Acta Psychologica 31 (1969) 189-193; ~' North-H~land Publishing Co., Amsterdam ~o~ to be r0produecd in any form withom written permission from the l~ublisher

THE E F F E C T OF DARK GAPS ON THE APPARENT LENGTH

OF A MOVING LIGHT LINE 1

GORDON STANLEY and JAGDISH K. DUA L'niversity of New Engt'and, Department o[ Psychology, Armidale,

NoS.W. 2351, Australia

ABSTRACT

Magnitude estim~t~ were obtained of the length of a rotating 12 cm ilia- minored arc and of the ~me arc with either a 3 cm black gap or t~o 3 cm black gags. ~ e d was varied flgm 66 to 165 cm/~c and throughout this range all three arcs manifi~sted apparent cor~tr~.ction a~ speed increased. The greatest deet~.a~e in apparem length occurred between the speeds of 66 and 99 cm/sec, at which speeds the gaps were ~;till detected. This result throws doubt on both the Ansbache.r and Stanley acco~mts of the effect.

The apparent contraction of an illuminated arc-line rotating in a path of rotation subtending ~ visual angle of 12 degrees to a central fixation

point was ret~orted by AnSBACnER (1944) and has subsequently become known as the Ansbacher Shrinkage Effect. In his study ANSBACrmR (1944) reported that when 11 cm of a 13 cm arc-line was blacked oat,

th:~ arc did not appear to shrink to any great extent when rotating at 165 cm/sec, whereas a complete arc-line seemed to contract to one- eighth of its stationary length at the same speed. However STANLaY (1964) asing both a gap of three em and also a section of hatching six cra long in the centres of 11 and 13 cm arcAincs fou~ld nn difference between these and standard arc-lines in their susc~ptlbflity to shrinka~;e. On the assumption that the gap would produce some central impover-

ishment of excitation, this result was considered to ran counter to ANSnACnER'S (1944) overlap and contrast explanation of the e!tect.

In STANLE~'S (1964) experiment ,Ss viewed the centre of the rotating di~c at a location 30 degrees to the left of a fixation point so that the arc

moved in a circular path ranging from approximate, Iv 25 to 35 degrees from the p,Ant of the S's fixation. The present expericaent was designed to examine the effects of blacking out central sectior.,s of the arc-line at

This study forms part of a project supported by a gr~mt from the Austral- ian Research Grants Committee to the first author.

189

190 G. STANLEY AND J. K. DUA

different speeds using experimental conditions more analogous to those used by ANSBACHEI~ (1944).

ANSBACI~ER (1944) assumed that the visual system as a whole acts interrait~ently, stimulatic, a being sampled during a 'pulsation time '. On this assumption the maximal region of brightness caused by the rotating arc-:line durir.~g one pulsation interval will Ix, at the centre of the seg- ment of the path of rotation traversed by the arc during that interval. Surrounding regions in the segment would not be continuously re- stimulated daring one pulsation interval. Ansbaeher assumed that this brighter certtral region 'eclipsed' the surrounding regions of lesser brightness, i~l follows from this account that if a section in the centre of the are is b:~cked out, then a sil~;ie central region of greater brightness will not be produced dm~ing oJ.le pulsation interval. Thus it may be predicted that contraction would not occur for are-lines with such gaps. In the present experiment two conditions of gapping were employed: (a) a central region of 3 em of a 12 cm arc-line was blacked out and (b) two regions of 3 cm each were blacked out at 2 cm intervals from the leading and trailing edges of the 12 cm arc. The standard 12 cm arc was also presented as a control.

~ E TttOD

Subjects

The Ss were 15 nora-psychology students who were paid volunteers and naive with respect to thte Ansbacher Effect. Atl were tested on the Eauseh and Lomb Orthorater and found to have 20/20 acuity and no c,~her visual defects.

Apparatus

The apparatus and testing conditions were essentially as described in STAYLEY (1968) except that an optical system using a partial m~rror enabled a small fixation light to appear before and during the exposure of the disc. The arc-line was 12 cm ~n length and 0°4 cm in width iocated at ~, radial distance of 21 cm f:cm the centre of the disc. Black c~cluding sections of 3 cm in length could be screwed onto the centre of the arc-line or at locations of 2 cm from each end of the e_rc-line. The disc rotated at either 30, 45, 60 or 75 r.p.m., the speed being monitored continuously by means of an electronic tachometer. The shutter was set to allow a one second exp..;~.~re of the arc.

DARKS GAPS AND MOVING LINES 19 1

Experimental design There were three main experimental conditions: (1) no-gap arc

(2) one-gap arc (3) two-gap arc. Each S was tested individually in three one-hour sessions on consecutive days. For each session Ss were presented with one of the three main conditions, the order being deter-. mined randc~mly. Speed of arc rotation was varied, 20 estimatcs being made at each of 66, 99, 132 and 165 em/sec. Order of p:'esentation of these 89 trials was random, witt; a run restriction of two.

In the instructic0ns Ss were informed that there may or may not bc gaps present in the arc and to estimate the length from the beginning of the light region and to include the length of the gaps in their overall estimate of length. They were also told to indicate if they detccte:l a gap and/or gaps when they gave their magnitude estimates.

RESULTS

Fig. 1 shows a plot of the mean estimates of arc-leaph for each e, xper- imental condition. As can be seen in the figure there is a decrease in

N

5

o - - o TWO gacs ~

2 - o-----o One gap ~" "~

: ~,,..----I No gaps

1!,--

I [ 1 J 66 99 132 165

Speed of a rc r o t a t i o n (cm/~ .ec )

Fig. 1. Mesa magni tude estimates of length plotted again,~,t speed of arc rotation for each length of arc.

apparent len~h as a function of speed for al! arcs. An analysis of variance yielded a significant speed effect (F(3, 42) -= 32.,;.3, p < 0.001),

192 O. STANLEY AND J. K. DUA

a signific;mt gr, p eff'ect (F(2~ 28 = 4.82, p ,< 0.05) and a significant speed X'gap imzracti.~n (F(3, 42), = 3.54, p -< G.01).

DISCUSSION

The data from thiis experiment confirm the earlier finding of STANLEY (1964) that shrinkage occurs under conditions where there are gaps in the arc-lh~e. This result provides further evidence for tile inadequacy of the origiztal ANSBACtmR (1944) account of the shrinkage effect. The present results also indicate that gaps do affect the extent of shrinkage, although 'the effect of gaps is complex, as suggested in fig. 1 and in the gaps X speed interaction.

To check on the possibility that metacortrast or i~lhibitory masking of the rear ~ection.,~ of arc may be producing the contraction with the gapped arcs~ a frequency count was made of the number of gaps detect- ed in each condition. Table 1 lists the frequency c f no gap judgments fo~

TABLE I

Frequency of no gap judgments (out of possible 300) ;'or each stimulus arc for each speed of rotation.

Speed (cmlsec) Stimulus 66 99 ! 32 165

No gap 236 249 262 262 One gap 11 71 148 197 Two gap 18 38 127 191

each stimul,us arc for each speed of rotation. It can be seen from this table that no gap responses increase significantly for the last two speeds. The frequency of correct gap identifications for the one and t~o gap arcs respectively were 249 and 26t, 220 and 196, 150 and 81,102 and 54 respeet~ve!y for the spee~ls of 66, 99, 132, 165 cm/sec. These frequency counts again sl~ow that relatively correct detection of the gaps breaks down for the last two speeds. However with the possible excep- tion of the two-gap arc, the apparent length functions depicted in fig. I show a relatively continuot~s decrease. If anything, the greatest decrease in apparent length occurs between the speeds of 66 and 99 cm/sec, at :dlich speeds the gaps are being detected. Hence the appare~lt contrac- tion of the gapped arcs cannot be dismissed simply on the grounds of

DARKS GAPS AND MOVING LINES 193

the possible eclipsing of d~e following sections of are due to a meta-

contras t process.

STANLEY (1967, lC68a) has suggested that the shrinkage effect may

, e cue to a brightness integrat ion and f, mncling process (B}iK~SY, 1967)

,~'hereby the exci tatory effects of the st imulat ion are pooled over a

give ~ t empora l / spa t i a l interval. The present finding that gaps can be

de~e:teti at speeds for which shrinkage occurs also casts serious doubt

on the adequacy of this account of the Ansbacher Effect.

REFERENCES

ANSBACHER. H. L., 1944. Distortion in the perception of real mo"ement. J. exp. Psychol. 34, I - 23.

SrANL~Y, G., 1964, A study of some variables influencing lhe Ansbacher shriek- age effect. Acta Psyehol, 22. 109--118.

- - - - - , 1967. Li~tt smnmation and the ~,~rceived length of moving lines. Aeta Psychol. 26, 260--264

.............. , 1968. Apparent lenglt~ of a rotating arc-line as a function of s~eed of rotation. Acre l:'sychol. 28, 398---403.

. . . . . 19S8. Exposure-time and file apparent Jertgtb of .m illuminated r,.~at- ing arc-line, Percept. & ?syehophys. 4, 71--72 (a).

VoN I~(:_K~,sY, G., t967. Sensory inhi;fitiom Prmcelon, N.J.: Princeton Univer~,ity PrL~,