,P5c

Examples

set.seed(1234)
x <- 0:20
a <- c(-1.00,  1.00,  1.00, -0.24,  6.00,  0.20, 13.00)
tmp <- TestData(x, a)
y <- tmp$resid# P7c(TRUE)
# Describes the model

# P7c(a)
# The sum of squared residuals

# P7c(a,x)
# The fitted thresholds for given parameters 'a' and times 'x'