Window discriminator

For confirming optical imaging results, single- and multi-unit responses to visual stimulation were recorded extracellularly to define the minimum spatial response field (RF), preferred orientation and direction of motion and ocularity using hand-held stimulation at random locations within the mapped area. To this end, glass electrodes (8–10 μm of tip diameter) were filled with 0.5–1 M NaCl (~5 MΩ). The pipettes were advanced perpendicularly to the cortical surface using a manual micro-drive fitted with a digital depth meter (10 μm accuracy, Sylvac SA, Crissier, Switzerland). Along each electrode track single and multi-units were recorded at several depths (300–2000 µm) from the cortical surface. To stabilize recordings, after advancing the electrode into the cortex, the chamber was filled with 3 % agar and covered with low-melting point paraffin (~43 °C, Merck, Darmstadt, Germany). Signals were amplified by AxoClamp-2A (Axon Instruments, Foster City, Canada) and filtered (0.3–10 kHz). Spike activity was monitored using a window discriminator (World Precision Instruments Inc., Sarasota, FL, USA) coupled to an audio monitor. The recorded units had large minimum discharge field (average: 9.5° ± 6.8° of diagonal length, N = 71 from six animals). In some cases RF had large size (maximum, 51°), especially at depths corresponding to layers 5/6.