L.H. van der Tweel

Contrast evoked responses in man

Abstract In man visually evoked responses (EPs) can be recorded that are specific to changes in spatial contrast and cannot be derived from luminance responses. No spatial contrast component could be demonstrated in the ERG. Contrast EPs depend on various parameters such as acuity, retinal location, size and configuration of spatial elements, time course of luminance change. They are affected by overlapping steady contrasts presented either monocularly or dichoptically. For a given condition the contrast EP is mainly determined by the instantaneous relative contrast irrespective whether this contrast is reached by an increase or decrease in luminance. The EPs to the appearance and disappearance of a pattern bear different relations to stimulus parameters and seem to originate from different populations of cortical cells. The responses to short appearances obey the contrast equivalent of Blochs law and correlate with psychophysics.

Human visual responses to sinusoidally modulated light

Abstract Sinusoidally modulated light, even at low percentages of modulation, gives rise to occipital responses, sometimes large. They may reproduce rather well the input sine wave in shape and in amplitude, especially between 9 and 15 c/sec for diffuse large field illumination and above 35 c/sec for strong illumination with small fields. The responses between 9 and 15 c/sec may be partly described by assuming a linear filter action. Resonance phenomena are observed, in which different types of stimulation yield compatible results. Responses and spontaneous activity seem to be additive to a considerable degree, as are the responses to stimulation of each eye. In the 9–15 c/sec frequency range, where the responses exhibit mainly the fundamental frequency, cancellation effects are found with out-of-phase stimulation of both eyes. That the large distortions found may be caused in early stages of the visual process, and are essential, is discussed. Responses have been recorded at many frequencies when the light was seen as fused by the subjects. Although subjective flicker threshold curves bear no clear relation to either the size or shape of the responses, it is hoped that the recordings of evoked responses with sinusoidally modulated light, combined with the techniques presented by Clynes et al. (1964), will be a valuable supplement to the current methods of investigation.

Spread of activation in the left ventricular wall of the dog. I.

Abstract A description is given of the technique adopted for the research of activation processes in the left ventricular wall. A two-channel cathode-ray tube oscillograph has two tubes for photographic registration and two tubes for continuous visual observation. Simplicity and sturdiness are important. Electrode needles are described which have a minimal lesion effect and allow the exact positioning of as many as eleven electrodes. These electrodes are situated in one ordinary injection needle. The technique of registration always consisted in taking as many leads as possible with one needle in one position.

Excitation of the left ventricular wall of the dog and goat.

Most theories concerning the genesis of the QRS complex have been based on evidence obtained from studies of the time relations and morphology of complexes registered a t the epicardial surface and, to a lesser degree, from the left and right ventricular cavities and endocardia1 surface. From these facts the time course of excitation in the ventricular septum, left and right ventricular walls, is deduced. Direct evidence concerning these points could not be obtained because of the impossibility of introducing electrodes into these areas without altering profoundly the physiological condition (excitability) of ventricular muscle cells. The commonly accepted theory, based on the experimental findings of Lewis and Rothschildl and elaborated by Wilson and his school, could be used “to predict the major features of the QRS complex of unipolar direct leads under a variety of circumstances.” Certain shortcomings were pointed out in an important paper by Wilson el a1.2: “We do not know in detail how the trabeculae and the papillary muscles are activated. Nor do we know how the junctions between the Purkinje system and the ordinary subendocardial muscle are distributed, or how deeply the Purkinje fibers penetrate the septa1 and free walls of the ventricles. The form of the QRS complex is undoubtedly affected by the communications between left and right ventricle through the septum and their variations, as well as by others, but we cannot a t present make proper allowances for them.” During the last eight years experiments have been designed and performed in an effort to solve these problems. Many problems in normal and pathological conditions could be solved if it were possible to introduce electrodes into the ventricular walls and septum in such a way as to cause only minimal injury and not alter the response of the cardiac muscle fibers to the excitation process. After many trials with different types of electrodes (plunge electrodes) and recording apparatus (direct writing apparatus, string galvanometer), only the following instrumental setup suited our purpose.

Spread of activation in the left ventricular wall of the dog. II: Activation conditions at the epicardial surface

Abstract Registration of unipolar epicardial leads of the exposed heart of the dog with a high fidelity oscillograph reveal several peculiarities in the form of the QRS complex. For most parts of the epicardial surface of the left ventricle, the intrinsic deflection consists of three parts; a slow portion after the zenith of the R wave, one near the nadir of the S wave, and between these two parts a fast portion. With differential leads it is proved that at the moment this part of the intrinsic deflection is written, the subepicardial muscle under the exploring electrode is activated. The differential spike coincides with the rapid part of the intrinsic deflection. In one instance this relation could be proved for the exposed human heart. This connection holds also for extrasystoles and for the gradually widening complexes during increasing anoxemia. By the use of multiple differential electrodes, results were obtained pointing to the existence on the epicardial surface of many activation waves propagated in a very small area. In accord with other authors it was found that the over-all direction of the activation is from apex to base. The concept of the intrinsic deflection was critically analyzed. Only the rapid part of the intrinsic deflection in curves obtained from the exposed heart can be called intrinsic. In precordial leads it is impossible to find a constant point in the “intrinsic deflection” that signals the arrival of the activation wave at the epicardial surface nearest the exploring electrode.

The Excitability Cycle of the Dog's Left Ventricle Determined by Anodal, Cathodal, and Bipolar Stimulation

Investigations are reported in which the excitability cycle of the dogs heart was tested successively by unipolar anodal and cathodal square wave shocks as well as by bipolar stimulation. The classical view that excitability decreases smoothly during the relative refractory phase was substantiated. The excitability for anodal stimulation reached its maximum of the cardiac cycle immediately after the absolute refractory period and exceeded cathodal excitability at that moment. Evidence is presented that the dip phenomenon described by Orias, Brooks and their associates is due to their arrangement of driving and testing electrodes. The coincidence of the dip phenomenon with the vulnerable period may be related to simultaneous spreading of two fronts of activation from cathode and anode of bipolar electrodes which would create favorable conditions for reentry.

Luminance Responses to Pattern Reversal

The stimuli used in many spatial contrast studies were accompanied by overall changes in luminance (for a review see Estevez & Remond, 1972). This implies that the responses to these stimuli might have been a mixture of components to (a) changes in spatial contrast and (b) changes in luminous flux. In reality the situation seems to be even more complicated. The response is not merely an algebraic sum of (a) and (b), but the existence of one change may influence the response to the other change (Spekreijse, van der Tweel & Zuidema, 1973). To overcome this problem methods of spatial contrast stimulation were employed in which the total luminous flux remained constant in time. Although spatial contrast responses can be isolated in this way, it should be realized that even in this situation a luminance response may be produced. This complication seems especially important for the evaluation of the ERG’s to spatial contrast variation. Such ERG’s were first measured by Riggs, Johnson & Schick, (1964) and are also studied concurrently with the VER’s by Armington, Corwin & Marsetta, (1971). Riggs introduced this stimulus to avoid stray-light so as to minimize scotopic response components.

Spread of activation in the left ventricular wall of the dog. III.: Transmural and intramural analysis

Abstract Important information as to the course of activation in the heart wall can be obtained from the transmural wall curve. This is but little influenced by the other parts of the heart, a fact that can be demonstrated by studying the influence of septum activation on it. The wall curve is highly asymmetric, and its fall has a very steep part. This fast portion is synchronous with the fast part of the corresponding unipolar epicardial lead. The P-R interval of the epicardial lead differs from that of the transmural lead: the former is 10 to 15 milliseconds longer than the latter. This is explained by the course of the cavity potential and the wall curve. The descending part of the cavity curve and the ascending part of the wall curve are nearly identical. The causes of this remarkable phenomenon are analyzed. The cavity potential comprises potential contributions from the wall activation and septum activation. The transmural potential originates nearly totally from activation of the free wall. Since the electrodes lie on either side of the activation front, the transmural potential is higher than that part of the cavity potential that originates from the wall activation. The contribution of the septum activation to the cavity potential makes the latter more or less equal to the transmural potential. In view of the fact that even the wall curve can give only a rough picture of the activation of the wall, the latter has been examined by means of needle electrodes. It was shown that despite the movements of the heart it is possible to fix the electrodes satisfactorily. The lesion effects following insertion of the needle disappear within 15 minutes at the most. The surrounding tissue remains sufficiently intact. Unipolar intramural leads and partial wall curves were registered synchronously. The latter were registered between an intramural leadpoint and a cavity electrode, placed close to the endocardium of the wall area in question. The unipolar intramural curves were strongly negative in a large part of the wall; R-waves occurred only in the outermost layers. The complex at the epicardial surface is the smallest of the series. The partial wall curves of the innermost layers show, in correspondence to this, a low voltage. Only in the outer layers does the shape of the partial wall curve approach that of the wall curve for the region in question. Both the unipolar intramural and the partial wall curves show larger or smaller fast portions; these are due to the activation of the layer in which the leadpoint in question is situated. A more accurate analysis can be achieved by taking bipolar leads from the closely-spaced leadpoints of an electrode needle and by the use of a differential needle electrode. In this way it has been shown that the innermost layers are more or less synchronously activated. In consequence of this the activation front in these layers is irregular in shape and the voltage does not reach the same values as in the outermost layers. In the bipolar leads this is shown by the small, irregular complexes. The activation of the outer layers, on the other hand, gives rise to regularly formed complexes, the width of which is proportional to the distance between the leadpoints. This indicates that the front is regular in form and sharply bounded. The velocity of propagation in these layers is approximately 50 cm./sec. Between the synchronously activated inner layers, which account for approximately two-fifths of the thickness of the wall and the outer layers there is in some areas a transitional region in which complexes, often regular in shape, are found which are indicative of activation in epi-endocardial direction. A hypothesis accounting for these facts is presented.

Electrical activity in sinus node and atrioventricular node

Summary Complicated multiphasic differential complexes of a low voltage are demonstrated locally in the anatomic sinus nodal region. This local sinus activity is, in differential leads, most characteristic in that part of the sinus node, where, in simultaneous unipolar leads, the initial negativity is found. Pure initial negativity is seen only in a circumscript, very small area of the sinus node. The initial negative complexes are complicated by multiple fast deflections of a low voltage, and by notching of the descending limb. A striking resemblance is seen with the unipolar complexes from the sinus node region in dogs, as published by Rijlant (1936). With the help of a new technique, a similar complicated pattern of local activity is found in the atrioventricular node. Complexes of this type have been seen in both nodal tissue areas and nowhere else in the auricular myocardium. All experiments are performed on the intact heart of mongrel dogs. We believe we have demonstrated the existence of an inherent local activity in the sinus node and in the atrioventricular node. A theory is postulated concerning the origin and the conduction of the activation in the auricle of the dog.

Spread of activation in the left ventricular wall of the dog. IV: Two and three dimensional analysis☆

Needle electrodes were used for the spatial analysis of the activation of the left ventricular wall of the dog. Two and three electrodes of this type were introduced perpendicular to the epicardial surface into the lateral part of the left ventricular wall. With two-needle electrodes the angle of the activation front with the epicardial surface in the outer layers of the lateral, left ventricular wall, propagating in an apico-basal direction was estimated and found to be small, approximately 5 to 10°. This and the fact that the activation front has small bulges explains the complicated activation pattern at the epicardial surface. The activation pattern of the inner layers of the lateral part was investigated with three needles. The inner layers are activated in a very short time interval, considered in an endo-epicardial direction. In an apico-basal direction the experimental findings show well-developed complexes, pointing to an excess of apico-basal propagation in small areas activated by single Purkinje fibers. This explanation must be considered as a working hypothesis. The lead points of one needle electrode introduced perpendicularly to the epicardial surface were also used as stimulating electrodes. In this way it was proved that stimulation in the endocardial layers gives rise to an activation wave in endo-epicardial direction of all layers of that part of the wall investigated. The bipolar complexes of a needle electrode had approximately the same form and duration in all layers, indicating a nonfunctioning Purkinje system in the inner layers of the investigated part. Epicardial stimulation showed reversed complexes in all intramural bipolar leads of the investigated area. In experiments with two-needle electrodes placed in an apico-basal direction it was shown that the two-needle complexes in the inner layers did not reverse their polarity if two of the lead points of the apical needle electrode were used as a stimulating electrode, but reversed their polarity if the basal needle electrode was used. It was made plausible that the Purkinje system in the neighborhood of the stimulating electrode was not activated but that this occurred at a distance from the needle.