Since the original description by Taylor, the term focal cortical dysplasia has been used to refer to a wide range of alterations of the cortical mantle. More recently, these conditions have been described from neuroimaging, neuropathological and genetic standpoints, generating several classifications. It is widely recognized that these classifications are unsatisfactory. We propose a simplified classification of focal cortical dysplasias based on easily recognized neuropathological characteristics. We retrospectively re-examined histological sections of cortex from 52 of 224 (23%) patients operated on for drug-resistant partial epilepsy in which cortical dysplasia was present but not associated with other brain pathologies except hippocampal sclerosis. Three subgroups were identified: (i) architectural dysplasia (31 patients) characterized by abnormal cortical lamination and ectopic neurones in white matter; (ii) cytoarchitectural dysplasia (six patients) characterized by giant neurofilament-enriched neurones in addition to altered cortical lamination; and (iii) Taylor-type cortical dysplasia (15 patients) with giant dysmorphic neurones and balloon cells (all but two patients) associated with cortical laminar disruption. The patients with architectural dysplasia had lower seizure frequency than those with cytoarchitectural and Taylor-type dysplasia, and the epileptogenic zone was mainly in the temporal lobe. In patients with Taylor-type dysplasia, the epileptogenic zone was mainly extratemporal, and interictal stereo-EEG was distinctive. MRI was unrevealing in 34% of patients, but distinctive signal alterations characterized most patients with Taylor-type dysplasia, while focal hypoplasia with MRI abnormalities was found in architectural dysplasia. Patients with Taylor-type dysplasia had the best outcome, with 75% seizure-free (Engel class Ia) after at least a year of follow-up compared with 50% of cytoarchitectural dysplasia and 43% of architectural dysplasia patients seizure-free. This three-category classification is based on easily recognized histopathological characteristics and avoids complicated terminology, while the distinctive ensemble of other characteristics defines clinically homogeneous groups.

Contents I. Introduction 765 II. Evolution of the Mechanistic View of the 766 Diazonium Group Replacement III. Homolytic Dediazoniation Reactions: A 769 General Description A. How the Homolytic Dediazoniation Takes 769 Place 1. Reduction at the Electrode 769 2. Radiolytic Approach 770 3. Photoinduced Electron Transfer 770 4. Reduction by Metal Cations 771 5. Anion-Induced Dediazoniation 772 6. Solvent-Induced Dediazoniation 775 7. Summary 776 B. Product Patterns of the Aryl Radical 777 1. Hydrogen Atom Abstraction 777 (Hydro-dediazoniation) 2. Reaction with X' (Halo-dediazoniation) 777 3. Other Sandmeyer-like Reactions 779 4. Addition to Olefins 779 5. Arylation of Aromatic Compounds 781 6. Conclusions 782 IV. Related Reactions 782 A. Intermediacy of the Aryl Radical 782 1. Reactions of Diaryliodonium Ions 783 2. SRN1 Reactions 784 B. Interchange of Product Patterns 786 V. Unrelated Reactions 788 VI. Acknowledgment 789 VII. References 789

The activity of 156 individual arm-related neurons was studied in the premotor cortex (area 6) while monkeys made arm movements of similar directions within different parts of 3-dimensional space. This study was aimed at describing the relationship between premotor cortical cell activity and direction of arm movement and assessing the coordinate system underlying this relationship. We found that the activity of 152 (97.4%) cells varied in an orderly fashion with the direction of movement, in at least some region of the work space. Premotor cortical cells fired most for a given preferred direction and less for other directions of movement. These preferred directions covered the directional continuum in a uniform fashion across the work space. It was found that, as movements of similar directions were made within different parts of the work space, the cells' preferred directions changed their orientation. Although these changes had different magnitudes for different cells, at the population level, they followed closely the changes in orientation of the arm necessary to move the hand from one to another part of the work space. This shift of cells' preferred direction with the orientation of the arm in space has been observed with similar characteristics in the motor cortex (see Caminiti et al., 1990). In both premotor and motor cortices, neuronal movement population vectors provide a good description of movement direction. Unlike the individual cell preferred directions upon which they are based, movement population vectors did not change their spatial orientation across the work space, suggesting that they remain good predictors of movement direction regardless of the region of space in which movements are made. The firing frequency of both premotor and motor cortical neurons varied significantly with the position occupied by the hand in space. These static positional effects were observed in 88.5% of premotor and 91.8% of motor cortical cells. In a second task, monkeys made movements from differing origins to a common end point. This task was performed within 3 different parts of space and was aimed at dissociating movement direction from movement end point. It was found that in both premotor and motor cortices virtually all cells were related to the direction and not to the end point of movement. These data suggest that premotor and motor cortices use common mechanisms for coding arm movement direction. They also provide a basis for understanding the coordinate transformation required to move the hand toward visual targets in space.

No abstract

The role of ring strain on the ease of ring formation over a wide range of ring sizes is discussed on the basis of a comparison of transition state versus product‐ring strain energies. A general procedure is illustrated for the assessment of transition‐state strain energies, based on experimental effective molarities (EMs) and an extra‐thermodynamic treatment of entropy data for ring closure. It is found that the ring product is a good model of the transition state of cyclisation for all but the shortest chains. Our earlier interpretation of this remarkable lack of correspondence between transition state and product‐ring strain energies in intramolecular nucleophilic substitutions of the very short chains, is compared with the results of recent theoretical calculations.