Adaptation and attentional selection.
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Adaptation, Physiological↗
Biomedical subjects
Geoffrey M Boynton
Publications and source records attributed to Geoffrey M Boynton.
Orientation-specific adaptation in human visual cortex.
Nearly all methods for analyzing and interpreting functional magnetic resonance imaging (fMRI) data assume that the fMRI signal behaves in an approximately linear manner. However, it has been shown that the mean fMRI response to a pair of briefly presented visual stimuli is significantly smaller than would be expected from the response to a single stimulus. This smaller response could be the result of either a nonlinearity in the fMRI signal or neuronal adaptation. We tested the neuronal adaptation hypothesis by measuring the fMRI response to sequential pairs of sinusoidal gratings that had either the same or orthogonal orientation. The adaptation hypothesis predicts that brain areas with orientation-selective neurons should show a more linear response when the stimulus pair is orthogonal than when the pair is identical. Our results show no orientation-specific adaptation effects in primary visual cortex (V1) but increasing effects along the hierarchy of visual areas (V2, V3, and V4V). A psychophysical contrast detection experiment, using similar oriented gratings as adapters, shows evidence of orientation-specific adaptation in the visual system. These results have implications for the interpretation of rapid event-related fMRI experiments, as well as for recently developed methods that use adaptation as a tool to measure the response properties of underlying neuronal subpopulations.
Adaptation, Physiological↗
Cortical magnification within human primary visual cortex correlates with acuity thresholds.
We measured linear cortical magnification factors in V1 with fMRI, and we measured visual acuity (Vernier and grating) in the same observers. The cortical representation of both Vernier and grating acuity thresholds in V1 was found to be roughly constant across all eccentricities. We also found a within-observer correlation between cortical magnification and Vernier acuity, further supporting claims that Vernier acuity is limited by cortical magnification in V1.
Adult↗
Global feature-based attention for motion and color.
We used a divided attention psychophysical task to test the hypothesis that visual attention to a stimulus feature(1) facilitates the processing of other stimuli sharing the same feature. Performance on a dual-task was significantly better when human observers divided attention across two spatially separate stimuli sharing a common feature (same direction of motion or same color) compared to opposing features. This attentional effect was dependent upon the presence of competing stimuli. These results are consistent with a spatially global feature-based mechanism of attention that increases the response of cortical neurons tuned to an attended feature throughout the visual field.
Adult↗
Long-term deprivation affects visual perception and cortex.
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Adult↗
Surface segmentation based on the luminance and color statistics of natural scenes.
The luminance and color of surfaces in natural scenes are relatively independent under certain linear transformations, with the luminance of a surface providing little information about the color of that surface, and vice versa. However, differences in luminance between two locations in a natural scene remain strongly associated with differences in color. We used the statistics of the spatiochromatic structure of natural scenes as the priors for a Bayesian model that decides whether or not two points within an image fall on the same surface. This model provides a biologically plausible algorithm for surface segmentation that models observer segmentations well.
Color↗
Color vision: how the cortex represents color.
Our understanding of how we see color has benefited from the long tradition of visual psychophysics. More recently, models and methods from psychophysics are guiding modern neuroimaging experiments on color vision. Combining the two techniques can lead to discoveries that neither can make alone.
Cerebral Cortex↗
Efficient design of event-related fMRI experiments using M-sequences.
Rapid event-related fMRI (erfMRI) allows estimation of the shape of hemodynamic responses (HDR) associated with transient brain activation evoked by various sensory, motor, and cognitive events. Choosing a sequence of events that maximizes efficiency of estimating the HDR is essential for conducting event-related brain imaging experiments, since increasing efficiency is essentially equivalent to reducing scanning time or increasing the strength of the principal magnetic field. The efficiency of an erfMRI design depends critically on the temporal arrangement of the sequence of events and the noise in the fMRI signal. We introduce to erfMRI a simple method for generating efficient event sequences based on maximum-length shift register sequences, or m-sequences. We show that under the assumption of white uncorrelated MRI noise, efficiency of erfMRI experimental designs that employ m-sequences exceeds efficiency of the best randomly generated sequences. This is true for single and multiple event type experiments, which allow either parallel events (overlapping events design) or designs in which only one event occurs at a time (nonoverlapping events design). HDR estimation efficiency afforded by m-sequences grows with the number of event types, and is greatest when event sequences are relatively short, albeit within commonly used scan times (i.e., 63-255 total events per scan). The improvement in efficiency, however, comes at a cost of constraints imposed by m-sequence generation rules, such as predetermined sequence lengths; for nonoverlapping events design m-sequence-based designs are not available for all possible numbers of event types. Nevertheless, designs that are available with m-sequences cover a large subset of commonly used erfMRI experimental designs. Under conditions of characteristic time-correlated fMRI noise, randomly generated sequences may yield efficiencies that exceed those afforded by m-sequences for single event-type designs, since in this case one can generate random sequences that partially decorrelate MRI noise by chance. Our simulations suggest that for designs of realistic sequence lengths that use more than one event type, m-sequence based designs tend to outperform random designs, thus making the knowledge of noise inessential. Finally, within an r-th order m-sequence (generated by a shift register of length r) all possible combinations of subsequences of length r occur, and thus these subsequences are exactly counterbalanced. This property is essential for minimizing effects of psychological and neuronal adaptation and expectation.
Brain↗
Global effects of feature-based attention in human visual cortex.
The content of visual experience depends on how selective attention is distributed in the visual field. We used functional magnetic resonance imaging (fMRI) in humans to test whether feature-based attention can globally influence visual cortical responses to stimuli outside the attended location. Attention to a stimulus feature (color or direction of motion) increased the response of cortical visual areas to a spatially distant, ignored stimulus that shared the same feature.
Attention↗