A flickering stimulus can appear brighter or of different colour than a steady stimulus of the same spectral composition and equal time-averaged intensity. The change in appearance is consistent with the distortion product of a nonlinearity in the human visual system. This nonlinearity is used for non-invasive dissection of the human visual system into prenonlinearity and post-nonlinearity linear stages, which affect the amplitude of the sinusoidal signals that they process. A five-channel Maxwellian system was used to generate cosine-windowed, amplitude-modulated, sinusoidally-flickering stimuli. The subjects adjusted the maximum modulation of the signal in order to set the threshold for the perception of flicker and colour or brightness change. The pre-nonlinearity filter was tested by varying the carrier frequency between 5 Hz and 60 Hz and the post-nonlinearity filter was measured by varying the amplitude-modulation frequency between 0.25 Hz and 5 Hz. The L- and M-cone pathways are separately measured by using a new combination of the silent-cone-substitution technique and the sandwich model. The distortion product at the output of the nonlinearity is measured by using a new matching method for measuring the colour and brightness changes. The results suggest that the pre-nonlinearity filter is band-pass and shows substantial adaptation with light intensity that can explain most of the adaptation in the visual pathways. The post-nonlinearity filter is low-pass and shows little or no adaptation with light intensity. The input-output function of the nonlinearity can be described as an expansive, quadratic function that rapidly saturates at high input levels.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:565296 |
| Date | January 2011 |
| Creators | Petrova, D. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1310438/ |
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