- Email: [email protected]
Right hemisphere superiority for initial stages of letter processing
Neuropsychologia. Vol. Ii. pp. 653 to 660. C Per gamon Press Ltd. 1979. Printed in Great
RIGHT
Britain.
HEMISPHERE SUPERIORITY FOR OF LETTER PROCESSING* JOSEPH B. HELLIGE and Department
of Psychology,
University
Los Angeles, (Received
INITIAL
STAGES
RON WEBSTER
of Southern California, CA 90007, U.S.A.
University
Park,
IS April 1979)
Abstract--When single letters, which could be perfectly recognized when presented alone, were embedded in an overlapping masking stimulus, observers recognized more letters from the left than from the right visual field. This left visual field-right hemisphere advantage persisted over short time intervals between the letter and the mask, regardless of which stimulus occurred first. Such results suggest that the right cerebral hemisphere is more efficient than the left at extracting relevant visual features of letters when the letters are perceptually degraded, even though letters are highly associated with language and, therefore, readily processed along verbal-analytic dimensions.
THERE is a considerable amount of clinical and experimental evidence that the right cerebral hemisphere is more efficient than the left at extracting complex visuospatial information
from visual displays [l-5]. Much of the evidence for this hypothesis comes from experiments in which stimuli are presented briefly to the left or right visual field and, therefore, projected directly to the right or left cerebral hemisphere, respectively. If the right cerebral hemisphere is specialized for processing visuospatial relationships, we might expect the left visual field-right hemisphere (LVF-RH) advantage for complex visuospatial stimuli to be enhanced when the stimuli are perceptually degraded, thereby making the extraction of visuospatial features and relationships more difficult. Several recent experiments have produced results suggesting that this is the case (e.g. [6-S]). dimensions, the initial Even when visual stimuli can be processed alon g verbal-analytic stages of information processing must involve such things as the extraction of relevant visual features and spatial relationships among those features. It is quite possible that the right cerebral hemisphere continues to be more efficient than the left cerebral hemisphere for these initial stages of processing. If so, then stimulus degradation effects similar to those reported for nonverbal tasks should be found for tasks that allow or even demand verbal processing. That is, as the stimuli become perceptually degraded, the visual laterality pattern should shift away from that pattern obtained with clear stimuli and in the direction of an LVF-RH advantage. There is, in fact, some experimental support for this hypothesis. HELLIGE [9] asked observers to indicate whether two simultaneously presented letters of different cases had the same name, a task which requires verbal processing. Presenting the letter pairs behind a grid of masking lines produced an LVF-RH advantage, even though a right visual field-left hemisphere (RVF-LH) advantage is typically found for tasks that *The research reported herein was supported National Science Foundation (BNS76-80093).
in part
653
by a research
grant
to the first author
from
the
644
JOSEPH
B.
HELLIGEand RON~~EBSTER
require verbal processing. BRYDENand XLL.\RD [IO] had observers identify single letters and varied visual field of letter presentation and type-face of the letters. For relatively familiar. printed type-faces, they found an RVF-LH recognition advantage. Honev,er, for elaborate, unfamiliar script type-faces they found an LC’F-RH recognition advantage, even though a verbal letter identification response vvas required. It seems likely that the extraction of relevant visuospatial relationships is more difficult for elaborate, unfamiliar type-faces. In a very different letter recognition paradigm, POLICH [I I] had observers indicate which one of four alternative letters was presented by pressin g one of four keys. A letter was either presented alone or flanked on either side by an outline square. No visual field differences vvere found in the letter-alone condition, but an LVF-RH reaction time advantage was found in the more visually complex condition involving letters plus flanking squares. The results just reviewed are interesting because they su,,UOest that, while the left hemisphere seems to be specialized for stages of verbal-analytic encoding, it is the right hemisphere that is more efficient at the relatively early stages of visuospatial processing-even for letters, which are highly associated with language [l2-141. It is important to employ converging operations to determine the generality and limits of such results. Accordingly, the present experiment was designed to examine further the effect on letter recognition of presenting a visual pattern mask, to examine effects of target-mask feature similarity, and to determine the temporal characteristics of any pattern mask effects. In the present experiment. observers were required to identify which one of 10 uppercase letters was presented on each trial. Viewin, n conditions were chosen so that letter identification was perfect from both the left and right visual fields. To make the extraction of visual features more difficult, a visual pattern mask was presented to both viewing fields at various times before and after the target letter. The mask either contained features that were similar to those of the target letter or contained features that were somewhat different from those of the target letter. If the right hemisphere is specialized for extracting the relevant features of letters from complex displays, then an LVF-RH advantage should be found when the target letters and mask occur simultaneously. When two visual stimuli occur very close together in time, they are treated by the central visual processing system as if they had been simultaneous [15-IS]. Therefore, the LVF-RH advantage should persist over short time intervals between the target letter and mask. Recent experiments indicate that letter identification is more difficult when the target letter and mask have similar features than when they have different features [15, 16). Therefore, the LVF-RH advantage may be larger and extend over longer temporal intervals in the similar-feature condition than in the dtfferent-feature condition. Finally, ability to identify the target letter should gradually improve as the time interval between the target letter and mask increases. As performance increases-that is, as the target letter becomes less degraded by the mask because of temporal separation-the LVF-RH advantage should decrease.
IMETHOD The observer sat at a table facing a 23 ? 15cm dark gray rear projection screen located about 49 cm away at eye level. In front of the observer on the table was a small gray box with a red button on top and a white card containing the IO target letters. A Gerbrands three-channel projection tachistoscope (Model Gl 176) equipped with a Gerbrands 300 series timer and a random access slide projector (Kodak Ektagraphic RA960) were used to rear project visual stimuli onto the display screen at the appropriate times. The time intervals used for the present experiment were well within the limits of the apparatus.
RIGHT
HEMISPHERE
SUPERIORITY
FOR INITIAL
STAGES
OF LETTER
PROCESSING
655
Slimulus materials Target stimuli consisted of 10 upper-case letters: 5 letters composed of straight lines (A, E, H, N, T) and five letters composed mainly of curved lines (B, D, G, Q, R). The target letters were photographed and prepared as 35 mm slides. When projected, each target appeared as a white letter on a dark gray background. The target letters subtended about 1.2’ of visual angle horizontally and vertically and were centered about 5.2” of visual angle from the center of the viewing screen. Two pattern masks were used, each appearing as white line segments on a dark gray background and each consisting of line segments that were the same stroke width as the stroke widths of the target letters. One mask was composed only of straight line segments and the other mask was composed only of curved line segments. Each pattern mask subtended about 4.7” visual angle horizontally and 3.5” of visual angle vertically. On each trial of the experiment, one of the pattern masks was projected simultaneously to both the LVF and RVF; that is, two identical masks were prepared on a single 35 mm slide so that, when projected, each mask was centered about 5.2” of visual angle to the left or right of the center of the viewing screen. Prior to each experimental trial a white fixation dot, subtending about 0.33” of visual angle, appeared in the center of the viewing screen. Both the target and mask fields had a luminance of 1.1 cd,‘m’ (0.3 ftL) and the fixation dot had a luminance of 0.33 cd,‘m2 (0.10 ftL). Procedure Observers were told that on each trial of the experiment they should fixate on the center of the viewing screen and press the red button in front of them when ready. When the button was pressed, a white dot appeared in the center of the screen and remained for 1 sec. Observers were told that LVF and RVF stimulation was equally likely so that their best strategy was to fixate their gaze on the central dot and maintain that fixation until after a target and mask had been presented. At the termination of the fixation dot, a target-then-mask trial (backward masking) or a mask-then-target trial (forward masking) was presented. Observers were required to say aloud one of the IO target letters as their best guess. Each observer received a total of four 210-trial blocks, two blocks during each of two If hr sessions separated by 24-48 hr. During each session, one 210-trial block used the straight segment mask, while the other 2IO-trial block used the curved segment mask, with mask-type order counterbalanced between observers. Each ‘IO-trial block consisted of 21 IO-trial blocks, one for each of the following target-mask intervals or stimulus onset asynchronies (SOAs): 0 msec; target preceding mask by IO, 20,30,40,50,60,80, 100, I20 and 180 msec; and mask preceding target by the same IO SOAs. The order of these 21 SOAs was randomized for each 210-trial block for each observer. All IO trials for a particular SOA were presented successively and consisted of one presentation of each of the IO target letters, five letters presented to the LVF and five letters presented to the RVF. Both visual field and specific letters were randomly ordered within these IO-trial blocks. For each SOA and mask type combination, letters that appeared in one visual field during the first session appeared in the opposite field during the second session. As a result, each observer received one presentation of each of the 10 targets in each visual field at each SOA for both mask types. Prior to the experimental trials, the observers were given approximately 5 min to adapt to the reduced level of illumination and were then shown each letter Rashed on the screen for 7 msec without any masking stimulus. No observer had difficulty identifying all IO letters under these conditions. Observers were also shown the masking stimuli and given a chance to ask questions about the procedure. During the experiment proper, the target and mask durations were both 7 msec. After approximately every 105 experimental trials, observers were given a short break and reminded of the importance of fixating appropriately on the center dot when it appeared. Subjects Eight men and eight women volunteers from introductory psychology courses at the University of Southern California participated in the present experiment. All subjects were right-handed, native speakers of English with normal or corrected-to-normal vision in both eyes. A preliminary analysis indicated that there were no effects of sex of subject so that all results will be presented collapsed over the sex variable.
RESULTS Figure 1 shows the percentage of correct target letter recognitions as a function of SOA. When the target letter and mask were presented simultaneously (SOA = 0 msec), 15 of 16 observers recognized more letters from the LVF-RH (percentage correct = 54.5) than from the RVF-LH (percentage correct = 39.5). In an analysis of variance for an SOA of 0 msec the main effect of visual field was highly significant, F (1, 15) = 18.107, P < 0.001. At this SOA observers also recognized slightly more targets in the Different feature condition
JOSEPHB. HELLICEand Ros WEBSTER
656
(49.5 :)A) than in the Same feature condition (44.5 “‘,), F (I,15)= 1.694, P < 0.05. For an SOA of 0 msec there uar no Same vs Different x Visual Field interaction, F < 1. As Fig. 1 shows, target recognition generally improved as SOA increased, in both forward and backward masking. The forward masking functions reached asymptote at an SOA of about -40 msec, while the backward masking functions reached asymptote at an SOA of +80 msec. These observations are supported statistically by the results of an analysis of variance with the following within-subjects factors: SOA (10-180 msec), Forward vs Backward masking presentation, Same vs Different target-mask feature relationship, and Visual Field of target presentation. Specifically, the effects just observed are supported by a significant main effect of SOA, F (9, 135) = 94.095, p < 0.001, a significant main effect of Forward vs Backward, F(1,15) = 39.688, P < 0.001, and a significant SOA x Forward vs Backward interaction, F (9, 135) = 21.782, P < 0.001.
SAME ‘0%
FEATURES
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80-
. LVF-RH 0
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,
,
, , , , , /
RVF-LH
, / , , [ ,
DIFFERENT
/
/
, /
FEATURES
4
I
I -
I80
,
I
I!/
I
- I20 -80 - 40 STIMULUS
ONSET
/
1, / 40/, 80, I
0
ASYNCHRONY
!
I
I20
I
I?”
IMSEC)
FIG. I. Percentage of correct target letter recognitions from the left (LVF-RH) and right (RVF-LH) visual fields as a function of stimulus onset asynchrony (SOA). Negative and positive SOAs refer to forward and backward masking, respectively. Results for Same-feature and Different-feature conditions are given in the upper and lower panels, respectively.
The LVF-RH target recognition advantage found at 0 msec SOA persisted across short nonzero SOAs in both forward and backward masking, producing a significant main effect of visual field averaged over nonzero SOAs, F (1,15) = 7.85 1, P < 0.02, and a significant Visual Field x SOP_ interaction, F(9, 135) = 3.383, P < 0.001. There was also a significant Visual Field x SOA x Forward vs Backward x Same vs Different interaction, F (9, 135) = 3.428, P < 0.001. This interaction was investigated further by subsequent, simpleeffects tests comparing the two visual fields at each nonzero SOA for both the Same-feature and Different-feature conditions (via further partitioning of the interaction sums of squares).
RIGHT
HE.VlSPHERE
SL-PERIORITY
FOR INITIAL
STAGES
OF LETTER
PROCESSISG
657
Target recognition was significantly higher on LVF-RH trials than on RVF-LH trials for SOAs of -40, -20, - 10, + 10 and +30 msec in the Same-feature condition and for SOAs of -30, -20, - 10 and +20 msec in the Different-feature condition, while there were no field differences at other nonzero SOAs (P < 0.05 for the set of simple-effects comparisons). Note that the LVF-RH advantage persisted over longer forward and backward masking SOAs in the more difficult Same-feature condition. The Different-feature target recognition advantage found at 0 msec SOA also persisted over short nonzero SOAs. As this is difficult to see in Fig. 1, the data are replotted in Fig. 2, collapsed over visual field. The pattern shown in Fig. 2 produced a significant main effect of Same vs Different, F (1, 15) = 15.044, P < 0.001 and a significant Same vs Different x SOA interaction, F (9, 135) = 2.032, P < 0.05. lOOLo
:
60-
2
5 g40-
.
DIFFERENT
f
LI -180
I -Ix)
I
I
-80
STIMULUS
Illllllll!lll 40
-40
0
ONSET
ASYNCHRONY
I 00
I
I
I
120
I80
I
(MSEC)
FIG. 2. Percentage of correct target letter recognitions from the Same-feature (SAME) and different-feature (DIFFERENT) conditions as a function of stimulus onset asynchrony (SOA). Negative and positive SOAs refer to forward and backward masking, respectively.
DISCUSSION The results of the present experiment are consistent with the results of the experiments reviewed in the introduction and support the hypothesis that the right hemisphere is more efficient than the left at extracting relevant visual features of letters when the letters are perceptually degraded by being embedded in a complex visual display. As shown in Fig. 2, letter identification was more difficult when the target and mask had the same features than when they had different features [15, 161. That is, the target letters were more perceptually degraded in the Same-feature condition. Consistent with the above hypothesis, the LVF-RH advantage persisted over slightly longer temporal intervals in this more degraded Samefeature condition than in the Different-feature condition, although at the shortest SOAs there was a consistent LVF-RH advantage for both feature relationship conditions. The present results, and the others reviewed earlier, suggest that the relative efficiency of the right and left cerebral hemispheres depends on the stage of letter processing under consideration [8-141. The right hemisphere appears to be more efficient at very early visuospatial processing stages, or at what BRYDEN and ALLARD [IO] refer to as preprocessing, while the left hemisphere is more efficient for somewhat later stages such as verbal encoding (e.g. attaching letter names to visual features) and verbal memory comparison (e.g. comparing two letter names) (7-121. Both HELLIGE [9] and BRYDEN and ALLARD [IO] have suggested that the overall visual field (or hemisphere) advantage observed in any particular experimental condition depends on which of these two general kinds of processing (visuospatial vs
658
JOSEPHB. HELLICEand RON WEBSTER
verbal-analytic) is more difhcult. Performance is likely to be better when the stimulus material is projected directly to the cerebral hemisphere that is the most efficient for whichever processing stages are most difficult. According to this hypothesis, no visual field differences will appear when all of the processing stages are very easy, an LVF-RH advantage will appear when the visuospatial stages are more difficult than the verbal-analytic stages, and an RVFLH advantage will appear when the visuospatial stages are less difficult than the verbalanalytic stages. The present results provide converging evidence for a shift toward an LVFRH advantage when letters are perceptually degraded. It is, therefore, very important to consider stimulus quality and visual complexity when evaluating visual field patterns across experiments and across treatment conditions within a single experiment. In the present experiment, the LVF-RH advantage was not limited to the case where the target letter and masking stimulus were presented simultaneously (i.e. SOA = 0 msec). Rather, the visual field difference persisted across short, nonzero SOAs in both forward and backward masking conditions. This persistence makes a great deal of sense because there is considerable evidence in the visual pattern masking literature that, when the target letter and masking stimulus have equal energy, the central visual processing system continues to treat the two stimuli as a single, composite image at short SOAs (up to about 30 msec) in both forward and backward masking [1.5-l 81. In contrast to forward masking and short backward masking SOAs, backward masking at longer SOAs (beyond about 30 msec) is thought to occur because ongoing central processing of the target is disrupted by the after-coming mask [15-l 81. That is, the target letter and mask are registered as separate visual stimuli rather than as a single montage. As shown in Fig. I, there were no visual field differences at backward masking SOAs where disruption of target recognition should be caused by this type of central interference. This was true despite the fact that letter recognition was clearly below asymptote in both visual fields up to an SOA of about 80 msec. That is, the absence of visual field differences is not caused by a ceiling effect. With observers as highly practised as those in the present experiment, MICHAELS and TURVEY [ 191, and WARD and Ross [IO] have also reported no visual field differences for letter recognition under conditions designed to produce only central interference masking. However, Ward and Ross did report that, when observers were less practised, the minimum SOA necessary to avoid central interference masking was shorter for the RVF-LH than for the LVF-RH. Note that this result constitutes an RVF-LH advantage, which is the opposite of the LVF-RH advantage found for very short SOAs in the present experiment. It appears, therefore, that the visual field advantage obtained during a visual masking experiment depends on whether the masking occurs because the central visual processing system is presented with a target-mask montage (SOAs from 0 to 20 msec) or because of central interference with ongoing target processing (backward masking SOAs above 20 or 30 msec). Interestingly, this hypothesis is consistent with the results of an experiment by MCKEEVER and SUBERI [21] which used single letters as targets and a surrounding circle as the mask and found an LVF-RH target recognition advantage at SOAs of 0 and 20 msec but an RVF-LH advantage at SOAs from 30 to 90 msec. It is possible that at very short SOAs the masking stimulus serves, primarily, to perceptually degrade the target, while at longer SOAS in backward masking the mask interferes with ongoing verbal encoding of the target. Additional visual laterality work using the masking paradigm will be necessary to test this notion and would serve to examine further the importance of temporal relationships for cerebral asymmetry.
RIGHT HE.MISPHERESCPERlORlTY FOR IXITLAL STAGES OF LETTER PROCESSI>G
659
REFERENCES 1. KIML.RA, D. Dual functional
of the brain in visual perception. ivewopsychologia 4, 275-285, 1966. 2. KIMURA, D. The asymmetry of the human brain. Scienr. Am. 228, 70-78, 1973. 3. LEVY, J. and REID, M. Variations in cerebral organization as a function of handedness, hand posture in writing, and sex. J. exp. Psychol. Gen. 107, 119-144, 1978. 4. NEBES, R. D. Hemispheric specialization in commissurotomized man. Psychol. Bull. 81, l-14, 1974. 5. RIZZOLATTI, G., UMILTA, C. and BERLUCCHI, G. Opposite superiorities of the right and left cerebral hemispheres in discriminative reaction time to physiognomical and alphabetic material. Brain 94, 431--M, 1971. 6. RIZZOLATTI. G. and BUCHTEL, H. A. Hemispheric superiority in reaction time to faces: A sex difference. C0rre.r 13, 300-305, 1977. 7. MOSCO~ITCH, M., SCULLION, D. and CHRISTIE, D. Early versus late stages of processing and their relation to functional hemispheric asymmetries in face recognition. J. exp. Psychol. Hum. Percepf. Perform. 2,401-416, 1976. 8. HELLIGE, J. B. Visual laterality and cerebral hemisphere specialization: Methodological and theoretical considerations. In Condirioning, Cognition, and LCfethodology: Contemporary Isslres in Experimental Psychology. J. B. SIDOWSKI (Editor). Erlbaum Press, Hillsdale. In press. 9. HELLIGE, J. B. Changes in same-different laterality patterns as a function of practice and stimulus quality. Percept. Psychophys. 20, X7-273, 1976. 10. BRYDEN, M. P. and ALLARD, F. Visual hemifield differences depend on type-face. Brain & Language 3, 191-200, 1976. 11. POUCH, J. M. Hemispheric differences in stimulus identification. Percept. Psychophys. 24, 49-57, 1978. 12. WILKINS, A. and STEWART, A. The time course of lateral asymmetries in visual perception of letters. J. exp. Psychol. 102, 905-908, 1974. 13. COHEN, G. Components of the laterality effect in letter recognition: Asymmetries in iconic storage. Q. JI exp. Psychol. 28, 105-l 14, 1976. 14. COHEN. G. Hemispheric differences in the utilization of advance information. In Atrenrion and Performance V. P. M. A. RABBITT and S. DORNIC (Editors). Academic Press, New York, 1975. IS. HELLIGE, J. B.. WALSH. D. A., LAWRENCE, V. W. and Cox, P. The importance of figural relationships between target and mask. Percqot. Psychophvr. 21, 285-286, 1977. 16. HELLIGE, J. B., WALSH, D. A., LAWRENCE, V. W. and PRASSE, M. Figural relationship effects and mechanisms of visual masking. J. exp. Psychol. Hum. Percept. Perform. 5, 88-100, 1979. Il. MASSARO, D. W. Experimental Psychology and Information Processing. Rand McNally, Chicago, 1975. 18. TURVEY, M. On peripheral and central processes in vision: Inferences from an information-processing analysis of masking with patterned stimuli. Psychology Rev. 80, l-52, 1973. 19. MICHAELS, C. F. and TL’RVEY, M. Hemiretinae and nonmonotonic masking functions with overlapping stimuli. Bull. Psychonomic Sot. 2, 163-164, 1973. 20. WAKE, T. B. and Ross, L. E. Laterality differences and practice effects under central backward masking conditions. ~2femory & Cognifion 5, 221-227, 1977. 21. MCKEEVER, W. F. and S~BERI, M. Parallel but temporally displaced visual half field metacontrast functions. Q. JI exp. Psychol. 26, 258-265, 1974.
R6sum6
asymmetry
: Quand des lettres qui pouvaient etre parfaitement
present&s
isolement etaient incorporks
les observateurs
reconnaissaient
reconnues
dans un stimulus de masquage,
plus de lettres dans le champ visuel
gauche que dans le droit. Cet avantage champ gauche - hemisphere droit persistait en cas
de brefs intervalles entre la lettre et le masque,
independamment de l'ordre de presentation
des stimulus. De tels resul-
tats suqqerent que l'h@misphSre cerebral droit est plus efficace que le gauche pour extraire des lettres quad n&e
les caract6ristiques
celles-ci sont perceptuellement
que ces lettres sont hautement associ&s
done ais&ment trait&s
visuelles provenant d&grad&s,
alors
avec le lanqaqe et
selon des dimensions verbales et analytiques.
660
JOSEPH B. HELLICE
and ROW WEBSTER
Deutschsprachige Zusammenfassung Wenn Einzelbuchstaben, die isoliert angeboten vollkommen richtig erfaBt werden konnten, eingebettet varen in einen iibergreifend maskierenden Stimulus, dann erkannten die Bsobachter mehr Buchstaben aus dem linken als aus dem rechten Gesichtsfeld. Diese Linksfeld/Rechtshemispharen-Uberlegenheitpersistlerte iiberkurze Zeltlntervalle zwischen dem Buchstaben und der Laskierung, unabhangig davon, welcher Stimulus zuerst erschien. Solche Ergebnisse sprechen dafiir,daB die rechte Hemisphgre leistungsfahiger ist als die linke beim Extrahieren relevanter visueller Zerkmale von Buchstaben, wenn diese hinsichtlich ihrer visuellen ErfaDbarkeit qualitatlv gemindert sind, und dies, obwohl Buchstaben an Sprache stark gebunden sind und daher gern nach verbal-andytischen Gesichtspunkten verarbeitet werden.
RIGHT
Britain.
HEMISPHERE SUPERIORITY FOR OF LETTER PROCESSING* JOSEPH B. HELLIGE and Department
of Psychology,
University
Los Angeles, (Received
INITIAL
STAGES
RON WEBSTER
of Southern California, CA 90007, U.S.A.
University
Park,
IS April 1979)
Abstract--When single letters, which could be perfectly recognized when presented alone, were embedded in an overlapping masking stimulus, observers recognized more letters from the left than from the right visual field. This left visual field-right hemisphere advantage persisted over short time intervals between the letter and the mask, regardless of which stimulus occurred first. Such results suggest that the right cerebral hemisphere is more efficient than the left at extracting relevant visual features of letters when the letters are perceptually degraded, even though letters are highly associated with language and, therefore, readily processed along verbal-analytic dimensions.
THERE is a considerable amount of clinical and experimental evidence that the right cerebral hemisphere is more efficient than the left at extracting complex visuospatial information
from visual displays [l-5]. Much of the evidence for this hypothesis comes from experiments in which stimuli are presented briefly to the left or right visual field and, therefore, projected directly to the right or left cerebral hemisphere, respectively. If the right cerebral hemisphere is specialized for processing visuospatial relationships, we might expect the left visual field-right hemisphere (LVF-RH) advantage for complex visuospatial stimuli to be enhanced when the stimuli are perceptually degraded, thereby making the extraction of visuospatial features and relationships more difficult. Several recent experiments have produced results suggesting that this is the case (e.g. [6-S]). dimensions, the initial Even when visual stimuli can be processed alon g verbal-analytic stages of information processing must involve such things as the extraction of relevant visual features and spatial relationships among those features. It is quite possible that the right cerebral hemisphere continues to be more efficient than the left cerebral hemisphere for these initial stages of processing. If so, then stimulus degradation effects similar to those reported for nonverbal tasks should be found for tasks that allow or even demand verbal processing. That is, as the stimuli become perceptually degraded, the visual laterality pattern should shift away from that pattern obtained with clear stimuli and in the direction of an LVF-RH advantage. There is, in fact, some experimental support for this hypothesis. HELLIGE [9] asked observers to indicate whether two simultaneously presented letters of different cases had the same name, a task which requires verbal processing. Presenting the letter pairs behind a grid of masking lines produced an LVF-RH advantage, even though a right visual field-left hemisphere (RVF-LH) advantage is typically found for tasks that *The research reported herein was supported National Science Foundation (BNS76-80093).
in part
653
by a research
grant
to the first author
from
the
644
JOSEPH
B.
HELLIGEand RON~~EBSTER
require verbal processing. BRYDENand XLL.\RD [IO] had observers identify single letters and varied visual field of letter presentation and type-face of the letters. For relatively familiar. printed type-faces, they found an RVF-LH recognition advantage. Honev,er, for elaborate, unfamiliar script type-faces they found an LC’F-RH recognition advantage, even though a verbal letter identification response vvas required. It seems likely that the extraction of relevant visuospatial relationships is more difficult for elaborate, unfamiliar type-faces. In a very different letter recognition paradigm, POLICH [I I] had observers indicate which one of four alternative letters was presented by pressin g one of four keys. A letter was either presented alone or flanked on either side by an outline square. No visual field differences vvere found in the letter-alone condition, but an LVF-RH reaction time advantage was found in the more visually complex condition involving letters plus flanking squares. The results just reviewed are interesting because they su,,UOest that, while the left hemisphere seems to be specialized for stages of verbal-analytic encoding, it is the right hemisphere that is more efficient at the relatively early stages of visuospatial processing-even for letters, which are highly associated with language [l2-141. It is important to employ converging operations to determine the generality and limits of such results. Accordingly, the present experiment was designed to examine further the effect on letter recognition of presenting a visual pattern mask, to examine effects of target-mask feature similarity, and to determine the temporal characteristics of any pattern mask effects. In the present experiment. observers were required to identify which one of 10 uppercase letters was presented on each trial. Viewin, n conditions were chosen so that letter identification was perfect from both the left and right visual fields. To make the extraction of visual features more difficult, a visual pattern mask was presented to both viewing fields at various times before and after the target letter. The mask either contained features that were similar to those of the target letter or contained features that were somewhat different from those of the target letter. If the right hemisphere is specialized for extracting the relevant features of letters from complex displays, then an LVF-RH advantage should be found when the target letters and mask occur simultaneously. When two visual stimuli occur very close together in time, they are treated by the central visual processing system as if they had been simultaneous [15-IS]. Therefore, the LVF-RH advantage should persist over short time intervals between the target letter and mask. Recent experiments indicate that letter identification is more difficult when the target letter and mask have similar features than when they have different features [15, 16). Therefore, the LVF-RH advantage may be larger and extend over longer temporal intervals in the similar-feature condition than in the dtfferent-feature condition. Finally, ability to identify the target letter should gradually improve as the time interval between the target letter and mask increases. As performance increases-that is, as the target letter becomes less degraded by the mask because of temporal separation-the LVF-RH advantage should decrease.
IMETHOD The observer sat at a table facing a 23 ? 15cm dark gray rear projection screen located about 49 cm away at eye level. In front of the observer on the table was a small gray box with a red button on top and a white card containing the IO target letters. A Gerbrands three-channel projection tachistoscope (Model Gl 176) equipped with a Gerbrands 300 series timer and a random access slide projector (Kodak Ektagraphic RA960) were used to rear project visual stimuli onto the display screen at the appropriate times. The time intervals used for the present experiment were well within the limits of the apparatus.
RIGHT
HEMISPHERE
SUPERIORITY
FOR INITIAL
STAGES
OF LETTER
PROCESSING
655
Slimulus materials Target stimuli consisted of 10 upper-case letters: 5 letters composed of straight lines (A, E, H, N, T) and five letters composed mainly of curved lines (B, D, G, Q, R). The target letters were photographed and prepared as 35 mm slides. When projected, each target appeared as a white letter on a dark gray background. The target letters subtended about 1.2’ of visual angle horizontally and vertically and were centered about 5.2” of visual angle from the center of the viewing screen. Two pattern masks were used, each appearing as white line segments on a dark gray background and each consisting of line segments that were the same stroke width as the stroke widths of the target letters. One mask was composed only of straight line segments and the other mask was composed only of curved line segments. Each pattern mask subtended about 4.7” visual angle horizontally and 3.5” of visual angle vertically. On each trial of the experiment, one of the pattern masks was projected simultaneously to both the LVF and RVF; that is, two identical masks were prepared on a single 35 mm slide so that, when projected, each mask was centered about 5.2” of visual angle to the left or right of the center of the viewing screen. Prior to each experimental trial a white fixation dot, subtending about 0.33” of visual angle, appeared in the center of the viewing screen. Both the target and mask fields had a luminance of 1.1 cd,‘m’ (0.3 ftL) and the fixation dot had a luminance of 0.33 cd,‘m2 (0.10 ftL). Procedure Observers were told that on each trial of the experiment they should fixate on the center of the viewing screen and press the red button in front of them when ready. When the button was pressed, a white dot appeared in the center of the screen and remained for 1 sec. Observers were told that LVF and RVF stimulation was equally likely so that their best strategy was to fixate their gaze on the central dot and maintain that fixation until after a target and mask had been presented. At the termination of the fixation dot, a target-then-mask trial (backward masking) or a mask-then-target trial (forward masking) was presented. Observers were required to say aloud one of the IO target letters as their best guess. Each observer received a total of four 210-trial blocks, two blocks during each of two If hr sessions separated by 24-48 hr. During each session, one 210-trial block used the straight segment mask, while the other 2IO-trial block used the curved segment mask, with mask-type order counterbalanced between observers. Each ‘IO-trial block consisted of 21 IO-trial blocks, one for each of the following target-mask intervals or stimulus onset asynchronies (SOAs): 0 msec; target preceding mask by IO, 20,30,40,50,60,80, 100, I20 and 180 msec; and mask preceding target by the same IO SOAs. The order of these 21 SOAs was randomized for each 210-trial block for each observer. All IO trials for a particular SOA were presented successively and consisted of one presentation of each of the IO target letters, five letters presented to the LVF and five letters presented to the RVF. Both visual field and specific letters were randomly ordered within these IO-trial blocks. For each SOA and mask type combination, letters that appeared in one visual field during the first session appeared in the opposite field during the second session. As a result, each observer received one presentation of each of the 10 targets in each visual field at each SOA for both mask types. Prior to the experimental trials, the observers were given approximately 5 min to adapt to the reduced level of illumination and were then shown each letter Rashed on the screen for 7 msec without any masking stimulus. No observer had difficulty identifying all IO letters under these conditions. Observers were also shown the masking stimuli and given a chance to ask questions about the procedure. During the experiment proper, the target and mask durations were both 7 msec. After approximately every 105 experimental trials, observers were given a short break and reminded of the importance of fixating appropriately on the center dot when it appeared. Subjects Eight men and eight women volunteers from introductory psychology courses at the University of Southern California participated in the present experiment. All subjects were right-handed, native speakers of English with normal or corrected-to-normal vision in both eyes. A preliminary analysis indicated that there were no effects of sex of subject so that all results will be presented collapsed over the sex variable.
RESULTS Figure 1 shows the percentage of correct target letter recognitions as a function of SOA. When the target letter and mask were presented simultaneously (SOA = 0 msec), 15 of 16 observers recognized more letters from the LVF-RH (percentage correct = 54.5) than from the RVF-LH (percentage correct = 39.5). In an analysis of variance for an SOA of 0 msec the main effect of visual field was highly significant, F (1, 15) = 18.107, P < 0.001. At this SOA observers also recognized slightly more targets in the Different feature condition
JOSEPHB. HELLICEand Ros WEBSTER
656
(49.5 :)A) than in the Same feature condition (44.5 “‘,), F (I,15)= 1.694, P < 0.05. For an SOA of 0 msec there uar no Same vs Different x Visual Field interaction, F < 1. As Fig. 1 shows, target recognition generally improved as SOA increased, in both forward and backward masking. The forward masking functions reached asymptote at an SOA of about -40 msec, while the backward masking functions reached asymptote at an SOA of +80 msec. These observations are supported statistically by the results of an analysis of variance with the following within-subjects factors: SOA (10-180 msec), Forward vs Backward masking presentation, Same vs Different target-mask feature relationship, and Visual Field of target presentation. Specifically, the effects just observed are supported by a significant main effect of SOA, F (9, 135) = 94.095, p < 0.001, a significant main effect of Forward vs Backward, F(1,15) = 39.688, P < 0.001, and a significant SOA x Forward vs Backward interaction, F (9, 135) = 21.782, P < 0.001.
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FIG. I. Percentage of correct target letter recognitions from the left (LVF-RH) and right (RVF-LH) visual fields as a function of stimulus onset asynchrony (SOA). Negative and positive SOAs refer to forward and backward masking, respectively. Results for Same-feature and Different-feature conditions are given in the upper and lower panels, respectively.
The LVF-RH target recognition advantage found at 0 msec SOA persisted across short nonzero SOAs in both forward and backward masking, producing a significant main effect of visual field averaged over nonzero SOAs, F (1,15) = 7.85 1, P < 0.02, and a significant Visual Field x SOP_ interaction, F(9, 135) = 3.383, P < 0.001. There was also a significant Visual Field x SOA x Forward vs Backward x Same vs Different interaction, F (9, 135) = 3.428, P < 0.001. This interaction was investigated further by subsequent, simpleeffects tests comparing the two visual fields at each nonzero SOA for both the Same-feature and Different-feature conditions (via further partitioning of the interaction sums of squares).
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Target recognition was significantly higher on LVF-RH trials than on RVF-LH trials for SOAs of -40, -20, - 10, + 10 and +30 msec in the Same-feature condition and for SOAs of -30, -20, - 10 and +20 msec in the Different-feature condition, while there were no field differences at other nonzero SOAs (P < 0.05 for the set of simple-effects comparisons). Note that the LVF-RH advantage persisted over longer forward and backward masking SOAs in the more difficult Same-feature condition. The Different-feature target recognition advantage found at 0 msec SOA also persisted over short nonzero SOAs. As this is difficult to see in Fig. 1, the data are replotted in Fig. 2, collapsed over visual field. The pattern shown in Fig. 2 produced a significant main effect of Same vs Different, F (1, 15) = 15.044, P < 0.001 and a significant Same vs Different x SOA interaction, F (9, 135) = 2.032, P < 0.05. lOOLo
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DISCUSSION The results of the present experiment are consistent with the results of the experiments reviewed in the introduction and support the hypothesis that the right hemisphere is more efficient than the left at extracting relevant visual features of letters when the letters are perceptually degraded by being embedded in a complex visual display. As shown in Fig. 2, letter identification was more difficult when the target and mask had the same features than when they had different features [15, 161. That is, the target letters were more perceptually degraded in the Same-feature condition. Consistent with the above hypothesis, the LVF-RH advantage persisted over slightly longer temporal intervals in this more degraded Samefeature condition than in the Different-feature condition, although at the shortest SOAs there was a consistent LVF-RH advantage for both feature relationship conditions. The present results, and the others reviewed earlier, suggest that the relative efficiency of the right and left cerebral hemispheres depends on the stage of letter processing under consideration [8-141. The right hemisphere appears to be more efficient at very early visuospatial processing stages, or at what BRYDEN and ALLARD [IO] refer to as preprocessing, while the left hemisphere is more efficient for somewhat later stages such as verbal encoding (e.g. attaching letter names to visual features) and verbal memory comparison (e.g. comparing two letter names) (7-121. Both HELLIGE [9] and BRYDEN and ALLARD [IO] have suggested that the overall visual field (or hemisphere) advantage observed in any particular experimental condition depends on which of these two general kinds of processing (visuospatial vs
658
JOSEPHB. HELLICEand RON WEBSTER
verbal-analytic) is more difhcult. Performance is likely to be better when the stimulus material is projected directly to the cerebral hemisphere that is the most efficient for whichever processing stages are most difficult. According to this hypothesis, no visual field differences will appear when all of the processing stages are very easy, an LVF-RH advantage will appear when the visuospatial stages are more difficult than the verbal-analytic stages, and an RVFLH advantage will appear when the visuospatial stages are less difficult than the verbalanalytic stages. The present results provide converging evidence for a shift toward an LVFRH advantage when letters are perceptually degraded. It is, therefore, very important to consider stimulus quality and visual complexity when evaluating visual field patterns across experiments and across treatment conditions within a single experiment. In the present experiment, the LVF-RH advantage was not limited to the case where the target letter and masking stimulus were presented simultaneously (i.e. SOA = 0 msec). Rather, the visual field difference persisted across short, nonzero SOAs in both forward and backward masking conditions. This persistence makes a great deal of sense because there is considerable evidence in the visual pattern masking literature that, when the target letter and masking stimulus have equal energy, the central visual processing system continues to treat the two stimuli as a single, composite image at short SOAs (up to about 30 msec) in both forward and backward masking [1.5-l 81. In contrast to forward masking and short backward masking SOAs, backward masking at longer SOAs (beyond about 30 msec) is thought to occur because ongoing central processing of the target is disrupted by the after-coming mask [15-l 81. That is, the target letter and mask are registered as separate visual stimuli rather than as a single montage. As shown in Fig. I, there were no visual field differences at backward masking SOAs where disruption of target recognition should be caused by this type of central interference. This was true despite the fact that letter recognition was clearly below asymptote in both visual fields up to an SOA of about 80 msec. That is, the absence of visual field differences is not caused by a ceiling effect. With observers as highly practised as those in the present experiment, MICHAELS and TURVEY [ 191, and WARD and Ross [IO] have also reported no visual field differences for letter recognition under conditions designed to produce only central interference masking. However, Ward and Ross did report that, when observers were less practised, the minimum SOA necessary to avoid central interference masking was shorter for the RVF-LH than for the LVF-RH. Note that this result constitutes an RVF-LH advantage, which is the opposite of the LVF-RH advantage found for very short SOAs in the present experiment. It appears, therefore, that the visual field advantage obtained during a visual masking experiment depends on whether the masking occurs because the central visual processing system is presented with a target-mask montage (SOAs from 0 to 20 msec) or because of central interference with ongoing target processing (backward masking SOAs above 20 or 30 msec). Interestingly, this hypothesis is consistent with the results of an experiment by MCKEEVER and SUBERI [21] which used single letters as targets and a surrounding circle as the mask and found an LVF-RH target recognition advantage at SOAs of 0 and 20 msec but an RVF-LH advantage at SOAs from 30 to 90 msec. It is possible that at very short SOAs the masking stimulus serves, primarily, to perceptually degrade the target, while at longer SOAS in backward masking the mask interferes with ongoing verbal encoding of the target. Additional visual laterality work using the masking paradigm will be necessary to test this notion and would serve to examine further the importance of temporal relationships for cerebral asymmetry.
RIGHT HE.MISPHERESCPERlORlTY FOR IXITLAL STAGES OF LETTER PROCESSI>G
659
REFERENCES 1. KIML.RA, D. Dual functional
of the brain in visual perception. ivewopsychologia 4, 275-285, 1966. 2. KIMURA, D. The asymmetry of the human brain. Scienr. Am. 228, 70-78, 1973. 3. LEVY, J. and REID, M. Variations in cerebral organization as a function of handedness, hand posture in writing, and sex. J. exp. Psychol. Gen. 107, 119-144, 1978. 4. NEBES, R. D. Hemispheric specialization in commissurotomized man. Psychol. Bull. 81, l-14, 1974. 5. RIZZOLATTI, G., UMILTA, C. and BERLUCCHI, G. Opposite superiorities of the right and left cerebral hemispheres in discriminative reaction time to physiognomical and alphabetic material. Brain 94, 431--M, 1971. 6. RIZZOLATTI. G. and BUCHTEL, H. A. Hemispheric superiority in reaction time to faces: A sex difference. C0rre.r 13, 300-305, 1977. 7. MOSCO~ITCH, M., SCULLION, D. and CHRISTIE, D. Early versus late stages of processing and their relation to functional hemispheric asymmetries in face recognition. J. exp. Psychol. Hum. Percepf. Perform. 2,401-416, 1976. 8. HELLIGE, J. B. Visual laterality and cerebral hemisphere specialization: Methodological and theoretical considerations. In Condirioning, Cognition, and LCfethodology: Contemporary Isslres in Experimental Psychology. J. B. SIDOWSKI (Editor). Erlbaum Press, Hillsdale. In press. 9. HELLIGE, J. B. Changes in same-different laterality patterns as a function of practice and stimulus quality. Percept. Psychophys. 20, X7-273, 1976. 10. BRYDEN, M. P. and ALLARD, F. Visual hemifield differences depend on type-face. Brain & Language 3, 191-200, 1976. 11. POUCH, J. M. Hemispheric differences in stimulus identification. Percept. Psychophys. 24, 49-57, 1978. 12. WILKINS, A. and STEWART, A. The time course of lateral asymmetries in visual perception of letters. J. exp. Psychol. 102, 905-908, 1974. 13. COHEN, G. Components of the laterality effect in letter recognition: Asymmetries in iconic storage. Q. JI exp. Psychol. 28, 105-l 14, 1976. 14. COHEN. G. Hemispheric differences in the utilization of advance information. In Atrenrion and Performance V. P. M. A. RABBITT and S. DORNIC (Editors). Academic Press, New York, 1975. IS. HELLIGE, J. B.. WALSH. D. A., LAWRENCE, V. W. and Cox, P. The importance of figural relationships between target and mask. Percqot. Psychophvr. 21, 285-286, 1977. 16. HELLIGE, J. B., WALSH, D. A., LAWRENCE, V. W. and PRASSE, M. Figural relationship effects and mechanisms of visual masking. J. exp. Psychol. Hum. Percept. Perform. 5, 88-100, 1979. Il. MASSARO, D. W. Experimental Psychology and Information Processing. Rand McNally, Chicago, 1975. 18. TURVEY, M. On peripheral and central processes in vision: Inferences from an information-processing analysis of masking with patterned stimuli. Psychology Rev. 80, l-52, 1973. 19. MICHAELS, C. F. and TL’RVEY, M. Hemiretinae and nonmonotonic masking functions with overlapping stimuli. Bull. Psychonomic Sot. 2, 163-164, 1973. 20. WAKE, T. B. and Ross, L. E. Laterality differences and practice effects under central backward masking conditions. ~2femory & Cognifion 5, 221-227, 1977. 21. MCKEEVER, W. F. and S~BERI, M. Parallel but temporally displaced visual half field metacontrast functions. Q. JI exp. Psychol. 26, 258-265, 1974.
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: Quand des lettres qui pouvaient etre parfaitement
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gauche que dans le droit. Cet avantage champ gauche - hemisphere droit persistait en cas
de brefs intervalles entre la lettre et le masque,
independamment de l'ordre de presentation
des stimulus. De tels resul-
tats suqqerent que l'h@misphSre cerebral droit est plus efficace que le gauche pour extraire des lettres quad n&e
les caract6ristiques
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que ces lettres sont hautement associ&s
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visuelles provenant d&grad&s,
alors
avec le lanqaqe et
selon des dimensions verbales et analytiques.
660
JOSEPH B. HELLICE
and ROW WEBSTER
Deutschsprachige Zusammenfassung Wenn Einzelbuchstaben, die isoliert angeboten vollkommen richtig erfaBt werden konnten, eingebettet varen in einen iibergreifend maskierenden Stimulus, dann erkannten die Bsobachter mehr Buchstaben aus dem linken als aus dem rechten Gesichtsfeld. Diese Linksfeld/Rechtshemispharen-Uberlegenheitpersistlerte iiberkurze Zeltlntervalle zwischen dem Buchstaben und der Laskierung, unabhangig davon, welcher Stimulus zuerst erschien. Solche Ergebnisse sprechen dafiir,daB die rechte Hemisphgre leistungsfahiger ist als die linke beim Extrahieren relevanter visueller Zerkmale von Buchstaben, wenn diese hinsichtlich ihrer visuellen ErfaDbarkeit qualitatlv gemindert sind, und dies, obwohl Buchstaben an Sprache stark gebunden sind und daher gern nach verbal-andytischen Gesichtspunkten verarbeitet werden.