Neuropsychologia 46 (2008) 3053–3060

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Neuropsychologia

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Does localisation blindsight extend to two-dimensional targets?

David P. Careya,∗, Arash Sahraiea, Ceri T. Trevethana, Larry Weiskrantza,b

a Vision Research Laboratories, School of Psychology, University of Aberdeen, Aberdeen AB24 2UB, Scotlandb Department of Experimental Psychology, University of Oxford, Department of Experimental Psychology, South Parks Road, Oxford OX1 3UD, United Kingdom

a r t i c l e i n f o

Article history:Received 20 December 2007Received in revised form 9 June 2008Accepted 12 June 2008Available online 26 June 2008

Keywords:BlindsightLocalisation

a b s t r a c t

Residual sensorimotor skills which survive compromise of the geniculostriate visual system may dependon activity of the dorsal stream of extrastriate occipitoparietal cortex. These circuits are crucial for con-trolling hand and eye movements to targets in a three-dimensional world. Remarkably, demonstrations ofabove chance localisation by hand and by eye in blindsight patients have used luminous targets that wereonly varied in one spatial dimension. These limitations result in experimental confounds. In the presentstudy we examined saccadic and manual localisation in a well-studied patient (DB) to positions that werevaried in 1 or 2 dimensions, using targets which control for luminance artefacts. We found that his goodmanual localisation without awareness in 1D conditions was relatively preserved when the targets werevaried in 2D. In stark contrast, saccadic performance was completely attenuated with 2D targets. These

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paradoxical results are difficult to reconcile with feedforward models of eye–hand coordination and withaccounts of localisation that depend on intact multidimensional representations of the visual fields in

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Under some conditions, people with blindness caused by lesionsf the geniculostriate system can process visual attributes that theyannot see. These residual visual abilities in the absence of anyisual experience have been investigated for over 30 years, sincearly description in non-human primates were supplemented byhe first demonstrations of “blindsight” in humans. The earliesttudies tended to focus on residual sensorimotor skills, such asocalising suddenly flashed targets that the patients had no aware-ess of. Later, the list of spared visual skills in some patients withisual field defects has grown to include chromatic processing, ori-ntation sensitivity and detection in 2-alternative forced choice2AFC) procedures (Cowey, 2004; Weiskrantz, 1996, 1997).

Early critics of blindsight were concerned with several poten-ial artefacts, including unconscious cueing of patients, non-visualues about target presence, and scattered light into intact visualeld (e.g. Campion, Latto, & Smith, 1983; reviewed in Cowey, 2004).ver subsequent years all of these concerns have been systemat-

cally eliminated as explanations of specific residual behaviours.

ne elegant example made use of the optic disc as a control for light

catter. Weiskrantz (1986) found that, for 2AFC detection, patientB’s performance fell to chance when targets were presented tois blindspot, even though that location was closer to intact visual

∗ Corresponding author at: Vision Research Laboratories, School of Psychology,niversity of Aberdeen, Aberdeen AB24 2UB, Scotland.

E-mail address: d.carey@abdn.ac.uk (D.P. Carey).

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028-3932/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.neuropsychologia.2008.06.015

© 2008 Elsevier Ltd. All rights reserved.

eld than several regions where residual visual performance wasearly perfect without any awareness. Because of the absencef photoreceptors at the optic disc, non-geniculostriate systemsediating any residual visual abilities could not receive visual infor-ation from the blindspot (also see Cowey & Weiskrantz, 1963;

chärli, Harman, & Hogben, 1999). As a consequence, that particularemonstration suffices to eliminate scattered light as an expla-ation for DB’s two alternative forced choice detection ability, at

east.After this “additional control” phase of blindsight research,

ore recent experiments have started to target the likely neu-al substrates of different classes of residual vision (e.g. Baseler,

orland, & Wandell, 1999; Holliday, Anderson, & Harding, 1997)nd their psychophysical boundary conditions (Sahraie et al., 2003).ther experiments have focussed on new methodologies to mea-

ure awareness in both monkey (Cowey & Stoerig, 1995; Cowey,lexander, & Stoerig, 2008) and human (Persaud, McLeod, & Cowey,007). In spite of some remaining disagreement over methodologi-al details, philosophical implications and so on, investigators agreehat the numerous retinofugal targets in addition to the geniculos-riate system must be mediating subclasses of these behaviours.mportant remaining challenges are to identify the specific non-

eniculostriate substrates for each distinct residual ability in turn.

By far the greatest consensus is that residual localisation abilitiesat least) are likely to depend on the superior colliculus–pulvinarnd/or occipitoparietal cortex (Baseler et al., 1999; Brown,roliczak, Demonet, & Goodale, 2008; Danckert & Rossetti, 2005;

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lickstein, 2000; Milner & Goodale, 1995; Ro, 2008; Ward,anziger, Owen, & Rafal, 2002). For instance, physiological studies

n monkeys have shown that many single units in area V5 (MT) withisual receptive fields continue to function after lesions of V1, show-ng that they receive non-geniculostriate input (Azzopardi, Fallah,ross, & Rodman, 2003; Rodman, Gross, & Albright, 1989). Theseon-geniculostriate regions typically contain two-dimensionalepresentations of the contralateral (and in some cases ipsilat-ral) visual fields (i.e. pulvinar see Adams, Hof, Gattass, Webster,Ungerleider, 2000; Bender, 1981; Jones, 2007; superior collicu-

us see Mays & Sparks, 1980; V5 see Gattass & Gross, 1981, alsoee Gross, 1991 for V5 responses in the absence of V1; V6; Galletti,attori, Gamberini, & Kutz, 1999; human parietal reach region andateral intraparietal region; Hagler, Riecke, & Sereno, 2008).

A popular account which links some of these non-eniculostriate visual structures to residual sensorimotor skillas first proposed by Milner and Goodale (1995). They suggestedreinterpretation of the classic Ungerleider and Mishkin (1982)odel of two visual cortical systems in the primate. In the orig-

nal account, circuits of the so-called dorsal stream, emanatingrom V1 and flowing through subregions of the occipitoparietalortex, were linked with spatial vision such as performance onhe landmark task and perceptual localisation. The ventral stream,lso emanating from V1 but coursing through extrastriate regionsf occipitotemporal cortex, was linked with object recognition. Inhe Milner and Goodale reformulation, the dorsal stream is notesponsible for spatial vision of all sorts. Instead, it deals withmplicit, unconscious sensorimotor circuits which operate in realime to control visual guidance of action. According to this model,or manual and saccadic localisation at least, the dorsal streamegions along with the colliculus and pulvinar are the most likelyediators of spared sensorimotor skills in the visually agnostic

atient DF (Goodale, Milner, Jakobson, & Carey, 1991) as wells patients who can localise without awareness (see Goodale &ilner, 2008, for their most recent review of their model). These

ircuits should be relatively intact after geniculostriate systemamage, and in theory, respond to targets anywhere in the blindisual field.

Evaluating this model of localisation in blindsight is difficult.emonstrations of intact localisation by hand movements in theortically blind remain relatively rare. Furthermore, these inves-igations have for the most part relied on targets defined byuminance (produced by small light bulbs or light emitting diodes).he optic disc control for 2AFC detection (mentioned above) hasot been used in localisation experiments, which means that lumi-ance artefact has yet to be ruled out for this particular residualisual behaviour in the cortically blind. Although some attemptso control for or identify contributions of scattered light haveeen made in studies of manual (Danckert et al., 2003; PereninJeannerod, 1975) and saccadic localisation (Zihl & Werth, 1984),

hese are the exception rather than the rule.A second limitation of studies of manual and saccadic local-

sation in the cortically blind is that they have tended to haverather restricted number of targets that vary in position alongsingle meridian (Bridgeman & Staggs, 1982; Danckert et al.,

003; Perenin & Jeannerod, 1978, 1978; Weiskrantz, Warrington,anders, & Marshall, 1974). One exception to this rule is Wessinger,endrich, and Gazzaniga (1997) who had patients select whichf four quadrants the target had appeared in. However, becausehese participants had hemianopia they were effectively required

o make two alternative forced choice judgements (on those trialshen they had no experience of a target in the two quadrants in

he intact visual field).Combining this reliance on targets which vary in only 1 dimen-

ion with the use of luminous targets means that unintended light

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gia 46 (2008) 3053–3060

catter into the good field may have accounted for residual local-sation in at least some of the patients. However, if sensorimotortructures outwith the geniculostriate system have mediated local-sation in some patients for visual targets within the scotoma, thoseesidual abilities should survive the use of targets which elimi-ate luminance spread to the intact field and vary in more thandimension in space.

The purpose of the present investigation was to evaluateaccadic and manual localisation in patient DB, whose remark-ble abilities were first documented in 1974 (Weiskrantz, 1986;eiskrantz et al., 1974). In the current experiments, we have devel-

ped targets which control for scattered light, monitored andecorded his eye position, and most importantly, compared andontrasted his abilities with targets that varied in space along oner 2 dimensions. If these saccadic and localisation abilities dependn the dorsal stream and its subcortical associates, then we wouldxpect comparable abilities for both one- and two-dimensional tar-et arrays for both the manual and the saccadic responses.

. Participants

DB (64 at the time of testing) suffered a left homonymous hemi-nopia after surgery for an arteriovenous malformation in 1973. In976 DB experienced some return of vision in his upper visual field,owever, when DB was tested using Humphreys perimetry (30-2

ull threshold program) in 2003 (using the same stimulus sizes asreviously), the fields revealed a complete left homonymous hemi-nopia. Nevertheless, all testing reported here was carried out in theower left quadrant, an area of visual field that had remained consis-ently blind (Weiskrantz, 1986; Trevethan, Sahraie, & Weiskrantz,007). DB’s performance was compared to that of five male con-rol participants (age range 60–70), and visual acuity (due to eyeracking requirements, all of the participants had to be capable ofetecting the targets without refractive correction, as DB could inis intact visual field).

. Apparatus and stimuli

Stimulus programs were prepared on an IBM compatible PC andere generated using a specialised SVGA graphics card (VSG 2/5,ambridge Research Systems, UK). Stimuli were presented on a1 in. monitor (SONY multiscan G520) at a refresh rate of 100 Hz.o control the hosting effect discussed by Cowey (2004), we mea-ured screen luminance at locations 1◦ away from the edges of a4◦ wide 100 cd/m2 white target on a 37 cd/m2 background andound no luminance artefacts. The screen background luminanceas 37 cd/m2 at the x y chromaticity of (0.309 0.353) and subtendedvisual angle of 26.6◦ × 20.6◦ at a distance of 350 mm to the partic-

pant’s eye. The monitor gamma corrections were carried out usingluminance meter (Optical, Cambridge Research Systems, UK) at56 linear steps.

A ‘Magic Touch’ touch screen (grid resolution: 2048 × 2048)hich was mounted on the screen of the computer monitor wassed to record manual localisation responses. An audio tone sig-alled a touch on the screen. Prior to each block of testing,alibration procedures were carried out to account for parallaxrrors caused by spatial separation of the computer monitor andhe touch screen. This procedure required presentation and sub-equent localisation of each element of a 4 × 4 equi-spaced grid

n the monitor. The position of each movement endpoint wasransformed using a linear transformation from the touch screeno the monitor based on calibration data. Stimulus presentationsnd subjective responses were automatically logged for each pre-entation block. During manual localisation, eye movements were

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aalways aware of the targets in his visual periphery. Fig. 4 shows the

ig. 1. The one-dimensional and two-dimensional target arrays. Note that we havexpanded the X axis to allow for the fixation position to be shown. The target size isot to scale and the depicted contrast is illustrative only.

onitored with a 50 Hz Picolo Eye Tracking Toolbox (Cambridgeesearch Systems, UK) with a spatial accuracy of ±0.1◦ (trackingccuracy 0.25◦–0.5◦). In addition, the video output of the eye trackeras displayed on a computer monitor out of sight of the partici-ant. In addition, one investigator continuously monitored fixationompliance by placing a cursor on a visible specular image on theclera.

We used 1◦ circular stimuli consisting of two black and whiteemi-circular discs with average luminance the same as the back-round grey (37 cd/m2). The contrast of the two semi-circles was0%. For the 1D blocks we used 100 ms presentations at 1 of 4 stimu-us locations at eccentricities of 28◦, 35◦, 42◦ & 48◦ from the fixationn the 45◦ lower left meridian (see Fig. 1, left panel). Ten presen-

ations at each location were given. For the 2D blocks, the sametimuli were used, presented at 1 of 5 stimulus locations, centredt 38◦ eccentricity from fixation, with 10◦ separation between tar-ets, three of which were located on the same diagonal meridian

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s was used in the 1D blocks, with identical endpoints (see Fig. 1,ight panel).

. Procedure

Pilot testing with DB allowed us to position stimulus arrays inis scotoma such that he was never aware of the targets (i.e. wasperating in Type 1 blindsight). The participant’s head was sta-ilised using a chin/head rest and a magnified image of the rightye was obtained for monitoring purposes. Calibration proceduresere carried out while the participant pointed to each of the 16re-determined grid locations on the monitor ensuring that therid extended beyond the area where responses were expected.articipants were required to make pointing movements to all loca-ions from a Velcro-defined home position (10 cm in front of theirody aligned with their midline). Participants were required toaintain fixation, and were told that an auditory tone (700 Hz,

50 ms) indicated presentation of the stimulus in the periphery,ut to withhold their pointing movement or saccade until the fixa-ion cross was removed. Participants were encouraged to move theiryes as well as their hands towards the place they saw or guessedhe target had been presented. Participants were encouraged to

ove quickly once the fixation point was extinguished. If morehat 2 s elapsed before the touch screen was contacted, an audi-ory error tone was produced which prompted a reminder aboutpeedy responses from the experimenter. Prior to testing, stimulusocations were demonstrated by the experimenter (DB observedhese in his good visual field). This was followed by two practicerials for each of the five locations. Errors in fixation were com-

ented upon by the experimenter, reinforcing that eye positionas being carefully monitored.

Participants were also required to report if they had anywareness of the stimulus presentation using a commentary keyaradigm (Weiskrantz, 1998). They were instructed to report ver-ally, if they had any awareness whatsoever of the stimulusresentation, otherwise to report unaware. They were instructedo point to a target location, irrespective of their awareness reports.his requirement meant that in unaware mode, they had to guesshe stimulus location.

For the manual localisation tests, we performed 10 trials for eachf the locations. Breaks were self-paced by the participants, and theouch screen calibration procedure was completed after any resteriod.

The procedures were similar for saccadic localisation (whichas examined after manual localisation). The calibration proce-ure involved saccadic localisation of the same 16-element grid.ye movements were monitored by the experimenter for stabilitynd absence of blinks during stimulus presentation. In additional tohe data collected from control participants, we tested DB’s sightedeld first for the saccadic experiments as we were uncertain abouthe ease of making saccades into a ganzfeld. We defined saccadicndpoints as the averaged x and y position of the first five sam-les post-saccade which were spatially distributed within 2◦ of onenother (see Fig. 2).

. Results

Fig. 3 shows raw and averaged manual localisation data for DBnd one of the five age and sex-matched controls (HS). HS was

ean absolute (i.e. unsigned) error (in degrees) for DB and the fiveontrol participants. DB’s absolute errors (AEs) for blindfield stim-lus presentations are clearly within the range of the age-matchedontrols, in spite of his complete lack of awareness as indicated by

3056 D.P. Carey et al. / Neuropsychologia 46 (2008) 3053–3060

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ig. 2. A sample of the 50 Hz raw data from the eye tracker and the obtained saccadicndpoint sample. Five frames of data within 2◦ of one another after the high velocityaccade are averaged to estimate endpoint on saccadic localisation trials.

is trial by trial awareness judgements (i.e. all of DB’s responsesere made in Type 1 mode).

In many single case studies, patient performance is so poor

hat no statistical analysis is used. Nevertheless, individuals drawnrom neurologically intact populations do rarely perform extremelyoorly on an individual test. In most single case studies where infer-ntial statistics are used, data are analysed using procedures whichre only appropriate for groups of subjects rather than groups of

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ig. 3. Manual localisation endpoints for 1d target array. DB’s performance is in the left panata from one block of trials. Lower panels depict mean endpoints for all trials.

ig. 4. Mean absolute error (degrees) for manual localisation. The control meanppears in grey, DB’s blind field in black.

ata points from a single individual (i.e. the latter are correlatedith one another while the former are not). The procedures intro-uced by Crawford and colleagues (Crawford & Garthwaite, 2002;rawford & Howell, 1998) provide an opportunity to make a moreppropriate parametric statistical analysis of such deficits in singlease studies. These methods are surprisingly robust to violations oformality (Crawford, Garthwaite, Azzalini, Howell, & Laws, 2006).herefore, we used Crawford and Garthwaite’s procedure (2002)n differences between an individual’s score and a control sam-le. Results of this modified t-test showed the localisation absoluterror of the control group (M = 1.98, S.D. = 0.76) was not signifi-

antly different to DB’s AE (mean AE = 1.51), t(5) = −0.568, p = 0.6,s, 2-tailed (Fig. 4).

Fig. 5 shows raw and averaged saccadic endpoint data for DBnd one of the four age- and sex-matched controls (CD) for the 1D

els and one control participant (HS) is in the right panels. Upper panels depict raw

D.P. Carey et al. / Neuropsychologia 46 (2008) 3053–3060 3057

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titwat the same eccentricities when they were embedded in the one-dimensional array. It is noteworthy that although DB was shown thearray at the beginning of the saccadic and manual tests, he main-tained good 1D localisation without seeing the arrays again, in spite

ig. 5. Saccadic localisation of targets that vary in 1D for DB and one of the controlsCD). Error bars = standard error of the mean.

rray. Although DB’s saccadic residual skill is clearly less dramatichan his implicit manual ability, his absolute errors are not sig-ificantly worse than controls using the Crawford and Garthwaiteest. The mean AE of the control group (M = 3.32, S.D. = 2.02) wasot significantly different to DB’s mean AE (AE = 4.46), t(5) = 0.517,= 0.633, ns, 2-tailed. His responses seem to be clustered into tworoups of fairly similar amplitudes, although his mean responseso respect the ordering of the targets. In spite of his undershooting,e is remarkably adept at saccading along the meridian defined byhe targets, in spite of the fact that he is only shown it at the start ofny block of trials. During testing there were no marks, reflections,r scratches which could have cued any of the participants abouthe targets, beyond the very edges of the touch screen, which wouldppear in peripheral vision after the saccade (Fig. 6).

For targets that varied in 2 dimensions, DB continued to localiseeasonably well when using his hand to point (although he seems toave a blindsight “dead zone” in the area of the upper right target;ee Fig. 7). In spite of his problems with this one target, there iso significant difference between control group mean manual AE2.409◦, S.D. = 0.416◦) and DB’s blind field AE (3.505◦, S.D. = 4.127◦;(5) = 2.405, p = 0.074, ns, 2-tailed).

These manual data are in stark contrast to DB’s attempts to make

accades to the two-dimensional target locations (see Fig. 8). Forhe 2D saccadic trials, we ran nine separate blocks of 25 trials, sepa-ated over 2 days in DB’s blind field. He performs significantly worsemean AE = 18.03) than the control group (AE M = 4.10, S.D. = 1.10;

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ig. 6. DB’s mean saccadic absolute errors compared to the five controls. Their means depicted in light grey, and DB’s blind field performance is depicted in black, hisighted field in white. He was not significantly different than the controls.

(5) = 11.56, p < .001, 2-tailed). Pairwise comparisons for each tar-et reveal he is significantly worse (p < 0.01) at localising all ofhem except the upper right one (which is clearly a default posi-ion given his tendency to undershoot the array). For the occasionshere his primary saccade reached the region of the target grid, hiserformance still remains very poor.

. Discussion

These data show that DB is perfectly capable of localising targetshat vary in 2 dimensions as long as he can respond by point-ng. On the contrary, his saccadic localisation for targets in thewo-dimensional array was extremely poor, even when contrastedith his preserved saccadic localisation of three target positions

ig. 7. DB’s mean manual endpoints for the targets which varied in 2 dimensions. In spitef his poor performance in his “blindsight dead zone” (see upper left target, green,nd DB’s associated mean which appears near the lower left target). Nevertheless,is mean absolute errors for the array were not significantly worse than the controls.

3058 D.P. Carey et al. / Neuropsycholo

Fig. 8. DB’s saccades to targets that varied in 2 dimensions. The left panel depicts theraw data for 40–44 trials per target (overlap prevents them from all being visible).Note that approximately half of his saccades fall well short of the target (shown inhaf

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alf black half colour) area. Nevertheless even those saccades which reach the areare unrelated to the targets. The rightward panel depicts his mean landing positionor each target. Standard deviations are shown rather than standard errors.

f the relatively large unconstrained space that he looked into oreached into after fixation offset. Recall that in many demonstra-ions of residual localisation, visual cues to possible target locationsemain present after the response is called for, under conditionshere the possible target positions (i.e. unlit, but clearly visible

ight emitting diodes) could be inspected in intact regions of visualeld. Such a procedure implies that processes akin to forced choiceemporal discriminations might be a necessary condition for resid-al vision in some patients other than DB.

To date, our search for localisation in a series of cortically blind

atients has not revealed localisation ability (without some aware-ess) in anyone other than DB. This absence is notable because mostf our patients have intact occipito-parietal cortex and have shownome residual abilities on 2AFC detection. Theoretically this group

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s a fairly optimal one for containing at least some patients withesidual localisation.

DB’s superior localisation using his hand confirms results fromhe original experiments, reported in Weiskrantz et al. (1974) andescribed in detail in Weiskrantz (1986). One of us (Weiskrantz,997) suggested that the freedom of the head in the original manualocalisation tests may have aided DB in those conditions, in contrast

ith the head-fixed conditions used in the original saccade tests.f gaze (i.e. head and eye) were used to localise, DB’s localisationbilities could improve to levels closer to the accuracy generatedy his hand movements. However, our data suggest that this sort ofxplanation for the manual-saccade discrepancy (in DB at least) isnlikely. In these experiments, DB made his manual responses fromhead-fixed position, identical to that used in the saccadic blocks.

n spite of this constraint, his manual localisation in all conditionss superior to what he achieves with saccades. These data suggesthat that earlier methodological difference is not necessary to finduperior manual localisation in DB. We have also informally testedB’s manual localisation when we request that he withholds sac-ades. Localising in the periphery shows no obvious decrements inis manual performance. Taken together, these observations castome doubt on previous hypotheses about the saccadic/manualistinction in localisation skill in DB.

This superior performance in manual localisation is difficulto reconcile with models of eye-to-hand coordinate transforma-ions from retinal to head-centred to arm-centred codes whichnjoy considerable popularity in neurophysiology and robotics. Forxample, Andersen and his coworkers have found evidence thatany of the cells in the lateral intraparietal area (LIP) which code

or saccades to auditory targets utilise an oculocentric coordinatecheme (i.e. a scheme centred on the eye; cells elsewhere in audi-ory cortex utilise a head-centred coordinate frame, as would bexpected for the purposes of auditory localisation). In fact, reach-elated activity in the parietal reach region (PRR; an area whicheems to code hand rather than eye movements) also seems toepresent targets in eye-centred coordinate frames (Buneo, Jarvis,atista, & Andersen, 2002). Andersen and colleagues conclude thatye-centred encoding “may be a fairly general way of represent-ng space and integrating different modalities within a particularpatial representation” (Andersen, Snyder, Batista, Bueno, & Cohen,998, p. 118). If this sort of position is correct, then, if anything, goodaccadic localisation coupled with poor manual localisation is theuch more probable kind of dissociation, since manual localisa-

ion in these schemes depends on eye-centred codes which haveeen computed much earlier in the hierarchy (for more discussionf these coordinate transformation schemes and some of their lim-tations, see Carey, Ietswaart, and Della Sala (2002)). Anatomically,t is possible that more medial posterior lesions affecting the PRRould impair pointing more than saccadic eye movement (see thenteresting case report of Trillenberg et al. (2008) who report thisattern, in some ways opposite to what we have shown in DB). Nev-rtheless, the dissociation between 1 and 2D target performancesing saccades suggests a slightly more complex explanation thanhe distinction between the anatomical locations of LIP and PRR.

In summary, our data suggest that localisation in blindsight, likehe many other residual visual behaviours that have captured muchf the limelight, is worthy of a more bottom-up series of investi-ations. By identifying the psychophysical limits of these skills innumber of different cases a clearer set of hypotheses about thenderlying mechanisms can be developed. For example, we know

ay not have depended on him being shown the target positionsrior to testing (or in his good visual field). Recent experimentsconducted 1 year after the current studies) have found that, as longs his dead zone in the superior region of the lower left quadrant

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s avoided, DB can manually localise additional targets introducednto testing blocks without his knowledge. These later results havelso confirmed the dead zone in DB under unique conditions in aifferent time period.

In conclusion, our results show that manual localisation inlindsight extends to targets which vary in 2 dimensions. Remark-bly, against strong theoretical predictions to the contrary, saccadicocalisation was grossly impaired using the 2D target arrays. Otherimits of DB’s manual and saccadic skills remain to be exploredn detail. We do not know if DB’s saccadic skills can be improvedy more incremental testing (i.e. gradual introduction of targetsdjacent to a one-dimensional meridian, for example). Neverthe-ess, his experience with 2D localisation with his hands was justs limited before our experiments, yet he performed quite welln those conditions outwith his typical experimental experience.nlike our experiments on 2 alternative forced choice detection,e have had little opportunity to derive the optimal psychophys-

cal parameters for localisation, which may turn out to be distinctor saccades versus movements of the hands. We chose suddenlyppearing small targets for these studies given their obvious con-ruence with known electrophysiology of the superior colliculusnd related structures and behavioural data in cortically blindatients (Sahraie et al., 2003; Sahraie, Trevethan, & MacLeod, 2008).owever, these data suggest the intriguing possibility that saccadicnd manual localisation ability (and inability) for various stimulusharacteristics, may not be immediately predictable based on activ-ty in the superior colliculus, pulvinar and dorsal stream structures,t least in the case of DB.

cknowledgements

Many thanks to DB for all of his patience and good humoururing testing and to Mr. James Urquhart for excellent technicalssistance. This research was supported by a grant from the Biotech-ology and Biological Sciences Research Council (BBS/B/05389) toPC and AS.

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