Dimitar Mitov

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Visually evoked responses (VERs) were recorded with vertical sinusoidal gratings presented in the on-off mode at rates of 0.5 and 1 Hz. Stimulus spatial frequency ranged from 0.5 to 16 c/deg and its contrast varied from near-threshold to the value of 0.3. At low spatial frequencies, 0.5 and 1 c/deg, the onset and offset VERs were of similar shape, magnitude(More)
We studied the stimulus-duration effect on the reaction time (RT) to gratings with different spatial frequencies. At each duration value, the product of the contrast and the duration, i.e. the contrast "energy" was kept constant. We found that at near-threshold "energy" levels the RT was constant for up to 15 ms at lower spatial frequencies and up to 30 ms(More)
Reaction time (RT) for detection of gratings of different spatial frequencies (SF, 0.87-13.87 c/d) and contrast (2-32 times above the detection threshold) was measured. It was found that at low stimulus contrast, 2 and 4 times above the detection threshold, the dependence of mean RT on SF is an "S"-type function with the two plateaus 50-60 ms apart. The(More)
Visual field symmetry in contrast sensitivity was investigated under conditions of spatial uncertainty. The stimuli were sinusoidal gratings of varying spatial frequency (SF). They were presented randomly in the left, right or in both visual hemifields. The percent detected gratings were slightly higher than the gratings in the left visual field except for(More)
The hypothesis that identification of line orientation is based on different mechanisms--a detector mechanism at large orientation differences and a computational one at small orientation differences--was tested in three experiments. The first two experiments compared reaction time and time of complete temporal summation (tc) in two tasks, line detection(More)
Visually evoked potentials (VEPs) were elicited by Gabor gratings with different lengths and widths at three spatial frequencies (SFs): low, 1.45 c/deg, medium, 2.9 c/deg and high, 5.8 c/deg and at a contrast 3 times above the detection threshold at each SF. An increase of grating length enhanced N1 amplitude at occipital and parietal positions stronger(More)
Changes in the components of visual event-related potentials (VERPs) depending on the difficulty of the identification counting tasks (“coarse” and “fine”) were studied in healthy humans. The basic finding is that much greater changes in the VERPs waves were observed within a 5 to 15 deg range than those within the range of 15 to 90 deg. The amplitude of(More)
The effects of the duration of presentation of stimuli on the reaction time for detection of gratings of different spatial frequencies were studied. The product of contrast and duration (the contrast "energy") was kept constant for each duration. These experiments showed that at near-threshold "energies," reaction times did not change with increases in the(More)