Variability of the relative preference for stimulus orientation and direction of movement in some units of the cat visual cortex (areas 17 and 18).
1. The responses to visual stimuli of single units in the cortex of cats anaesthetized with pentobarbitone were recorded extracellularly with glass micropipettes. All had receptive field centres more than 5 degrees and most lay between 5 and 20 degrees from the area centralis. Most units were in Area 18 but some were in the corresponding field representation in Area 17. 2. A quantitative method is described in which the visual stimuli (slits or light bars) were presented repetitively by mechanical means in each of four orientations of the stimulus and in two directions of movement for each orientation. The responses were analysed quantitatively and criteria are described for classification of units according to their preference for particular orientations or directions of movement of the stimulus. 3. Some units were studied continuously for up to 2 hr using the quantitative technique. In Area 18, of nineteen units, eighteen showed changes in their preference for direction of stimulus movement and, in seven of eleven units, the orientation preference changed. In Area 17 direction preference changed in eight of nine units and orientation preference in six of seven. In some cases both orientation and direction preference altered. 4. The relationship of these changes to alterations in the 'spontaneous activity' of the cortical units and to variations in the depth of anaesthesia are considered. Neither would appear to be the sole cause of the phenomenon.