ALBERT

Notes: Abstract The correspondence between afferent discharges and sinusoidal length modulations (at 0.2–10 cps) was evaluated by average Lissajous displays of rate vs. length in isolated fast-adapting stretch receptor organs (FAO) of crayfish. Frequency effects (Diez Martínez et al., 1983) were compared using modulation depths and background lengths of, respectively, 1–10% of and 0.3–40% over resting length, well within physiological ranges. The receptor remained silent with shallow and slow stimuli. With greater depths, more frequencies became effective, discharges occupied more of each cycle, and peak and overall rates increased. At large depths, increases were smaller or were substituted by decreases (overstretch). With greater background lengths at 0.2 and 1 cps, individual and peak Lissajous rates increased, as did the overall value; eventually, the flat extension disappeared. At 0.2 cps, Lissajous plots became ellipses; at 1 cps, loops became counter-clockwise. At 3 and 10 cps, rates augmented only with small increases; beyond, peak rates remained unchanged or dropped. In short, each characteristic of the transduction is apparent only within a domain defined by the stimulus features as well as by the way stimuli and afferent discharges are evaluated. Each domain overlaps partially with others but falls short of that used in nature. For example: the stimulus-response relation was linear with low frequencies, large background lengths and moderate depths when observing average Lissajous plots; saturations were present with the low frequency, shallow modulations without discharges; clockwise loops occurred with particular frequencies, background lengths and depths, but not with others. Hence, any simple and succinct description is only a local rule that holds exclusively within restrictive constraints. Any generalization beyond this domain constitutes a misleading oversimplication.

Notes: Abstract Slowly adapting stretch receptor (SAO) pacemaker neurons, driven with periodic tugs, were analyzed by way of Poincaré mappings (Appendix). Two behaviors were apparent, i) Intermittency characterized previously unclear situations: discharges shifted irregularly between prolonged epochs where spike phases (relative to tugs) and intervals barely changed (slid), and brief bursts with marked variations (skipped), ii) Locking was well-known: phases and intervals remained almost fixed, regardless of the initiation. Changing frequencies, map domains with locking (ordered according to spikes/tugs ratios), alternated with intermittent ones. The best fit for any experimental map was a curve, not straight but certainly unidimensional, continuous and monotonic; it varied characteristically with frequency. This suggested relations called diffeomorphisms, implying periodicity and quasi-periodicity. Outcomes, expanding previous knowledge and meaningful biologically, were i)a precise, exhaustive behavior list (including between behavior transitions) and ii)a thorough understanding or model. This, in turn, provides norms for more specific models (single-variable ones suffice), constraints upon basic mechanisms (one variable, reflecting several real ones combined, should behave as the phase), and forecasts for future experimentation (e.g., unexamined tug frequencies and amplitudes).

Notes: Abstract The experimentally observed influence of regularly arriving tugs upon the AP discharge of the slowly-adapting stretch receptor organs (SAO) of crayfish was compared to a model of pacemaker excitatory synaptic interactions (Segundo and Kohn 1981). Criteria for compliance referred to facets as A) the excitation, B) the postulates, and C) the behavior. A) Excitation was implied primarily by the tug initially increasing the AP rate (it subsequently decreased it). B) The pacemaker AP discharges, and with more reason the electronically driven tugs, were considered acceptably regular sequence (postulate i). Tugs advanced the next AP (postulate ii); the “delay function” plost of delays vs. phases, i.e. interval shortenings vs. the time from the last AP to the tug, were close to theV of postulate iii (Fig. 2), even though the shortest phases tended to postopone the next AP and the longest ones did not trigger immediately but with an around 5 ms latency. These effects were displayed also as “old phase vs. new phase” plots (Figs.2 and 3). The interval following that with the tug tended to be lengthened, but the pre-tug timing was not recovered (Fig. 4). C) Behavior during a train of excitatory events, both in model and experiments, went through very similar initial settlings and eventual steady-states. The latter were characterized in the model by 1. an average excitatory vs. excited rate display formed by an endless number of segments with all positive rational slopes separated by negative-going discontinuities, 2. locking in the sense of preferential phases, and 3. periodic repetition of the same phases and inter-AP intervals. Experimental results were compatible with this (Figs. 5 and 7–9). Such behavior was absent when the tug sequence was highly irregular (Fig. 9). The initial settling, in the SAO as in the model, depended jointly on the first phaseΦ 1 and the intertug intervalE. If the former was under λ, it went through one or two monotonic phase-decreasing stages (one smaller, the other larger, than λ), or through a single increasing one, depending onE being smaller or greater than, respectively, an estimated but never actually observedE * leading to unstable lockings. If the initial phase was greater than λ, settling withE's underrN+λ involved jumps between larger than and smaller than λ phases; withE's overrn+λ, it involved an intermediate stable locking with Φ=E-rN. The model-embodiment correspondence in terms of postulates was judged acceptable in spite of discrepancies like the imperfect regularities, the dispersion and extra segment in the delay function (Fig. 2), and the post-tug interval lengthening (Fig. 4). The correspondence in terms of behavior turned out to be quite precise (see above) and at times surprising because of its counterintuitiveness. This reiterates the epistemologically interesting issue of why the generality of model applicability.

Notes: We report the case of an Indonesian patient who required urgent bone marrow transplantation for acute leukaemia and who received successive transplants from two siblings. The first transplant failed while the second was successful. There were some uncertainties in serological typing due to the presence of cross-reacting HLA-B alleles, lack of paternal typing and the use of Caucasoid sera for Indonesian patients. Distinction between the two donors was also difficult. Interestingly, the use of a new DNA technique identified the presence of differences between the patient and the first unsuccessful donor but not the second successful donor.

Notes: In this letter we report the measurement of the field enhancement at the tip of a scanning tunneling microscope, by means of the detection of the optical rectification current. A field enhancement factor between 1000 and 2000 is obtained for highly oriented pyrolytic graphite and between 300 and 600 for gold. Field enhancement factors found are strongly dependent on the particular tip used. The magnitude of the emitted light at the field enhanced region, calculated from the measured optical voltage, could be easily detected by a simple photodiode. © 1998 American Institute of Physics.

Notes: Thermal expansion of the sample or tip in a laser-assisted scanning tunneling microscope (STM) junction can obstruct the study of other phenomena related with STM–light interactions, when photoinduced currents are analyzed. In this article, we show that the thermal contribution is proportional to the average tunneling current and that this can be used to distinguish it from other contributions. Simultaneous tunneling current–voltage and photoinduced current–voltage curves are recorded for highly oriented pyrolitic graphite and gold samples with a Pt tip. We have done the measurements for two different polarizations of the incident beam. We show that the I–V curves can be used to discriminate between different mechanisms that appear, producing photoinduced currents. © 1997 American Institute of Physics.

Notes: Real time detection of pulsed laser surface melting was performed by analyzing the photoacoustic signals produced on the samples. Comparison between the amplitudes of the transversal and longitudinal waves allowed us to identify the fluence thresholds for surface melting. The method was tested with AISI 304 stainless steel samples and the results obtained were checked against direct metallographic analysis. © 1997 American Institute of Physics.

Notes: The absorption recovery of a photoexcited InGaP epitaxial film 0.4 μm thick was investigated using the pump-probe laser technique and found to have a time constant of 55 ps at room temperature. Measurements done in the temperature range of 300–50 K show the decay of the photoexcited carrier distribution to be dominated by ambipolar diffusion and surface recombination. The measured absorption recovery time constant corresponds to an ambipolar diffusion coefficient D(approximately-greater-than)2.8 cm2/s and a surface recombination velocity of S(approximately-greater-than)4×105 cm/s at room temperature.