Responses of the human visual cortex and LGN to achromatic and chromatic temporal modulations: an fMRI study

In this study, we investigate how the responses of the human visual pathway to temporal frequency are modified as information transfers between the lateral geniculate nucleus (LGN) and primary visual cortex (V1) and to the extrastriate areas of the dorsal and ventral streams (V2, V3, VP, V3A, V4, and MT). We use high-field fMRI (4 T) to record simultaneously the responses of these areas across temporal frequency for chromatic stimuli (L/M-cone opponent and S-cone opponent) and stimuli of high and low achromatic contrasts. We find that: (1) the LGN has relatively low-pass responses for temporal frequency at both high and low achromatic contrasts, indicating that LGN cell spiking activity is not well reflected in the BOLD response. In addition, M cell-like temporal responses were not found, even at low contrasts. (2) Responses in V1 and extrastriate areas V2, V3, VP, and V3A display a progressively low-pass dependence on temporal frequency for achromatic stimuli (2-16 Hz) and are flat for chromatic stimuli (2-8 Hz), showing little overall difference between chromatic and achromatic cortical temporal filtering. (3) Strongly differential effects are found between dorsal and ventral stream processing by the level of MT and V4. V4 shows a significant low-pass temporal dependence for all achromatic and chromatic stimuli, whereas MT has temporally high-pass or flat responses for achromatic and chromatic stimuli. MT was the only visual area that showed M cell-like responses. We conclude that the dorsal and ventral pathways of human vision progressively develop characteristic differences in temporal processing that affect both chromatic and achromatic stimuli.