was compared with performance on the same letters presented in pairs. We found the ... in which black letters m the white background were presented, but.
ACTA NEUROBIOL. EXP. 1979, 39: 469-475 Lecture delivered at the Warsaw Colloquium on Instrumental Conditioning and Braln Research M a y 1979
SEARCH FOR STRUCTURES INVOLVED IN INTEGRATION OF LETTERS IN PAIRS Wanda BUDOHOSKA and Anna GRABOWSKA Department of Neurophysiology, Nencki Institute of Experimental Biology, Warsaw, Poland
Abstract. Two experiments aimed at finding, at what &age of perceptual processing the positive i~nteractions between letters, observed in some previous studies, are established. Single letters and pairs of letters were exposed. The task of the subjects was to recognize letters. In Experiment I bright letters on a black background were used instead of black letters on bright background used in previous experiments. Such change of brightness within the stimuli did not influence the character of interactions between letters. This suggests that the positive interactions are connected with higher level of visual system. In Experiment I1 patients with damage of various areas of the brain cortex were tested. The facilitation between two letters did not occur i~npatients with prestriate lesions, although it can be observed in patients with different localization of lesions. This suggests that prestriate cortex is involved in positive interactions between letters. INTRODUCTION
Our studies concerning the i~nteractionsbetween elements of compound visual patterns have shown that the nature of these interactions depends on the subject's famliliarity with the perceived stimuli. In the case of familiar complex patterns there occurred a facilitation between their components (1-3, 5). In the case of patterns composed of elements
that did not combine into a familiar pattenn, a negative interaction was observed (2, 5). A task used in some d these studies was to identify letters that were presented very briefly (1, 4). Performance on single letters was compared with performance on the same letters presented in pairs. We found the letters in first position of pairs to be subjected to facilitation from the letters in second position. This finding raised the question as to the level of perceptual analysis at which the interaction takes place. In Experiment I white letters on the {blackbackgroulnd were used. The shape of these stimuli was the same as i~nprevious experiments (1, 4) in which black letters m the white background were presented, but overall illumination as well as the distribution of light differed. It seemed interesting to find whether such a change in the experimental conditions could influence the interactions between two letters. This would provide the possibility of inferring whether the observed interactions depend on the stimulus shape or on the light disbribution within the stimuli. In Experiment I1 we investigated ilnteractions between letters in neurosurgical patients with different localization of brain lesions. The aim of this research was to provide information concerning the structures engaged in the integration of the elements of compound visual patterns. METHODS
In Experiment I, 13 subjects with normal vision were run and in Experiments 11, 11 patients hospitalized at a neurosurgery clinic: 4 with occipital lesions, 3 with temporal lesions, 2 with parietal lesions and 2 with frontal lesions. The patients did not demonstrate any disorders in visual functions examined by means of standard clinical tests. All subjects were able to perceive, recognize, identify, and appropriately report the graphic characters used in the experiments. In Experiment I, ten upper case, printed letters (ABCEFLOPTZ), white on a black background, and in Experiment 11, seven upper case letters (ABCEFOP), black on a white background, were used. The characters were presented either separately or in pairs (100 combinations of 10 letters in Experiment I and 49 combinations of 7 letters in Experiment I1 were used). Each of the letters was shown with the same frequency as single stimuli and as the first or s e m d elements within a pair. The letters used in Experiment I were 53' in height and those in Exper~mentI1 were 1'10' in height. The space separating two letters was about 9'. The luminance of stimuli was kept constant for a given person and was adjusted by means of Kodak neutral filters to a level which provided 60-80°/o correct recognition.
A Kodak projector, model Camusel 140 with Laffayet shutter was used. The stimuli were presented foveally. The time of letter exposure was 20 ms i n Experiment I and 17 ms in Experiment 11. A proper fixat'ilon was provided by a light spot projected just above the area in which the test stimuli were to appear. The subjects were to report after each presentation what letter or pair of letters was s h o r n on the screen. Single letters and pairs of letters were presented in separate sessions. The order of sessions was random. Single letters were presented randomly at positions equivalent to the positions of left and right letters in pairs. The right and left side exposures were balanced. The stimuli were presented every .8 s in series of 20 items in Experiment I and of 10 items In Experiment 11. The series were separated by 2 min intervals. Each of the Experiments was preceded by 10 min of dark adaptation (Experiment I) or light adaptation (Experiment XI). RESULTS AND DISCUSSION
Experiment I The analysis of data was based on the number of errors made by subjects in the identification of letters exposed in each of the two positions. The principal goal of the experiment was to determine the interaction between the letters in pains, and so an attempt was made to detect differences in performance on letters presented separately and the same letters presented in pairs. For each subject, the difference between the number of e m r s made with single letters (sl) and the number of errors made with paired letters (pl), was divided by the sum of the same (sl-pl) . values to yield the person's identification difference score (IDS) (slf ~ 1 ) Such a score was computed separately for letters in first and in second positions. The wore was positive in case of superior identyfication of paired letters compared with single letters (positive interaction) and it was negative in case of superior identification of single letters (negative interaction). The score equaled zero if there was no difference in number of errolrs in the two conditions. The values for each subject are shown in Fig. 1. The scores are positive in 12 subjects for letters in the 1st position and in 8 subjects for letters in the 28nd position; a t-test revealed significant facilitation of paired letters 'only in the case of the first position (P < 0.01). These data are very similar to those obtained m i n experiments with black latters on white background (1, 4) This shows that facilitation does not depend on the illumination or distribution of light over the stimulus, but rather on the shape of the stimuli. This would suggest that the in-
teractions occur at higher levels of the nervous system. Nevertheless the results of Experiment I give no indication as to which structures of nervous system are involved i n this process. Some evidence on these lines seems to emerge from Experiment I1 which was conducted on patients with brain lesions in different regions of the cortex. II p o s i t i o n
I position
Fig. 1. Identification difference scores for lettefts exposed in I and I1 positions in particular subjects (numbers in bars). For detailed explanation see text.
Experiment I1
The data obtained for each of the subjects are given in Table I. The scores were computed in the same way as i n Experiment I. Each patient's performance was analyzed individually, silnce the pooling of data obtained
Identification difference scores for patients with cerebral lesion Subject
Hemisphere left right left right left left left left right left left
Area
Indentification difference scores I IIposition Iposition
occipito-parietal occipito-parietal occipito-temporal occipito-temporal temporal temporal temporal-parietal parietal parieto-frontal frontal frontal
from patients with different lesions seemed unwarranted, even for lesions in the same cerebral lobe. In order to determine whether or not the performance of the patients differed significantly from the performance of normal subjects tested in similar conditi~ons,we tabulated the data obtained from normals run in our laboratory jn several experiments of the same kind (e.g., 1, 4), and estimated the hypothetical distribution of their IDS. These scores ranged from -0.066 to 4-0.658 for letters in first position and from -0.243 to +0.443 for letters in second position. The scores in Table I that exceed these limits (set in bold type) can be taken to differ significantly from the performance of normals with probability below 0.05. All patients with lesions tin occipital cortex identified letters in first position with significantly lower efficiency than normals (the fourth person on the boded of statistical significance). In all these cases their performance on paired letters was infenior to performance m single letters, which means that perception was impaired, mot facilitated, as in the case of normals. All remaining patients were better at identifying paired letters than single letters i n the first position, the same as in the case of normals. As t o identification of letters in the second position, our patients performed about the same as normals except for one case (patient 7), where secbnd position identification was inferior to single letter identification. The absence of positive interaction between simultaneously presented letters in cases of occipital lesions can be interpreted as reflecting some kind of disorder in the synthesis of visual elements into complex patterns.
These patients seemed to be perceiving the two letters independently of each other, not as component of one pattern. Description of the operated areas and the absence of disorders typical for striate cortex lesions (e. g., decreased acuity of vision or moms in the visual field) indicate
Fig. 2. Damaged area of the brain cortex in 11 patients. The pictures show only the whole area of the surgery (in most cases a tumor was removed), since more precise estimation of the damage region is not possible.
that mainly the prestriate region (18 and 19 area) of the visual cortex was damaged (Fig. 2). This might suggest that the prestriate region serves bo integrate the elements of the visual field into complex patterns. Such an interpretation is consistent with clinical data reported by other authors (6) postulating that partial lesions in this region result in impaired ability of integral perception of entire visual patterns. It is interesting that our patients did not reveal clinical symptoms of visual agnosia: they had no difficulties in identifying objects that were crossed out, or drawn in outline, or overlapping, in naming two objects shown simultaneously, or in describing complicated situational pictures. It may be speculated that the visual impairment recorded in our experiments could be due to the short time of exposure which sufficed for only a near-threshold identification. Normally, when the same patients are free to examine the scene fmor longer periods, they can take advantage of the redundancy of information and thus perform perfectly. It seems
that by thmughly examilning this kind of interaction between letters we might be able to discover disorders in visual functions which are not detected by ordinary clinical tests. Ln summary, our experiments indicte that posibive interaction between letters takes place at a level of visual processing at which information about the brightness of stimuli is neglected and the shape of stimuli is analysed. This interaction might occur in the prestriate region of the occipital cortex. Experiment I1 was conducted in cooperation with E. Fersten and J. Szumska from Neurosurgery Clinic of Polish Academy of Sciences. REFERENCES 1. BUDOHOSKA, W., GRABOWSKA, A. and JABEONOWSKA, K. 1975. Interaction between two letters. Acta Neurobiol. Exp. 35: 115-123. 2. BUDOHOSKA, W. and CELINSKI, M. 1975. Interaction between two simple visual patterns. Acta Neurobiol. Exp. 35: 125-137. 3. BUDOHOSKA, W., GRABOWSKA, A. and JABEONOWSKA, K. 1979. The effect
of interaction between elements of familiar and unfamiliar patterns. Pol.
Psychol. Bull. 2 (in press). 4. GRABOWSKA, A. and BUDOHOSKA, W. 1979. Interaction between two spatialy separated letters presented in succession. Acta Neurobiol. Exp. (in press). 5. JABEONOWSKA, K. and BUDOHOSKA, W. 1976. Hemispheric differences in
the visual analysis of the verbal and non-verbal material in children. Acta Neurobiol. Exp. 36: 693-701. 6. LURIA, A. C. 1973. The working brain. An introduction to neuropsychology. Penguin Books Ltd. Harmondsworth, England, 360 p. Wanda BUDOHOSKA and Anna Pasteura 3, 02-093 Warsaw, Poland.
GRABOWSKA,
Nencki Institute
of
Experimental
Biology,
