Effects of expectation on face perception and its association with expertise.

Perceptual decisions are derived from the combination of priors and sensorial input. While priors are broadly understood to reflect experience/expertise developed over one's lifetime, the role of perceptual expertise at the individual level has seldom been directly explored. Here, we manipulate probabilistic information associated with a high and low expertise category (faces and cars respectively), while assessing individual level of expertise with each category. 67 participants learned the probabilistic association between a color cue and each target category (face/car) in a behavioural categorization task. Neural activity (EEG) was then recorded in a similar paradigm in the same participants featuring the previously learned contingencies without the explicit task. Behaviourally, perception of the higher expertise category (faces) was modulated by expectation. Specifically, we observed facilitatory and interference effects when targets were correctly or incorrectly expected, which were also associated with independently measured individual levels of face expertise. Multivariate pattern analysis of the EEG signal revealed clear effects of expectation from 100 ms post stimulus, with significant decoding of the neural response to expected vs. not stimuli, when viewing identical images. Latency of peak decoding when participants saw faces was directly associated with individual level facilitation effects in the behavioural task. The current results not only provide time sensitive evidence of expectation effects on early perception but highlight the role of higher-level expertise on forming priors.

California encephalitis virus endemicity in the Yukon Territory, 1972.

Sera from 218 of 1574 (14%) small mammals collected in the Yukon Territory between 14 May and 13 August 1972 neutralized a Yukon strain of California encephalitis virus (snowshoe-hare subtype). These included 133 of 319 (42%) snowshoe hares (Lepus americanus), 84 of 1243 (7%) ground squirrels (Citellus undulatus) and 1 of 12 (8%) tree squirrels (Tamiasciurus hudsonicus). California encephalitis virus (snow-shoe hare subtype) was isolated from four pools of unengorged Aedes communis mosquitoes collected near Whitehorse (61 degrees N., 135 degrees W.) and on one occasion each from pools of the same species collected at Hunker Creek (64 degrees N., 138 degrees W.) and at mile 125, Dempster Highway (66 degrees N., 138 degrees W.) during July 1972. Replication of a Yukon strain of California encephalitis virus was observed in wild-caught Culiseta inornata and Aedes canadensis mosquitoes after intrathoracic injection and holding at temperatures of 80 degrees , 50 degrees and 40 degrees F.