\name{betacells}
\alias{betacells}
\alias{betacells.extra}
\docType{data}
\title{Beta Ganglion Cells in Cat Retina} 
\description{
  Point pattern of cells in the retina, each cell classified as `on' or `off'.
  A bivariate point pattern.
} 
\format{
  \code{betacells} is an object of class \code{"ppp"}
  representing the point pattern of cell locations.
  Entries include
  \tabular{ll}{
    \code{x} \tab Cartesian \eqn{x}-coordinate of cell \cr
    \code{y} \tab Cartesian \eqn{y}-coordinate of cell \cr
    \code{marks} \tab factor with levels \code{off} and \code{on} \cr
                 \tab indicating ``off'' and ``on'' cells
  }
  See \code{\link{ppp.object}} for details of the format.
  Cartesian coordinates are given in microns.

  \code{betacells.extra} is a list with one component \code{area}
  which is the vector of areas (in square microns) of the cells
  in the pattern.
}
\usage{data(betacells)}
\source{
  W\"assle et al (1981), Figure 6(a),
  scanned and processed by Stephen Eglen
  \email{S.J.Eglen@damtp.cam.ac.uk}
}
\section{Notes}{
  This is a new, corrected version of the old dataset
  \code{\link{ganglia}}. See below.
  
  These data represent a pattern of beta-type ganglion cells
  in the retina of a cat recorded by W\"assle et al. (1981).
  Beta cells are associated
  with the resolution of fine detail in the cat's visual system.
  They can be classified anatomically as ``on'' or ``off''.
 
  Statistical independence of the arrangement of the
  ``on''- and ``off''-components
  would strengthen the evidence for Hering's (1878) `opponent theory'
  that there are two separate channels for sensing
  ``brightness'' and ``darkness''.
  See W\"assle et al (1981). There is considerable current interest
  in the arrangement of cell mosaics in the retina, see
  Rockhill et al (2000).

  The dataset is a multitype point pattern giving the locations
  and types (``on'' or ``off'') of beta cells observed
  in a rectangle of dimensions \eqn{750 \times 990}{750 x 990} microns.
  Coordinates are given in microns (thousandths of a millimetre).
  
  The original source is Figure 6 of W\"assle et al (1981),
  which is a manual drawing of the beta mosaic observed
  in a microscope field-of-view of a whole mount of the retina.
  Thus, all beta cells in the retina were effectively projected onto the same
  two-dimensional plane.
  
  The data were scanned in 2004 by Stephen Eglen from
  Figure 6(a) of W\"assle et al (1981).
  Image analysis software was used to identify the soma (cell
  body). The \eqn{x,y} location of each cell was taken to be the
  centroid of the soma. The type of each cell (``on'' or `off'')
  was identified by referring to Figures 6(b) and 6(d).

  The area of each soma (in square microns) was also computed,
  and is provided in the dataset \code{betacells.extra}.

  Note that this is a corrected version of the \code{\link{ganglia}}  
  dataset provided in earlier versions of \pkg{spatstat}.
  The earlier data \code{\link{ganglia}} were not faithful to the scale
  in the original paper and contain some scanning errors.
}
\examples{
   data(betacells)
   plot(betacells)
   plot(betacells$window, main="beta cells")
   symbols(betacells$x, betacells$y,
       circles=sqrt(betacells.extra$area/pi),
       inches=FALSE, add=TRUE)
}
\references{
Hering, E. (1878) Zur Lehre von Lichtsinn. Vienna.

Van Lieshout, M.N.M. and Baddeley, A.J. (1999)
Indices of dependence between types in multivariate point patterns.
\emph{Scandinavian Journal of Statistics} \bold{26}, 511--532.

Rockhill, R.L., Euler, T. and Masland, R.H. (2000)
Spatial order within but not between types of retinal neurons.
\emph{Proc. Nat. Acad. Sci. USA} \bold{97}(5), 2303--2307.

W\"assle, H., Boycott, B. B. & Illing, R.-B. (1981).
Morphology and mosaic of on- and off-beta cells in the cat retina and
some functional considerations.
\emph{Proc. Roy. Soc. London Ser. B} \bold{212}, 177--195.
}
\keyword{datasets}
\keyword{spatial}