The Jaganese Journal of Psychonomic Science

2010, Vol.29, No. I, l-16

Introduction

Early studies in visual psychophysics have demon-

strated that perception of line or dot segments and

gratings depends on their alignment relative to the

cardinal axes (i.e., horizontal and vertical). Perhaps

Jastrow (1893) was the first to show this kind of fact,

asking subjects to view a line segment and then

immediately set another segment to lie at the same

orientation. He found that the judgments of horizon-

tal or vertical segments were more accurate and less

variable than oblique segments. Craig and Lichten-

stein (1953) claimed that oblique segments that were

viewed with steady fixation disappeared more read-

ily and for longer durations than did horizontal or

vertical segments. Leibowitz, Meyers, and Grant

(1955) reported that single dots that were flashed to

+ Department of Psychology, University ofDhaka, Bangladesh (e-mail: karim. akmr.monscho06@gmail.com)

** Graduate School of Human and Socio-environ-ment Studies, Kanazawa University, Japan(e-mail: hkoj ima@kenroku.kan az aw a-u.ac.jp)

Original Article

an eccentric position (i.e., relative to fixation) were

more accurately localized if they were positioned on

or near a cardinal axis. Leibowitz (1953), Campbell,

Kulikowski, and Levinson (1966), and Regan and

Price (1986) found that grating resolution/acuity

was better for horizontal and vertical orientations

than when the gratings were at a tilt. This has been

replicated in a more recent fMRI study with gratings

(Furmanski & Engel, 2000) and in psychophysical

studies on stereo-acuity (Mustillo, Francis, Oross,

Fox, &.Orban, 1988) and vernier acuity (Saarinen &

Levi, 1995b; Skrandies, Jedynak, & Fahle,2001;Wes-

theimer & Beard, 1998).

The superior accuracy, perceptibility and dis-

criminability can be accompanied by differentials in

variance. For example, Sulzer and Zener (1953) ob-

served variability in orientation judgment to be

minimal for horizontal, near minimal for vertical and

maximal for oblique segments. Bouma and Andries-

sen (1968) found minimal variability in collinearityjudgment at horizontal and vertical positions and

maximal variability at 45o. This has been confirmed

in a more recent study on collinearity judgment

Perceptual asymmetry in vernier offset discrimination

-a similar trend between the cardinal and oblique orientations-

A. K. M. Rezaul Kentu* and Haruyuki KoJtue**

Uniuersity of Dhaka* and Kanazawa Uniueristy**

Our previous study has demonstrated that in the cardinal orientation, offset detection dependson the relative position of the vernier bars, i.e., how the left and right or upper and lower vernierbars are displaced from each other in space (Karim & Kojima, 2008, 2010). The present studyexamined whether the fact can be generalized to the oblique orientation. Experiment 1 demon-strated that for a pair of vernier bars, one displaced from the other and arranged side-by-side witha large gap between them at l45o orientation observers were, on average, somewhat better atdiscriminating an offset if the relative position of the lower right-hand bar was to the right of theupper left-hand bar than vice versa. A similar asymmetry was evident in Experiment2, whereobservers judged vernier offset for a pair of bars at -45" orientation. In that case, average acuitywas significantly better when the relative position of the upper right-hand bar was to the right ofthe lower left-hand bar than the opposite. The asymmetries in performance were consistent across

the two orientations and did not diminish with learning, thus indicating configuration-speciflcprocessing of the vernier stimuli.

Key words: perceptual asymmetry, vernier, configuration, cardinal, oblique, invariance

Copyright 2010. The Japanese Psychonomic Society. All rights reserved.

The Japanese Journal of Psychonomic Science Vol. 29, No. 1

(Greene, Frawley, & Swimm,2000). All these asym-

metries or differentials are collectively known as the

"oblique effect" (Appelle, I972).

In addition to the oblique effect, there is evidence

of some other kinds of perceptual asymmetry. For

example, in orientation and angular position judg-

ments line segments or dots are seen as being closer

to the horizontal or vertical than is true (Bouma &

Andriessen, 1968; Jastrow, 1893; Sittig & De Graaf,

1994). It has been reported that subjects are good at

finding a target when the light is lit from the top-left

(Sun & Perona, 1998). This perceptual asymmetry is

most prominent in foveal vision (Mamassian,

Jentzsch, Bacon, & Schweinberger, 2003) and can be

modifled or reduced by training or visual experience

(Adams, Graf, & Ernst, 2004: Champion & Adams,

2007).

In a recent study, we tested humans' perception of

spatial offset by means of alignment judgment

(Karim & Kojima, 2008, 2010). The study indicated

within-cardinal asymmetry depending on how the

left and right or upper and lower vernier lines were

displaced from each other. Specifically, we demon-

strated that for a pair of horizontal line segments

arranged side-by-side with a large gap between them

observers were, on average, somewhat better at dis-

criminating a vertical offset if the right-hand seg-

ment was below the left-hand segment than vice

versa. We observed a similar kind of asymmetry

when observers judged horizontal offset for a pair of

vertically oriented line segments, one above the

other. In that case, performance was better when the

lower segment was to the left of the upper segment

than the opposite. Consistent with other studies

(Adams et al., 2004: Champion & Adams, 2007) we

also showed that perceptual asymmetry reduced

more or less with training. This means that the

asymmetric phenomena are modifiable through

learning. These findings motivated us to carry out

the present study with a two-fold purpose: to see (1)

whether the within-cardinal configurational asym-

metry in line alignment judgment can be generalized

to the oblique orientation and (2) whether it can be

shaped by learning as in the cardinal orientation. To

achieve these purposes, we conducted two experi-

ments using the line vernier stimuli at *45' and

- 45o orientations respectively.

Experiment I

Method

Obseruers. Eight paid undergraduate students of

normal, or corrected to normal vision, participated in

this experiment. All the observers were naive to the

experimental purpose and did not have any history

of psychophysiological or neurological illness.

Stimuli and apparatus. Vernier stimuli, each com-

prising of two light bars either aligned or misaligned

at a 145" orientation, were generated using Borland

C+ + Builder 6. The offset sizes of the misaligned

verniers were 130, +90, +150, +210 and !270arcsec. One of the bars was displaced to the right (- )

or left (+ ) at right angles to the other, constant, bar.

The stimuli were white against a black background,

with a feature separation of 15 arcmin (Figure 1).

The width and length of each bar were 0.5 and 15

arcmin respectively. The luminance of the stimulus

and background were measured by a luminance me-

ter (TOPCON, BM-3). The Michelson contrast of each

stimulus was 0.98 (L-u*:90.43 cd/mz: Lmin:0.81 cd/m2). For displaying the stimuli we used a 2l-inch

color monitor (Eizo, FlexScan T962) of 1280x1024pixels and 85 Hz with a high-speed graphic card (3

Dlabs, Wildcat III 6110). From a viewing distance of

1.82 m, the angular resolution of each pixel was 30

arcsec.

Procedures. At the beginning, observers took a few

practice trials to get some practical knowledge of

how to respond with a keyboard. Then they were

run in two experimental sessions using the method

of constant stimuli. In one session, we presented five

possible vernier stimuli of the rightward offset (- )

and five aligned (null offset) verniers in random or-

der, each one repeating 80 times, at the centre of the

visual field. Similarly, we presented five possible

stimuli of the leftward offset (+) and five aligned

verniers in another session. The order of the two

sessions was counterbalanced between the observers

and between the training days for each observer. In

each session, the stimulus duration and response

stimulus interval were 100 ms and 1000 ms respec-

A. K. M. R. KanIvr and Haruyuki KolIue: Perceptual asymmetry in vernier offset discrimination

Figure 1. Schematic of the vernier stimuli used in Experiment 1. (a) Stimuli with rightward offset ofthe lower right-hand bar, (b) Stimuli with leftward offset of the lower right-hand bar, (c) Stimulus withnull-offset, (d) Stimuli with rightward offset of the upper left-hand bar, (e) Stimuli with leftward offsetof the upper left-hand bar. Vernier separation is defined in arcminutes as the parallel distance betweenthe endpoints of the upper left-hand and lower right-hand bars and vernier offset is defined inarcsecods as the perpendicular distance between the two bars. The dotted lines indicate theapproximate positions of the displaced bar with different offset sizes ("C" and "D" are used here andsubsequently where necessary to represent the "Constant" and "Displaced" bars respectively).

(e)(d)

tively. Observers in a dark room viewed the stimuli

binocularly using a chin and forehead rest from the

distance mentioned above. Because there was no

additional fixation point in order to avoid unwanted

positional cues available from that point, we asked

observers to always attend to the gap between the

vernier features (the centre of the display). They

indicated by a key press (F or J) whether the features

were aligned or misaligned, where incorrect re-

sponses were followed by an auditory feedback. The

two response keys were counterbalanced between

the observers.

The experiment was continued for six days, each

day covering 1600 trials in total (800 in each session).

A half of the observers were introduced to the stim-

uli with the lower-right bar being displaced right-

ward or leftward (Figure la,b) and the remaining

half with the upper-left bar being displaced in either

direction (Figure 1d,e). From the point of intersec-

tion of the polar axes (0, 0), the constant bar was

always at the same distance across the left-upper and

right-lower visual fields. However, observers were

not informed of which bar would be constant and

which one be displaced.

Data Drocessing and statistical analysis. In any

first hand data in-psychophysical experiment, the

clude perceiver sensitivity, but

with normal guessing habit and

often confounded

response bias. The

habitual guess or guessing bias and response bias are

reflected in observer's false alarm-such as responding

to a null-offset vernier as an offset one. So, we

calculated in each experimental session the propor-

tion of false alarm and the proportion of correct

offset detection at each offset for individual observ-

ers. Then, we excluded each observer's guessing bias

and response bias following two crucial steps as

below.

First, to exclude potential guessing bias we con-

verted subjective data in each training day using

Equation 1.

The Japanese Journal of Psychonomic Science Vol. 29, No. 1

v,: fff xloo (1)

Where, )zs represents the corrected number of "Yes"

responses to a vernier stimulus, and Yo and FA repre-

sent the observed proportion of "Yes" responses to a

vernier stimulus and the proportion of false alarm("Yes" response to a null-offset) respectively, in an

experimental session. Here, "Yes" response indicates

responding to any vernier stimulus as offset. This

equation converts any false alarm to '0' and 100%

correct offset detection to '100'.

Second, if an observer response were biased to-

ward a particular vernier configuration (non-uniform

distribution of response biases between the two con-

figurations) the proportion of false alarm would be

ultimately higher in that session as compared to the

session of an opposite configuration. So, we calcu-

lated subjective response biases in each training day

using Equation 2.

Response Bias:(FA,-FAt) x 100 (2)

Where, FA, and FA1 represent the proportions of

false alarm in the rightward and leftward offset ses-

sions respectively. The response bias score could

thus range between - 100 and * 100, with negative

and positive values reflecting leftward and right-ward biases respectively. A score approaching zero

indicates no response bias.

The response bias scores so obtained were ana-

lyzed in a series of one sample /-tests. If any ob-

server's response bias toward a particular displace-

ment/configuration were found to be signiflcant, the

frequency of biased responses was subtracted on

day-by-day basis from the guess-free frequency (see

Equation 1) of "Yes" responses to an offset vernier.

After such exclusion of guessing bias and response

bias, each observer's data in the rightward and left-

ward displacements were separately fitted by theprobit model (using XLSTAT;Addinosoft USA). Ac-

cording to Yesr/No paradigm, offset detection thresh-

olds were calculated at 50% correct detection of the

vernier misalignment. In order to minimize the

effect of presentation order (of the configuration) on

any pair of subjective scores, we averaged the

threshold data on every two successive days. Thus

in six days of training we obtained three pairs of

scores for each observer. We analyzed each ob-

server's threshold data in matched sample /-test and

group data in repeated measures ANOVA followed

by the post-hoc LSD test where appropriate. If the

sphericity assumption was violated for repeated

measures data we carried out these ANOVA using

the Greenhouse-Geisser (when e <.75) or Huynh-

Feldt (when l) e ).75) correction.

Results

Response bias. Figure 2 shows mean subjective

response biases calculated over the training days

(left panels) and mean daily response biases for the

two observer groups (right panels). When subjected

to a series of one-sample /-tests we found that the

average response bias scores were significantly diffe-

rent from zero for 02 (t(5): -6.217, D:.002) and 06(f(S): -2.572,2:.040) only. So, before fitting anypsychometric functions for determining the offset

detection thresholds we excluded, on day-by-day ba-

sis, the response biases for these two observers.

As shown in the right panels, the mean response

bias scores obtained on every two successive days

were not significantly different from zero for the twoobserver groups who were introduced to the upper-

left bar (Figure 2a; t(3): -.231, D:.832 in d1,d2; t(3):-.51I,p:.645 in d3, d4 and /(3):.19U,2:.857 ind5, dO) and lower-right bar (Figure 2b; t(3): - 1.197,

D:.317 in dld2; /(3): - 1.404, D:.255 in d3, d4 and

f(3):.015, 2:.989 in d5, d6) displacements respec-

tively.

Offset direction, s|atial configuration and training

effects. Figure 3a, b displays the daily offset detec-

tion thresholds for individual observers and the cor-

responding aggregates for the two groups who expe-

rienced stimuli with the upper-left and lower-rightbar displacements respectively. A series of matched

sample /-tests applied to the individual observer data

demonstrated that one (O2) of the four observers

experiencing the upper-left bar displacement (Figure

3a) had significantly lower thresholds if the offset

was rightward rather than left ward (t(2):16.806,2:.004). Two of them (O1 and 03) showed a fairlysimilar trend (t{27:2.tnr, p:.104 for Ol; t(2):4.023,

b:.057 for O3). One (06)of the four observers expe-

A. K. M. R. Kanru and Haruyuki Koitva: Perceptual asymmetry in vernier offset discrimination

0

-10

-20

-20

0

-10

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(b)

Figure 2. Response biases (MeantCls) in Experiment 1. (a) Percent biases in the upper-left bardisplacement; Left panel-subjective biases of four observers; Right panel-group mean biases on everytwo successive days of training. (b) Percent biases in the lower-right bar displacement; Left panel-subjective biases of other four observers; Right panel-group mean biases on every two successivedays of training. The positive (+) and negative (-) values indicate biases to rightward and leftwardoffsets respectively. Error bars reflect 95% confidence intervals (CIs) of the mean differences.

20

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riencing the lower-right bar displacement (Figure 3b)

had significantly lower thresholds if the offset was

leftward rather than rightward (t(2):-11.046, D:.008). One (O7) of them showed a fairly similar trend

(t(2): -2.575,p:.123) and another one (Ob) an oppo-

site trend (t(2):3.12n,1 :.089). However, line graphs

of the aggregated data for the two groups show that

average thresholds were lower when the upper-left

bar was displaced to right (Figure 3a; right most last

panel) and the lower-right bar was displaced to left as

compared to the opposite displacements (Figure 3b;

right most last panel). Though the differences were

non-significant in both the upper-left bar (F(1, 3):3.639, P:.152) and the lower-right bar (F(1, 3):.825,

D:.431) displacements, the trends were conflguratio-

nally identical irrespective of which bar was dis-

placed. This led us to plot the grand averages by

configuration (Figure 3d) in addition with plotting

by offset direction (Figure 3c), taking training as a

common factor.

We analyzed the observers'threshold data in two-

way repeated measures ANOVA with offset direction

and training as within-subjects factors. The spheric-

ity assumption was violated for training factor and

interaction, so the Greenhouse-Geisser correction

was applied. It revealed that the main effect of

training was significant (Greenhouse-Geisser cor-

rected F(1.285, 8.996):8.578, e :.643,2:.013). But

the mean effect of offset direction (F(1, 7;:.514, P:.497) and the effect of interaction between offset

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The Japanese Journal of Psychonomic Science Vol. 29, No. 1

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TFigure 3. Observer performances by offset direction, configuration, and training in Experiment 1. (a)

Four observers' individual thresholds averaged on every two successive days and their aggregate inthe upper-left bar displacement. (b) Other four observers' individual thresholds averaged on every twosuccessive days and their aggregate in the lower-right bar displacement. (c) Offset direction-wise meanthresholds for all observers on every two successive days irrespective of which bar was displaced. (d)

Configuration-wise mean thresholds for all observers on every two successive days irrespective ofwhich bar was displaced. Error bars reflect standard errors (SEs) of the means.

d1,2 (B,4 (F.6Training day

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A. K. M. R. KenIu and Haruyuki Kolttrla: Perceptual asymmetry in vernier offset discrimination

direction and training were not significant (Green-

house-Geisser corrected F(1.462, 10.232):.693, € :.73L,P:.478). We also analyzed the data by the same

statistical procedures considering configuration and

training as within-subjects factors. We found that

the main effect of configuration was fairly large (F(1,

7):4.324, D:.076) and that of training was sig-

nificant (Greenhouse-Geisser corrected F(|.285,

8.996):8.578, e:.643,2:.013). But the effect of

interaction between the two factors wase not sig-

nificant (Huynh-Feldt corrected rq(1.940, 13.583):

.864, e:.784, f :.440). A further analysis of the

training effect done by the post hoc LSD test re-

vealed that the threshold was, fairly lower on the 3rd

and 4th day (41[: 187.890, SE:4.867,2:.066) than

on the lst and 2nd day (M:202.I39, SE:8.608), the

effect being significant on the Sth and 6th day of

training (M: 177.72I, SE:6.493, D:.0I2).Finally, we examined the configurational diffe-

rences in both pre- and post-training. We considered

the first two days' training as pre-training and the

last two days' training as post-training. The pre-

training mean threshold difference between the twoconfigurations was about 15 arcsec (S-o:8.67) and

the post-training difference was about 21 arcsec (S-E

:9.26) (Figure 3d). Matched sample /-test revealed

that the pre-training mean difference was fairly large

(t(Z): 1.7 26, D :.128) and the post-training difference

was nearly significant (t(7):2.244, 2:.060). The

post-training higher mean difference was unex-

pected, however, it was not significantly differentfrom the pre-training mean difference (t(7):1.157, P

-.285).

Experiment 2

Method

Obseruers. Twelve naive and paid adults of nor-

mal, or corrected to normal vision, participated in

this experiment.

Stimuli and apparatus. We used line vernier stim-

uli, either aligned or misaligned, at -45o orientation(Figure not shown). The offset sizes, stimulus con-

trast, feature separation, feature width and length

were all identical to the first experiment.

Procedures. Following the setup and procedures

identical to the first experiment, this experiment was

run for twelve days.

Data processi,ng and statistical analysis. The data

were fitted by the probit model after excluding sub-

jective guess and response bias where necessary fol-

lowing the same procedures as in Experiment 1.

Then, offset detection thresholds were calculated at

5096 correct detection of the vernier misalignment.

In order to reduce the effect of presentation order (of

the configuration) on any pair of subjective scores,

the threshold data were averaged on every two suc-

cessive days. Thus in twelve days of training sixpairs of scores were obtained for each observer.

Then, inferential analyses of the data were done

following the same statistical tools as in Experiment

1. One of the observers (O9) showing higher false

alarm than correct detection even at larger offsets

(i.e., response by guessing) was considered unreliable

and hence excluded from the analysis.

Results

Response bias. Figure 4 shows mean subjective

response biases calculated over the training days

(left panels) and mean response biases for the twoobserver groups on every two successive days (rightpanels). When subjected to a series of one sample

/-test we found that the average bias scores were

significantly different from zero for 01 (t(11):

-2.7I0, p:.020), 02 (t!l):-3.395, 2:.006) and

010 ((11):2.877, D:.015) only. So, before fittingany psychometric functions for calculating the offset

detection thresholds we excluded, on day-by-day ba-

sis, the response biases for these three observers.

As shown in the right panels, the response bias

scores obtained on every two successive days were

not significantly different from zero for the two ob-

server groups who were introduced to the upper-

right bar (Figure 4a: t(5): - 1.358, D:.232 in dl.d2;

t(5): -.326, p:.758 in d3.d4; t(5): -.599, p:.575 ind5.d6;t(5): - I.640,P:.162 in d7.d3;t(5): -.489,P:.645 in dg.d10 and t(5):-1.301, D:.250 in d11.d12)

and lower-left bar (Figure 4b: t(4):.660, 2:.546 in dl.d2;[({):.324,P:.762 in d3.d4; /(4):.336, D:.754 ind5.d6; t(4):2.6nt, D:.057 in d7.d3; t(4):1.f,f$, p:.240 in dg.d10 and t(4): -.665, D:.543 in d11.d12)

The Japanese Journal of Psychonomic Science Vol. 29, No. I

-20

-20

(b)

Figure 4. Response biases (MeantCls) in Experiment 2. (a) Percent biases in the upper-right bardisplacement; Left panel-subjective biases of six observers; Risht panel-group mean biases on everytwo successive days of training. (b) Percent biases in the lower-left bar displacement; Left panel-subjective biases of other flve observers; Right panel-group mean biases on every two successive daysof training. The positive (*) and negative (-) values indicate biases to rightward and leftward offsetsrespectively. Error bars reflect 95% confldence intervals (CIs) of the mean differences.

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displacements respectively. The right panels also

indicate that the distribution of mean response bias

scores across the training days (whatever the degree)

was constantly leftward when the upper-right bar

was displaced (Figure 4a) and almost rightward

when the lower-left bar was displaced (Figure 4b).

These refer to the response biases toward an identi-

cal vernier configuration which did not reduce withtraining.

Offset direction, spatial confi,guration and training

effects. Figure 5a, b displays the daily offset detec-

tion thresholds for individual observers and the cor-

responding aggregates for the two groups who expe-

rienced stimuli with the upper-right and lower-left

bar displacements respectively. A series of matched

sample /-tests applied to the individual observer data

demonstrated that three (O1, 02 and 05) of the six

observers experiencing the upper-right bar displace-

ment (Figure 5a)had significantly lower thresholds ifthe offset was rightward rather than leftward (f(5):

5.832,0:.002 for 01; /(5):6.9Ut,p:.002 for 02 and

/(5):9.2f 0, 2<.001 for O5). One (O10) of the six

observers experiencing the lower-left bar displace-

ment (Figure 5b) had significantly lower thresholds

if the offset was leftward rather than rightward (t(5)

:-6.925, P:.001). Though other observers of the

two groups did not show any significant difference a

few of them showed a differential trend to some

degree (e.9., 03, 04 and O11). The line graphs of the

aggregated data for the two groups show that aver-

A. K. M. R. Kanru and Haruyuki Koltua: Perceptual asymmetry in vernier offset discrimination

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Figure 5. Observer performances by offset direction, conflguration, and training in Experiment 2. (a)

Six observers' individual thresholds averaged on every two successive days and their aggregate in theupper-right bar displacement. (b) Other five observers' individual thresholds averaged on every twosuccessive days and their aggregate in the lower-left bar displacement. (c) Offset direction-wise meanthresholds for all observers on every two successive days irrespective of which bar was displaced. (d)

Configuration-wise mean thresholds for all observers on every two successive days irrespective ofwhich bar was displaced. Error bars reflect standard errors (SEs) of the means.

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10 The Japanese Journal of Psychonomic Science Vol. 29, No. 1

age thresholds were lower when the upper-right bar

was displaced to right (Figure 5a; right most last

panel) and the lower-left bar was displaced to left as

compared to the opposite displacements (Figure 5b;

right most last panel). This asymmetry was sig-

nificant in the upper right bar displacement (F(1,5):

10.902, 0:.021) and non-significant in the lower left

bar displacement F(I, 4):1.9I2, b:.239) scenarios.

However, the trends were configurationally identical

irrespective of which bar was displaced. This led us

to plot the grand averages by configuration (Figure

5d) in addition with plotting by offset direction (Fig-

ure 5c), taking training as a common factor.

As in Experiment 1, we analyzed the observers'

threshold data in two-way repeated measures

ANOVA which revealed that the main effect of offset

direction was non-significant (F(1, 10):.611, 2:.453). However, the main effects of both configur-

ation and training were significant (F(1, 10):10.595,

P:.009 for configuration, Greenhouse-Geisser cor-

rected F(2.628, 26.27 8) : 11.862, e : .526,p ( .00 1 for

training). The effect of their interaction was non-

significant (Greenhouse-Geisser corrected F(2.740,

27 ,402): 1.881, e : .548, p:.160). A further analysis

of the training effect done by the post hoc LSD test

revealed that the 5th and 6th days' mean threshold

(M:166.999, SE:8.102) was significantly lower

than the 1st and 2nd days' mean threshold (M:202.065, 5E:7.295,p<.001). This improvement was

maintained, on average, at the 7th and 8th (M:157.974, SE:8.512, p<.001), 9th and l}th (M:155.247, SE:8.590,p<.001) and 1lth and LZth (M:15I.246, SE:7 .425, p <.001) days' follow-ups.

Finally, we examined the configurational diffe-

rences in both pre- and post-training. We considered

the first two days' training as pre-training and the

last two days' training as post-training. The pre-

training average threshold difference between the

two configurations was about 36 arcsec, the diffe-

rence being reduced to 22 arcsec in the post-training

(Figure 5d). Matched sample t-test revealed that

both these mean differences were statistically sig-

nificant (t(10):4.309, p:.002 for pre-training, (t(10)

:2.393, 2 :.038 for post-training).

Figure 6. Schematic of the vernier configura-tions in comparison. (a) 0', 90', +45" and

- 45" oriented configurations in whichaverage performance was better. (b) 0', 90",+ 45" and - 45" oriented configurations inwhich average performance was worse(Source of 0" and 90' oriented configura-tions: Karim & Kojima,2010).

General Discussion

In this study, experiments at the oblique orienta-

tion showed that there might be signiflcant or nearly

significant effect of spatial configuration, but no

effect of offset direction on average vernier acuity.

Specifically, Experiment I demonstrated that for a

pair of bars arranged side-by-side with a large gap

between them at * 45" orientation, observers were

somewhat better at discriminating an offset if the

relative position of the upper left-hand bar was to the

right of the lower right-hand bar than vice versa,

regardless of which bar they experienced as dis-

placed. That is, vernier acuity was better when the

Upper Left bar was to the Right of the Lower Right

bar (ULR-LRL configuration) than when the Upper

Left bar was to the Left of the Lower Right bar

(ULL-LRR configuration) (Figure 3d). Similarly, Ex-

periment 2 which was conducted at -45o orienta-

tion revealed that performance was significantly bet-

ter when the Upper Right bar was to the Right of the

Lower Left bar (URR-LLL conflguration) than when

the Upper Right bar was to the Left of the Lower

Left bar (URL-LLR configuration) (Figure 5d). The

results are in consistent with what we demonstrated

in a previous study at cardinal orientations (Karim &

Kojima, 2010). A rotation of the vernier configur-

ations (Figure 6) 45" clockwise indicates that conflg-

uration LU-RD (a, horizontal) corresponds to conflg-

A. K. M. R. KnnIlt and Haruyuki Kolrue: Perceptual asymmetry in vernier offset discrimination 11

urations ULR-LRL (a, *45'), UR-LL (a, vertical), and

URR-LLL (a, -45") directly in mental representa-

tion. Similarly, conflguration LD-RU (b, horizontal)

represents configurations ULL-LRR (b, +45"), UL-

LR (b, vertical), and URL-LLR (b, -45"). Thus there

is no difference between the cardinal and oblique

trends of the configurational asymmetry.

Possible mechanisms of the asymmetry. As we have

excluded observers' guessing bias and response bias

prior to fitting psychometric functions and calculat-

ing thresholds, the asymmetry can be explained by

neither of these variables. The contribution of eye

movement is also unlikely as the stimulus duration

was very brief (100 ms). Nevertheless, if there had

been any eye movements it might not have any role

in vernier acuities (Kessy, 1960), because vernier

lines have internal orientation information and may,

therefore, be less susceptible to orientational or an-

gular noise created by head tilt or eye torsion

(Waugh & Levi, 1993). So, the present results

definitely refer to perceptual bias or preference.

As vernier acuity is accomplished by the activityin orientation tuned neural channels (Campbell &

Kulikowski, 1966;Phillips & Wilson, 1984), such per-

ceptual bias might originate from neural selectivity

for a specific orientation created by the feature offset

or for a specific vernier frame created by the lumi-

nance edges and other surrounding spatial cues. Vis-

ual response properties are thought to develop in

two distinct phases: an experience independent

phase in which the basic neural circuits become es-

tablished and organized into cortical maps, and a

subsequent phase of plasticity in which initial cir-

cuits are elaborated and refined by experience (Crair,

Gillespie, & Stryker, 1998; Hubel & Wiesel, 1963;

Katz & Crowley, 2002; Sengpiel & Kind, 2002). How-

ever, it is not clear in the present study whether the

reported perceptual bias/preference can be attrib-

uted to the first candidate because until today we do

not know any inborn corresponding asymmetry of

neural organizations in the visual cortex. The sec-

ond candidate instead explains the asymmetry better

because most aspects of spatial vision (e.g., vernier

acuity, grating acuity) are quite immature in the

human neonate (Skoczenski & Norcia, 1999)and neu-

ral organization of the human visual system may be

influenced by early visual experience (Freeman,

Mitchell, & Millodot,I972: Freeman & Thibos, 1973;

Mitchell, Freeman, Millodot, & Haegerstrom, 1973).

In our study, when a roughly 50% of the observers

showed more or less better performances in a par-

ticular vernier configuration, 1096 of them (O5 in

Exp.l and 08 in Exp.2) showed fairly an opposite

trend and the remaining 40% did not show any

asymmetry at all (Figure 3a,b; Figure 5a, b). This

might be because the experiential worlds which play

a critical role in visual development (Chapman, Go-

decke, & Bonhoeffer, 1999; Coppola & White, 2004)

are not necessarily equal for all.

The above statistics also remind us of an incidence

of handedness. Annett (1967) reported that the pro-

portions of left-handers, mixed-handers and right-handers are approximately 2-5%, 25.1-3796, and

58.4-71.5%. As is often argued on the causality of

handedness, some kind of genetic factors, evolu-

tional/developmental factors such as language later-

ality, as well as cultural factors such as the prejudice

against left-handed, might be related to perceptual

asymmetry. However, because the present study

lacks in handedness data it is difficult to answer

whether the asymmetric vernier performance can be

associated with handedness or cerebral lateraliza-

tion. The existing literature in visual psychophysics

or visual neurology also cannot explain the phe-

nomenon directly. Nonetheless, we can get impor-

tant clues of the underlying reason from some re-

lated studies (Freimuth & Wapner, 1979; Levy, 1976.,

Mead & Mclaughlin, 1992). These studies have

shown importance of handedness in other visuospa-

tial tasks, with considerable performance variation

being found between dextrals and sinistrals. Spe-

cifically, in a study of aesthetic judgment of pictures

Levy (1976) found that asymmetric pictures judged

to have centers of interest to the right of the center

were preferred over their enantiomorphs (mirror-

reversed pictures), the effect being true for dextrals,

but not for sinistrals. Levy (1976) explained her

findings by the hemispheric activation model which

suggests that differential activation of a hemisphere

produced an attentional orientation bias to the con-

L2 The Japanese Journal of Psychonomic Science Vol. 29, No. I

tralateral visual field (cf. Kinsbourne,l9T4). Assum-

ing that the right hemisphere is more activated than

the left hemisphere when a dextral is engaged in

visuospatial task, she argued that such an atten-

tional bias to the left visual field would make a

symmetrical picture look unbalanced. If, on the

other hand, a picture were asymmetric in the direc-

tion opposite to the attentional bias, the net percep-

tual result would be a state of balance and the aes-

thetic response would be positive. Pictures that have

areas of greater interest or weight in their righthalves are then preferred because their asymmetries

counteract the asymmetry produced by the atten-

tional bias to the left visual fleld. Freimuth and

Wapner (1979), on the other hand, reported that dex-

tral subjects preferred paintings with left-to-right

directional properties. According to them, direc-

tional cues that suggest a left-to-right sequence may

draw attention to the rightward portions of the pic-

ture, but unlike Levy's view such an attentional

draw need not necessarily create the perceptual bal-

ance. Freimuth and Wapner's model can explain the

results of the present study better than does the

model of Levy. In our study, we have shown that a

large part of the observers performed better in the

rightward asymmetry (offset) of the vernier stimuli

at both *45' and -45o orientations. In addition, a

small part of the observers showed an opposite trend.

This led us to assume that for a visuospatial stimulus

suggesting directional cues, visual attention proba-

bly becomes clockwise (left-to-right directional) bi-

ased in the dextrals and anticlockwise (right-to-left

directional) biased in the sinistrals. In line with this

hypothesis, Mclaughlin, Dean, and Stanley (1983)

reported that dextral subjects preferred asymmetric

paintings that contained a rightward position of the

area of major interest and that sinistrals behaved in

an exactly opposite fashion. Similarly, Mead, and

Mclaughlin (1992) demonstrated that paintings con-

taining left-to-right directional cues were preferred

by both dextrals and inverted sinistrals. However,

the phenomenon is still unclear and needs further

investigation to generalize to other visuospatial

stimuli as such as vernier stimuli.

Possible mechanisms of the orientation-inuariance.

By definition, the line vernier features represent cate-

gorical (e.g., above/below, left/right), not coordinate(exact distances) spatial relation (cf. Kosslyn, 1987).

The categorical spatial relations afford invariant ab-

stract spatial relations useful for viewpoint inde-

pendent object recognition and prototypical spatial

location coding (Bullens & Postma, 2008; Jager &

Postma, 2003; Kosslyn, 1987). This invariance is

possibly achieved at some stage(s) of higher level of

cortical processing. Early cortical neurons having

small receptive fields show selectivity for specific

stimulus orientation and motion direction (Hubel &

Wiesel, 1968) and for specific spatial and temporal

frequency of luminance gratings (DeValois, Albrecht,

& Thorell, 1982: Foster, Gaska, Nagler, & Pollen,

1985;Holub & Morton-Gibson, 1981). Higher cortical

neurons, in contrast, have large receptive fields and

respond preferentially to particular shapes, textures

or patterns of complex objects (e.g., David, Hayden, &

Gallant, 2006 Hegd6 & Essen, 2000; Kobatake &

Tanaka, 1994; Logothetis, Pauls, & Poggio, 1995;

Merigan & Pham, 1998; Pasupathy & Connor, 2001;

Sheinberg & Logothetis, 2001; Tamura & Tanaka,

2001). In addition, unlike early visual (lower level)

processing which is orientation-specific in favor of

the cardinal (e.g., Campbell et al., 1966; Furmanski &Engel,2000;Saarinen & Levi, 1995b; Skrandies et al.,

2001: Westheimer & Beard, 1998), late (higher level)

processing is normally orientation-invariant (e.g.,

Biederman & Cooper, 1991; Biederman & Gerhard-

stein, 1993; McMullen & Farah, 1991; Murray, Joli-

coeur, McMullen, & Ingleton, 1993; Takano, 1989;

Tarr & Pinker, 1990). These differential response

properties of lower and higher level cortical neurons

led us to assume two possibilities for the orientation-

invariant configurational preference in our study.

One possibility is that the preference might occur

primarily at early stages of processing the spatial

frame structured by the luminance edges of the ver-

nier bars and other surrounding spatial connections.

Once the preference is thus gained for a specific

configuration the signal is then dispatched to subse-

quent higher stage(s) where it is controlled by orien-

tation-invariant mechanism. Alternatively, the pref-

erence might be directly mediated by configuration-

A. K. M. R. KanIn and Haruyuki Kotlrue: Perceptual asymmetry in vernier offset discrimination 13

seiective higher order processing which is orienta-

tion-invariant. The higher cortical neurons respond

not only to complex objects, but also to simple stim-

uli such as edges and bars (Cadieu, Kouh, Pasupathy,

Connor, Riesenhuber, & Poggio, 2007: Desimone &

Schein, 1987; Desimone, Albright, Gross, & Bruce,

1984; Pollen, Przybyszewski, Rubin, & Foote, 2002)

as in our study. The role of higher cortex to achieve

vernier acuity has also been suggested in previous

studies (c.f., Beard, Levi, & Klein , lgg7: Saarinen &Levi, 1995a,b; Sterken, Toet, & Yap, 1994) claiming

that the neurons on higher levels of processing may

use complex features to discriminate vernier offsets

(Fahle, 2004). Thus, a higher order mechanism is

suggested to operate either directly or indirectly to

achieve conflguration preference invariantly at diffe-

rent orientations.

Learning uernier acuity and configurational asymme-

try. Training improved vernier performance sig-

nificantly, but inter-individual differences were strik-

ing in our study. In the first experiment, for example,

02, 03 and 05 showed remarkable fall of vernier

thresholds in the course of training whereas 01 and

07 did not show any improvement (Figure 3a,b). The

large individual variations were also observed withextended period of training (Figure 5a,b; Exp.2). This

is in agreement with the previous similar kind of

studies (Fahle & Edelman, 1993; Karim & Kojima,

2QIQ; McKee & Westheimer, 1978; Saarinen & Levi,

1995a). For instance, McKee and Westheimer (1978)

reported that after 2000-2500 trials, the range of the

individual decrease in vernier thresholds was from

2% to 70%. An important aspect of the present

findings is that average vernier acuity improved

with training, but configurational differences per-

sisted from the beginning to the end of training

course in both the experiments (Figure 3d; Figure

5d). In addition, we unexpectedly found a littlehigher post-training mean difference than the pre-

training mean difference in Exp.l (not in Exp.2).

These demonstrations make an important difference

from our previous study in which the mean asymme-

try markedly decreased with at least extended period

of training (Karim & Kojima, 2010). This implies

that the asymmetry in oblique orientation is less

sensitive to training than the asymmetry in cardinal

orientation. it can be interpreted by the same mecha-

nism of the oblique effect (Furmanski & Engel, 2000).

That is, a much lower percentage of Vl neurons are

tuned to oblique than to cardinal orientation (Cop-

pola, White, Fitzpatrick, & Purves, 1998; DeValois,

Yund, & Hepler, 1982: Furmanski & Engel, 2000; Li,

Peterson, & Freeman, 2003). In addition, neurons

with oblique preferences exhibit wider orientation

tuning widths than neurons with cardinal prefer-

ences (Kennedy & Orban, 1979: Nelson, Kato, &Bishop, 1977;Orban & Kennedy, 1981;Rose & Blake-

more, I97 4). Thus, the asymmetry might reduce

with training at a slower rate in the oblique than in

the cardinal orientation.

In brief, consistent with our previous study incardinal orientation (Karim & Kojima, 2008, 2010)

the present study demonstrated configuration-specifi

response property in oblique orientation. However,

unlike the previous study we did not find here any

diminishing trend of the average asymmetry withtraining, indicating a cardinal versus oblique orienta-

tion difference in response to training. Though we

do not know clearly about the origin of the asymmet-

ric response, the results advance our understanding

that the relationship between vernier elements even

of simple configuration might be crucial for visual

spatial judgment.

Acknowledgements

This work was supported by the grants from Ka-

nazawa University COE program: Innovative Brain

Science; initiated by the Japan Ministry of Educa-

tion, Culture, Sports, Science and Technology. We

sincerely thank the anonymous reviewers who made

a number of constructive comments to improve the

quality of this paper.

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-Received Oct.7,2009; Accepted July 12,2010-