letters to nature..............................................................

Genome sequence of theultrasmall unicellular red alga
Cyanidioschyzon merolae 10D
Motomichi Matsuzaki1, Osami Misumi2, Tadasu Shin-i3,
Shinichiro Maruyama4, Manabu Takahara5, Shin-ya Miyagishima2,
Toshiyuki Mori2, Keiji Nishida6, Fumi Yagisawa6, Keishin Nishida2,
Yamato Yoshida2, Yoshiki Nishimura6, Shunsuke Nakao6,
Tamaki Kobayashi6, Yu Momoyama6, Tetsuya Higashiyama6,
Ayumi Minoda4, Masako Sano3, Hisayo Nomoto3, Kazuko Oishi3,
Hiroko Hayashi3, Fumiko Ohta3, Satoko Nishizaka3, Shinobu Haga3,
Sachiko Miura3, Tomomi Morishita3, Yukihiro Kabeya7,
Kimihiro Terasawa7, Yutaka Suzuki8, Yasuyuki Ishii9, Shuichi Asakawa9,
Hiroyoshi Takano10, Niji Ohta7, Haruko Kuroiwa2, Kan Tanaka4,
Nobuyoshi Shimizu9, Sumio Sugano8, Naoki Sato7, Hisayoshi Nozaki6,
Naotake Ogasawara11, Yuji Kohara3 & Tsuneyoshi Kuroiwa2
1
Department of Biomedical Chemistry, Graduate School of Medicine, Universityof Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan2
Department of Life Science, College of Science, Rikkyo (St Paul’s) University,
3-34-1 Nishiikebukuro, Toshima-ku, Tokyo 171-8501, Japan
3
Center for Genetic Resource Information, National Institute of Genetics,
1111 Yata, Mishima, Shizuoka 411-8540, Japan
4
Institute of Molecular and Cellular Biosciences, University of Tokyo, 1-1-1 Yayoi,
Bunkyo-ku, Tokyo 113-0032, Japan
5
National Institute of Livestock and Grassland Science, 2 Ikenodai, Tsukuba,
Ibaraki 305-0901, Japan
6
Department of Biological Sciences, Graduate School of Science, University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
7
Department of Biochemistry and Molecular Biology, Faculty of Science, Saitama
University, 255 Shimo-Ohkubo, Sakura-ku, Saitama, Saitama 338-8570, Japan
8
Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku,
Tokyo 108-8639, Japan
9
Department of Molecular Biology, Keio University School of Medicine,
35 Shinanomachi, Shinjyuku-ku, Tokyo 160-8582, Japan10
Department of Biological Science, Graduate School of Science and Technology,
Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
11
Graduate School of Biological Science, Nara Institute of Science and Technology,
8916-5 Takayama, Ikoma, Nara 630-0101, Japansynthetic eukaryotes, and contains a minimal set of small membrane-bounded compartments; for example, a microbody(peroxisome), a single Golgi apparatus with two cisternae, coatedvesicles, a single endoplasmic reticulum, and a few lysosome-likestructures, as well as a small volume of cytosol (Fig. 1)6. One of
the main points of interest that we discuss regarding this alga
focuses on the origin, evolution and fundamental traits (forexample, multiplication and differentiation) of single- as well as
double-membrane-bounded organelles in plant cells. C. merolae,
with its complete genomic information, provides an excellent
opportunity for addressing such basic questions using microarrayand proteome analyses. In addition, from an evolutionary perspective, C. merolae has other noteworthy properties that allow us to
study the origin of eukaryotic cells, primary endosymbiosis between
cyanobacteria and eukaryotic hosts, and secondary endosymbiosis
between red algae and their hosts.
Samples of C. merolae 10D were isolated7 from the hot springalgal collection provided by G. Pinto (Naples University). The entire
C. merolae genome was sequenced using the random sequencing
method (see Methods). We obtained 16,520,305 base pairs (bp;approximately 99.98% of the estimated total length) of the nuclear
genome sequence (Fig. 2, Table 1, and Supplementary Fig. 1 andSupplementary Table 1) with 46 gaps. The genome is distributed
among the 20 chromosomes and ranges in size from approximately
0.42 to 1.62 Mb. No significant deviation in statistical parameters,such as base composition and gene density, were observed among
the chromosomes (Supplementary Table 1). The overall GþC
composition was 55.0%. The dinucleotide CpG in the C. merolae
genome was exceptionally over-represented (1.151) compared withthe expected value from observations of GþC content; it is generally
underrepresented in other eukaryote genomes (Table 1).
The putative repeat unit of telomeres in C. merolae is.............................................................................................................................................................................

Small, compact genomes of ultrasmall unicellular algae provide
information on the basic and essential genes that support the livesof photosynthetic eukaryotes, including higher plants1,2. Here we
report the 16,520,305-base-pair sequence of the 20 chromosomesof the unicellular red alga Cyanidioschyzon merolae 10D as thefirst complete algal genome. We identified 5,331 genes in total, of
which at least 86.3% were expressed. Unique characteristics ofthis genomic structure include: a lack of introns in all but 26
genes; only three copies of ribosomal DNA units that maintainthe nucleolus; and two dynamin genes that are involved only inthe division of mitochondria and plastids. The conserved mosaic
origin of Calvin cycle enzymes in this red alga and in green plants
supports the hypothesis of the existence of single primary plastid
endosymbiosis. The lack of a myosin gene, in addition to theunexpressed actin gene, suggests a simpler system of cytokinesis.
These results indicate that the C. merolae genome provides amodel system with a simple gene composition for studying theorigin, evolution and fundamental mechanisms of eukaryotic
cells.
C. merolae is a small (2 mm diameter) unicellular organism that
inhabits sulphate-rich hot springs (pH 1.5, 45 8C) (Fig. 1). The cellsof C. merolae offer unique advantages for studies of mitochondrialand plastid (chloroplast) divisions1–4 because they do not have arigid cell wall and contain just one nucleus, one mitochondrion and
one plastid, divisions of which can be highly synchronized by light/
dark cycles5. This alga also has the smallest genome of all photoNATURE | VOL 428 | 8 APRIL 2004 | www.nature.com/nature

Figure 1 The unicellular red alga C. merolae 10D. Phase contrast-fluorescent images of
the interphase (a) and dividing (b) cells show localization of nuclear (top), mitochondrial(middle) and plastid DNA (bottom in blue/white) after DAPI staining. The plastids emit red
autofluorescence. The schematic dividing cell (c) contains a nucleus, a V-shaped
mitochondrion, a dumb-bell-shaped plastid, a microbody and a Golgi apparatus, divisionsof which can be highly synchronized by light/dark cycles.

©2004 Nature Publishing Group

653

letters to nature

Figure 2 Representation of C. merolae chromosomes. a, Chromosome 1 and mapping of
the BAC clones (50 kb and 100 kb). The chromosome is represented as a bar withpseudo-colour assignments of local GþC contents. Side rods represent genes and genelike elements (left for transcription towards the top, and right for the bottom). Telomere
and subtelomeric elements are indicated respectively as semicircles and purple
654

rectangles at each end of the chromosome. b, A bird’s-eye view of 20 chromosomes
showing GþC contents, genes, subtelomeric elements (designations on the side: P, H, L,
A, E, and so on), and RNA genes. A putative centromeric (AþT-rich) region is located oneach chromosome.

©2004 Nature Publishing Group

NATURE | VOL 428 | 8 APRIL 2004 | www.nature.com/natureletters to nature
Table 1 Nuclear and organellar genomes of C. merolae and comparison to other genomes
FeatureC. merolae

S. pombe

S. cerevisiae

A. thaliana

P. falciparum...................................................................................................................................................................................................................................................................................................................................................................

NucleusNo. of chromosomes
Sequenced length (bp)GþC content (%)
CpG occurrence (Obs./Exp.)
No. of genes
Mean gene length† (bp)Gene density (bp per gene)
Per cent coding
Genes with introns (%)
No. of introns
Mean length of intron (bp)
Mean length of exons (bp)
No. of tRNA genes
No. of 5S rRNA genes
No. of 18S, 5.8S and 28S rRNA units
Mitochondrion
Genome size (bp)
No. of protein genes
Density (bp per protein genes)
Plastid
Genome size (bp)
No. of protein genes
Density (bp per protein genes)
Genes with introns (%)

20
16,520,305
55.0
1.151
5,331
1,552
3,099
44.9
0.5
27
248
1,540
30
3
3

3
12,462,637*
36.0
0.886
4,929
1,426
2,528
57.5
43.0
4,730
81
ND
174
30
200–400

16
12,495,682*
38.3
0.803
5,770
1,424
2,088
70.5
5.0
272
NA
ND
274
100–150
100–1505
115,409,949*
34.9
0.724
25,498
1,310
4,526
28.8
79.0
107,784
170
170
620
1,000
700–800

14
22,853,764
19.4
0.765
5,268
2,283
4,338
52.6
53.9
7,406
179
949
43
3
7

32,211
34
947

19,43110
1,943

85,779292,958

366,924
58
6,326

5,967
3
1,989

149,987
208
721
0

NA
NA
NA
NA

NA
NA
NA
NA

154,478
79
1,955
18

29,422309810...................................................................................................................................................................................................................................................................................................................................................................
Data for the other organisms are from ref. 31. ND, not determined; NA, not applicable.
* Ribosomal DNA repeats were not sequenced.
† Excluding introns.

GGGGGGAAT, and as far as could be determined experimentally, it
is found on both ends of the chromosomes. In addition, several
sequence elements up to 20 kilobases (kb) in length were duplicatedin 30 of the 40 putative subtelomeric regions (Fig. 2, Supplementary
Fig. 1). Each chromosome has, in varying degrees, a single AþT-richregion on its mid-section. As chromosomal centromeric regions
generally have a biased base composition, this AþT-rich region
possibly defines centromeres (Fig. 2). The centromeres were confirmed via immunological experiments using antibodies against
CENP-A, which was identified in the C. merolae genome (data notshown). Unlike many other eukaryotes, the C. merolae genome does
not contain tandem repeated arrays of ribosomal RNA (rRNA)
genes (Fig. 2). A single rRNA gene unit (18S-5.8S-28S) was
discovered on three separate loci. The three units were virtually
identical in sequence. Moreover, C. merolae has only three copies of
the 5S rRNA gene, the sequences of which are also almost identical.

Figure 3 Comparison of the functional classification of C. merolae proteins with otherorganisms. Columns represent the proportion of proteins assigned to KOG classification ofeach organism: C. merolae, S. cerevisiae, S. pombe and A. thaliana in a left-to-rightfashion. The actual numbers of proteins assigned to each classification are given in
Supplementary Table 3.
NATURE | VOL 428 | 8 APRIL 2004 | www.nature.com/nature

Therefore, C. merolae has the smallest set of rRNA genes among all
eukaryotes thus far studied. These results might be related to theexistence of a single small nucleolus without nucleolus-associatedchromatin. Furthermore, they also promote studies on the origin
and formation of the nucleolus, because even prokaryotic cells withmore than three copies of rRNA gene units do not have a nucleolus.
A full-length complementary DNA (cDNA) library was used tomap expressed genes within the C. merolae genome. Fortunately,
99.85% of the expressed sequence tags (ESTs) were mapped on the
genome sequence. In addition, many cDNA clones encoded a single
open reading frame (ORF) bridging both end sequences. Thissuggests that most C. merolae genes lack introns. The predictedgenes were automatically annotated using several databases (seeMethods and Supplementary Information). As a result, 5,331 genes
were identified, and 86.3% of them had corresponding ESTs
(Supplementary Table 2). The number of genes in the C. merolae
genome is similar to those found in yeasts and malarial parasites,
despite the great ecological differences between these species(Table 1). Furthermore, the genes of the C. merolae genome areremarkable for their paucity of introns. Only 26 genes (0.5% of theprotein genes) contained introns, and all but one of them had onlya single intron. These introns had strict consensus sequences(Supplementary Fig. 2 and Supplementary Table 3).
Figure 3 summarizes the repertoire of C. merolae proteins on thebasis of their assignment to eukaryotic clusters of orthologous
groups (KOGs)8. Of the 4,771 predicted proteins, 2,536 wereassigned to KOGs, by emulating the NCBI KOGnitor service(http://www.ncbi.nlm.nih.gov/COG/new/kognitor.html). The distribution of the functional classification of C. merolae was compared with those of other free-living unicellular eukaryotes, such as
Saccharomyces cerevisiae9 and Schizosaccharomyces pombe10, and ahigher plant Arabidopsis thaliana11. The distribution was on thewhole similar to both yeasts which have similar genome size
although C. merolae cells contain plastids. The lowered proportionof genes for ‘secondary metabolites biosynthesis, transport andcatabolism’ found in these unicellular organisms, as comparedwith that of A. thaliana, might reflect their simple cellular organizations (Fig. 3, Supplementary Table 4).
In C. merolae, the division of double-membrane-bounded mitochondria and plastids involves a dynamic trio: an FtsZ ring ofbacterial origin, electron dense mitochondrial/plastid dividing rings(MD and PD rings), and eukaryotic mechanochemical dynamin©2004 Nature Publishing Group

655

letters to naturerings. Four genes representing mitochondrial FtsZ (FtsZ2-1 and
FtsZ2-2) and plastid FtsZ (FtsZ1-1 and FtsZ1-2) were identified12,13.
A large gene family consisting of more than 10 members encoding
functionally diverse dynamins with a wide range of membrane
pinching roles have been found in other organisms; however, only
two dynamin genes (C. merolae Dnm1 and Dnm2) are found, with arole in the later stages of the mitochondrion14 and plastid13 division,respectively. These findings suggest that plastids and mitochondria
divide in a similar way, using very common systems consisting of theamalgamation of bacterial and eukaryotic rings. The dynamic trio ofplastid division is conserved in lower algae to higher plants15,16. Withmitochondrial divisions, however, whilst dynamin rings are retainedin higher organisms, FtsZ and MD rings are not clearly observed, and
it is possible that they were replaced by other systems during
eukaryotic evolution13. MD/PD ring genes are yet unknown,
although their identification should be accelerated by works suchas this. Although the microbody, a single-membrane-bounded organelle, divides by binary fission in C. merolae17, it lacks Pex11p, which
is a known key regulator of microbody division and proliferation18.
The following proteins related to cell motility and cytokinesis
were encoded in C. merolae (Supplementary Table 5); one set of
tubulin, two actins, five proteins of the kinesin family, and several
intermediate filament proteins. However, no genes encoding myosinor proteins containing dynein motor domains were found. The
absence of the myosin gene is consistent with the fact that electron
microscopy and immuno-detection19 techniques did not detectmicrofilaments of actin; cDNA clones for actin genes were also not
obtained. In the red alga Cyanidium caldarium RK-1,which is closely
related to C. merolae but has a genome double the size7, cells divide
using a contractile ring of actin filaments19. C. merolae cells therefore
seem to divide using a system that is simpler than that of actomyosin.
C. merolae has noteworthy properties, which are relevant for
examining the origin of eukaryotes, and primary and secondary
plastid endosymbiosis20. Only 30 transfer RNAs (tRNAs) weredetected in the nuclear genome using the program tRNAscan-SE
with relaxed parameter settings (Fig. 2). Some of these tRNA genes
showed possible archaeal features, namely, ectopic introns andanticodon GAU for tRNA-Ile (Supplementary Fig. 3). Four ofthese tRNA genes seemed to have introns in the D-loop region,
whereas the introns of eukaryotic tRNA genes are limited to a site 3 0
to the anticodon. As ectopic tRNA introns have been reported insome archaeal genomes21, this could explain the paucity of detectedtRNA genes in C. merolae; tRNAscan-SE might have overlooked
other tRNAs owing to the existence of unknown types of ectopic
introns. Another point to note is that C. merolae possesses a single
tRNA-Ile with anticodon GAU, which has not been observed ineukaryotes, but only in prokaryotes21.
Standard sets of photosystem genes, including those encoding
phycobilisome components, were observed in C. merolae. Many ofthem (11 PSI genes and 17 PSII genes) are encoded in the plastid
genome22, while PsbO, P, U, Z as well as a distant PsbQ homologue
are encoded in the nuclear genome. Although only PsbU and PsbZ
were previously identified in red algal PSII, the localization of PsbP
and putative PsbQ in PSII, as recently suggested in Synechocystis
sp. PCC680323 is an interesting subject of proteomic study. The
genes psaH, N, X, as well as psbS and ndh genes are not encoded ineither the plastid or the nuclear genomes. Therefore, the photosystems of C. merolae lack various mechanisms for dissipatingexcessive light energy.
Enzymes of the Calvin cycle in plants are known to be a mosaic
of enzymes originating from cyanobacteria-like ancestors of anendosymbiont and its eukaryotic host24. Red algal ribulose-1,5bisphosphate carboxylase/oxygenase (Rubisco) is known to be a
product of horizontal gene transfer25. The origin of other Calvincycle enzymes is essentially identical in C. merolae and A. thaliana(Supplementary Fig. 4 and Supplementary Table 6). It is highly
probable that the complex and mosaic origin of Calvin cycle
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enzymes derived from common ancestors of green plants and red
algae, and no essential changes occurred after the separation of thetwo lineages. This is strong support for the concept of a single eventof primary plastid endosymbiosis. Among the known translocon
proteins of plastids, Toc34, Toc75, Tic20, Tic22 and Tic110 wereencoded in the C. merolae genome, but other proteins such as
Toc159, Tic40 and Tic55 were not found. Results of phylogenetic
analysis of the five translocon components (to be published elsewhere) also suggest the concept26.
Another aspect of the comparative genomics of the red algal
genome is secondary endosymbiosis. Cryptophytes are thought toretain a remnant of the endosymbiotic red algal nucleus, thenucleomorph, in the periplastidic compartment. The sequencingof the cryptophyte alga Guillardia theta nucleomorph genome
revealed a number of curious architectural features that might beshared by the genome of red algae27. C. merolae chromosomes
showed multiple subtelomeric duplications, but did not containrRNA gene clusters such as those of the nucleomorph genome. Thisimplies that the telomeric rRNA gene clusters observed in the
nucleomorph genome, as well as other prominent genome structures such as overlapped genes, appeared after secondary symbiosis.
It is also notable that ectopic tRNA introns are also reported innucleomorph tRNAs28. Details of the comparisons with the nucleomorph genome will be presented elsewhere.
Light signal transduction is critical for the growth and differentiation of photoautotrophic organisms. As the division of C. merolae
cells is synchronized by light, an elaborate mechanism for lightsignal transduction must exist. Several putative blue light receptor(cryptochrome) genes were found in C. merolae, whereas no genes
encoding phytochromes and phototropins were identified. As
bacterial phytochrome genes are only found in some species of
cyanobacteria with large genomes29, the ancestor of plastids might
be an ancestral cyanobacterium without phytochromes. This alsosuggests that the phytochromes of higher plants might not be of
cyanobacterial origin. In higher plants, various signalling pathways(such as the two-component system consisting of histidine kinases
and response regulators as well as a MAP kinase cascade) are
involved in the signal transduction of various hormones, and in
the development of organs. In C. merolae, the presence of only asingle candidate for histidine kinase and a dozen MAP kinaserelated molecules is suggested. However, there are no response
regulators other than those that are plastid-encoded, trimeric G
protein and adenylate cyclase. Thus, C. merolae appear to use only alimited repertoire of signal transduction mechanisms, which corroborates the lack of cell differentiation in this alga.
C. merolae is an alga in which all of the three genome compartments—nucleus, mitochondrion (32,211 bp) 30 and plastid(149,987 bp)22—have been sequenced. Such information is a prerequisite for future studies on proteomics, expression analysis using
microarrays, and structural biology with heat-stable proteins thatare unique among eukaryotes. All of this information will, in turn,help elucidate the origin, evolution and fundamental mechanismsof the single- as well as double-membrane-bounded organelles, andultimately all photosynthetic eukaryotes. In addition, this hot
spring alga will be useful in analysing the mechanisms of heat andacid tolerance in eukaryotic cells.
A

Methods
Whole genome shotgun sequencing
We sequenced the C. merolae genome by the whole genome random sequencing method(see Supplementary Information for details). About 335,000 insert ends were sequenced,
which covered the genome 11 times. BAC libraries with two subsets were constructed and alarge-scale full-length cDNA library from cells cultured under various growth conditions
prepared. The sequences were assembled using Phrap, further examined by referring toanother assembly using ARACHNE, and edited using CONSED. The scaffolds were built
within the hybridization groups using read-pair information from the BAC, shotgun and
cDNA clones. The gaps between the contigs were closed by primer walking PCR, andmate-pair clone and BAC clone sequencings.

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NATURE | VOL 428 | 8 APRIL 2004 | www.nature.com/natureletters to nature
Gene identification and annotation
We principally used two strategies for gene prediction and combined the results (seeSupplementary Information for details). (1) Each read-pair of cDNA clones was mappedon the contigs using BLAST and putatively transcribed regions were determined byclustering the mapped pairs. (2) ORFs likely to encode a protein showing similarity toknown proteins or having known motifs were identified respectively by the BLASTP
program with GenBank nr database, or a HMMER program with a Pfam database. A
functional classification was performed based on the NCBI KOG. The tRNA genes weredetected using the tRNAscan-SE program with relaxed parameters (-X 15 -I -36).
Received 13 November 2003; accepted 5 February 2004; doi:10.1038/nature02398.
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Supplementary Information accompanies the paper on www.nature.com/nature.
NATURE | VOL 428 | 8 APRIL 2004 | www.nature.com/nature

Acknowledgements We thank many colleagues for discussions, especially K. Kita, Y. Watanabe,
H. Fujiwara and T. Q. Ueda. We also thank Trans New Technology, Inc. for providing
computational resources. This work was supported by Grants-in-Aid for Scientific Research on
Priority Areas “Genome” from the Ministry of Education, Culture, Sports, Science, andTechnology of Japan, and a Grant-in-Aid from the Promotion of Basic Research Activities forInnovative Biosciences (ProBRAIN).
Competing interests statement The authors declare that they have no competing financial
interests.
Correspondence and requests for materials should be addressed to T.K. (tsune@rikkyo.ne.jp) or
M.M. (mzaki@biol.s.u-tokyo.ac.jp). Chromosome sequences were submitted to DDBJ withaccession numbers AP006483–AP006502 (chromosome 1–20) and AP006600–AP006614(unassigned contigs). Sequences and annotation are available at
http://merolae.biol.s.u-tokyo.ac.jp/ or http://dolphin.lab.nig.ac.jp/publish/.

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Long-lasting sensitization to a given
colour after visual search
Chia-huei Tseng1*, Joetta L. Gobell1* & George Sperling1,2
1
Department of Cognitive Sciences, and 2Neurobiology and Behavior, andInstitute of Mathematical Behavioral Sciences, University of California, Irvine,
California 92697, USA* Present addresses: Center of Cognitive Science, Rutgers University, New Brunswick, New Jersey 08854,
USA (C.-h.T.); Department of Psychology and Center for Neural Science, New York University, New York,New York 10003, USA (J.L.G.).............................................................................................................................................................................

Visual attention enables an observer to select specific visual
information for processing. In an ambiguous motion task inwhich a coloured grating can be perceived as moving in either of
two opposite directions depending on the relative salience of twocolours in the display, attending to one of the colours influences
the direction in which the grating appears to move1. Here, we use
this secondary effect of attention in a motion task to measure theeffect of attending to a specific colour in a search task. Observers
performed a search task in which they searched for a target letter in
a 4 3 4 coloured matrix. Each of the 16 squares within a matrix
was assigned one of four colours, and observers knew that thetarget letter would appear on only one of these colours throughout
the experiment. Observers performed the ambiguous motion task
before and after the search task. Attending to a particular colour
for a brief period in the search task profoundly influenced theperceived direction of motion. This effect lasted for up to one
month and in some cases had to be reversed by practising searches
for the complementary colour, indicating a much longer-persisting effect of attention than has been observed previously.
To investigate the consequences of attending to a particularcolour, we designed a search task that requires observers to attend
to a particular colour and to ignore all other colours. The task is toreport the location of a target letter among other ‘distracter’
characters. Each trial consists of ten consecutive 4 £ 4 matrices
displayed rapidly (Fig. 1a). Each of the 16 squares of a matrix is
randomly assigned one of four colours (red, green, yellow or blue).
The initial frame rate was two matrices per second and it wasincreased as observers’ performance improved to as high as 19
matrices s21 for the fastest observer. At the average final speed of 10
frames s21, each colour display was shown for 50 ms and then the
to-be-searched letters and numbers appeared superimposed on thecoloured squares for an additional 50 ms. The purpose of advancing
the colour matrix relative to the target was to induce the observers to
use the colour to find the target. A target colour, either red or green,
was assigned to each observer for the duration of the first phase of
the experiment.
At the beginning of each trial, observers were shown a randomly©2004 Nature Publishing Group

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