stanka stimulirajuceg flesa. Ako sc, na primer, razmatra slubj prelaza s difuzne na »strukturizovanu
POSEBNI OTISAK
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N E U ROPSI H IJATRIJA CASOPIS ZA NEUROLOGIJU, PSIHIJATRIJU I GRANICNA PODRUCJA
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VOL. 18.- NO. 4 ZAGREB 1970.
Department of Physics, School of Medicine Beograd, Yugoslavia
n. Ristanovic
EFFECT OF VISUAL FIELD STRUCTURES ON POTENTIALS EVOKED ON MAN*
It has been noticed that the diversity of flash-presented structures of a contoured visual field containing intersecting, high-contrast edges or bars brings about the variations only in amplitude and in latency of the »specific« (prominent, well defined and fairly reproducible) components which had already been indicated in the averaged response to flash presentation of diffuse (blank-field) light (Ristanovic, 1971a). Very little work has, however, so far been done on the most significant structure parameters of a patterned field being capable of producing changes in the »Specific« waves of the response to continuously illuminated stimulus field. Both Spehlmann and MacKay et al. reported a regular increase in amplitude of the large, positive (»late«) wave (the peak latency of which being about 200 msec after the flash onset) with increasing number of contrast bmders between black and white areas in chequerboard patterns presented to the human. Having extended the range of the pattern unit sizes of the chequerboard patterns, Rietveld et al. stressed the fact that there was a maximum deflection for both Z and »inverted« r wave if a particular unit size was used, and emphasized as well that the further change in density of black to white interfaces caused a precipitate drop in amplitude of the response. They also confirmed Spehlmann's finding that the degree of variations in amplitude of the individual peaks of the prominent components depended only on the total number of contrast borders in the field but not on their combined length. In addition, they found that the influence of unit square size on both the amplitudes was strikingly similar. The purpose of the report presented in this paper is to examine the contrast border density influence on peak-to-peak amplitudes of the pattern response and extend that idea on a class of various, effective patterns. It is believed that the present report might contribute the evidence * The data described in this paper are drawn from a series of experiments performed at the Department of Communications of Keele University (England) in 1968.
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on the visual system function characterized by the property of visual content discrimination of the structure presented, a.s well as to prove useful evidence on the evolution of biologic information processing design. Material
and
methods
A series of experiments was carried out on a single subject over a period of one month. The subject was located in a semidarkened soundproof room viewing the squared stimulus field of a tachistoscope. Having situated at a distance of 1 meter from the subject's eye, the filed subtended an anglre of about 9° at the eye and the subject fixed at a dim fixation spot at the centre of the field. A series of intermittent patterned tlashes of 30 msec duration separated by the blank-field illumination periods (of 480 msec each) were used as stimuli. Patterned fields are produced by placing suitable black-white photographic transparencies in front of the back-illum~nated opal glass screen ot the tachistoscope. A Mnemotron CAT computer was used for averaging 200 s~gnals in a run, the signals led off in a unipolar lead from a row of scalp electl."odes placed in the midline of the occipital region. Characteristics of the optical system for stimulus presentation as well as the general set-up for stimulus detection have been described in detail elsewhere (MacKay et al.) and partly by the author (Ristanovic, 1969). /
Results
If r E and EZ amplitudes are considered to determine the efficiency of a pattern structure [the indications of the »specific« components are used according to the notation proposed earlier (Ristanovic, 197la)], on a basis of the results obtained from the txperiments with the series of regular chequerboard patterns, the efficiency, for a given electrode position, of that structure (structure A) as a function of unit square side length is shown in Fig. 1. If the efficiency is evaluated conventionally in terms of the peak-to-peak amplitudes of diphasic r E waveforms, it can be seen from Fig. 1 that the corresponding curve reaches its maximum at a side length of about 0.75 em (subtending the angle of 26 minutes of arc at the subject's eye), while the maximum of the EZ amplitudes curve is reached at a longer side length of about 1 em (sub tending the angle of 34 min of arc). A similar difference of about 20 mm between the peak positions of another pair of curves can be observed in Fig. 2 indicating the results of an experimental session when a series of patterns of structure B (involving regular chequerboard patterns the black units being omitted from every second line and column) was presenting to the subject in the same way. In that cas-; r C amplitudes reach their maximum value at the smaller pattern unit (the side length being about 0.70 em), and the side length of the pattern for the maximum EZ amplitude is approximated to be only 0.50 em (subtending the angle of 17 min of arc). The same happens when patterns of structure C (comprising chequerboard half-field patterns) were used for stimulus presentation (Fig. 3) .
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UNIT SQUARE SIDE LENGTH (em)
Fig. 1. rE and EZ amplitudes as functions of unit square side length. The ampli:udes are measured on a succession of fullydeveloped averaged responses to presentation of whole-field chequerboard patterns with different interface densities. The EEG signals are recorded unipolarly from the electrode on the inion, the reference being on the right earlobe (the left earlobe was grounded). Each response represents an average of 200 signals in a run. The computer analysis time is 500 msec, and the period of flash repetition is 510 rnOn« and >>Off« signals travel along two different thannels (Clynes et al.) , and with the inference that one cortical cell is corresponded to one receptive field but the opposite cannot be truth (Hubel). Summary In the present paper an attempt was made to examine the relation of certain simpler and more efficient structures to corresponding averaged visually evoked pattern responses recorded transcranially on a single human. The amplitudes of diphasic r E and EZ complexes of the response were chosen as a measures of the cortical activity degree. On the basis of a number of experiments carried out over a period of one month the following conclusions can be summarized: 1. The side length of the retinal projection of a pattern unit bringing about the maximum cortical activity depends on whether the patterned flash, eliciting the response, begins or ends: the side length appears to be longer if one deals with r E amplitudes, that is, if there exist numerous »Off« transitions over the retinal projections of these units. 2. The maximum efficiency side length of a patter~· made up from congruent units in a spatially regular patterned field depends on the contrast border density of interfaces regardless of the structure over that field. According to that assumption the maximum efficiency side length of a chequerboard pattern unit is proved to b e shorter than a unit of a chequerboard pattern in which the units are omitted from every second line and column. 3. The overall efficiency of a structure presented to the subject in a given technique increases if the ratio of the combined area of the black units of a pattern of the structure to that of the white units of the pattern tends to unity. Therefore the most efficient structure appears to be that whose ratio rnentioned is equal to 1. 4. The overal efficiency also depends on electrode position used. Maximum efficiency side length for a given complex, however, remains the same if measur"J from different electrodes on the scalp.
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D. Rlstanovic
UTICAJ STRUKTURE VIZUELNOG POLJA NA HUMANE IZAZVANE POTENCIJALE Sadrzaj
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Primeceno je da odredeni parametri strukture vidnog polja uticu na oblik »averaziranog«' vizuelno izazvanog odgovora zapisanog s covekovog skalpa, ali do sada nije izvrseno mnogo eksperimenata koji bi ukazali na bitnije zakonitosti ovog uticaja. UoCivsi jos ranije da kvalitativne i kvantitativne promene u strukturi vidnog polja uticu uglavnom na vrednosti amplituda i latencija vrhova »specificnih« komponenti odgovora, mi smo u ovom radu razmotrili osnovne zakone uticaja gustine kontrastnih ivica uzajamno istovetnih i pravilno rasporedenih jedinica modela na amplhude »Specifienih« komponenti odgovora za relativno siroku klasu efikasnih struktura. U tom su smislu subjektu ogleda, lociranom u antisonornoj sobi, tahistoskopski prezentirane tri vrste slucajno odabranih struktura u vidu serija modela sastavljenih od jedinica Cije se dimenzije sukcesivno smanjuju, a odgovarajuCi izazvani potencijali odvodeni s kontaktne elektrode na inionu, zatim pojacavani i najzad kompjuterski sumirani u cilju eliminisanja stetnog uticaja spontane mozdane aktivnost;. Amplitude karakteristicnih difazicnih kompleksa r E i EZ ovakvih odgovora odabrane su kao mere efikasnosti odgovarajucih modela na stepen izazvane kortikalne aktivnosti. Na osnovu serije eksperimenata izvrsenih na jednom subjektu u jednomesecnom periodu zakljuceno je sledece: 1. Duzina projekcije ivice proizvoljne jedinice nekog modela na retini koja uslovljava maksimalnu kortikalnu aktivnost zavisi od smera i naCina promene karaktera osvetljenosti retine u momentu nastanka i prestanka stimulirajuceg flesa. Ako sc, na primer, razmatra slubj prelaza s difuzne na »s trukturizovanu
