Multifractality of decomposed EEG during imaginary and real visual-motor tracking

We test the possible multifractal properties of dominant EEG frequency components, when a subject tracks a path on a map, either only by eyes (imaginary movement - IM) or by visual-motor tracking of discretely moving spot in regular (RM) and Brownian time-step (BM) (real tracking of moving spot). We check the hypotheses that the fractal properties of filtered EEG (1) change with respect to the law of spot movement; (2) differ among filtered EEG components and scalp sites; (3) differ among real and imaginary tracking. Sixteen right-handed subjects begin to perform IM, next--real spot tracking (RM and BM) following a moving spot on streets of a citymap displayed on a computer screen, by push forward/backward a joystick. Multichannel long-lasting EEG is band-pass filtered for theta, alpha, beta and gamma oscillations. The Wavelet-Transform-Modulus-Maxima-Method is applied to reveal multifractality [local fractal dimensions Dmax(h)] among task conditions, frequency bands and sites. Non-parametric statistical estimation of the fractal measures h (Dmax) is finally applied. Multifractality is established for all experimental conditions, EEG components and sites as follows among filtered components - anticorrelation (h(Dmax) < 0.5) in beta and gamma, and long-range correlation (h(Dmax) > 0.5) for theta and alpha oscillations; among tasks--for RM and BM, h (Dmax) differ significantly whereas IM resembles mostly RM; among sites--no significant difference for local fractal properties is established. The results suggest that for both imaginary and real visual-motor tracking a line, multifractal scaling, specific for lower and higher EEG oscillations, is a very stable intrinsic one for the activity of large brain areas. The external events (task conditions) insert weak effect on the scaling.

Peripheral and central delay in processing high spatial frequencies: reaction time and VEP latency studies

Visually evoked potentials (VEP) and reaction time (RT) were recorded under stimulation with sinusoidal gratings. Grating spatial frequency (SF) was 0.5, 5 or 12 cd and grating contrast was varied. Consistent with previous findings, both VEP latency and RT increased with the increase of grating SF and with the decrease of grating contrast. It was found, in addition, that RT and VEP latency increased by approximately the same amount when SF increased from 0.5 to 5 cd, thus suggesting that the main source of the RT delay at 5 cd in comparison with RT at 0.5 cd is of peripheral origin. However, in comparison with the data at 0.5 and 5 cd, RT at 12 cd increased much more than VEP latency. We conclude that the RT delay at high SF involves a substantial central component in addition to the peripheral delay.

An apparatus for measuring the optical density of human ocular media to blue light

The paper describes an apparatus constructed by the authors and aimed at evaluating the optical density of human ocular media by a non-invasive psychophysical method described previously (Sample et al. 1988). The device has two light sources within the wavelength bands of 437 +/- 6 nm and 557 +/- 6 nm, respectively. They are a square-wave alternated at 1 Hz. The radiance of each can be controlled by the subject and adjusted at visibility threshold or at subjective equiluminance. Upon scotopic vision, the ratio between the two thresholds (or ratio of radiances at equiluminance) is a function of the ocular optical density at the short wavelength. The psychophysical procedures of adjustment of the 437 nm source at the absolute threshold and at equality with the 557 nm source had been tested with volunteers aged between 24 and 67 years. The dependence of blue-light sensitivities on age, obtained with the two procedures, suggests, in agreement with the literature, two main sources of sensitivity decline with age: senile myosis and ocular-media density increase at the short wavelength end of the visible spectrum.

Flicker masking depends on the method of measurement

Luminance flicker masks sinusoidal gratings of low spatial frequency (SF), the effect being attributed to the transiency of response within this SF range. An alternative point of view is that the lower number of cycles of such gratings makes them more susceptible to flicker masking. To verify this possibility, flicker masking of low-SF gratings (1 c/deg) of varying size was studied by the methods of human visually evoked potentials (VEP), reaction time (RT) and contrast threshold. Difference between the effects of flicker masking on contrast threshold, RT and VEP was found: RT was delayed and VEP was reduced in amplitude by flicker with both small and large gratings while contrast threshold was only increased with the small gratings. The decrease of grating duration increased the masking effect on contrast threshold. Moreover, contrast threshold was a unimodal function of flicker frequency with brief test gratings and a bimodal one with long-lasting gratings. It is concluded that: (1) the previously reported effects of luminance flicker masking on RT and VEP are not an artefact of the reduced number of cycles; (2) at the SF tested, both RT and VEP depend on the transient components of the response, while contrast sensitivity depends on both transient and sustained components of the response.

Spatial-frequency specific contrast gain and flicker masking of human transient VEP

We studied the effects of grating contrast and luminance-flicker masking on the early waves of human visually evoked potentials (VEPs) recorded at the onset-offset of sinusoidal gratings of varying spatial frequencies (SFs). At high SFs, the response waveform was simple and VEP was dominated by a negative wave (N110). At low SFs, several positive-negative deflections were recorded, the earliest dominating wave being positive (P90). The amplitude of P90 was saturated at a contrast of about 0.1 and it was attenuated by flicker masking. Masking involved to a lesser extent the waves following P90. It was weaker at the flicker frequency of 5 Hz than at 10 and 20 Hz. No flicker masking was found at SFs higher than 2-4 c/deg. At medium and high SFs, VEPs were obtained at higher contrast levels. No saturation (max contrast tested 0.5) and no flicker masking of N110 were observed. These results suggest that the early VEP components recorded at low and high SFs are related to different types of neuronal activity. Correlation between VEP properties and properties of magnocellular and parvocellular pathways is considered with an emphasis on recent morphological data about the human retina.