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Notes: Summary Meinecke (1989, Exp. 1, cond. HO) showed that the detectability of a visual target embedded in a linear noise array decreases with increasing retinal eccentricity, while the reaction time (RT) of the hits increases. One of the most interesting features of her results was that the RT of the correct rejections is consistently larger than the RT for signals presented near the fovea. This finding suggests that initially visual attention is concentrated near the fixation point and then diffuses across the stimulus array to perform a serial, exhaustive search. We present a diffusion model of early visual-search processes that quantitatively describes this evolution of attention in time and space; in contrast to most previous conceptions, it is based on a genuine relation between the spatial and temporal dimensions of the search processes performed. The model predicts quantitatively both detection performance and RT. We conducted an experiment similar to that of Meinecke (1989), but with an additional variation of the presentation time. All the main features of the 189 predictions could be explained by the model. The interpretation of the four model's parameters is discussed in some detail and compared with previous estimates of the microscopic search speed derived from alternative models. Finally, we consider some possible modifications related to results of Kehrer (1987, 1989), and some generalizations to multi target detection and two-dimensional stimulus arrays.

Notes: Abstract In Go/No-Go detection tasks, responses to redundant targets are typically faster than responses to either of these targets alone. One explanation of this redundant-targets effect is from race models, which assume statistical facilitation due to the activation of more than one processing channel under the redundant-targets condition. J. O. Miller (1982, 1986) has derived an upper boundary for the amount of facilitation these models can predict and has found this boundary to be consistently violated in bimodal divided-attention tasks and in letter-detection tasks. Thus, until recently race models were thought to be unable to predict the amount of facilitation commonly observed. Mordkoff and Yantis (1991) have challenged this conclusion and showed that no facilitation beyond the predictions of race models is observed if certain types of contingency within the experimental design are removed. The present study tries to replicate this basic finding and to generalize it to conditions with (a) nonsimultaneous signal presentation, and (b) negative interstimuius contingency benefit. Several important predictions of Mordkoff and Yantis' interactive race model were found to hold, but for nonsimultaneous signals presentations consistent violations of Miller's upper bound were found under certain conditions. The implications of the present results for models of divided attention are discussed.

Notes: Summary The popular, but low-power, two-stimulus signal-detection paradigms are extended to generate response-criterion-dependent psychometric functions. Maximumlikelihood estimation within a sufficiently powerful experimental design is described, and theoretical conditions for the resulting psychometric functions to be monotonically increasing are presented. These results are applied to an experiment on the detection of spatial-density differences in briefly presented random-dot patterns. The results confirm, extend, and complement current notions put forth by H. B. Barlow and his co-workers on mechanisms of internal spatial-density representation and the efficiency of intermediate levels of visual processing.

Notes: Summary Modulation of the current generated by the Na+/K+ pump by membrane potential and protein kinases was investigated in oocytes of Xenopus laevis. In addition to a positive slope region in the current-voltage (I–V) relationship of the Na+/K+ pump, a negative slope region has been described in these cells (Lafaire & Schwarz, 1986) and has been attributed to a voltage-dependent apparent K m value for pump stimulation by external [K+] (Rakowski et al., 1991). To study this feature in more detail. Xenopus oocytes were used for comparative analysis of the negative slope of the I–V relationship of the endogenous Na+/K+ pump and of the Na+/K+ pump of the electric organ of Torpedo californica expressed in the oocytes. The effects of stimulation of protein kinases A and C on the negative slope were also analyzed. To investigate the negative slope over a wide potential range, experiments were performed in Na+-free solution and in the presence of high concentrations of Ba2+ and tetraethylammonium, to block all nonpump related K+-sensitive currents. Pump currents and pump-mediated fluxes were determined as differences of currents or fluxes in solutions with and without extracellular K+. The voltage dependence of the K m value for stimulation of the Na+/K+ pump by external [K+] shows significant species differences. Over the entire voltage range from -140 to +20 mV, the K m value for the Na+/K+ pump of Torpedo electroplax is substantially higher than for the endogenous pump and exhibits more pronounced voltage dependence. For the Xenopus pump, the voltage dependence can be described by voltage-dependent stimulation by external [K+] and can be interpreted by voltage-dependent K+ binding, assuming that an effective charge between 0.37 and 0.56 of an elementary charge is moved in the electrical field. An analogous evaluation of the voltage dependence of the Torpedo pump requires the assumption of movement of two effective charges of 0.16 and 1.0 of an elementary charge. Application of 1,2-dioctanoyl-sn-glycerol (diC8. 10–50 μm). which is known to stimulate protein kinase C, reduces the maximum activity of the Xenopus pumps in the oocyte membrane by 40% and modulates the voltage dependence of K+ stimulation. For the endogenous Xenopus pump, the apparent effective charge increased from 0.37 to 0.51 of elementary charge and the apparent K m at 0 mV increased from 0.46 to 0.83 mm. For the Torpedo pump, one of the apparent effective charges increased from 1.0 to 2.5 of elementary charge. Injection of cAMP (final concentration 50 μm), which stimulates protein kinase A, has an effect opposite to stimulation of protein kinase C. The activity of the Xenopus Na+/K+ pump is elevated by 80%, and the voltage dependence of K+ stimulation becomes less pronounced. For the endogenous pump the apparent effective charge decreased from 0.56 to 0.38 of elementary charge and the apparent K m at 0 mV decreased from 0.78 to 0.65 mm. Also for the Torpedo pump, the effective charges and apparent affinities became reduced. The data suggest that species differences in voltage-dependent stimulation of the Na+/K+ pump by external K+ can account for differences in the steepness of the negative slope in the I–V relationships observed in different preparations. In addition, they suggest that the voltage dependence and the maximum activity of the Na+/K+ pump can be modulated by activation of protein kinases.

Notes: Abstract Exact predictions for two-pulse visual temporal integration data are derived from the Bouman-van der Velden quantum coincidence model for threshold vision. The predictions of the model start with complete summation for superposed pulses, then pass to a transition zone of partial integration, and finally reach the level of probability summation for pulses presented with large interstimulus intervals. From our results we can clearly reject the assumption of constant integration times with the basic model. We thus generalize the coincidence model to allow for variable integration times, derive the corresponding predictions for two-pulse integration data, and compare these predictions to published data currently available. It is shown that detectors of low order of coincidence generally underestimate the actual reduction of threshold intensity (or equivalently the corresponding increase of the detection probability) for two pulses as compared to the singlepulse performance.

Notes: Abstract Currents generated by the Na+/K+ ATPase were measured under voltage clamp in oocytes of Xenopus laevis. The dependence of pump current on external [Na+] was investigated for the endogenous Xenopus pump as well as for wild-type and mutated pumps of electroplax of Torpedo californica expressed in the oocytes. The mutants had α-subunits truncated before position Lys28 (αΔK28) or Thr29 (αΔT29) of the N-terminus. The currents generated by all variants of pump molecules in the presence of 5 mM K+ show voltage-dependent inhibition by external [Na+]. The apparent K1 values increase with membrane depolarisation, and the potential dependence can be described by the movement of effective charges in the electrical potential gradient across the membrane. Taking into account Na+-K+ competition for external binding to the E2P form, apparent K1 values and effective charges for the interaction of the Na+ ions with the E2P form can be estimated. For the Xenopus pump the effective charge amounts to 1.1 of an elementary charge and the K1 value at 0 mV to 44 mM. For the wild-type Torpedo pump, the analysis yields values of 0.73 of an elementary charge and 133 mM, respectively. Truncation at the N-terminus removing a lysinerich cluster of the a-subunit of the Torpedo pump leads to an increase of the effective charge and decrease of the K1 value. For αΔK28, values of 0.83 of an elementary charge and 117 mM are obtained, respectively. If LyS28 is included in the truncation (α·T29), the effective charge increases to 1.5 of an elementary charge and the apparent K1 value is reduced to 107 mM. The K, values for pump inhibition by external Na+, calculated by taking into account Na+-K+ competition, are smaller than the K/12 values determined in the presence of 5 mM [K+]. The difference is more pronounced for those pump variants that have higher Km, values. The variations of the parameters describing inhibition by external [Na+] are qualitatively similar to those described for the stimulation of the pumps by external [K+] in the absence of extracellular [Na+]. The observations may be explained by an acess channel within the membrane dielectric that has to be passed by the external Na+ and K+ ions to reach or leave their binding sites. The potential-dependent access and/or the interaction with the binding sites shows species differences and is affected by cytoplasmic lysine residues in the N-terminus.