Topographic Maps fMRI: Theory and Practice Spring 2010

Macaque Retinotopy Source: Tootell et al., 1982

Retintopy: Flickering Checkerboard EXPANDING RINGS Retintopy: Flickering Checkerboard ROTATING WEDGES 8 Hz flicker (checks reverse contrast 8X/sec) good stimulus for driving visual areas subjects must maintain fixation (on red dot) Source: Jody Culham

EXPECTED RESPONSE PROFILE OF AREA RESPONDING TO STIMULUS To analyze retinotopic data: Analyze the data with a set of functions with the same profile but different phase offsets. For any voxels that show a significant response to any of the functions, color code the activation by the phase offset that yielded maximum activation (e.g., maximum response to foveal stimulus = red, maximum response to peripheral stimulus = green) time = 0 time = 20 sec time = 40 sec 20 40 60 time = 60 sec TIME  Source: Jody Culham

Retintopy: Eccentricity calcarine sulcus left occipital lobe right occipital lobe foveal area represented at occipital pole peripheral regions represented more anteriorly Source: Jody Culham

Retintopy on Flattened Occipital Lobe 1) virtually cut off the occipital lobe (remember, it’s a cup shape and the lateral surface is on the side we can’t see from this viewpoint) occipital pole 2) cut along calcarine sulcus upper calcarine sulcus 3) unfold and flatten the cortical surface lateral surface (note: retinotopic areas do extend onto the lateral surface but are not shown here in this schematic) left occipital lobe occipital pole lower calcarine sulcus Source: Jody Culham

Retintopy: Eccentricity Movie calcarine sulcus occipital pole Movie: eccentricity.mpeg http://cogsci.ucsd.edu/~sereno/phasemovie2.mpg Source: Marty Sereno’s web page

Retintopy in V1: Polar Angle calcarine sulcus horizontal meridian (HM) VM VM HM HM VM VM left occipital lobe right occipital lobe vertical meridian (VM) left-right hemifields reverse (left field to right hemisphere) upper-lower hemifields reverse (upper field to below calcarine) horizontal meridian lies ~along calcarine (not always exactly) Source: Jody Culham

Polar Angle and Eccentricity in V1 calcarine sulcus left occipital lobe right occipital lobe retinotopic areas are like polar coordinates: eccentricity and polar angle Source: Jody Culham

Polar Angle in V1, V2 and beyond VM } V3 HM } V2 upper calcarine sulcus } V1 upper horizontal meridian (HM) } V1 lower VM HM } V2 lower HM VM } VP VM left occipital lobe vertical meridian (VM) V2 is mirror image map of V1 V1-V2 border occurs at vertical meridian V2-V3 border occurs at horizontal meridian situation gets more complex in higher-tier areas (V4v, V3A) that have representations of whole hemifield Source: Jody Culham

Retinotopy Source: Sereno et al., 1995

Retinotopy: Polar Angle Movie calcarine sulcus occipital pole Movie: phase.mpeg http://zakros.ucsd.edu/~sereno/movies/phasemovie1b.mpg Source: Marty Sereno’s web page

Other Sensory “-topies” Touch: Somatotopy Servos et al., 1998 red = wrist; orange = shoulder Audition: Tonotopy cochlea Sylvian fissure temporal lobe Movie: tonotopy.mpeg http://cogsci.ucsd.edu/~sereno/downsweep2.mpg Source: Marty Sereno’s web page

Face/Place-o-topy faces activate foveal area (more for foveal than peripheral faces) places activate peripheral area (more for peripheral than foveal places) Source: Levy et al., 2001

Marty Sereno’s web page Saccadotopy delayed saccades move saccadic target systematically around the clock Source: Sereno et al., 2001 http://kamares.ucsd.edu/~sereno/LIP/both-closeup+stim.mpg Marty Sereno’s web page

Sulcal Formation Although sulci vary considerably from person to person (even in identical twins), there is considerable regularity in where the folds occur… Why? Source: Van Essen, 1997 David Van Essen proposes that as the brain develops, areas that are richly interconnected will be pulled together to form a gyrus (and those that are weakly interconnected form sulci).

Development of Sulci Sulci appear at predictable points in fetal development with the most prominent sulci (e.g., Sylvian fissure) appearing first. Source: Ono, 1990

Sulcal Formation: V1-V2 The V1/V2 border provides one example of two richly interconnected areas that form a gyrus. This arrangement also explains why maps in V1 and V2 are mirror images of each other! calcarine sulcus Source: Van Essen, 1997

Comparative Neuroanatomy The complexity of sulci increased throughout evolution Source: Comparative Mammalian Brain Collection

Interspecies Comparisons Figure H shows the macaque monkey visual areas morphed onto human cortex based on the placement of sulcal landmarks (Van Essen et al., 2001) Can we assume humans are just morphed monkeys? In some areas the human cortical surface area is slightly larger than in the macaque (e.g., visual cortex: 2X); in others it is considerably larger (e.g., parietal cortex: 20X) Are individual areas larger? Are there more areas?