Decision-Making Time Cells in Hippocampal Dorsal CA1.

Sequential neural dynamics encoded by "time cells" play a crucial role in hippocampal function. However, the role of hippocampal sequential neural dynamics in associative learning is an open question. In this manuscript, we used two-photon Ca2+ imaging of dorsal CA1 pyramidal neurons in head-fixed mice performing a go-no-go associative learning task. We found that pyramidal cells responded differentially to the rewarded or unrewarded stimuli. The stimuli were decoded accurately from the activity of the neuronal ensemble, and accuracy increased substantially as the animal learned to differentiate the stimuli. Decoding the stimulus from individual pyramidal cells that responded differentially revealed that decision-making took place at discrete times after stimulus presentation. Lick prediction decoded from the ensemble activity of cells in dCA1 correlated linearly with lick behavior indicating that sequential activity of pyramidal cells in dCA1 constitutes a temporal memory map used for decision-making in associative learning.

Some aspects of the lateralization of echoed sound in man. II. The role of the stimulus spectrum.

The lateralization of clicks and their "echoes" was investigated with a view to determining the role of spectral characteristics in lateralization. Lateralization-discrimination performance was measured in a number of two-interval, two-alternative forced-choice experiments using three pairs of binaural clicks designed to elucidate how spectral cues are used in lateralization. The stimulus in one observation interval comprised a diotic click followed, after the interclick interval (ICI), by a dichotic click with either (1) an interaural time delay or (2) an interaural amplitude difference. The dichotic click was in turn followed, after an ICI of the same size, by another diotic click. In the second observation interval, the signals to the two ears were interchanged. The stimulus has the property that the signals delivered to the two ears had either (1) identical energy-density spectra but nonzero interaural-phase differences (IPDs) or (2) zero IPDs but nonidentical energy-density spectra. Under certain circumstances, observers perceived these stimuli as arising from the side of the head opposite that which would be predicted from the direction of the interaural cue in the temporal waveform. Joint consideration of the psychophysical data and the spectral characteristics of the stimuli strongly suggest a spectral "dominance region" for lateralization near 750 Hz, observers' lateralization performance was determined predominantly by the IPD cues from this region. In general, the results demonstrate that echoes of transients that arrive within about 2-3 ms of an initial transient are not suppressed, but have a substantial effect on lateralization through their contribution to the resultant spectral characteristics. The results contradict models that represent the precedence effect as the temporary suppression or inhibition of directional information in echoes over 2-3 ms after an initial transient.

Some aspects of the lateralization of echoed sound in man. I. The classical interaural-delay based precedence effect.

The precedence effect in two-click stimuli was investigated by measuring observers' sensitivity to interaural time delays (ITDs) as a function of interclick interval (ICI). A two-interval two-alternative forced-choice discrimination paradigm was used in two stimulus configurations: type I, a dichotic click with a given ITD preceded a diotic click; and type II, a dichotic click followed a diotic click. Threshold ITDs were measured in each configuration for a finely sampled distribution of ICIs that ranged from 0.1 to 25.6 ms. Performance was characterized by the "threshold elevation factor" (TEF) which normalized each of the observers' type I and type II ITD thresholds relative to their ITD threshold for a single dichotic click. The finer sampling of ICIs revealed two novel results: First, for two observers, sensitivity to ITD in the later arriving ITD (type II) oscillated in a consistent and systematic way with changes in ICI. Second, when the ICI reached 12.8 ms, ITD thresholds in the type I and type II configurations were equal but nearly a factor of 2 greater than for a single dichotic click. Some aspects of the data are consistent with the phenomenon of binaural adaptation.

Soluble cytochromes and ferredoxins from the marine purple phototrophic bacterium, Rhodopseudomonas marina.

Four soluble c-type cytochromes, the high redox potential 4-Fe-S ferredoxin known as HiPIP, a large molecular weight 2-Fe-S ferredoxin and a 4-Fe-S 'bacterial' ferredoxin, were isolated from extracts of two strains of Rps. marina. Cytochrome c-550, cytochrome c' and cytochrome c-549 were previously described, and we have extended their characterization. Cytochrome c-558, which has not previously been observed in Rps. marina, appears to be a low-spin isozyme of the more commonly observed high-spin cytochrome c'. HiPIP, which was not observed in previous work, was found to be abundant in Rps. marina. The 2-Fe-S ferredoxin, which has previously been observed only in Rps. palustris, has a native size greater than 100 kDa and a subunit size of 17 kDa. The 'bacterial' ferredoxin appears to have only a single four-iron-sulfur cluster. We examined photosynthetic membranes by difference spectroscopy and found abundant c-type cytochromes. Approximately one-quarter of the heme can be reduced by ascorbate and the remainder by dithionite. There is 2 nm difference between the high-potential heme (554 nm) and the low (552 nm). These characteristics resemble those of the tetraheme reaction center cytochrome of Rps. viridis. In addition to the electron transfer components, we found small amounts of a fluorescent yellow protein which has spectral resemblance to a photoactive yellow protein from Ec. halophila.

Effect of nucleotide structure on cardiac myosin subfragment 1 transient kinetics.

Transient kinetic data of the hydrolysis of several nucleotides (TTP, CTP, UTP, GTP) by cardiac myosin subfragment 1 (S1) were analyzed to obtain values for the equilibrium constant for nucleotide binding and rate constants for the S1-nucleotide isomerization and the subsequent nucleotide hydrolysis as well as the magnitudes of the relative fluorescence enhancements of the myosin that occur upon isomerization and hydrolysis. These data are compared with data from a previous study with ATP. Nucleotide binding is found to be relatively insensitive to nucleotide ring structure, being affected most by the group at position C6. Isomerization and hydrolysis are more sensitive to nucleotide structure, being inhibited by the presence of a bulky group at position C2. Kinetic parameters decrease as follows: for binding, GTP greater than UTP approximately TTP greater than ATP greater than CTP; for isomerization, ATP greater than UTP approximately TTP approximately CTP greater than GTP; for hydrolysis, ATP greater than TTP greater than CTP approximately UTP greater than GTP. Fluorescence enhancements appear to be most dependent upon the relative values of the individual rate constants.