Spike-timing-dependent synaptic modification induced by natural spike trains

any tendency for the induced squint itself to cause a visual impairment (strabismic amblyopia) in the deviating eye would actually reduce the likelihood of the nondeprived eye's continuing to dominate the cortex. In six kittens (three MDB and three MDS), visual acuity in the previously deprived eye was determined daily by the jumping-stand method 6,7. Kittens were trained to discriminate between a vertical and a horizontal grating, the spatial frequency of which was increased in accordance with an ascending method of limits. The nondeprived eye was occluded when the previously deprived eye was tested. The visual acuity of the nondeprived eye was also determined during and after the recovery period, either directly or by measurement of the acuity with both eyes open 6. After at least 14 days of recovery, ocular dominance and orientation maps in the primary visual cortex of all ten kittens were obtained by optical imaging of intrinsic signals. For six kittens (three MDB, three MDS), both behavioural tests and optical imaging experiments were performed. Anaesthesia was induced with an intramuscular injection of ketamine (20– 40 mg kg 21) and xylazine (2– 4 mg kg 21). Animals were intubated and artificially ventilated (50– 60% N 2 O, 40 – 50% O 2 , 0.9 – 1.2% halothane). Electrocardiogram, electroencephalogram, end-tidal CO 2 and rectal temperature were monitored continuously. Optical imaging of primary visual cortex was performed as described previously 25. Images were captured with either a cooled slow-scan charge-coupled device camera or an enhanced differential imaging system (ORA 2001 or Imager 2001; Optical Imaging Inc.), with the camera focused ,500 mm below the cortical surface. Visual stimuli, produced by a stimulus generator (VSG; Cambridge Research Systems), consisted of high-contrast, sinusoidally modulated gratings (0.2 –0.6 cycle deg 21) of four different orientations, drifting at a temporal frequency of 2 Hz, presented to the two eyes separately in randomized sequence, interleaved with trials in which the screen was blank. Single-condition responses (averages of 32 – 96 trials per eye and orientation) were divided (1) by responses to the blank screen, and (2) by the sum of responses to all four orientations ('cocktail blank') 25 to obtain iso-orientation maps. Signal amplitude was displayed on an eight-bit greyscale. Ocular-dominance maps were obtained by dividing the sum of responses to all four orientations through one eye by the similar sum of responses through the other eye. Resulting maps were high-pass filtered (cutoff …