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A Bartels

Publications and source records attributed to A Bartels.

26 records · Page 2Linked to original sources
PubMed1998

The theory of multistage integration in the visual brain.

The theory of multistage integration is based on evidence that the visual brain consists of several parallel multistage processing systems, each specialized for a given attribute such as colour or motion. Each stage of a given system processes information at a distinct level of complexity. Our theory supposes that activity at any stage of a given multistage processing system is perceptually explicit--that is to say, it requires no further processing to generate a conscious experience. This activity can be integrated, or bound, with the perceptually explicit activity at any given stage of another or the same multistage processing system. Such binding is therefore not a process that generates a conscious experience, but rather one that brings different conscious experiences together. Many perceptual advantages result from such a flexible and dynamic integrative system. Conversely, there would be disadvantages to limiting perception and binding to hypothetical 'terminal' stages of such processing systems or to hypothetical 'integrator' areas. Although we formulate our hypothesis in terms of the visual brain, we believe it might form a general principle of brain functioning.

Animals↗
PubMed1998

The autonomy of the visual systems and the modularity of conscious vision.

Anatomical and physiological evidence shows that the primate visual brain consists of many distributed processing systems, acting in parallel. Psychophysical studies show that the activity in each of the parallel systems reaches its perceptual end-point at a different time, thus leading to a perceptual asynchrony in vision. This, together with clinical and human imaging evidence, suggests strongly that the processing systems are also perceptual systems and that the different processing-perceptual systems can act more or less autonomously. Moreover, activity in each can have a conscious correlate without necessarily involving activity in other visual systems. This leads us to conclude not only that visual consciousness is itself modular, reflecting the basic modular organization of the visual brain, but that the binding of cellular activity in the processing-perceptual systems is more properly thought of as a binding of the consciousnesses generated by each of them. It is this binding that gives us our integrated image of the visual world.

Animals↗
PubMed1998

The asynchrony of consciousness.

We present below a simple hypothesis on what we believe is a characteristic of visual consciousness. It is derived from facts about the visual brain revealed in the past quarter of a century, but it relies most especially on psychophysical evidence which shows that different attributes of the visual scene are consciously perceived at different times. This temporal asynchrony in visual perception reveals, we believe, a plurality of visual consciousnesses that are asynchronous with respect to each other, reflecting the modular organization of the visual brain. We further hypothesize that when two attributes (e.g. colour and motion) are presented simultaneously, the activity of cells in a given processing system is sufficient to create a conscious experience of the corresponding attribute (e.g. colour), without the necessity for interaction with the activities of cells in other processing systems (e.g. motion). Thus, any binding of the activity of cells in different systems should be more properly thought of as a binding of the conscious experiences generated in each system.

Animals↗
PubMed1994

The spatiotemporal transfer function of crayfish lamina monopolar neurons.

1. The graded, synaptic potentials of first-order visual interneurons (lamina monopolar cells) were examined with intracellular recordings. The spatiotemporal properties were characterized with drifting sine wave gratings and annuli. 2. Annulus-elicited inhibition is maximal for annulus-test pulse intervals of approximately 140 ms and declines exponentially. The inhibition declines with increasing annular internal radii (ri). 3. Grating responses were examined with respect to spatial and temporal frequency. The gratings elicit sinusoidal signals that are approximately linear with contrast. 4. Variations in spatial frequency produce response functions with a low-pass or modest band-pass characteristic, which are described by a difference of Gaussians sensitivity profile. The central Gaussian approximates the sensitivity profile of photoreceptors. The inhibitory Gaussian is similar to the inhibitory field estimated with annulus pulses. The peak of the inhibitory Gaussian is approximately 18% of the peak excitatory Gaussian. 5. Variations in temporal frequency generally produce transfer functions with a band-pass characteristic and a peak at 1.0 Hz. These data were described by a difference of exponentials function convolved with a low-pass filter that approximates the photoreceptor response. The inhibitory time course estimated from these data was similar to that of the annulus measurements. 6. The spatiotemporal properties of lateral inhibition are consistent with inhibitory action by the lamina amacrine neurons. The proposed model is spatiotemporally inseparable and nonrecurrent. 7. Eleven of 20 monopolar cells tested exhibited a strong orientation preference with a bias to the vertical. Photoreceptors exhibit little or no orientation preference.

Animals↗
PubMed1992

Non-invasive determination of systolic blood pressure by heart sound pattern analysis.

A new concept of non-invasive blood pressure measurement by heart sound pattern analysis is described. The known diagnostic criterion of the 'accentuated' second heart sound of a hypertensive patient is here converted into a computer-aided pattern-recognition process for the second heart sound, applicable over the entire scale of blood pressure. After a 'learning phase', during which the second heart sound is recorded, analysed and correlated with a set of systolic blood pressure values of the individual patient, the computer is able to determine systolic blood pressure of the same patient from the acoustic spectrum of the second heart sound with sufficient precision. The method is in principle suited for automatically repeated blood pressure measurements, but further development is still needed for conversion into a widely practicable procedure.

Blood Pressure↗
PubMed1990

[Control of the placement of a central venous catheter using Doppler ultrasound].

Precise placement of central venous catheters is necessary to prevent complications and assure proper functioning. Chest X-ray is the current standard method of locating the catheter tip. This is usually not feasible in the operating room setting, particularly after the induction of anesthesia. Intravascular ECG registration using the catheter as a lead and identification of intra-atrial P-waves has been suggested as an alternative. In the present study we evaluated the use of Doppler sonography as a noninvasive method of locating the catheter tip and detecting faulty placement. Two hundred patients scheduled for insertion of a central venous catheter took part in this study. The catheters were inserted via standard routes (internal or external jugular vein, basilar or cephalic vein). A Doppler sonographic device with a 2 mHz probe was used (Parke Electronics 915L). The probe was applied to the right sternal border and affixed at the position where the characteristic venous flow sound was most distinct. The signals were displayed visually, subjected to spectral analysis, and also recorded for later evaluation. A rapid injection of 2-5 ml isotonic saline causes turbulences which can readily be heard and recognized without special training. The position of every catheter was later confirmed by radiography, and in 159 patients the intraatrial ECG method was subjected to direct comparison with the sonographic method. The turbulences due to the injected fluid were found to cause an increased amplitude at frequencies above 350 Hz. If the catheter tip was positioned correctly there was no discernable time lag between the start of the injection and perception of turbulences.(ABSTRACT TRUNCATED AT 250 WORDS)

Catheterization, Central Venous↗