Perceptual processing is facilitated by ascribing meaning to novel stimuli

storage and circadian rhythms. Memory and learning have been studied in squirrels that cache their food, sometimes not returning until the next year. Singing mice provide a new model for studying speech and learning. Studies of wild rodents will undoubtedly give us a window into the genetics underlying phenotypic variation, further promoted by genome sequencing projects that extend beyond the usual model species (see www. genome.gov). All of this diversity can be traced back to the first fossil rodents (Ischyromyoidae) from the late Paleocene of Asia. These primitive rodents, although donning a beaver-like skull, had the teeth and feet of a squirrel, and skeletal features suggestive of an arboreal lifestyle. And from so squirrely a beginning evolved endless rodential forms most beautiful — species as distinct as ungulate-like capybaras, raccoon-like viscachas, rabbit-like springhares, and otter-like muskrats. Rodents are an evolutionary success story— they were here long before us and these opportunistic survivors will certainly be here long after we are gone. A B C D E Current Biology Figure 5. Digit reduction in desert-dwelling rodents. (A) Gerbil (Meriones sp.). (B) Ord's kangroo rat (Dipodomys ordii). (C) Merriam's kangaroo rat (Dipo-domys merriami). (D) Jerboa (Allactaga sp.). (E) Greater Egyptian jerboa (Jaculus orientalis). Digits I and V are shown in yellow; digits II, III, and IV are shown in blue. Allactaga and Jaculus orientalis have lost digits I and V, and digits II, III, and IV have fused together. (Adapted with permission from Berman 1985 / Blackwell publishing.) Photograph of a Desert kangaroo rat provided by E. Bartov (top); photograph of a Northern three-toed jerboa provided by K. Cooper (bottom). (2004). Phylogeny and divergence-date estimates of rapid radiations in muroid rodents based on multiple nuclear genes. Can the interpretation of a visual stimulus (normally conceived as a late visual process) influence the recognition of that same stimulus (normally conceived as an early visual process)? Access of meaning from vision can be extremely rapid [1–3]. If the visual processing of meaningful stimuli is supported by top-down feedback from conceptual representations [4,5], then meaningful stimuli may be processed more efficiently than meaningless stimuli. A difficulty with testing this prediction is that meaningfulness is often confounded with familiarity. It is well established that familiar stimuli are easier to process than unfamiliar stimuli [6]. In visual search tasks, finding a target among unfamiliar non-targets (such as s) is much more effortful than searching among familiar non-targets …