Acute Amnestic Syndrome and Ischemic Stroke: A Case Series.

PURPOSE OF REVIEW: Stroke is an uncommon cause of amnesia. We describe in detail 3 cases of anterograde amnesia and confabulation secondary to acute ischemic stroke and review the available literature. RECENT FINDINGS: In our case series, all 3 patients presented with anterograde amnesia and 2 of 3 copresented with prominent confabulation. These symptoms were recognized in delayed fashion, and no patients received IV tissue plasminogen activator (tPA). Although stroke infarct topology was variable, all 3 patients had infarction of the fornix. Long-term follow-up was obtained in 2 of 3 patients: both had persistent memory impairment and were no longer functionally independent. SUMMARY: Acute onset anterograde amnesia and confabulation may uncommonly represent acute ischemic stroke. Delays in this diagnosis typically exclude patients from emergent stroke treatment or timely diagnostic stroke evaluation. Clinicians should maintain a high degree of suspicion for ischemic stroke in this setting, especially in patients with comorbid vascular risk factors. Memory impairment secondary to ischemic stroke can produce considerable long-term disability.

Sequential mechanisms underlying concentration invariance in biological olfaction.

Concentration invariance-the capacity to recognize a given odorant (analyte) across a range of concentrations-is an unusually difficult problem in the olfactory modality. Nevertheless, humans and other animals are able to recognize known odors across substantial concentration ranges, and this concentration invariance is a highly desirable property for artificial systems as well. Several properties of olfactory systems have been proposed to contribute to concentration invariance, but none of these alone can plausibly achieve full concentration invariance. We here propose that the mammalian olfactory system uses at least six computational mechanisms in series to reduce the concentration-dependent variance in odor representations to a level at which different concentrations of odors evoke reasonably similar representations, while preserving variance arising from differences in odor quality. We suggest that the residual variance then is treated like any other source of stimulus variance, and categorized appropriately into "odors" via perceptual learning. We further show that naïve mice respond to different concentrations of an odorant just as if they were differences in quality, suggesting that, prior to odor categorization, the learning-independent compensatory mechanisms are limited in their capacity to achieve concentration invariance.