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Andrew Stockman, Mark Williams, Hannah Smithson; Flicker-clicker: cross modality matching experiments. Journal of Vision 2004;4(11):86. doi: 10.1167/4.11.86.
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We used cross-modality matching between a visual flicker and an auditory “clicker” to investigate (i) the nature of visual encoding at those central sites at which visual and auditory signals are compared and (ii) the responses of different visual pathways at low to moderate temporal frequencies.
The experiments employed two principal methods. In the first, the clicker frequency was matched to the perceived visual flicker rate; while, in the second, the clicks were temporally aligned with a distinct phase of the flicker cycle.
Four frequency ranges were evident: (i) below c. 2.0 Hz, the visual and auditory stimuli could be reliably matched in phase; (ii) between 2.0 and c. 3.5 Hz, a phase-dependent synchronicity of the flicker was apparent, suggesting that some phase information survives up to 3.5 Hz; (iii) between 3.5 and c.12.5 Hz, confirming earlier work by others, the stimuli could be accurately matched in frequency but not in phase; whereas (iv) above 12.5 Hz, the flicker rate was increasingly underestimated by the clicker. Above 3.5 Hz, the apparent visual flicker rate was strongly driven by the clicker.
These flicker-clicker matching methods can be used to contrast the properties of the different visual pathways. At frequencies below 2.0 Hz, the phase method has allowed us to quantify the perceptual delays between chromatic and luminance signals, as well as the speeding up of the visual response with adaptation. Apparently, flicker below 3.5 Hz is encoded by a moment-by-moment variation of the neural signal that retains phase information and survives as far as those central sites at which the visual and auditory signals can be compared, whereas above 3.5 Hz all that survives is rate information.
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