Neural circuits for chromatic and temporal signals in human visual cortex

The retina encodes chromatic and temporal information into several parallel pathways. These neural pathways, with differential sensitivities to color and temporal frequency, communicate their signals to distinct layers of primary visual cortex (V1). We know little, however, about the organization of neural circuits that communicate chromatic and temporal signals in human visual cortex. Here I use functional magnetic resonance imaging (fMRI) to study the architecture of this chromatic and temporal circuitry.; In Chapter 1, I develop new techniques to improve the fMRI spatial resolution to 1.6 mm or finer. These include a volume-based auto-focus method to correct off-resonance. Functional regions within visual cortex are identified by visual field mapping. I quantitatively evaluate the reliability and stability of visual field maps against stimulus variations (Chapter 2).; Using these methods, Chapter 4 shows the human secondary visual cortex (V2) can be divided into distinct types of stripe-like regions with differential stimulus responses. One set of stripes, color-slow, responds preferentially to chromatic (red-green or blue-yellow) stimuli and at lower (1.5 Hz) temporal frequencies. The complementary set of stripes, luminance-fast, responds preferentially to luminant (light-dark) stimuli and at higher (7.5 Hz) frequencies. The two sets of stripes interleave, forming a periodic pattern perpendicular to the V1/V2 border (periodicity: 8-15 mm).; Chapter 5 shows that ventral occipital (VO) cortex is more responsive to low than high temporal frequencies, and very responsive to both chromatic and luminant stimuli. Dorsal regions MT+ and V3A, on the other hand, respond very weakly to some colors, and are very responsive to all temporal frequencies. In summary, chromatic and temporal information in distinct layers of V1 separates into color-slow stripes and luminance-fast stripes within V2. Signals from these stripes may serve as the primary input to VO regions (color-slow), and dorsal regions MT+ and V3A (luminance-fast), respectively.