The ability to accurately remember distinct episodes is supported by high-level sensory discrimination. Performance on mnemonic similarity tasks, which test high-level discrimination, declines with advancing age in humans and these deficits have been linked to altered activity in hippocampal CA3 and dentate gyrus. Lesion studies in animal models, however, point to the perirhinal cortex as a brain region critical for sensory discriminations that serve memory. Reconciliation of the contributions of different regions within the cortical-hippocampal circuit requires the development of a discrimination paradigm comparable to the human mnemonic similarity task that can be used in rodents. In the present experiments, young and aged rats were cross-characterized on a spatial water maze task and two variants of an object discrimination task: one in which rats incrementally learned which object of a pair was rewarded and different pairs varied in their similarity (Experiment 1), and a second in which rats were tested on their ability to discriminate a learned target object from multiple lure objects with an increasing degree of feature overlap (Experiment 2). In Experiment 1, aged rats required more training than young to correctly discriminate between similar objects. Comparably, in Experiment 2, aged rats were impaired in discriminating a target object from lures when the pair shared more features. Discrimination deficits across experiments were correlated within individual aged rats, though, for the cohort tested, aged rats were not impaired overall in spatial learning and memory. This could suggest discrimination deficits emerging with age precede declines in spatial or episodic memory, an observation that has been made in humans. Findings of robust impairments in object discrimination abilities in the aged rats parallel results from human studies, supporting use of the developed tasks for mechanistic investigation of cortical-hippocampal circuit dysfunction in aging and disease.

No abstract

The prominent sensory recruitment model argues that visual working memory (WM) is maintained via representations in the same early visual cortex brain regions that initially encode sensory stimuli, either in the identical neural populations as perceptual representations or in distinct neural populations. While recent research seems to reject the former (strong) sensory recruitment model, the latter (flexible) account remains plausible. Moreover, this flexibility could explain a recent result of high theoretical impact (Harrison & Bays, The Journal of Neuroscience, 38 (12), 3116-3123, 2018)a failure to observe interactions between items held in visual WMthat has been taken to reject the sensory recruitment model. Harrison and Bays (The Journal of Neuroscience, 38 (12), 3116-3123, 2018) tested the sensory recruitment model by comparing the precision of memoranda in radially and tangentially oriented memory arrays. Because perceptual visual crowding effects are greater in radial than tangential arrays, they reasoned that a failure to observe such anisotropy in WM would reject the sensory recruitment model. In the present Registered Report or Replication, we replicated their study with greater sensitivity and extended their task by controlling a potential strategic confound. Specifically, participants might remap memory items to new locations, reducing interactions between proximal memoranda. To combat remapping, we cued participants to report either a memory item or its precise location with this report cue presented only after a memory maintenance period. Our results suggest that, similar to visual perceptual crowding, location-bound visual memoranda interact with one another when remapping is prevented. Thus, our results support at least a flexible form of the sensory recruitment model.

No abstract