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Hand-centred coding of target location in visuo-spatial working memory
\ PERGAMON
Neuropsychologia 26 "0888# 384Ð491
Hand!centred coding of target location in visuo!spatial working memory S[ Chie.a \ D[A[ Allportb\ M[ Woodinb a
Dipartimento di Fisiolo`ia Umana e Funzioni Biolo`iche Inte`rate {Filippo Bottazzi|\ Seconda Universita di Napoli\ Via Costantinopoli 05\ 79027\ Napoli\ Italy b Department of Experimental Psycholo`y\ University of Oxford\ UK Received 00 December 0886^ accepted 1 July 0887
Abstract The aim of the present experiment was to determine the frame of reference used to encode a visually speci_ed target location\ in a simple\ visuo!spatial short!term!memory task\ using delayed manual pointing[ Subjects were required to remember the location of a single\ visually presented stimulus\ over delays of 2 and 29 s\ and to recall its location by pointing to it with the tip of a stylus with eyes closed[ There were two starting positions for the hand] one near the body\ proximal to the target area\ the other far from the body\ i[e[\ beyond the target area[ Two groups of twelve subjects participated in the experiment[ One group observed the target locations in the light\ the other group observed the target LEDs in the dark[ The results showed a large "between 09Ð29)# and consistent undershoot of the remembered target location with respect to the hand|s starting position\ in the sagittal axis\ both when the hand started from the near and from the far position[ That is\ from the far starting position\ pointing errors lay consistently beyond the actual target location as seen from the subject|s viewpoint[ Further\ this undershoot error in the remembered target location increased over time delay[ These results are in favour of the hypothesis that the visual target locations were encoded in a hand!centred frame of reference in spatial short!term memory[ They also implicate the use of motor! or action!related representations in visuo!spatial working memory[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
0[ Introduction Behavioural and neurophysiological evidence suggests that primate and human spatial location coding is based on a variety of di}erent reference frames ð1\ 29\ 22\ 27Ł[ Two fundamentally di}erent types of reference system are usually considered\ egocentric and allocentric[ The _rst type codes locations with reference to the position of the body\ or of speci_c parts of the body\ the second codes locations with reference to the surrounding environment[ Woodin and Allport ð37Ł have demonstrated a clearcut behavioural dissociation between memory for a target location _xed with respect to the stationary environment "{allocentric| location# vs memory for a target location _xed with respect to the subject|s body "{egocentric| location#\ where the environmental reference frame was provided only by vestibular and proprioceptive cues[ There is also behavioural evidence for egocentric ref!
Corresponding author[ Fax] 99 28 70 4554719^ E!mail] schie. Ýunina[it
erence frames centred on di}erent body parts\ depending on the subject|s task[ Soechting and Flanders ð31\ 32Ł asked subjects to point to the remembered location of a target presented visually[ They found that pointing errors were more consistently centred on the shoulder than on the eyes or head[ However\ in a subsequent study\ Soe! chting et al[ ð33Ł showed that subjects may use a head! centred reference frame\ when asked to point to locations relative to the head\ and a shoulder!centred reference frame\ when asked to point to locations relative to the shoulder[ Flanders et al[ ð00Ł proposed a model in which the spatial representation of a target location for manual pointing evolves from a retinocentric to a head!centred and a shoulder!centred representation[ Behavioural evi! dence for a hand!centred representation was found by Tipper et al[ ð20\ 34Ł when subjects pointed at targets in a 2!D environment[ The authors asked their subjects to reach for a target stimulus while avoiding an irrelevant distractor[ When the distractor was presented in the space near the hand\ it produced interference e}ects "and {nega! tive priming|# dependent on its relation to the starting position of the hand\ irrespective of its visual "i[e[\ view! point!dependent# position[ Evidence for a hand!centred co!ordinate system in the planning of simple\ 1!D reach!
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ing movements has been reported also by Gordon et al[ ð19Ł "ðsee also 07Ł#[ Egocentric representation of space is obtained by inte! grating\ continuously\ visual and proprioceptive infor! mation[ Neurophysiological studies have shown a series of co!ordinate transformations\ from a retinotopic to a head!centred and body!centred spatial representation[ Early visual processing in the primate visual system appears to be coded in retinotopic co!ordinates ð8Ł[ How! ever\ a retinotopic representation of visual space is not appropriate for guiding bodily actions in space\ since the position of the eyes and head change continuously[ More stable spatial representations\ adapted to the needs of action planning and memory\ are progressively elab! orated in the posterior parietal cortex\ prefrontal cortex and elsewhere[ A head!centred co!ordinate rep! resentation may be obtained by combining both the reti! nal location of a visual stimulus and information signalling the position of the eye ð0Ł\ while a body!centred representation may be reached by combining the retinal and eye position signals with head position information ð5Ł[ Coding systems apparently centred on the face\ trunk\ arm\ and other body parts have been reported in inferior premotor cortex\ or area F3 ð01Ð03\ 10Ð12Ł[ Area F3 receives a major input from the inferior parietal lobule ð18\ 23Ł[ Graziano et al[ ð10\ 12Ł found neurons in ventral premotor cortex and in putamen that responded to visual stimuli in the space adjacent to the hand or arm[ Fur! thermore\ the visual receptive _elds of these neurons moved when the arm moved[ The authors ð10\ 12Ł sug! gested that these cells code space in arm!centred co!ordi! nates[ They reported other cell populations responsive to visual stimuli adjacent to the face or head\ with receptive _elds "RFs# that moved\ likewise\ with the head[ More recently\ Graziano and colleagues ð11Ł showed that some of these neurons continued a sustained response even after the lights were turned o} and the stimulus was no longer visible\ i[e[\ these neurons showed {object per! manence|[ For some cells\ when the relevant bodypart was moved passively\ in the dark\ such as to carry the previously seen but now invisible stimulus out of the cell|s RF\ the cell abruptly ceased responding^ comple! mentarily\ when the bodily movement brought the pre! viously seen stimulus into the cell|s RF\ still in the dark\ the cell started to respond vigorously[ These neurons thus plausibly play a role in action!related spatial working memory ð28Ł[ The majority of the studies referred to above "with the exception of ð11 and 37Ł# have investigated spatial representation in immediate or ongoing sensory!motor processing[ Behavioural studies of spatial representation in human short!term memory\ on the other hand\ have developed a separate research tradition\ in which the idea of multiple reference frames has received relatively little attention[ In the original formulation of their {working memory| "WM# model\ Baddeley and Hitch ð2Ł postulated
the existence of a unitary\ limited!capacity {visuo!spatial scratch!pad| "VSSP# for the temporary representation and manipulation of all visuo!spatial information^ more recent versions restate this postulate ð3\ 4Ł[ Other studies\ using selective interference paradigms\ suggested the need to fractionate the VSSP into two components\ one {vis! ual| \ the other {spatial| ð16\ 17\ 35Ł[ Within {spatial| short! term memory\ a further distinction\ also based on selec! tive interference e}ects\ has been made between memory for the subject|s own\ egocentric body postures or move! ments\ vs exocentric positional coding of objects ð39\ 30Ł[ Moreover\ with respect to memory for exocentric object location\ a further behavioural dissociation has been demonstrated between a positional map {per se|\ and the assignment of particular objects!to!locations ð24Ł[ The present study attempts to link together these two separate research traditions[ Its aim is to identify the spatial reference frame"s# used in human short!term memory in a simple\ egocentric positional memory task[ As such\ it forms part of a series of ongoing studies of spatial representation in human visuo!spatial working memory ð7\ 36\ 37Ł[ Spatial representation involves a multiplicity of di}er! ent coding systems\ as the foregoing review makes clear[ To isolate a particular spatial coding system exper! imentally\ it is essential to constrain the subject|s task in such a way as to reduce "if possible\ to eliminate# the use of all alternative coding strategies[ For example\ if memory for egocentric positional coding is to be inves! tigated "as here#\ it is necessary to exclude\ so far as possible\ all allocentric reference points in the memory recall conditions0[ In the present study\ this was done by requesting the subjects to point to a remembered location with their eyes closed\ that is\ without allocentric visual guidance[ Further\ in one of two conditions\ the target location was presented in darkness\ as a luminous spot\ thus excluding also any other\ external visual reference points at input\ as well as at recall[ Subjects were required to remember the location of a single\ visually presented stimulus\ over delays of 2 and 29 s\ and then to point to the remembered location[ In previous studies of pointing to a remembered location\ the standard hand starting position was directly in front of the subject\ that is\ between the subject|s body and target location[ As a result\ it is not possible to tell whether the spatial reference frame that is used to code the target location is centred at the body "eye\ head\ trunk\ shoulder# or at the hand[ One way to disentangle the possible spatial reference frames is simply to change the starting position of the hand\ without changing the subject|s viewpoint ð20\ 34Ł[ For this purpose we used two di}erent starting positions for the hand] one on the body 0
Gravitational cues provide a major allocentric spatial reference[ In the present study\ position coding orthogonal to the gravitational axis was studied[
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midline near the subject|s body\ the other far from the body\ practically at full arm extension\ beyond the target locations[ Stimulus presentation was made either in the dark\ providing purely egocentric visual information\ or in the light\ in which both egocentric and allocentric information was available[ Pointing to the remembered location\ however\ was done with eyes closed in both cases[ The rationale of the experiment is as follows[ As is well known\ pointing to a visually presented target\ without visual guidance during the act of pointing\ typically results in spatial undershoot errors that increase sys! tematically with target distance ð7\ 25\ 31Ł[ On the other hand\ when the target location is speci_ed proprio! ceptively\ for example by active or passive movement of the hand\ undershoot errors are not observed or are gre! atly reduced ð31Ł[ This _nding has led to the suggestion that the undershoot may result from systematic error in the sensorimotor transformation from a visual rep! resentation of the target location to a representation in terms of intrinsic co!ordinates of the hand and arm at their intended target position\ with respect to the shoulder ð31\ 32Ł "see also ð04Ł#[ However\ it remains open to ques! tion whether the critical transformation may be into a co!ordinate system centred on the body "e[g[\ trunk or shoulder# or on the hand[ The present experiment pro! vides a critical test between these alternatives[ The predictions are as follows[ If the locations to be pointed to are encoded in an egocentric system centred on the body "head\ trunk\ shoulder#\ pointing errors should undershoot with reference to the body\ regardless of whether the hand starting position was near or far[ On the other hand\ if the target location is encoded with respect to the hand\ pointing errors should tend to under! shoot with reference to the hand starting position^ i[e[\ from the far starting position\ pointing errors should tend to lie beyond the true target location\ with reference to the body[ There is a second question to which this experiment is addressed[ This concerns the available coding domains in visuo!spatial short!term memory "s[t[m[#[ If spatial s[t[m[ "over more than 0 s or so# is predominantly or even exclusively visual or perceptual in nature\ as some current approaches might suggest ð4\17Ł "see also ð08\ 21Ł#\ then we should expect the target location to be retained "e[g[\ over a 29 s interval# in a visual egocentric reference frame[ The sensori!motor transformation into intrinsic hand and arm "motor or joint!angle# co!ordinates should then be executed only at the time of the required act of poin! ting[ If this were so\ the size of any systematic undershoot errors\ resulting from inaccuracies in this transformation\ should be independent of time delay[ An alternative possibility\ however\ is that the target location is recoded immediately into intrinsic hand:arm co!ordinates\ and retained\ over the imposed time delay\ in the form of a potential action\ a motor schema ð13\ 28Ł\ directed toward
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a particular spatial location[ In this case\ any observed decline in pointing accuracy as a function of time delay should likewise re~ect the intrinsic parameters of the encoded movement*e[g[\ in terms of its intended extent and direction[ Moreover\ if these movement parameters are encoded independently ð6\ 09\ 05\ 06\ 19\ 14\ 15Ł\ we may expect them also to show independent loss of accuracy over s[t[m[ delay[ Neurophysiological evidence suggests that\ in action planning\ movement extent may be encoded in terms of the amplitude of set!related pre! motor activity ð15\ 26Ł[ If so\ we might expect such a representation to be subject to time!dependent decay of activation[ In this case\ undershoot errors should become systematically larger with increasing s[t[m[ delay[ The experiment provides a test of this somewhat speculative prediction also[
1[ Method 1[0[ Subjects Two groups of twelve subjects participated in the experiment[ The _rst group performed the experiment in the light[ Subjects in this group were seven women and _ve men^ mean age 12[7^ S[D[2[1^ ten subjects were right! handed and two left!handed[ The second group per! formed the experiment in the dark[ There were eight women and four men^ mean age 13[7^ S[D[ 3[4^ all the subjects were right!handed[ The subjects were naive to the task[
1[1[ Apparatus Subjects sat in a comfortable chair in front of a table on which a digitizing tablet was placed[ The tablet mea! sured 329 mm "width#×469 mm "depth# and had an active surface of 294×347 mm[ It was contacted with a non!inking electronic stylus[ When in contact with the active surface\ the position of the stylus tip was sampled at a rate of 49 Hz[ Data were recorded in X "horizontal# and Y "vertical# co!ordinates with a measuring accuracy of 9[14 mm[ The tablet surface was covered with a thin white card\ on which the starting position was drawn in black ink "a 2 mm diameter spot# either at 19 cm from the trunk "near hand position# or at 53 cm "far hand position#\ along the midsagittal axis[ Testing was conducted either in the light or in the dark[ In the _rst case\ the room was uniformly lit^ in the second case\ the room was darkened\ and the subjects wore dark glasses so as to eliminate any possible extraneous visual cues[
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When testing was carried out in the light\ the target stimulus consisted of a ~at\ black disc\ 4 mm in diameter^ when in the dark\ the stimulus was a red luminous spot "LED#\ 4 mm in diameter[ There were three target distances] 27 cm from the trunk "07 cm from the near hand position^ 15 cm from the far hand position#\ 31 cm from the trunk "11 cm from both hand starting positions#\ and 35 cm from the trunk "15 from the near hand position and 07 cm from the far hand position#[ Target locations were along the midsagittal axis[
1[2[ Procedure At the beginning of each trial\ the subject held the stylus with her or his preferred hand and closed her eyes[ The experimenter placed the stimulus on the tablet sur! face and the subject|s hand on the starting position[ The subject opened her eyes\ looked at the target stimulus for 1 s\ and then closed her eyes[ When the retention interval had elapsed\ the experimenter verbally gave the order to go[ The subject was instructed to move the tip of the stylus on the tablet at a natural speed and to stop with the stylus in the remembered target position\ still with eyes closed[ When the hand starting distance was near\ the subject thus performed an extension movement\ when far a ~exion movement[ At the end of the subject|s move! ment\ the experimenter brought the subject|s hand back to the starting position[ The experimenter ensured that subjects never saw the end point of their movement[ The time delays were 2 and 29 s[ The combination of hand starting position "near vs far# and time delay "2 vs 29 s# resulted in four experimental conditions[ A total of 37 trials was given per subject "two hand starting positions×two time delays×three target distances×four presentations#[ There was one block of 01 trials for each experimental condition[ The order of the four blocks was counterbalanced across the subjects[ Target distances were randomized[ Pointing errors were de_ned as the distance of the end point of stylus movement from the center of the target[ Errors were decomposed into their sagittal "i[e[\ straight ahead# and transverse components[ The dependent vari! ables employed were] absolute "unsigned# and constant "signed# sagittal and transverse errors[ For constant sag! ittal errors\ the sign was de_ned with reference to the hand starting position[ Sagittal errors farther than the target location "i[e[\ overshoot errors\ with reference to the starting position# were assigned "¦# values\ whereas errors nearer than the target location "undershoot errors# were assigned "−# values[ For constant transverse errors\ pointing errors to the right of the target location with respect to the subject|s viewpoint were given "¦# values\ whereas errors to the left of the target location were given "−# values[ Data for each experimental condition were
pooled across target distances[ For each condition\ the means of each variable were calculated for each subject[ 2[ Results The mean values and the standard deviations of each variable are shown in Table 0[ Mixed!design 1×1×1 ANOVAs were conducted on each dependent variable\ with Lighting "light vs dark# as a between!subjects factor\ and Delay "2!s vs 29!s# and Starting position of the hand "near vs far# as the within! subjects factors[ Paired comparisons were performed using NewmanÐKeuls procedure[ 2[0[ Sagittal errors The principal data concern the constant "i[e[\ signed# sagittal errors[ As Table 0 shows\ these took the form\ consistently\ of undershoot errors "i[e[\ negative values of constant sagittal error#\ in all conditions[ Note that in each case the undershoot is coded with respect to the hand starting position[ That is\ as seen from the subject|s viewpoint\ from the near starting position undershoot errors fell on the near side of the target locations^ from the far starting position\ on the contrary\ the subject|s pointing errors were consistently on the far side of the target locations[ Overall\ 71[2) of responses were sag! ittal undershoot errors[ Constant sagittal error was sig! ni_cantly greater than zero in each of the eight conditions in Table 0\ by one!sample\ two!tailed t!tests "1f 00#[ "See Table 0[# ANOVA showed that the size of the constant sagittal error "undershoot error# was signi_cantly a}ected by Delay "F"0\11# 00[20\ P ³ 9[994# "2!s −20[3 mm^ 29!s −31[8 mm#[ No other main e}ects were signi_! cant[ However\ the interaction between Lighting and Starting position approached signi_cance "F"0\11# 2[74\ P 9[95#[ Post!hoc comparisons showed that the undershoot error was largest "P ³ 9[94# when the target locations were presented in the dark\ with the hand in the near starting position "dark\ near −43[1 mm^ dark\ far −14[1 mm^ light\ near −23[7 mm^ light\ far −23[2 mm#[ Absolute "i[e[\ unsigned# sagittal error was a}ected by both Delay and Starting position "Delay "F"0\11# 09[86\ P ³ 9[994^ 2!s 39[4 mm^ 29!s 37[4 mm^ Starting position] F"0\11# 4[85\ P ³ 9[94] near 49[3 mm^ far 27[5 mm#[ There was a signi_cant interaction between Lighting and Starting position "F"0\11# 09[58\ P ³ 9[994#[ Post!hoc comparisons showed that absolute sagittal error was greater with tar! get locations presented in the dark\ when the hand started at the near position\ than in the other conditions "P ³ 9[90# "dark\ near 52[1 mm^ dark\ far 24[7 mm^ light\ near 26[4 mm^ light\ far 30[4 mm#[
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S[ Chief_ et al[:Neuropsycholo`ia 26 "0888# 384Ð491 Table 0 Mean values of sagittal and transverse errors in pointing with eyes closed to a remembered target location "standard deviations in brackets# Lighting "during stimulus presentation#
Light
Starting position of the hand
Near
Delay Constant "i[e[\ signed# sagittal error "mm# Absolute "i[e[\ non!signed# sagittal error "mm# Constant "i[e[\ signed# transverse error "mm# Absolute "i[e[\ non!signed# transverse error "mm#
Dark Far
Near
Far
2s
29 s
2s
29 s
2s
29 s
2s
29 s
−15[5 "10[8# 20[9 "05[4# −0[7 "00[7# 09[6 "5[1#
−32[0 "11[6# 32[8 "10[5# 2[1 "04[2# 02[4 "7[0#
−29[9 "18[4# 28[9 "05[1# 1[5 "8[2# 7[8 "3[8#
−27[4 "15[0# 32[8 "07[7# −0[9 "00[1# 09[8 "4[2#
−35[3 "28[7# 46[3 "08[7# 9[7 "03[1# 01[3 "6[1#
−50[8 "30[7# 58[9 "17[7# 0[5 "06[1# 04[4 "6[3#
−11[4 "23[8# 23[5 "11[3# 3[8 "00[2# 00[9 "5[8#
−17[9 "22[1# 26[9 "13[3# 2[3 "09[9# 09[8 "5[2#
For constant sagittal errors\ positive values were assigned to errors farther than the target "i[e[\ overshoot errors# and negative values to errors nearer than the target location "i[e[\ undershoot errors#\ with reference to hand starting position[ For constant transverse errors\ positive values were assigned to errors to the right of and negative values to errors to the left of target location[ Signi_cance levels of departure from zero are shown for constant sagittal and transverse error "two!tailed t!test\ 1f 00^ P ³ 9[94^ P ³ 9[90#[
2[1[ Transverse errors No signi_cant main e}ects were found on constant transverse error[ The values obtained did not di}er sig! ni_cantly from zero in any condition\ by one!sample\ two!tailed t!tests "1f 00#[ Absolute transverse error was signi_cantly a}ected by Delay "F"0\11# 4[56\ P ³ 9[94^ 2!s 09[6 mm^ 29! s 01[6 mm#[ No other e}ects were signi_cant[
body!centred or hand!centred*is the target end! point of the movement represented< "1# During the retention interval\ is the memory rep! resentation of the target location encoded in a visual "i[e[\ extrinsic# reference frame\ to be transformed into intrinsic "arm! and hand!movement# co!ordi! nates only at the time of pointing< Or is the target location recoded immediately into intrinsic "motoric# co!ordinates\ and represented in this form in short! term memory over at least a 2Ð29 s delay<
2[2[ Correlation of absolute sagittal and transverse error The correlation coe.cients "Sperman|s rho# between absolute sagittal and transverse errors were calculated individually for each subject and for each condition[ Sig! ni_cant correlations "P ³ 9[94#\ either positive or nega! tive\ were found in only ten cases out of 85 "_ve positive and _ve negative#[ At each delay the mean correlation coe.cients between sagittal and transverse error approxi! mated to zero "At 2!s delay] dark\ near r 9[98^ dark\ far r −9[90^ light\ near r 9[09^ light\ far r −9[91^ at 29!s delay] dark\ near r −9[97\ dark\ far r 9[98^ light\ near r 9[09^ light\ far r −9[90#[
3[ Discussion This experiment was directed to two questions] "0# How is the motor act of pointing with eyes closed to a remembered\ visually presented target location spatially represented< More speci_cally\ in terms of what spatial reference frame*viewpoint!speci_c\
Question "0# will be considered _rst[ The present experiment provides two kinds of behavioural evidence for a hand!centred encoding of the movement] "a# There was a consistent undershoot of the target distance\ as measured from the starting position of the hand\ both when the hand started from the near or from the far side of the target area[ That is\ as seen from the subjects| viewpoint\ their errors fell respectively on the near or the far side of the target location\ depending on whether they pointed from a near or far starting position[ It is not easy to see how this result could be reconciled with a representation of the movement end!point in terms of its distance from the body "eye\ head\ trunk or shoulder#[ It is entirely consistent\ however\ with a hand!centred encoding of the target location[ "b# The trial!by!trial error in the remembered target location varied independently in terms of its distance and direction from the hand starting position[ Chie. and Allport ð7Ł found a similar independence in the coding of the distance and direction of a location to
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be pointed to after a delay[ These data parallel those found for immediate planning of movement distance and direction\ both in behavioural ð09\ 19Ł and neu! rophysiological studies ð6\ 05\ 06\ 14\ 15Ł[ Such data are clearly consistent with movement planning in a hand! "or e}ector!# centred co!ordinate system ð19\ 34Ł[ These two types of evidence\ "a# and "b# above\ for a hand!centred representation of the target location were observed both when the target was viewed in the light\ when simultaneous vision of both hand and target location might favour spatial computation of the target location with reference to the hand\ and in the dark\ when only vision of the target was possible[ In the light\ undershoot errors were of approximately equal mag! nitude "mean 23 mm# from both near and far starting positions[ In the dark\ however\ the undershoot was larger when the hand started from the near position[ This result may depend on systematic misperception of target distance "in extrinsic co!ordinates# in the dark\ such that the target location was perceived as nearer to the body than it actually was[ In support of this hypothesis are the following observations] "0# there was no such e}ect when the target was presented in the light^ "1# the mean sagittal error "undershoot error# from the far hand position was actually smaller\ in the dark\ than the corresponding error in the light^ indeed\ mean undershoot error in the dark\ overall\ was comparable to that when the target stimuli were seen in the light[ The evidence relevant to question "1#\ concerning the nature of spatial location coding in short!term memory\ depends on comparisons between the 2! and 29!s delay conditions[ "a# Undershoot errors increased systematically over time delay\ in all four conditions[ "b# However\ the trial!by!trial independence of distance and direction error\ as measured from the hand star! ting position\ remained unchanged even at the 29!s delay[ Both these results are consistent with the hypothesis that the to!be!remembered target location was recoded into intrinsic "i[e[\ hand!movement related# spatial co! ordinates\ at the start of each trial\ and that target location was retained in this form over the 29!s delay[ The argument is as follows[ As mentioned in the Introduction\ there is neurophysiological evidence suggesting that\ in action planning\ movement extent may be encoded in terms of the amplitude of premotor activity\ perhaps mediated via control of peak movement velocity ð15\ 26Ł[ We hypothesized that\ over the delay imposed by a reten! tion interval\ such activation coding of movement extent should be subject to time!dependent decay[ Accordingly\ we predicted that\ in these conditions\ undershoot of a remembered reach should become systematically greater
with increasing short!term memory delay[ This prediction was con_rmed[ As predicted\ moreover\ the direction of undershoot was dependent on the starting position of the hand[ Clearly\ a visually coded\ i[e[\ viewpoint!depen! dent\ short!term memory ð4\ 16Ł could not account for increasing constant error over time delay in opposite directions\ from the subject|s viewpoint\ contingent sim! ply on the hand starting!position[ "Of course\ the data do not exclude the possibility that subjects also maintained a visual s[t[m[ representation^ however\ this latter rep! resentation could not be the one determining delayed pointing accuracy#[ There is one further argument to the same conclusion[ Suppose that\ contrary to the hypothesis proposed\ short! term memory for the target location did in fact depend on a visual "i[e[\ viewpoint!dependent# egocentric rep! resentation\ to be transformed into motor "hand!centred# co!ordinates only at the time of pointing recall[ This visual s[t[m[ code might be expected to show increasing spatial uncertainty\ observable as random\ trial!by!trial\ absolute error in any direction\ as a function of time delay[ Accordingly\ as the amplitude of the visual pos! ition!error increased over time relative to the intrinsic motor error "which should remain constant#\ the charac! teristic independence in the coding of movement distance and direction\ seen in the uncorrelated sagittal and trans! verse error\ should be increasingly masked or overlaid\ resulting in an increasingly positive correlation in the measured absolute error on these two axes[ However\ although the absolute error increased sig! ni_cantly over time delay on both the sagittal and trans! verse axes\ at 29 s these two measures remained completely uncorrelated\ as they had been at 2 s[ In other words\ the decline in s[t[m[ position accuracy over time represents the independent loss of information specifying movement direction and movement distance\ not the decay of a visual s[t[m[ code[ Current models of spatial short!term memory ð4\ 16Ł evidently require modi_cation to accommodate these and related results ð7\ 30\ 37Ł[ As observed by one of the anonymous referees of this paper\ our results may illustrate a dissociation between what a subject experiences in doing a "delayed# per! ceptual!motor task\ and what s:he actually uses to guide his:her actions[ The referee reports forming a visual image of the target location when performing a task of this kind[ However\ it appears that this visual rep! resentation is not what is actually used to do the task[ Motoric or movement!related coding of spatial location\ as revealed by the conditions of this experiment\ is clearly one among a number of possible spatial s[t[m[ codes[ Under di}erent conditions\ hand!centred coding of target position may not be an e}ective strategy*e[g[\ if the hand!position is changed between initial perception of the target position and subsequent recall[ Further stud! ies\ related to these questions\ are in progress in our laboratory[
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Acknowledgements The preparation of this paper was supported\ in part\ by project grant No G 8210061 N from the U[K[ Medical Research council\ to D[ A[ Allport and M[ Woodin[
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References ð12Ł ð0Ł Andersen RA\ Essick GK\ Siegel RM[ Encoding of spatial location by posterior parietal neurons[ Science 0874^129]345Ð7[ ð1Ł Andersen RA[ Coordinate transformation and motor planning in posterior parietal cortex[ In] Gazzaniga MS\ editor[ The cognitive neurosciences[ Cambridge MA] MIT Press\ 0884]408Ð21[ ð2Ł Baddeley AD\ Hitch GJ[ Working memory[ In] Bower G\ editor[ The psychology of learning and motivation\ Vol[ VII[ New York] Academic Press\ 0863]36Ð89[ ð3Ł Baddeley AD[ Working memory[ Oxford] Oxford University Press\ 0875[ ð4Ł Baddeley AD[ Working memory[ In] Gazzaniga MS\ editor[ The cognitive neurosciences[ Cambridge MA] MIT Press\ 0884]644Ð 53[ ð5Ł Brotchie PR\ Andersen RA\ Snyder LH\ Goodman SJ[ Head pos! ition signals used by parietal neurons to encode locations of visual stimuli[ Nature 0884^264]121Ð4[ ð6Ł Caminiti R\ Johnson PB\ Galli C\ Ferraina S\ Burnod Y[ Making arm movements within di}erent parts of space] the premotor and motor cortical representation of coordinate system for reaching to visual targets[ The Journal of Neuroscience 0880^00]0071Ð86[ ð7Ł Chie. S\ Allport DA[ Independent coding of target distance and direction in visuo!spatial working memory[ Psychological Research 0886^59]133Ð49[ ð8Ł De Yoe EA\ van Essen C[ Concurrent processing streams in mon! key visual cortex[ Trends Neurosciences 0877^00]108Ð15[ ð09Ł Favilla A\ Hening W\ Ghez C[ Trajectory control in targeted force impulses[ VI[ Independent speci_cation of response amplitude and direction[ Experimental Brain Research 0878^64]179Ð83[ ð00Ł Flanders M\ Tillery SI\ Soechting JF[ Early stages in a sen! sorimotor transformation[ Behavioural Brain Sciences 0881^04]298Ð51[ ð01Ł Fogassi L\ Gallese V\ di Pellegrino G\ Fadiga L\ Gentilucci M\ Luppino G\ Matelli M\ Pedotti A\ Rizzolatti G[ Space coding by premotor cortex[ Experimental Brain Research 0881^87]575Ð89[ ð02Ł Fogassi L\ Gallese V\ Fadiga L\ Luppino M\ Matelli M\ Rizzolatti G[ Coding of peripersonal space in inferior premotor cortex "area F3#[ Journal of Neurophysiology 0885^65]030Ð46[ ð03Ł Gentilucci M\ Rizzolatti G[ Cortical motor control of arm and hand movements[ In] Goodale MA\ editor[ Vision and action] the control of grasping[ Ablex N[J[] Norwood\ 0889[ pp[ 036Ð51[ ð04Ł Gentilucci M\ Negrotti A[ Dissociation between perception and visuomotor transformation during reproduction of remembered distances[ Journal of Neurophysiology 0883^61]1915Ð29[ ð05Ł Georgopoulos AP\ Kalaska JF\ Caminiti R\ Massey JT[ On the relation between the direction of two!dimensional arm movements and cell discharge in primate motor cortex[ The Journal of Neu! roscience 0871^1]0416Ð26[ ð06Ł Georgopoulos AP\ Delong MR\ Crutcher MD[ Relations between parameters of step!tracking movements and single cell discharge in the globus pallidus and subthalamic nucleus of the behaving monkey[ The Journal of Neuroscience 0872^2]0475Ð87[ ð07Ł Ghez C\ Gordon J\ Ghilardi MF\ Sainburg R[ Contributions of vision and proprioception to accuracy in limb movements[ In] Gazzaniga MS\ editor[ The cognitive neurosciences[ Cambridge MA] MIT Press\ 0884]438Ð53[ ð08Ł Goodale MA\ Jakobson LS\ Keillor JM[ Di}erences in the visual
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control of pantomimed and natural grasping movements[ Neu! ropsychologia 0883^21]0048Ð67[ Gordon J\ Ghilardi MF\ Ghez C[ Accuracy of planar reaching movements] I[ Independence of direction and extent variability[ Experimental Brain Research 0883^88]86Ð000[ Graziano MA\ Gross CG[ Multiple pathways for processing visual space[ In] Inui T\ McClelland JL\ editors[ Attention and per! formance XVI[ Cambridge MA] MIT Press\ 0885]070Ð196[ Graziano MA\ Hu XT\ Gross CG[ Coding the locations of objects in the dark[ Science 0886^166]128Ð30[ Graziano MA\ Yap GS\ Gross CG[ Coding of visual space by premotor neurons[ Science 0883^155]0943Ð6[ Jeannerod M\ Arbib MA\ Rizzolatti G\ Sakata H[ Grasping objects] The cortical mechanisms of visuomotor transformation[ Trends in Neurosciences 0884^07]203Ð19[ Kalaska JF\ Caminiti R\ Georgopoulos AP[ Cortical mechanisms related to the direction of two!dimensional arm movements] relations in parietal area 4 and comparison with motor cortex[ Experimental Brain Research 0872^40]136Ð59[ Kurata K[ Premotor cortex of monkeys] set! and movement! related activity re~ecting amplitude and direction of wrist move! ments[ Journal of Neurophysiology 0882^58]076Ð199[ Logie RH\ Marchetti C[ Visuo!spatial working memory] Visual\ spatial or central executive< In] Logie RH\ Denis M\ editors[ Men! tal images in human cognition[ Amsterdam] North Holland Press\ 0880]094Ð04[ Logie RH[ Visuo!spatial working memory[ Erlbaum\ 0884[ Matelli M\ Camarda\ R\ Glickstein M\ Rizzolatti G[ A}erent and e}erent projections of the inferior area 5 in the macaque monkey[ Journal of Comparative Neurology 0875^140]170Ð87[ McNaughton BL\ Knierim JJ\ Wilson MA[ Vector encoding and the vestibular foundations of spatial cognition] Neuro! physiological and computational mechanisms[ In] Gazzaniga MS\ editor[ The cognitive neurosciences[ Cambridge MA] MIT Press\ 0884[ Meegan DV\ Tipper SP[ Reaching into cluttered visual environ! ments] Spatial and temporal in~uences of distracting objects[ Quar! terly Journal of Experimental Psychology 0887^40A]114Ð38[ Milner AD\ Goodale MA[ The visual brain in action[ Oxford] Oxford University Press\ 0884[ Paillard J[ Motor and representational framing of space[ In] Pail! lard J\ editor[ Brain and space[ Oxford] Oxford University Press\ 0880[ Petrides M\ Pandya DN[ Projections of the frontal cortex from the posterior region in the rhesus monkey[ Journal of Comparative Neurology 0875^117]094Ð005[ Postma A\ De Haan EHF[ What was where< Memory for object locations[ Quarterly Journal of Experimental Psychology 0885^38A]067Ð88[ Prablanc C\ Echallier JF\ Komilis E\ Jeannerod M[ Optimal response of eye and hand motor systems in pointing at a visual target[ I[ Spatio!temporal characteristics of eye and hand move! ments and their relationships when varying the amount of visual information[ Biological Cybernetics 0868^24]002Ð13[ Riehle A\ MacKay WA\ Requin J[ Are extent and force inde! pendent movement parameters< Preparation! and movement! related neuronal activity in the monkey cortex[ Experimental Brain Research 0883^88]45Ð63[ Rizzolatti G\ Riggio L\ Sheliga BM[ Space and selective attention[ In] Umilta C\ Moscovitch M\ editors[ Attention and performance XV[ Cambridge MA] MIT Press\ 0883[ Rizzolatti G\ Fadiga L\ Fogassi L\ Gallese V[ The space around us[ Science 0886^166]089Ð0[ Smyth MM\ Pearson NA\ Pendleton LR[ Movement and working memory] patterns and positions in space[ Quarterly Journal of Experimental Psychology 0877^39A]386Ð403[
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ð30Ł Smyth MM\ Pendleton LR[ Working memory for movements[ Quarterly Journal of Experimental Psychology 0878^30A]124Ð149[ ð31Ł Soechting JF\ Flanders M[ Sensorimotor representations for poin! ting to targets in three!dimensional space[ Journal of Neuro! physiology 0878^51]471Ð483[ ð32Ł Soechting JF\ Flanders M[ Errors in pointing are due to approxi! mations in sensorimotor transformations[ Journal of Neuro! physiology 0878^51]484Ð597[ ð33Ł Soechting JF\ Tillery SIH\ Flanders M[ Transformation from head! to shoulder centred representation of target direction in arm movements[ Journal of Cognitive Neuroscience 0889^1]21Ð32[ ð34Ł Tipper SP\ Lortie C\ Baylis GC[ Selective reaching] evidence for
action!centred attention[ Journal of Experimental Psychology] Human Perception and Performance 0881^07]780Ð894[ ð35Ł Tresch MC\ Sinnamon HM\ Seamon JG[ Double dissociation of spatial and object visual memory] evidence from selective inter! ference in intact human subjects[ Neuropsychologia 0882^20]100Ð 8[ ð36Ł Woodin ME\ Heil J[ Skilled motor performance and working memory in rowers] body patterns and spatial positions[ Quarterly Journal of Experimental Psychology 0885^38A]246Ð67[ ð37Ł Woodin ME\ Allport A[ Independent reference frames in human spatial memory] body!centred and environment!centred coding in near and far space[ Memory + Cognition 0888\ in press[
Neuropsychologia 26 "0888# 384Ð491
Hand!centred coding of target location in visuo!spatial working memory S[ Chie.a \ D[A[ Allportb\ M[ Woodinb a
Dipartimento di Fisiolo`ia Umana e Funzioni Biolo`iche Inte`rate {Filippo Bottazzi|\ Seconda Universita di Napoli\ Via Costantinopoli 05\ 79027\ Napoli\ Italy b Department of Experimental Psycholo`y\ University of Oxford\ UK Received 00 December 0886^ accepted 1 July 0887
Abstract The aim of the present experiment was to determine the frame of reference used to encode a visually speci_ed target location\ in a simple\ visuo!spatial short!term!memory task\ using delayed manual pointing[ Subjects were required to remember the location of a single\ visually presented stimulus\ over delays of 2 and 29 s\ and to recall its location by pointing to it with the tip of a stylus with eyes closed[ There were two starting positions for the hand] one near the body\ proximal to the target area\ the other far from the body\ i[e[\ beyond the target area[ Two groups of twelve subjects participated in the experiment[ One group observed the target locations in the light\ the other group observed the target LEDs in the dark[ The results showed a large "between 09Ð29)# and consistent undershoot of the remembered target location with respect to the hand|s starting position\ in the sagittal axis\ both when the hand started from the near and from the far position[ That is\ from the far starting position\ pointing errors lay consistently beyond the actual target location as seen from the subject|s viewpoint[ Further\ this undershoot error in the remembered target location increased over time delay[ These results are in favour of the hypothesis that the visual target locations were encoded in a hand!centred frame of reference in spatial short!term memory[ They also implicate the use of motor! or action!related representations in visuo!spatial working memory[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
0[ Introduction Behavioural and neurophysiological evidence suggests that primate and human spatial location coding is based on a variety of di}erent reference frames ð1\ 29\ 22\ 27Ł[ Two fundamentally di}erent types of reference system are usually considered\ egocentric and allocentric[ The _rst type codes locations with reference to the position of the body\ or of speci_c parts of the body\ the second codes locations with reference to the surrounding environment[ Woodin and Allport ð37Ł have demonstrated a clearcut behavioural dissociation between memory for a target location _xed with respect to the stationary environment "{allocentric| location# vs memory for a target location _xed with respect to the subject|s body "{egocentric| location#\ where the environmental reference frame was provided only by vestibular and proprioceptive cues[ There is also behavioural evidence for egocentric ref!
Corresponding author[ Fax] 99 28 70 4554719^ E!mail] schie. Ýunina[it
erence frames centred on di}erent body parts\ depending on the subject|s task[ Soechting and Flanders ð31\ 32Ł asked subjects to point to the remembered location of a target presented visually[ They found that pointing errors were more consistently centred on the shoulder than on the eyes or head[ However\ in a subsequent study\ Soe! chting et al[ ð33Ł showed that subjects may use a head! centred reference frame\ when asked to point to locations relative to the head\ and a shoulder!centred reference frame\ when asked to point to locations relative to the shoulder[ Flanders et al[ ð00Ł proposed a model in which the spatial representation of a target location for manual pointing evolves from a retinocentric to a head!centred and a shoulder!centred representation[ Behavioural evi! dence for a hand!centred representation was found by Tipper et al[ ð20\ 34Ł when subjects pointed at targets in a 2!D environment[ The authors asked their subjects to reach for a target stimulus while avoiding an irrelevant distractor[ When the distractor was presented in the space near the hand\ it produced interference e}ects "and {nega! tive priming|# dependent on its relation to the starting position of the hand\ irrespective of its visual "i[e[\ view! point!dependent# position[ Evidence for a hand!centred co!ordinate system in the planning of simple\ 1!D reach!
9917Ð2821:88:,Ðsee front matter Þ 0888 Elsevier Science Ltd[ All rights reserved PII] S 9 9 1 7 Ð 2 8 2 1 " 8 7 # 9 9 9 7 1 Ð 6
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ing movements has been reported also by Gordon et al[ ð19Ł "ðsee also 07Ł#[ Egocentric representation of space is obtained by inte! grating\ continuously\ visual and proprioceptive infor! mation[ Neurophysiological studies have shown a series of co!ordinate transformations\ from a retinotopic to a head!centred and body!centred spatial representation[ Early visual processing in the primate visual system appears to be coded in retinotopic co!ordinates ð8Ł[ How! ever\ a retinotopic representation of visual space is not appropriate for guiding bodily actions in space\ since the position of the eyes and head change continuously[ More stable spatial representations\ adapted to the needs of action planning and memory\ are progressively elab! orated in the posterior parietal cortex\ prefrontal cortex and elsewhere[ A head!centred co!ordinate rep! resentation may be obtained by combining both the reti! nal location of a visual stimulus and information signalling the position of the eye ð0Ł\ while a body!centred representation may be reached by combining the retinal and eye position signals with head position information ð5Ł[ Coding systems apparently centred on the face\ trunk\ arm\ and other body parts have been reported in inferior premotor cortex\ or area F3 ð01Ð03\ 10Ð12Ł[ Area F3 receives a major input from the inferior parietal lobule ð18\ 23Ł[ Graziano et al[ ð10\ 12Ł found neurons in ventral premotor cortex and in putamen that responded to visual stimuli in the space adjacent to the hand or arm[ Fur! thermore\ the visual receptive _elds of these neurons moved when the arm moved[ The authors ð10\ 12Ł sug! gested that these cells code space in arm!centred co!ordi! nates[ They reported other cell populations responsive to visual stimuli adjacent to the face or head\ with receptive _elds "RFs# that moved\ likewise\ with the head[ More recently\ Graziano and colleagues ð11Ł showed that some of these neurons continued a sustained response even after the lights were turned o} and the stimulus was no longer visible\ i[e[\ these neurons showed {object per! manence|[ For some cells\ when the relevant bodypart was moved passively\ in the dark\ such as to carry the previously seen but now invisible stimulus out of the cell|s RF\ the cell abruptly ceased responding^ comple! mentarily\ when the bodily movement brought the pre! viously seen stimulus into the cell|s RF\ still in the dark\ the cell started to respond vigorously[ These neurons thus plausibly play a role in action!related spatial working memory ð28Ł[ The majority of the studies referred to above "with the exception of ð11 and 37Ł# have investigated spatial representation in immediate or ongoing sensory!motor processing[ Behavioural studies of spatial representation in human short!term memory\ on the other hand\ have developed a separate research tradition\ in which the idea of multiple reference frames has received relatively little attention[ In the original formulation of their {working memory| "WM# model\ Baddeley and Hitch ð2Ł postulated
the existence of a unitary\ limited!capacity {visuo!spatial scratch!pad| "VSSP# for the temporary representation and manipulation of all visuo!spatial information^ more recent versions restate this postulate ð3\ 4Ł[ Other studies\ using selective interference paradigms\ suggested the need to fractionate the VSSP into two components\ one {vis! ual| \ the other {spatial| ð16\ 17\ 35Ł[ Within {spatial| short! term memory\ a further distinction\ also based on selec! tive interference e}ects\ has been made between memory for the subject|s own\ egocentric body postures or move! ments\ vs exocentric positional coding of objects ð39\ 30Ł[ Moreover\ with respect to memory for exocentric object location\ a further behavioural dissociation has been demonstrated between a positional map {per se|\ and the assignment of particular objects!to!locations ð24Ł[ The present study attempts to link together these two separate research traditions[ Its aim is to identify the spatial reference frame"s# used in human short!term memory in a simple\ egocentric positional memory task[ As such\ it forms part of a series of ongoing studies of spatial representation in human visuo!spatial working memory ð7\ 36\ 37Ł[ Spatial representation involves a multiplicity of di}er! ent coding systems\ as the foregoing review makes clear[ To isolate a particular spatial coding system exper! imentally\ it is essential to constrain the subject|s task in such a way as to reduce "if possible\ to eliminate# the use of all alternative coding strategies[ For example\ if memory for egocentric positional coding is to be inves! tigated "as here#\ it is necessary to exclude\ so far as possible\ all allocentric reference points in the memory recall conditions0[ In the present study\ this was done by requesting the subjects to point to a remembered location with their eyes closed\ that is\ without allocentric visual guidance[ Further\ in one of two conditions\ the target location was presented in darkness\ as a luminous spot\ thus excluding also any other\ external visual reference points at input\ as well as at recall[ Subjects were required to remember the location of a single\ visually presented stimulus\ over delays of 2 and 29 s\ and then to point to the remembered location[ In previous studies of pointing to a remembered location\ the standard hand starting position was directly in front of the subject\ that is\ between the subject|s body and target location[ As a result\ it is not possible to tell whether the spatial reference frame that is used to code the target location is centred at the body "eye\ head\ trunk\ shoulder# or at the hand[ One way to disentangle the possible spatial reference frames is simply to change the starting position of the hand\ without changing the subject|s viewpoint ð20\ 34Ł[ For this purpose we used two di}erent starting positions for the hand] one on the body 0
Gravitational cues provide a major allocentric spatial reference[ In the present study\ position coding orthogonal to the gravitational axis was studied[
S[ Chief_ et al[:Neuropsycholo`ia 26 "0888# 384Ð491
midline near the subject|s body\ the other far from the body\ practically at full arm extension\ beyond the target locations[ Stimulus presentation was made either in the dark\ providing purely egocentric visual information\ or in the light\ in which both egocentric and allocentric information was available[ Pointing to the remembered location\ however\ was done with eyes closed in both cases[ The rationale of the experiment is as follows[ As is well known\ pointing to a visually presented target\ without visual guidance during the act of pointing\ typically results in spatial undershoot errors that increase sys! tematically with target distance ð7\ 25\ 31Ł[ On the other hand\ when the target location is speci_ed proprio! ceptively\ for example by active or passive movement of the hand\ undershoot errors are not observed or are gre! atly reduced ð31Ł[ This _nding has led to the suggestion that the undershoot may result from systematic error in the sensorimotor transformation from a visual rep! resentation of the target location to a representation in terms of intrinsic co!ordinates of the hand and arm at their intended target position\ with respect to the shoulder ð31\ 32Ł "see also ð04Ł#[ However\ it remains open to ques! tion whether the critical transformation may be into a co!ordinate system centred on the body "e[g[\ trunk or shoulder# or on the hand[ The present experiment pro! vides a critical test between these alternatives[ The predictions are as follows[ If the locations to be pointed to are encoded in an egocentric system centred on the body "head\ trunk\ shoulder#\ pointing errors should undershoot with reference to the body\ regardless of whether the hand starting position was near or far[ On the other hand\ if the target location is encoded with respect to the hand\ pointing errors should tend to under! shoot with reference to the hand starting position^ i[e[\ from the far starting position\ pointing errors should tend to lie beyond the true target location\ with reference to the body[ There is a second question to which this experiment is addressed[ This concerns the available coding domains in visuo!spatial short!term memory "s[t[m[#[ If spatial s[t[m[ "over more than 0 s or so# is predominantly or even exclusively visual or perceptual in nature\ as some current approaches might suggest ð4\17Ł "see also ð08\ 21Ł#\ then we should expect the target location to be retained "e[g[\ over a 29 s interval# in a visual egocentric reference frame[ The sensori!motor transformation into intrinsic hand and arm "motor or joint!angle# co!ordinates should then be executed only at the time of the required act of poin! ting[ If this were so\ the size of any systematic undershoot errors\ resulting from inaccuracies in this transformation\ should be independent of time delay[ An alternative possibility\ however\ is that the target location is recoded immediately into intrinsic hand:arm co!ordinates\ and retained\ over the imposed time delay\ in the form of a potential action\ a motor schema ð13\ 28Ł\ directed toward
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a particular spatial location[ In this case\ any observed decline in pointing accuracy as a function of time delay should likewise re~ect the intrinsic parameters of the encoded movement*e[g[\ in terms of its intended extent and direction[ Moreover\ if these movement parameters are encoded independently ð6\ 09\ 05\ 06\ 19\ 14\ 15Ł\ we may expect them also to show independent loss of accuracy over s[t[m[ delay[ Neurophysiological evidence suggests that\ in action planning\ movement extent may be encoded in terms of the amplitude of set!related pre! motor activity ð15\ 26Ł[ If so\ we might expect such a representation to be subject to time!dependent decay of activation[ In this case\ undershoot errors should become systematically larger with increasing s[t[m[ delay[ The experiment provides a test of this somewhat speculative prediction also[
1[ Method 1[0[ Subjects Two groups of twelve subjects participated in the experiment[ The _rst group performed the experiment in the light[ Subjects in this group were seven women and _ve men^ mean age 12[7^ S[D[2[1^ ten subjects were right! handed and two left!handed[ The second group per! formed the experiment in the dark[ There were eight women and four men^ mean age 13[7^ S[D[ 3[4^ all the subjects were right!handed[ The subjects were naive to the task[
1[1[ Apparatus Subjects sat in a comfortable chair in front of a table on which a digitizing tablet was placed[ The tablet mea! sured 329 mm "width#×469 mm "depth# and had an active surface of 294×347 mm[ It was contacted with a non!inking electronic stylus[ When in contact with the active surface\ the position of the stylus tip was sampled at a rate of 49 Hz[ Data were recorded in X "horizontal# and Y "vertical# co!ordinates with a measuring accuracy of 9[14 mm[ The tablet surface was covered with a thin white card\ on which the starting position was drawn in black ink "a 2 mm diameter spot# either at 19 cm from the trunk "near hand position# or at 53 cm "far hand position#\ along the midsagittal axis[ Testing was conducted either in the light or in the dark[ In the _rst case\ the room was uniformly lit^ in the second case\ the room was darkened\ and the subjects wore dark glasses so as to eliminate any possible extraneous visual cues[
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When testing was carried out in the light\ the target stimulus consisted of a ~at\ black disc\ 4 mm in diameter^ when in the dark\ the stimulus was a red luminous spot "LED#\ 4 mm in diameter[ There were three target distances] 27 cm from the trunk "07 cm from the near hand position^ 15 cm from the far hand position#\ 31 cm from the trunk "11 cm from both hand starting positions#\ and 35 cm from the trunk "15 from the near hand position and 07 cm from the far hand position#[ Target locations were along the midsagittal axis[
1[2[ Procedure At the beginning of each trial\ the subject held the stylus with her or his preferred hand and closed her eyes[ The experimenter placed the stimulus on the tablet sur! face and the subject|s hand on the starting position[ The subject opened her eyes\ looked at the target stimulus for 1 s\ and then closed her eyes[ When the retention interval had elapsed\ the experimenter verbally gave the order to go[ The subject was instructed to move the tip of the stylus on the tablet at a natural speed and to stop with the stylus in the remembered target position\ still with eyes closed[ When the hand starting distance was near\ the subject thus performed an extension movement\ when far a ~exion movement[ At the end of the subject|s move! ment\ the experimenter brought the subject|s hand back to the starting position[ The experimenter ensured that subjects never saw the end point of their movement[ The time delays were 2 and 29 s[ The combination of hand starting position "near vs far# and time delay "2 vs 29 s# resulted in four experimental conditions[ A total of 37 trials was given per subject "two hand starting positions×two time delays×three target distances×four presentations#[ There was one block of 01 trials for each experimental condition[ The order of the four blocks was counterbalanced across the subjects[ Target distances were randomized[ Pointing errors were de_ned as the distance of the end point of stylus movement from the center of the target[ Errors were decomposed into their sagittal "i[e[\ straight ahead# and transverse components[ The dependent vari! ables employed were] absolute "unsigned# and constant "signed# sagittal and transverse errors[ For constant sag! ittal errors\ the sign was de_ned with reference to the hand starting position[ Sagittal errors farther than the target location "i[e[\ overshoot errors\ with reference to the starting position# were assigned "¦# values\ whereas errors nearer than the target location "undershoot errors# were assigned "−# values[ For constant transverse errors\ pointing errors to the right of the target location with respect to the subject|s viewpoint were given "¦# values\ whereas errors to the left of the target location were given "−# values[ Data for each experimental condition were
pooled across target distances[ For each condition\ the means of each variable were calculated for each subject[ 2[ Results The mean values and the standard deviations of each variable are shown in Table 0[ Mixed!design 1×1×1 ANOVAs were conducted on each dependent variable\ with Lighting "light vs dark# as a between!subjects factor\ and Delay "2!s vs 29!s# and Starting position of the hand "near vs far# as the within! subjects factors[ Paired comparisons were performed using NewmanÐKeuls procedure[ 2[0[ Sagittal errors The principal data concern the constant "i[e[\ signed# sagittal errors[ As Table 0 shows\ these took the form\ consistently\ of undershoot errors "i[e[\ negative values of constant sagittal error#\ in all conditions[ Note that in each case the undershoot is coded with respect to the hand starting position[ That is\ as seen from the subject|s viewpoint\ from the near starting position undershoot errors fell on the near side of the target locations^ from the far starting position\ on the contrary\ the subject|s pointing errors were consistently on the far side of the target locations[ Overall\ 71[2) of responses were sag! ittal undershoot errors[ Constant sagittal error was sig! ni_cantly greater than zero in each of the eight conditions in Table 0\ by one!sample\ two!tailed t!tests "1f 00#[ "See Table 0[# ANOVA showed that the size of the constant sagittal error "undershoot error# was signi_cantly a}ected by Delay "F"0\11# 00[20\ P ³ 9[994# "2!s −20[3 mm^ 29!s −31[8 mm#[ No other main e}ects were signi_! cant[ However\ the interaction between Lighting and Starting position approached signi_cance "F"0\11# 2[74\ P 9[95#[ Post!hoc comparisons showed that the undershoot error was largest "P ³ 9[94# when the target locations were presented in the dark\ with the hand in the near starting position "dark\ near −43[1 mm^ dark\ far −14[1 mm^ light\ near −23[7 mm^ light\ far −23[2 mm#[ Absolute "i[e[\ unsigned# sagittal error was a}ected by both Delay and Starting position "Delay "F"0\11# 09[86\ P ³ 9[994^ 2!s 39[4 mm^ 29!s 37[4 mm^ Starting position] F"0\11# 4[85\ P ³ 9[94] near 49[3 mm^ far 27[5 mm#[ There was a signi_cant interaction between Lighting and Starting position "F"0\11# 09[58\ P ³ 9[994#[ Post!hoc comparisons showed that absolute sagittal error was greater with tar! get locations presented in the dark\ when the hand started at the near position\ than in the other conditions "P ³ 9[90# "dark\ near 52[1 mm^ dark\ far 24[7 mm^ light\ near 26[4 mm^ light\ far 30[4 mm#[
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S[ Chief_ et al[:Neuropsycholo`ia 26 "0888# 384Ð491 Table 0 Mean values of sagittal and transverse errors in pointing with eyes closed to a remembered target location "standard deviations in brackets# Lighting "during stimulus presentation#
Light
Starting position of the hand
Near
Delay Constant "i[e[\ signed# sagittal error "mm# Absolute "i[e[\ non!signed# sagittal error "mm# Constant "i[e[\ signed# transverse error "mm# Absolute "i[e[\ non!signed# transverse error "mm#
Dark Far
Near
Far
2s
29 s
2s
29 s
2s
29 s
2s
29 s
−15[5 "10[8# 20[9 "05[4# −0[7 "00[7# 09[6 "5[1#
−32[0 "11[6# 32[8 "10[5# 2[1 "04[2# 02[4 "7[0#
−29[9 "18[4# 28[9 "05[1# 1[5 "8[2# 7[8 "3[8#
−27[4 "15[0# 32[8 "07[7# −0[9 "00[1# 09[8 "4[2#
−35[3 "28[7# 46[3 "08[7# 9[7 "03[1# 01[3 "6[1#
−50[8 "30[7# 58[9 "17[7# 0[5 "06[1# 04[4 "6[3#
−11[4 "23[8# 23[5 "11[3# 3[8 "00[2# 00[9 "5[8#
−17[9 "22[1# 26[9 "13[3# 2[3 "09[9# 09[8 "5[2#
For constant sagittal errors\ positive values were assigned to errors farther than the target "i[e[\ overshoot errors# and negative values to errors nearer than the target location "i[e[\ undershoot errors#\ with reference to hand starting position[ For constant transverse errors\ positive values were assigned to errors to the right of and negative values to errors to the left of target location[ Signi_cance levels of departure from zero are shown for constant sagittal and transverse error "two!tailed t!test\ 1f 00^ P ³ 9[94^ P ³ 9[90#[
2[1[ Transverse errors No signi_cant main e}ects were found on constant transverse error[ The values obtained did not di}er sig! ni_cantly from zero in any condition\ by one!sample\ two!tailed t!tests "1f 00#[ Absolute transverse error was signi_cantly a}ected by Delay "F"0\11# 4[56\ P ³ 9[94^ 2!s 09[6 mm^ 29! s 01[6 mm#[ No other e}ects were signi_cant[
body!centred or hand!centred*is the target end! point of the movement represented< "1# During the retention interval\ is the memory rep! resentation of the target location encoded in a visual "i[e[\ extrinsic# reference frame\ to be transformed into intrinsic "arm! and hand!movement# co!ordi! nates only at the time of pointing< Or is the target location recoded immediately into intrinsic "motoric# co!ordinates\ and represented in this form in short! term memory over at least a 2Ð29 s delay<
2[2[ Correlation of absolute sagittal and transverse error The correlation coe.cients "Sperman|s rho# between absolute sagittal and transverse errors were calculated individually for each subject and for each condition[ Sig! ni_cant correlations "P ³ 9[94#\ either positive or nega! tive\ were found in only ten cases out of 85 "_ve positive and _ve negative#[ At each delay the mean correlation coe.cients between sagittal and transverse error approxi! mated to zero "At 2!s delay] dark\ near r 9[98^ dark\ far r −9[90^ light\ near r 9[09^ light\ far r −9[91^ at 29!s delay] dark\ near r −9[97\ dark\ far r 9[98^ light\ near r 9[09^ light\ far r −9[90#[
3[ Discussion This experiment was directed to two questions] "0# How is the motor act of pointing with eyes closed to a remembered\ visually presented target location spatially represented< More speci_cally\ in terms of what spatial reference frame*viewpoint!speci_c\
Question "0# will be considered _rst[ The present experiment provides two kinds of behavioural evidence for a hand!centred encoding of the movement] "a# There was a consistent undershoot of the target distance\ as measured from the starting position of the hand\ both when the hand started from the near or from the far side of the target area[ That is\ as seen from the subjects| viewpoint\ their errors fell respectively on the near or the far side of the target location\ depending on whether they pointed from a near or far starting position[ It is not easy to see how this result could be reconciled with a representation of the movement end!point in terms of its distance from the body "eye\ head\ trunk or shoulder#[ It is entirely consistent\ however\ with a hand!centred encoding of the target location[ "b# The trial!by!trial error in the remembered target location varied independently in terms of its distance and direction from the hand starting position[ Chie. and Allport ð7Ł found a similar independence in the coding of the distance and direction of a location to
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be pointed to after a delay[ These data parallel those found for immediate planning of movement distance and direction\ both in behavioural ð09\ 19Ł and neu! rophysiological studies ð6\ 05\ 06\ 14\ 15Ł[ Such data are clearly consistent with movement planning in a hand! "or e}ector!# centred co!ordinate system ð19\ 34Ł[ These two types of evidence\ "a# and "b# above\ for a hand!centred representation of the target location were observed both when the target was viewed in the light\ when simultaneous vision of both hand and target location might favour spatial computation of the target location with reference to the hand\ and in the dark\ when only vision of the target was possible[ In the light\ undershoot errors were of approximately equal mag! nitude "mean 23 mm# from both near and far starting positions[ In the dark\ however\ the undershoot was larger when the hand started from the near position[ This result may depend on systematic misperception of target distance "in extrinsic co!ordinates# in the dark\ such that the target location was perceived as nearer to the body than it actually was[ In support of this hypothesis are the following observations] "0# there was no such e}ect when the target was presented in the light^ "1# the mean sagittal error "undershoot error# from the far hand position was actually smaller\ in the dark\ than the corresponding error in the light^ indeed\ mean undershoot error in the dark\ overall\ was comparable to that when the target stimuli were seen in the light[ The evidence relevant to question "1#\ concerning the nature of spatial location coding in short!term memory\ depends on comparisons between the 2! and 29!s delay conditions[ "a# Undershoot errors increased systematically over time delay\ in all four conditions[ "b# However\ the trial!by!trial independence of distance and direction error\ as measured from the hand star! ting position\ remained unchanged even at the 29!s delay[ Both these results are consistent with the hypothesis that the to!be!remembered target location was recoded into intrinsic "i[e[\ hand!movement related# spatial co! ordinates\ at the start of each trial\ and that target location was retained in this form over the 29!s delay[ The argument is as follows[ As mentioned in the Introduction\ there is neurophysiological evidence suggesting that\ in action planning\ movement extent may be encoded in terms of the amplitude of premotor activity\ perhaps mediated via control of peak movement velocity ð15\ 26Ł[ We hypothesized that\ over the delay imposed by a reten! tion interval\ such activation coding of movement extent should be subject to time!dependent decay[ Accordingly\ we predicted that\ in these conditions\ undershoot of a remembered reach should become systematically greater
with increasing short!term memory delay[ This prediction was con_rmed[ As predicted\ moreover\ the direction of undershoot was dependent on the starting position of the hand[ Clearly\ a visually coded\ i[e[\ viewpoint!depen! dent\ short!term memory ð4\ 16Ł could not account for increasing constant error over time delay in opposite directions\ from the subject|s viewpoint\ contingent sim! ply on the hand starting!position[ "Of course\ the data do not exclude the possibility that subjects also maintained a visual s[t[m[ representation^ however\ this latter rep! resentation could not be the one determining delayed pointing accuracy#[ There is one further argument to the same conclusion[ Suppose that\ contrary to the hypothesis proposed\ short! term memory for the target location did in fact depend on a visual "i[e[\ viewpoint!dependent# egocentric rep! resentation\ to be transformed into motor "hand!centred# co!ordinates only at the time of pointing recall[ This visual s[t[m[ code might be expected to show increasing spatial uncertainty\ observable as random\ trial!by!trial\ absolute error in any direction\ as a function of time delay[ Accordingly\ as the amplitude of the visual pos! ition!error increased over time relative to the intrinsic motor error "which should remain constant#\ the charac! teristic independence in the coding of movement distance and direction\ seen in the uncorrelated sagittal and trans! verse error\ should be increasingly masked or overlaid\ resulting in an increasingly positive correlation in the measured absolute error on these two axes[ However\ although the absolute error increased sig! ni_cantly over time delay on both the sagittal and trans! verse axes\ at 29 s these two measures remained completely uncorrelated\ as they had been at 2 s[ In other words\ the decline in s[t[m[ position accuracy over time represents the independent loss of information specifying movement direction and movement distance\ not the decay of a visual s[t[m[ code[ Current models of spatial short!term memory ð4\ 16Ł evidently require modi_cation to accommodate these and related results ð7\ 30\ 37Ł[ As observed by one of the anonymous referees of this paper\ our results may illustrate a dissociation between what a subject experiences in doing a "delayed# per! ceptual!motor task\ and what s:he actually uses to guide his:her actions[ The referee reports forming a visual image of the target location when performing a task of this kind[ However\ it appears that this visual rep! resentation is not what is actually used to do the task[ Motoric or movement!related coding of spatial location\ as revealed by the conditions of this experiment\ is clearly one among a number of possible spatial s[t[m[ codes[ Under di}erent conditions\ hand!centred coding of target position may not be an e}ective strategy*e[g[\ if the hand!position is changed between initial perception of the target position and subsequent recall[ Further stud! ies\ related to these questions\ are in progress in our laboratory[
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Acknowledgements The preparation of this paper was supported\ in part\ by project grant No G 8210061 N from the U[K[ Medical Research council\ to D[ A[ Allport and M[ Woodin[
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