On the Benussi effect in a kinetic field

A study is reported of observers' perception of displays in which a black square (side 57 min of arc) is transformed into a rectangle (side 6.6 deg) by the outward motion of the right vertical edge. This display is seen as a rectangle that lengthens horizontally to the right. If a vertical bar is superimposed on the path of the rectangle, it is seen as two separate shapes on either side of the bar, one motionless and the other expanding in horizontal direction. This is an example of the Benussi effect in a kinetic field [Vicario, 1997, in La Percezione degli Eventi Eds G B Vicario, E Zambianchi (Milan: Guerini) pp 266 ^ 274].We tested the conditions in which 12 observers (80 trials each) perceived a stationary shape on the left side of the vertical bar, varying the position of the bar (across 5 values) and the velocity of expansion of the rectangle (across 4 values). The results indicate that a stationary shape is perceived on the left side of the bar only when the expanding process on the right of the bar lasts more than 380 ms, regardless of the position of the bar or the velocity of expansion.