Convergent evidence demonstrates the suppressive effects of noise masks D Baker |
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Department of Psychology, University of York, United Kingdom
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| The noise masking paradigm is widely used to assess visual deficits by measuring detection of targets embedded in broadband white noise. Recent work (Baker & Meese, 2012, Journal of Vision, 12(10):20, 1-12) demonstrates that unwanted suppression from such masks can contaminate estimates of internal variability. The magnitude of suppression can be assessed using a contrast matching paradigm, which measures the perceived contrast of a grating embedded in noise. For both dynamic and counterphase flickering noise at a range of temporal frequencies (1-19Hz), perceived contrast was reduced most severely (a factor of >4) at higher temporal frequencies. This is consistent with threshold elevation results for orthogonal grating masks (Meese & Holmes, 2007, Proc R Soc B, 274: 127-136). A second line of evidence comes from steady state visual evoked potential (SSVEP) measurements of the contrast response function to sine-wave gratings (1c/deg, 5Hz flicker) at the occipital pole (Oz). There was a marked reduction in the grating response when a high contrast noise mask was added at a temporal frequency (7Hz) that is distinct in the Fourier spectrum of the EEG. The implications of gain control suppression, as well as suggestions for how best to estimate internal noise, will be discussed. |
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