Uncinate fascicle section leaves delayed matching-to-sample intact, with both large and small stimulus sets

An earlier study found that rhinal cortex ablations in the monkey (Macaca fascicularis) impaired delayed matching-to-sample only when the stimuli in the experiment came from a large population of possible stimuli, not when the stimulus population was small. The present experiment tested the idea that delayed matching-to-sample with a small stimulus population selectively engages the direct projection from visual association cortex to the prefrontal cortex, bypassing the rhinal cortex. This selective involvement could explain the preservation, after rhinal cortex ablations, of memory for items drawn from a small stimulus population. We trained monkeys preoperatively in delayed matching-to-sample with large and small stimulus populations, exactly as in the earlier study, then examined the effect of sectioning the cortico-cortical pathway between visual association cortex and prefrontal cortex, the uncinate fascicle. Uncinate fascicle section had no effect on postoperative performance of delayed matching-to-sample, with either large or small stimulus populations. These data give no support to the idea that preserved matching with a small stimulus population after rhinal lesions reflects the selective involvement in this task of the direct projection from visual association cortex to prefrontal cortex. Further, they strengthen the idea (derived from earlier studies of uncinate fascicle section) that the uncinate fascicle does not play a general role in visual memory or perception, but instead has a specialized function in the processing of conditional instruction cues.