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Dissociating perceptual and motor effects of prism adaptation in neglect

March 2010
Neuroreport 21(6):436-41

March 2010
21(6):436-41

DOI:10.1097/WNR.0b013e328338592f

Source
PubMed

Authors:

Christopher Lee Striemer

MacEwan University

James Danckert

University of Waterloo

Prism adaptation reduces some symptoms of neglect; however the mechanisms underlying such changes are poorly understood. We suggest that prisms influence neglect by acting on dorsal stream circuits subserving visuomotor control, with little influence on perceptual aspects of neglect. We examined prism adaptation in three neglect patients and a group of healthy controls on line bisection and landmark tasks. Neglect patients showed a dramatic reduction in the rightward bias for line bisection, but absolutely no change in their leftward bias on the landmark task, which is a perceptual equivalent to bisection. However, in controls, prisms produced 'neglect-like' deficits on both the line bisection and landmark tasks. These data suggest that prisms influence visually guided actions more so than perception in neglect.

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Dissociating perceptual and motor effects of prism

adaptation in neglect

Christopher L. Striemer

and James Danckert

Prism adaptation reduces some symptoms of neglect;

however the mechanisms underlying such changes are

poorly understood. We suggest that prisms influence

neglect by acting on dorsal stream circuits subserving

visuomotor control, with little influence on perceptual

aspects of neglect. We examined prism adaptation in three

neglect patients and a group of healthy controls on line

bisection and landmark tasks. Neglect patients showed a

dramatic reduction in the rightward bias for line bisection,

but absolutely no change in their leftward bias on the

landmark task, which is a perceptual equivalent to bisection.

However, in controls, prisms produced ‘neglect-like’

deficits on both the line bisection and landmark tasks.

These data suggest that prisms influence visually guided

actions more so than perception in neglect. NeuroReport

21:436–441 c2010 Wolters Kluwer Health | Lippincott

Williams & Wilkins.

NeuroReport 2010, 21:436–441

Keywords: attention, neglect, parietal lobe, perception–action, prism

adaptation

Department of Psychology, Centre for Brain and Mind, Social Sciences Centre,

University of Western Ontario, London and

Department of Psychology,

University of Waterloo, Waterloo, Ontario, Canada

Correspondence to James Danckert, PhD, Department of Psychology, University

of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada

Tel: + 1 519 888 4567 x37014; fax: + 1 519 746 8631;

e-mail: jdancker@uwaterloo.ca

Received 1 February 2010 accepted 3 February 2010

Introduction

Lesions to right parietal cortex often lead to neglect – a

disorder in which patients are unaware of contralesional

space [1]. One method that has been shown to improve

neglect symptoms is prism adaptation (PA) [2], in which

patients point to targets while wearing prisms that shift

vision 101rightward. Before PA, neglect patients’

subjective straight-ahead judgments are biased right of

center. During adaptation patients adjust their pointing

leftward to compensate for the rightward prism shift.

After 5 min, patients’ straight-ahead judgments are shifted

leftward, closer to center. After adaptation, patients

improve on clinical and experimental tasks including tactile

perception, occulomotor biases, and spatial attention (for a

reviewseeRef.[3]).

Although prisms improve some neglect symptoms, the

mechanisms for these effects are unknown. How does

visuomotor adaptation, relying on mechanisms in cere-

bellar, parietal, and motor cortex [4,5], improve such a

broad range of symptoms? The dual pathways’ model

of vision suggests that visuomotor control is subserved

by the dorsal stream (V1 to posterior parietal cortex),

whereas conscious perception is subserved by the

ventral stream (V1 to inferior temporal cortex) [6]. One

explanation for the effects of prisms is that they alter

dorsal stream behaviors – the pathway most active during

adaptation [4]. That is, leftward realignment signals,

generated in the right cerebellum, are sent to left

posterior parietal cortex [3,7]. The realignment signals

encourage the patient to shift exploratory motor beha-

viors leftward into previously neglected space, resulting

in a decrease in some symptoms [3]. This would explain

many of the effects of prisms on tasks requiring overt

motor responses with the adapted limb (e.g., line bisection,

cancellation, figure copying). Furthermore, many of the

‘perceptual’ after effects of prisms such as covert attention

[8] or visual imagery mental imagery [9,10] may be related

to effects of PA on eye movements [11–13], which are

also controlled by the dorsal stream [6]. This could allow

patients to mentally scan a previously learned image [14]

without necessitating any changes in bottom-up visual

perception. Importantly, the dorsal stream is typically

undamaged in neglect [1]. Furthermore, recent imaging

[15] and patient studies [16] implicate the dorsal stream as

being critical for generating the beneficial effects of prisms.

Consistent with the view that prisms influence the dorsal

stream, recent studies have shown dissociations in

neglect between benefits post prisms for motor tasks,

with unaltered performance on perceptual tasks [12,13].

Ferber and colleagues [13] used a chimeric faces (i.e., one

half smiling, the other neutral) task to show that before

prisms, fixations in a neglect patient were biased to the

right half of faces. When asked which of the two

chimerics seemed ‘happier’, the patient chose the face

smiling on the right – a bias opposite that of controls

[17]. After prisms the patient’s eye movements were now

biased towards fixating the left side of faces. Even though

the patient’s eye movements now explored the previously

neglected side of faces, he continued to choose the right-

smiling face as being ‘happier’. Although some suggest

that this may be specific to faces [18], others have shown

the same dissociation with simple shapes [12].

If prisms improve neglect by acting on the dorsal stream

there should be a greater effect on behaviors requiring

motor responses. In contrast, tasks relying on perception

436 Vision, central

0959-4965 c2010 Wolters Kluwer Health | Lippincott Williams & Wilkins DOI: 10.1097/WNR.0b013e328338592f

may be less likely to show benefits. We tested this using

the line bisection and landmark tasks – a perceptual

equivalent of bisection – in three neglect patients. The

landmark task requires patients to judge which end

of a horizontal line is closest to a bisection mark [19].

Responses reflect a perceptual judgment of the spatial

extent of the line. Patients consistently reporting that

the left end is closest to the bisection, even when it is at

center, reflects a perceptual distortion such that the left

half of the line is seen to be shorter. Although deficits on

bisection are considered to reflect the same bias, the fact

that this task requires a motor response distinguishes it

from the landmark task. That is, while performance on

both tasks reflects the same underlying perceptual bias,

improvement after prisms may only be expected for the

task that has a motor response (i.e., bisection).

Methods

Eight, right-handed, healthy, older individuals [three

males, mean age = 67 (± 5.5) years] with no history of

neurological or psychiatric illness were recruited from the

community. Three neglect patients were examined using

cancellation, figure copying, and bisection. For bisection,

deviations from center were calculated as a percentage of

total line length (leftward deviations coded as negative).

Neglect was considered to be present if mean bisections

deviated from center by 5% or more. For cancellation,

neglect was considered present if 10% or more of left

targets were omitted. Neglect was scored as present on

figure copying by visual inspection (Fig. 1).

NS was an 80-year-old, right-handed female who

presented with neglect after a stroke affecting right

parietal white matter and the right thalamus (Fig. 1). Her

bisections were biased rightward by 11% and had 75%

leftward omissions on cancellation. Neglect was evident

on figure copying. NS was right handed and had no

hemiplegia.

RR was a 66-year-old male who suffered a right hemi-

sphere stroke involving fronto-parietal white matter,

parietal cortex, and basal ganglia (Fig. 1). His bisection

was biased rightwards by 12%, had 90% leftward

omissions on cancellation, and neglect was evident on

figure copying. RR was right handed and was hemiplegic.

SQ was a 79-year-old female who suffered a stroke

affecting right occipito-temporal and medial parietal

cortex (Fig. 1). Her bisection was biased rightwards

by 12.8%, had 77% omissions of leftward targets on

cancellation, and neglect was evident on figure copying.

SQ was right handed, had a left hemianopia, and showed

no signs of hemiplegia.

All three patients showed a strong leftward bias on

the landmark task pre-PA (% of ‘left’ responses for

RR = 100%, NS = 100%, SQ = 90%) indicating they all

perceived the left half of the line to be shortest. Informed

consent was obtained from all patients before testing

and the protocol was approved by institutional ethics

committees in accordance with the Helsinki Declaration.

Apparatus and procedure

The bisection task consisted of 10 trials with a single

black line (length = 236 mm) presented horizontally on

a sheet of 8.5 1100 paper aligned to the patients’

midline. Patients used a pen to mark where they thought

center was.

The landmark task consisted of 16 black lines (length =

200 mm). Each line was placed horizontally on a sheet of

8.5 1100 paper. Ten lines had a vertical bisection mark

(height = 10 mm) placed at center, with six lines bisected

1, 3, or 5 mm to the left or right. Lines were presented

randomly and patients were told that no bisection

appeared at center. On half the trials, patients indicated

verbally which end of the line was closest to bisection.

On the other half of trials they indicated which end

was further away from bisection. This eliminated the

influence of response biases (for analysis, responses were

reverse coded when patients indicated which end was

further away). Only trials in which the bisection was at

center were included for analysis.

For PA, patients pointed to targets left and right of

midline while wearing prisms that shifted vision to 101

right. Patients pointed to targets every 2–3 s for 5 min.

Controls completed an identical procedure with left

shifting prisms as previous studies find robust effects

with this direction of shift in controls [20]. Controls were

adapted for 10 min as a previous study indicates that

longer exposure periods are needed for ‘neglect like’

patterns of performance to emerge in controls [20]. To

measure the effects of PA, patients and controls made

five pointing movements with eyes closed to a location

straight-ahead of their body (i.e., proprioceptive straight-

ahead judgment). These straight-ahead judgments were

made once before adaptation (baseline), once immedi-

ately postadaptation (post), and once at the end of

testing (late). Endpoints of movements were recorded by

the experimenter. For all tasks, patients and controls used

their right hand. Patients and controls first completed

one session of line bisection, landmark, and straight-

ahead pointing before being exposed to prisms, after

which they completed the same three tasks. Line

bisection and landmark tasks were counterbalanced

before and after PA. Straight-ahead pointing was repeated

a third time at the end of testing to determine whether

participants were still adapted.

Data analysis

Bisection was analyzed in terms of the percentage

deviation from center relative to total line length

(leftward deviations coded as negative). The landmark

Prisms influence action but not perception Striemer and Danckert 437

task was analyzed in terms of the percentage of ‘left’

responses (i.e., bisection was closer to the left end of

the line). For controls, to examine the effects of PA on

subjective straight-ahead, line bisection, and landmark

tasks, we ran paired samples t-tests. We did the same

thing for patients at both the group and individual

level. Finally, each patient’s performance pre and post-PA

was compared directly to controls using a modified

independent samples t-test, in which the patient is

treated as an individual sample not contributing to within

group variance [21]. Straight-ahead judgments were

measured as deviations from center (leftward deviations

coded as negative) and converted to degrees of visual

angle. Data were analyzed separately for each patient

using paired samples t-tests to contrast pre vs. post, and

pre vs. late straight-ahead pointing.

Fig. 1

Patient NS

Patient RR

Patient SQ

Pre

Post

Late

Pre

Post

Late

−20246810

Degrees of visual angle

12 14

−20 2 4 6 810

Degrees of visual angle

12 14

−20 2 4 6 810

Degrees of visual angle

12 14

Patient scans (left), figure copying performance for the daisy (middle), and subjective straight-ahead judgments made preprisms (Pre), postprisms

(Post) and at the late session postprisms (Late). Patients NS (top) and RR (middle) both have computed tomography scans, whereas patient SQ

(bottom) has a sagittal MRI. Note that for both NS and RR scans are presented in radiological convention with the right hemisphere appearing on the

left side of the scan. Straight-ahead judgments are presented in degrees of visual angle (left is negative).

438 NeuroReport 2010, Vol 21 No 6

Results

Straight-ahead pointing

Controls

Straight-ahead judgments were shifted right after left

prisms [mean (standard error) pre = 3.4 (1.5) vs. mean

post = 10.3 (0.9), t(7) = 5.5, P= 0.001]. Controls had

begun to deadapt at the end of testing [post mean = 10.3

(0.9) vs. late mean = 6.9 (1), t(7) = 4.6, P< 0.01], how-

ever, straight-ahead judgments were still to the right of

judgments made pre-PA [pre mean = 3.4 (1.5) vs. late

mean = 6.9 (1), t(7) = 3.2, P< 0.05]. Thus, controls were

still adapted at the end of testing.

Straight-ahead judgments for two of the three patients

(RR and SQ) were shifted leftward immediately post-PA,

and all three patients were shifted significantly left-

ward at the late test (all P< .05; Fig. 1). Thus, at the

last test all patients were still significantly adapted to

prisms.

Line bisection

For controls pre-PA bisections were slightly left of center

(mean bias = – 0.21%). In all but one control, mean

bisections were shifted right post-PA [mean bias = 1.43%,

t(7) = 3.5, P< 0.01; Fig. 2].

For all three patients there was a leftward shift in

bisection post-PA [NS, t(9) = 2.8, P<0.05; RR,t(9) = 3.2,

P< 0.05; SQ, t(9) = 8.6, P< 0.001; Fig. 2]. In addition,

Crawford’s t-test on pre-PA bisections indicated that all

three patients were well outside the range of controls

(Table 1). When postexposure performance was com-

pared in the same way, results showed that all three

patients were well within the range of controls (Table 1).

Landmark task

For controls the percentage of ‘left’ responses was 63.8%

pre-PA (Fig. 2). This was not significantly altered post-PA

at the group level [mean % (standard error) ‘left’

responses =71.25% (10.3), t(7) = – 1.2, P=0.3]. To ex-

amine whether the null effect on landmark performance

resulted from deadaptation before completion of the

bisection task (owing to the motor response required), we

compared landmark performance pre vs. post-PA for

controls who completed the bisection task first post-PA,

or the landmark task first post-PA. This analysis revealed

a significant increase in ‘left’ responses post-PA for

controls who completed the landmark task first post-PA

[pre–post difference = 18%; t(4) = 3.09, P= 0.037] com-

pared with a tendency to make fewer ‘left’ responses

in those who completed the landmark task after bisec-

tion post-PA [pre–post difference = – 10%; t(2) = 1.73,

P= 0.23; Fig. 2].

Two patients (NS and RR) responded ‘left’ on 100% of

trials pre and post-PA. SQ responded ‘left’ on 87.5% of

trials pre-PA and on 90% of trials post-PA. Thus, all three

patients showed a strong bias towards seeing the left half

of lines as shorter than the right, a bias that was unaltered

by PA (Fig. 2). Note that deadaptation cannot explain the

null effect in patients as adaptation remained stable,

or significantly increased by the late test in all three

patients (Fig. 1).

Discussion

We examined the effect of PA on perceptual and motor

performance using the bisection and landmark tasks in

three neglect patients and a group of controls. Results

showed that for neglect patients, PA reduced rightward

biases in bisection, but had no effect on leftward

perceptual biases on the landmark task (Fig. 2). Thus,

despite the fact that bisections were shifted in the

opposite direction to the prism shift, there was no

influence on how the spatial extent of lines was per-

ceived. The dissociation between perceptual and motor

influences of PA shown here is consistent with other

studies showing that PA shifts eye movements left

without altering the perception of stimuli in neglected

space [12,13]. These data are consistent with the notion

that PA influences dorsal stream processes without

influencing perceptual biases in the ventral stream in

patients with neglect.

Performance on line bisection and landmark tasks in

younger healthy controls typically reveal a leftward over-

estimation of line length (i.e., pseudoneglect) that results

in a leftward bias in bisection, and a tendency to say the

prebisection mark is closer to the right end of the line.

Consistent with earlier studies on perceptual biases in

elderly patients [22,23], our controls showed only a small

leftward bias in bisection and no bias in the landmark

task. Irrespective of the presence of pseudoneglect

before PA, leftward PA resulted in a ‘neglect-like’ pattern

of performance for line bisection (i.e., a rightward shift)

and, for those who completed the landmark task first

postprisms, an increase in the percentage of ‘left’

responses in the landmark task. This is consistent with

earlier studies indicating that prisms can influence both

perceptual and motor biases in healthy individuals

[20,24,25]. These results are informative because they

suggest that the landmark task used here was sensitive

enough to detect any effects of PA in patients where such

effects existed. In addition, they suggest it is possible

for prisms to influence perception in individuals without

brain damage, perhaps through intact connections be-

tween the dorsal and ventral streams. Importantly, the

failure to alter perceptual biases in the face of changes

to visuomotor performance in neglect may be precisely

because of the fact that damage to inferior parietal cortex

disrupts the normal interaction between the dorsal and

ventral streams [6], eliminating the path by which prisms

could influence perception in neglect.

Prisms influence action but not perception Striemer and Danckert 439

Fig. 2

Healthy controls

Line bisection

Pre

Post

Pre

Pre Pre

Post

−1−0.5 0.5 1.5 2.5 100 80 60 40 20 0321

−2

−4

100

−2

100

Percentage of ‘left’ responses

Deviation (percentage of line length)

Percentage of ‘left’ responses

2345678

−20 2 4 6 8 10 12 14 16 18 100 80 60 40 20 0

Deviation (percentage of line) Percentage of ‘left’ responses

Percentage of ‘left’ responses

Participants

Deviation (percentage of line length)

Participants

Line bisection

SQ NS RR

Participants

SQ NS RR

Neglect patients

12345678

Participants

∗

∗∗

∗

Landmark

Post

Pre Post

Upper panel: healthy older control data. Mean [standard error (SE)] line bisection performance pre (open circle) and postadaptation (filled circle)is

to the left, with mean (SE) percentage of ‘left’ responses for the landmark task pre (open bar) and post (filled bar) adaptation to the right. Below are

the data for individual participants on each task. *The participant performed the landmark task first post adaptation. Lower panel: neglect patient

data: mean (SE) line bisection performance (i.e., deviation from center represented as a percentage of total line length, rightward deviations codedas

positive) pre (open circles) and postprisms (filled black circles) is presented on the left. Mean (SE) percentage of ‘left’ responses for pre (open bar)

and postprisms (filled bar) is presented to the right. Below this are the individual performances of patients with neglect on each task. *Significant

difference at P< 0.05.

440 NeuroReport 2010, Vol 21 No 6

Conclusion

Many of the beneficial effects of prisms observed in

neglect may arise through interactions with the dorsal

‘action’ stream but may fail to influence perceptual biases

evident in neglect.

Acknowledgements

This study was supported by Heart and Stroke Founda-

tion of Canada Postdoctoral award, and a Natural Sciences

and Engineering Research Council of Canada PhD award

to C.S. This study was also supported by a Heart and

Stroke Foundation of Canada Grant-in-Aid and Natural

Sciences and Engineering Research Council of Canada

Discovery and Canada Research Chair (Tier II) awards

to J.D. The authors would like to thank Phil Hatcher for

his assistance with collecting the control data.

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Table 1 Mean (standard error) line bisection performance

(deviation from centre as a % of total line length; rightward

deviations codes as positive) for controls and each patient

pre and postprisms

Preprisms Postprisms

Control, mean (SD) – 0.21 (3.05) 1.43 (3.2)

Patient NS 11.02 0.88

Crawford’s t-test t= – 10.496, P= 0.005 t=0.49, P=0.438

Pbelow 99.49% 43.79%

Patient RR 6.81 – 0.98

Crawford’s t-test t=–6.56,P= 0.033 t= 2.13, P= 0.25

Pbelow 96.68% 25.07%

Patient SQ 18.98 3.49

Crawford’s t-test t=–17.54,P= 0.0003 t= – 1.82, P= 0.28

Pbelow 99.97% 71.72%

‘Pbelow’ refers to the percentage of the healthy population with scores falling

below the patient’s as determined by Crawford’s t-test.

SD, standard deviation.

Prisms influence action but not perception Striemer and Danckert 441

Effects of prism adaptation and visual scanning training on perceptual and response bias in unilateral spatial neglect

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Full-text available

Jan 2023

Background Unilateral spatial neglect (USN) is a complex neurological syndrome that often reduces rehabilitation outcomes, prolongs patients' hospital stays, and decreases their quality of life. However, the current therapies for USN have varying efficacy. We will explore a new treatment option that combines prism adaptation (PA) with eye movement training (EMT) for the treatment of USN after stroke. Methods We will conduct a single-blind, prospective, randomized controlled trial to assess the efficacy of the combined intervention (PA & EMT) on USN in an inpatient rehabilitation setting. The study aims to recruit 88 patients with USN after an ischemic or hemorrhagic stroke. Participants will be randomly assigned to the following four groups: (1) PA group (n = 22), (2) EMT group (n = 22), (3) PA and EMT group (n = 22), and (4) control group (n = 22). All groups will receive 10 sessions of interventions over 2 weeks, 5 times per week. Blinded assessors will conduct a baseline assessment, a post-intervention assessment, and a follow-up assessment (2 weeks post-intervention). The primary outcome measure will use the Behavioral Inattention Test-Conventional Subset (BIT-C) and Catherine Bergego Scale (CBS) to assess the levels of USN. Secondary outcome measures will assess the patient's ability to perform activities of daily living using the Modified Barthel Index (MBI). Patients who completed all treatment and assessment sessions will be included in the final analysis. Discussion This study will explore the effects of 10 sessions of combined interventions (PA & EMT) on USN and functional capacity. This study has the potential to identify a new, evidence-based treatment option and provide new ideas for the treatment of USN. Ethics and dissemination The study protocol has been approved by the Nanchong Central Hospital. Written informed consent will be obtained from all the participants. The results of this study will be disseminated to the public through scientific conferences and a peer-reviewed journal. Trial registration ChiCTR, ChiCTR2100049482. Registered on 2 August 2021, http://www.chictr.org.cn/showproj.aspx?proj=130823.

Using Prism Adaptation to Alleviate Perception of Unilateral Tinnitus: A Case Study

Article

Oct 2022
CORTEX

Tinnitus is described as an uncomfortable sound or noise heard by an individual in the absence of an external sound source. Treating this phantom perception remains difficult even if drug and nondrug therapies are used to alleviate symptoms. The present case study aimed to investigate whether prism adaptation could induce beneficial aftereffects in a tinnitus sufferer. A 75-year-old man, R. B., with chronic unilateral tinnitus in the left ear reported a self-estimation of parameters of his tinnitus—discomfort, pitch and loudness—and performed a manual line-bisection task to study the consequences of lateralized auditory disorder on spatial representation. Aftereffects of prism adaptation were assessed using a sensorimotor open-loop pointing task. In parallel, a control group completed the line-bisection task and the open-loop pointing task before and after lens exposure, under the same experimental condition as those of R. B. Throughout the pretests, the patient assessed his tinnitus at a constant medium pitch (around 3000 Hz), and he was biased toward the affected ear in both the sensorimotor task and the estimation of the subjective center in the manual line-bisection task. Although both optical deviations were effective, an exposure to prism adaptation to a rightward optical deviation (i.e., toward the unaffected ear) produced stronger aftereffects. In posttests, the tinnitus pitch decreased to 50 Hz and the subjective center was shifted toward the right side (i.e., unaffected ear side). Furthermore, the line-bisection task seemed to reflect the changes in the tinnitus perception, and spatial representation could be a new tool to assess tinnitus indirectly. Our findings suggest that prism adaptation may have benefits on unilateral tinnitus and open a new avenue for its treatment.

Clinical Assessment of Unilateral Spatial Neglect Dissociations and Heterogeneities: A Narrative Synthesis

Article

Full-text available

Jul 2022

Alma Guilbert

Objective: Unilateral spatial neglect (USN) is a neuropsychological syndrome whose symptoms differ considerably between patients. Several heterogeneities and dissociations have been described between processing stages, spatial regions, reference frames, or sensory modalities. However, in clinical practice, current routine tests rarely assess the different USN subtypes. There is a real lack of practice guidance for clinicians to help them choosing the right USN assessment tools for specific USN aspects. Method: The present article is a narrative synthesis based on a systematic literature search of (a) the dissociations and heterogeneities that can appear in USN and of (b) the different clinical tools available for their assessment. Results: Several tools have been developed to better assess USN heterogeneities. This review highlights their potential relevance and advocates for their widespread use in a clinical practice context. However, it also raised the issue of the lack of standardized clinical tools for the evaluation of some USN aspects. Conclusion: While USN heterogeneities and dissociations are difficult to assess in current clinical practice, there is a real need to improve their assessment. This will allow clinicians to establish the individual USN profile of each patient not only in terms of severity but also of impaired aspects in order to provide them the rehabilitation program that suits the best their needs. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Choosing Sides: Impact of Prismatic Adaptation on the Lateralization of the Attentional System

Article

Full-text available

Jun 2022

Seminal studies revealed differences between the effect of adaptation to left- vs. right-deviating prisms (L-PA, R-PA) in normal subjects. Whereas L-PA leads to neglect-like shift in attention, demonstrated in numerous visuo-spatial and cognitive tasks, R-PA has only minor effects in specific aspects of a few tasks. The paucity of R-PA effects in normal subjects contrasts with the striking alleviation of neglect symptoms in patients with right hemispheric lesions. Current evidence from activation studies in normal subjects highlights the contribution of regions involved in visuo-motor control during prism exposure and a reorganization of spatial representations within the ventral attentional network (VAN) after the adaptation. The latter depends on the orientation of prisms used. R-PA leads to enhancement of the ipsilateral visual and auditory space within the left inferior parietal lobule (IPL), switching thus the dominance of VAN from the right to the left hemisphere. L-PA leads to enhancement of the ipsilateral space in right IPL, emphasizing thus the right hemispheric dominance of VAN. Similar reshaping has been demonstrated in patients. We propose here a model, which offers a parsimonious explanation of the effect of L-PA and R-PA both in normal subjects and in patients with hemispheric lesions. The model posits that prismatic adaptation induces instability in the synaptic organization of the visuo-motor system, which spreads to the VAN. The effect is lateralized, depending on the side of prism deviation. Successful pointing with prisms implies reaching into the space contralateral, and not ipsilateral, to the direction of prism deviation. Thus, in the hemisphere contralateral to prism deviation, reach-related neural activity decreases, leading to instability of the synaptic organization, which induces a reshuffling of spatial representations in IPL. Although reshuffled spatial representations in IPL may be functionally relevant, they are most likely less efficient than regular representations and may thus cause partial dysfunction. The former explains, e.g., the alleviation of neglect symptoms after R-PA in patients with right hemispheric lesions, the latter the occurrence of neglect-like symptoms in normal subjects after L-PA. Thus, opting for R- vs. L-PA means choosing the side of major IPL reshuffling, which leads to its partial dysfunction in normal subjects and to recruitment of alternative or enhanced spatial representations in patients with hemispheric lesions.

Revisiting sensorimotor adaptation : new insights from inter-task transfer of after-effects in prism adaptation

Thesis

Full-text available

Dec 2020

Lisa Fleury

The plasticity that characterizes our nervous system enables us to realize smooth and precise movement despite varying demands. The understanding and distinction of processes enabling to modify existing movements when facing a perturbation (adaptation) or to learn new ones (learning) represents a crucial challenge. Sensorimotor adaptation is reflected not only by the gradual error reduction during the perturbation but mostly by the presence of after-effects once the perturbation is removed. The nature of after-effects provides information concerning the modifications entailed in the sensorimotor system and thus the type of processes involved. The aim of this thesis was to shed light on the comprehension of sensorimotor plasticity processes. More precisely, we hypothesized that the transfer of after-effects to a task that has not been performed under the perturbation might uncover the contribution of distinct processes during sensorimotor adaptation. Therefore, the objective was to isolate the characteristics of processes favouring transfer. In addition, we aimed at investigating the role of the cerebellum in these processes. To complete these objectives, we used a behavioural approach through a prism adaptation paradigm, using googles laterally shifting the visual field. Our first study showed a transfer of after-effects acquired during pointing prism exposure to a throwing task. However, the transfer from throwing to pointing was possible only for experts in throwing. Thus, the expertise on the task performed during prism exposure strongly influenced the nature of processes at work to face the perturbation. In the second study, we demonstrated that prism adaptation by motor imagery of pointing movements under exposure leaded to substantial transfer to throwing movements. This was the case only for participants with high motor imagery abilities. Finally, using tDCS, the two last studies emphasized the possible role of the cerebellum in after-effects transfer. Altogether, these results underline the interest to study inter-task transfer of after-effects to unravel the contribution of distinct processes during sensorimotor adaptation, beyond the classical measure of after-effects. We propose that the development of transferable after-effects relies on sustained adjustments of sensorimotor system internal models. These modifications may be guided by sensory prediction errors and controlled by cerebellar regions. In a translational view, these fundamental findings paves the way for clinical perspectives of research to optimize the transfer of compensations from rehabilitation context to other daily life situations with patients with motor disorders.

A scoping review and critique of the Input-Output subtyping dimension of spatial neglect

Article

Full-text available

Apr 2024
CORTEX

Investigating premotor reaching biases after prism adaptation

Article

Aug 2023
NEUROPSYCHOL REHABIL

Prism adaptation (PA) is both a visuomotor learning task and potential treatment for spatial neglect after stroke. While PA's aftereffects can improve neglect symptoms, therapeutic benefits vary across individuals, possibly due to differences in neglect subtypes. Neglect symptoms can be described along an information processing pathway, yielding perceptual (input) and premotor (output) neglect subtypes. There is some evidence that PA mainly benefits persons with premotor neglect. We investigated whether PA modulates the premotor stage of information processing by examining whether PA could induce a premotor bias in healthy adults. We measured perceptual and premotor biases using a speeded reach task that compares the initiation time of leftward and rightward reaches to lateralized targets from different hand start positions. Using a randomized mixed experimental design, 30 right-handed healthy adults completed this speeded reach task before and after either left-shifting (n = 15) or right-shifting (n = 15) PA. As hypothesized, left-shifting PA speeded initiation time specifically for reaches in the rightward direction, regardless of target location (p = .02, ηp2 = .18), suggesting that PA induced a premotor bias in the direction of the prism aftereffect. These findings have implications for PA's underlying mechanisms, which can inform visuomotor learning theories and PA's use as a treatment for spatial neglect.

Porter des lunettes pour entendre différemment ? Les effets consécutifs de l'adaptation prismatique visuomanuelle sur la perception auditive

Thesis

Full-text available

Mar 2022

Clémence Bonnet

La plasticité sensorimotrice se définit comme notre capacité à produire une réponse motrice appropriée face à des modifications corporelles ou environnementales. Le paradigme expérimental classique pour l’étudier est l’adaptation prismatique qui consiste à pointer des cibles visuelles en portant des lunettes prismatiques qui dévient le champ visuel latéralement ou verticalement. Ses effets consécutifs ne sont pas restreints au comportement sensorimoteur et s’étendent à d’autres modalités sensorielles non-sollicitées lors de l’exposition, comme l’audition. Il existe une représentation mentale de la fréquence auditive au sein de laquelle les basses fréquences (i.e., sons graves) sont associées avec l’espace gauche et les hautes fréquences (i.e., sons aigus) avec l’espace droit. Le premier objectif de ce travail de thèse est d’étudier l’organisation de la représentation mentale de la hauteur des sons et sa modulation par l’adaptation prismatique latérale et verticale. Le second objectif est de s’intéresser aux effets de l’adaptation prismatique latérale sur l’attention divisée auditive chez des individus sains. Enfin, le troisième et dernier objectif a une portée clinique et vise à tester les effets de l’adaptation prismatique sur la perception d’un acouphène unilatéral.Indépendamment de l’expertise musicale, la première étude a mis en évidence l’existence d’une pseudonégligence auditive orientée vers les basses fréquences au sein d’un spectre auditif élargi. Ce biais initial est décalé vers les hautes fréquences après adaptation prismatique de déviation gauche. La seconde étude s’est intéressée à l’adaptation prismatique verticale. Nous avons montré les premiers effets sensorimoteurs après adaptation verticale aux deux déviations optiques, ainsi que la première modulation du biais de pseudonégligence auditive après adaptation prismatique de déviation vers le bas. Dans la troisième étude, nous avons observé de nouveaux résultats avec des effets de l’adaptation prismatique de déviation gauche sur l’attention divisée auditive chez des individus sains. La quatrième et dernière investigation était une étude de cas qui a fourni des résultats préliminaires et prometteurs sur la modulation de la perception de l’acouphène unilatéral après adaptation prismatique latérale. Les résultats innovants de ces travaux de thèse témoignent des effets intermodaux de l’adaptation prismatique et plus particulièrement dans le domaine de l’audition. L'exposition aux prismes engendre des modifications dans la perception de la hauteur des sons et l’orientation de l’attention auditive. De plus, même si des études complémentaires sont nécessaires, la modulation de la perception de l’acouphène après adaptation prismatique ouvre de nouvelles voies de recherches thérapeutiques.

The Visual Brain in Action

Article

Full-text available

Jan 1997

First published in 1995, this book presents a model for understanding the visual processing underlying perception and action, proposing a broad distinction within the brain between two kinds of vision: conscious perception and unconscious 'online' vision. It argues that each kind of vision can occur quasi-independently of the other, and is separately handled by a quite different processing system. For this new edition, the text from the original edition has been left untouched, standing as a coherent statement of the authors' position. However, a very substantial epilogue has been added to the book, which reviews some of the key developments that support or challenge the views that were put forward in the first edition. The new chapter summarizes developments in various relevant areas of psychology, neuroscience, and behaviour. It supplements the main text by updating the reader on the contributions that have emerged from the use of functional neuroimaging, which was in its infancy when the first edition was written. Neuroimaging, and functional MRI in particular, has revolutionized the field by allowing investigators to plot in detail the patterns of activity within the visual brains of behaving and perceiving humans. The authors show how its use now allows scientists to test and confirm their proposals, based largely on evidence accrued from primate neuroscience in conjunction with studies of neurological patients.

Pseudoneglect: A review and meta-analysis of performance factors in line bisection tasks

Article

Full-text available

Jan 2000
NEUROPSYCHOLOGIA

An exhaustive qualitative (vote-counting) review is conducted of the literature concerning visual and non-visual line bisection in neurologically normal subject populations. Although most of these studies report a leftward bisection error (i.e., pseudoneglect), considerable between-study variability and inconsistency characterize this literature. A meta-analysis of this same literature is performed in which the total quantitative data set, comprising 73 studies (or sub-studies) and 2191 subjects, is analyzed with respect to 26 performance factors. The meta-analytic results indicate a significant leftward bisection error in neurologically normal subjects, with an overall effect size of between -0.37 and -0.44 (depending on integration method), which is significantly modulated to varying degrees by a number of additional task or subject variables. For example, visual bisection tasks, midsagittal-pointing tasks and tactile bisection tasks all lead to leftward errors, while kinesthetic tasks result in rightward errors. Tachistoscopic forced-choice testing methods reveal much greater estimates of bisection error (effect size = -1.32) than do manual method-of-adjustment procedures (effect size= -0.40). Subject age significantly modulates line bisection performance such that older subjects err significantly rightward compared to younger subjects, and to veridical line midpoint. Male subjects make slightly larger leftward errors than do female subjects. Handedness has a small effect on bisection errors, with dextrals erring slightly further to the left than sinistral subjects. The hand used to perform manual bisection tasks modulated performance, where use of the left hand lead to greater leftward errors than those obtained using the right hand. One of the most significant factors modulating bisection error is the direction in which subjects initiate motor scanning (with either eye or hand), where a left-to-right scan pattern leads to large leftward errors while a right-to-left scan pattern leads to rightward errors.

Prism adaptation to rightward optical deviation rehabilitates left hemispatial neglect

Article

Full-text available

Oct 1998

A large proportion of right-hemisphere stroke patients show hemispatial neglect-a neurological deficit of perception, attention, representation, and/or performing actions within their left-sided space, inducing many functional debilitating effects on everyday life, and responsible for poor functional recovery and ability to benefit from treatment. The frequent parietal locus of the lesion producing neglect reflects the impairment of coordinate transformation used by the nervous system to represent extrapersonal space. Given that adaptation to a visual distortion can provide an efficient way to stimulate neural structures responsible for the transformation of sensorimotor coordinates, the aim of our study was to investigate the effect of prism adaptation on various neglect symptoms, including the pathological shift of the subjective midline to the right. All patients exposed to the optical shift of the visual field to the right were improved on their manual body-midline demonstration and on classical neuropsychological tests. Unlike other physiological manipulations used to improve neglect, this improvement lasted for at least two hours after prism removal and thus could be useful in rehabilitation programmes. The positive effect found for both sensorimotor and more cognitive spatial functions suggests that they share or depend on a common level of space representation linked to multisensory integration.

Cognitive bias induced by visuo-motor adaptation to prisms: A simulation of unilateral neglect in normal individuals?

Article

Full-text available

Jul 2000
NEUROREPORT

Unilateral neglect has been recently shown to be improved following a short period of adaptation to wedge prisms. The present study investigates whether visuo-motor adaptation can generate a bias in normals on line bisection tasks classically used to assess unilateral neglect. Our results show that adaptation to left-deviating prisms induces a stronger rightward bias for the perceptual than the motor line bisection task. This bias is in the same direction as the one observed in unilateral neglect. No significant effect is produced by adaptation to right-deviating prisms. Our data confirm that the plasticity of inter-sensory and sensori-motor coordinations affects higher levels of space representation. These asymmetric results may reflect the inherent bias of the brain's structural organisation and provide an empirical explanation for the left-sided predominance of unilateral neglect.

Eye scanpaths during visual imagery reenact those of perception of the same visual scene

Article

Mar 2002

Eye movements during mental imagery are not epiphenomenal but assist the process of image generation. Commands to the eyes for each fixation are stored along with the visual representation and are used as spatial index in a motor-based coordinate system for the proper arrangement of parts of an image. In two experiments, subjects viewed an irregular checkerboard or color pictures of fish and were subsequently asked to form mental images of these stimuli while keeping their eyes open. During the perceptual phase, a group of subjects was requested to maintain fixation onto the screen’s center, whereas another group was free to inspect the stimuli. During the imagery phase, all of these subjects were free to move their eyes. A third group of subjects (in Experiment 2) was free to explore the pattern but was requested to maintain central fixation during imagery. For subjects free to explore the pattern, the percentage of time spent fixating a specific location during perception was highly correlated with the time spent on the same (empty) locations during imagery. The order of scanning of these locations during imagery was correlated to the original order during perception. The strength of relatedness of these scanpaths and the vividness of each image predicted performance accuracy. Subjects who fixed their gaze centrally during perception did the same spontaneously during imagery. Subjects free to explore during perception, but maintaining central fixation during imagery, showed decreased ability to recall the pattern. We conclude that the eye scanpaths during visual imagery reenact those of perception of the same visual scene and that they play a functional role.

Prism adaptation overcomes pseudoneglect for the greyscales task

Article

May 2009
CORTEX

Visuomotor adaptation to left-shifting prisms can affect performance for a variety of tasks in neurologically intact (normal) participants. This study examined whether visuomotor adaptation affects performance on the greyscales task in normal participants. Forty-two normal participants completed a greyscales task before and after adaptation to either: left-shifting prisms, right-shifting prisms or control spectacles that did not shift the visual scene. Participants demonstrated a leftward bias (i.e., selected the stimulus that was darker on the left as being darker overall) that was reversed by a short period of visuomotor adaptation to left-shifting prisms. In contrast, this bias was unaffected by adaptation to right-shifting prisms and control spectacles. The findings demonstrate that a simple visuomotor task can alter the distribution of spatial attention for the greyscales task in normal participants.

Direct effects of prismatic lenses on visuomotor control: An event-related functional MRI study

Article

Nov 2008
EUR J NEUROSCI

Exposure to prisms has long been used to explore the control of visually guided actions primarily because adaptation requires the recalibration of misaligned reference frames due to perturbed visual input (i.e. eye-in-head and hand-centered reference frames must be realigned). To date, the only neuroimaging study to explore the direct effects of prisms on pointing used positron emission tomography and found increased activation only in right parietal cortex. We used event-related functional MRI to examine the effects of prisms on visuomanual pointing. Results demonstrated changes in activity in the anterior cingulate, the anterior intraparietal region and in a medial region of the right cerebellum. Specifically, activity in these regions was higher for the first few pointing trials made while viewing targets through prisms when directly contrasted to the last few trials. These results highlight that a more extensive network of cortical and cerebellar regions is involved in recalibrating visuomotor commands in the face of perturbed visual input.

Reversed Perceptual Asymmetry for Faces in Left Unilateral Neglect

Article

Dec 1993

Twelve patients with left unilateral neglect and 12 matched controls were examined on two tests of face perception. In the chimeric faces task, subjects were required to make a judgement of happiness on pairs of photographic chimeras, while in the face-matching task, they were required to indicate which of two symmetrical face composites more closely resembled the original. Whereas controls showed a significant leftward perceptual bias on both tasks, left neglect patients showed an even stronger rightward (reversed) perceptual bias. Patients with and without left-sided visual field defects exhibited the reversed asymmetry, and the degree of perceptual bias shown by patients was not related to the severity of their symptoms as measured by standard clinical tasks. There was no relationship between the extent of rightward bias exhibited by patients or controls on the two tasks. These results contribute to our understanding of the mechanisms underlying unilateral neglect. Moreover, the tasks themselves may be employed as a simple and sensitive adjunct to the clinical assessment of this disorder.

Line bisection errors in visual neglect: Misguided action or size distortion?

Article

Feb 1993
NEUROPSYCHOLOGIA

The rightward line bisection errors made by patients with visuospatial neglect can be explained as due to a spatially misdirected response, which would be predicted on either of two accounts. An alternative view, however, is that such patients actually misperceive the left half of a horizontal line as being shorter than the right half. We have tested this possibility directly in three neglect patients, by giving them prebisected lines: they were found to judge a central transection mark as lying nearer to the left end of the lines. We were also able to test one of the patients on a series of size comparisons using computer-generated patterns. She was found to judge horizontal lines as shorter in the left half of visual space than in the right. This was also true for comparisons of the areas of nonsense figures. However she did not make such constant errors when comparing the lengths of vertical lines. It is suggested that an attentional deficit in left hemispace may result in the underestimation of horizontal extent. This would act in combination with misdirected reaching to determine the magnitude of line bisection errors.

Role of posterior parietal cortex in the recalibration of visually guided reaching

Article

Nov 1996

Visually guided reaching requires complex neural transformations to link visual and proprioceptive inputs with appropriate motor outputs. Despite the complexity of these transformations, hand-eye coordination in humans is remarkably flexible, as demonstrated by the ease with which reaching can be adapted to distortions in visual feedback. If subjects attempt to reach to visual targets while wearing displacing prisms, they initially misreach in the direction of visual displacement. Given feedback about their reaching errors, however, they quickly adapt to the visual distortion. This is shown by the gradual resumption of accurate reaching while the prisms remain in place, and by the immediate onset of reaching errors in the opposite direction after the prisms have been removed. Despite an abundance of psychophysical data on adaptation to prisms, the functional localization of this form of sensorimotor adaptation is uncertain. Here we use positron emission tomography (PET) to localize changes in regional cerebral blood flow (rCBF) in subjects who performed a prism-adaptation task as well as a task that controlled for the sensory, motor and cognitive conditions of the adaptation experiment. Difference images that reflected the net effects of the adaptation process showed selective activation of posterior parietal cortex contralateral to the reaching limb.

Dissociating perceptual and motor effects of prism adaptation in neglect

Abstract

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