nblast {nat.nblast}

Calculate similarity score for neuron morphologies

Description

Uses the NBLAST algorithm that compares the morphology of two neurons. For more control over the parameters of the algorithm, see the arguments of NeuriteBlast.

Usage

nblast(
  query,
  target = getOption("nat.default.neuronlist"),
  smat = NULL,
  sd = 3,
  version = c(2, 1),
  normalised = FALSE,
  UseAlpha = FALSE,
  OmitFailures = NA,
  ...
)

Arguments

query

the query neuron.

target

a neuronlist to compare neuron against. Defaults to options("nat.default.neuronlist"). See nat-package.

smat

the scoring matrix to use (see details)

sd

Standard deviation to use in distance dependence of NBLAST v1 algorithm. Ignored when version=2.

version

the version of the algorithm to use (the default, 2, is the latest).

normalised

whether to divide scores by the self-match score of the query

UseAlpha

whether to weight the similarity score for each matched segment to emphasise long range neurites rather then arbours (default: FALSE, see UseAlpha section for details).

OmitFailures

Whether to omit neurons for which FUN gives an error. The default value (NA) will result in nblast stopping with an error message the moment there is an error. For other values, see details.

...

Additional arguments passed to NeuriteBlast or the function used to compute scores from distances/dot products. (expert use only).

Details

when smat=NULL options("nat.nblast.defaultsmat") will be checked and if NULL, then smat.fcwb or smat_alpha.fcwb will be used depending on the value of UseAlpha.

When OmitFailures is not NA, individual nblast calls will be wrapped in try to ensure that failure for any single neuron does not abort the whole nblast call. When OmitFailures=FALSE, missing values will be left as NA. OmitFailures=TRUE is not (yet) implemented. If you want to drop scores for neurons that failed you will need to set OmitFailures=FALSE and then use na.omit or similar to post-process the scores.

Note that when OmitFailures=FALSE error messages will not be printed because the call is wrapped as try(expr, silent=TRUE).

Internally, the plyr package is used to provide options for parallelising NBLAST and displaying progress. To display a progress bar as the scores are computed, add .progress="natprogress" to the arguments (non-text progress bars are available – see create_progress_bar). To parallelise, add .parallel=TRUE to the arguments. In order to make use of parallel calculation, you must register a parallel backend that will distribute the computations. There are several possible backends, the simplest of which is the multicore option made available by doMC, which spreads the load across cores of the same machine. Before using this, the backend must be registered using registerDoMC (see example below).

Value

Named list of similarity scores.

NBLAST Versions

The nblast version argument presently exposes two versions of the algorithm; both use the same core procedure of aligning two vector clouds, segment by segment, and then computing the distance and absolute dot product between the nearest segment in the target neuron for every segment in the query neuron. However they differ significantly in the procedure used to calculate a score using this set of distances and absolute dot products.

Version 1 of the algorithm uses a standard deviation (argument sd) as a user-supplied parameter for a negative exponential weighting function that determines the relationship between score and the distance between segments. This corresponds to the parameter \sigma in the weighting function:

f=\sqrt{|\vec{u_{i}}\cdot\vec{v_{i}}|\exp\left(-d_{i}^{2}/2\sigma^{2}\right)}

This is the same approach described in Kohl et al 2013 and the similarity scores in the interval (0,1) described in that paper can exactly recapitulated by setting version=1 and normalised=TRUE.

Version 2 of the algorithm is described in Costa et al 2014. This uses a more sophisticated and principled scoring approach based on a log-odds ratio defined by the distribution of matches and non-matches in sample data. This information is passed to the nblast function in the form of a scoring matrix (which can be computed by create_scoringmatrix); a default scoring matrix smat.fcwb has been constructed for Drosophila neurons.

Which version should I use? You should use version 2 if you are working with Drosophila neurons or you have sufficient training data (in the form of validated matching and random neuron pairs to construct a scoring matrix). If this is not the case, you can always fall back to version 1, setting the free parameter (sd or \sigma) to a value that encapsulates your understanding of the location precision of neurons in your species/brain region of interest. In the fly brain we have used \sigma=3 microns, since previous estimates of the localisation of identifiable features of neurons (Jefferis, Potter et al 2007) are of this order.

UseAlpha

In NBLAST v2, the alpha factor for a segment indicates whether neighbouring segments are aligned in a similar direction (as typical for e.g. a long range axonal projection) or randomly aligned (as typical for dendritic arbours). See Costa et al. for details. Setting UseAlpha=TRUE will emphasise the axon, primary neurite etc. of a neuron. This can be a particularly useful option e.g. when you are searching by a traced fragment that you know or suspect to follow an axon tract.

References

Kohl, J. Ostrovsky, A.D., Frechter, S., and Jefferis, G.S.X.E (2013). A bidirectional circuit switch reroutes pheromone signals in male and female brains. Cell 155 (7), 1610–23 doi:10.1016/j.cell.2013.11.025.

Costa, M., Ostrovsky, A.D., Manton, J.D., Prohaska, S., and Jefferis, G.S.X.E. (2014). NBLAST: Rapid, sensitive comparison of neuronal structure and construction of neuron family databases. bioRxiv preprint. doi:10.1101/006346.

Jefferis G.S.X.E., Potter C.J., Chan A.M., Marin E.C., Rohlfing T., Maurer C.R.J., and Luo L. (2007). Comprehensive maps of Drosophila higher olfactory centers: spatially segregated fruit and pheromone representation. Cell 128 (6), 1187–1203. doi:10.1016/j.cell.2007.01.040

See Also

nat-package, nblast_allbyall, create_scoringmatrix, smat.fcwb

Examples

# load sample Kenyon cell data from nat package
data(kcs20, package='nat')
# search one neuron against all neurons
scores=nblast(kcs20[['GadMARCM-F000142_seg002']], kcs20)
# scores from best to worst, top hit is of course same neuron
sort(scores, decreasing = TRUE)
hist(scores, breaks=25, col='grey')
abline(v=1500, col='red')

# plot query neuron
open3d()
# plot top 3 hits (including self match with thicker lines)
plot3d(kcs20[which(sort(scores, decreasing = TRUE)>1500)], lwd=c(3,1,1))
rest=names(which(scores<1500))
plot3d(rest, db=kcs20, col='grey', lwd=0.5)

# normalised scores (i.e. self match = 1) of all neurons vs each other
# note use of progress bar
scores.norm=nblast(kcs20, kcs20, normalised = TRUE, .progress="natprogress")
hist(scores.norm, breaks=25, col='grey')
# produce a heatmap from normalised scores
jet.colors <- colorRampPalette( c("blue", "green", "yellow", "red") )
heatmap(scores.norm, labCol = with(kcs20,type), col=jet.colors(20), symm = TRUE)

## Not run: 
# Parallelise NBLASTing across 4 cores using doMC package
library(doMC)
registerDoMC(4)
scores.norm2=nblast(kcs20, kcs20, normalised=TRUE, .parallel=TRUE)
stopifnot(all.equal(scores.norm2, scores.norm))

## End(Not run)