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K. Mori, T. Saito, F. A. Dodge, R. B. Barlow; Visual Processing of High Contrast Objects in the Brain. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3312.
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© ARVO (1962-2015); The Authors (2016-present)
Understand the central neural mechanisms underlying visual detection of high contrast objects in Limulus.
To record brain cell activities we first surgically exposed the Limulus brain in situ with intact optic nerves and removed surrounding tissues. We then inserted a plastic chamber under the brain for stability and perfusion. We recorded intracellularly responses from neurons in the second visual ganglion, the medulla. Visual stimuli were displayed on LCDs placed in front of the lateral eyes. Upon encountering a contrast cell, we mapped its receptive field using a white bar moved on a black background. We then recorded responses to vertical bars (widths: 6º or 12º) of varying contrasts moving on a gray background with constant velocity. Bar contrast ranged from -1.0 and +1.0 (C = log10 (Io / Ib ), where Io is the intensity of the object and Ib is the intensity of the background).
Contrast cells maintained firing to a uniform gray background and were inhibited by moving vertical bars having either positive and negative contrasts. Changing the contrast of bars did not significantly change the response of the cells. Contrast cells had receptive fields measuring 10 ± 2 (SD, n = 8) degrees horizontally and 8 ± 3 degrees vertically. They were widely mapped from the anterior to posterior borders of the field of view of the lateral eye (~120º), and narrowly and slightly below the horizon of the lateral eye (~20º).
Object contrast inhibits Limulus brain cells regardless of the sign of the contrast - a response property reminiscent of suppressed-by-contrast cells detected in the mammalian visual system. With receptive fields widely distributed near the horizon, contrast cells appear well adapted for the initial stage of mate detection.
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