SUMMARY1. Cholinergic drugs were infused into the retinal circulation of the rabbit while we analysed the receptive field properties of directionally sensitive retinal ganglion cells. Physostigmine eliminated the trigger feature, directional specificity, of both types (on-centre and on-off) of these cells. In this respect the action of physostigmine (an ACh potentiator) was very like that of picrotoxin (a GABA antagonist). Therefore, a detailed analysis of the receptive field properties of directionally sensitive ganglion cells was made to analyse the effects of physostigmine and picrotoxin.2. Size specificity and radial grating inhibition were not abolished by physostigmine, but were often affected by picrotoxin. The optimal velocity in the preferred direction (as measured by maximum firing frequency) was not much changed by physostigmine, but was higher during infusion of picrotoxin. Infusion of nicotine, a depolarizing ACh agonist which increases the activity of retinal ganglion cells, revealed the presence of inhibition to movement in the null direction. The null direction response during picrotoxin started slightly later than this inhibition. The null direction response during physostigmine was weaker and started later still. Mecamylamine and dihydro-,B-erythroidine, nicrotinic receptor antagonists, totally blocked the effect of physostigmine and reduced the control light response by about half.3. From this analysis, it appears that on-off ACh release onto directionally sensitive cells provides a substantial excitation which, when potentiated by physostigmine, overcomes or outlasts the null direction GABA inhibition within the receptive field. The spatial extent of GABA inhibition is asymmetric to and larger than the spatial extent of ACh excitation. Similar pathways appear to be involved in both the on-centre and on-offdirectionally sensitive ganglion cells, yet the on-centre cell pathway may receive an additional input which suppresses the ACh excitation at light offset. Possible schemes for the cellular mechanism of directional sensitivity are discussed in light of these results and recent anatomical and pharmacological findings.

The receptive fields of directionally sensitive ganglion cells in the rabbit retina were analyzed. Several types of experiment showed that each point within the receptive field of the cell is inhibited by a fairly wide area of points around it, lying on each side of the preferred-null axis as well as along the preferred-null axis in the preferred direction. The excitatory or responsive receptive field of these cells has an inhibitory surround: this inhibitory surround appears to be simply an extension of the inhibition that occurs within the center of the receptive field. Points toward the edge of the responsive receptive field are inhibited from an area around them which extends into the center of the receptive field and also into the inhibitory surround. Directionally sensitive retinal ganglion cells respond to moving spots better than to moving bars. This is particularly true for objects moved perpendicularly to the preferred-null axis. In some cells a spot moved perpendicularly to the preferred-null axis will give a substantial response, whereas a bar moved in the same direction will give no response at all. This phenomenon can be explained by the inhibitory area which surrounds each point within the receptive field; since this inhibitory area is asymmetrical, it is also responsible for the cell's directional sensitivity. When two bars oriented perpendicular to the preferred null axis are flashed, one after the other, the response to the second bar is nearly always reduced by the presentation of the first bar. This is true for many temporal and spatial sequences corresponding to movement in the preferred direction, as well as those corresponding to movement in the null direction. However, there are temporal and spatial sequences, corresponding to movement in the preferred direction, for which the response to the second bar is unaffected by the presentation of the first bar. The time delay for this does not vary from cell to cell--it is always approximately 20 ms for on-off directionally sensitive cells and approximately 180 ms for on directionally sensitive cells. The spatial separation does vary from cell to cell, between 0.13 degrees and 1.2 degrees in 11 on-off directionally sensitive cells. This spatial separation, which gives linear summation of the response to two bars flashed 20 ms apart in the preferred direction, is correlated with the speed of movement which gives the best response for a bar moved through the receptive field in the preferred direction.

SUMMARY1. Kittens were reared in the dark from birth except for a period each day when they were put inside a stationary transparent cylinder, around which a drum, with vertical black and white stripes on the inside, rotated in one direction. After the end of the period of exposure, we recorded a sample of single cells from their visual cortices, and analysed each cell for direction and orientation sensitivity and other properties.2. Two kittens were placed inside the drum, rotating rightward, for 2 hr each weekday from 3j to 7 weeks of age. A greater proportion of the directionally sensitive cells in their cortices showed a preference for rightward movement. 3. Six other kittens were placed inside the drum for 1 hr each weekday from 2 to 12 weeks of age with the drum rotating leftward up to a particular changeover age, then rightward until 12 weeks. The changeover point occurred at 21, 26, 28, 33, 35 and 51 days for different kittens. A changeover earlier than 4 weeks of age led to a preponderance of cells preferring rightward movement. A changeover later than 5 weeks of age led to a preponderance of cells preferring leftward movement. Comparison of these results with others on monocular deprivation suggests that the peak of the critical period for directional deprivation may occur earlier than the peak of the critical period for monocular deprivation.4. None of the samples of cells showed a preponderance of cells specific for vertical orientations. It is unclear whether this negative effect resulted from the presence of some horizontal contours during exposure, or some more fundamental cause.

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