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can drive convergence in avian spectacular plumages through this radiation. Proc. Natl. Acad. Sci. USA 101, 11040–11045. morphology [15,16]. long period of evolutionary isolation. 10. Hackett, S.J., Kimball, R.T., Reddy, S., As suggested by Fleischer and Bowie, R.C., Braun, E.L., Braun, M.J., The Loss of an Ancient Lineage his colleagues [4], the Hawaiian Chojnowski, J.L., Cox, W.A., Han, K.L, Harshman, J., et al. (2008). A phylogenomic The misleading caused by lineage is best classified study of reveals their evolutionary history. this convergent has been into its own new family, the Mohoidae. Science 320, 1763–1768. 11. Fuchs, J., Fjeldsa, J., Bowie, R.C.K., Voelker, G., rectified by the new DNA-based Sadly, this is the only avian family and Pasquet, E. (2006). The African warbler genus analyses, which reveal the surprising known to have gone extinct in Hyliota as a lost lineage in the Oscine uniqueness of these Hawaiian birds. its entirety in the past several tree: Molecular support for an African origin of the . Mol. Phyl. Evol. 39, 186–197. The last sighting of a Hawaiian centuries. Its demise therefore 12. Cibois, A., and Cracraft, J. (2004). Assessing Chaetoptila occurred in 1859, and three represents the loss of a particularly the ‘‘Tapestry’’: phylogenetic relationships of the Muscicapoidea inferred of the four species were likewise divergent evolutionary lineage from nuclear DNA sequences. Mol. Phyl. Evol. extinct by the mid-1900s. The final [19,20], one that we only now 32, 264–273. surviving species, Moho braccatus, recognize for its true level of 13. Willerslev, E., and Cooper, A. (2005). Ancient DNA. Proc. Royal. Soc. Lond. B 272, 3–16. persisted in the highlands of Kauai into uniqueness. 14. Millar, C.D., Huynen, L., Subramanian, S., the late 1980s, but is now almost Mohandesan, E., David, M., and Lambert, D.M. (2008). New developments in ancient genomics. certainly also extinct. The poignancy of References Trends Ecol. Evol. 23, 386–393. this lineage’s decline is captured in 1. Futuyma, D.J. (2005). Evolution (Sunderland, 15. Beecher, W.J. (1951). Convergence in the recordings of the haunting song of the MA: Sinauer Associates). Coerebidae. Wilson Bull. 63, 274–287. 2. Harmon, L.J., Kolbe, J.J., Cheverud, J.M., and 16. Burns, K.J., Hackett, S.J., and Klein, N.K. (2003). last (and mate-less) male Kauaii ‘O’o, Losos, J.B. (2005). Convergence and the Phylogenetic relationships of Neotropical which can be played online from the multidimensional niche. Evolution 59, 409–421. honeycreepers and the evolution of feeding 3. Wake, D.B. (1991). Homoplasy: the result of morphology. J. Avian Biol. 34, 360–370. Cornell Lab of Ornithology’s sound natural selection or evidence of design 17. Boyer, A.G. (2008). patterns in the archive (http://www.birds.cornell.edu/ limitations? Am. Nat. 138, 543–567. avifauna of the Hawaiian Islands. Divers. macaulaylibrary/). Like so much of the 4. Fleischer, R.C., James, H.F., and Olson, S.L. Distrib. 14, 509–517. (2008). of Hawaiian 18. Price, J.P., and Clague, D.A. (2002). How old is native Hawaiian avifauna, these and Australo-Pacific from the Hawaiian biota? Geology and phylogeny honeyeaters were doomed by a lethal distant songbird ancestors. Curr. Biol. 18, suggest recent divergence. Proc. Roy. Soc. B 1927–1931. 269, 2429–2435. combination of human-caused 5. James, H.F., and Olson, S.L. (1991). 19. Faith, D.P. (1992). Conservation evaluation and stressors [17]. Descriptions of thirty-two new species of birds phylogenetic diversity. Biol. Cons. 61, 1–10. The new phylogenetic evidence from the Hawaiian Islands: Part II. . 20. Erwin, D.H. (2008). Extinction as the loss of Ornithological Monographs 46, 1–88. evolutionary history. Proc. Natl. Acad. Sci. USA places the split between the Hawaiian 6. Fleischer, R.C., and McIntosh, C.E. (2001). 105, 11520–11527. honeyeaters and their living relatives at Molecular systematics and biogeography of the Hawaiian avifauna. Studies Avian Biol. 22, about 15 million years ago, a period 51–60. Fuller Evolutionary Biology Program, Cornell coincident with the arrival in the 7. Fuller, E. (2001). Extinct Birds (Ithaca, NY: Lab of Ornithology, Cornell University, islands of the -pollinated Cornell University Press). 159 Sapsucker Woods Road, Ithaca, 8. Dickinson, E.C. (2003). The Howard and Moore plants that likely fostered their NY 14850, USA. Complete Checklist of the Birds of the World, E-mail: [email protected] nectivorous specializations [4,18]. The 3rd edition (London: Christopher Helm). 9. Barker, F.K., Cibois, A., Schikler, P., Hawaiian honeyeaters evolved their Feinstein, J., and Cracraft, J. (2004). Phylogeny nectar-feeding adaptations and and diversification of the largest avian DOI: 10.1016/j.cub.2008.11.006

Perceptual Learning: Complete location or orientation of the lines between training and test greatly Transfer across Retinal Locations reduces or abolishes the effect of practicing Vernier discriminations or other simple discrimination tasks [3–5]. A newly developed ‘double training’ technique demonstrates that practice- Such specificity is unlikely to be dependent improvement in the discrimination of basic visual features will present with more complex stimuli, transfer to a location that has been trained with a different discrimination. such as faces [6,7], that are not restricted to a single retinal location. Dominic M. Dwyer distinctions between complex Research reported by Xiao et al. [8] in multimodal stimuli that are not this issue of Current Biology questions Gibson [1] defined perceptual possible for novices [2] to the fact whether perceptual learning with learning as ‘‘any relatively permanent that extensive practice with very simple visual stimuli is genuinely and consistent change in the simple discriminations, for example specific to particular retinal locations perception of a stimulus array, between the Vernier offsets of two by demonstrating that, with following practice or experience sets of lines, leads to improved appropriate training methods, with this array’’. This definition performance [3]. improvements in discrimination can encompasses practice-dependent There is much evidence that transfer completely across locations. improvements in performance perceptual learning with simple stimuli The issue of whether or not ranging from the observation that can be very specific to the training perceptual learning is specific to experienced wine-tasters can make situation: changes in the retinal trained locations or stimuli is of Dispatch R1135 critical importance because early A B C D visual cortex contains neurons that are more selective to the position and orientation of stimuli than are neurons further down the visual processing stream. Thus, the apparent specificity + + + + of perceptual learning to particular orientations or locations was considered to be very strong evidence that the neural mechanisms E Test after phase 1 training underpinning the improvements must Test after phase 2 training 40 involve the early visual cortex (for example [9,10]). So consistent have the observations of stimulus 30 specificity been that they have, quite justifiably, been taken as a grounding 20 constraint on both empirical investigations and theoretical accounts of perceptual learning (for 10 example [10,11]). Despite this, it was noted over a decade ago that the 0 specificity in perceptual learning might lie in what is learnt, rather than where Mean % improvement the learning takes place [12], and this –10 possibility has only now been directly investigated [8]. –20 A simplified version of the Ori1 Ori1 Ori2 Ori2 Ori1 Ori2 double-training version of the Vernier Loc1 Loc2 Loc1 Loc2 Loc2 Loc1 discrimination task used by Xiao et al. [8] is shown in Figure 1. Initially Current Biology training was given with lines of one orientation in one location (for example, Figure 1. A simplified version of Experiment 3 from [8]. vertical lines in the upper left visual (A) Phase 1 training with Vernier discriminations using vertical lines in the upper left quadrant; quadrant). In line with previous results, orientation 1-location 1 (Ori1Loc1 in (E), which shows the percent improvement over baseline this produced an improvement in in this condition). (B) The transfer test condition orientation 1-location 2 (Ori1Loc2): Vernier discrimination performance, but this discrimination with the original training stimulus at a new location. (C) Transfer test condition improvement did not transfer to orientation 2-location 1 (Ori2Loc1): Vernier discrimination with a new stimulus at the original location. (D) Phase 2 training with Vernier discriminations using horizontal lines in the horizontal lines tested at the original lower left quadrant (orientation 2-location 2, Ori2Loc2). The transfer tests shown in (B) and location or to the vertical lines tested at (C) were performed after both phase 1 and phase 2 training (left and right sides of (E), respec- a different location such as the lower tively). Note, in the actual experiment the locations and line orientations were counterbal- left visual quadrant (see the leftmost anced. (E) Summary results as mean percentage improvement in discrimination performance three bars in Figure 1E). over baseline. The left side shows performance after phase 1 training and the right side shows Subsequently, training was given performance after phase 2 training. with a second orientation at a second location — for example, horizontal locations when the two discriminations [13,14]), but that the critical site for lines in the lower left visual were trained concurrently and when the process rather than the outcome quadrant — before testing with the different types of discrimination, of perceptual learning must be assignment of line orientation to rather than different orientations of more central or complete transfer training location reversed. This lines, were used. across locations would be second phase of training dramatically The complete transfer of impossible. Location transfer also changed the results of the critical training-dependent improvement in implies that a reconsideration is transfer tests. Discrimination with the discrimination from one retinal needed of the role played by vertical lines in the lower quadrant was location to another directly challenges non-retinotipic central brain now as good as it was in the upper the idea that location specificity is a key mechanisms (for example [15]). quadrant at which it had actually been feature of perceptual learning. In turn, These have previously been trained. Discrimination with horizontal this questions the common belief that considered in light of the fact that lines was as good in the upper retinotopically organised early visual attention constrains perceptual quadrant, where it had not been cortex is the neural site for perceptual learning (for example [16]), but the trained, as it was in the lower quadrant learning and suggests instead that results of Xiao et al. [8] suggest that where it had been trained (see the more central mechanisms are involved. they might actually underpin the rightmost three bars Figure 1E). Other This is not to say that perceptual learning itself rather than simply its experiments confirmed that learning does not influence primary attentional modulation. double-training allowed for complete visual cortex, as there is direct Retinotopic specificity would transfer of perceptual learning across evidence that it does (for example constrain not only neural models of Current Biology Vol 18 No 24 R1136 perceptual learning but also functional consequences than have previously 13. Schoups, A., Vogels, R., Qian, N., and Orban, G. (2001). Practising orientation identification ones. A number of theorists have been supposed. improves orientation coding in V1 neurons. argued that perceptual learning should Nature 412, 549–553. be considered as independent of more 14. Schwartz, S., Maquet, P., and Frith, C. (2002). References Neural correlates of perceptual learning: A general processes which would not be 1. Gibson, E.J. (1963). Perceptual learning. Annu. functional MIR study of visual texture restricted to stimulus-specific features Rev. Psychol. 14, 29–56. discrimination. Proc. Natl. Acad. Sci. USA 99, 2. Solomon, G.E.A. (1997). Conceptual change 17137–17142. or locations (for example [10,17]). and wine expertise. J. Learn. Sci. 6, 41–60. 15. Mukai, I., Kim, D., Fukunaga, M., Japee, S., Relaxing this constraint gives 3. Fahle, M. (1997). Specificity of learning Marrett, S., and Ungerleider, L.G. (2007). additional support to the alternative curvature, orientation, and vernier Activations in visual and attention-related discriminations. Vision Res. 37, 1885–1895. areas predict and correlate with the degree view that more general mechanisms 4. Shiu, L.P., and Pashler, H. (1992). Improvement of perceptual learning. J. Neurosci. 27, such as association formation or in line orientation discrimination is retinally 11401–11411. local but dependent on cognitive set. Percept. 16. Ahissar, M., and Hochstein, S. (1993). categorization can make significant Psychophys. 52, 582–588. Attentional control of early perceptual-learning. contributions to perceptual learning 5. Karni, A., and Sagi, D. (1991). Where practice Proc. Natl. Acad. Sci. USA 90, 5718–5722. (for example [18–20]). Similarly, by makes perfect in texture-discrimination - 17. Hall, G. (1991). Perceptual and Associative evidence for primary visual-cortex plasticity. Learning (Oxford: Oxford University Press). demonstrating that learning with Proc. Natl. Acad. Sci. USA 88, 4966–4970. 18. McLaren, I.P.L., and Mackintosh, N.J. (2000). simple stimuli can be independent of 6. Mundy, M.E., Honey, R.C., and Dwyer, D.M. An elemental model of associative learning: (2007). Simultaneous presentation of similar I. Latent inhibition and perceptual learning. location in the same way as more stimuli produces perceptual learning in human Anim. Learn. Behav. 28, 211–246. complex stimuli, Xiao et al.’s [8] work picture processing. J. Exp. Psychol. 33, 19. Goldstone, R. (2003). Learning to perceive raises the possibility that perceptual 124–138. while perceiving to learn. In Perceptual 7. Bruce, V., and Burton, A.M. (2002). Learning Organization in Vision: Behavioral and learning with simple and complex new faces. In Perceptual Learning, T. Poggio Neural Perspectives, R. Kimchi, stimuli might rely on at least partially and M. Fahle, eds. (Cambridge, MA: MIT Press), M. Behrmann, and C. Olson, eds. pp. 317–334. (Mahwah, NJ: Lawrence Erlbaum Associates), overlapping mechanisms. 8. Xiao, L., Zhang, J., Wang, R., Klein, S.A., pp. 233–280. In summary, by implicating central, Levi, D.M., and Yu, C. (2008). Complete transfer 20. Mundy, M.E., Dwyer, D.M., and Honey, R.C. rather than peripheral, mechanisms for of perceptual learning across retinal locations (2006). Inhibitory associations contribute to enables by double training. Curr. Biol. 18, perceptual learning in humans. J. Exp. Psychol. perceptual learning with simple visual 1922–1926. 32, 178–184. stimuli the demonstration of complete 9. Fahle, M. (2004). Perceptual learning: A case for transfer across retinal locations raises early selection. J. Vision 4, 879–890. 10. Fahle, M. (2002). Introduction. In Perceptual School of Psychology, Cardiff University, many interesting possibilities. In Learning, M. Fahle and T. Poggio, eds. Tower Building, Park Place, Cardiff CF10 particular, that there might be more (Cambridge, MA: MIT Press), pp. ix–xx. 11. Dosher, B.A., and Lu, Z.L. (1999). Mechanisms 3AT, UK. commonalities between perceptual of perceptual learning. Vision Res. 39, E-mail: [email protected] learning with simple and complex 3197–3221. 12. Mollon, J.D., and Danilova, M.V. (1996). Three stimuli and between general cognitive remarks on perceptual learning. Spatial Vision mechanisms and their perceptual 10, 51–58. DOI: 10.1016/j.cub.2008.10.037

Gene Expression: Dialing Up Differential expression is then due to the various affinities of each the Frequency promoter within the single input module. By encoding signal information Cells often respond to external signals by altering their gene expression. within TF concentrations and response The external signaling information is transduced and typically encoded in information within promoters, cells concentrations of relevant transcription factors. A recent study demonstrates are capable of executing regulatory that, by encoding this information in the frequency with which genes ‘see’ programs that coordinate the timing a transcription factor, the expression of hundreds of genes can be modulated of expression of hundreds of genes. in a linearly proportional manner. For example, if the TF within a single input module is autoregulated by Narendra Maheshri levels in a different way, depending on itself or its targets, the external the details of the promoter. A gene signal triggers a slow rise of the TF, The single input module is a prevalent regulatory function is a compact which turns on high-affinity (low K) network motif in genetic regulatory mathematical way to represent the genes early and low-affinity (high K) networks that allows cells to respond response of each gene to different TF genes late. Some examples of this to external signals through the concentrations [1]. These responses strategy include precise timing in coordinated regulation of hundreds of are typically hyperbolic or sigmoidal developmental systems [2], flagellar genes. This module consists of and can be described by a Hill-like biosynthesis in Escherichia coli [3], ½TFn a transcription factor (TF) that directly function: w k n , where and host and viral gene expression ½TF + Kn regulates the expression of many k corresponds to the strength of post-infection [4]. downstream genes. Typically, external the promoter, K is the affinity of However, what if the goal is to signal information is encoded in the TF–promoter binding, and the Hill double the expression of all concentration of the TF. Each coefficient n captures the degree of downstream genes in response to downstream gene responds to TF cooperativity in TF–promoter binding. a change in an external signal?