Time Perception in Spatial Neglect: A Distorted Representation?

Abstract

It has been proposed that time, space, and numbers share the same metrics and cortical network, the right parietal cortex. Several recent investigations have demonstrated that the mental number line representation is distorted in neglect patients. The aim of this study is to investigate the relationship between time and spatial configuration in neglect patients.
Fourteen right-brain damaged patients (six with neglect and eight without neglect), as well as eight age-matched healthy controls, performed a time discrimination task. A standard tone (short: 700 ms and long: 1,700 ms) had to be confronted in duration to a test tone. Test tone differed of 100, 200, and 300 ms respect to the standard tone duration.
Neglect patients performed significantly worse than patients without neglect and healthy controls, irrespective of the duration of the standard tone.
These results support the hypothesis that mental representations of space and time both share, to some extent, a common cortical network. Besides, spatial neglect seems to distort the time representation, inducing an overestimation of time durations.

Full text

Time Perception in Spatial Neglect: A Distorted Representation?
Marco Calabria
IRCCS Centro San Giovanni di Dio Fatebenefratelli
Sophie Jacquin-Courtois
INSERM UMR-S 864 “Espace et Action”, Universite´ Claude
Bernard Lyon I, and Institut Fe´de´ratif de Neurosciences de Lyon
Antonio Miozzo
University of Brescia
Yves Rossetti
INSERM UMR-S 864 “Espace et Action”, Universite´ Claude
Bernard Lyon I, and Institut Fe´de´ratif de Neurosciences de Lyon
Alessandro Padovani
University of Brescia
Maria Cotelli
IRCCS Centro San Giovanni di Dio Fatebenefratelli
Carlo Miniussi
IRCCS Centro San Giovanni di Dio Fatebenefratelli and University of Brescia
Objective: It has been proposed that time, space, and numbers share the same metrics and cortical
network, the right parietal cortex. Several recent investigations have demonstrated that the mental
number line representation is distorted in neglect patients. The aim of this study is to investigate the
relationship between time and spatial configuration in neglect patients. Method: Fourteen right-brain
damaged patients (six with neglect and eight without neglect), as well as eight age-matched healthy
controls, performed a time discrimination task. A standard tone (short: 700 ms and long: 1,700 ms) had
to be confronted in duration to a test tone. Test tone differed of 100, 200, and 300 ms respect to the
standard tone duration. Results: Neglect patients performed significantly worse than patients without
neglect and healthy controls, irrespective of the duration of the standard tone. Conclusion: These results
support the hypothesis that mental representations of space and time both share, to some extent, a
common cortical network. Besides, spatial neglect seems to distort the time representation, inducing an
overestimation of time durations.
Keywords: time perception, spatial neglect, brain damaged patients, theory of magnitude, parietal cortex
Converging evidence for similarities in the processing of space,
numbers, and time has recently led to the Theory of Magnitude
(ATOM) (Walsh, 2003). This theory suggests that these three
domains share similar mechanisms, including a common metric for
action, the same accumulator principles and the same neural basis,
the inferior parietal cortex. The relation between numbers and
space is well established and has been demonstrated both in
healthy volunteers and in brain damaged patients (for a review see
Hubbard et al., 2005). In general, healthy participants tend to have
a spatially oriented number representation. This “mental number
line” is typically organized such that small digits are represented
toward the left side of the imagined space, while large digits are
represented toward the right side. Previous findings strongly sug-
gest that number magnitude is represented with a spatial orienta-
tion that develops progressively from left to right.
To date, less attention has been dedicated to the link between
spatial processing and time perception. In behavioral studies, sub-
jects who observe downscaled environments (i.e., a miniature
version) undergo systematic shifts in their experience of time (De
Long, 1981; Mitchell & Davis, 1987). Specifically, they experi-
ence a compression in temporal estimation that is proportional to
the scale-model environments being observed.
Recently, researchers have proposed a spatial-temporal associ-
ation of response codes (STEARC), as a ‘mental time line,’ or a
systematic relation between spatial and time codes for action
(Ishihara, Keller, Rossetti, & Prinz, 2008). In this work, Ishihara
and colleagues (2008) asked participants to estimate the onset time
of the last click of a given sequence. In the congruent condition,
left-sided responses indicated ‘early,’ whereas right-sided response
indicated “later” appearance of the probe. In the noncongruent
condition, the stimulus-response contingency was reversed. The
authors identified a facilitatory effect for compatible responses
(“left”–“early,” “right”–“late”) compared to noncompatible re-
sponses (left–late, right– early), which suggests a space-time rep-
resentation relationship that has the potential to influence percep-
tion and action. The STEARC is an analogous of the SNARC
effect (spatial-numerical association of response codes), such as in
the parity judgment task, participants are quicker to respond to
smaller numbers (e.g., 1 and 2) with the left hand and to larger
numbers (e.g., 8 and 9) with the right hand.
Marco Calabria, IRCCS Centro San Giovanni di Dio Fatebenefratelli;
Sophie Jacquin-Courtois and Yves Rossetti, INSERM UMR-S 864
“Espace et Action”, Universite´ Claude Bernard Lyon I, and Hospices
Civils de Lyon, Institut Fe´de´ratif de Neurosciences de Lyon, Mouvement
et Handicap; Antonio Miozzo and Alessandro Padovani, Clinical Neurology
Department, University of Brescia; Maria Cotelli, IRCCS Centro San Gio-
vanni di Dio Fatebenefratelli; Carlo Miniussi, IRCCS Centro San Giovanni di
Dio Fatebenefratelli and Department of Biomedical Sciences and Biotechnol-
ogies, National Institute of Neuroscience, University of Brescia.
Correspondence concerning this article should be addressed to Marco
Calabria, IRCCS Centro San Giovanni di Dio Fatebenefratelli, Via
PILASTRONI, 4; 25125 Brescia, Italy. E-mail: calabria.marc@gmail.com
Neuropsychology © 2011 American Psychological Association
2011, Vol. 25, No. 2, 193–200 0894-4105/11/$12.00 DOI: 10.1037/a0021304
193

Vicario, Caltagirone, and Oliveri (2007) showed that optoki-
netic stimulation can influence time estimation. Subjects were
required to make time comparison of time intervals before and
after optokinetic stimulation (leftward or rightward). They found a
directional bias in time perception with only rightward stimulation
inducing an overestimation of time perception compared with
baseline and leftward optokinetic stimulation, suggesting a mental
linear representation of time intervals.
Also neuropsychological research on hemispatial neglect has
considered this issue. Hemispatial neglect is a syndrome charac-
terized by a deficit in interpreting and processing stimuli presented
to the left hemispace, which follows damage to the right hemi-
sphere, and in particular to the inferior parietal lobule (Husain &
Nachev, 2006). Studies with neglect patients have demonstrated
impaired temporal dynamics of spatial attention (Becchio & Ber-
tone, 2006), but few studies to date have investigated time discrim-
ination or interval-production tasks. Basso and colleagues (1996)
tested a patient with hemispatial neglect on short-duration estimation
tasks (300 vs. 700 ms). The patient overestimated durations of stimuli
presented in the leftward space of the right space and underestimated
durations of stimuli presented in the non-neglected field. More re-
cently, Danckert et al. (2007) investigated the ability of neglect
patients to estimate time intervals of seconds.
They found that, compared to healthy controls (HCs), neglect
patients grossly underestimated all durations for intervals greater
than one second. Right-hemisphere damaged patients without ne-
glect also tended to underestimate durations, but their responses
were less inaccurate than those of patients with neglect.
Recently, Cappelletti, Freeman, and Cipolotti (2009) described
a patient, with a right hemisphere lesion. This patient showed
important deficit of time estimation (underestimation), but not of
number and space processing. Nevertheless, an interaction in his
behavior between numbers and time representations as well as
between numbers and space representations was present. Once
again, these results suggest that space, numbers, and time partially
overlap even if they could be impaired selectively.
More recently, some evidence for the spatial representation of
time has been presented in studies that use prism adaptation
(Frassinetti, Magnani, & Oliveri, 2009). Prism adaptation is a
technique to rehabilitate spatial deficits in neglect patients (e.g.,
Luaute, Halligan, Rode, Jacquin-Courtois, & Boisson, 2006). Ne-
glect patients are initially biased to the right, but after sensorimotor
adaptation to the rightward prismatic displacement of the visual
field, this symptom can improve markedly (Rossetti et al., 1998).
Frassinetti et al. (2009) used this technique to shift time perception
in healthy participants. They found that, after prism adaptation,
participants reported an underestimation (if exposed to leftward
deviation) or overestimation (if exposed to rightward deviation) of
time durations, suggesting that time perception may be modified
after sensorimotor adaptation.
The aim of the present study is to investigate the possible
relationship between time and space processing in neglect patients.
We used a time discrimination task, involving a comparison of
duration between a standard auditory stimulus and a test tone. Two
groups of right brain-damaged patients were tested, one with
hemispatial neglect, and the other without hemispatial neglect. In
addition, we tested a group of healthy controls. We compared the
responses to two different durations of standard stimulus (700
or 1,700 ms). Because it has been reported that suprasecond
durations are processed by cortical structures (Mauk & Buono-
mano, 2004), we expected that, for durations longer than one
second, a larger bias would be present in neglect patients, similar
to what happens for spatial elaboration. In particular, if the time
representation follows the same rules as the spatial one it is expected
that the magnitude of the over- or underestimation bias of is linearly
related to the length of the stimulus. As in line bisection has been
shown the longer the line to bisect, the greater the magnitude of the
neglected space (e.g., Halligan & Marshall, 1988).
Moreover, to test the presence of specific lateralization effects in
time discrimination, the presentation of auditory stimuli was lat-
eralized. Standard and test tones were randomly presented con-
tralaterally (e.g., standard at right ear and test at left ear, or vice
versa). At this regard, we expected that time discrimination in the
left space would be more severely impaired than in the right space,
for neglect patients as compared to non-neglect patients and HCs.
Method
Apparatus and Stimuli
Tones were delivered at a suprathreshold level using earphones.
The first stimulus, termed the ‘standard,’ was presented to one ear,
and the second stimulus, termed the ‘test,’ was presented to the
other ear. Presentation order was counterbalanced between the two
sides. Each trial comprised a series of two stimuli, a standard and
a test tone. Standard tones could be of two durations: “short” or
“long” (700 vs. 1,700 ms, respectively). Test tones were presented
at multiple offsets from the standard tones, using differences of
100, 200, or 300 ms. In other words, given a standard of 700 ms,
the test tone could be any of 400, 500, 600, 800, 900, or 1000 ms.
The interstimulus interval was 500 ms and the intertrial interval
was 800 ms. The experiment comprised four blocks of 48 trials
each, for a total of 192 trials, of which 96 featured short and 96
featured long tones.
Participants were required to decide whether the test tone was
shorter or longer than the standard tone. Responses were recorded
via a response-box. Participants used the right hand to respond, or
in the case of motor deficits, the experimenter recorded the re-
sponses (for five patients with neglect). Accuracy was stressed as
being of greatest importance, and there were no response time
restrictions. Stimuli were created using the Test Tone Generator
and presented with Presentation software package, version 11.0
(http://www.neurobs.com). This software was used both to deliver
tones and to record subjects’ responses.
Participants
Eight right-brain damaged patients without hemispatial neglect
(N⫺) (mean age ⫽70.0 ⫾10.9 years; mean educa-
tion ⫽6.9 ⫾2.4 years) and six right-brain damaged patients who
exhibited hemispatial neglect (N⫹) (mean age ⫽60.3 ⫾11.1
year; mean education ⫽9.3 ⫾2.6 years) took part in our exper-
iment. Patients were recruited at the Neurological Unit of the
University of Brescia, Italy, and at the Service de Re´e´ducation
Neurologique, Hoˆ pital Henry Gabrielle, Hospices Civils de Lyon,
France. Eight age-matched healthy controls (HC) were also tested
(mean age ⫽67.2 ⫾4.5 years; mean education ⫽9⫾3.2 years).
Clinical details are listed in Tables 1 and 2. All participants were
right-handed. All gave informed consent and the experimental
194 CALABRIA ET AL.

protocol was approved by the Ethics Committee of IRCCS San
Giovanni di Dio Fatebenefratelli, Brescia, Italy.
All right-brain damaged patients were assessed for visuospatial abili-
ties (Lezak et al., 2004). The neuropsychological battery included:
A. Line bisection tasks. Line bisection of three different
lengths (5, 10, and 25 cm) presented on A4 format paper and
the Schenkenberg test were performed. The cut-off score for
line bisection was calculated on the basis of an age-matched
Table 1
Clinical and Demographic Characteristic of Patient Groups
Patients Gender
Age
(years)
Education
(years) Aethiology
Time from lesion
(days) Lesion site
Patients with spatial
neglect (N⫹)
RB M 65 8 ISCHAEMIC 477 Fronto-parietal
CP M 60 5 ISCHAEMIC 35 Territory of middle cerebral
artery and basal ganglia
RF M 80 13 ISCHAEMIC 150 Temporo-occipital
DO F 49 10 HAEMORRAGIC ⫹LOBECTOMY 328 Temporal and partial
fronto-parietal
LMI M 53 10 ISCHAEMIC 210 Fronto-temporo-parietal
VR F 55 10 ISCHAEMIC 113 Fronto-parietal
Patients without spatial
neglect (N⫺)
MB F 88 8 ISCHAEMIC 15 Frontal and basal ganglia
PG M 72 5 ISCHAEMIC 19 Periventricular white matter
PM F 68 11 HAEMORRAGIC 28 Capsulo-thalamic and basal
ganglia
ZP F 76 5 ISCHAEMIC 27 Anterior foreharm of
internal capsule
GS M 49 8 ISCHAEMIC 350 Frontal
TM M 67 8 ISCHAEMIC 16 Frontal
CF M 73 5 ISCHAEMIC 172 Superior intraparietal lobule
MA M 67 5 ISCHAEMIC 180 Fronto-temporale
Table 2
Neuropsychological Assessment and PSE of Patient Groups
Neuropsychological assessment
Time
discrimination
task
Patients
Visual
extinction
Auditory
extinction
a
Line
bisection
b
Shenkenberg
c
Albert test
d
Star
cancellation
d
PSE right side
e
Patients with spatial
neglect (N⫹)
RB ⫹⫹26 17 (4) 2 37 55
CP ⫹⫺13 28 (4) 3 31 24
RF ⫹⫹42 31 (8) 3 35 15
DO ⫹⫹13 35 (2) 6 29 74
LMI ⫹⫹28 26 (2) 3 37 89
VR ⫹⫺14 18 (5) 4 18 171
Patients without spatial
neglect (N⫺)
MB ⫺⫺⫺13 0 1
PG ⫺⫺⫺25 0 0 ⫺44
PM ⫺⫺⫺32 0 1 ⫺20
ZP ⫺⫺⫺35 0 1 ⫺30
GS ⫺⫺ 24 0 0 11
TM ⫺⫺ 23 0 0 ⫺28
CF ⫺⫺⫺31 0 1 ⫺24
MA ⫺⫺⫺13 0 1 ⫺19
a
Cuf-off ⬍60% double tones detected.
b
Cuf-off ⬍7 mm deviation (“⫹” indicates rightward bias, “⫺” indicates leftward bias).
c
Cuf-off ⬍15 mm
deviation (“⫹” indicates rightward bias, “⫺” indicates leftward bias); in parentheses the number of omissions.
d
Cuf-off ⬍2 omissions on the left side
of the sheet.
e
PSE: point of subjective equivalence; difference between PSE and standard tone duration is reported.
195TIME PERCEPTION IN SPATIAL NEGLECT

sample of 10 subjects (2 SDs below the mean of control
subjects’ performance). For the Schenkenberg test, we con-
sidered hemispatial neglect to be present when participants
exhibited a mean rightward deviation of greater than 1.53 cm.
B. Cancellation tasks. The Albert Test and the Star Cancel-
lation test were used. For both tests, we considered a cut-off
score of two omissions on the left hemispace of the paper.
C. Drawing. A drawing-copy test and the Clock Test of
drawing for memory were performed. A score was considered
pathological when left-sided elements of the drawing were
omitted.
D. Reading task. We asked participants to read aloud eight
sentences of different lengths presented separately on a sheet
of paper (cut-off score: 2 errors).
In addition, the Mini Mental State Examination Test was per-
formed to assess possible cognitive decline. Patients were ex-
cluded if they scored below the cut-off of 24/30.
Clinical evaluation was supplemented by an assessment of mo-
tor, somatosensory, and visual deficit awareness, using the Bisiach,
Vallar, Perani, Papagno, and Berti (1986) questionnaire. Auditory
extinction was also assessed by delivering 10 tones unilaterally
(right and left) and 10 tones under a dichotic listening condition.
The tone duration was 500 ms, and auditory extinction was con-
sidered if the patient missed more than 60% of the left stimuli
under the dichotic condition.
Statistical Analysis
Mixed repeated measure ANOVAs were performed on the per-
centage of correct responses. Within-subject factors included:
Tone Series (short vs. long), Side of Standard Tone Presentation
(right vs. left), Time Offset (100, 200, and 300 ms). The between-
subjects factor was the Group (N⫹,N⫺, and HC).
For the point of subjective equivalence (PSE, for its calculation
see below), it was conducted a mixed repeated measure ANOVA
considering the standard tone (700 vs. 1,700) as within-subject
factor and the Group (N⫹,N⫺, and HC) as between-subjects
factor.
The assumption of sphericity was assessed using Mauchly’s
Test, and the Geisser-Greenhouse correction for the number of
degrees of freedom was adopted when necessary. Size effects are
reported as partial eta squared values (␩
p
2
).
Post hoc analyses were performed by applying the False Dis-
covery Rate (FDR) together with the Benjamini and Hochberg
(1995) procedure for multiple comparisons. This technique has the
advantage of controlling for Type I errors and is applicable for
both within- and between-subjects comparisons. Statistical analy-
ses were performed with SPSS (SPSS, Inc., Chicago) and the alpha
value for significance was set equal to 0.05.
Results
The correct response percentages were analyzed to compare
between-group performance. We found a significant main effect of
Group [F(2, 19) ⫽52.52, p⬍.001 ␩
p
2
⫽0.85]. Post hoc analyses
revealed that N⫹patients (62.4 ⫾18.4) scored significantly worse
than N⫺(78.2 ⫾15.2, p⬍.0001) and HC (85.2 ⫾14.1; p⬍
.0001); moreover, N⫺patients were significantly worse than HC
(p⫽.003).
A main effect of Tone Series [F(1, 19) ⫽36.23, p⬍.001,
␩
p
2
⫽0.66] was found. All participants exhibited better perfor-
mances for the short Tone Series (81.1 ⫾17.2) than for long ones
(60.5 ⫾17.4), but there was no difference between groups (no
significant interaction of Tone series and Group). The main effect
of Time Offset was also significant [F(1.98, 37.66) ⫽49.28, p⬍
.001, ␩
p
2
⫽0.72), demonstrating a linear effect as the Offset
between standard and test tones increased.
Interestingly, the interaction between Side of Presentation ⫻
Time Offset ⫻Group was statistically significant [F(2.94,
27.97) ⫽4.51, p⫽.01, ␩
p
2
⫽0.32]. Post hoc analyses showed
that the N⫹patients recorded performances that were signifi-
cantly weaker than HC for all the Time Offsets considered.
Namely, for the right side: at 100 ms (N⫹⫽
58.1 ⫾15.6, HC ⫽73.2 ⫾6.9; p⫽.03), 200 ms (N⫹⫽
67.9 ⫾16.6, HC ⫽93.2 ⫾8.0; p⫽.007), and 300 ms
(N⫹⫽66.7 ⫾21.7, HC ⫽98.2 ⫾5.1; p⫽.0009). For the left
side: at 100 ms (N⫹⫽49.7 ⫾9.7, HC ⫽76.9 ⫾11.2; p⫽.0006),
200 ms (N⫹⫽57.8 ⫾16.1, HC ⫽95.1 ⫾5.8; p⬍.0001), and
300 ms (N⫹⫽77.6 ⫾14.3, HC ⫽94.3 ⫾7.8; p⫽.002). Even
if the two groups (i.e., N⫹and HC) performed similarly for both
sides, higher differences were found on the left side; for this reason
the Side of Presentation ⫻Time Offset ⫻Group interaction was
significant. That is, the difference, on the right side, between N⫹
and HC was 15.1 ms at 100 ms Time Offsets, and 30.5 ms at 200
ms Time Offsets. While on the left side these differences were
higher: 27.2 ms and 40.3 ms at 100 and 200 ms Time Offsets,
respectively.
Moreover N⫹individuals performed significantly worse than
N⫺patients for all Time Offsets ( p⬍.05), whereas the N⫺
performance metrics were not significantly different in terms of
Time Offset to those of HCs (see Figure 1).
Point of Subjective Equivalence (PSE)
We calculated the PSE, defined as the perceived duration (of the
test tone) in relation to the reference duration (of the standard
tone), for each participant. The PSE was calculated following this
method. First, we calculated the probability of the “test longer”
responses against the actual duration of the stimulus. The proba-
bility goes from 0 to 100%, where 50% stands for the subjective
perception of the stimulus duration equal to the test duration. Then,
data were fitted to a logistic function and PSE (in milliseconds)
was estimated for each participant trough the regression ⫻param-
eter (using the standard durations, such as 700 and 1,700 ms).
If the PSE is inferior to the standard tone duration it means that
there is an underestimation of the perceived test tone, and if the
PSE is longer an overestimation is assumed. So, the PSE estimates
the subjective perception of each participant when the tone is 700
or 1,700 ms long. For instance, if the standard tone duration is 700
ms and the PSE is 650 ms, this means that participant had per-
ceived the duration of the tone 50 ms shorter, indicating an
underestimation.
For the standard of 700 ms and the test on the right, the PSE for
the N⫹patients was 772 (⫾81), 671 (⫾26) for N⫺, and 683
(⫾31) for HC. When the test was presented on the left the PSE, or
196 CALABRIA ET AL.

the N⫹patients was 692 (⫾81), 691 (⫾26) for N⫺, and 690
(⫾31) for HC. The analysis showed a significant overestimation of
the duration (72 ms) of N⫹patients compared to N⫺(⫺29 ms,
p⫽.03) and HC (⫺17 ms, p⫽.04), when the test tone was on the
right. No difference between groups was found when the test tone
was presented on the left.
For the standard of 1,700 ms and the test on the right, the PSE
for the N⫹patients was 1769 (⫾94), 1628 (⫾91) for N⫺and 1638
(⫾105) for HC. When the test was presented on the left the PSE,
or the N⫹patients was 1667 (⫾110), 1638 (⫾126) for N⫺and
1634 (⫾65) for HC. The analysis showed a significant overesti-
mation of the duration (69 ms) of N⫹patients compared to N⫺
(⫺72 ms, p⫽.04) and HC (⫺62 ms, p⫽.04), when the test tone
was on the right. No difference between groups was found when
the test tone was presented on the left (see Figure 2).
Correlations
Spearman’s rank correlation coefficients were calculated con-
sidering the spatial bias in line bisection and the N⫹’s perfor-
mance in time discrimination task. For line bisection it was con-
sidered the spatial rightward bias from the geometric midpoint for
the three length of lines (5, 10 and 25 cm). It was considered the
percentage of correct responses for the three time offsets (100,
200, and 300 ms), when the test tone was delivered on the right and
the left side, respectively. Positive correlations between the per-
centage of correct responses for the time offset of 300 ms on the
right and the rightward bias in bisection of 10 cm (r⫽.87) and
of 25 cm (r⫽.93) lines were found. Negative correlations be-
tween the percentage of correct responses for the time offset of 300
ms on the left and the rightward bias in bisection of 10 cm (r⫽
⫺0.82) and of 25 cm (r⫽⫺0.81) lines were found. These results
indicate that in N⫹larger the spatial bias poorer the performance
in the time discrimination task when the test tone was delivered on
the left, but not on the right.
In N⫹patients, PSE negatively correlated with the magnitude of
the rightward bias only when the test tone was presented on the left
(r⬎⫺0.71). This indicates that the larger is the spatial bias on line
bisection, the larger is the underestimation of the time duration on
the left side.
Discussion
Recent evidence has suggested that numbers, space, and time
share a common cognitive mechanism and neural substrate in the
parietal cortex (Walsh, 2003). Distortion of space and numbers has
been previously demonstrated in brain damaged patients by several
studies (e.g., Hubbard et al., 2005; Zorzi, Priftis, & Umilta, 2002)
strengthening the idea of a direct relationship between the two.
Conversely, to the best of our knowledge, the relationship
between time and space has not been widely investigated in
patients with visuospatial deficits, such as hemispatial neglect
patients (Basso et al., 1996). In the current study, a time discrim-
ination task was performed with a group of neglect patients, and
the data were compared to a group of right-brain damaged patients
without spatial neglect and a group of HCs. In general, we con-
cluded that neglect patients were more severely impaired in terms
of time discrimination compared to both HCs and right-brain
damaged patients without visuospatial deficits. These results are
consistent with previous studies that investigated temporal pro-
cessing in neglect patients (for a review see Becchio & Bertone,
2006). These studies have suggested that neglect patients have
deficits in processing time information because of their inability to
integrate information across time or in temporal processing (e.g.,
Husain, Shapiro, Martin, & Kennard, 1997).
It was not found any effect of tone series between groups. All
the groups had a lower performance in processing long tones
(1,700 ms) in comparison to short tones (700 ms), but this effect
was of the same size in all groups.
Neglect patients performed worse than controls both when the
test tone was presented on the left and when it was presented on
the right. Only small differences in effect-size were found: neglect
patients performed worse when the test tone was on the left than on
the right for short time intervals (i.e., 100 and 200 ms). It is known
that, in neglect patients, left-side stimuli are often not reported or
undetected (Heilman, Watson, & Valenstein, 2005) because of the
right parietal damage and its consequential attentional impair-
ments. By assuming a parallelism between space and time repre-
sentation, we hypothesized that time discrimination might be more
severely impaired when the target stimuli were presented on the
left side, as left-sided targets are impaired in spatial processing.
Space and time processing are thought to share the same neural
network, namely the right parietal region (Bueti & Walsh, 2009;
Walsh, 2003), which is one of the typical lesion sites associated
Figure 1. Percentage of correct responses in the time discrimination task,
as a function of the side of test tone presentation. Bars display means and
standard errors. N⫹⫽patients with spatial neglect (n⫽6); N⫺⫽patients
without spatial neglect (n⫽8); HC ⫽healthy controls (n⫽8).
197TIME PERCEPTION IN SPATIAL NEGLECT

with spatial neglect (Husain & Nachev, 2006). Conversely, our
behavioral results suggest that time discrimination is always im-
paired irrespective of the side of presentation.
However, when we looked at the qualitative aspect of the time
distortion, such as under- or overestimation, interesting results
were found. We calculate the PSE for each participant to see the
subjective estimation of the perceived time duration of the test
tone. It is supposed that shorter PSE respect to the standard tone is
consistent with an underestimation and vice versa. It was found
that N⫹patients have an overestimation of about 70 ms whereas
N⫺and controls showed underestimation, when the test tone was
presented on the right. Conversely, when the test tone was pre-
sented on the left the slope function of the PSE changed (it was
flatter than HC group), and no difference to HC for the PSE was
observed for N⫹.
These results could be compatible with an impaired clock mech-
anism (Gibbon, Church, & Meck, 1984). The internal clock is an
accumulator that sums pulses produced by a pacemaker. The
output of the accumulator is compared to reference in the memory,
and time estimation is made. In this case, impaired attentional
resource in N⫹affects the accumulation resulting in a higher
production of the pulses, and consequently an overestimation. This
overestimation was observed when the test tone was on the right,
so not in the neglected space. So, it is not the case. How can this
overestimation on the right be explained? Beside the accumulator,
in the model of Gibbon et al. (1984) reference memory and
working memory are included. Reference memory is related to
the representation of the time and permits the comparison with the
output of the accumulator, while working memory allows the
temporal storing of the information. It is likely that one of these
two systems is affected in the N⫹patients. When the standard
tone is in the memory and its representation is distorted (i.e.,
smaller in N⫹) the consequence is an overestimation of the test
tone perception, as showed by present results. The distortion of the
standard tone occurred in the neglected space and consequently the
test tone perceived on the right was altered, inducing an overesti-
mation on the right side, as our results shown. Interestingly, the
underestimation in controls mimics what it is classically found in
line bisection. Neurologically normal subjects bisect space during
line bisection systematically on the left of the geometrical mid-
point, a phenomenon called “pseudoneglect” (Jewell & McCourt,
2000). The same bias for time perception and spatial representation
suggests, to some extent, a common mechanism already in healthy
participants.
Basso et al. (1996) found an overestimation in the leftmost part
of right space, in which neglect was present as well. These oppo-
site results could be explained by the different methodology. We
presented the stimuli each to one ear, whereas Basso et al. (1996)
required classifying a shorter or longer stimulus after a phase of
training. Moreover in Basso et al. (1996) the stimuli were pre-
sented in the visual modality, while in our study we used auditory
modality. Alternatively these different results could be because of
the organization of the sensory modalities, that is, the time dis-
crimination of N⫹for auditory material is less specific to one
hemisphere because a less lateralized representation of time is
present in the auditory space.
Even if different pattern of time estimation has been found in
our participants, it appeared that N⫹showed lower performance in
time discrimination compared to N⫺and HC. So, our results may
also be explained in terms of a general deficit in temporal dynam-
ics (see Husain et al., 1997). When healthy participants are re-
quired to detect two serially presented targets, they need about 400
ms between the two targets to be able to report both. When neglect
patients are confronted with the same task, they exhibit a longer
attentional blink effect than HCs (Husain et al., 2001; Husain et al.,
1997). This suggests that neglect patients suffer from a deficit in
0
0,5
1
-300 -200 -100 +100 +200 + 300
0
0,5
1
-300 -200 -100 +100 +200 +300
Proportion of longer standard tone
Right side Left side
0.5 0.5
Time offsets
HC
N -
N +
0
0,5
1
-300 - 200 -100 +100 +200 +300
0
0,5
1
-300 -200 -100 +100 +200 +300
Proportion of longer standard tone
Right side Left side
0.5 0.5
Time offsets
0
0,5
1
-300 - 200 -100 +100 +200 +300
0
0,5
1
-300 -200 -100 +100 +200 +300
Proportion of longer standard tone
Right side Left side
0.5 0.5
Time offsets
HC
N -
N +
Figure 2. PSE for each group in the time discrimination task, as a function of the side of test tone presentation.
N⫹⫽patients with spatial neglect (n⫽6); N⫺⫽patients without spatial neglect (n⫽8); HC ⫽healthy
controls (n⫽8).
198 CALABRIA ET AL.

detecting stimuli over time. Neglect patients fail to correctly retain
the representation of the stimuli, and in turn they fail to compare
the various interval durations presented in the task. This may lead
to deficits in the estimation of time duration, as shown by our
results.
Battelli, Walsh, Pascual-Leone, and Cavanagh (2008) also sug-
gest different role of the parietal lobe, the so called “when”
pathway. According to these authors the parietal lobe is involved
in the spatial attentional processing and this function is strongly
contralateral. Conversely, the control of the right parietal lobe over
temporal attention is bilateral. This bilateral control could be
impaired in N⫹patients, reflecting a lower performance in time
discrimination both in the right and in the left side of the space.
In general, we found that right-brain damaged patients without
neglect performed significantly worse than HCs but better than
spatial neglect patients. However, when the presentation side was
taken into account in our analyses, patients without neglect per-
formed as well as HCs did. This again confirms the interdepen-
dence of spatial and time codes in perception. Moreover, the
correlational analysis support that the larger is the rightward bias
in N⫹the larger is the impairment in time discrimination.
We cannot specifically determine whether the lesion site was
related to performance, because only general data of structural
imaging are available of our patients. However, almost all our
neglect patients exhibited damage involving those temporo-pari-
etal regions that are related to spatial functions (Husain & Nachev,
2006). Even though right hemisphere involvement in time process-
ing remains under debate (Funnell, Corballis, & Gazzaniga, 2003;
Handy, Gazzaniga, & Ivry, 2003; Kagerer, Wittmann, Szelag, &
Steinbuchel, 2002), imaging studies in brain damaged patients
(Harrington & Haaland, 1999) have identified a right hemispheric
superiority in time perception, specifically involving the fronto-
parietal network. From a speculative point of view, this could
support he notion that time and space are not only functionally
linked, but they also share the same neural network.
In conclusion, the results of the present study support the exis-
tence of an overlapping perception of time and space. Neglect
patients, who are known to be impaired in spatial processing, also
exhibit impairment in terms of time discrimination. These data
support the hypothesis of a “distortion” of the representation in the
reference memory or impaired working memory in time process-
ing that is potentially located in the right parietal cortex, a neural
network involved in spatiotemporal processing of perception and
action.
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Received February 17, 2010
Revision received August 11, 2010
Accepted August 12, 2010 䡲
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