Papers by Milena Mihaylova
Proceeding of the Bulgarian Academy of Sciences, 2013
Visually evoked potentials (VEPs) to stimuli-gratings with varying length and width were recorded... more Visually evoked potentials (VEPs) to stimuli-gratings with varying length and width were recorded at 3 spatial frequencies (SFs), 1.45, 2.9 and 5.8 c/deg and contrast three times above the detection threshold for each SF. The amplitude of the first negative VEP wave, N1, increased to a greater extent with increase of stimulus length than with increase of stimulus width at higher and medium SFs, 5.8 and 2.9 c/deg. However, at low SF, 1.45 c/deg, the effects of the grating length and the width on N1 amplitude were similar. Moreover, the difference between the effects of stimulus length and width on the amplitude of the second VEP component, P1, was not observed at all three SFs studied. The results obtained, along with the psychophysical data showing stronger effect of stimulus length in comparison with width on the detection threshold [ 1, 2 ], might be interpreted as neurophysiological evidence that the underlying mechanisms are arrays of elongated receptive fields.
Vision Research, 2002
Saleh and Bonnet [Fechner Day 98, p. 344] have shown that, upon parafoveal stimulation and up to ... more Saleh and Bonnet [Fechner Day 98, p. 344] have shown that, upon parafoveal stimulation and up to 6.5 c/deg, reaction time (RT) is a function of grating contrast multiplied by grating period. The present experiments extend these findings to foveal stimulation within a wider spatial-frequency (SF) range and to stimuli of different duration. Both RT and latency of visually evoked potentials (VEP) were measured. The findings might be explained by the following assumption: Most RT and VEP latency variations across the SF range are a result of local intensity factors (retinal contrast and width of grating bars). Residual RT variations were found that might be due to processing of high SFs by slower mechanisms than those processing low and medium SFs.
Vision Research, 1998
Visually evoked potentials (VEP) and reaction time (RT) were recorded under stimulation with sinu... more 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.
Vision Research, 2003
We studied spatial summation for Scone ON and OFF signals as a function of retinal eccentricity i... more We studied spatial summation for Scone ON and OFF signals as a function of retinal eccentricity in human subjects. Scone isolation was obtained by the two-colour threshold method of Stiles, modified by adding blue light to the yellow background. Test stimuli were blue light increments or decrements within a circular area of variable size. These were presented for 100 ms at 0 to 20 deg along the horizontal temporal retinal meridian. Ricco's area of complete spatial summation was measured from the threshold vs. area curves. This was nearly constant and approximately the same for both types of stimuli within the 0-5 deg range and increased beyond this range. The decremental area increased faster, suggesting that separate mechanisms, presumably ON and OFF, integrate Scone increments and decrements. The results appear to provide new evidence for the existence of separate Scone ON and OFF pathways. We compare the data with known morphology of primate retina and assume that, if Scone decrements are detected via separate OFF cells, these should differ in density and dendritic field size from the Scone ON cells, but only in the retinal periphery.
Journal of Vision, 2010
Research has shown that the processing time for discriminating illusory contours is longer than f... more Research has shown that the processing time for discriminating illusory contours is longer than for real contours. We know, however, little whether the visual processes, associated with detecting regions of illusory surfaces, are also slower as those responsible for detecting luminance-defined images. Using a speed-accuracy trade-off (SAT) procedure, we measured accuracy as a function of processing time for detecting illusory Kanizsa-type and luminance-defined squares embedded in 2D static luminance noise. The data revealed that the illusory images were detected at slower processing speed than the real images, while the points in time, when accuracy departed from chance, were not significantly different for both stimuli. The classification images for detecting illusory and real squares showed that observers employed similar detection strategies using surface regions of the real and illusory squares. The lack of significant differences between the x-intercepts of the SAT functions for illusory and luminance-modulated stimuli suggests that the detection of surface regions of both images could be based on activation of a single mechanism (the dorsal magnocellular visual pathway). The slower speed for detecting illusory images as compared to luminance-defined images could be attributed to slower processes of filling-in of regions of illusory images within the dorsal pathway.
Acta neurobiologiae experimentalis, 2015
Visually evoked potentials (VEPs) were elicited by Gabor gratings with different lengths and widt... 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 than the increase of grating width at aspect ratios (length : width) above 4:1. The stronger effect of stimulus length than width was reflected also in the amplitude of the later P1 component at central and parietal positions. The larger effect of stimulus length than width on the VEP amplitude was SF specific: it was stronger at 5.8 c/deg, smaller at 2.9 c/deg and vanished at 1.45 c/deg. The results obtained suggest anisotropy in the physiological mechanisms that underlie grating perception and involve bottom- up processes initiated in the occipital cortex.
Uploads
Papers by Milena Mihaylova