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Biomedical subjects

V Virsu

Publications and source records attributed to V Virsu.

51 records · Page 3Linked to original sources
PubMed1977

Psycholphysical 'measurement' of cortical colour mechanisms: reply of Meyer.

In a paper recently published in this journal, Meyer criticised our study on relationships between channels for colour and spatial frequence for not being able to demonstrate a size aftereffect not specific to colour, a McCollough effect not specific to size, or the functions of cortical colour mechanisms. In fact, our study attempted none of these demonstrations in the sense suggested by Meyer because the first would have been impossible for empirical reasons, the second for conceptual reasons, and the third for methodological reasons. Instead, our study yielded evidence that at least three different types of perceptual channel underlie our capacity to perceive the size and colour of objects.

Afterimage↗
PubMed1975

Central inhibitory interactions in human vision.

Contrast threshold and perceived orientation of a line segment were measured when another line segment was simultaneously presented either to the same or the other eye; the angle between the two line segments was varied. The presence of the masking line elevated the contrast threshold under both conditions and the threshold increased similarly both in monoptic and dichoptic masking when the masking angle was made smaller. The presence of the masking line affected also the perceived orientation of the test line. The effect was similar both in monoptic and dichoptic masking; the largest change of perceived orientation occurred at about 15 degrees masking angle and the change was smaller at other angles. The effect disappeared within a short distance when the masking line was removed farther from the test line. The similarity of the monoptic and dichoptic threshold elevations demonstrates that there are lateral inhibitory interactions between central neural units in the human visual system. It is likely that the interacting units mediate the perception of contour orientation, for the threshold elevation functions were consistent with concurrent changes of perceived orientation. The results are evidence for the hypothesis that inhibition between orientation detectors is a factor in the perceptual expansion of acute angles.

Humans↗
PubMed1975

Human auditory evoked responses during hangover.

Auditory evoked responses (AER) to trains of 6 click stimuli (1 click/sec) were studied in 9 subjects under hangover, tired control, and normal control conditions in order to find out whether the symptoms of hyperexcitability during hangover have a correlate in the characteristics of the AER. In addition, the audiograms were measured. AERs to the first click in a stimulus train were markedly smaller during hangover than in the other 2 states. The amplitude levels of the AERs during the repetition of the click stimulus were, however, similar under all three conditions. The audiograms obtained in the three states were similar except for a very slight decrease of auditory threshold sensitivity during hangover as compared with the tired control condition. The results show that the effects of hangover on AERs resemble those of alcohol intoxication. The symptoms of hyperexcitability during hangover cannot be explained in terms of increased peripheral sensitivity.

Acoustic Stimulation↗
PubMed1975

Dark adaptation and short-wavelength backgrounds decrease perceived size.

The effects of background luminance, contrast, and background wavelength on the perceived size of small line figures were studied at mesopic levels of light adaptation. Perceived size diminished at low levels of background luminance. The effect disappeared at high levels of luminance. Perceived size of luminous circles increased as a logarithmic function of background luminance when the background intensity did not exceed 25 td(1). The strength of the size effect decreased as a function of circle diameter from 0-125 to 2 deg of visual angle(2). Perceived size of small luminous circles, subtending less than 0-5 deg, also increased as a function of contrast at low values of contrast but at very high values of contrast there was a decrease in perceived size. Background luminance had the same effect on the perceived size of circles as on the perceived size of spatial cycles in gratings. Control experiments led to the conclusion that dark adaptation is the primary source of the size effects. The main evidence for this conclusion was obtained from a demonstration that the same background luminance produced either an increase or a decrease in perceived size, depending on the adaptational state of the eye. It was also found that a shift from cone vision to rod vision contributes to the effects, for a stimulus looked smaller on a short-wavelength background than on a long-wavelength background. The size effects can be predicted from the changes of receptive-field properties of single neurones under corresponding conditions of stimulation, if it is assumed that the perception of size is mediated by size-specific channels formed of single neurones. Stimulation that leads to an activation of small receptive fields appears to indicate to the brain the presence of small retinal images. If small receptive fields are experimentally made responsive to larger retinal images, an underestimation of size results.

Dark Adaptation↗