Critical flicker frequency and cortical alpha.

The purpose of this pape r is to check the s t a r t l i ng ly high correlation (0.855) which C. Chyat te (1954) has found between critical flicker f requency and a lpha index. Chya t te ' s work was carr ied out in our labora tory three years ago with the same equipment as used in this s tudy and in a previous one by F. P. Fr iedl (1954). I t was on the basis of this lat ter s tudy tha t we became uneasy about Chya t te ' s correlation, which is cer ta inly the highest yet repor ted between measures of cortical a lpha and any other variable. T h e same correlation a t ta ined the very inconspicuous value of 0.]4 in Fr ied l ' s subsequent investigation. Both Chyat te and Fr iedl used a Strobotac Model 631 B with a cardboard shield reducing the i l luminated area to a circular field of 2.5 era. in diameter. By means of a t ranslucent piece of bond pape r the i l lumination of this area was kept un i form at about 0.5 foot candles at the source. The same procedure was followed in the present investigation. In all three studies C F F was measured under conditions of light adapta t ion (LA) . Fo r our subjects, however, C F F measures a f te r 15 nfin. of dark adapta t ion (DA) are also available. The measure of cortical potentials was obtained on the Grass electroencephalograph (Model I I I B) f rom two 1 S, min. records, with a 15 see. interval between recordings dur ing which the subjects were asked to open their eyes and again make themselves comfortable. The actual records were obtained, of course, while the subjects kept their eyes closed and t r ied to relax as much as possible. Monopolar recordings (occipital to ear lobe) were used. The a lpha index is the per eent t ime dur ing which the regular alpha pa t t e rn persists in a given record. The subjects of this s tudy were 56 normal, male college students whose ages ranged f rom 19 to 22 years. They were drawn f rom a larger populat ion in such a manner as to over-represent the two extremes on the Tay lor ' s (1953) Scale of Manifest Anxiety. I t was hypothesized tha t the divergence between the correlations obtained by Chyatte, on the one hand, and by Friedl, on the other, could be due to the anxiety variable which was not included in the former study. Table I gives the comparable results of the three studies. A significant difference (at the .05 level) was found between the mean C F F values of this s tudy and those of Chya t te ' s (t ---2.19). The difference in the mean alpha indices repor ted by Chyat te and by Fr iedl is also significant at the .05 level (t ~2.34). We do not feel, however, that these minor discrepancies can account for the very impressive difference betwee~ the respective correlations. Our own result agrees with that of Fr iedl almost completely and stands in marked contrast to Chyat te 's . Snedecor 's (1946) Chi square test for correlation coefficients yields a value of 42.27 for 2 df which is significant beyond the .00l level. Our hypothesis tha t the difference between Chya t te ' s very high correlation and Fr ied l ' s very low correlation might be due to differences in anxiety-proneness between the two samples was not substantiated. For 24 cases with low Taylor-scores (0-8). the correlation between C F F and a lpha itidex was 0.07; for 26 cases with high Taylor-scores (13-39) it amounted to -0 .02 . These two values are neither significantly di f ferent f~om each other nor f rom zero. Therefore, we f ind ourselves unable to account for the huge difference between Chya t te ' s result and that obtained in the two subsequent studi,,s. I t is t rue that Chya t te ' s subjects were, on the average, older (mean ---26.6 yoars) than ours (mean ~ 20.8 years ; t ~ 6.82) but they were not s ignif icantly older than Fr ied l ' s subjects (mean ~ 25.2 years ; t ~ 1.40).