1. Receptive-field properties of single neurons in cat's cortical area 18 were studied before and after partial bilateral lesions of area 17. 2. The majority of cells recorded from animals with intact visual cortex exhibited orientation selectivity, directional selectivity, and could be independently activated through either eye. All cells responded well to moving targets and nearly all of them exhibited broadly tuned preferences with respect to speed of the target. Over 45% of cells responded optimally or exclusively at very fast (above 50 degrees/s) speeds. 3. The majority of neurons recorded from animals with intact visual cortex responded weakly but clearly to appropriately oriented localized stationary stimuli flashed on and off. About one-third of the cells responded with mixed on-off discharges from all over their receptive field. In the receptive fields of 10% of cells, separate on- and off-discharge regions could be revealed. In the receptive fields of the remaining cells, only on- or only off-discharge regions could be revealed. 4. The majority of neurons recorded after ablation of area 17 were orientation selective; 50% of the cells were also direction selective. All neurons responded well to moving targets; about 65% of them responded optimally or exclusively at very fast target speeds. 5. Destruction of the dorsolateral part of contralaterial area 17 and most of contralateral area 18 caused significant reduction in proportion of cells in area 18 which could be activated through either eye. 6. The majority of neurons recorded after ablation responded to appropriately oriented localized stationary stimuli flashed on and off. Cells with mixed on-off discharge regions all over the receptive field with separate on- and off-discharge regions and with only on- or only off-discharge regions were found. 7. It is concluded that the processing of afferent visual information in area 18 is, to a great extent, independent of the information carried to this area by associational fibers from cells of area 17.

To comply with the author's request we are publishing below Fig. 7 which had been missing in the manuscript of the above paper due to an unfortunate mishap.

SUMMARY1. Using a technique described previously, we have applied pressure to the optic nerve of a cat sufficient to cause conduction block of the t1 response (the response of the Y optic nerve fibres). A greater pressure, usually sufficient to cause a transient block of the t2 response (the response of the X fibres), leads to degeneration of the Y axons caudal to the block. This is demonstrated by the disappearance of the t1 response in this region after 4-5 days and by the presence in electron micrographs of degenerating large (Y) fibres. Some small fibres also show degeneration, but the medium (X) fibres are largely spared.2. The time course of loss of response in the Y fibres is similar whether the loss is due to a pressure block or to enucleation, suggesting that the pressure block as used by us causes a disruption of the axon. If the pressure is great enough to block part of the t2 response (X fibres) there is also a similarity in time course of loss of response to that following enucleation.3. Both for the enucleated and the pressure-blocked cat the t2 response fails about 1 day before the t1 response. This is in apparent disagreement with the morphological findings in the literature, confirmed here, indicating an earlier degeneration of the larger fibres.4. The post-synaptic response in the lateral geniculate nucleus to the t1 input (the r1 response) also fails about 1 day before the t1 response. In the visual cortex the loss of the r1 response reveals more clearly than is normally possible an r2 response, the response of the X optic radiation fibres.5. The response in the optic nerve or tract to a bright flash of light is dominated by the response of the Y fibres. When these are blocked the response is greatly reduced.

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