Nrumprycholog,a. Vol 25, No 6. pp. 957-963. 1987. PrInted I” Great Bntam.

0028-3932/X7 S3.00 + 0.00 4-1 ,987 Pergamon Journals Ltd.

VISUAL FIELD DIFFERENCES IN THE MAGNITUDE OF THE TILT AFTER-EFFECT*

ANNA GRABOWSKA

Nencki Institute of Experimental Biology, Laboratory of Psychophysiology, 3, Pasteur str. 02-093, Warsaw. Poland

(Receiced 17 January 1985; accepted 19 Notlember 1986)

Abstract-Following monocular adaptation to a grating tilted 10” clockwise from the vertical, subjects judged the orientation of a single bar tilted l-7” clockwise from the vertical presented monocularly for 30 msec in the left or in the right visual field. The effect of counter-rotation of the subjectively perceived orientation was observed. The value of tilt at which the apparent orientation of the bar changed from clockwise to anticlockwise was established for each individual. The effects of the right vs the left field of exposure and of the ipsiocular (the same eye adapted and tested) vs interocular transfer (different eyes adapted and tested) condition of testing on the value of the tilt after-effect (TAE) were analysed. The results showed higher values of TAE for the left field presentations. A significant loss of TAE was observed for the interocular transfer condition. The hypothesis is discussed that stronger tilt after-effect produced by the right hemisphere results from wider spreading inhibition in the net of orientation specific neurons of this hemisphere.

MEYER [ 131 has demonstrated that pink McCollough effect hues perceived by subjects after exposure to a green square-wave grating were stronger in the left visual field (right hemisphere) than in the right visual field (left hemisphere). Since McCollough illusion is regarded as an effect related to the orientation specific neurones of the primary visual cortex, this finding suggested that the hemispheric differences may arise even at such a relatively low level of processing.

The experiment to be presented here was aimed at establishing whether there exists hemispheric asymmetry in another post-adaptation after-effect, namely in tilt after-effect (TAE). This phenomenon has also been neuropsychologically localized in the visual sensory areas and has been lately supposed to be a consequence of inhibitory interactions between orientation detectors in the visual cortex [2, 11, 201.

In studies of the tilt after-effect [ 12,23,24] a grating or a line tilted several degrees from the vertical is presented for a couple of minutes. After this adaptation period another grating or a line, this time vertical, is presented. Due to adaptation the test grating or the line is perceived not as vertical but as apparently tilted in the direction opposite to the original adapting stimulus. If a clockwise tilted adapting stimulus is used, the vertical line is perceived as anticlockwise tilted and if the adapting stimulus is anticlockwise oriented the test line is perceived as apparently clockwise oriented.

Many studies have shown that the tilt after-effect transfers interocularly, i.e. it can be observed in the situation when one eye is adapted and the other tested [3,7, 15,241. GIBSON

[7] and CAMPBELL and MAFFEI [3] observed almost complete transfer, i.e. similar values of

* Part ofthe results ofthe present study were published in an abstract form in Behar. Brain Res. 12,195%196,1984.

957

958 ANNA GRABOWSKA

TAE when the same eye was adapted and tested and when one eye was adapted and the other tested. Other authors reported, however, that TAE measured with the eye which had not been previously adapted was reduced in magnitude [S, 15, 23, 241. Testing for lateral differences in TAE both in ipsiocular and in interocular (transfer) conditions seemed therefore interesting. As several authors reported that transfer was greater from the dominant to the non-dominant eye than vice versa [14, 151 in present experiment the eye dominance factor has also been taken into consideration.

METHOD In carlier studies dealing with TAE, the task of the subjects was to adjust the apparently non-vertical line in

vertical position [7.9] or to align another line. the orientation of which could be changed, with the test line 112,231. Such settings made by subjects after a period of adaptation differed several degrees from those made with no prior adaptation. To provide the possibility of the operation of adjustment, time of exposure of the test line was rather long.

As the lateral presentation of stimuli commonly used to study hemisphere differences requires very brief exposures. I had to modify these methods of measurement of the tilt after-effect. I assumed that if, following the adaptation procedure. a vertical line appears as tilted, there must be a position oftilt at which the truly tilted line will be perceived as apparently vertical. If therefore, after a period of adaptation to a clockwise tilted grating, we briefly present lines of various clockwise tilt. some of them (namely those of a very small clockwise orientation) should be perceived as anticlockwise tilted and some (those of a greater clockwise orientation) still as clockwise tilted. Certainly. the more tilted the clockwise line perceived as an anticlockwise one--the stronger the tilt after-effect. An angular value of tilt where apparent orientation of the bar changes from clockwise to anticlockwise can be regarded as the point where subjective verticality is perceived. A shift ofthe perceived verticality after exposure to an adapting grating, could then bc used as an index of TAE. Finding this value, from the mathematical point of view, is exactly the bame as determining psychophysical thresholds and therefore for its estimation we followed the Spearman distribution method [25]. Such threshold values were established for each of our subjects. The magnitude of TAE was dcfined as the dinercnce between the angular values of a tilt where suhjectibe verticality was perceived after exposure to adapting grating and those in non-adaptation condition.

The experiment consisted offourexperimental and two control sessions. Theexperimental sessions started always with an initial 3 min adaptation period to a square wave grating of2 c/degree tilted 10’ clockwise from the vertical, coverIng a circular area with a diameter of 5.3 (Fig. I). Then the grating was removed and a pulsating red light

adapting grating test bar

I;K,. I, An evamplc ofadaptIng and test stirnull used 111 the experiment. The adapting grating is tilted IO clochwiac and the teht bar 7 clockwihc from the vertical. The dot in the middle marks the lixation

point.

li\arlon point \\a\ presented. The subject.\ \cerc instructed to tixate It and then to elicit ;I short (30 msec)cxposure of a bar h\ preshlng a button held in their handa. The bars had the same width ~5 the barb ofgratmg and various (1 7 ) clock\vjrc tilts. They ucrc prcscnrcd randomlv on the left OI- on the right side of a circular licld of the same diamctcr a:, the adapting lield.* The cli\tancc from thctisation point to the middle of the bar suhtendcd a visual angle of 2

VISUAL FIELD DIFFERENCES IN THE MAGNITUDE OF THE TILT AFTER-EFFECT 959

The subjects had to judge whether the bar was clockwise or anticlockwise tilted. Each experimental session consisted of a series of 70 such short exposures of bars of various tilts presented in random order, providing five estimations for each of the seven possible bar tilts in each visual half-field. After each of the test exposures, a period of4 set darkness was allowed during which the subjects made their estimations. Since the effect of adaptation could decrease with time, each of the consecutive trials was preceded by a period of 15 set readaptation to grating. The initial adaptation and readaptation periods were similar to those commonly used in experiments on TAE [lo, 23, 241. During the adaptation periods observers were instructed to move their gaze across the grating to prevent generation of after images.

As both the adaptation grating and the test bars were presented monocularly, four experimental sessions containing four combinations of the adapted and tested eye were conducted (Fig. 2). In two of them, the same eye, either dominant or non-dominant was adapted and tested (ipsiocular conditions). In the other, two different eyes were adapted and tested (interocular transfer conditions).

IPSIOCULAR INTEROCULAR

D eye D eye D eye ND eye

ND eye ND eye ND eye D eye

FIG. 2. Experimental conditions. A monocular adaptation of D (dominant) or ND (non-dominant) eye to a grating was followed by short exposures of bars randomly on the left or right side of a circular field. The bars were presented either to the same eye as the adapting grating (ipsiocular condition) or

to the other eye (interocular condition).

In addition, two control sessions which did not contain adaptation procedure were conducted. Series of bars of seven different orientations (l-3” clockwise, l-3” anticlockwise and vertical) were presented monocularly for 30 msec randomly on the left and the right side of the circular field. In one session, the dominant eye and in the other the non-dominant eye was tested. Again, the subjects were asked to decide whether the perceived bar was clockwise or anticlockwise tilted, and the angular values of the perceived verticality were calculated in a similar way to that of the experimental sessions. These values, usually very small, were then treated as a baseline for the estimations of TAE in experimental conditions.

The stimuli were presented in a three channel Scientific Prototype tachistoscope. The order of all six sessions was randomized across the subjects. Since the tilt after-effect persists, the subjects did not begin another session until at least 4 hr had passed after finishing the last one. Prior to the collection of the data, the subjects were familiarized with the apparatus and stimuli and given some practice in making judgements of the bar tilt.

Twelve adults-8 females and 4 males, ranging from 2435 yr of age served as subjects. The observers had adequate vision. All of them were right-handed and none had any known history of left-handedness. The eye dominance was determined by two sighting-dominance tests and the subects’ reports. In one of the tests, we checked which eye was chosen in a task demanding monocular viewing by looking through a telescope. In the other, observers viewed an object with both eyes through a tube (which was kept at a certain distance), and then closed the eyes alternately. The eye with which the objects remained visible was determined as dominant. Eight subjects showed right eye dominance and four left eye dominance.

960 AWA GKAHOWSKA

The post-adaptation data

RESULTS

In all of the tested subjects, adaptation to clockwise tilted grating produced a shift in the anticlockwise direction in the perceived orientation of the test bars. A sample of the obtained data is given in Table I.

Table 1. An example of data obtained by one of the subjects in an interocular transfer condition. The numbers represent frequency of evaluation of each of the seven bars (each presented five times in the left and right visual fields), as clockwise or anticlockwise tilted.

The estimations of subjective verticality by the Spearman distribution method were 4.3 and 3.1’ anticlockwise for the left and right visual fields, respectively. As in non-adapt-

ation control condition the analogous values were 0.1 anticlockwise for both visual fields, the magnitude of TAE in the considered example was equal to 4.2’ in the left visual held

and to 3.0‘ in the right visual field

Visual fields

Subjectively perceived

orientation Stimuli (clockwise tilt in degrees)

1 2’ 3’ 4‘ 5 6’ I

Left Clockwise 0 0 0 3 4 4 5 Anticlockwise 5 5 5 2 1 I 0

Right Clockwise 0 1 2 4 5 5 5 Anticlockwise 5 4 3 I 0 0 0

The values of the tilt after-effect were determined separately for each subject in each of the four experimental sessions. The influence of various exposure conditions on the magnitude of the tilt after-effect was assessed by 2 x 2 x 2 three-way analysis of variance (A x B x C x S type) with the field of exposure of the test line (left or right), ipsiocular vs interocular condition of testing (the same eye or different eyes adapted and tested) and eye dominance (the tested eye dominant or non-dominant) as factors. Since the results obtained by subjects who showed left and right eye dominance were very similar, the two groups of subjects were combined.

The analysis of variance (Table 2) indicated significant effects of the first two variables. The interaction between the two factors was also significant at the level of PcO.05. Table 3 shows the values of the tilt after-effect averaged over all the subjects, obtained in various experimental conditions. There is a marked advantage of the left side presentations over the right side presentations in the strength of the produced tilt after-effect. The difference is

Table 2. ANOVA F ratios showing effects of visual field, ipsi- vs interocular testing and ocular dominance on the magnitude of TAE

Factors F 4 P

A (L visual field/R visual held) 13.24 l/l 1 0.005 B (ipsiocular/interocular) 12.5 l/11 0.005 C (dominant/non-dominant) 2.42 l/l 1 “.S.

AB 5.97 l/l 1 0.05 AC 0.6 l/l I “.S.

BC 1.6 l/11 “2.

ABC 0.0 l/11 “.S.

WSLJAL FlELD DlFFERENCES IN THE MAGNITUDE OF THE TILT AFTER-EFFECT 961

Table 3. The values of TAE averaged over subjects and corresponding standard deviations (in brackets) obtained in different conditions of the experiment

Left visual field Right visual field Ipsiocular condition Interlocular transfer Ipsiocular condition Interlocular transfer

Dominant Non-dominant Dominant Non-dominant Dominant Non-dominant Dominant Non-dominant

eve eve eve eye cyc cyc cyc w

4.13 4.19 2.81 3.66 3.13 2.96 2.33 2.9 (1.01) (1.18) (1.06) (1.48) (0.92) (0.82) (1.46) (1.13)

4.16 3.24 3.05 2.62 (0.84) (0.91) (0.60) (0.84)

3.70 2.84 (0.78) (0.66)

present both when the same eye was adapted and tested and, though (as implied by the interaction) to a smaller degree, in interocular transfer condition. It is also evident that transfer yielded a significant loss of the tilt after-effect. Although the analysis of variance did not show either a significant effect of the eye dominance or interactions between this and the other two factors, it can be noticed that in transfer condition where the non-dominant eye was tested (dominant eye adapted), the magnitude of TAE was higher than in the situation where the dominant eye was tested (the non-dominant eye adapted). This tendency fits well with what has been reported by other authors.

The non-adaptation control data

The data were analysed for significance by two way analysis of variance, the side of the exposure and eye dominance being the main factors. The analysis did not show a significant effect of either factor (Fwas equal to 0.16,0.92 and 0.18 for the two factors and the interaction between them, respectively). The mean shifts from the vertical were 0.59” and 0.49” in the anticlockwise direction for the LVF and RVF presentations, respectively.

DISCUSSION

Following adaptation to a grating oriented 10” clockwise from the vertical, subjects perceived a briefly presented bar of a small clockwise tilt as anticlockwise tilted. The magnitude of the tilt after-effect was comparable to values reported by other authors [3, 7, 12, 23, 241. The data indicated significantly higher values of TAE for the left hemifield presentations of the test bar than for the right one. Since the exposures of the test bar to the left/right visual fields tested the effects of adaptation in the right/left hemispheres, the results suggest that the right hemisphere produced stronger tilt after-effect. The difference between the two hemifields was absent in non-adaptation condition. The results therefore cannot be accounted for by lateral differences in discrimination of line orientation demonstrated by several authors [ 18, 221.

It should be mentioned that since in the present experiment only the clockwise oriented adapting grating was used, the conclusions must be limited to that particular situation. One may, however, surmise that similar effects could be also obtained with anticlockwise oriented grating.

Current studies provide evidence that the mechanisms underlying the tilt after-effect and tilt illusion (a similar phenomenon but in simultaneously presented lines of different tilt) are

962 ANNA GKABOWSKA

identical and that both phenomena result from lateral inhibition between orientation detectors in the visual cortex [2,4,6, 15, 19-211. It is assumed that when a line stimulus of a given orientation is exposed a population of neurons tuned to that and similar orientations is excited. The excited neurons send inhibitory signals to units tuned to neighbouring orientations, thus reducing their sensitivity. Due to such inhibitory interactions a prior or simultaneous inspection of an additional stimulus of nearby orientation can change the distribution of excitation in the population of orientation detectors yielded by the exposure of the test stimulus. Consequently the perceived orientation of the test stimulus can be distorted.

According to this hypothesis the magnitude of TAE should depend on the amount (spread) of inhibition exerted by the excited neurons on their neighbours. The present finding of stronger TAE in the right hemisphere can be, therefore, well explained by postulating wider spreading inhibition in that hemisphere.

This hypothesis seems to be attractive since it fits well with some other findings, for example, that of the right hemisphere predominance in a line orientation discrimination task. One of the effects of inhibitory interactions is narrowing of the band of orientations to which a given neuron responds [4] which in turn causes the system of cells to be more discriminative for different orientations. Due to stronger inhibitory interactions the right hemisphere might discriminate better between orientations. Such finding has been reported by several authors [ 18,223. The same idea could be extended to other feature detecting cells, for example, cells which respond to colours. Similar inhibitory interactions could improve discriminations between colours. In fact, there are studies which show the advantage of the right hemisphere in colour perception [S, 16, 171.

Another finding in this experiment was that TAE was reduced in magnitude in the interocular transfer condition in comparison to the situation where the same eye was adapted and tested. This effect has been demonstrated by other authors [ 1.5,23,24]. To explain this reduction it has been supposed that the magnitude of TAE is determined by a weighted average of adapted and unadapted channels [ 1, 15, 241. The after-effect measured with the eye which has not been previously adapted is reduced in magnitude by the contribution of the activity of the unadapted monocular channel that could be activated only by monocular input to that eye. The same factor may also account for the slightly smaller advantage of the left side presentations over the right one in the interocular transfer condition than in the ipsiocular condition.

The present results yielded also some implications for the discussion concerning the level of processing at which the hemispheric differences emerge. They suggest that the hemispheres may differ at a relatively early stage of visual processing subserved by the orientation specific neurons of the visual cortex.

Ackno~oledgem~nts-The author thanks Professor M. A. Jeeves and the referees for their valuable comments which helped to improve the paper and Dr L. Szymaliski for his advice and assistance in statistic matters.

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