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Incheol Kang, Joseph Malpeli; A comparison of behavioral contrast sensitivity with the constrast sensitivities of X and Y geniculate cells in the awake cat. Journal of Vision 2005;5(8):461. doi: 10.1167/5.8.461.
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© ARVO (1962-2015); The Authors (2016-present)
To determine if individual LGN cells are sensitive enough to account for cat dim-light vision, the contrast sensitivity of single cells and behavioral contrast sensitivity were measured at adaptation levels from the lowest scotopic to the mid-mesopic. Cats were trained to fixate a centrally-presented laser spot, to make saccades to Gabor functions appearing 8 deg to the right or left, and to ignore patches of uniform sine gratings. Temporal frequency was 4 Hz for both Gabor functions (SD = 1.5 deg) and extended sine gratings (15×15 deg), and both were upward drifting. Behavioral contrast sensitivity was determined with a staircase procedure, and psychometric functions derived for 0, 1/8, 1/4, 1/2, 1, 2 and 4 cyc/deg at adaptation levels spanning 8 log units above absolute luminance threshold. Single cells were isolated at mesopic levels, and characterized as X or Y with counterphasing gratings. The cats were dark adapted, and spikes evoked by drifting sine gratings of spatial frequency 1/8, 2 and 4 cyc/deg were recorded at increasing levels of adaptation. Signal detection theory was used to estimate thresholds from the mean firing rate (F0) and first harmonic (F1). We find that the most sensitive LGN cells can account for the observed behavior under all conditions tested - i.e., there is no need to invoke post-thalamic averaging across a large population of cells at these adaptation levels or spatial frequencies. For F1 responses, behavioral sensitivity for 1/8 cyc/deg closely matched the most sensitive Y cells, whereas for 2 cyc/deg, the most sensitive X cells were as sensitive as the cat. For 1/8 cyc/deg in the scotopic range, Y cells were approximately 1 log unit more sensitive than X cells over a wide range of contrasts. Cats performed above chance at 4 cyc/deg, with contrast thresholds of about 75% for luminance levels above 1 cat troland. Surprisingly, this behavior appears to be supported not by X cells, but by the F0 response of Y cells. Supported by NIH EY02695.
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