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Is space representation distorted in neglect?
\ PERGAMON
Neuropsychologia 26 "0888# 6Ð04
Is space representation distorted in neglect< Hans!Otto Karnath \ Susanne Ferber Department of Neurology\ University of Tubingen\ Hoppe!Seyler Str[ 2\ 61965 Tubingen\ Germany Received 4 January 0887^ accepted 17 May 0887
Abstract It has been argued that neglect of contralateral stimuli following brain damage might be associated with either a compressed or an anisometric neural representation of space along the earth!horizontal axis[ Two di}erent models have been put forward[ One model proposes a uniform compression of subjective space\ while the other envisages an expansion on one side of space and a compression on the other[ We tested these models by determining neglect patients| perception of spatial distances in the horizontal plane[ The models di}er concerning the expected degree of under! vs overestimation of distances in the left and right hemispace[ In the _rst experiment\ patients were asked to position ten red LEDs equidistantly along a semicircle\ which was located horizontally in front of them at eye level[ A second experiment compared the patients| subjective perception of short\ medium and long distances extending into left and right hemispace[ We found no evidence for any compression or expansion\ nor for anisometry along the earth!horizontal axis[ These _ndings argue against a distortion of subjective space along the horizontal axis in patients with neglect which could account for their failure to orient towards and to explore the contralesional parts of space[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Keywords] Human^ Neglect^ Brain damage^ Distance perception^ Space representation^ Parietal lobe
0[ Introduction The characteristic disturbance of patients with neglect is a de_cient response to stimuli located contralateral to the lesion and a failure to explore the contralesional part of space[ In searching for the underlying mechanism leading to neglect\ altered neural representations of space have been discussed[ One model has speculated that the whole frame of exploratory behaviour is rotated around an earth!vertical body axis towards the ipsilesional side ð00Ł[ Alternative hypotheses have proposed either a com! pressed or an anisometric representation of space along the earth!horizontal axis in patients with neglect[ Halligan and Marshall ð7Ł argued for a linear\ uniform compression of subjective visual space along the hori! zontal axis[ They investigated a patient with left!sided neglect and hemianopia with a visuospatial localization task[ An arrow was presented either at the bottom or top of a monitor[ The arrow always appeared at a location opposite one of a row of numbers "0 to 04#[ The subject|s task was to estimate visually the corresponding spatial position on the row of numbers to which the arrow was pointing[ The patient|s responses revealed a systematic Corresponding author[ Fax] ¦38!6960!18!4159^ e!mail] karnathÝ uni!tuebingen[de
bias towards the right[ The magnitude of this bias tended to decrease from the left to the right side of the monitor[ The authors interpreted the _nding as evidence of a linear\ uniform compression of subjective visual space along the horizontal axis[ They likened the compressed perception of the surround to a compression of a spring that is _xed at the ipsilesional right hand side and {{pushed in|| from the contralesional left side ðFig[ 0"A0#Ł[ The model maintains Euclidean geometry\ i[e[ there is no relative distortion within subjective space[ Halligan and Marshall ð7Ł did not di}erentiate between the subject|s perception of the surround and the repre! sentation of space[ This distinction rather is essential for an alternative model that proposed a non!Euclidean\ anisometric representation of space along the horizontal axis in patients with neglect ðFig[ 0"B#Ł[ Nevertheless\ if one applies the same rationale to the model of Halligan and Marshall\ it could be speculated that the rep! resentation of space that would lead to a compressed perception of the surround\ would be a uniform expan! sion as illustrated in Fig[ 0"A1#[ Like Halligan and Marshall ð7Ł\ Milner ð02Ł speculated that patients with neglect might perceive their visual sur! roundings as being compressed[ However\ he suggested that this compression might progressively increase from the right to the left side ðsee Fig[ 0"B0#Ł[ Such a disturbed
9917!2821:87:,*see front matter Þ 0887 Elsevier Science Ltd[ All rights reserved[ PII] S 9 9 1 7 ! 2 8 2 1 " 8 7 # 9 9 9 6 9 ! 9
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Fig[ 0[ Two models of neglect patients| perception of the visual array "A0:B0# and of the assumed neural representation of space "A1:B1# that would lead to such a disturbed perception[ The subject|s body orientation is illustrated as seen from above^ the body is represented by an ellipse\ the head by a circle[ On the semicircle positioned in front of the subject\ markers are exposed {{equidistantly|| according to the di}erent hypotheses[ Model A suggests a uniform compression of the visual surround along the horizontal axis {{pushed in|| from the con! tralesional left side "A0^ Halligan and Marshall ð7Ł#[ The representation of space that would lead to such an altered perception would be a uniform expansion "A1#[ Model B proposed a progressively increasing leftward compression of the visual array "B0^ Milner ð02Ł#\ which would follow from an expansion of space representation on the contralesional and a compression on the ipsilesional side according to a logarithmic transformation "B1^ Bisiach et al[ ð1Ł#[
perception of the world would result from a distortion of space representation in which the left side was expanded and the right side compressed\ as illustrated in Fig[ 0"B1#[ Bisiach and co!workers ð1Ł have recently proposed a dis! tortion of just this kind in patients with neglect[ They assumed an expansion of space representation on the contralesional side and a compression on the ipsilesional side\ according to a logarithmic transformation[ Using a modi_ed version of the line bisection task\ they asked right brain!damaged patients with neglect to bisect the empty space between two dots corresponding to the endpoints of a previously shown line\ and then to set
both endpoints of an equally long "horizontal# virtual line on the basis of its midpoint that was printed at the center of a sheet of paper[ In executing the latter of these tasks\ the patients misplaced the endpoints of the virtual line leftwards with respect to one another and to the centrally printed midpoint[ Interestingly\ this resulted in the same disproportion usually yielded by neglect patients on the canonical version of the line bisection task\ i[e[ the longer segment lay on the contralesional side of the {{midpoint||[ The authors explained these _ndings by a progressive expansion of space representation on the con! tralesional and a compression on the ipsilesional side[ According to this model ðFig[ 0"B1#Ł\ subjectively equal distances should correspond to increasing real distances as one passes from the ipsilesional right to the con! tralesional left side of space[ The aim of the present study was to test between the hypotheses of a compressed\ expanded or anisometric space representation in neglect[ To distinguish between the di}erent models\ the patients| perception of subjective equidistance in the horizontal plane was determined in a _rst experiment[ Subjects had to adjust ten red LEDs equidistantly along a semicircle\ which was positioned horizontally in front of them at eye level[ Half of these LEDs had to be positioned on the subjects| left and the other half on their right side[ The results were then compared with the two models illustrated in Fig[ 0 to see which model would _t the observed LED positions along the semicircle best[ The models further di}er concerning the expected degree of under! vs overestimation when subjects judge spatial distances in the left and right hemispaces "see Fig[ 1#[ A second experiment therefore compared the patients| perception of three di}erent reference distances "short\ medium\ long# on the left and right of the subjects| mid! sagittal plane[ Two vertically presented green LEDs indi! cated each reference distance which then had to be matched to a pair of horizontally arranged red test LEDs[ The model of Halligan and Marshall ð7Ł predicts that patients should adjust these horizontal distances by a constant increased proportion irrespective of their exten! sion "short\ medium\ long# and irrespective of the hemispace where the distances have to be adjusted ðcf[ Figure 1"A#Ł[ Alternatively\ with an expansion on the contralesional and compression on the ipsilesional side\ as assumed by Bisiach et al[ ð1Ł\ one would expect an under!adjustment of spatial distances the longer the dis! tance extends toward the ipsilesional right and an over! adjustment the longer the distance extends toward the leftward part of egocentric space ðcf[ Figure 1"B#Ł[ 1[ Methods 1[0[ Patients Seven consecutively admitted patients with unilateral right hemispheric lesions and spatial neglect but without
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8
trol subjects gave informed consent to participate in the study[ 1[1[ Apparatus and procedure Both experiments were conducted in complete dark! ness[ Subjects sat upright in an armchair which provided an adjustable support for the back[ Head and body axes were aligned[ The head was _xed with a bandage to a head rest[ In addition\ one experimenter manually held the head steady[
Fig[ 1[ Pattern of expected under! and overestimation of spatial dis! tances in the horizontal plane according to the two models of distorted space representation in neglect[ The di}erent predictions concerning the patients| distance perception when matching three reference distances "short\ medium\ long# in the left and right hemispace are presented[ The open circles represent the physical distance of the vertically presented reference LEDs[ The _lled circles illustrate the expected distance judge! ments in the horizontal plane as they should occur left and right of the patients| midsagittal plane according to the two di}erent models of distorted space representation "models A and B^ Fig[ 0#[
visual _eld defects were investigated[ Five of these pat! ients participated in both experiments\ the other two pat! ients in only one of the experiments "cf[ Table 0#[ Nine patients with unilateral right hemispheric lesions but without neglect or visual _eld defects served as controls[ Seven of these patients participated in the _rst experiment and _ve of them in the second experiment "cf[ Table 0#[ Clinical and demographic characteristics of all patients are listed in Table 0[ As an additional control group\ 4 healthy subjects were investigated in the _rst experiment[ The median age of this group was 59 years "range 42Ð66#[ A second group of 4 di}erent healthy subjects served as a further control group in the second experiment[ The median age of this group was 53 years "range 46Ð54#[ The patients and con!
1[1[0[ Experiment 0 In order to distinguish between the above hypotheses suggesting altered space representation in the horizontal plane "see Fig[ 0#\ subjects had to position ten red LEDs equidistantly along a semicircle\ which was positioned horizontally in front of them at eye level[ To avoid exter! nal cues during the adjustment of the LED positions\ stimulation had to be reduced to a minimum[ Therefore\ during the whole experiment subjects saw only two of the ten red LEDs at a time[ These two red LEDs were horizontally arranged and presented at di}erent locations either on the subject|s left or right side[ Before each pres! entation\ the subjects _rst saw two green LEDs indicating the distance that had to be reproduced between the red LEDs[ During the whole experiment\ subjects thus saw nothing in the dark except either the two green reference LEDs or two of the red LEDs[ While the distance between the green reference LEDs was constant throughout the experiment\ the two red LEDs were presented with vary! ing distances that had to be adjusted[ Subjects sat at the center of a horizontally oriented semicircle "diameter 84 cm# which served as the guide rail for the two _xed green LEDs in the center and the ten freely movable red LEDs\ _ve at each side "cf[ Figure 2#[ The horizontal distance between the two green ref! erence LEDs was constant at 6[4>[ These two reference LEDs were presented symmetrically to the left and right of each subject|s individual {{straight ahead|| body position[ "The latter had been determined immediately before the present experiments using the technique described previously ð00Ł#[ The experiment began with the two green reference LEDs being illuminated for 09 s[ 1Ð4 s after these green LEDs were switched o}\ two red LEDs were switched on[ The distance between the red LEDs varied in di}erent trials[ The subjects| task was to adjust the distance between the two red LEDs to match the distance between the two green reference LEDs presented before[ The dis! tance was adjusted by the experimenter moving only the more eccentric red LED according to the verbal instruc! tions "closer:farther# given by the subjects[ No time limit was used[ The subjects indicated when the correct dis! tance was reached[ The two red LEDs were then switched o}\ their separation was measured and the next trial
09
Table 0 Demographic and clinical data for the right brain damaged patients with neglect "NEG# and without neglect "RH# Patient
¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
Experiment 1
¦ ¦ ¦ ¦ ¦ ¦
¦ ¦ ¦ ¦ ¦
Sex
M F M F M F M M M F M F M F F M
Age "years#
52 41 55 49 53 58 37 60 45 65 49 60 48 64 61 16
Etiology
infarct OP grade IV glioma infarct infarct infarct infarct hemorrhage OP grade IV glioma OP grade IV glioma OP grade IV glioma OP grade IV glioma infarct OP grade IV glioma infarct infarct OP grade IV glioma
Lesion location
temporo!parietal parietal parietal temporo!parietal temporo!parietal temporo!parietal parietal temporo!occipital temporal fronto!parietal temporo!parietal fronto!parietal temporal temporo!parietal parietal frontal
Days since _rst clinical symptoms
00 02 6 033 1 00 029 87 6 45 59 40 06 12 40 04
Neglect
yes yes yes yes yes yes yes no no no no no no no no no
Letter cancel! lation
Copying
Clock face
left
right
left
right
left
right
9 11 03 0 01 9 08 17 13 17 29 15 29 29 17 29
19 29 14 13 14 12 29 18 16 17 18 16 29 17 13 29
− "−# ¦ "−# − ¦ − ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
− "−# ¦ "−# ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
¦ ¦ "¦# ¦ ¦ ¦ − ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
Experiment 0 or 1] participation in experiments 0 or 1[ OP] operation:surgery[ Letter cancellation ð01Ł] number of correct target letters "nmax 29# on each half of the test sheet[ Copying] the copying task featured four di}erent objects on a horizontally oriented DIN A 3 sheet[ Two were placed on the right\ two on the left side of the test sheet[ ¦\ complete copy of all objects^ "−# omission of elements located at the objects| left side^ −\ complete omission of at least one object located on one side of the test sheet[ Clock face] ¦\ all numerals positioned\ "¦# numerals in incorrect positions^ −\ omission of numerals[
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NEG0 NEG1 NEG2 NEG3 NEG4 NEG5 NEG6 RH0 RH1 RH2 RH3 RH4 RH5 RH6 RH7 RH8
Experiment 0
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started by illuminating the two green reference LEDs again[ Trials were arranged in blocks of four[ In each block\ the two red LEDs were presented to the left or to the right of the subject with the side alternating from block to block[ In the _rst block on each side\ one of the red LEDs had the same position as the green reference LED on that side[ The second red LED had a more eccentric position[ Its distance from the _rst red LED was pseudo! randomly varied in the four trials of a block\ ranging from 1> to 04>[ Following these four distance judgements\ the experimenter computed the average distance\ placed the more eccentric red LED at that distance from the other\ and fastened both red LEDs "now switched o}# to the guide rail[ The more eccentric of these newly!_xed LEDs served as the _xed red LED for the next block of trials on that side[ It was paired with a second\ even more eccentrically located red LED[ Again\ the position of the latter was pseudo!randomly varied over four trials such that the red LEDs were presented at four di}erent distances as described above[ This procedure was repeated until _ve red LEDs "not illuminated and thus not visible to the subjects except for the two LEDs currently being adjusted# were each positioned on the subject|s left and right side[ 1[1[1[ Experiment 1 The hypotheses suggesting altered space representation in neglect also di}er concerning the expected pattern of under! and overestimation when neglect patients judge horizontal distances in the left and right hemispace "see Fig[ 1#[ The second experiment therefore compared the patients| perception of three reference distances "short\ medium\ long#\ each on the left or right of the subjects| midsagittal plane[ Two vertically oriented green LEDs indicated these reference distances which subsequently had to be matched to a pair of horizontally oriented red test LEDs[ The subjects sat at the center of two semicircles "diameter 84 cm#[ As in experiment 0\ one semicircle was positioned horizontally in front of the subjects at their individual eye level[ A second semicircle was ver! tically oriented and corresponded with the subject|s physical midsagittal plane[ The vertical semicircle served for presenting the green reference LEDs\ while the hori! zontal semicircle was used as the guide rail for the two red test LEDs[ Each trial began with the two green\ vertically pre! sented reference LEDs being illuminated for a 09 s inter! val[ The reference LEDs were presented at one of three spatial distances "short] 6[4> 01[4 cm : medium] 03[6> 14 cm : long] 16[7> 49 cm#[ One of the LEDs was located half of the reference distance above eye level\ the other LED half of the distance below eye level[ When the green LEDs were switched o}\ the two hori!
00
zontally oriented red test LEDs were switched on after a random interval of 1Ð4 s in complete darkness[ One of these red LEDs always corresponded with the subject|s physical midsagittal plane at eye level[ The second LED was presented in an eccentric position either on the left or on the right side[ Varying from trial to trial\ the initial distance between these two red LEDs was either 29Ð 49) larger or 29Ð49) smaller than the actual reference distance between the two green LEDs[ The subject|s task was to exactly match the previously presented reference distance to the pair of red test LEDs[ The distance was adjusted by the experimenter moving the more eccentric red LED according to the verbal instructions "clo! ser:farther# given by the subject[ No time limit was used[ The subject indicated when the exact distance was reached[ The two red LEDs were then switched o}\ the adjusted distance between them was measured and the next trial started after a 09 s interval in complete darkness when two vertically oriented green reference LEDs were presented again[ Each of the three reference distances "short\ medium\ long# was presented eight times\ in a pseudo!randomized order[ The subsequent presentation of the horizontally arranged red test LEDs was on the subject|s left four times and on the right side four times[ The adjusted distances on these trials were averaged for each subject[
2[ Results 2[0[ Experiment 0 Distances judged as {{equidistant|| at the di}erent spa! tial locations along the horizontal semicircle were rela! tively constant in both control groups[ The _ve subjects without brain damage adjusted a distance of 6[5> "SD 9[1># averaged over the four spatial locations of LED presentation on the left and a distance of 6[4> "SD 9[2># averaged over the four spatial locations of LED presentation on the right[ Comparable results were observed for the eight right brain!damaged control patients without neglect[ They adjusted the pairs of LEDs with an average distance of 6[3> both on the left "SD 9[6># and on the right "SD 9[7>#[ Figure 2 illustrates the judgements of each individual patient with neglect as well as the average distances of the whole group along the horizontal semicircle[ The patients showed no regular pattern that could be attri! buted to a uniform compression or to a logarithmic trans! formation "compare Figs[ 0 and 2#[ On both sides of the body|s midsagittal plane\ the distance judgements of the patients with neglect lay within the range of the controls and close to the actual distance of the green reference LEDs[ The patients adjusted the pairs of LEDs at an average separation of 7[0> "SD 0[9># on the left and at a separation of 6[3> "SD 9[8># on the right[
01
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F 9[59\ p 9[450^ hemispace] F 2[96\ p 9[099# and the interactions "patient group by hemispace] F 0[62\ p 9[100^ patient group by spatial location] F 9[57\ p 9[553^ patient group by hemispace by spatial location] F 9[70\ p 9[457# were not signi_cant[ Given a log! arithmic scaling of space "as proposed by model B^ Fig[ 0#\ a comparison between the four corresponding eccen! tricities of LED presentation in the left vs the right hemispace would yield the largest di}erences[ Post hoc comparisons "t!tests for paired samples# within the spatial location factor between these four corresponding eccen! tricities\ however\ revealed no signi_cant results[ 2[1[ Experiment 1
Fig[ 2[ Positions of ten red LEDs " _lled circles# judged as {{equidistant|| along a horizontally oriented semicircle situated in front of the patients with neglect[ The patients| body orientation is illustrated as seen from above^ the body is represented by an ellipse\ the head by a circle[ On the semicircle\ the two green reference LEDs "open circles# were presented with a spatial distance of 6[4> at the subject|s {{straight ahead|| position "S#[ The results of each individual patient with neglect "NEG0! 5# are presented\ as well as the group average[
A statistical analysis was carried out using a repeated measures analysis of variance with {{patient group|| as a {{between|| factor and with {{hemispace|| "left vs right# and {{spatial location|| "four eccentricities of LED presentation in each hemispace# as {{within|| factors[ The main e}ect of spatial location was found to be signi_cant "F 4[24\ p 9[992#[ The other e}ects "patient group]
Figure 3 presents the average distance estimates made by the three groups of subjects when they judged short\ medium and long distances in the left and right hemi! space[ The comparison between neglect patients and con! trols showed no consistent pattern of over! and:or underestimation of spatial distances in the patients with neglect that could be attributed to a uniform compression\ a uniform extension\ or a logarithmic trans! formation of subjective space "compare Figs[ 1 and 3#[ Rather\ there was a tendency in all subject groups to overestimate the distances in both left and right hemi! space[ This _nding is known as the horizontalÐvertical illusion ð6\ 06Ł which describes that subjects overestimate the apparent length of a vertical line relative to that of a horizontal line of equal physical extent[ For purposes of statistical analysis\ we calculated the di}erences between the subjectively judged distance and the respective reference distance for each subject[ Using these di}erence scores\ indicating the extent of each sub! ject|s over! or underestimation\ a repeated measures analysis of variance was performed using {{patient group|| as a between!subjects factor and {{hemispace|| "left vs right# and {{reference distance|| "short\ medium\ long# as within!subjects factors[ Only the main e}ect of hemispace was found to be signi_cant "F 5[33\ p 9[917#\ re~ecting the fact that in all subject groups the overestimation of distances on the left "mean ¦0[3>\ SD 0[2># was slightly larger than the overestimation on the right "mean ¦9[7>\ SD 0[2>#[ No other e}ects "patient group] F 1[72\ p 9[091^ reference distance] F 9[63\ p 9[377# or interactions "patient group by hemispace] F 9[00\ p 9[785^ patient group by reference distance] F 0[90\ p 9[313^ patient group by hemispace by reference dis! tance] F 9[48\ p 9[565# were signi_cant[ 3[ Discussion The present study aimed at comparing the hypotheses of a compressed\ expanded\ or anisometric space rep! resentation along the earth!horizontal axis\ each of which
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Fig[ 3[ Average distance judgements of the group of non!brain!damaged patients "NBD#\ of the right brain!damaged controls "RH# and of the patients with neglect[ The patients| judgements of three reference distances "short\ medium\ long# in the left and right hemispace are presented[ The open circles represent the physical separation of the green\ vertically presented reference LEDs[ The _lled circles illustrate the adjusted separations of pairs of red LEDs in the earth!horizontal plane[
has been proposed in neglect patients "see Figs[ 0Ð1#[ Two experiments investigated the di}erent predictions derived from these hypotheses on the patients| perception of spatial distances in the left and right hemispace[ In the _rst experiment\ patients with neglect had to adjust ten red LEDs equidistantly along a horizontally oriented semicircle[ A second experiment compared the patients| subjective perception of short\ medium and long ref! erence distances in the left and right hemispace[ In a group of seven consecutively admitted patients with neg! lect without visual _eld defects\ we found no consistent pattern along the horizontal axis that could be attributed
02
to a uniform compression of the visual array ð7Ł\ a uni! form expansion of space representation or an expansion of space representation on the contralesional and com! pression on the ipsilesional side ð1\ 02Ł[ The present _ndings seem to contrast with recent observations that were taken as evidence for a rep! resentational anisometry in the horizontal plane[ In a modi_ed line bisection task\ Bisiach et al[ ð1\ 2Ł observed that patients with left!sided neglect misplaced the two endpoints of a virtual line in a mental imagery task leftwards with respect to one another and with respect to a centrally printed midpoint[ Chokron et al[ ð3Ł used a line extension task[ Only one half of a prebisected line was presented and the subjects were required to construct the missing part of the line in such a way that the {{bisec! tion|| mark became the middle of the whole line[ In two patients with left!sided neglect\ the authors found an underconstruction of the missing right half and an over! construction of the missing left half[ Milner and Harvey ð03Ł investigated size perception of objects that were pre! sented on the left and right side of a computer screen[ In three patients with neglect "two of them were also hemianopic#\ they found that objects in the contralesional leftward parts of the visual array were perceived as hori! zontally shorter than those in the ipsilesional part[ The four studies report a similar observation[ They concur in that the contralesional object ð03Ł or the con! tralesional segment of a real or a virtual object ð2\ 3Ł had to be physically longer to be perceived as equal in length with the ipsilesional one by the patients with neglect[ Two interpretations have been put forward to explain these _ndings[ Bisiach et al[ ð1Ł speculated that the mis! perception of object size is evoked by a distorted rep! resentation of space\ namely by an expansion of subjective space on the contralesional side and a com! pression on the ipsilesional side[ An alternative expla! nation for the perceptual shrinkage of object size\ however\ was o}ered by Milner and colleagues ð03\ 04Ł[ Instead of a distortion of subjective space\ i[e[ of distances between objects in space\ they argued that there could be a failure in patients with neglect to construct adequate representations of the objects themselves in contralesional parts of egocentric space[ In our view\ this latter explanation is a more convincing one\ since the experiments of Milner and colleagues\ like those of Bisiach et al[ and Chokron et al[\ investigated processes of object perception\ rather than of distance perception between separate objects in space[ The present results indirectly support the interpretation of Milner and colleagues[ We found no evidence for a distortion of subjective space in patients with neglect[ The patients perceived horizontal distances in the left and right hemispace in just the same way as did the controls[ Since our study aimed at investigating the representation of space and not of single objects in space\ subjective dis! tance perception was determined explicitly\ without using
03
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a real object or a virtual\ mentally imagined object as in the previous experiments ð2\ 3\ 03Ł[ Rather\ we measured the subjective perception of distances between two LEDs in an otherwise completely darkened room[ Thereby\ a di}erent mental strategy was induced[ Whereas subjects in the previous studies were explicitly instructed either to compare the size of two objects or to focus on the spatial relations within a single object\ subjects in our study were instructed to concentrate on the spatial distance between two distinct objects "the two LEDs#[ Our instructions would thus probably have discouraged a strategy of for! ming a single Gestalt of the two spots of light presented in the dark[ It is possible that disturbed processes in object per! ception might occur independently of a disturbance of space representation in neglect or\ at least\ might have a di}erent signi_cance in space and in object perception[ Evidence has been reported that "a# encoding of spatial relations of objects with respect to the body and "b# encoding of locations of object parts relative to that object may have di}erent neural correlates ð0\ 05Ł[ In agreement with such _ndings\ it has been observed that the failure to respond to contralateral stimuli in neglect patients can operate in an object!centered ð5Ł as well as a body!centered frame of reference[ This might indicate that distinct cognitive processes of visual selection and perception can selectively be disturbed with brain damage[ Humphreys and Riddoch ð8Ł have accordingly argued for two forms of spatial representation[ One was thought to be concerned with the relation between parts of single objects\ the other with the relations between separate objects[ They described two neglect patients with bilateral lesions who showed neglect for one side of indi! vidual objects in the left hemi_eld when items in the display were coded as parts of a single object\ but for entire objects in the right hemi_eld when items were coded as separate perceptual objects[ This proposal is strengthened by the recent obser! vations of Bisiach et al[ ð1Ł using mentally imagined lines in a bisection task[ That study showed that the neglect patients| altered perception of these virtual objects could not be compensated by optokinetic stimulation[ The same type of visual stimulation\ however\ does in~uence the disturbed perception of body orientation in patients with neglect ð09Ł[ Taking both Bisiach et al[|s ð1Ł and Karnath|s ð09Ł _ndings together\ the data could indicate that pro! cesses involved in object perception are independent of those involved in identifying the location of an object with respect to the subject|s body[ The failure to orient towards and to explore the contralesional parts of space after right "predominantly parietal# brain damage might be distinct from those de_cits observed once an object of interest has been located and attention is focused on it[ An alternative explanation for the _nding that an object or a segment of an object had to be physically longer on the contralesional side in patients with neglect
to be perceived as equal in length with the one on the ipsilesional side ð2\ 3\ 03Ł should be discussed[ The term micropsia has been used to describe a disturbance in the appreciation of the apparent size of objects[ Objects appear shrunken relative to their true physical size[ This disturbance can a}ect only one hemi_eld and this {{hemimicropsia|| may occur without any clinical signs of neglect[ Only a few studies have yet reported precise lesion locations in these patients[ Cohen et al[ ð4Ł described a case with left!sided hemimicropsia who su}ered from a lesion in the right temporo!occipital region and a case with right!sided hemimicropsia with bilateral parieto!occipital lesions[ When drawing\ the _rst case spontaneously tended to compensate for his per! ceptual asymmetry by drawing the left half of objects slightly larger than the right half[ When asked to correct six truly symmetrical drawings of objects\ he either expanded the left part or reduced its right part[ The second case was tested with a visual matching task similar to that used by Milner and Harvey ð03Ł[ A pair of hori! zontally aligned circles was presented on a computer screen and the patient had to decide which circle was larger[ The patient displayed a perceptual bias such that the right circle had to be slightly larger than the left circle to be perceived as equal in size[ While Milner and Harvey ð03Ł described a disturbance of size perception in their patients which concerned speci_cally the horizontal dimension of objects\ Cohen et al[ ð4Ł did not test horizontal and vertical object dimen! sions separately[ Moreover\ in Cohen et al[|s cases it remained unclear which frame of reference was a}ected by the perceptual disturbance[ Although the authors clai! med that the de_cit was restricted to one half of the patients| visual _eld\ they did not report any empirical evidence supporting this notion[ The matching task con! ducted with the second case de_nitely did not test for di}erent reference frames\ while the authors| description of the _rst case even argues instead for a nonretinal reference frame as was the case in Milner and Harvey|s ð03Ł patients[ In a free vision condition\ i[e[ with frequent spontaneous changes of _xation\ this patient tended to draw objects "or one half of single objects# larger on the left than on the right side[ Since both phenomena\ i[e[ neglect of contralaterally located objects and altered size perception of objects "or of parts of objects# located in the contralateral hemispace\ can be observed together ð2\ 3\ 03Ł as well as occurring separately ð4Ł\ future studies are needed to see whether the two phenomena are associated with a disturbance of the same or of di}erent functional systems[ To conclude\ the present experiments revealed no evi! dence for a distorted representation of egocentric space along the earth!horizontal axis that could account for the neglect patients| failure to orient towards and to explore the contralesional parts of space[ The results were clear in showing no uniform compression of subjective space\
H[!O[ Karnath\ S[ Ferber:Neuropsycholo`ia 26 "0888# 6Ð04
nor a combined extension on the contralesional and com! pression on the ipsilesional side[ On both sides of ego! centric space\ the distance judgements of the patients with neglect lay within the normal range[ It should be emphasized that the present _ndings do not directly con! tradict those reported on object perception discussed above ð1Ð3\ 03\ 04Ł[ Instead\ they may re~ect a di}erent cognitive and perhaps even a di}erent neuronal process ð0\ 05Ł\ namely the encoding of space with respect to the body and the determination of relations between separate objects in egocentric space[
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Acknowledgements ð09Ł
This investigation was supported by grants from the Deutsche Forschungsgemeinschaft and the Bun! desministerium fur Bildung\ Wissenschaft\ Forschung und Technologie awarded to the _rst author[ We are grateful to David Milner and Edoardo Bisiach for their thoughtful comments on an earlier draft of this manu! script[ We are also indebted to David Milner for his help with the language[
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References ð04Ł ð0Ł Andersen RA\ Snyder LH\ Li C!S\ Stricanne B[ Coordinate trans! formations in the representation of spatial information[ Current Opinion in Neurobiology 0882^2]060Ð065[ ð1Ł Bisiach E\ Pizzamiglio L\ Nico D\ Antonucci G[ Beyond unilateral neglect[ Brain 0885^008]740Ð746[ ð2Ł Bisiach E\ Rusconi ML\ Peretti VA\ Vallar G[ Challenging current
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accounts of unilateral neglect[ Neuropsychologia 0883^21]0320Ð 0323[ Chokron S\ Bernard J!M\ Imbert M[ Length representation in normal and neglect subjects with opposite reading habits studied through a line extension task[ Cortex 0886^22]36Ð53[ Cohen L\ Gray F\ Meyrignac C\ Dehaene S\ Degos J!D[ Selective de_cit of visual size perception] two cases of hemimicropsia[ J[ Neurol[ Neurosurg[ Psychiatry 0883^46]62Ð67[ Driver J\ Baylis GC\ Goodrich SJ\ Rafal RD[ Axis!based neglect of visual shapes[ Neuropsychologia 0883^21]0242Ð0254[ Finger FW\ Spelt DK[ The illustration of the horizontal!vertical illusion[ Journal of Experimental Psychology 0836^26]132Ð149[ Halligan PW\ Marshall JC[ Spatial compression in visual neglect] a case study[ Cortex 0880^16]512Ð518[ Humphreys GW\ Riddoch MJ[ Attention to within!object and between!object spatial representations] multiple sites for visual selection[ Cognitive Neuropsychol[ 0883^00]196Ð130[ Karnath H!O[ Optokinetic stimulation in~uences the disturbed perception of body orientation in spatial neglect[ J[ Neurol[ Neuro! surg[ Psychiatry 0885^59]106Ð119[ Karnath H!O[ Spatial orientation and the representation of space with parietal lobe lesions[ Philosophical Transactions of the Royal Society "London# 0886^B241]0300Ð0308[ Mesulam\ M[!M[\ Attention\ confusional states and neglect[ In Principles of Behavioral Neurology\ ed[ M[!M[ Mesulam[ Davis\ Philadelphia\ 0874\ pp[ 014Ð57[ Milner\ A[ D[\ Animal models for the syndrome of spatial neglect[ In Neurophysiological and Neuropsychological Aspects of Spatial Neglect\ ed[ M[ Jeannerod[ Elsevier Science\ Amsterdam\ 0876\ pp[ 148Ð177[ Milner AD\ Harvey M[ Distortion of size perception in visuo! spatial neglect[ Current Biology 0884^4]74Ð78[ Milner AD\ Harvey M\ Roberts RC\ Forster SV[ Line bisection errors in visual neglect] misguided action or size distortion< Neuro! psychologia 0882^20]28Ð38[ Olson CR\ Gettner SN[ Object!centered direction selectivity in the macaque supplementary eye _eld[ Science 0884^158]874Ð877[ ber geometrisch!optische Tauschungen[ Jahres! Oppel\ J[ J[\ U bericht des Frankfurter Vereins\ 0743Ð0744\ 44\ 26Ð36[
Neuropsychologia 26 "0888# 6Ð04
Is space representation distorted in neglect< Hans!Otto Karnath \ Susanne Ferber Department of Neurology\ University of Tubingen\ Hoppe!Seyler Str[ 2\ 61965 Tubingen\ Germany Received 4 January 0887^ accepted 17 May 0887
Abstract It has been argued that neglect of contralateral stimuli following brain damage might be associated with either a compressed or an anisometric neural representation of space along the earth!horizontal axis[ Two di}erent models have been put forward[ One model proposes a uniform compression of subjective space\ while the other envisages an expansion on one side of space and a compression on the other[ We tested these models by determining neglect patients| perception of spatial distances in the horizontal plane[ The models di}er concerning the expected degree of under! vs overestimation of distances in the left and right hemispace[ In the _rst experiment\ patients were asked to position ten red LEDs equidistantly along a semicircle\ which was located horizontally in front of them at eye level[ A second experiment compared the patients| subjective perception of short\ medium and long distances extending into left and right hemispace[ We found no evidence for any compression or expansion\ nor for anisometry along the earth!horizontal axis[ These _ndings argue against a distortion of subjective space along the horizontal axis in patients with neglect which could account for their failure to orient towards and to explore the contralesional parts of space[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Keywords] Human^ Neglect^ Brain damage^ Distance perception^ Space representation^ Parietal lobe
0[ Introduction The characteristic disturbance of patients with neglect is a de_cient response to stimuli located contralateral to the lesion and a failure to explore the contralesional part of space[ In searching for the underlying mechanism leading to neglect\ altered neural representations of space have been discussed[ One model has speculated that the whole frame of exploratory behaviour is rotated around an earth!vertical body axis towards the ipsilesional side ð00Ł[ Alternative hypotheses have proposed either a com! pressed or an anisometric representation of space along the earth!horizontal axis in patients with neglect[ Halligan and Marshall ð7Ł argued for a linear\ uniform compression of subjective visual space along the hori! zontal axis[ They investigated a patient with left!sided neglect and hemianopia with a visuospatial localization task[ An arrow was presented either at the bottom or top of a monitor[ The arrow always appeared at a location opposite one of a row of numbers "0 to 04#[ The subject|s task was to estimate visually the corresponding spatial position on the row of numbers to which the arrow was pointing[ The patient|s responses revealed a systematic Corresponding author[ Fax] ¦38!6960!18!4159^ e!mail] karnathÝ uni!tuebingen[de
bias towards the right[ The magnitude of this bias tended to decrease from the left to the right side of the monitor[ The authors interpreted the _nding as evidence of a linear\ uniform compression of subjective visual space along the horizontal axis[ They likened the compressed perception of the surround to a compression of a spring that is _xed at the ipsilesional right hand side and {{pushed in|| from the contralesional left side ðFig[ 0"A0#Ł[ The model maintains Euclidean geometry\ i[e[ there is no relative distortion within subjective space[ Halligan and Marshall ð7Ł did not di}erentiate between the subject|s perception of the surround and the repre! sentation of space[ This distinction rather is essential for an alternative model that proposed a non!Euclidean\ anisometric representation of space along the horizontal axis in patients with neglect ðFig[ 0"B#Ł[ Nevertheless\ if one applies the same rationale to the model of Halligan and Marshall\ it could be speculated that the rep! resentation of space that would lead to a compressed perception of the surround\ would be a uniform expan! sion as illustrated in Fig[ 0"A1#[ Like Halligan and Marshall ð7Ł\ Milner ð02Ł speculated that patients with neglect might perceive their visual sur! roundings as being compressed[ However\ he suggested that this compression might progressively increase from the right to the left side ðsee Fig[ 0"B0#Ł[ Such a disturbed
9917!2821:87:,*see front matter Þ 0887 Elsevier Science Ltd[ All rights reserved[ PII] S 9 9 1 7 ! 2 8 2 1 " 8 7 # 9 9 9 6 9 ! 9
7
H[!O[ Karnath\ S[ Ferber:Neuropsycholo`ia 26 "0888# 6Ð04
Fig[ 0[ Two models of neglect patients| perception of the visual array "A0:B0# and of the assumed neural representation of space "A1:B1# that would lead to such a disturbed perception[ The subject|s body orientation is illustrated as seen from above^ the body is represented by an ellipse\ the head by a circle[ On the semicircle positioned in front of the subject\ markers are exposed {{equidistantly|| according to the di}erent hypotheses[ Model A suggests a uniform compression of the visual surround along the horizontal axis {{pushed in|| from the con! tralesional left side "A0^ Halligan and Marshall ð7Ł#[ The representation of space that would lead to such an altered perception would be a uniform expansion "A1#[ Model B proposed a progressively increasing leftward compression of the visual array "B0^ Milner ð02Ł#\ which would follow from an expansion of space representation on the contralesional and a compression on the ipsilesional side according to a logarithmic transformation "B1^ Bisiach et al[ ð1Ł#[
perception of the world would result from a distortion of space representation in which the left side was expanded and the right side compressed\ as illustrated in Fig[ 0"B1#[ Bisiach and co!workers ð1Ł have recently proposed a dis! tortion of just this kind in patients with neglect[ They assumed an expansion of space representation on the contralesional side and a compression on the ipsilesional side\ according to a logarithmic transformation[ Using a modi_ed version of the line bisection task\ they asked right brain!damaged patients with neglect to bisect the empty space between two dots corresponding to the endpoints of a previously shown line\ and then to set
both endpoints of an equally long "horizontal# virtual line on the basis of its midpoint that was printed at the center of a sheet of paper[ In executing the latter of these tasks\ the patients misplaced the endpoints of the virtual line leftwards with respect to one another and to the centrally printed midpoint[ Interestingly\ this resulted in the same disproportion usually yielded by neglect patients on the canonical version of the line bisection task\ i[e[ the longer segment lay on the contralesional side of the {{midpoint||[ The authors explained these _ndings by a progressive expansion of space representation on the con! tralesional and a compression on the ipsilesional side[ According to this model ðFig[ 0"B1#Ł\ subjectively equal distances should correspond to increasing real distances as one passes from the ipsilesional right to the con! tralesional left side of space[ The aim of the present study was to test between the hypotheses of a compressed\ expanded or anisometric space representation in neglect[ To distinguish between the di}erent models\ the patients| perception of subjective equidistance in the horizontal plane was determined in a _rst experiment[ Subjects had to adjust ten red LEDs equidistantly along a semicircle\ which was positioned horizontally in front of them at eye level[ Half of these LEDs had to be positioned on the subjects| left and the other half on their right side[ The results were then compared with the two models illustrated in Fig[ 0 to see which model would _t the observed LED positions along the semicircle best[ The models further di}er concerning the expected degree of under! vs overestimation when subjects judge spatial distances in the left and right hemispaces "see Fig[ 1#[ A second experiment therefore compared the patients| perception of three di}erent reference distances "short\ medium\ long# on the left and right of the subjects| mid! sagittal plane[ Two vertically presented green LEDs indi! cated each reference distance which then had to be matched to a pair of horizontally arranged red test LEDs[ The model of Halligan and Marshall ð7Ł predicts that patients should adjust these horizontal distances by a constant increased proportion irrespective of their exten! sion "short\ medium\ long# and irrespective of the hemispace where the distances have to be adjusted ðcf[ Figure 1"A#Ł[ Alternatively\ with an expansion on the contralesional and compression on the ipsilesional side\ as assumed by Bisiach et al[ ð1Ł\ one would expect an under!adjustment of spatial distances the longer the dis! tance extends toward the ipsilesional right and an over! adjustment the longer the distance extends toward the leftward part of egocentric space ðcf[ Figure 1"B#Ł[ 1[ Methods 1[0[ Patients Seven consecutively admitted patients with unilateral right hemispheric lesions and spatial neglect but without
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8
trol subjects gave informed consent to participate in the study[ 1[1[ Apparatus and procedure Both experiments were conducted in complete dark! ness[ Subjects sat upright in an armchair which provided an adjustable support for the back[ Head and body axes were aligned[ The head was _xed with a bandage to a head rest[ In addition\ one experimenter manually held the head steady[
Fig[ 1[ Pattern of expected under! and overestimation of spatial dis! tances in the horizontal plane according to the two models of distorted space representation in neglect[ The di}erent predictions concerning the patients| distance perception when matching three reference distances "short\ medium\ long# in the left and right hemispace are presented[ The open circles represent the physical distance of the vertically presented reference LEDs[ The _lled circles illustrate the expected distance judge! ments in the horizontal plane as they should occur left and right of the patients| midsagittal plane according to the two di}erent models of distorted space representation "models A and B^ Fig[ 0#[
visual _eld defects were investigated[ Five of these pat! ients participated in both experiments\ the other two pat! ients in only one of the experiments "cf[ Table 0#[ Nine patients with unilateral right hemispheric lesions but without neglect or visual _eld defects served as controls[ Seven of these patients participated in the _rst experiment and _ve of them in the second experiment "cf[ Table 0#[ Clinical and demographic characteristics of all patients are listed in Table 0[ As an additional control group\ 4 healthy subjects were investigated in the _rst experiment[ The median age of this group was 59 years "range 42Ð66#[ A second group of 4 di}erent healthy subjects served as a further control group in the second experiment[ The median age of this group was 53 years "range 46Ð54#[ The patients and con!
1[1[0[ Experiment 0 In order to distinguish between the above hypotheses suggesting altered space representation in the horizontal plane "see Fig[ 0#\ subjects had to position ten red LEDs equidistantly along a semicircle\ which was positioned horizontally in front of them at eye level[ To avoid exter! nal cues during the adjustment of the LED positions\ stimulation had to be reduced to a minimum[ Therefore\ during the whole experiment subjects saw only two of the ten red LEDs at a time[ These two red LEDs were horizontally arranged and presented at di}erent locations either on the subject|s left or right side[ Before each pres! entation\ the subjects _rst saw two green LEDs indicating the distance that had to be reproduced between the red LEDs[ During the whole experiment\ subjects thus saw nothing in the dark except either the two green reference LEDs or two of the red LEDs[ While the distance between the green reference LEDs was constant throughout the experiment\ the two red LEDs were presented with vary! ing distances that had to be adjusted[ Subjects sat at the center of a horizontally oriented semicircle "diameter 84 cm# which served as the guide rail for the two _xed green LEDs in the center and the ten freely movable red LEDs\ _ve at each side "cf[ Figure 2#[ The horizontal distance between the two green ref! erence LEDs was constant at 6[4>[ These two reference LEDs were presented symmetrically to the left and right of each subject|s individual {{straight ahead|| body position[ "The latter had been determined immediately before the present experiments using the technique described previously ð00Ł#[ The experiment began with the two green reference LEDs being illuminated for 09 s[ 1Ð4 s after these green LEDs were switched o}\ two red LEDs were switched on[ The distance between the red LEDs varied in di}erent trials[ The subjects| task was to adjust the distance between the two red LEDs to match the distance between the two green reference LEDs presented before[ The dis! tance was adjusted by the experimenter moving only the more eccentric red LED according to the verbal instruc! tions "closer:farther# given by the subjects[ No time limit was used[ The subjects indicated when the correct dis! tance was reached[ The two red LEDs were then switched o}\ their separation was measured and the next trial
09
Table 0 Demographic and clinical data for the right brain damaged patients with neglect "NEG# and without neglect "RH# Patient
¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
Experiment 1
¦ ¦ ¦ ¦ ¦ ¦
¦ ¦ ¦ ¦ ¦
Sex
M F M F M F M M M F M F M F F M
Age "years#
52 41 55 49 53 58 37 60 45 65 49 60 48 64 61 16
Etiology
infarct OP grade IV glioma infarct infarct infarct infarct hemorrhage OP grade IV glioma OP grade IV glioma OP grade IV glioma OP grade IV glioma infarct OP grade IV glioma infarct infarct OP grade IV glioma
Lesion location
temporo!parietal parietal parietal temporo!parietal temporo!parietal temporo!parietal parietal temporo!occipital temporal fronto!parietal temporo!parietal fronto!parietal temporal temporo!parietal parietal frontal
Days since _rst clinical symptoms
00 02 6 033 1 00 029 87 6 45 59 40 06 12 40 04
Neglect
yes yes yes yes yes yes yes no no no no no no no no no
Letter cancel! lation
Copying
Clock face
left
right
left
right
left
right
9 11 03 0 01 9 08 17 13 17 29 15 29 29 17 29
19 29 14 13 14 12 29 18 16 17 18 16 29 17 13 29
− "−# ¦ "−# − ¦ − ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
− "−# ¦ "−# ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
¦ ¦ "¦# ¦ ¦ ¦ − ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦
Experiment 0 or 1] participation in experiments 0 or 1[ OP] operation:surgery[ Letter cancellation ð01Ł] number of correct target letters "nmax 29# on each half of the test sheet[ Copying] the copying task featured four di}erent objects on a horizontally oriented DIN A 3 sheet[ Two were placed on the right\ two on the left side of the test sheet[ ¦\ complete copy of all objects^ "−# omission of elements located at the objects| left side^ −\ complete omission of at least one object located on one side of the test sheet[ Clock face] ¦\ all numerals positioned\ "¦# numerals in incorrect positions^ −\ omission of numerals[
H[!O[ Karnath\ S[ Ferber:Neuropsycholo`ia 26 "0888# 6Ð04
NEG0 NEG1 NEG2 NEG3 NEG4 NEG5 NEG6 RH0 RH1 RH2 RH3 RH4 RH5 RH6 RH7 RH8
Experiment 0
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started by illuminating the two green reference LEDs again[ Trials were arranged in blocks of four[ In each block\ the two red LEDs were presented to the left or to the right of the subject with the side alternating from block to block[ In the _rst block on each side\ one of the red LEDs had the same position as the green reference LED on that side[ The second red LED had a more eccentric position[ Its distance from the _rst red LED was pseudo! randomly varied in the four trials of a block\ ranging from 1> to 04>[ Following these four distance judgements\ the experimenter computed the average distance\ placed the more eccentric red LED at that distance from the other\ and fastened both red LEDs "now switched o}# to the guide rail[ The more eccentric of these newly!_xed LEDs served as the _xed red LED for the next block of trials on that side[ It was paired with a second\ even more eccentrically located red LED[ Again\ the position of the latter was pseudo!randomly varied over four trials such that the red LEDs were presented at four di}erent distances as described above[ This procedure was repeated until _ve red LEDs "not illuminated and thus not visible to the subjects except for the two LEDs currently being adjusted# were each positioned on the subject|s left and right side[ 1[1[1[ Experiment 1 The hypotheses suggesting altered space representation in neglect also di}er concerning the expected pattern of under! and overestimation when neglect patients judge horizontal distances in the left and right hemispace "see Fig[ 1#[ The second experiment therefore compared the patients| perception of three reference distances "short\ medium\ long#\ each on the left or right of the subjects| midsagittal plane[ Two vertically oriented green LEDs indicated these reference distances which subsequently had to be matched to a pair of horizontally oriented red test LEDs[ The subjects sat at the center of two semicircles "diameter 84 cm#[ As in experiment 0\ one semicircle was positioned horizontally in front of the subjects at their individual eye level[ A second semicircle was ver! tically oriented and corresponded with the subject|s physical midsagittal plane[ The vertical semicircle served for presenting the green reference LEDs\ while the hori! zontal semicircle was used as the guide rail for the two red test LEDs[ Each trial began with the two green\ vertically pre! sented reference LEDs being illuminated for a 09 s inter! val[ The reference LEDs were presented at one of three spatial distances "short] 6[4> 01[4 cm : medium] 03[6> 14 cm : long] 16[7> 49 cm#[ One of the LEDs was located half of the reference distance above eye level\ the other LED half of the distance below eye level[ When the green LEDs were switched o}\ the two hori!
00
zontally oriented red test LEDs were switched on after a random interval of 1Ð4 s in complete darkness[ One of these red LEDs always corresponded with the subject|s physical midsagittal plane at eye level[ The second LED was presented in an eccentric position either on the left or on the right side[ Varying from trial to trial\ the initial distance between these two red LEDs was either 29Ð 49) larger or 29Ð49) smaller than the actual reference distance between the two green LEDs[ The subject|s task was to exactly match the previously presented reference distance to the pair of red test LEDs[ The distance was adjusted by the experimenter moving the more eccentric red LED according to the verbal instructions "clo! ser:farther# given by the subject[ No time limit was used[ The subject indicated when the exact distance was reached[ The two red LEDs were then switched o}\ the adjusted distance between them was measured and the next trial started after a 09 s interval in complete darkness when two vertically oriented green reference LEDs were presented again[ Each of the three reference distances "short\ medium\ long# was presented eight times\ in a pseudo!randomized order[ The subsequent presentation of the horizontally arranged red test LEDs was on the subject|s left four times and on the right side four times[ The adjusted distances on these trials were averaged for each subject[
2[ Results 2[0[ Experiment 0 Distances judged as {{equidistant|| at the di}erent spa! tial locations along the horizontal semicircle were rela! tively constant in both control groups[ The _ve subjects without brain damage adjusted a distance of 6[5> "SD 9[1># averaged over the four spatial locations of LED presentation on the left and a distance of 6[4> "SD 9[2># averaged over the four spatial locations of LED presentation on the right[ Comparable results were observed for the eight right brain!damaged control patients without neglect[ They adjusted the pairs of LEDs with an average distance of 6[3> both on the left "SD 9[6># and on the right "SD 9[7>#[ Figure 2 illustrates the judgements of each individual patient with neglect as well as the average distances of the whole group along the horizontal semicircle[ The patients showed no regular pattern that could be attri! buted to a uniform compression or to a logarithmic trans! formation "compare Figs[ 0 and 2#[ On both sides of the body|s midsagittal plane\ the distance judgements of the patients with neglect lay within the range of the controls and close to the actual distance of the green reference LEDs[ The patients adjusted the pairs of LEDs at an average separation of 7[0> "SD 0[9># on the left and at a separation of 6[3> "SD 9[8># on the right[
01
H[!O[ Karnath\ S[ Ferber:Neuropsycholo`ia 26 "0888# 6Ð04
F 9[59\ p 9[450^ hemispace] F 2[96\ p 9[099# and the interactions "patient group by hemispace] F 0[62\ p 9[100^ patient group by spatial location] F 9[57\ p 9[553^ patient group by hemispace by spatial location] F 9[70\ p 9[457# were not signi_cant[ Given a log! arithmic scaling of space "as proposed by model B^ Fig[ 0#\ a comparison between the four corresponding eccen! tricities of LED presentation in the left vs the right hemispace would yield the largest di}erences[ Post hoc comparisons "t!tests for paired samples# within the spatial location factor between these four corresponding eccen! tricities\ however\ revealed no signi_cant results[ 2[1[ Experiment 1
Fig[ 2[ Positions of ten red LEDs " _lled circles# judged as {{equidistant|| along a horizontally oriented semicircle situated in front of the patients with neglect[ The patients| body orientation is illustrated as seen from above^ the body is represented by an ellipse\ the head by a circle[ On the semicircle\ the two green reference LEDs "open circles# were presented with a spatial distance of 6[4> at the subject|s {{straight ahead|| position "S#[ The results of each individual patient with neglect "NEG0! 5# are presented\ as well as the group average[
A statistical analysis was carried out using a repeated measures analysis of variance with {{patient group|| as a {{between|| factor and with {{hemispace|| "left vs right# and {{spatial location|| "four eccentricities of LED presentation in each hemispace# as {{within|| factors[ The main e}ect of spatial location was found to be signi_cant "F 4[24\ p 9[992#[ The other e}ects "patient group]
Figure 3 presents the average distance estimates made by the three groups of subjects when they judged short\ medium and long distances in the left and right hemi! space[ The comparison between neglect patients and con! trols showed no consistent pattern of over! and:or underestimation of spatial distances in the patients with neglect that could be attributed to a uniform compression\ a uniform extension\ or a logarithmic trans! formation of subjective space "compare Figs[ 1 and 3#[ Rather\ there was a tendency in all subject groups to overestimate the distances in both left and right hemi! space[ This _nding is known as the horizontalÐvertical illusion ð6\ 06Ł which describes that subjects overestimate the apparent length of a vertical line relative to that of a horizontal line of equal physical extent[ For purposes of statistical analysis\ we calculated the di}erences between the subjectively judged distance and the respective reference distance for each subject[ Using these di}erence scores\ indicating the extent of each sub! ject|s over! or underestimation\ a repeated measures analysis of variance was performed using {{patient group|| as a between!subjects factor and {{hemispace|| "left vs right# and {{reference distance|| "short\ medium\ long# as within!subjects factors[ Only the main e}ect of hemispace was found to be signi_cant "F 5[33\ p 9[917#\ re~ecting the fact that in all subject groups the overestimation of distances on the left "mean ¦0[3>\ SD 0[2># was slightly larger than the overestimation on the right "mean ¦9[7>\ SD 0[2>#[ No other e}ects "patient group] F 1[72\ p 9[091^ reference distance] F 9[63\ p 9[377# or interactions "patient group by hemispace] F 9[00\ p 9[785^ patient group by reference distance] F 0[90\ p 9[313^ patient group by hemispace by reference dis! tance] F 9[48\ p 9[565# were signi_cant[ 3[ Discussion The present study aimed at comparing the hypotheses of a compressed\ expanded\ or anisometric space rep! resentation along the earth!horizontal axis\ each of which
H[!O[ Karnath\ S[ Ferber:Neuropsycholo`ia 26 "0888# 6Ð04
Fig[ 3[ Average distance judgements of the group of non!brain!damaged patients "NBD#\ of the right brain!damaged controls "RH# and of the patients with neglect[ The patients| judgements of three reference distances "short\ medium\ long# in the left and right hemispace are presented[ The open circles represent the physical separation of the green\ vertically presented reference LEDs[ The _lled circles illustrate the adjusted separations of pairs of red LEDs in the earth!horizontal plane[
has been proposed in neglect patients "see Figs[ 0Ð1#[ Two experiments investigated the di}erent predictions derived from these hypotheses on the patients| perception of spatial distances in the left and right hemispace[ In the _rst experiment\ patients with neglect had to adjust ten red LEDs equidistantly along a horizontally oriented semicircle[ A second experiment compared the patients| subjective perception of short\ medium and long ref! erence distances in the left and right hemispace[ In a group of seven consecutively admitted patients with neg! lect without visual _eld defects\ we found no consistent pattern along the horizontal axis that could be attributed
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to a uniform compression of the visual array ð7Ł\ a uni! form expansion of space representation or an expansion of space representation on the contralesional and com! pression on the ipsilesional side ð1\ 02Ł[ The present _ndings seem to contrast with recent observations that were taken as evidence for a rep! resentational anisometry in the horizontal plane[ In a modi_ed line bisection task\ Bisiach et al[ ð1\ 2Ł observed that patients with left!sided neglect misplaced the two endpoints of a virtual line in a mental imagery task leftwards with respect to one another and with respect to a centrally printed midpoint[ Chokron et al[ ð3Ł used a line extension task[ Only one half of a prebisected line was presented and the subjects were required to construct the missing part of the line in such a way that the {{bisec! tion|| mark became the middle of the whole line[ In two patients with left!sided neglect\ the authors found an underconstruction of the missing right half and an over! construction of the missing left half[ Milner and Harvey ð03Ł investigated size perception of objects that were pre! sented on the left and right side of a computer screen[ In three patients with neglect "two of them were also hemianopic#\ they found that objects in the contralesional leftward parts of the visual array were perceived as hori! zontally shorter than those in the ipsilesional part[ The four studies report a similar observation[ They concur in that the contralesional object ð03Ł or the con! tralesional segment of a real or a virtual object ð2\ 3Ł had to be physically longer to be perceived as equal in length with the ipsilesional one by the patients with neglect[ Two interpretations have been put forward to explain these _ndings[ Bisiach et al[ ð1Ł speculated that the mis! perception of object size is evoked by a distorted rep! resentation of space\ namely by an expansion of subjective space on the contralesional side and a com! pression on the ipsilesional side[ An alternative expla! nation for the perceptual shrinkage of object size\ however\ was o}ered by Milner and colleagues ð03\ 04Ł[ Instead of a distortion of subjective space\ i[e[ of distances between objects in space\ they argued that there could be a failure in patients with neglect to construct adequate representations of the objects themselves in contralesional parts of egocentric space[ In our view\ this latter explanation is a more convincing one\ since the experiments of Milner and colleagues\ like those of Bisiach et al[ and Chokron et al[\ investigated processes of object perception\ rather than of distance perception between separate objects in space[ The present results indirectly support the interpretation of Milner and colleagues[ We found no evidence for a distortion of subjective space in patients with neglect[ The patients perceived horizontal distances in the left and right hemispace in just the same way as did the controls[ Since our study aimed at investigating the representation of space and not of single objects in space\ subjective dis! tance perception was determined explicitly\ without using
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H[!O[ Karnath\ S[ Ferber:Neuropsycholo`ia 26 "0888# 6Ð04
a real object or a virtual\ mentally imagined object as in the previous experiments ð2\ 3\ 03Ł[ Rather\ we measured the subjective perception of distances between two LEDs in an otherwise completely darkened room[ Thereby\ a di}erent mental strategy was induced[ Whereas subjects in the previous studies were explicitly instructed either to compare the size of two objects or to focus on the spatial relations within a single object\ subjects in our study were instructed to concentrate on the spatial distance between two distinct objects "the two LEDs#[ Our instructions would thus probably have discouraged a strategy of for! ming a single Gestalt of the two spots of light presented in the dark[ It is possible that disturbed processes in object per! ception might occur independently of a disturbance of space representation in neglect or\ at least\ might have a di}erent signi_cance in space and in object perception[ Evidence has been reported that "a# encoding of spatial relations of objects with respect to the body and "b# encoding of locations of object parts relative to that object may have di}erent neural correlates ð0\ 05Ł[ In agreement with such _ndings\ it has been observed that the failure to respond to contralateral stimuli in neglect patients can operate in an object!centered ð5Ł as well as a body!centered frame of reference[ This might indicate that distinct cognitive processes of visual selection and perception can selectively be disturbed with brain damage[ Humphreys and Riddoch ð8Ł have accordingly argued for two forms of spatial representation[ One was thought to be concerned with the relation between parts of single objects\ the other with the relations between separate objects[ They described two neglect patients with bilateral lesions who showed neglect for one side of indi! vidual objects in the left hemi_eld when items in the display were coded as parts of a single object\ but for entire objects in the right hemi_eld when items were coded as separate perceptual objects[ This proposal is strengthened by the recent obser! vations of Bisiach et al[ ð1Ł using mentally imagined lines in a bisection task[ That study showed that the neglect patients| altered perception of these virtual objects could not be compensated by optokinetic stimulation[ The same type of visual stimulation\ however\ does in~uence the disturbed perception of body orientation in patients with neglect ð09Ł[ Taking both Bisiach et al[|s ð1Ł and Karnath|s ð09Ł _ndings together\ the data could indicate that pro! cesses involved in object perception are independent of those involved in identifying the location of an object with respect to the subject|s body[ The failure to orient towards and to explore the contralesional parts of space after right "predominantly parietal# brain damage might be distinct from those de_cits observed once an object of interest has been located and attention is focused on it[ An alternative explanation for the _nding that an object or a segment of an object had to be physically longer on the contralesional side in patients with neglect
to be perceived as equal in length with the one on the ipsilesional side ð2\ 3\ 03Ł should be discussed[ The term micropsia has been used to describe a disturbance in the appreciation of the apparent size of objects[ Objects appear shrunken relative to their true physical size[ This disturbance can a}ect only one hemi_eld and this {{hemimicropsia|| may occur without any clinical signs of neglect[ Only a few studies have yet reported precise lesion locations in these patients[ Cohen et al[ ð4Ł described a case with left!sided hemimicropsia who su}ered from a lesion in the right temporo!occipital region and a case with right!sided hemimicropsia with bilateral parieto!occipital lesions[ When drawing\ the _rst case spontaneously tended to compensate for his per! ceptual asymmetry by drawing the left half of objects slightly larger than the right half[ When asked to correct six truly symmetrical drawings of objects\ he either expanded the left part or reduced its right part[ The second case was tested with a visual matching task similar to that used by Milner and Harvey ð03Ł[ A pair of hori! zontally aligned circles was presented on a computer screen and the patient had to decide which circle was larger[ The patient displayed a perceptual bias such that the right circle had to be slightly larger than the left circle to be perceived as equal in size[ While Milner and Harvey ð03Ł described a disturbance of size perception in their patients which concerned speci_cally the horizontal dimension of objects\ Cohen et al[ ð4Ł did not test horizontal and vertical object dimen! sions separately[ Moreover\ in Cohen et al[|s cases it remained unclear which frame of reference was a}ected by the perceptual disturbance[ Although the authors clai! med that the de_cit was restricted to one half of the patients| visual _eld\ they did not report any empirical evidence supporting this notion[ The matching task con! ducted with the second case de_nitely did not test for di}erent reference frames\ while the authors| description of the _rst case even argues instead for a nonretinal reference frame as was the case in Milner and Harvey|s ð03Ł patients[ In a free vision condition\ i[e[ with frequent spontaneous changes of _xation\ this patient tended to draw objects "or one half of single objects# larger on the left than on the right side[ Since both phenomena\ i[e[ neglect of contralaterally located objects and altered size perception of objects "or of parts of objects# located in the contralateral hemispace\ can be observed together ð2\ 3\ 03Ł as well as occurring separately ð4Ł\ future studies are needed to see whether the two phenomena are associated with a disturbance of the same or of di}erent functional systems[ To conclude\ the present experiments revealed no evi! dence for a distorted representation of egocentric space along the earth!horizontal axis that could account for the neglect patients| failure to orient towards and to explore the contralesional parts of space[ The results were clear in showing no uniform compression of subjective space\
H[!O[ Karnath\ S[ Ferber:Neuropsycholo`ia 26 "0888# 6Ð04
nor a combined extension on the contralesional and com! pression on the ipsilesional side[ On both sides of ego! centric space\ the distance judgements of the patients with neglect lay within the normal range[ It should be emphasized that the present _ndings do not directly con! tradict those reported on object perception discussed above ð1Ð3\ 03\ 04Ł[ Instead\ they may re~ect a di}erent cognitive and perhaps even a di}erent neuronal process ð0\ 05Ł\ namely the encoding of space with respect to the body and the determination of relations between separate objects in egocentric space[
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Acknowledgements ð09Ł
This investigation was supported by grants from the Deutsche Forschungsgemeinschaft and the Bun! desministerium fur Bildung\ Wissenschaft\ Forschung und Technologie awarded to the _rst author[ We are grateful to David Milner and Edoardo Bisiach for their thoughtful comments on an earlier draft of this manu! script[ We are also indebted to David Milner for his help with the language[
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