File:PremotorTheoryOfAttention Figure1.jpg

Schematic drawing of the experimental set up used by Rizzolatti, Riggio, Dascola, Umiltà (1987) to test the presence of the Vertical meridian effect. The visual display comprised one fixation box and four boxes for stimulus presentation. The stimulus boxes were marked by an adjacent digit (1-4). The fixation box was always shown at the geometrical centre of the screen, whereas the position of the other boxes varied according to different blocks (horizontally in the upper or lower hemifield, vertically in the right or left hemifield). The cue used to for directing attention was a digit (1-4), presented in the fixation box and indicating a probability of 80% for the target to appear in the relative box (valid trial) and 20% in one of the other possible boxes (invalid trials). Participants had to respond to target appearance by pressing a key with their right index finger. The importance of maintaining fixation was stressed and eye movements were monitored. The presence of the Vertical meridian effect was specifically tested by comparing reaction times to stimulus presentation in two locations that were at the same distance from the cued box (e.g., box 2) but in different hemifields. This comparison was significant and showed that reaction times were faster in the same (e.g., stimulus in box 1) than in the opposite hemifield (e.g., stimulus in box 3), indicating that passing from one hemifield to the other caused an additional delay of about 21 ms, regardless of which meridian had to be crossed.