Biomedical subjects

Adrian L Williams

Publications and source records attributed to Adrian L Williams.

5 recordsLinked to original sources

PubMed2005

Attentional modulation in the human visual cortex: the time-course of the BOLD response and its implications.

Throughout the visual areas of the brain, the sensory response to a stimulus is enhanced by attending to the stimulus. Neurophysiological studies in primates show that such enhancement is marked in posterior parietal cortex and some anterior occipital areas, but much more modest in the earliest processing stages, such as the primary visual cortex (V1). In contrast, human fMRI studies show large and robust attentional modulation in all visual areas, including V1. We investigate the possibility that, in the case of fMRI, the BOLD (blood oxygen level dependent) response may be increased not only by local attention-related increases in neural activity, but also by local blood-flow increases caused by remote control systems that anticipate an impending need for oxygen at the attended location. Such changes could be much more rapid than the rather slow response to oxygenation change that typifies the BOLD response. We have employed a paradigm that isolates the component of the BOLD response due to attentional modulation and the component due to the mere presence of a visual stimulus. The results show that the temporal profiles of the BOLD responses in human V1 to the onset of a stimulus and to the onset of attention are extremely similar. The time-course of the attention-related BOLD response is not consistent with the action of remote, anticipatory control mechanisms and suggests that the modulatory effect of attention seen in human V1 with fMRI probably reflects genuine changes in local neural activity that are considerably larger than in non-human primates.

Adult↗

PubMed2004

Negative BOLD in the visual cortex: evidence against blood stealing.

The positive BOLD (blood oxygen level-dependent) response elicited in human visual cortex by a localized visual stimulus is accompanied by a reduction in the BOLD response in regions of the visual cortex that represent unstimulated locations in the visual field. We have suggested previously that this negative BOLD reflects attention-related suppression of neural activity, but it might also be explained in terms of "blood stealing," i.e., hemodynamic changes that have no neural correlate. We distinguish two possible hemodynamic effects of this type: (1). blood flow reduction caused by locally reduced pressure in vessels that share their blood supply with nearby dilated vessels; and (2). blood flow reduction caused by active constriction of vessels under neural control. The first is ruled out as an explanation of negative BOLD by showing that a visual stimulus that stimulates primary visual cortex in one hemisphere can cause extensive suppression in the other hemisphere i.e., it is not a local phenomenon. Negative BOLD most likely reflects suppression of neural activity, but could also reflect an active blood flow control system.

Attention↗

PubMed2003

Surround modulation measured with functional MRI in the human visual cortex.

Visual context profoundly influences 1) the responses of mammalian visual neurons and 2) the perceptual sensitivity of human observers to localized visual stimuli. We present data from functional MRI studies demonstrating contextual modulation in the human visual cortex. Subjects viewed a circular grating patch that was continuously present. A surround grating was added in an ON-OFF block design to reveal its effect on the central region. Stimulus-correlated activation was quantified and visualized on a flattened map of the occipital gray matter. Modulation was measured in a region of interest activated by the central grating alone. The observed effects were predominantly suppressive, consistent with the effects typically found in single neurons and perception. Suppression was greatest when the surround and center had the same orientation and was reduced or absent when it was orthogonal. When spatial phase was manipulated, suppression was greatest for in-phase center/surround gratings and much reduced or reversed (facilitation) for opposite-phase stimuli. With eccentric stimulus presentation, suppression was reduced and facilitation became more common. The findings provide a direct demonstration of the existence of powerful and stimulus-specific surround effects in human visual cortex.

Adult↗

PubMed2002

Role of retinal afferents in regulating growth and shape of the lateral geniculate nucleus.

Segregated binocular maps in the dorsal lateral geniculate nucleus (LGN) develop from a stage where they initially overlap. Sophisticated computational models have been used to describe the dynamics of three-dimensional LGN shape changes that play a role in segregation. These models have revealed specific nuclear growth vectors associated with the process of ocular segregation in the LGN (Williams and Jeffery [2001] J Comp Neurol 430:332-342.). In this study, we use similar techniques to determine whether retinal innervation contributes to the dynamics of shape maturation in the ferret LGN. In this animal, 90% of the retinal innervation of the mature LGN comes from the contralateral eye. If one eye is removed before segregation, the projection from the remaining eye remains diffuse and nuclear growth is stunted. Here, we quantify this effect and show that removing the contralateral projection before segregation has a profound impact on LGN size but changes its ultimate shape by only 12%. The impact on shape on the other side of the brain where the ipsilateral projection is removed, which accounts for only 10% of its innervation in maturity, is less than 2%. Hence, retinal innervation plays a minor role in determining mature LGN shape. Although in both hemispheres, the ultimate shape of the nucleus is close to normal, removal of the larger projection disrupts normal growth vectors, with almost none being present in the 5 days after enucleation, when the normal nucleus expands markedly. Hence, the effect of enucleation is to delay shape maturation. Growth vectors absent after removal of the smaller projection are mainly confined to those in what would be the binocular region.

Afferent Pathways↗

PubMed2002

Task-related changes in cortical synchronization are spatially coincident with the hemodynamic response.

Using group functional Magnetic Resonance Imaging (fMRI) and group Magnetoencephalography (MEG) we studied two cognitive paradigms: A language task involving covert letter fluency and a visual task involving biological motion direction discrimination. The MEG data were analyzed using an adaptive beam-former technique known as Synthetic Aperture Magnetometry (SAM), which provides continuous 3-D images of cortical power changes. These images were spatially normalized and averaged across subjects to provide a group SAM image in the same template space as the group fMRI data. The results show that frequency-specific, task-related changes in cortical synchronization, detected using MEG, match those areas of the brain showing an evoked cortical hemodynamic response with fMRI. The majority of these changes were event-related desynchronizations (ERDs) in the 5-10 Hz and 15-25 Hz frequency ranges. Our study demonstrates how SAM, spatial normalization, and intersubject averaging enable group MEG studies to be performed. SAM analysis also allows the MEG experiment to have exactly the same task design as the corresponding fMRI experiment. This new analysis framework represents an important advance in the use of MEG as a cognitive neuroimaging technique and also allows mutual cross-validation with fMRI.

Adult↗