Figure 2. The saccade/fixation sequence according to the competition-interaction theory. In abscissa (space scale) the foveal and parafoveal stimulus position (Sf and Sp, respectively) at time t0 (continuous axis) and time intervals t0, t1,t2 (dotted axis) are depicted. In ordinate on the left is represented the activation level of the P-system corresponding to the central-vision-related fixate center (green) and of the M-system corresponding to the peripheral-vision-related move center (red). The ordinate on the left refers to the saccadic speed. The green and red curves describe the reciprocal interaction of the progressive activation of the two programming stations (green= fixation system=P-system; red= saccadic system=M-system). The blue dashed curve, referred to the blue dotted x-axis (time scale), represents the saccadic speed. Note that the starting point of the saccade is the last fixation Sf. When the syllable Sf is fixated at time t0, P activation is at its maximum while M activation is at its minimum, resulting in a stable fixation. However, due to the presence of a second, peripheral syllable Sp, M activation rises more and more, gradually inhibiting the P-system until it matches and then overcomes its activation level (I) at time t1. So, at time t1 the prevailing effect of the peripheral-vision-related move center disrupts the fixation and promotes the saccadic triggering. FEM speed increases, keeping abreast of the M-activation level up to a maximum, while the P system is concurrently inhibited. From that moment on, a new progressive P-activation of the central vision-fixate center starts to oppose the effect of the magnocellular peripheral move-center: it follows that the speed of the actual FEM lowers gradually, keeping abreast of the decreased magnocellular activation. When the activation level of the P system overcomes the M level (II), the speed of the saccade slows down further and its “landing” is forthcoming. The landing site in Sp has become, therefore, the new fixation point (being now placed on the fovea). In the meanwhile, the complete P activation has overwhelmed the M-system, determining a total inhibition of the magnocellular peripheral move-center. A new cycle takes place as soon as the M activation starts again to rise and at the same time P activation begins to decrease proportionally. The inhibition/interaction mechanism is open to cognitive modulation


The Game of Reading: When Saccades and Fixations Play on a Seesaw

Carlo Aleci

American Journal of Medical and Biological Research. 2014, 2(2), 31-36 doi:10.12691/ajmbr-2-2-1