The magnocellular (M) and parvocellular (P) subdivisions of primate LGN are

The magnocellular (M) and parvocellular (P) subdivisions of primate LGN are known to process complementary types of visual stimulus information but a method for noninvasively defining these subdivisions in humans has proven elusive. the known anatomical business of the M and P subdivisions. In test-retest studies the relative responses of individual voxels to M-type and P-type stimuli were reliable across scanning sessions on individual days and across sessions at different field strengths. The ability to functionally identify magnocellular and parvocellular regions of human LGN with fMRI opens possibilities for investigating the functions of these subdivisions in human visual perception in individual populations with suspected abnormalities in one of these subdivisions and in visual cortical processing streams arising from parallel thalamocortical pathways. = 0.05 × is check size and is distance from fixation in degrees of visual angle. The checkerboard pattern covered half of the screen except for the central 0.6° of visual angle which contained background gray luminance (50% contrast luminance 105 cd/m2 (3T) or 1019 cd/m2 (7T)). The other half of the screen also contained the gray background. A white fixation point subtending 0.2° of visual angle appeared at the center of the screen throughout the run and subjects were instructed to maintain fixation while passively viewing the stimuli. For each run the checkerboard pattern alternated between the left and right halves of the screen 16 s (7T) or 13.5 s (3T) per side and was presented for 8 (7T) or 11 (3T) left-right cycles. Physique 1 LGN M/P localization methods. (A) A flickering checkerboard stimulus that alternated between the left and right visual hemifields was used to localize the LGN. (B) Voxels were selected that responded selectively to contralateral visual field activation. … An M/P localizer stimulus (Physique 1C) was designed to elicit differential responses from voxels with greater M-layer representation and voxels with greater P-layer representation based on findings from monkey electrophysiology (observe Kleinschmidt et al. 1996 and Liu et al. 2006 for related methods). The M/P localizer consisted of 16-s (7T) or 18-s (3T) blocks of “M stimuli” “P stimuli” and blank (fixation point only) stimuli. The M and P LY2940680 stimuli were both full-field sinusoidal gratings with sinusoidal counterphase flicker. The outer borders of the stimulus faded into gray to avoid sharp visual edges at the stimulus boundaries. The gratings were presented at one of 6 orientations (0° 30 60 90 120 or 150°) and changed to a new random orientation every 3 s in order to drive different populations of LGN neurons with different spatial receptive fields throughout the block. The M stimulus was a 100% luminance contrast black-white grating with LY2940680 a LY2940680 spatial frequency of 0.5 cpd and a flicker frequency of 15 Hz. The P stimulus was a low luminance-contrast high color-contrast S1PR2 red-green grating with a spatial frequency of 2 cpd and a flicker frequency of 5 Hz. A spatial frequency of 2 cpd was selected for the P stimulus because contrast sensitivity for isoluminant stimuli is usually attenuated at high spatial frequencies (De Valois and De Valois 2000 The blank stimulus was a gray screen of imply luminance. The reddish and green levels of the P stimulus were set to be near-isoluminant by performing heterochromatic flicker photometry outside the scanner. Specifically subjects adjusted the luminance of a green disk to match a 100% reddish disk on a neutral gray background by minimizing the belief of flicker as the two disks alternated at a frequency of 7.5 Hz. Two subjects (S2 and S3) performed flicker photometry and the average green value (39%) from these subjects was utilized for all scanning sessions. Although we did not perform flicker photometry in the scanner for all subjects (due to time constraints as well as a concern about adapting subjects to the reddish and green stimuli before the M/P localizer scans) we verified that this green luminance value obtained outside the scanner was affordable for both scanner displays by obtaining flicker photometry data from two subjects around the 7T display (mean of 41% green) and one subject around the 3T display (49% green). Since the values needed to accomplish isoluminance vary across subjects and across the visual field our main objective was to create LY2940680 a standard low luminance contrast stimulus that would enable relative activation of the M vs. P subdivisions. On each run 15 blocks (6 M 6 P and 3 blank) were offered in pseudorandom order with the.