The retinotectal projection is the predominant model for studying molecular mechanisms controlling development of topographic axonal connections. Our analyses of topographic mapping of retinal ganglion cell (RGC) axons in chick optic tectum indicate that a primary role for guidance molecules is to regulate topographic branching along RGC axons, a process that imposes unique requirements on the molecular control of map development. We show that topographically appropriate connections are established exclusively by branches that form along the axon shaft. Initially, RGC axons overshoot their appropriate termination zone (TZ) along the anterior-posterior (A-P) tectal axis; temporal axons overshoot the greatest distance and nasal axons the least, which correlates with the nonlinear increasing A-P gradient of ephrin-A repellents. In contrast, branches form along the shaft of RGC axons with substantial A-P topographic specificity. Topography is enhanced through the preferential arborization of appropriately positioned branches and elimination of ectopic branches. Using a membrane stripe assay and time-lapse microscopy, we show that branches form de novo along retinal axons. Temporal axons preferentially branch on their topographically appropriate anterior tectal membranes. After the addition of soluble EphA3-Fc, which blocks ephrin-A function, temporal axons branch equally on anterior and posterior tectal membranes, indicating that the level of ephrin-As in posterior tectum is sufficient to inhibit temporal axon branching and generate branching specificity in vitro. Our findings indicate that topographic branch formation and arborization along RGC axons are critical events in retinotectal mapping. Ephrin-As inhibit branching along RGC axons posterior to their correct TZ, but alone cannot account for topographic branching and must cooperate with other molecular activities to generate appropriate mapping along the A-P tectal axis.

Adina Roskies course. Department of Linguistics and Philosophy As I see it, there are two main divisions of neuroethics: Massachusetts Institute of Technology the ethics of neuroscience and the neuroscience of eth-Cambridge, Massachusetts 02139 ics. Each of these can be pursued independently to a large extent, but perhaps most intriguing is to contemplate how progress in each will affect the other. The past several months have seen heightened interest The Ethics of Neuroscience in the intersection of ethics and neuroscience. In the The ethics of neuroscience can be roughly subdivided popular press, the topic grabbed headlines in a May into two groups of issues: (1) the ethical issues and issue of The Economist (2002) and was featured in a considerations that should be raised in the course of New York Times editorial (Safire, 2002). Professional designing and executing neuroscientific studies and (2) societies were a step ahead, staging several meetings evaluation of the ethical and social impact that the redevoted to ethics and neuroscience since the beginning sults of those studies might have, or ought to have, on of this year. In January 2002, Neuron and the AAAS existing social, ethical, and legal structures. Let me call, sponsored a symposium entitled "Understanding the for convenience, the first the "ethics of practice," and Neural Basis of Complex Behaviors: The Implications the second the "ethical implications of neuroscience." for Science and Society," which brought together a For the most part, the ethics of practice is where tradipanel with expertise in the neurosciences, policy, ethics, tional bioethics, as applied to neuroscience, resides. It and the law to discuss the recent advances in the neuroincludes familiar issues like optimal clinical trial design, sciences and their potential implications for science and guidelines for use of fetal tissues or stem cells or cloning, society. The Royal Institution in London sponsored privacy rights to results of testing for neurological dis-"Neuroscience Future" in March (see conference sumease, and so on. However, the ethics of practice includes mary by J. Waldbauer on NMTF conference website some questions peculiar to neuroethics. For instance, [URL below]), and in May the Dana Foundation, in collabin a liberal democratic society such as ours self-determioration with Stanford University and UCSF, sponsored nation is highly prized, and hence the importance of a conference boldly entitled "Neuroethics: Mapping the informed consent is central to medical practice and Field" (NMTF). The name "neuroethics" implies such a medical ethics. But neurodegenerative diseases and field exists, an "unexplored continent lying between the psychiatric disorders may impair cognition so that intwo populated shores of ethics and of neuroscience," formed consent, as generally conceived, may be imposin the words of Al Jonsen, an organizer of the NMTF sible. What guidelines should be in place for treatment conference.or experimental participation in these cases? We also My aim here i...

Abstract& To distinguish areas involved in the processing of word meaning (semantics) from other regions involved in lexical processing more generally, subjects were scanned with positron emission tomography (PET) while performing lexical tasks, three of which required varying degrees of semantic analysis and one that required phonological analysis. Three closely apposed regions in the left inferior frontal cortex and one in the right cerebellum were significantly active above baseline in the semantic tasks, but not in the nonsemantic task. The activity in two of the frontal regions was modulated by the difficulty of the semantic judgment. Other regions, including some in the left temporal cortex and the cerebellum, were active across all four language tasks. Thus, in addition to a number of regions known to be active during language processing, regions in the left inferior frontal cortex were specifically recruited during semantic processing in a task-dependent manner. A region in the right cerebellum may be functionally related to those in the left inferior frontal cortex. Discussion focuses on the implications of these results for current views regarding neural substrates of semantic processing. &

voice in a crowd; binding across time is required for interpreting object motion; and cross-modal binding is Since its original formulation as a theoretical problem required to associate the sound of a ball striking a bat (von der Malsburg, 1981), "the binding problem" has with the visual percept of it, so that both are effortlessly captured the attention of researchers across many disciperceived as being aspects of a single event. I like to plines, including psychology, neuroscience, computarefer to these sorts of problems as perceptual binding tional modeling, and even philosophy. Despite the isproblems, since they involve unifying aspects of persue's prominence in these fields, what "binding" means cepts. In addition, there are cognitive binding problems: is rarely made explicit. In this paper, I will briefly survey they include relating a concept to a percept, such as the many notions of binding and will introduce some linking the visual representation of an apple to all the issues that will be explored more fully in the reviews semantic knowledge stored about it (it is edible, how it that follow. tastes, used in pies, etc.); cross-modal identification, such as being able to identify an item that has previously What Is Binding?only been seen by how it feels; and memory reconstruc-The canonical example of binding is the one suggested tion, the linking of previously encoded information to by Rosenblatt (1961; see also von der Malsburg, 1999 form a structured and unified representation. While this [this issue of Neuron]), in which one sort of visual feature, perceptual/cognitive distinction is somewhat artificial, such as an object's shape, must be correctly associated it serves to highlight the fact that binding occurs in many with another feature, such as its location, to provide a different kinds of brain processes. The reviews that folunified representation of that object. Such explicit assolow tend to focus on the visual binding problem, alciation, or "binding," becomes especially important though not exclusively. It behooves us to be as explicit when more than one visual object is present, in order as possible about the nature of the particular problem to avoid incorrect combinations of features belonging under investigation, for while it is likely that most differto different objects, otherwise known as "illusory conent forms of binding problems are solved via common junctions" (Triesman and Schmidt, 1982). Considerable mechanisms, that is only a parsimonious assumption, psychological evidence exists for the occurrence of illuand ultimately it is an empirical issue. sory conjunctions (see Wolfe and Cave, 1999 [this is-On the other side of the coin, it is worthwhile to resue]), suggesting that in certain cases, binding is indeed member that something as complex as binding, writ a problem for the brain. In addition, evidence from neularge, may not have a single mechanistic solution. The roanatomy and neurophysiology indicates that propotential mechanisms for binding suggested in these cessing streams in the...

Recent developments in the neuropsychology of criminal behavior have given rise to concerns that neuroimaging evidence (such as MRI and functional MRI [fMRI] images) could unduly influence jurors. Across four experiments, a nationally representative sample of 1,476 jury-eligible participants evaluated written summaries of criminal cases in which expert testimony was presented in support of a mental disorder as exculpatory. The evidence varied in the extent to which it presented neuroscientific explanations and neuroimages in support of the expert's conclusion. Despite suggestive findings from previous research, we found no evidence that neuroimagery affected jurors' judgments (verdicts, sentence recommendations, judgments of the defendant's culpability) over and above verbal neuroscience-based testimony. A meta-analysis of our four experiments confirmed these findings. In addition, we found that neuroscientific evidence was more effective than clinical psychological evidence in persuading jurors that the defendant's disorder reduced his capacity to control his actions, although this effect did not translate into differences in verdicts.

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Retinotopic map development in nonmammalian vertebrates appears to be controlled by molecules that guide or restrict retinal axons to correct locations in their targets. However, the retinotopic map in the superior colliculus (SC) of the rat is developed instead by a topographic bias in collateral branching and arborization. Temporal retinal axons extending across alternating membranes from the topographically correct rostral SC or the incorrect caudal SC of embryonic rats preferentially branch on rostral membranes. Branching preference is due to an inhibitory phosphatidylinositol-linked molecule in the caudal SC. Thus, position-encoding membrane-bound molecules may establish retinotopic maps in mammals by regulating axon branching, not by directing axon growth.

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