Giedrius T. Buracas

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.

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.

The Effect of Spatial Attention on Contrast Response Functions in Human Visual Cortex

Previous electrophysiology data suggests that the modulation of neuronal firing by spatial attention depends on stimulus contrast, which has been described using either a multiplicative gain or a contrast-gain model. Here we measured the effect of spatial attention on contrast responses in humans using functional MRI. To our surprise, we found that the modulation of blood oxygenation level-dependent (BOLD) responses by spatial attention does not greatly depend on stimulus contrast in visual cortical areas tested [V1, V2, V3, and MT+ (middle temporal area)]. An additive model, rather than a multiplicative or contrast-gain model best describes the attentional modulations in V1. This inconsistency with previous single-unit electrophysiological data has implications for the population-based neuronal source of the BOLD signal.

Caffeine alters the temporal dynamics of the visual BOLD response.

The blood oxygenation level-dependent (BOLD) responses to visual stimuli, using both a 1-s long single trial stimulus and a 20-s long block stimulus, were measured in a 4-T magnetic field both before and immediately after a 200-mg caffeine dose. In addition, resting levels of cerebral blood flow (CBF) were measured using arterial spin labeling. For the single trial stimulus, the caffeine dose significantly (p<0.05) reduced the time to peak (TTP), the time after the peak at which the response returned to 50% of the peak amplitude (TA50), and the amplitude of the poststimulus undershoot in all subjects (N=5). Other parameters, such as the full-width half-maximum (FWHM) and the peak amplitude, also showed significant changes in the majority of subjects. For the block stimulus, the TTP, TA50, and the time for the response to reach 50% of the peak amplitude (T50) were significantly reduced. In some subjects, oscillations were observed in the poststimulus portion of the response with median peak periods of 9.1 and 9.5 s for the single trial and block responses, respectively. Resting CBF was reduced by an average of 24%. The reproducibility of the results was verified in one subject who was scanned on 3 different days. The dynamic changes are similar to those previously reported for baseline CBF reductions induced by hypocapnia and hyperoxia.

Contribution of area MT to perception of three-dimensional shape: a computational study.

Successful recognition and manipulation of objects in ones visual environment is critically dependent upon the ability to recover three-dimensional (3D) surface geometry from two-dimensional (2D) retinal images. The relative motion of image features, caused by relative displacement of object and observer, has characteristic properties that betray components of the 3D source geometry (distance, tilt, slant and curvature) and is among the most valuable sources of information used for 3D surface recovery by the primate visual system. We have considered the behavior of motion-sensitive neurons in primate visual cortex and found that their properties closely resemble those of differential motion operators that can be used to formally characterize the 3D shape of a smooth moving surface. Our analysis has led us to identify a set of three orders of filters for differential motion detection. These filters behave in a manner that is strikingly similar to the spatial and velocity tuning profiles of a sub-population of neurons--those possessing antagonistic motion surrounds--in the middle temporal visual area (MT). On the basis of this analysis, we suggest that MT neurons subserve 3D surface recovery from relative motion cues.

Gauging sensory representations in the brain

The stream of information that enters a sensory system is a product of the ecological niche of an organism and the way in which the information is sampled. The most salient characteristic of this sensory stream is the rich temporal structure that is caused by changes in the environment and self motion of sensors (for example, rapid eye or whisker movements). In recent years, substantial progress has been made in understanding how such rapidly varying stimuli are represented in the responses of sensory neurons of a large variety of sensory systems. The crucial observation that has emerged from these studies is that individual action potentials convey substantial amounts of information, which permits the discrimination of rapidly varying stimuli with high temporal precision.

Attention strongly increases oxygen metabolic response to stimulus in primary visual cortex.

Top-down attention enhances neural processing, but its effect on metabolic activity in primary visual cortex (V1) is unclear. Combined blood flow and oxygenation measurements provide the best tool for investigating modulations of oxidative metabolism. We measured the human V1 response to a peripheral low contrast stimulus using fMRI and found a larger fractional modulation of blood flow with attention compared to the blood oxygenation level dependent (BOLD) response, thus indicating a much larger modulation of oxygen metabolism than was previously thought. These findings point to different aspects of neural activity driving flow and metabolic changes to different degrees. We propose that V1 flow is driven strongly but not exclusively by the initial sensory-driven neural activity, which dominates the response in the unattended condition, while V1 oxygen metabolism is driven strongly by the overall neural activity, which is modulated by top-down signals related to attention.

On-line psychophysical data acquisition and event-related fMRI protocol optimized for the investigation of brain activation in response to gustatory stimuli

An experimental method for event-related functional magnetic resonance imaging that allows for the presentation of several chemosensory stimuli in the oral cavity during the same run, the collection of psychophysical measures (intensity or pleasantness) during the presentation of the stimuli, and the analysis of the data in an event-related fashion are described. The automatic pumps used to present taste stimuli allowed for multiple tastes to be delivered in small amounts under computer control. Psychophysical ratings of pleasantness or intensity were collected after each presentation of a taste stimulus and water, with the general labeled magnitude scale, using a joystick that controlled the movement of an arrow on the visual display. Performing these cognitive tasks required that the participant remained focused, and aided in the interpretation of the data collected. The perceived pleasantness differed across stimuli for all conditions; however, pleasantness ratings for the same stimulus displayed consistency, over the duration of the run and before each scan on separate days. Activation in response to sucrose and caffeine while the participant rated pleasantness was found in the insula, frontal operculum, rolandic operculum and orbitofrontal cortex which is consistent with previous taste fMRI studies.

Modulation of neuronal responses during covert search for visual feature conjunctions

While searching for an object in a visual scene, an observers attentional focus and eye movements are often guided by information about object features and spatial locations. Both spatial and feature-specific attention are known to modulate neuronal responses in visual cortex, but little is known of the dynamics and interplay of these mechanisms as visual search progresses. To address this issue, we recorded from directionally selective cells in visual area MT of monkeys trained to covertly search for targets defined by a unique conjunction of color and motion features and to signal target detection with an eye movement to the putative target. Two patterns of response modulation were observed. One pattern consisted of enhanced responses to targets presented in the receptive field (RF). These modulations occurred at the end-stage of search and were more potent during correct target identification than during erroneous saccades to a distractor in RF, thus suggesting that this modulation is not a mere presaccadic enhancement. A second pattern of modulation was observed when RF stimuli were nontargets that shared a feature with the target. The latter effect was observed during early stages of search and is consistent with a global feature-specific mechanism. This effect often terminated before target identification, thus suggesting that it interacts with spatial attention. This modulation was exhibited not only for motion but also for color cue, although MT neurons are known to be insensitive to color. Such cue-invariant attentional effects may contribute to a feature binding mechanism acting across visual dimensions.

The Neural Substrates of Spoken Idiom Comprehension.

To examine the neural correlates of spoken idiom comprehension, we conducted an event-related functional MRI study with a ‘rapid sentence decision’ task. The spoken sentences were equally familiar but varied in degrees of ‘idiom figurativeness’. Our results show that ‘figurativeness’ co-varied with neural activity in the left ventral dorsolateral prefrontal cortex, which involved two different clusters: one cluster engaged Brocas area (Brodmann area, BA: 44 & 45) and adjacent regions (BA 11 & 47); the other cluster involved the superior and medial frontal gryrus (BA 8 & 9). Moreover, the comparison between unambiguous, explicit idiomatic and literal sentences revealed left-sided cluster activity in Brocas area and BA 46 & 47. The contrast between ambiguous idiomatic and literal sentences engaged the superior and medial of the frontal midline (BA 9 & 10), but with left-sided prevalence. Finally, we report a very consistent deactivation in the superior parietal cortex by explicit idiomatic sentences as compared to literal sentences. Our findings are discussed in context of a neurological model of spoken language processing.