Mu-ming Poo and colleagues study signaling pathways involved in the turning of axonal growth cones. They find that myelin-associated glycoprotein (MAG) mediates a repulsive response of the growth cone by signaling through both the Nogo receptor and the p75 neurotrophin receptor. The background image shows a section of monkey dorsal root ganglion, traversed by a myelin-containing neuronal tract. In the foreground image, intracellular calcium rises in a Xenopus axon growth cone as it is repelled by a gradient of MAG. See page 1302.

Differentiating neurons extend multiple processes, yet only one is destined to become the axon. Using the powerful genetic tools available in C. elegans, Yishi Jin and colleagues found that syd-1 is important for localizing presynaptic components to the developing axonal compartment, an important step in distinguishing the axon from dendrites. See pages 1107 and 1137.

Female frogs can recognize individual male calls differing in only the number of sound repetitions that they contain. Gary Rose and colleagues now report that single neurons in the midbrain of these frogs show sensitivity to the number and timing of repeated sounds. These 'counting neurons' responded only after a certain number of sounds had been presented and would 'reset' to zero if the interval between repeated sounds was too short or too long. Photograph courtesy of Photo Researchers. See page 934.

Recognizing faces is something we do with great skill. Using magnetoencephalography in subjects viewing face and non-face images, Jia Liu and colleagues found that this process occurs in two stages. The authors conclude that subjects very rapidly categorize a stimulus as a face, beginning 100 ms after image presentation, and then recognize its individual identity, beginning 70 ms later. The cover shows cloud-like faces at different levels of image degradation, similar to those used in the study. Image created by Javid Sadr. See page 910.

Banerjee and colleagues study neuronal pathfinding in the Drosophila optic lobe. They show that photoreceptor axon target choice is controlled by an interaction between the transcription factor Runt and the ubiquitously expressed nuclear protein Brakeless. Shown here are labeled retinal axons (green) innervating the optic lobe, lamina precursor cells and lamina neurons (red) and glial cells (blue). See page 746.

Sleep is known to be important for learning, but how it exerts this effect remains mysterious. Mednick and colleagues now report that performance on a visual shape discrimination task deteriorated across multiple training sessions within a given day, but this deterioration could be prevented or reversed by a brief midday nap. This benefit was specific to the trained task and to the region of the visual field that was engaged by it. Photo courtesy of Photo Researchers. See pages 618 and 677.

The timing of neuronal activity is proposed to be important for binding features of a complex sensory stimulus. Christensen and colleagues recorded simultaneously from pairs of projection neurons in the pheromone-receptive macroglomerular complex of male sphinx moths and found more synchrony of responses to a specific odor component among neurons that innervated the same rather than separate glomeruli. This synchrony was enhanced by inhibitory influences from neighboring glomeruli responding to a different, but chemically similar pheromone. Photograph courtesy of Photo Research. See pages 505 and 557.

Even when the events around us are random, it can be hard to resist the temptation to make predictions about upcoming events based on apparent patterns (for instance, betting on black in roulette because the previous five runs have come up black). Gregory McCarthy and colleagues now suggest that the prefrontal cortex is involved in making such moment-to-moment predictions. The authors used functional MRI to image brain activity while subjects viewed random sequences composed of two stimuli and found that prefrontal cortex was activated when a particular stimulus violated an apparent pattern. See pages 394 and 485.

Radial glia, which guide the migration of newly born neurons in the developing cortex, are also neuronal and glial precursors. Magdalena Götz and colleagues now show that Pax6 is necessary and sufficient for the neurogenic potential of glial cells. Pax6 can also instruct neurogenesis even in astrocytes from the postnatal cortex. See page 308.

Winston and colleagues used event-related fMRI to examine brain activation while subjects assessed either trustworthiness or apparent age of unknown faces. Explicit trustworthiness judgments evoked enhanced activity in the right superior temporal sulcus, whereas increased activity in bilateral amygdala and right insula was seen during presentation of faces rated as untrustworthy regardless of the task. The findings suggest a functional dissociation between intentional and automatic judgments of trustworthiness. See pages 192 and 277.

A classic function of acetylcholinesterase is the hydrolytic cleavage of acetylcholine, which leads to clearing of the neurotransmitter from the synaptic cleft. Uwe Strhle and colleagues have now examined zebrafish mutants in which acetylcholinesterase activity was completely abolished. The mutation was lethal at early larval stages and resulted in impaired muscle fiber development and premature sensory neuron death, suggesting a possible non-classical role for acetylcholinesterase in neuromuscular development. See page 111

Mutations in the unconventional myosin VIIa can cause deafness. Cornelis Kros and colleagues now report that mutations in the Myo7a gene alter the mechano-electrical properties of hair cell stereocilia such that the transducer channels no longer open under normal physiological stimulation. The authors suggest that myosin VIIa tethers membrane components to the stereocilia actin core, which is important for maintaining the tensional force needed to link stereocilia movement to the transduction apparatus. Photograph courtesy of Dr. R. Holme. See pages 3 and 41.