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Showing papers on "Orientation column published in 1994"


Journal ArticleDOI
TL;DR: This study investigated the cortical sources of the early components of the pattern‐onset visual evoked potential (VEP) and found the C1 component was found to change its polarity and topography systematically as a function of stimulus position in a manner consistent with the retinotopic organization of the striate cortex.
Abstract: This study investigated the cortical sources of the early (50-250 ms) components of the pattern-onset visual evoked potential (VEP) VEPs were recorded in response to a small circular checkerboard stimulus that was flashed over a range of visual field positions Temporally and spatially overlapping VEP components were distinguished by differences in retinotopic sensitivity and scalp topography, and by inverse dipole modeling The C1 component (50-80 ms) was found to change its polarity and topography systematically as a function of stimulus position in a manner consistent with the retinotopic organization of the striate cortex The P1 component (comprised of the P75 and P100 subcomponents) had a time course that overlapped the C1 but could be distinguished from the C1 by its differing topography and reduced sensitivity to stimulus position The P1 generators were localized to the lateral extrastriate cortex Inverse dipole models were consistent with these striate and extrastriate source locations for the C1 and P1, respectively The N1 component (120-180 ms) was found to originate from several spatially distinct generators that differed in their retinotopic organization © 1995 Wiley-Liss, Inc

562 citations


Journal ArticleDOI
TL;DR: This hypothesis leads robustly to development of simple cell receptive fields selective for orientation and spatial frequency, and to the continuous and periodic arrangement of preferred orientation across the cortex.
Abstract: Neurons in the primary visual cortex of higher mammals respond selectively to light/dark borders of a particular orientation. The receptive fields of simple cells, a type of orientation-selective cell, consist of adjacent, oriented regions alternately receiving ON-center and OFF-center excitatory input. I show that this segregation of inputs within receptive fields can occur through an activity-dependent competition between ON-center and OFF-center inputs, just as segregation of inputs between different postsynaptic cells into ocular dominance columns appears to occur through activity-dependent competition between left-eye and right-eye inputs. These different outcomes are proposed to result, not from different mechanisms, but from different spatial structures of the correlations in neural activity among the competing inputs in each case. Simple cells result if ON-center inputs are best correlated with other ON-center inputs, and OFF with OFF, at small retinotopic separations, but ON-center inputs are best correlated with OFF-center inputs at larger separations. This hypothesis leads robustly to development of simple cell receptive fields selective for orientation and spatial frequency, and to the continuous and periodic arrangement of preferred orientation across the cortex. Input correlations determine the mean preferred spatial frequency and degree of orientation selectivity. Estimates of these correlations based on measurements in adult cat retina (Mastronarde, 1983a,b) produce quantitative predictions for the mean preferred spatial frequencies of cat simple cells across eccentricities that agree with experiments (Movshon et al., 1978b). Intracortical interactions are the primary determinant of cortical organization. Simple cell spatial phases can play a key role in this organization, so arrangements of spatial phases and preferred orientations may need to be studied together to understand either alone. Possible origins for other cortical features including spatial frequency clusters, afferent ON/OFF segregation, blobs, pinwheels, and opponent inhibition within simple cell receptive fields are suggested. A number of strong experimental tests of the hypothesis are proposed.

377 citations


Journal ArticleDOI
TL;DR: Results support the proposal that visual-spatial attention modulates neural activity in extrastriate visual cortex but does not affect the initial evoked response in striate cortex, and account for the voltage topographies produced by both attended and unattended stimuli with low residual variance.
Abstract: In a study of the neural processes that mediate visual attention in humans, 32-channel recordings of event-related potentials were obtained from 14 normal subjects while they performed a spatial attention task. The generator locations of the early C1, P1, and Nl components of the visual evoked response were estimated by means of topographic maps of voltage and current source density in conjunction with dipole modelling. The topography of the C1 component (ca. 85 ms post-stimulus) was consistent with a generator in striate cortex, and this component was unaffected by attention. In contrast, the P1 and Nl components (ca. 95 and 170ms) exhibited current density foci at scalp sites overlying lateral extrastriate cortex and were larger for attended stimuli than for unattended stimuli. The voltage topographies in the 75–175 ms latency range were modeled with a 5-dipole configuration consisting of a single striate dipole and left-right pairs of dipoles located in lateral extrastriate and inferior occipito-temporal areas. This model was found to account for the voltage topographies produced by both attended and unattended stimuli with low residual variance. These results support the proposal that visual-spatial attention modulates neural activity in extrastriate visual cortex but does not affect the initial evoked response in striate cortex.

359 citations


Journal ArticleDOI
TL;DR: The results indicate that feedback connections to early visual cortex derive from a widespread network of areas, including limbic-associated cortices, and also indicate that some cortical feedback connections may not be strictly reciprocal.
Abstract: Although there have been reports of sparse projections from temporal areas TE, TF, and even TH to area V1, it is generally believed that cortical afferents to V1 originate exclusively from prestriate areas. Injections of anterograde tracers in anterior occipital and temporal areas, however, consistently produce labeled terminals in area V1. In order to confirm these results and display the full range of foci projecting to V1, we injected V1 in two monkeys with the retrograde tracer fast blue. Feedback connections were found, as expected, from several prestriate areas (V2, V3, V4, and MT). These originate from neurons in layers 3A and 6. Connections were also found from several more distal regions, namely, areas TEO, TE, TF, TH, and from cortex in the occipitotemporal and superior temporal (STS) sulci. Filled neurons occurred in two small foci in the caudal intraparietal sulcus. These more distal feedback connections tend to originate only from layer 6. An additional injection of the retrograde tracer diamidino yellow in area V2 of one animal revealed a similarly widespread network of feedback connections. In some areas (in the STS and in TEO), 10-15% of fluorescent neurons were double-labeled. These results indicate that feedback connections to early visual cortex derive from a widespread network of areas, including limbic-associated cortices. These connectional patterns testify to the massive recursiveness of anatomical pathways. As there are no reports of projections from V1 to anterior temporal cortices, our results also indicate that some cortical feedback connections may not be strictly reciprocal.

198 citations


Book ChapterDOI
01 Jan 1994
TL;DR: This chapter will concentrate on cytochrome oxidase, and review what this endogenous metabolic marker has revealed about the primate visual cortex.
Abstract: Few regions in the brain have received as much attention and scrutiny as the visual cortex, whose structural and functional organization provides an ideal model for understanding cerebral cortex in general. Binocularity in the visual system further permits experimental manipulations of a single eye input with the other eye serving as a useful internal reference point. In the last century, the visual cortex has consistently been used as a fertile testing ground for virtually every new neurobiological technique and innovation. As a result, much of its anatomical, neurochemical, and functional organizations have been examined. Of special significance is the discovery by Hubel and Wiesel (1968) of ocular dominance columns, orientation columns, and the exquisite system of functional modules in the primate striate cortex. Our understanding of the visual system has reached new heights over the last decade, with new techniques based on brain metabolism, enzyme histochemistry, immunohistochemistry, voltage-sensitive dyes, and brain imaging having been applied rigorously to the study of the visual cortex (e.g., Kennedy et al., 1976; Wong-Riley, 1979b; Horton and Hubel, 1981; Hendrickson et al., 1981; Tootell et al., 1982, 1988a–e; Hockfield et al., 1983; Horton, 1984; Carroll and Wong-Riley, 1984; Wong-Riley and Carroll, 1984a,b; Livingstone and Hubel, 1984a; Hendry and Jones, 1986; Blasdel and Salama, 1986; Wong-Riley et al., 1989a,b; Ts’o et al., 1990; Beaulieu et al., 1992). In this chapter, I will concentrate on cytochrome oxidase, and review what this endogenous metabolic marker has revealed about the primate visual cortex. The species on which most of the studies are based is the macaque monkey, but other primate species including man will be described when appropriate. A comparative study with other mammalian species has been reported previously (Wong-Riley, 1988).

115 citations


BookDOI
01 Jan 1994
TL;DR: The Organization of the Primary Visual Cortex in the Macaque A. Peters, et al., and the Role of Area 17 in the Transfer of Information to Extrastiate Visual Cortex are described.
Abstract: The Organization of the Primary Visual Cortex in the Macaque A. Peters. Substrates for Interlaminer Connections in Area V1 of Macaque Monkey Cerebral Cortex J.S. Lund. GABA Neurons and Their Role in Activity-Dependent Plasticity of Adult Primal Visual Cortex E.G. Jones, et al. Primate Visual Cortex M.T.T. Wong-Riley. The Afferent, Intrinsic, and Efferent Connections of Primary Visual Cortex in Primates V.A. Casagrande, J.H. Kaas. The Organization of Feedback Connections from Area V2 (18) to V1 (17) K.S. Rockland. What Does In vivo Optical Imaging Tell Us about the Primary Visual Cortex in Primates? R.D. Frostig. The Role of Area 17 in the Transfer of Information to Extrastiate Visual Cortex J. Bullier, et al. Computational Studies of the Spatial Architecture of Primate Visual Cortex E.L. Schwartz. Motion Processing in Monkey Striate Cortex G.A. Orban. Temporal Codes for Colors, Patterns, and Memories J.W. McClurkin, et al. The Human Primary Visual Cortex R.O. Kuljis. The Role of Striate Cortex M. Rizzo. Index

90 citations


Journal ArticleDOI
TL;DR: Results indicate that even at the time of eye opening, prior to extensive visual experience, most cells receive patterns of synaptic inputs consistent with a clustered organization of horizontal connections that functionally link iso-orientation columns.
Abstract: In cat striate cortex, patchy horizontal axonal projections link columns of similar orientation specificity. To assess the physiological correlates of such clustered projections, a new multisite stimulation technique was used to functionally map the pattern of horizontal synaptic inputs onto single layer 2/3 cells within tangential slices of developing ferret visual cortex. Twenty-four separate sets of horizontal fibers were stimulated within a 1200 microns strip of cortex, while evoked synaptic responses were recorded using whole-cell patch methods. For most cells, input maps demonstrated the presence of clustered horizontal connections in which multiple strong and weak synaptic responses were alternately evoked across the stimulated cortical region. Recordings from up to nine cells in a single slice revealed that patterns of synaptic input were closely correlated for cells in close proximity, and that this correlation decreased with distance, with no correlation at distances greater than 500 microns. To determine whether these physiological results were consistent with the known anatomical linkage of iso-orientation columns by clustered horizontal connections, mathematical analysis and computer simulations were performed upon orientation tuning maps obtained from optical imaging of activity-dependent intrinsic signals in mature ferret visual cortex. Optical imaging revealed an organization of iso-orientation domains consisting of broad regions of cortex across which orientation preference smoothly varied, together with “orientation centers” around which orientation preference was arranged in a pinwheel manner. The distribution of synaptic connections between different cortical sites was simulated by a model of functionally linked iso- orientation columns. Simulated synaptic input maps, generated by the same stimulation and recording arrangements used in our experimental protocol, accurately reproduced the observed patterns of clustered inputs onto experimentally recorded cells. These results indicate that even at the time of eye opening, prior to extensive visual experience, most cells receive patterns of synaptic inputs consistent with a clustered organization of horizontal connections that functionally link iso-orientation columns.

76 citations


Journal ArticleDOI
TL;DR: This work measured dichoptic contour interaction in this region of the visual field in humans by having observers report the orientation of a test letter "T" presented to this region, in the presence of flanking T's presented around the blind spot of the fellow eye.

68 citations


Journal ArticleDOI
TL;DR: A threshold and expansive nonlinearity in the spike-generating mechanism enhances selectivity for stimulus features such as orientation, direction, and spatial frequency.

52 citations


Book ChapterDOI
TL;DR: The Von der Malsburg model is a classic example as discussed by the authors, and it has been applied to other developmental phenomena such as the development of lamination in the LGN, the formation of visual maps in experimentally altered auditory cortex, and the mapping of visual and auditory maps in the optic tectum.
Abstract: What makes a useful model of neural development? One important contribution of modeling is to demonstrate that proposed biological mechanisms can be sufficient to account for experimental results. The Von der Malsburg model is a classic example. But such demonstrations alone do not provide tools to experimentally distinguish one mechanism from another. To draw such distinctions, the connection between measurable biological quantities and developmental outcomes must be established. Perhaps the most important task for the future of developmental modeling is to deepen the connection between theory and experiment. Experimentally, this requires detailed and difficult measurements or experimental perturbations of the correlations among inputs and the intracortical connectivity existing during development. Simultaneous measurement of the maps of spatial phase and orientation of mature simple cells will provide important information for the understanding of orientation column development. Theoretically, the number of open problems is enormous. How will inclusion of additional plasticity mechanisms, such as sprouting and retraction of synapses or plasticity of intracortical connections, alter the analytical understanding thus far achieved? What precisely determines the width of orientation columns in the model presented here? Can the relationship between ocular dominance and orientation columns be understood from developmental rules in a testable way? The existing framework may be extended to a three-dimensional cortex and to more complex models of intracortical connectivity. It may also be applied to other developmental phenomena including the development of lamination in the LGN (Shatz and Stryker, 1988; Hahm et al., 1991), the formation of visual maps in experimentally altered auditory cortex (Roe et al., 1990, 1992), and the mapping of visual and auditory maps in the optic tectum (Knudsen and Brainard, 1991; Brainard and Knudsen, 1993). For each system the goal is to develop testable predictions as to the patterns of activity and connectivity that could or could not lead to the results observed given a proposed mechanism of plasticity. Incorporation of deeper levels of biophysical realism will extend, deepen, and perhaps fundamentally alter the framework presented here. An important goal for the future will be to understand the computational and functional significance of developmental rules. Activity-dependent, competitive mechanisms of synaptic plasticity appear to play an important role in many processes of late neural development, where an initially rough connectivity pattern refines to a precise, mature pattern. A prominent example is the formation of ocular dominance columns in the visual cortex of many mammals. These processes may be modeled at several levels.(ABSTRACT TRUNCATED AT 400 WORDS)

41 citations


Journal ArticleDOI
TL;DR: Three computational rules are sufficient to generate model cortical maps that simulate the interrelated structure of cortical ocular dominance and orientation columns: a noise input, a spatial band pass filter, and competitive normalization across all feature dimensions.

Journal ArticleDOI
TL;DR: Whether the striate cortex of the guinea pig projects to the pretactal nucleus of the optic tract (NOT), the first postretinal station of the horizontal optokinetic pathway, is determined, and, if so, the anatomical organization of this cortico‐NOT projection is analyzed.
Abstract: The primary goal of this study was to determine whether the striate cortex (Oc 1) of the guinea pig projects to the pretectal nucleus of the optic tract (NOT), the first postretinal station of the horizontal optokinetic pathway, and, if so, to analyze the anatomical organization of this cortico-NOT projection. Other goals of this investigation are to identify other pretectal nuclear projections from the visual cortex in the guinea pig, and to determine whether there is any visuotopic organization in this pathway. Axonal tracers (biocytin or 3H-leucine) were injected into the striate cortex (Oc 1), and the tissue processed with histochemical or light autoradiographic techniques. All subcortical terminal labeling is ipsilateral in the basal ganglia and thalamic nuclei. Furthermore, projections are traced to the ipsilateral brainstem, including two areas of the pretectal complex: (1) one in the NOT, extending in some cases to the adjacent lateral portion of the posterior pretectal nucleus (PPN), and (2) one in the pars compacta of the anterior pretectal nucleus (APNc). The terminal fields in the APN are consistently located rostrally in the dorsolateral portion of the nucleus, independently of the injection site in Oc 1, whereas in the NOT the terminal fields shift slightly after injections placed in different locations in the striate cortex. A correlation of the injection sites in Oc 1 and terminal fields in the NOT reveals a loose topographic organization in the cortico-NOT projection; accordingly, the rostrocaudal axis of the striate cortex projects to the lateromedial axis of the NOT, with a 90 degrees rotation, whereas lateral parts of the striate cortex project diffusely throughout the rostrocaudal extent of the NOT. These data show for the first time that the NOT in the guinea pig receives a substantial projection from the visual cortex. Given the fact that in the guinea pig the optokinetic nystagmus shares some of the characteristics found in cat and monkey (i.e., consistent initial fast rise in the slow phase velocity and reduced asymmetry in monocular stimulation), the present findings lend support to the hypothesis that a cortical input to the NOT is a necessary condition for these oculomotor properties to be present.

Journal Article
TL;DR: The results suggest that caudo-lateral cortex in rats corresponds to the inferotemporal cortex of primates.
Abstract: In Experiment I, bilateral ablations of the caudolateral cortex involving Krieg's area 36 impaired discrimination of visual patterns but not delayed alternation. In Experiment II, the same type of lesions retarded postoperative learning to discriminate embedded visual patterns. In rats from the Experiment II tracers of axonal transport gave no signs of damage of the connections of the primary visual cortex. In agreement with this, Nissl stain of the dorsal lateral geniculate nuclei showed no neuronal loss or gliosis. These results suggest that caudo-lateral cortex in rats corresponds to the inferotemporal cortex of primates.

Proceedings Article
01 Jan 1994
TL;DR: The model shows how patterned lateral connections in the cortex may develop based on correlated activity and explains why lateral connection patterns follow receptive field properties such as ocular dominance.
Abstract: A neural network model for the self-organization of ocular dominance and lateral connections from binocular input is presented. The self-organizing process results in a network where (1) afferent weights of each neuron organize into smooth hill-shaped receptive fields primarily on one of the retinas, (2) neurons with common eye preference form connected, intertwined patches, and (3) lateral connections primarily link regions of the same eye preference. Similar self-organization of cortical structures has been observed experimentally in strabismic kittens. The model shows how patterned lateral connections in the cortex may develop based on correlated activity and explains why lateral connection patterns follow receptive field properties such as ocular dominance.

Book ChapterDOI
Matthew Rizzo1
01 Jan 1994
TL;DR: This chapter examines converging lines of evidence in the context of what they tell us about the role of human striate cortex in vision from the study of dysfunction in patients with specific lesions of visual pathways.
Abstract: Much of what is known about the role of human striate cortex in vision comes from the study of dysfunction in patients with specific lesions of visual pathways, from the retina to the occipital lobe and the adjoining temporal and parietal regions. That evidence depends on neuro-ophthalmological, neuropsychological, and psychophysical techniques. The neuroanatomy is provided, in vivo, by modern neuroimaging techniques such as magnetic resonance imaging (MRI) (Damasio and Damasio, 1989; Damasio and Frank, 1992), and (less often) at autopsy. Positron emission tomography (PET) studies offer another window on regional localization and visual function. Comparative anatomical studies on the functional organization of the visual system, particularly in the monkey, also provide insights into the corresponding organization in the human. This chapter examines these converging lines of evidence in the context of what they tell us about the role of human striate cortex [which is also referred to as the primary visual cortex, the calcarine cortex, area 17 of Brodmann (1909), and more recently as area V1] (see Fig. 1). The emphasis in this chapter is on human brain lesion studies. In the introductory section, we start with a definition of human visual areas and deficits, and follow with comments on the human brain lesion method applied to vision, on human—monkey homologies, and on subcortical inputs to V1.

Book ChapterDOI
TL;DR: The original abstraction of the visual environment is replaced in this chapter by real visual images, which allows the BCM neuron to be trained and tested with static real two-dimensional images.
Abstract: Publisher Summary The Bienenstock, Cooper, and Munro (BCM) theory of synaptic plasticity models the development of orientation selectivity and binocular interaction in primary visual cortex, and has successfully reproduced kitten visual deprivation experiments. To better compare the consequences of the BCM theory with experiment, the original abstraction of the visual environment is replaced in this chapter by real visual images. Circular regions from the left and right retinas covering the same visual space are used to generate input to a single BCM neuron. The lateral geniculate nucleus is assumed to simply relay signals generated in the retina to the visual cortex. Each ganglion selects an antagonistic center-surround receptive field generated by a difference of two Gaussians, and ganglion cell activity is restricted to be positive. This extension allows the BCM neuron to be trained and tested with static real two-dimensional images. The visual environment is represented by 24 gray scale natural images, which can be shifted across the artificial retinas. In this environment, the BCM neuron develops receptive similar to simple cells found in primary visual cortex. It displays adjacent excitatory and inhibitory bands, when tested with spot stimuli, and orientation selectivity when tested with bar stimuli.

Journal ArticleDOI
TL;DR: In the primary visual cortex of an immobilized awake cat, nearly one-third of the neurons studied were found to respond to flashing cruciform light stimuli 1.5–4 times better than to single stimulations with the strips of preferred orientation.
Abstract: In the primary visual cortex of an immobilized awake cat, nearly one-third of the neurons studied (8 out of 22) were found to respond to flashing cruciform light stimuli 1.5–4 times better than to single stimulations with the strips of preferred orientation. It is suggested that such neurons can detect angles and line intersections.

Journal ArticleDOI
TL;DR: Results showed that striate neurons are dependent on the adjacent cells1 excitability, and modification of responses to stationary targets suggests that lateral interactions play a role in the generation of discharges to fixed stimuli.
Abstract: The goal of this study was to examine the role of horizontal connections in rabbit striate neurons. Anaesthetized rabbits were prepared in the usual fashion for single-cell recordings in area 17 of the visual cortex. We compared responses evoked by moving and stationary stimuli prior to, during and after recovery from lateral microinjection of either lidocaine (n = 61), gamma-aminobutyric acid (GABA, n = 18) or bicuculline (n = 8) 2 mm from the recording site. This procedure allows evaluation of the contribution of neighbouring neurons in visual responses. Results showed that striate neurons are dependent on the adjacent cells' excitability. Modification of responses to stationary targets suggests that lateral interactions play a role in the generation of discharges to fixed stimuli. Lateral inactivation preferentially influenced non-directional over direction-selective units. This influence usually resulted in the non-directional neuron becoming directional by attenuation of the visually driven response in one direction. Simple and complex cells tended to be influenced differently by lateral inactivation. Simple cells became less responsive, whereas complex cells became more responsive. This dichotomy among cellular types suggests that simple cells receive mainly excitatory horizontal influences, while complex cells are contacted mostly by lateral inhibitory inputs.


Book ChapterDOI
01 Jan 1994
TL;DR: Findings show that different stimulus attributes are processed at least partially in parallel and multiplexed temporal codes may be used by visual neurons to transmit multiple messages about different stimulus qualities.
Abstract: Recent findings on the structural and functional properties of the striate cortex of cats and primates are briefly reviewed. In particular these findings show that: i) different stimulus attributes are processed at least partially in parallel, ii) the responses of single neurones to a given visual stimulus are context-dependent, iii) some degree of neural plasticity is present even in the adult visual cortex, iv) multiplexed temporal codes may be used by visual neurons to transmit multiple messages about different stimulus qualities. Some properties of extrastriate visual areas are suggestive of further stages of visual processing that may be relevant for pattern recognition.

01 Jan 1994
TL;DR: In a recent paper in Science, Matthew Dalva and Lawrence Katz describe experiments in which a novel scanning-laser 'photostimulation'method is used to follow developmental changes in theorganization of functional connections in primary visual cortex, and the expectation is that this method will allownot not only developing, but also adult, cortical circuits to beunderstood in considerably greater detail.
Abstract: If you want to understand how a complex electronicdevice works, you might start by obtaining a circuitdiagram. A sufficiently knowledgeable engineer could usethe diagram not only to determine what the device cando, but also to build a copy of it. When it comes to thebrain, things are not so simple. For several decades,neurobiologists have struggled to understand how themammalian brain carries out the computations thatmediate perception, cognition and behavior, and how thecircuits responsible for these computations emerge duringdevelopment. A major hindrance to these studies hasbeen the limited ability of available methods to describethe actual patterns of connections between neurons.Despite these technical limitations, a good deal is knownabout the organization and development of complexneural circuits. In the cerebral cortex of adult mammals,individual neurons are known to make precise axonalprojections to specific cortical layers (Fig. 1) [1-3], aswell as to functionally distinct compartments formingcolumns that run perpendicular to the cortical layers[4-6]. The latter compartments are exemplified by theocular dominance and orientation columns in the visualcortex, which are respectively specific for the leftor right eye and for the orientation of a contour in avisual stimulus.The mechanisms guiding the development of thesecircuits have been studied intensively by a number ofinvestigators (for recent reviews see [7-9]). Anatomicalstudies have revealed that the axonal arbors of developingneurons undergo considerable activity-dependent reorga-nization as they strive to find functionally appropriatecolumns with which to connect, while layer-specificprojections are formed with great precision from theoutset [9]. It is believed that during development ofcolumn-specific projections, transient connections areformed that allow neurons to use activity patterns todetect their correct synaptic partners.However, the anatomical studies that provide the basisfor our understanding of how such circuits developprovide only indirect information about whether con-nections are being formed at higher densities in somelocations than others. Crucial information about subtlechanges in connectivity that might occur as maturationproceeds is lacking, but is required to assess the role ofactivity-dependent mechanisms in shaping cortical cir-cuitry. Recent methodological advances are at lastmaking it possible to obtain this important information.In a recent paper in Science [10], Matthew Dalva andLawrence Katz of Duke University describe experimentsin which a novel scanning-laser 'photostimulation'method is used to follow developmental changes in theorganization of functional connections in primary visualcortex. The expectation is that this method will allownot only developing, but also adult, cortical circuits to beunderstood in considerably greater detail.In a 1979 Scientific American article entitled "ThinkingAbout the Brain" [11], Francis Crick stated that "amethod that would make it possible to inject one neuronwith a substance that would then clearly stain allthe neurons connected to it, would be invaluable".Studies of neural circuitry using photostimulation dependon the use of latent, 'caged' forms of neurotransmitterFig. 1. Neurons in the cerebral cortexform precise patterns of axonal projec-tions. The diagram shows the patterns ofaxonal (blue) and dendritic (red)arborizations formed by neurons withcell bodies in different layers of theprimary visual cortex. Although the pro-jection patterns show considerablespecificity, there is a good deal of ambi-guity about the actual patterns of con-nections. The resolution of thisambiguity should be facilitated by thenew technique of scanning laser photo-stimulation [10] described in the text.(Adapted from [1].)1010