ULTRADIAN VARIATIONS IN A LATERALIZED LETTERS RECOGNITION AND POINTS ENUMERATION TASK - NON-STATIONARITY, STABILITY AND INDIVIDUAL-DIFFERENCES

Introduction Broughton (1975) hypothesized that the 90 minute Basic Rest Activity Cycle (BRAC) is associated with an alternation in the relative efficiency of the cerebral hemispheres. Investigations testing daytime ultradian rhythms in cognitive performance with non-lateralized verbal and spatial tasks (Klein and Armitage, 1979; Kripke, Fleck, Mullaney dz Levy, 1983 ; Armitage, Moffitt, Hoffman & Pigeau, 1985) yielded contrasting results. In order to have a more direct evaluation of hemispheric functioning, the present study assessed performance in a letters recognition (LR) and in points enumeration (PE) task, in which stimuli were laterally presented to the left and to the right visual hemifields (LVHF and RHVF) (Bertini, Violani, Zoccolotti, Altomare, Doricchi & Evangelisti, 1985). Furthermore morning V.S. afternoon statiomniety, stability of ultradian variations in two different days and modulating effects by individual characteristics were assessed. Methods Ten right-handed subjects (5 males and 5 females), were individually tested on the two lateralized tasks (LR and PE) in a soundproof chamber at 15’ intervals from 10.00 am to 6.00 p.m. The same procedure was repeated the following day. For each day of each subject 8 hour time series of lateralized performance scores in LR and PE tasks were constructed and submitted to power spectra analysis centered at 48,24, 16..6 cycles/day (c/d) frequenciesIn order to evaluate morning-afternoon stationariety each time series was divided in two 4 hour series. Results Anovas were carried out on power spectra of the performance scores for each task in each hemifield. In order to assess the significance of the variance explained by the ultradian frequencies, the power spectra at the BRAC frequency (16 c/d, corresponding to a 90 minute period) at the two adjacent frequencies (12 c/d and 24 c/d) and the mean power at all other frequencies considered, were computed andconsidered as levels of the factor “Frequency”. Three-way 2x2x4 (SEX x SESSION x FREQUENCY) anovas failed to reveal any main effect or interactions involving the factor “FREQUENCY”. Four-way 2x2x2x4 (SEX x SESSION x PART OF DAY x FREQUENCY) on power spectra of the 4h. time series revealed a significant PART OF DAY x FREQUENCY interaction F(3,24)= 4.68; p=.Oll for LVHF performance in the LR task, indicating that, in the morning, power was greater in the 16 c/d band (a 90 minute period), while in the afternoon it was greater in the 12 c/d band ( a 120 minute period). A significant SEX x PART OF DAY x FREQUENCY interaction [F(3,24)=6.31; p=.OO2] was found for RVHF in the PE task, indicating that only in males there were more power in the 16 c/d band in the morning and in the 12 c/d band during the afternoon. Anovas including “group factor” based on mean individual differences in Morning-Evening Type, EPI neuroticism, sleep quality and dream recall assessed on a 7 days morning diary, failed to show any modulating effects on the power spectra. Conclusion Considering the entire 8 hour time series no ultradian 16 c/d (a 90 minute period) rhythm was found in cognitive performances in lateralized verbal and spatial tasks. But considering the part of day (momingafternoon), a significant 16 c/d rhythm was shown in the morning. This phenomenon concerns the nondominant hemisphere for verbal functions in the LR task and, at least for males, the non-dominant hemisphere for spatial functions in the PE task. This ultradian rhythm is stable over two successive experimental sessions and does