Activity of primate inferotemporal neurons related to a sought target in pair-association task.

Visual long-term memory in primates has been assessed by using the pair-association (PA) task, in which a subject retrieves and chooses the paired associate of a cue picture. Our previous studies on single neurons in the anterior inferotemporal (AIT) cortex suggested their roles in representing paired associates in the mind. To test the possibility that the delay activity of AIT neurons is related to a particular picture as a sought target, we devised the PA with color switch (PACS) task. In the PACS task, the necessity for memory retrieval and its initiation time were controlled by a color switch in the middle of the delay period. A control task, in which there is no color switch, corresponds to the conventional delayed matching-to-sample (DMS) task where the monkey chooses the same picture as a cue. We found that AIT neurons started to respond just after the color switch in the PACS task, when the cue-optimal picture's associate was presented as a cue. In contrast, they showed no response change in the DMS task. We confirmed that this effect is not due to the visual response to colors. Furthermore, when the cue-optimal picture was presented as a cue, these neurons showed suppression after the color switch in the PACS task. These results suggest that the activity of AIT neurons mediates gating mechanisms that preferentially pass information about a sought target, even when the sought target is retrieved from long-term memory.

Repair of x-ray-induced DNA double-strand breaks in specific Not I restriction fragments in human fibroblasts: joining of correct and incorrect ends.

An assay that allows measurement of absolute induction frequencies for DNA double-strand breaks (dsbs) in defined regions of the genome and that quantitates rejoining of correct DNA ends has been used to study repair of dsbs in normal human fibroblasts after x-irradiation. The approach involves hybridization of single-copy DNA probes to Not I restriction fragments separated according to size by pulsed-field gel electrophoresis. Induction of dsbs is quantitated from the decrease in the intensity of the hybridizing restriction fragment and an accumulation of a smear below the band. Rejoining of dsbs results in reconstitution of the intact restriction fragment only if correct DNA ends are joined. By comparing results from this technique with results from a conventional electrophoresis assay that detects all rejoining events, it is possible to quantitate the misrejoining frequency. Three Not I fragments on the long arm of chromosome 21 were investigated with regard to dsb induction, yielding an identical induction rate of 5.8 X 10(-3) break per megabase pair per Gy. Correct dsb rejoining was measured for two of these Not I fragments after initial doses of 80 and 160 Gy. The misrejoining frequency was about 25% for both fragments and was independent of dose. This result appears to be representative for the whole genome as shown by analysis of the entire Not I fragment distribution. The correct rejoining events primarily occurred within the first 2 h, while the misrejoining kinetics included a much slower component, with about half of the events occurring between 2 and 24 h. These misrejoining kinetics are similar to those previously reported for production of exchange aberrations in interphase chromosomes.