The experiment was conducted under bright room illumination (four 25-w fluorescent lamps located in the wall opposite to the chambers). The habituation phase and CS1 exposure period were conducted in square–shaped chambers made of wood (sides and height: 23 cm). These chambers were different (in shape, color and material) from the cages where animals were kept whenever not undergoing experimental procedures. The apparatus employed for the assessment of tactile preferences consisted of three interconnected chambers (28 × 20 × 21 cm each): a neutral central area made of white Plexiglas and lined with Formica and two end compartments. One of these compartments was also employed as CS2 during Phase 2: it contained a vertical striped pattern on the walls with a sandpaper floor (Gatorgrit, USA, coarse: 60). The other compartment had a horizontal striped wall pattern with a smooth floor (the reverse side of a sandpaper sheet). Sandpaper was replaced after each animal underwent phase 2 conditioning and after testing. A preliminary experiment conducted in our lab indicated the absence of age-related differences in bias for the distinctive chamber.
