Visual masking was measured in two human psychophysical experiments which showed that the contrast threshold to a grating test stimulus and the increment threshold to a spatially homogeneous (flash) test stimulus were elevated when they were preceded, respectively, by a grating or flash masking stimulus over a range of stimulus onset asynchronies (SOAs) from 55 to 200 milliseconds. The amplitude of the visual evoked cortical potential (VECP) to a test stimulus was then measured in anesthetized, paralyzed cats under the stimulus conditions used in the psychophysical experiments. The amplitude of the VECP was reduced when the test stimulus was presented after the masking stimulus at SOAs shown to produce masking in the psychophysical experiments. It was concluded that the reduction in VECP amplitude could be used as a physiological measure of visual masking and experiments were conducted to identify the properties and neural locus of the masking effects. / Visual masking was produced by both monoptically and dichoptically presented gratings and flashes. Masking produced dichoptically must be attributed to cortical mechanisms. However, the degree of masking was significantly weaker for dichoptically presented stimuli, suggesting that masking produced monoptically was both peripheral and central in origin. / Masking produced by monoptically presented gratings was greatest when the contours of the masking and test stimuli had the same spatial phase and orientation and was reduced by half when the orientation of the contours differed 6(DEGREES) to 15(DEGREES). A cortical neural locus for orientation-selective masking was demonstrated by simultaneous recordings of evoked potentials in the optic tract and visual cortex, which showed the degree of masking measured in the optic tract was independent of test stimulus orientation. / The findings support an integration theory of masking which posits that masking results when the masking and test stimuli evoke responses in the same neural elements and produce a degradation of the response to the test stimulus. Masking and test stimuli with contours differing in orientation would excite separate populations of orientation-selective cortical neurons and thereby produce little masking. / Source: Dissertation Abstracts International, Volume: 44-02, Section: B, page: 0643. / Thesis (Ph.D.)--The Florida State University, 1983.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_75062 |
| Contributors | TOOTLE, JOHN STANLEY., Florida State University |
| Source Sets | Florida State University |
| Detected Language | English |
| Type | Text |
| Format | 140 p. |
| Rights | On campus use only. |
| Relation | Dissertation Abstracts International |
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