On the basis of his extensive and elegant anatomical investigations on the visual cortex, Poliak (1932) suggested that a mathematical projection of the retina on the cerebral cortex must exist. Talbot & Marshall (1941) used physiological methods to map the central part of the visual field on to the posterolateral surface ofthe cortex in the monkey. They devised an index of cortical representation expressed as the increment of the angle, measured radially from the centre of gaze, which is represented on each millimetre of cortex. We have confirmed their observations and have extended the mapping to the buried visual cortex in the horizontal and vertical calcarine fissures. We have preferred to use the reciprocal of their index and to call it the cortical magnification, M. When this is measured along radii and at right angles to them, it provides the empirical quantitative relation which Polyak wanted. It also defines the shape and size of the visual receptive field. We have made such a surface, folded it and compared it with the calcarine cortex of the monkey. A preliminary account of this work has been published (Daniel & Whitteridge, 1959). METHODSOut of twenty-two monkeys and baboons, we have had satisfactory records from the posterolateral surface of the cortex in fourteen, from the cortex of the calcarine fissure in eight. The first satisfactory experiments were carried out in 1952, but subsequently there were a number of unsuccessful experiments due to haemorrhage and to prolonged lowering of the blood pressure by pentobarbitone. The last eight experiments on three baboons, three large macaques, a cynomolgus and a vervet monkey have all been successful. We have found operation on small macaques very difficult, as unavoidable blood loss on opening the skull is liable to cause serious lowering of the blood pressure. We have been careful to induce anaesthesia slowly, using Dr C. G. Phillips's method of putting the animal in a small cage in a glass-fronted tank and pumping in nitrous oxide. The moment the animal appears to be unconscious, we have taken it out, given Nembutal (sodium pentobarbitone; Abbott Laboratories) 40 mg/kg, or hexobarbitone intraperitoneally, or chloralose 50-60 mg/kg intravenously. The animal is then put back into the small cage in room air and, after recovering from the nitrous oxide, it quietly becomes drowsy without further handling. In

SUMMARY1. We have studied the neuronal circuitry and structure-function relationships of single neurones in the striate visual cortex of the cat using a combination of electrophysiological and anatomical techniques.2. Glass micropipettes filled with horseradish peroxidase were used to record extracellularly from single neurones. After studying the receptive field properties, the afferent inputs of the neurones were studied by determining their latency of response to electrical stimulation at different positions along the optic pathway. Some cells were thus classified as receiving a mono-or polysynaptic input from afferents of the lateral geniculate nucleus (l.g.n.), via X-or Y-like retinal ganglion cells.3. Two striking correlations were found between dendritic morphology and receptive field type. All spiny stellate cells, and all star pyramidal cells in layer 4A, had receptive fields with spatially separate on and off subfields (S-type receptive fields). All the identified afferent input to these, the major cell types in layer 4, was monosynaptic from X-or Y-like afferents.4. Neurones receiving monosynaptic X-or Y-like input were not strictly segregated in layer 4 and the lower portion of layer 3. Nevertheless the X-and Y-like l.g.n. fibres did not converge on any of the single neurones so far studied.5. Monosynaptic input from the l.g.n. afferents was not restricted to cells lying within layers 4 and 6, the main termination zones of the l.g.n. afferents, but was also received by cells lying in layers 3 and 5.6. The projection pattern of cells receiving monosynaptic input differed widely, depending on the laminar location of the cell soma. This suggests the presence of a number of divergent paths within the striate cortex.7. Cells receiving indirect input from the l.g.n. afferents were located mainly within layers 2, 3 and 5. Most pyramidal cells in layer 3 had axons projecting out ofthe striate cortex, while many axons of the layer 5 pyramids did not.8. The layer 5 cells showed the most morphological variation of any layer, were the most difficult to activate by electrical stimulation, and contained some cells which responded with the longest latencies of any cells in the striate cortex. This suggests that they were several synapses distant from the J.g.n. input.9. The majority of cells in layers 2, 3, 4 and 6 had the same basic S-type receptive field structure.

We have used microanatomy derived from single neurons, and in vivo intracellular recordings to develop a simplified circuit of the visual cortex. The circuit explains the intracellular responses to pulse stimulation in terms of the interactions between three basic populations of neurons, and reveals the following features of cortical processing that are important to computational theories of neocortex. First, inhibition and excitation are not separable events. Activation of the cortex inevitably sets in motion a sequence of excitation and inhibition in every neuron. Second, the thalamic input does not provide the major excitation arriving at any neuron. Instead the intracortical excitatory connections provide most of the excitation. Third, the time evolution of excitation and inhibition is far longer than the synaptic delays of the circuits involved. This means that cortical processing cannot rely on precise timing between individual synaptic inputs.

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There are numerous hypotheses for the role of the axon collaterals of pyramidal cells. Most hypotheses predict that pyramidal cells activate specific classes of postsynaptic cells. We have studied the postsynaptic targets of two layer III pyramidal cells, that were of special interest because of their clumped axon arborization near, and also 0.4-1.0 mm from the cell body, in register in both layers III and V. 191 terminations from four sites (layers III and V, both in the column of the cell and in distant clumps) were analysed by electron microscopy. Only one bouton contacted a cell body and that was immunoreactive for GABA. The major targets were dendritic spines (84 and 87%), and the remainder were dendritic shafts. Of these 13 were classed as pyramidal-like (P), 8 smooth cell-like (S) and three could not be classified. Four of five S types, but none of the seven P types tested were immunoreactive for GABA, supporting the fine structural classification. The putative inhibitory cells therefore formed not more than 5% of the postsynaptic targets, and their activation could only take place through the convergence of pyramidal cells onto a select population of GABA cells. The results show that the type of pyramidal cells with clumped axons studied here make contacts predominantly with other pyramidal cells. Thus the primary role of both the intra and intercolumnar collateral systems is the activation of other excitatory cells.