performed over the visual cortex (a

performed over the visual cortex (approximately 7.0–8.0 mm pos- terior to bregma, 3.0–4.0mm lateral to the midline for primary visual cortex). Dura mater was dissected away and the cortex was bathed with silicone oil to prevent it from drying and reducing pulsations. Similarly, glycerine was administered periodically to prevent the cornea from drying. Body temperature was maintained at 37.0 ± 0.2 ◦C.
Flashing stimuli were used to investigate response properties of the cells in visual cortex. The stimulus was presented by a light-emitting diode (LED) located 1 cm in front of the eye. Twenty stimuli delivered at frequencies of 1, 2, 4 and 8 Hz were used for our adaptation studies because cells of aged rats display a criti- cal flicker frequency (CFF) threshold about 7 Hz [8]. Extracellular action potentials of neurons were recorded with glass electrodes filled with NaCl (2 M), with an impedance of 4.0–5.0 M