When monkeys perform a delayed match-to-sample task, some neurons in the anterior inferotemporal cortex show sustained activity following the presentation of specific visual stimuli, typically only those that are shown repeatedly. When sample stimuli are shown in a fixed temporal order, the few images that evoke delay activity in a given neuron are often neighboring stimuli in the sequence, suggesting that this delay activity may be the neural correlate of associative long-term memory. Here we report that stimulus-selective sustained activity is also evident following the presentation of the test stimulus in the same task. We use a neural network model to demonstrate that persistent stimulus-selective activity across the intertrial interval can lead to similar mnemonic representations (distributions of delay activity across the neural population) for neighboring visual stimuli. Thus, inferotemporal cortex may contain neural machinery for generating long-term stimulus-stimulus associations.

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1H and 31P NMR spectral analysis of a chromomycin/d(ATGCAT)2 complex provides strong evidence for a nonintercalative mode of drug binding. Investigation of the imino proton region of the duplex suggests a protection of one of the two guanine imino protons from fast exchange with the bulk water up to at least 45 degrees C by the drug. Subsequent one-dimensional nuclear Overhauser enhancement experiments place the exchangeable chromomycin chromophoric hydroxyl proton less than 0.45 nm from this guanine imino proton and the chromophore 7-methyl less than 0.45 from the internal thymine 6-proton and/or the guanine 8-proton. 1H two-dimensional NMR reveals that the duplex retains a right-handed B conformation but there are distortions at the TGC region of one chain and large deviations in the chemical shift of protons relative to the uncomplexed duplex in the other chain in the same TGC region. The data suggest that the chromomycin chromophore is oriented such that the hydrophilic side of the ring system is proximal to the helix center in the major groove near the TG region while the aromatic side of the ring is oriented away from the helix but is partially protected from the solvent by the aliphatic chain, which bends back over the two aromatic protons. Changes in the 31P spectrum of the duplex on binding of the drug are different from the effect of either actinomycin or netropsin on nucleic acid fragments.

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