Perception & Psychophysics1975, Vol. 17 (5), 497-503

Foreground and backgroundin dynamic spatial orientation

THOMAS BRANDT, EUGENE R. WIST. and JOHANNES DICHGANSNeurologische Universitatskfinilc mit Abteilung fUr Neurophusioloqie, Freiburg i. Br., West Germany

The dependency of visually induced self-motion sensation upon the density of moving contrasts as wellas upon additional stationary contrasts in the foreground or background was investigated. Using twodifferent optokinetic stimuli, a disk rotating in the frontoparallel plane, and the projection of horizontallymovingstripes onto a cylindrical screen, it was found that: (1) visually induced self-motion depends uponthe density of moving contrasts randomly distributed within the visual field and, with a single contrastarea of 1/4 %, is saturated when about 30% of the visual field is moving; (2) additional stationary contrastsinhibit visually induced self-motion, proportional to their density; and (3) the location in depth of thestationary contrasts has a significant effect upon this inhibition. Their effect is considerable when locatedin the background of the moving stimuli but weak when appearing in the foreground. It is concluded thatdynamic visual spatial orientation relies mainly on information from the seen periphery, both retinal anddepth.

The perception of self-motion is based mainly onthe integration of visual and vestibular motioninformation. It has been shown, for example. thatvisual information is essential for the perception ofconstant-velocity self-motion. since the vestibularsystem detects body acceleration only (Brandt,Dichgans, & Koenig, 1973; Dichgans & Brandt,1974). The perception of self-motion, resulting, i.e.,from rotation of the visual surround about theobserver (circularvection or CV). was considered anillusion ("railroad illusion") in the older literature(Fischer & Kommuller, 1930; Helmholtz, 1896). andas a result its functional importance has beenneglected. Clearly, when the body, for example,moves to the left at constant velocity, the visual fieldmoves to the right, thus providing appropriateinformation concerning the direction and velocity ofseIf-motion. That vision is essential for the sensationof constant-velocity body motion in vehicles can easilybe detected if one closes the eyes, in which instancethe sensation of motion ceases. Neurophysiologicalevidence for the underlying visual-vestibularintegration has been provided by microelectrodestudies of units in the vestibular nucleus in severalspecies. Convergence of visual and vestibular motioninformation on these units has been found in thegoldtish (Dichgans, Schmidt. & Graf, 1973), rabbit(Dichgans & Brandt. 1972; Leopold. Schmidt. Sterc,& Dichgans, 1973), and monkey (Henn, Young, &

Support of this research was provided by the DeutscheForschungsgemeinschaft (SFB 70. "Hirnforschung und Sinnes­physiologic"). Eugene R. Wist is on sabbatical leave from theWhitely Psychology Laboratories. Franklin and Marshall College.This study was conducted while Dr. Wist was a "U.S. SeniorScientist Awardee" of the German Alexander von HumboldtFou ndation.

Finley, 1973). For all three species, bothconstant-velocity visual stimuli and angular accelera­tion of the body were found to modulate unit activity.

In contrast to vestibular information, visual motioninformation can be interpreted as either object motionor self-motion. The present study is concerned withthe critical role of stationary contrasts in determiningwhether object- or self-motion perception will occur,since under normal environmental conditions bothmoving and stationary contrasts are typically present.In riding a car, for example, there exist for theobserver the potentially conflicting stimuli providedby a stationary foreground (the framed window) and amoving background (the rearward motion of thevisual scene outside). Yet self-motion rather thanobject motion is perceived.

The present study was thus designed to investigatethe interaction of simultaneously present moving andstationary contrasts on self-motion perception. Twoexperiments were conducted. In the first, the effect ofcontrast density in the moving field in the absence aswell as in the presence of stationary contrasts was ex­amined. In the second experiment, the effect of thelocation in depth of the stationary in relation to themoving contrasts was investigated. Our hypothesis,based on preliminary observations as well as on theabove considerations, was that when stationary andmoving contrasts are simultaneously present at differ­ent distances from the observer, self-motion perceptionis more affected when either the stationary or themoving contrasts are located in the background asopposed to the foreground. This hypothesis impliesthat dynamic spatial orientation (in this case,self-motion perception) relies mainly upon back­ground information, whereas object-motion percep­tion depends predominantly upon foreground

497

498 BRANDT, WIST, AND DICHGANS

information. Thus. in analogy to the finding that theretinal periphery is dominant in determiningself-motion perception (Brandt. Dichgans, & Koenig.1973; Dichgans & Brandt. 1974), the depth peripheryis dominant as well.

METHODS

SubjectsFour male subjects were employed in the first experiment. and

five in the second. All had previous experience in motion perceptionstudies.

Apparatus and Experimental ProcedureFor technical reasons. the two experiments had to be performed

using two separate stimulation set-ups. as described below.Rotating disk system for measuring Induced tilt. A visual

surround rotating around the observer's line of sight induces anapparent body rotation opposite in direction and causes a limitedtilt of the apparent upright. The amount of this tilt can bemeasured in terms of the angle by which the test edge is displaced inthe direction of the rotating stimulus. so as to compensate for theperceived tilt in the opposite direction. Therefore. the amount ofinduced tilt can serve to quantify the potency of the stimulus ininducing apparent self-motion as well as a shift of apparent vertical(Dichgans, Diener. & Brandt. 1974; Dichgans. Held, Young &Brandt. 1972; Held. Dichgans, & Bauer. 1975).

A large, transparent. motor-driven disk, which could be rotatedaround the observer's line of sight. was mounted JO em in front ofthe subject's eyes (Figure l a). It subtended 132 deg of visual anglewhen viewed binocularly with the subject's head aligned with itscenter. A bite board was used to immobilize the head. Thetransparent rotating disk's surface was randomly covered by

Figure J. Schematic drawing of the two stimulus arrangements.(A) Rotating disk system with stationary background disk andcentral adjustment disk for indication of subjective vertical.(B) Projection of horizontaUy moving stripes in front of a stationarybackground pattern.

colored circles (green. red. yellow. and blue in equal proportions>.each subtending ',% of (he total disk area. Five different stimulusdisplays. carrying a total number of 4. 20. 40. 120, or 288 movingcircles. covering 1%. S%. 10%.30%. and 72% of the disk's area.respectively. were presented (figure 2). The disk was rotated at aconstant angular speed of SO deg/sec. while a second. stationarydisk was located 5 ern behind it. This latter disk was visible throughthe spaces between the colored circles on the transparent rotatingdisk, The effect of three different stationary backgrounds wasinvestigated: a plain white ground (no circles>. 8 circles (covering2% of the area), and 288 circles (covering 72% of the area>.

Finally. a third adjustment disk. subtending a visual angle of18 deg. was mounted on a coaxial shaft at the center just in front ofthe large disk. It carried a straight edge. produced by dividing itssurface into black and white semicircular areas. A fixation pointwas located at the midpoint of the edge. The orientation of theedge. which served as an indicator for subjective vertical. could becontrollcd by the observer by turning a potentiometer. the output ofwhich was simultaneously recorded on a strip chart. The resolutionof the measurements thus taken was I deg.

Measurements of the induced tilt of the apparent vertical wereobtained by means of continuous tracking of the visual vertical for aJO-sec stimulation period under 30 randomly presented stimulusconditions: 2 rotation directions (clockwise or counterclockwise) x 5rotating disk displays (I %. 5%. 10%. 30%. or 72% of surfacecovered with circles) x J background conditions (white background.2% or 72% of surface covered with circles). Four subjects receivedfive trials for each condition; thus 20 measurements for each of the30 stimulus cond itions were obtained. For each 30-sec record. themaximum deviation. which lasted a minimum of 8 sec. wasmeasured in degrees. The mean deviation of apparent vertical (AV)in degrees was calculated from these values tor each of the stimulusconditions.

Projection of horizontally moving stripes for measuringcircularvection latencies and magnitudes. As described earlier.rotation of the entire visual surround around a vertical axis inducesan apparent body rotation (eV) indistinguishable from real(vestibularly detected) body rotation (Brandt. Wist. & Dichgans,1971). The latencies from stimulus onset and the perceived velocityof circu larvection can be used as a measure of the potency of anoptokinetic stimulus in inducing self-referred motion perception.

A regular pattern of vertical black and white stripes. subtending7 deg of visual angle. was projected by means of a Tenniesoptokinetic stimulator on a cylindrical screen (Figure Ib) , Thesubjects sat in an upholstered chair titted with a head support sothat the distance of their eyes from the cylindrical screen was equalto its radius of curvature (79.5 em). The visual angle of the entirevisible stimulus field subtended 165 deg horizontaIly and 100 degvertically. A small stationary 0.5-deg disk. serving as a fixationpoint. was fixed in the subjects' median plane at eye level. either onthe surface of the screen or 24 em in front of it.

The perceived distance of the moving stripes was decreased to theplane of the nearer fixation point by means of the Pulfrich effect. Asis well known from the work of Lit (1949) and Pulfrich (1922). themagnitude of the change in perceived depth is a function of bothtilter density (attenuation of luminance in one eye) and stimulusspeed. The perceived distance of a unidirectionally movinghorizontal stimulus is decreased when the direction of motion istoward the tilter-covered eye. Preliminary experiments werenecessary in order to determine the stripe velocity at which theindividual subject. with one eye covered by a neutral density tilter of1.5 log units of attenuation. perceived the stripes moving in theplane of the nearer fixation point. The angular stripe velocity wasdetermined separately for each subject (range between 85 and100 dog/sec) and was employed in the experiment proper to insurethat the stripes appeared to move in a plane 24 cm in front of thescreen. In addition to the moving stripes. a similar, but stationary.pattern of vertical. opaque, black stripes could be affixedeither in the foreground (48 em in front of the screen) or in thebackground (in the plane of the projection screen).

Figure 2. Stimulus displaysused in rotating disk experimentarranged in order of increasingdensity (1%, 5%, 10%, 30%, and72% of the total surface).Adjustment disk is shown at thecenter of each display.

DYNAMIC SPATIAL ORIENTATION 499

This stationary stripe pattern was presented under twoconditions: For the equal angular size conditions. the physical sizeof the background pattern was enlarged in order to subtend thesame angular size as the foreground pattern (compare F and B, inFigure 4). For the equal physical size conditions. the angular size ofthe" stationary stripe pattern was reduced in the background(compare F and B1 in Figure 4).

Measurements of circularvection latencies were obtained bymeans of a stopwatch. The subject was instructed to start the watchas soon as the stripes began to move and to stop it as soon as heexperienced CV. Circularvection latencies were recorded for thefollowing stimulus conditions: no stationary stripes (0), stationarystripes in foreground (F). stationary stripes in background of thesame physical size (Bl ) , or the same angular size (B,). Each of thesefour conditions was presented twice, once with the stripes moving tothe left and once to the right, for a total of eight conditions (seeFigure 4). The live subjects received five trials on each of the eightrandomly presented conditions.

Continuous tracking 0.( perceived circularvection velocity over aI-min stimulation period was achieved by the subject's indicatingthe magnitude of CV velocity through turning a potentiometerwhose output was continuously recorded on a strip chart (Brandt,Dichgans, & Buchelc, 1974).

The subject was shown the standard condition consisting of thestripes moving in the plane of the screen with no stationarycontrasts present (Condition 0 in Figure 5). He was told that theCV speed he experienced had the arbitrary values of 10, which wasrepresented by setting the potentiometer dial to a position midwaybetween the zero and maximum positions. He was instructed toindicate his perceived speed of self-motion continuously for thefollowing six stimulus conditions (see Figure 5) by adjusting thepotentiometer accordingly: no stationary stripes present with themoving stripes either in the plane ofthe screen (0) or 24 cm nearer(OF), stationary stripes present in the foreground with the stripesmoving within the plane of the screen (FOO) or 24 ern nearer (FOF),and stationary stripes in the background with the moving stripeseither in the plane of the screen (80) or 24 em nearer (BF). Whenthe stationary stripes appeared in the foreground, their distancefrom the screen was always 48 em (i.e.. 31.5 em from the subject'seyes). while the background distance was identical to that of thescreen (79.5 em from the subject's eyes). These six experimentalconditions were presented in a random order to each subject. Twotrials were given per condition, one with stripe movement to the leftand one with movement to the right in a counterbalanced order,

The standard condition (0) served as a rnod cIus. For allmeasurements, the fixation point was positioned in the depth planeof the moving stripes. Even though it was the case that, withfixation here, the foreground stripes appeared slightly blurred ascompared to the background stripes, preliminary studies showedthis to have no effect on the phenomena observed. Motionperception was unaltered when fixation was on either theforeground or background stripes rather than on this fixation point.

RESULTS

Magnitude of Induced Tilt as a Function ofNumber and Total Area of Moving Contrasts

The magnitude of visually induced self-motion asmeasured by tilt of subjective vertical depends uponthe number of moving visual contrasts, randomlydistributed within the visual field. It is striking thatonly four circles attached to the transparent diskrotating in front of an unstructured white background(1 % of total area) are able to induce an average shiftof 2.5 deg in perceived upright. An additionalincrease of the percentage of moving circles within therotating disk leads to a negatively accelerated increasein induced tilt. as can be seen in Figure 3. Tilt reachessaturation at an average deviation from the truevertical of about 13 deg when 30% of the movingdisk's surface is covered by colored circles. A furtherincrease of the total number of moving contrasts to72% does not result in a significant further increase ofinduced tilt. The tendency of the counter clockwiseeffect to be slightly larger than the clockwise effect,evident in Figure 3, has previously been observed(Held, Dichgans, & Bauer, 1975).

Simultaneously Presented Stationary ContrastsInhibit the Apparent Tilt

If the stationary white background disk carrieseight colored circles (2% of its surface), then the

500 BRANDT,WIST,AND DICHGANS

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Figure 3. Mean magnitude of Induced illt of apparent vertical asa fuacdon of the denalty of the colored circles on the rotating disk.AbKIaa l'epftllenta the total area of the disk In percent covered bythe colored circles. Each circle constituted 0.25% of the total areaof the disk. Resulta for three background conditions are shown:lOUd circles for plain white background; open circles for 2%bacqr-nd (8 RatlolUU'Y clrdes); open lIQuares for 72%bacqr-nd (288 stationary circles). Since, In preJlm.lnarynperlmenta, no meuurable change In Induced illt or CV could befCMUld for the 72% background condition In the presence of 4, 20,and 40 moving circles (l %, 5"10, and 10%, respectively), theseconditions were omitted here. Top: clockwise data. Bottom:counterclockwise data.

amount of induced tilt produced by thesimultaneously presented rotary stimulus is signifi­cantly reduced, This inhibition is about one-half forthe 1% and 5% stimulus conditions and at leastone-third for the 30% and 72% conditions (Figure 3).No apparent tilt could be observed up to 120 circles(30%) rotated in front of 288 (72%) stationary ones.The inhibitory effects of such a powerful stationarybackground could be weakened only if the totalnumber of moving contrasts was also increased to 288circles. It must be taken into account that in thisstimulus situation 72% of the stationary backgroundwas occluded by the moving contrasts, thus restrictinginformation about the stationary visual environment.

Increase of Circularvection Latencies as a Result ofa Simultaneously Presented StationlUYBackground

The results of the first experiment show aninhibitory effect of stationary contrasts onself-referred motion perception. The differentialeffect of additional stationary contrasts located eitherin the background or foreground of the CV stimuluswas tested in the second experiment. As can be seen inFigure 4. the mean CV latency of 7 sec for the controlcondition (0, no stationary contrasts present) remainsunaffected when the moving pattern is viewed througha stationary foreground (F). If, however, thestationary pattern is affixed to the screen in the

background. then CV latencies are significantly (t =3.59, df = 4, P < .5) enhanced to mean values of12-13 sec (B1) , thus indicating an inhibitory effect onCV due to the presence of stationary contrasts in thebackground. CV latencies are further increased if thephysical size of the stationary background pattern isincreased (B~ in order to maintain the same angularsize as for the stationary foreground pattern.

Visuatly Induced Self-Motion is Inhibited by aSimultaneously Presented Stationary Background

The inhibition of circularvection by additionalstationary contrasts in the background can also beseen qualitatively in the tracking experiments(Figure 5). While a stationary foreground only slightlydiminishes the perceived velocity ofCV, it is markedlyreduced with a stationary background (compareFigure 5: FOF vs. BF). With stationary backgroundstripes, not only are latencies enhanced, but once CVhas been induced it is weak and unstable. interruptedby intervals in which exclusively object motion insteadof self-referred motion, is perceived.

No interruptions in CV were found for the top threeconditions in Figure 5. The total duration of CVinhibition (interruption after CV onset) within each ofthe 60-sec records for the bottom three conditions inthis tigure was calculated . Mean CV inhibitiondurations were 4.09, 1.25, and 24.5 sec for ConditionsFOF, BO, and BF, respectively. t tests for relatedmeasures indicated that significantly more CV

i..4

Figure 4. Mean latencies (bars) and standard deviations (solidUnes) of c1rculanection onset for vertical stripes moving to the leftor the right for the four different stimulus situations involvingstationary contrasts. On abscissa: 0 = moving stripes only, Inplane of screen; F = moving stripes In plane of screen withstationary foreground stripes; 8 1 = moving stripes In plane ofscreen with stationary background stripes of same physicaldlmensloDl as In F; B, = moving stripes In plane of screen withstationary background stripes of same angular size as In F. CVlatencies are maximal with the stationary background (BI • 8 2) ,

Figure 5. Individual records of continuous tracking ofcircuIarvection speed with stripe movement to the left or to the right(N = 5). The stimulus conditions are depicted schematically at theleft. Open bars in each symbol represent the moving stripes eitherin front of or in the plane of the screen, while the dark barsrepresent the stationary stripes in either the foreground orbackground. Condition 0 served as the standard situation in whichthe mean perceived velocity is indicated with an arbitrary value of10. These records show that background information is critical indetermining visually induced self-motion. CV is relatively stableover the entire I-min stimulation period In the absence of stationarycontrasts (0, OF) and even in the presence of a stationaryforeground (FOO, FOFj. However, if the stationary contrasts appearin the background (BF), CV is weak and unstable and frequentlyinterrupted by periods of object-referred motion perception.

inhibition existed when the stationary contrasts werelocated in the background (BF) than when they werelocated in either the foreground (FOF) (t = 2.90, df =4. p < .05) or in the plane of the moving stripes (BO)(t = 3.12, df = 4, P < .05). No significant differencein CY inhibition duration was found betweenCondition FOF, in which the stationary stripesappeared in the foreground, and Condition BO, inwhich they appeared in the plane of the moving stripes(t = 1.36, df = 4. n.s.).

A comparison of Conditions FOFand BF shows thatthe obtained inhibitory effect of the stationarybackground contrasts on CY was not a matter of theocclusion of moving by stationary stripes. Thiscomparison makes clear that the occluding stationarystripes located in the foreground cause very little CYinhibition. In contrast. the stationary background.when passed and not occluded by the projected

DYNAMIC SPATIAL ORIENTATION 501

moving pattern, produces marked CY inhibition.Finally, a comparison of BO and BF, in which themonocularly projected retinal images are identical,indicates that the inhibitory effect is determined bythe perceived depth interval between the moving andstationary contrasts.

As can be seen from the individual tracking tracesin Figure 5. the perceived velocity of CY decreases ifthe perceived distance of the moving stripes isdecreased by use of the Pulfrich effect, althoughangular speed is held constant (0 vs. OF). Thisfinding confirms the results and conclusions of anearlier study (Wist. Diener, Dichgans, & Brandt,1975). that linear rather than angular velocity isperceived.

DISCUSSION

In the present study, it was demonstrated that themagnitude of induced self motion as measured by tiltangle increases with an increase in the proportion ofmoving circles within the visual field. Thisrelationship may not be simply a matter of thenumber (density) of moving contrasts, but might bealso due to an increase of the total moving- area,causing a summation of simultaneously excitedmovement-detecting receptive fields. An area effecthas been previously obtained in experiments usingrestriction of coherent areas of the moving stimulus(Brandt, Dichgans, & Koenig, 1973; Held, Dichgans,& Bauer_, 1975). The present results-althoughusing a different stimulus display-elosely correspondto the earlier findings, showing that the tilt effectincreases linearly with a logarithmic increase of thetotal moving area. This correspondence, however.holds only up to a percentage of the total area of 30% .At this point, saturation was reached with therandomly distributed moving contrasts in ourexperiment. But this was not the case in theexperiments of Held et al. (1975).

The important difference between the twoexperiments is that the latter involved annulardisplays, in which the highly structured stimulus areawas confined to a specific location within the visualfield. Within this area, the number of contrasts perunit area was held constant. In the presentexperiment. area was increased by augmenting thenumber of small discrete contrasts randomlydistributed over the visual field. This procedure,besides stimulating a greater number of motionreceptive fields within a given frame, possibly involvesan additional mechanism different from the areaeffect: Increasing the proportion of moving contrastsin the visual field increases the density of theelements. Therefore. the results may also be due to aspatial frequency effect, Such an effect may beinferred from Brown's (1930 results, showing thatmoving detection thresholds are lowered as a functionof increasing spatial frequency. A spatial frequencyeffect has also been shown to exist for subjective

502 BRANDT, WIST. AND DlCHGANS

velocity of object-referred motion (Diener. Wist.Dichgans, & Brandt. Note I). A more precisespecification of the relative contributions of stimulusarea and the number of moving contrasts (spatialfrequency) cannot be made on the basis of the presentdata.

The inhibitory effect of simultaneously presentedstationary contrasts was expected. It simply reflectsthe two visual mechanisms of static and dynamicspatial orientation. In a normal environment. bothmechanisms interact. The interaction. as has beenshown. is determined by the proportional distributionof stationary and moving visual contrasts.Furthermore. in addition to this proportionaldistribution. spatial arrangement in depth is critical.

In accordance with the hypothesis presented in theintroduction. background information is of greatersignificance than foreground information. Movementin the background induces apparent self-motion.while. if the foreground moves. the stationarybackground markedly inhibits CV. In the reversedcondition. the stationary foreground causes much lessinhibition of the background induced CV. Therefore.it is appropriate to extend the earlier findings of adominance of the retinal periphery (Brandt.Dichgans. & Koenig. 1973; Dichgans & Brandt.1974; Held. Dichgans. & Bauer. 1975) for inductionof circularvection and tilt of perceived vertical to thethird stimulus dimension: depth.

Consequently. visually induced self-motion andspatial orientation rely mainly on the informationfrom the seen periphery. both the retinal and thedepth periphery.

The existence of such a mechanism is obviouslybiologically adaptive. Self-motion should be perceivedwhen sizable portions of the depth and/or retinalperiphery are in motion. since this is the inevitableconsequence of movernentof the body in space. even ifthe foreground is stationary as in passive movement ina vehicle. Stationary contrasts in the retinal and depthperiphery. with movement restricted to contrasts inthe foreground. will normally exist when an objectmoves. and should thus be associated withobject-motion perception. The foreground within thecenter of the visual field should typically be reservedfor the observation and fixation of moving objectslocalized in body rather than external spatialcoordinates. thus allowing for reaching and graspingas well as avoidance movements. Interesting in thisregard is the observation made in the present studythat stationary contrasts located in the foreground areperceived to move with the observer during CV. thusindicating a localization in terms of body coordinates.Fischer and Kornmuller (1930) also reported thisphenomenon. This conceptual scheme implies. then.

that the two functions of spatial localization andspatial orientation are associated with differentregions in space. the former mainly with the centralforeground and the latter mainly with the retinal anddepth periphery. Periphery-determined spatialorientation would appear to take advantage of thephysical fact that the density per unit visual angle ofobjects in the background is greater than in theforeground (Texture gradient. Gibson. 196M. andconsequently provides a larger and therefore morereliable sample of the visual surround.

REFERENCE NOTE

I. Diener. H. Ch .. Wist. E.. Dichgans , J.. & Brandt. Th. Thespatial frequency effect on perceived velocity. Submitted forpublication.

REFERENCES

BRANDT. TH.. DKHGANS. J., &: BUCHELE, W. Motion habuuation:Inverted sclt-motion perception and optokinetic after-nvstagmus .Expcrimeutal Brain Research. 1974. 21. 337-352.

BRANDT. TH.. DICHGANS. 1.. & KOENIG. E. Ditterentialc1kch 01 ccnt ral versus peripheral vision on egocentric- andcxocentric monon pcrcc pt ion. 1::.1:/'CI1'meI/IUI Bruin Rcsrarch.1473. lb. 4-0·491.

BRANDT. TH .. WIST, E.. &: DICHGANS, J. Optisch induziertePscudocoriolis-Ettckte und Circularvcktion: Ein Beitrag zuropusch-vcstibularcn lnteraktion. Archil, Fir Psychiatrie WId

Ncrvcukrunkhcuen, 1471. 2t4. 305·3tN.BROWN. J. F. The thresholds for visual movement. Psychologische

Forschung; 1931. 14.249-208.DICHGANS. J.. &: BRANDT. TH. Visual-ves tibular interaction and

motion perception. In J. Dichgans and E. Bini (Eds.).Ccrrbrul control ot t"C movenunts and motion perception.Bihliotheca Ophthalmologica 82. Basel: Karger. 1972.I'p. 327-338.

DICHGANS. J.. & BRANDT. TH. The psychophysics of visuallyinduced perception 01 self- motion and tilt. In F. O. Schmidtand F. G. Worden (Eds.). The neurosciences (Vol. III).Cambridge. Mass: M.LT. Press. 1974. Pp. 123·129.

DICHGANS. J .. DIENER. H. CH.. & BRANDT. TH. Optok inetico­graviccptivc interaction in different head positions. ActuOto-larvngologica. 1974.78.391-398.

DICHGANS. J.. HELD. R.. YOUNG. L. R.. & BRANDT. TH. Movingvisual scenes influence the apparent direction of gravity. Science,1972. 178. 1217-1219.

DICHGANS. J .. SCHMIDT. C. L.. & GHAF. W. Visual inputimproves the speedometer function 01 the vestibular nucleiin the goldfish. Experimental Brain Research, 1973. 18.319-322.

FISCHER. M. H.. & KORNMULLER. A. E. Optokinetischauxgclostc Bcwegungswahmehruungen und optokinetischerNystagmus. Journal [iir Psychologic und Neurologic, 1930. 41.273-3011.

GIBSON. J. J. The S<'lIses as pcrccptuul systems. Boston:Houghton Miftlin. 1960.

HELD. R.. DICHGANS. J.. & BAUER. J. Characteristics ofmoving visual scenes influencing spatial orientation, VisionRrscurclt. 1975. 15,357-365.

HELMHOLTZ, H. VON. Handbuch der physiologischen Optik,Hamburg und Leipzig: Voss, 18%. (Treatise on physiologicaloptics, English translation. New York: Dover, 1962.)

HENN, V., YOUNG, L. R., & FINLEV, CH. Visual input in thenucleus vestibularis of the alert monkey. Pfliigers Archiv,1973. Suppl. 343. R 86.

LEOPOLD, H. c.. SCHMIDT, C. L., STERC, J., & DICHGANS, 1.Der Eintluss optokinetischer Reize auf das Entladungs·verhalten vestibularer Kernneurone. PflUgers Archiv, 1973,Suppl. 339, R 77.

LIT, A. The magnitude of the Pulfrich stereo phenomenon as afunction of binocular differences of retinal illuminance atscotopic and photopic levels. American Journal of Psychology,1949,62,159·181.

DYNAMIC SPATIAL ORIENTATION 503

MACH, E. Grundlinien der Lehre von den Bewegungs­emfindungen, Leipzig: Engelmann, 1875.

PuLFRlCH, C. Die Stereoskopie im Dienst der isochromen undheterochromen Photometrie. Naiurwissenschaften, 1922, 10,553-564.

WIST, E., DIENER, H. CH., DICHGANS, I., « BRANDT, TH.Perceived distance and the perceived speed of self-motion:Linear versus angular velocity? Perception & Psychophysics.1975. in press.

(Received for publication October 9, 1974;revision received February 12, 1975.)