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Geraint Rees

Publications and source records attributed to Geraint Rees.

24 records · Page 2Linked to original sources
PubMed2005

Saccades differentially modulate human LGN and V1 responses in the presence and absence of visual stimulation.

Saccades occur several times each second in normal human vision. The visual image moves across the retina at high velocity during a saccade, yet no blurring of the visual scene is perceived . Active suppression of visual input may account for this perceptual continuity, but the neural mechanisms underlying such saccadic suppression remain unclear. We used functional MRI to specifically examine responses in the lateral geniculate nucleus (LGN) and primary visual cortex (V1) during saccades. Activity in both V1 and LGN was strongly modulated by saccades. Furthermore, this modulation depended on whether visual stimulation was present or absent. In complete darkness, saccades led to reliable signal increases in V1 and LGN, whereas in the presence of visual stimulation, saccades led to suppression of visually evoked responses. These findings represent unequivocal evidence for saccadic suppression in human LGN and retinotopically defined V1 and are consistent with the earliest site of saccadic suppression lying at or before V1.

Geniculate Bodies↗
PubMed2004

Responses of human visual cortex to uniform surfaces.

Surface perception is fundamental to human vision, yet most studies of visual cortex have focused on the processing of borders. We therefore investigated the responses of human visual cortex to parametric changes in the luminance of uniform surfaces by using functional MRI. Early visual areas V1 and V2/V3 showed strong and reliable increases in signal for both increments and decrements in surface luminance. Responses were significantly larger for decrements than for increments, which was fully accounted for by differences in retinal illumination arising from asymmetric pupil dynamics. Responses to both sustained and transient changes of illumination were transient. Signals in early visual cortex scaled linearly with the magnitude of change in retinal illumination, as did subjects' subjective ratings of the perceived brightness of the stimuli. Our findings show that early visual cortex responds strongly to surfaces and that perception of surface brightness is compatible with brain responses at the earliest cortical stages of processing.

Humans↗
PubMed2004

Neural correlates of attentional capture in visual search.

Much behavioral research has shown that the presence of a unique singleton distractor during a task of visual search will typically capture attention and thus disrupt target search. Here we examined the neural correlates of such attentional capture using functional magnetic resonance imaging in human subjects during performance of a visual search task. The presence (vs. absence) of a salient yet irrelevant color singleton distractor was associated with activity in the superior parietal cortex and frontal cortex. These findings imply that the singleton distractor induced spatial shifts of attention despite its irrelevance, as predicted from an AC account. Moreover, behavioral interference by singleton distractors was strongly and negatively correlated with frontal activity. These findings provide direct evidence that the frontal cortex is involved in control of interference from irrelevant but attention-capturing distractors.

Adult↗
PubMed2002

Neural correlates of conscious and unconscious vision in parietal extinction.

Brain areas activated by stimuli in the left visual field of a right parietal patient suffering from left visual extinction were identified using event-related functional magnetic resonance imaging. Left visual field stimuli that were extinguished from awareness still activated the ventral visual cortex, including areas in the damaged right hemisphere. An extinguished face stimulus on the left produced robust category-specific activation of the right fusiform face area. On trials where the left visual stimulus was consciously seen rather than extinguished, greater activity was found in the ventral visual cortex of the damaged hemisphere, and also in frontal and parietal areas of the intact hemisphere. These findings extend recent observations on visual extinction, suggesting distinct neural correlates for conscious and unconscious perception.

Aged↗