The LGN is the thalamic station in the projection of the visual pathway from retina to visual cortex and has been traditionally viewed as a gateway for sensory information. Its topographic organization and neuronal response properties have been extensively studied in nonhuman primates, but are poorly understood in humans. I will review a series of studies aimed at elucidating functional roles of the human LGN in perception and cognition using fMRI. Together, these studies indicate the need to revise the traditional view and to consider the LGN as an early gatekeeper in controlling attention and conscious perception.
Functional LGN topography [1] was studied by presenting periodic flickering checkerboard stimuli that evoked a traveling wave of activity. We found that the contralateral visual hemifield was represented with the lower field in the medial-superior portion and the upper field in the lateral inferior portion of each LGN. The fovea was represented in posterior and superior portions, with increasing eccentricities represented more anteriorly. This topography is strikingly similar to that of the macaque.
Selective attention has been shown to modulate neural activity in both extrastriate and striate cortex. We studied the poorly understood role of earlier, subcortical structures in attentional processing and found that attention modulated neural responses in the human LGN in several ways: it enhanced neural responses to attended stimuli, attenuated responses to ignored stimuli and increased baseline activity in the absence of visual stimulation, suggesting a role as a gatekeeper in controlling attentional response gain [2].
Finally, we studied the role of the human LGN in conscious perception by investigating the level at which competing inputs to the eyes, as perceived in binocular rivalry, can be resolved [3]. Neural activity in the LGN correlated strongly with the subjects' reported percepts, suggesting a mechanism by which LGN layers that process the input from one particular eye are selectively enhanced or suppressed. These finding suggest a role for the LGN as an early gatekeeper of visual awareness.
Schneider, K. A., Richter, M., Kastner, S. (2004). Retinotopic organization and functional subdivisions of the human lateral geniculate nucleus: A high-resolution fMRI study. J. Neurosci., 24, 8975–8985.
O'Connor, D. H., Fukui, M. M., Pinsk, M. A., Kastner, S. (2002). Attention modulates responses in the human lateral geniculate nucleus. Nature Neurosci, 5, 1203–1209.
Wunderlich, K., Schneider, K. A., Kastner, S. (2005): Neural correlates of binocular rivalry in the human LGN. Nature Neurosci, 8, 1595–1602.