Exp Brain Res
DOI 10.1007/s00221-010-2327-y
R ES EA R C H A R TI CLE
A body-centred frame of reference drives spatial priming
in visual search
Keira Ball · Daniel Smith · Amanda Ellison ·
Thomas Schenk
Received: 22 October 2009 / Accepted: 5 June 2010
© Springer-Verlag 2010
Abstract Spatial priming in visual search is a well-documented phenomenon. If the target of a visual search is presented at the same location in subsequent trials, the time
taken to Wnd the target at this repeated target location is signiWcantly reduced. Previous studies did not determine which
spatial reference frame is used to code the location. At least
two reference frames can be distinguished: an observerrelated frame of reference (egocentric) or a scene-based
frame of reference (allocentric). While past studies suggest
that an allocentric reference frame is more eVective, we
found that an egocentric reference frame is at least as eVective as an allocentric one (Ball et al. Neuropsychologia47(6):1585–1591, 2009). Our previous study did not
identify which speciWc egocentric reference frame was used
for the priming: participants could have used a retinotopic or
a body-centred frame of reference. Here, we disentangled the
retinotopic and body-centred reference frames. In the retinotopic condition, the position of the target stimulus, when
repeated, changed with the Wxation position, whereas in the
body-centred condition, the position of the target stimulus
remained the same relative to the display, and thus to the
body-midline, but was diVerent relative to the Wxation position. We used a conjunction search task to assess the generality of our previous Wndings. We found that participants relied
K. Ball · D. Smith · A. Ellison · T. Schenk
Department of Psychology,
Durham University, Durham, UK
K. Ball (&)
Cognitive Neuroscience Research Unit,
Wolfson Research Institute, Durham University,
Queen’s Campus, University Boulevard,
Stockton on Tees, TS17 6BH Durham, UK
e-mail: [email protected]
on body-centred information and not retinotopic cues. Thus,
we provide further evidence that egocentric information, and
speciWcally body-centred information, can persist for several
seconds, and that these eVects are not speciWc to either a feature or a conjunction search paradigm.
Keywords Egocentric · Frames of reference · Spatial
memory · Conjunction search · Priming
Introduction
DiVerent frames of reference are employed when considering the locations of objects. Mou et al. (2008) state that two
components are used to deWne an object’s location: a reference direction (e.g. in front of, to the East of) and a reference object (e.g. the viewer, the post box). If the reference
object is the viewer, the reference frame is egocentric, and
if the reference object is a landmark the frame of reference
is allocentric (Burgess et al. 2004; Witt et al. 2008). Both of
these frames of reference are integral to our interactions
with the world.
Milner and Goodale (1993) argue that egocentric representations support visuomotor control: to accurately perform any motor action the subject must know how the
location of the target object corresponds with the position
of their eVector. They also argue that these representations
are short-lived as there would be no advantage to storing
this information after the motor action has been completed
(Milner and Goodale 1993, 2006; Westwood et al. 2003).
Research from various areas supports the notion that egocentric dorsal stream representations are highly transient.
For example, the observations that the visuomotor abilities
of visual agnosia (ventral stream damage) and optic ataxia
(dorsal stream damage) patients are diVerentially aVected
123
Exp Brain Res
by the introduction of a delay support the notion that dorsal
stream information is highly transient (Goodale et al. 1994;
Himmelbach and Karnath 2005; Milner et al. 2001; Milner
and Goodale 2006). The contrasting eVects of visual illusions on action and perception are also taken as evidence
that dorsal stream representations have a short duration; for
example, while Aglioti et al. (1995) observed that visual
illusions do not aVect motor performance when an action is
executed immediately, Westwood and Goodale (2003)
found that they do have a signiWcant eVect on motor performance after a delay of 2-s. This suggests that after a delay
the ventral stream, rather than the dorsal stream, provides
the visual information for the control of the movement, and
as perceptual information is susceptible to visual illusions,
motor performance is aVected.
To investigate whether egocentric representations can be
stored, research has evaluated whether the egocentric information learnt in an initial phase can be transferred to a subsequent testing phase. Some studies of topographical
memory have observed that changing the egocentric information between the learning and testing phases disrupts
performance less than changing the allocentric information
between the two phases and therefore conclude that egocentric information cannot be stored (Burgess et al. 2004;
Simons and Wang 1998; Wang and Simons 1999). Additionally, there is a body of research investigating the processes involved in updating egocentric representations. It is
thought that the updating of egocentric representations
needs to be continuous in order to take account of object
and/or observer motion, and this indicates that there would
be no advantage to storing them (Mou et al. 2004; Wang
and Simons 1999; Wang and Spelke 2000). This again suggests that egocentric representations have a short time span.
Conversely, other studies report that changes in egocentric
position between the learning and testing phases do aVect
the recognition performance of object locations, thus
implying that egocentric representations can persist over
time and aVect the subsequent processing of spatial
locations (Christou and BuelthoV 1999; Diwadkar and
McNamara 1997; Finlay et al. 2007; Shelton and McNamara
2004).
Spatial priming in visual search is believed to be a suitable method to investigate the duration of egocentric representations as it allows memory for target locations to be
tested over short time spans. Priming refers to the prior
experience with a stimulus exerting an eVect on subsequent
encounters with that same stimulus. It is believed that a
memory representation of the Wrst trial is stored and is subsequently retrieved when that same trial is presented at a
later point in time. This leads to more eYcient processing
of the second presentation of that stimulus, which is
indexed by faster detection times (Huang et al. 2004; Shore
and Klein 2001). Robust priming eVects have been
123
observed when the location of a target is repeated across
trials, known as position priming (for example, Campana
and Casco 2009; Kristjansson 2008; Kristjansson et al.
2005; Kumada and Humphreys 2002; Tanaka and Shimojo
2000). However, it remained unclear in which frame of reference participants were coding the location of the target.
For example, while the absolute locations of targets were
repeated, the formations of the search arrays were also
maintained across trials which meant that the target’s location relative to the other items (allocentric position) was
also repeated. For example, Geyer et al. (2006) presented
the search elements in a four by four grid; see also Hilstrom
(2000) and Maljkovic and Nakayama (2000). While
Maljkovic and Nakayama (1996) went some way to addressing whether it is the target’s relative location or absolute
location that is being primed, the absence of a condition
where only the absolute location of a target was repeated
meant that it was not possible to determine whether repetition
in absolute or relative coordinates is more eYcient.
To investigate this issue, in our previous paper (Ball
et al. 2009) we compared the eVect of repeating the target
location in either an allocentric, egocentric or combined
allocentric-egocentric reference frame. To achieve this, we
introduced a visual “landmark” which consisted of two parallel, tilted lines, presented in close proximity. This landmark was used in all conditions and trials, but always
appeared at a diVerent position within the search display.
In the case of the allocentric condition, the target stimulus
(a line, whose orientation diVered from both the landmark
and the distractors), when repeated, was always at the same
position relative to the landmark but at diVerent positions
within the search display. In the egocentric condition, the
target location, when repeated, was at the same location
within the search display, and since participants did not
move relative to the projected display, it was also at the
same position relative to the observer’s body. However, in
this condition, the location of the target stimulus was not
repeated relative to the landmark. In the combined condition, the landmark position did not change between subsequent trials and thus the target position, when repeated, was
the same both relative to the landmark and relative to the
observer’s body. We observed signiWcant priming in all
three conditions, but interestingly the priming eVects found
for the egocentric condition were greater than those for the
allocentric condition.
Evaluating whether egocentric information contributes
to the spatial priming eVect in a visual search task allowed
us to provide some information about whether egocentric
information is as transient as previously thought. We
observed that when the egocentric coordinates of the location of the target were repeated from one trial to the next,
with a minimum delay of 2,200 ms, we found a signiWcant
priming eVect. We also found that the priming eVect was
Exp Brain Res
cumulative, building up over six target present trials interspersed with target absent trials. Therefore, we rejected the
notion that egocentric information cannot be stored for
more than one or 2-s.
Our previous experiment, however, did not identify
which speciWc egocentric reference frame was used for spatial priming. The two possibilities are that participants used
either a retinotopic reference frame or a body-centred reference frame. Since the Wxation position was always the same
(i.e. in the centre of the screen), a repeated target’s location
remained constant with respect to both the Wxation position
and the rest of the observer’s body. Consequently, it was
not possible to disentangle the eVects of retino-centric and
body-centric frames of reference. In the present experiment, we examined these two reference frames separately
and in combination. To achieve this, we used an allocentric
priming condition and three diVerent egocentric priming
conditions: eye-centred, body-centred, and body- and eyecentred. In the eye-centred condition, the location of the
Wxation spot changed between trials and the position of the
repeated target changed accordingly to ensure that its position relative to the current Wxation position remained constant. In the body-centred condition, the Wxation position
also varied between trials, but the position of the target was
repeated. This meant that the target position was repeated
relative to the body but changed relative to the position of
the eye at the beginning of the trial. This condition also
allowed us to address another question which our previous
study left unanswered. During the egocentric condition in
our previous study, the Wxation position was always the
same. This meant that when the target position was
repeated, and thus presented at the same location within the
search display, its position relative to the Wxation position
was also the same. In this situation, the gaze shift, which
was required to foveate the target in the Wrst and the
repeated trial, was of the same direction and amplitude. In
principle, participants could have used exactly the same
eye-movements in the Wrst and second trial, and it could
thus be argued that the reduction in search times for the
repeated trials reXects the fact that for those trials no new
eye movement had to be programmed. In this current
experiment, such a strategy would be possible in the eyecentred condition but not in the body-centred condition,
since in the latter condition the position of the target relative to the Wxation spot diVers between trials, meaning that
for each trial the direction and amplitude of the target-foveating saccade will have to be computed de novo. Thus, we
were able to compare the priming eVect in the eye-centred
condition with that in the body-centred condition, allowing
us to evaluate the extent to which the priming eVect is
determined by the oculomotor strategy described earlier.
The Wnal condition combined the body-centred and eyecentred reference frame. To achieve this, the Wxation
position remained constant across trials, meaning that if the
position of the target remained the same relative to the
body, it also remained the same relative to the Wxation
position. This condition was eVectively the same as the
egocentric condition used in our previous study.
If body-centred information is the more important frame
of reference of the two, there should be little diVerence
between priming in the body-centred condition and priming
in the body- and eye-centred condition. Likewise if eyecentred information is the most relevant, then eye-centred
and body- and eye-centred priming should be similar.
One further aim of our study was to investigate whether
egocentric priming eVects could also be found in other
forms of visual search. Therefore, we used a conjunction
search paradigm (i.e. the target was deWned by a combination of features, e.g. orientation and colour) to extend our
previous Wndings, which were obtained with a simple feature search paradigm (i.e. the target was deWned by a single
feature, namely orientation).
Method
Participants
Twenty-seven naïve participants (5 male) from Durham
University took part in this experiment and received course
credit. Ethical approval was obtained from the Psychology
Research Ethics Committee at Durham University, and participants gave informed consent. Participants all had normal
or corrected-to-normal vision.
Stimuli
Two sets of stimuli were presented in each trial. First, a letter
was presented and participants were instructed to report the
identity of this letter. The font size of the letter was such
that participants could only recognise its identity if they
foveated it. The purpose of this was to control where participants were looking at the start of each trial and to ensure
that they were not lingering at the location of the previous
target. To obtain the ideal font size, we determined the
smallest font size that each participant could read at the
adopted observer distance. The font sizes used varied
between 8 and 16 (corresponding to visual angles 0.3°, or
2/1,200, and 0.7°, or 2/5,143, respectively). For each
participant, we established that for letters of such a small
font size, accuracy of letter identiWcation dropped below
20% when they Wxated on any location other than the location at which the letter was presented.
During the second part of a trial a search array, consisting
of red and green lines on a black background, was presented.
The target line was a green backslash (oriented at ¡20° from
123
Exp Brain Res
Fig. 1 Illustrations of the
priming conditions and trial
sequence. a Schematic of stimuli
for the four priming conditions.
Please note the target stimulus is
a green, backward slash.
(1) Allocentric (A) priming
condition: in trial 1 and trial 2
the target is to the left of the
anchor but it occupies diVerent
positions relative to the observer
and the Wxation spot.
(2) Egocentric-body (EB) priming condition: when the egocentric body position is repeated,
the target occupies the same
absolute position on the screen,
but it has no constant relationship with either the anchor or the
Wxation spot. (3) Egocentriceyes (EE) priming condition: the
position of the target relative to
the Wxation spot is the same in
trial 1 and trial 2 but the target
has a diVerent position relative
to the anchor and the observer’s
body. (4) Egocentric-body and
eyes combined (EBE): the target
occupies the same absolute
position on the screen and has
the same location
relative to the Wxation spot.
The target has no relationship
with the location of the anchor.
b The sequence and timing
of each trial
A
Trial 1 fixation
Trial 1
Trial 2 fixation
Trial 2
i Allocentric condition
+
+
ii Egocentric-body
condition
+
+
iii Egocentric-eyes
condition
+
+
iv Egocentric-combined
body and eyes
condition
+
+
B
Start trial fixation 1000 ms
+
Letter 500 ms
A
Fixation 500 ms
+
Search display,
until response
or 5000 ms
End trial blank
500 ms
vertical) and distractors were a combination of green forward
slashes (oriented at 20° from vertical) and red backslashes
(see Fig. 1a). Each visual search array consisted of 13 lines:
in target present trials there were 12 distractors (6 red backslashes and 6 green forward slashes) and one target, and in target absent trials there were 13 distractors (6 red backslashes
and 7 green forward slashes); thus, there were the same number of red and green items in present and absent displays
(6 red, 7 green). In all search arrays, two green distractors
123
were placed close together and acted as a landmark for the
allocentric priming condition. The stimuli were projected
onto a blank wall. Participants sat 2.2 metres away from the
wall throughout the experiment. The search arrays measured
»20° (2/18) both horizontally and vertically. These were
placed onto black backgrounds so the whole stimulus display
measured 50° (2/7.2) horizontally and 40° (2/9) vertically.
The luminance of the black background was 6.7 candelas
per square metre, while the stimuli lines were 10.6 cd/m2.
Exp Brain Res
The experiment was completed in a semi-lit room, and the
level of lighting used was such that the edges of the projected
image were not discernable from the wall. This setup was
used to ensure that no other stable visual cues, such as the
edge of a computer monitor, were available as potential
points of reference for allocentric coding.
There were four priming conditions: allocentric, egocentric-body, egocentric-eyes, and egocentric-body -eyes
combined (see Fig. 1a). In the allocentric (A) condition,
the location of the target was positioned relative to the
landmark but at diVerent positions relative to the
observer’s body and the Wxation spot. In the egocentricbody (EB) condition, the target maintained the same position relative to the observer’s body but it occupied diVerent
positions relative to the landmark and the Wxation spot.
In the egocentric-eyes (EE) condition, the target maintained the same position relative to the Wxation spot while
occupying diVerent locations relative to the landmark and
the observer’s body. In the egocentric-body-eyes combined (EBE) condition, the target occupied the same location relative to both the observer’s body and the Wxation
spot, but at diVerent positions relative to the landmark. In
this condition, it was necessary that the Wxation spot
stayed in the same location for all the trials of a particular
sequence. The placement of the stimulus lines was not
constrained in any other way, that is, the placement of the
distractor lines was random within the search area (see
Fig. 1a). The location of the Wxation spot relative to the
centre of the screen was not diVerent across the four priming conditions (M = 19.83 cm, SD = 11.3, range 0.45–
59.3 cm).
a given sequence. Interspersed within a sequence there
were also 2 target absent trials; thus, each sequence consisted of 7 trials. The order of the target present and target
absent trials was randomised across sequences. For each
priming condition, there were 20 diVerent sequences, with a
new priming position being used for each sequence, making
a total of 140 trials per priming condition (100 present, 40
absent). The experimental trials were divided into 5 blocks,
with each block containing 4 sequences (28 trials) of each
priming condition (the 28 trials were grouped together,
with a 3 s blank screen separating the diVerent priming conditions). The orders of the priming conditions within a
block, and the order of the blocks, were randomised across
participants.
Data analysis
All analyses are concerned with participants’ reaction times
to decide whether the target line was present or absent.
Incorrect answers (4.3% of trials) and outliers (responses
with reaction times more than two standard deviations
above or below the mean, 4.5% of correct trials) were
removed. All data were tested for normality using the
Shapiro–Wilk statistic; the data were normal unless
otherwise stated. Data violating this assumption was normalised using the log function. When this transformation
could not be used to normalise the data (i.e. when values
were negative) a Wilcoxon Signed Ranks test was used.
Inferential statistics used a signiWcance level of P < .05,
except when multiple comparisons were performed, when a
Bonferonni correction was applied.
Procedure
At the beginning of each trial, a Wxation cross was presented for 1,000 ms. The Wxation cross was then replaced
with a letter (randomly chosen from a set of 5; presentation
duration of 500 ms). Participants had to report the letter to
the experimenter. Following the presentation of the letter,
the Wxation cross was presented again for 500 ms. Next, the
search display was presented, and participants had to decide
whether the target line was present in the display, and make
a key press response accordingly. Participants were
instructed to respond as accurately and as rapidly as possible. No feedback about whether their response was correct
was provided. Once participants had pressed a response
key, the projected display went blank for 500 ms and the
next trial was initiated. There was a minimum of 2,500 ms
between two consecutive search displays. The trial procedure is shown in Fig. 1b.
The target stimulus was present in just over 70% of trials. To induce position-priming, we designed sequences of
trials where a given target position was used 5 times within
Results
For each participant, the smallest font size they could read
when Wxating was established prior to the experimental
trials (8 participants used font size 8; 9 used font size 10;
2 used font size 12; 4 used font size 14; 4 used font size
16). The accuracy of letter reporting was recorded during
the experimental trials and was 99.4% across all participants, indicating that participants Wxated correctly at the
beginning of each trial. Trials where the participant failed
to correctly report the letter were not included in the
analysis.
Participants were highly accurate in their responding to
the visual search stimuli (present trials 93% correct, absent
trials 92% correct). Search times to target absent trials
(M = 1,083.31, SD = 363.7) were signiWcantly slower than
those to target present trials (M = 901.02, SD = 284.2),
t(26) = 10.13; P < .05 (normalised data). This was observed
in all four priming conditions (Table 1).
123
Exp Brain Res
Table 1 Mean search times (ms) for present and absent searches
Egocentric—body
and eyes combined
Present
Absent
873.1 (260.3)
1,066.1 (329.4)
Egocentric—body
915.8 (305.3)
1,108.6 (389.4)
Egocentric—eyes
956.5 (307.2)
1,094.8 (384.8)
Allocentric
858.7 (272.4)
1,063.8 (360.8)
Note: Standard deviations are shown in parentheses
Immediate priming eVects
Figure 2 compares the search times to the Wrst two present
trials of a sequence when they directly followed one
another (i.e. when there were no intervening absent trials).
A 2 £ 4 repeated measures ANOVA (normalised data)
with the factors Repetition (Wrst present trial, second present trial) and Priming Condition (EBE, EB, EE, A) revealed
a signiWcant main eVect of Repetition, F(1,26) = 10.88;
P < .05, such that search times were faster on the second
presentation of a target position, a signiWcant main eVect of
Priming Condition, F(3,78) = 16.36; P < .05, and a signiWcant Repetition by Priming Condition interaction,
F(3,78) = 7.51; P < .05.
With regards to the signiWcant main eVect of Priming
Condition, post-hoc tests (2-tailed t-tests) revealed that participant’s responses were faster in the A condition compared to the other conditions (A vs. EBE: t(26) = ¡4.60;
P < .008; A vs. EB: t(26) = ¡4.52; P < .008; A vs. EE:
t(26) = ¡7.50; P < .008). Search times in the three egocentric priming conditions were not signiWcantly diVerent from
Fig. 2 Response times for the
Wrst two present trials of a sequence when they directly followed each other. Error bars
represent the within-subjects
standard error of the mean
one another (EBE vs. EB: P = .273; EBE vs. EE: P = .011;
EB vs. EE: P = .109).
Post-hoc tests (2-tailed t-tests) revealed that the diVerence in search times between the Wrst presentation and the
second presentation of a target location was only signiWcant
in the EBE condition, t(26) = 5.15; P < .0125, with a mean
reduction of 97.46 ms (EB: P = .288, reduction of
19.69 ms; EE: P = .130, increase of 12.73 ms; A: P = .132,
reduction of 24.70 ms). This suggests that the interaction
between Repetition and Priming Condition was driven by
signiWcantly greater priming in the EBE condition compared to the other conditions (EBE vs. A: Z = ¡2.64;
P < .016, r = ¡.359; EBE vs. EE: Z = ¡3.27; P < .016,
r = ¡.445; EBE vs. EB: Z = ¡2.81; P < .016, r = ¡.382).
Figure 2 illustrates this interaction.
Cumulative priming eVects
Within each sequence of trials, the target stimulus was at a
given position 5 times. Search times to non-primed trials
(1st trials of a sequence, M = 886.77, SD = 266.8) were signiWcantly slower than those to the primed trials (2nd, 3rd,
4th, and 5th trials in a sequence, M = 836.75, SD = 255.6),
t(26) = 5.98; P < .05 (normalised data); indicating priming.
This was true for all conditions except the EE condition
where there was no diVerence between primed and nonprimed trials (Table 2).
Figure 3 shows the search times to the 5 presentations of
a target position for the four priming conditions. Search
time data were subjected to a 5 £ 4 repeated measures
ANOVA (normalised data) with the factors Presentation
Number (1st, 2nd, 3rd, 4th, 5th presentation) and Priming
1st present trial of a sequence
2nd present trial of a sequence
950
Mean search time (ms)
900
850
800
750
700
Egocentric - Body and
Eyes
123
Egocentric - Body
Egocentric - Eyes
Priming Condition
Allocentric
Exp Brain Res
Table 2 Mean search times (ms) for non-primed trials (Wrst trials of a
sequences) and primed trials (trials 2–5 of a sequence), and the diVerence between trials 1 and 5 of a sequence
Non primed
trials (1)
Primed
trials (2–5)
DiVerence
between
trials 1 and 5
Egocentric—body
897.3 (253.1) 795.0 (238.8)
and eyes combined
118.57 (78.7)
Egocentric—body
922.6 (293.6) 841.33 (273.0) 110.79 (91.9)
Egocentric—eyes
908.4 (296.7) 912.0 (277.3)
¡19.66 (85.4)
Allocentric
818.8 (249.2) 798.7 (245.5)
27.17 (65.0)
Note: Standard deviations are shown in parentheses
Egocentric- Body and Eyes
Egocentric- Body
Egocentric- Eyes
Allocentric
Mean search times (ms)
950
900
850
800
750
1
2
3
4
5
Position in sequence
Fig. 3 Mean search times to present trials as a function of their
position in the sequence. Error bars represent within-subjects standard
error of the mean
Condition (EBE, EB, EE, A). This analysis revealed a signiWcant main eVect of Priming Condition, F(3,78) = 39.27;
P < .05; a signiWcant main eVect of Presentation Number,
F(4,104) = 17.07; P < .05; and a signiWcant Priming Condition by Presentation Number interaction, F(12,312) = 8.00;
P < .05.
With regard to the signiWcant main eVect of Priming
Condition, post-hoc tests (2-tailed t-tests) revealed that participant’s responses were slower in the EE condition compared to the other conditions (EE vs. EBE: t(26) = ¡8.62;
P < .008; EE vs. EB: t(26) = ¡6.71; P < .008; EE vs. A:
t(26) = ¡9.47; P < .008). Participants were also slower in
the EB condition compared to both the EBE and A conditions (t(26) = ¡3.22; P < .008 and t(26) = ¡4.70; P < .008,
respectively). There was no diVerence in search times in the
EBE and the A conditions (P = .226).
Inspection of Fig. 3 suggests that search times decreased
as the number of presentations of a target location increased
in the EBE and EB conditions, but not in the EE or A conditions. Direct comparisons between the search times to the
Wrst and Wfth present trials of a sequence provided a measure of cumulative priming for each of the conditions and
are shown in Table 2. With regard to egocentric cumulative
priming, the diVerences between the Wrst and Wfth present
trials of a sequence were signiWcant in the EBE,
t(26) = 8.62; P < .0125, and the EB condition, t(26) = 6.59;
P < .0125, but not in the EE priming condition (P = .130).
This diVerence was also not signiWcant in the A priming
condition (P = .030). Cumulative priming was signiWcantly
greater for the EBE condition compared to the A condition
(Z = ¡4.11, P < .008, r = ¡.559) and the EE condition
(Z = ¡4.06, P < .008, r = ¡.552); however, there was no
signiWcant diVerence in the magnitude of cumulative priming between the EBE condition and the EB condition
(P = .456). There was greater priming in the EB condition
than in the A condition (Z = ¡3.41, P < .008, r = ¡.464)
and the EE condition (Z = ¡3.72, P < .008, r = ¡.507).
There was no diVerence between the A and the EE conditions (P = .021).
It could be argued that the diVerences in the magnitudes
of the priming eVects are a result of the variable initial
search times (see Fig. 3). While search times to the Wrst
present trial of a sequence are faster in the allocentric condition compared to those in the three egocentric conditions,
F(3,78) = 14.31; P < .05 (normalised data), at this point
there is no diVerence between the search times in the three
egocentric conditions (P = .566). Therefore, the search
times at the start of a sequence cannot explain the diVerent
amounts of immediate and cumulative priming observed in
the three egocentric conditions. Additionally, there is no
diVerence between the four priming conditions when only
the Wrst present trial of the Wrst sequence of a group of four
sequences are compared (P = .505, normalised data).
Given that a signiWcant cumulative allocentric priming
eVect was found in our previous study (Ball et al. 2009), the
lack of signiWcant cumulative allocentric priming in this
study came as a surprise. It was observed that there was
greater variability, in the form of higher standard deviations, in participants’ search times to the conjunction
searches compared to the features searches reported by Ball
et al. (2009). It is plausible that this increase in variability
was suYcient to mask the cumulative allocentric priming
eVect. This is further supported by the observation that the
cumulative allocentric priming eVects are of similar magnitudes in both studies (current experiment: 27.2 ms and previous paper: 24.7 ms). Furthermore, the observation that
search times in the allocentric condition are faster than
those in the egocentric conditions could also provide an
explanation for the reduced allocentric priming eVects: it is
possible that the already fast initial search times did not
allow much scope for a further reduction in search times
due to spatial priming. However, we would like to stress
that the main focus of this study was to examine egocentric
priming and to establish which speciWc egocentric frame of
reference was being used.
123
Exp Brain Res
Discussion
The aim of this experiment was to investigate which
speciWc egocentric frame of reference was responsible for
driving the priming eVects we previously reported (Ball
et al. 2009). We replicated our previous Wnding that an egocentric frame of reference can drive spatial priming in
visual search (the EBE condition here is equivalent to the
egocentric condition in the previous experiment). As there
was a minimum delay of 2,500 ms between search arrays,
and the priming eVects were cumulative, building up over
sequences of Wve target present trials interspersed with target absent trials, the data therefore conWrm that egocentric
representations can be stored for at least a few seconds.
Here, we compared two egocentric conditions, one where
the target location was speciWed in body-centred coordinates, and one where the target location was deWned using
eye-centred coordinates. We observed signiWcantly greater
priming when body-centred coordinates deWned the target
location. When the target had the same location relative to
the Wxation spot, and thus to the position of the observer’s
eye, repetition had no consistent eVect on search times;
therefore, we are able to reject the proposal that participants
learnt that a speciWc saccade from the Wxation spot would
take them to the target location. There was no diVerence
between the body-centred condition and the combined condition (body- and eye-centred); therefore, we propose that
the most relevant frame of reference seems to be the body.
Alternatively, it might be thought that the presence of
allocentric information could account for the priming eVects
observed in the egocentric-body condition. However, we
went to great lengths to prevent any interference from allocentric landmarks: the stimuli were projected onto a blank
wall, thus removing any frame of reference information that
presenting the stimuli on a computer monitor might create,
and the level of lighting in the room was such that the edges
of the projected image were not discernable from the wall.
We therefore think it is most likely that in this condition
participants used their own body as frame of reference.
In this current experiment, a conjunction of features
(orientation and colour) deWned the target, whereas in our
previous experiment the target was deWned solely by its
orientation. In observing signiWcant priming eVects here we
provide evidence that our previously observed priming
eVects are not speciWc to either a feature or a conjunction
search paradigm. Interestingly, we observed greater cumulative priming in the egocentric condition when using a
conjunction rather than a feature search paradigm. This
might reXect the increased diYculty of the conjunction
search. In this case, greater diYculty translates into
longer search times, meaning that during a conjunction
search there is greater scope for search time reductions with
repetition.
123
Our observations conWrm our previous Wnding that egocentric information can be stored for a couple of seconds, a
suggestion at odds with the perception–action model which
argues that egocentric representations have a short time
span. Milner and Goodale (1993, 2006) argue that the completion of motor actions relies on the use of egocentric representations, and that it is the dorsal stream that subserves
this function. While we cannot directly show that the egocentric information used in our experiment derives from the
dorsal stream, there is some evidence that this is the case.
For example, functional imaging data show that diVerent
brain regions are recruited in allocentric and egocentric
processing (Committeri et al. 2004; Zaehle et al. 2007);
Schenk (2006) found that the allocentric processing abilities of a patient with ventral stream damage were impaired,
while her egocentric coding was unaVected; and the symptoms of optic ataxia patients, who have damage to dorsal
stream areas, manifest themselves in degraded visuomotor
coordination abilities (Ellis and Young 1996; Jakobson
et al. 1991). It has also been found that patients with parietal lobe damage show greater impairments at completing a
spatial navigation task from an egocentric perspective compared to frontal patients (Seubert et al. 2008). Therefore,
research from a variety of sources suggests that egocentric
information derives from the dorsal stream.
Furthermore, both imaging and patient studies suggest
that dorsal stream areas are involved in position priming.
For example, Kristjánsson et al. (2007) observed that when
the location of a target was repeated, activation in the inferior parietal and frontal areas was suppressed relative to
when the colour of the target item was repeated across trials
(see also Geng et al. 2006). Furthermore, Kristjansson et al.
(2005) observed that while feature priming (colour) of a
target was not impaired in patients with hemispatial neglect
as a result of fronto-parietal damage, they displayed deWcits
in position priming, again pointing to the crucial role of
dorsal stream areas in position priming. Taken together, the
Wndings suggest that the information being used in our
experiment derives from the dorsal stream.
Our conclusions are also consistent with recent Wndings
that the dorsal stream is involved during a delay; for example, Connolly et al. (2003), using fMRI, observed dorsal
stream activations when participants completed a motor
action after a delay. Furthermore, Cohen et al. (2009) using
TMS found that not only is the anterior intraparietal sulcus,
an area of the dorsal stream, involved in immediate grasping but it is also required in the completion of the same
movements after a delay. Cohen et al. (2009) suggest that
the ventral stream is not solely responsible for mediating
grasping movements after a delay, and that the dorsal
stream has some involvement.
There is also a body of research looking at the role of
spatial frames of reference in patients with unilateral
Exp Brain Res
neglect. A robust Wnding seems to be that neglect can be
trunk-centred, with left neglect patients failing to report
stimuli presented to the left of the midline of their body
(Beschin et al. 1997; Karnath et al. 1991, 1993). Furthermore, when the patient’s body is rotated to the left, the
extent of neglect is reduced, suggesting the relevance of
coding information relative to the body. However, spatial
neglect can also be based on retinotopic coordinates (Behrmann et al. 2002; Vuilleumier et al. 1999). In contrast,
studies looking at the role of the head as a reference-point
for unilateral neglect did not yield conclusive results.
In conclusion, our work with visual search adds to the
existing literature on the use of egocentric coding in a number of other cognitive tasks; for example, scene and object
recognition, mental imagery and rotation, and number line
processing (Conson et al. 2009; Creem-Regehr et al. 2007;
Tao et al. 2009; Waller 2006). SpeciWcally, we are now
able to provide more detailed information about the nature
of the egocentric coordinates that are used for spatial priming in a visual search task. We showed that participants did
not remember the relationship between the location of the
target and the eye movement required to locate it, but rather
they relied on body-centred information. Our Wndings
provide further evidence that egocentric coding is relevant not just for visuomotor control but also for a number
of other cognitive functions, including attentional control
and memory.
References
Aglioti S, DeSouza JFX, Goodale MA (1995) Size-contrast illusions
deceive the eye but not the hand. Curr Biol 5:679–685
Ball K, Smith D, Ellison A, Schenk T (2009) Both egocentric and allocentric cues support spatial priming in visual search. Neuropsychologia 47(6):1585–1591
Behrmann M, Ghiselli-Crippa T, Sweeney JA, Dimatteo I, Kass R
(2002) Mechanisms underlying spatial representation revealed
through studies of hemispatial neglect. J Cogn Neurosci
14(2):272–290
Beschin N, Cubelli R, Della Sala S, Spinazzola L (1997) Left of what?
The role of egocentric coordinates in neglect. J Neurol Neurosurg
Psychiatry 63(4):483–489
Burgess N, Spiers HJ, Paleologou E (2004) Orientational manoeuvres
in the dark: dissociating allocentric and egocentric inXuences on
spatial memory. Cognition 94(2):149–166
Campana G, Casco C (2009) Repetition eVects of features and spatial
position: evidence for dissociable mechanisms. Spat Vis 22:325–
338
Christou CG, BuelthoV HH (1999) View dependence in scene recognition after active learning. Mem Cogn 27:996–1007
Cohen N, Cross ES, Tunik E, Grafton ST, Culham JC (2009) Ventral
and dorsal stream contributions to the online control of immediate
and delayed grasping: a TMS approach. Neuropsychologia
47(6):1553–1562
Committeri G, Galati G, Paradis AL, Pizzamiglio L, Berthoz A,
Le Bihan D (2004) Reference frames for spatial cognition: diVerent
brain areas are involved in viewer-, object-, and landmark-centered
judgements about object location. J Cogn Neurosci 16(9):
1517–1535
Connolly JD, Andersen RA, Goodale MA (2003) fMRI evidence for a
‘parietal reach region’ in the human brain. Exp Brain Res
153:140–145
Conson M, Mazzarella E, Trojano L (2009) Numbers are represented
in egocentric space: eVects of numerical cues and spatial reference frames on hand laterality judgements. Neurosci Lett
452(2):176–180
Creem-Regehr SH, Neil JA, Yeh HJ (2007) Neural correlates of two
imagined egocentric transformations. NeuroImage 35(2):916–927
Diwadkar VA, McNamara TP (1997) Viewpoint dependence in scene
recognition. Psycholo Sci 8(4):302–307
Ellis AW, Young AW (1996) Human cognitive neuropsychology:
a textbook with readings. Psychology Press Ltd, Hove
Finlay CA, Motes MA, Kozhevnikov M (2007) Updating representations of learned scenes. Psychol Res 71:265–276
Geng JJ, Eger E, RuV CC, Kristjansson A, Rotshtein P, Driver J (2006)
On-line attentional selection from competing stimuli in opposite
visual Welds: eVects on human visual cortex and control
processes. J Neurophysiol 96(5):2601–2612
Geyer T, Müller HJ, Krummenacher J (2006) Cross-trial priming in
visual search for singleton conjunction targets: role of repeated
target and distractor features. Percept Psychophys 68(5):736–749
Goodale MA, Jakobson LS, Keillor JM (1994) DiVerences in the visual
control of pantomimed and natural grasping movements.
Neuropsychologia 32(10):1159–1178
Hilstrom A (2000) Repetition eVects in visual search. Percept
Psychophys 62(4):800–817
Himmelbach M, Karnath HO (2005) Dorsal and ventral stream interactions: contributions from optic ataxia. J Cogn Neurosci
17(4):632–640
Huang L, Holcombe AO, Pashler H (2004) Repetition priming in visual search: episodic retrieval, not feature priming. Mem Cogn
32(1):12–20
Jakobson LS, Archibald YM, Carey DP, Goodale MA (1991) A
kinematic analysis of reaching and grasping movements in a patient
recovering from optic ataxia. Neuropsychologia 29(8):803–809
Karnath HO, Schenkel P, Fischer B (1991) Trunk orientation as the
determining factor of the ‘contralateral’ deWcit in the neglect syndrome and as the physical anchor of the internal representation of
body orientation in space. Brain 114(Pt 4):1997–2014
Karnath HO, Christ K, Hartje W (1993) Decrease of contralateral neglect by neck muscle vibration and spatial orientation of trunk
midline. Brain 116(Pt 2):383–396
Kristjansson A (2008) “I know what you did on the last trial”—a selective review of research on priming in visual search. Front Biosci
13:1171–1181
Kristjansson A, Vuilleumier P, Malhotra P, Husain M, Driver J (2005)
Priming of colour and position during visual search in unilateral
spatial neglect. J Cogn Neurosci 17(6):859–873
Kristjánsson Á, Vuilleumier P, Schwartz S, Macaluso E, Driver J
(2007) Neural basis for priming of pop-out revealed with fMRI.
Cereb Cortex 17:1612–1624
Kumada T, Humphreys GW (2002) Cross-dimensional interference
and cross-trial inhibition. Percept Psychophys 64:493–503
Maljkovic V, Nakayama K (1996) Priming of pop-out: II. The role of
position. Percept Psychophys 58(7):977–991
Maljkovic V, Nakayama K (2000) Priming of popout: III. A short-term
implicit memory system beneWcial for rapid target selection. Vis
Cogn 7(5):571–595
Milner AD, Goodale MA (1993) Visual pathways to perception and
action. In: Hicks TP, MolotchnikoV S, Ono T (eds) Progress in
brain research, vol 95. Elsevier, Amsterdam, pp 317–337
Milner AD, Goodale MA (2006) The visual brain in action, 2nd edn.
Oxford University Press Inc, Oxford
123
Exp Brain Res
Milner AD, Dijkerman HC, Pisella L, McIntosh RD, Tilikete C,
Vighetto A et al (2001) Grasping the past: delay can improve
visuomotor performance. Curr Biol 11:1896–1901
Mou W, McNamara TP, Valiquette CM, Rump B (2004) Allocentric
and egocentric updating of spatial memories. J Exp Psychol Learn
Mem Cogn 30(1):142–157
Mou W, Xiao C, McNamara TP (2008) Reference directions and reference objects in spatial memory of a brieXy viewed layout.
Cognition 108:136–154
Schenk T (2006) An allo-centric rather than perceptual deWcit in
patient D.F. Nat Neurosci 9(11):1369–1370
Seubert J, Humphreys GW, Muller HJ, Gramann K (2008) Straight
after the turn: the role of the parietal lobes in egocentric space
processing. Neurocase 14(2):204–219
Shelton AL, McNamara TP (2004) Orientation and perspective dependence in route and survey learning. J Exp Psychol Learn Mem
Cogn 30:158–170
Shore DI, Klein RM (2001) On the manifestations of memory in visual
search. Spat Vis 14:59–75
Simons DJ, Wang RF (1998) Perceiving real-world viewpoint changes. Psychol Sci 9(4):315–320
Tanaka Y, Shimojo S (2000) Repetition priming reveals sustained
facilitation and transient inhibition in reaction time. J Exp Psychol
Human Percept Perform 26(4):1421–1435
123
Tao W, Liu Q, Huang X, Tao X, Yan J, Teeter CJ et al (2009) EVect of
degree and direction of rotation in egocentric mental rotation of
hand: an event-related potential study. Neuroreport 20(2):180–185
Vuilleumier P, Valenza N, Mayer EUG, Egrave Ne, Perrig S et al
(1999) To see better to the left when looking more to the right:
EVects of gaze direction and frames of spatial coordinates in unilateral neglect. J Int Neuropsychol Soc 5(01):75–82
Waller D (2006) Egocentric and nonegocentric coding in memory for
spatial layout: evidence from scene recognition. Mem Cogn
34(3):491–504
Wang RF, Simons DJ (1999) Active and passive scene recognition
across views. Cognition 70(2):191–210
Wang RF, Spelke ES (2000) Updating egocentric representations in
human navigation. Cognition 77(3):215–250
Westwood DA, Goodale MA (2003) Perceptual illusion and the realtime control of action. Spat Vis 16:243–254
Westwood DA, Heath M, Roy EA (2003) No evidence for accurate
visuomotor memory: systematic and variable error in memory
guided reaching. J Motor Behav 35(2):127–133
Witt JK, Ashe J, Willingham DT (2008) An egocentric frame of reference
in implicit motor sequence learning. Psychol Res 72(5):542–552
Zaehle T, Jordan K, Wustenberg T, Baudewig J, Dechent P, Mast FW
(2007) The neural basis of the egocentric and allocentric spatial
frame of reference. Brain Res 1137:92–103