Fixation Control in Tachistoscopic Studies of Laterality Effects: Comment and Data Relevant to Hines' Experiment

A report of three experiments designed to test the impact of the fixation digits on RVF superiority. In Experiment I, 10 Ss were assigned to a "fixation digit" condition and 10 to a "fixation dot" condition. Stimulus words were presented binocularly in a Scientific Prototypes Tachistoscope. Results indicate a highly significant main effect of VHFs favoring recognitions in the RVF. The inclusion of the fixation control procedure did not affect the overall difficulty of the task nor the magnitude of RVF superiority. In Experiment II, 10 more Ss (all 30 Ss were right handed) were presented with 40 bilateral word presentations, 20 of which had fixation digits and 20 of which did not. Bilateral stimulus pairs were arranged so that each word appeared twice in each VHF, once with and once without fixation digits. Viewing was binocular, and apparatus was as in Experiment I. More total recognitions occurred in the digit-absent condition. Results indicate that the interaction between type of trials and VHFs was not significant. Experiment III was identical in every respect to Experiment II, except that all 40 trials were with fixation digits present. Total recognition levels did not differ between Experiments II and III, nor did overall recognitions interact with VHFs for the two experimental treatments. An overall main effect for VHF was, of course, significant. In all conditions studied, significant RVF superiority obtained. In unilateral presentation, no significant effects were produced by the presence of fixation digits. In the case of bilateral presentation with fixation digits present on half the trials, no significant increment in RVF superiority was seen for digit-present trials and the only difference between performances on trials with and without digits was a lower overall level of recognition with digits present. Results indicate that fixation digits do not produce, nor indeed significantly augment, RVF superiority in either unilateral of bilateral presentation. They argue clearly against Hines' conclusion that fixation digits artifactually produce RVF superiority by providing a character for the initiation of a left-to-right readout (See LLBA VII/2, abstract #7301591.). When the S must fixate accurately, LVF superiority does not obtain with an "empty" fixation point; LVF superioity with bilateral presentation occurs only when Ss can fixate inaccurately.