Biomedical subjects

David Ress

Publications and source records attributed to David Ress.

7 recordsLinked to original sources

PubMed2006

Laminar profiles of functional activity in the human brain.

Functional magnetic resonance imaging (fMRI) data were obtained in human visual cortex using sub-millimeter voxels at a field strength of 3 T. Reliable functional signals were largely confined to the gray matter and these responses measure the retinotopic organization of visual cortex. Functional signals were further characterized with respect to their laminar position within the cortical gray matter. The laminar response profiles during our visuospatial attention task, normalized for cortical thickness, had a stereotypical shape, with a peak in the superficial gray matter and declining in the deeper layers. The thickness of the sheet producing functional signals was in excellent agreement with the estimated structural thickness of the gray matter throughout early visual cortex (error < 0.5 mm). Thickness measurements were highly repeatable from session-to-session (error < 0.4 mm). Hence, it is feasible and useful to use high-resolution fMRI to measure laminar activity profiles. The ability to distinguish signals arising in different lamina has significant potential scientific and clinical applications.

Brain↗

PubMed2006

Neural correlates of sustained spatial attention in human early visual cortex.

Attention is thought to enhance perceptual performance at attended locations through top-down attention signals that modulate activity in visual cortex. Here, we show that activity in early visual cortex is sustained during maintenance of attention in the absence of visual stimulation. We used functional magnetic resonance imaging (fMRI) to measure activity in visual cortex while human subjects performed a visual detection task in which a variable-duration delay period preceded target presentation. Portions of cortical areas V1, V2, and V3 representing the attended part of the visual field exhibited sustained increases in activity throughout the delay period. Portions of these cortical areas representing peripheral, unattended parts of the visual field displayed sustained decreases in activity. The data were well fit by a model that assumed the sustained neural activity was constant in amplitude over a time period equal to that of the actual delay period for each trial. These results demonstrate that sustained attention responses are present in early visual cortex (including primary visual cortex), in the absence of a visual stimulus, and that these responses correlate with the allocation of visuospatial attention in both the spatial and temporal domains.

Attention↗

PubMed2006

High-resolution imaging reveals highly selective nonface clusters in the fusiform face area.

A region in ventral human cortex (fusiform face area, FFA) thought to be important for face perception responds strongly to faces and less strongly to nonface objects. This pattern of response may reflect a uniform face-selective neural population or activity averaged across populations with heterogeneous selectivity. Using high-resolution functional magnetic resonance imaging (MRI), we found that the FFA has a reliable heterogeneous structure: localized subregions within the FFA highly selective to faces are spatially interdigitated with localized subregions highly selective to different object categories. We found a preponderance of face-selective responses in the FFA, but no difference in selectivity to faces compared to nonfaces. Thus, standard fMRI of the FFA reflects averaging of heterogeneous highly selective neural populations of differing sizes, rather than higher selectivity to faces. These results suggest that visual processing in this region is not exclusive to faces. Overall, our approach provides a framework for understanding the fine-scale structure of neural representations in the human brain.

Adult↗

PubMed2005

Topographic maps of visual spatial attention in human parietal cortex.

Functional magnetic resonance imaging (fMRI) was used to measure activity in human parietal cortex during performance of a visual detection task in which the focus of attention systematically traversed the visual field. Critically, the stimuli were identical on all trials (except for slight contrast changes in a fully randomized selection of the target locations) whereas only the cued location varied. Traveling waves of activity were observed in posterior parietal cortex consistent with shifts in covert attention in the absence of eye movements. The temporal phase of the fMRI signal in each voxel indicated the corresponding visual field location. Visualization of the distribution of temporal phases on a flattened representation of parietal cortex revealed at least two distinct topographically organized cortical areas within the intraparietal sulcus (IPS), each representing the contralateral visual field. Two cortical areas were proposed based on this topographic organization, which we refer to as IPS1 and IPS2 to indicate their locations within the IPS. This nomenclature is neutral with respect to possible homologies with well-established cortical areas in the monkey brain. The two proposed cortical areas exhibited relatively little response to passive visual stimulation in comparison with early visual areas. These results provide evidence for multiple topographic maps in human parietal cortex.

Attention↗

PubMed2003

Functional brain imaging using a blood oxygenation sensitive steady state.

Blood oxygenation level dependent (BOLD) functional MRI (fMRI) is an important method for functional neuroimaging that is sensitive to changes in blood oxygenation related to brain activation. While BOLD imaging has good spatial coverage and resolution relative to other neuroimaging methods (such as positron emission tomography (PET)), it has significant limitations relative to other MRI techniques, including poor spatial resolution, low signal levels, limited contrast, and image artifacts. These limitations derive from the coupling of BOLD functional contrast to sources of image degradation. This work presents an alternative method for fMRI that may over-come these limitations by establishing a blood oxygenation sensitive steady-state (BOSS) that inverts the signal from deoxygenated blood relative to the water signal. BOSS fMRI allows the imaging parameters to be optimized independently of the functional contrast, resulting in fewer image artifacts and higher signal-to-noise ratio (SNR). In addition, BOSS fMRI has greater functional contrast than BOLD. BOSS fMRI requires careful shimming and multiple acquisitions to obtain a precise alignment of the magnetization to the SSFP frequency response.

Artifacts↗

PubMed2003

Neuronal correlates of perception in early visual cortex.

We used functional magnetic resonance imaging (fMRI) to measure activity in human early visual cortex (areas V1, V2 and V3) during a challenging contrast-detection task. Subjects attempted to detect the presence of slight contrast increments added to two kinds of background patterns. Behavioral responses were recorded so that the corresponding cortical activity could be grouped into the usual signal detection categories: hits, false alarms, misses and correct rejects. For both kinds of background patterns, the measured cortical activity was retinotopically specific. Hits and false alarms were associated with significantly more cortical activity than were correct rejects and misses. That false alarms evoked more activity than misses indicates that activity in early visual cortex corresponded to the subjects' percepts, rather than to the physically presented stimulus.

Action Potentials↗

PubMed2002

What does fMRI tell us about neuronal activity?

In recent years, cognitive neuroscientists have taken great advantage of functional magnetic resonance imaging (fMRI) as a non-invasive method of measuring neuronal activity in the human brain. But what exactly does fMRI tell us? We know that its signals arise from changes in local haemodynamics that, in turn, result from alterations in neuronal activity, but exactly how neuronal activity, haemodynamics and fMRI signals are related is unclear. It has been assumed that the fMRI signal is proportional to the local average neuronal activity, but many factors can influence the relationship between the two. A clearer understanding of how neuronal activity influences the fMRI signal is needed if we are correctly to interpret functional imaging data.

Action Potentials↗