Francesco Di Russo

Cortical sources of the early components of the visual evoked potential

This study aimed to characterize the neural generators of the early components of the visual evoked potential (VEP) to isoluminant checkerboard stimuli. Multichannel scalp recordings, retinotopic mapping and dipole modeling techniques were used to estimate the locations of the cortical sources giving rise to the early C1, P1, and N1 components. Dipole locations were matched to anatomical brain regions visualized in structural magnetic resonance imaging (MRI) and to functional MRI (fMRI) activations elicited by the same stimuli. These converging methods confirmed previous reports that the C1 component (onset latency 55 msec; peak latency 90–92 msec) was generated in the primary visual area (striate cortex; area 17). The early phase of the P1 component (onset latency 72–80 msec; peak latency 98–110 msec) was localized to sources in dorsal extrastriate cortex of the middle occipital gyrus, while the late phase of the P1 component (onset latency 110–120 msec; peak latency 136–146 msec) was localized to ventral extrastriate cortex of the fusiform gyrus. Among the N1 subcomponents, the posterior N150 could be accounted for by the same dipolar source as the early P1, while the anterior N155 was localized to a deep source in the parietal lobe. These findings clarify the anatomical origin of these VEP components, which have been studied extensively in relation to visual‐perceptual processes. Hum. Brain Mapping 15:95–111, 2001.

Identification of the neural sources of the pattern-reversal VEP

This study aimed to characterize the neural generators of the early components of the visual-evoked potential (VEP) to pattern-reversal gratings. Multichannel scalp recordings of VEPs and dipole modeling techniques were combined with functional magnetic resonance imaging (fMRI) and retinotopic mapping in order to estimate the locations of the cortical sources giving rise to VEP components in the first 200 ms poststimulus. Dipole locations were seeded to visual cortical areas in which fMRI activations were elicited by the same stimuli. The results provide strong evidence that the first major component of the VEP elicited by a pattern-reversal stimulus (N75/P85) arises from surface-negative activity in the primary visual cortex (area V1). Subsequent waveform components could be accounted for by dipoles that were in close proximity to fMRI activations in the following cortical areas: P95 (area MT/V5), P125/N135 (area V1), N150 (transverse parietal sulcus, TPS), N160 (ventral occipital areas VP, V4v, and V4/V8), and N180 (dorsal occipital areas V3A/V7). These results provide a detailed spatiotemporal profile of the cortical origins of the pattern-reversal VEP, which should enhance its utility in both clinical and basic studies of visual-perceptual processing.

Spatiotemporal analysis of the cortical sources of the steady-state visual evoked potential

This study aimed to characterize the neural generators of the steady‐state visual evoked potential (SSVEP) to repetitive, 6 Hz pattern‐reversal stimulation. Multichannel scalp recordings of SSVEPs and dipole modeling techniques were combined with functional magnetic resonance imaging (fMRI) and retinotopic mapping in order to estimate the locations of the cortical sources giving rise to the SSVEP elicited by pattern reversal. The time‐varying SSVEP scalp topography indicated contributions from two major cortical sources, which were localized in the medial occipital and mid‐temporal regions of the contralateral hemisphere. Colocalization of dipole locations with fMRI activation sites indicated that these two major sources of the SSVEP were located in primary visual cortex (V1) and in the motion sensitive (MT/V5) areas, respectively. Minor contributions from mid‐occipital (V3A) and ventral occipital (V4/V8) areas were also considered. Comparison of SSVEP phase information with timing information collected in a previous transient VEP study (Di Russo et al. [ 2005 ] Neuroimage 24:874–886) suggested that the sequence of cortical activation is similar for steady‐state and transient stimulation. These results provide a detailed spatiotemporal profile of the cortical origins of the SSVEP, which should enhance its use as an efficient clinical tool for evaluating visual‐cortical dysfunction as well as an investigative probe of the cortical mechanisms of visual‐perceptual processing. Hum. Brain Mapp, 2007.

Neural Substrates of Perceptual Enhancement by Cross-Modal Spatial Attention

Orienting attention involuntarily to the location of a sudden sound improves perception of subsequent visual stimuli that appear nearby. The neural substrates of this cross-modal attention effect were investigated by recording event-related potentials to the visual stimuli using a dense electrode array and localizing their brain sources through inverse dipole modeling. A spatially nonpredictive auditory precue modulated visual-evoked neural activity first in the superior temporal cortex at 120140 msec and then in the ventral occipital cortex of the fusiform gyrus 1525 msec later. This spatio-temporal sequence of brain activity suggests that enhanced visual perception produced by the cross-modal orienting of spatial attention results from neural feedback from the multimodal superior temporal cortex to the visual cortex of the ventral processing stream.

Neural basis of auditory-induced shifts in visual time-order perception

Attended objects are perceived to occur before unattended objects even when the two objects are presented simultaneously. This finding has led to the widespread view that attention modulates the speed of neural transmission in the various perceptual pathways. We recorded event-related potentials during a time-order judgment task to determine whether a reflexive shift of attention to a sudden sound modulates the speed of sensory processing in the human visual system. Attentional cueing influenced the perceived order of lateralized visual events but not the timing of event-related potentials in visual cortex. Attentional cueing did, however, enhance the amplitude of neural activity in visual cortex, which shows that attention-induced shifts in visual time-order perception can arise from modulations of signal strength rather than processing speed in the early visual-cortical pathways.

Automatic gain control contrast mechanisms are modulated by attention in humans: evidence from visual evoked potentials

This study investigated the effect of attention on the contrast response curves of steady-state visual evoked potentials (VEPs) to counter-phased sinusoidal gratings. The 1 cyc/deg gratings were modulated either in luminance or chromaticity (equiluminant red-green). The luminance grating counter-phased at 9 Hz (to favour activation of the magno-cellular system), and the chromatic grating at 2.5 Hz (to favour activation of the parvo-cellular system). Attention was directed towards the gratings (displayed in the left visual field) by requiring subjects to detect and respond to randomly occurring changes in contrast. In the control condition, attention towards the grating was minimised by requiring subjects to detect a target letter amongst distracters briefly flashed in the contra-lateral visual field. Attention increased VEP amplitudes for both luminance and chromatic stimuli, more so at high than at low contrasts, increasing the slope of the contrast amplitude curves (over the non-saturating range of contrasts). The estimates of contrast threshold from extrapolation of amplitudes were unaffected by attention. Attention also changed the VEP phases, but only for luminance gratings, where it acted to reduce the magnitude of phase advance with contrast. Attention had no effect on the average phases for chromatic gratings. The results are consistent with the notion that attention acts on cortical gain control mechanisms, which are known to be different for the magno- and parvo-cellular systems.

Fixation stability and saccadic latency in élite shooters

This study tested the hypothesis that elementary visuo-motor functions involved in visual scanning, as measured by fixation and saccadic tasks, are better in a group of high-level clay target shooters (N=7) than in a control group (N=8). In the fixation task, subject were told to keep fixation as still as possible on a target for 1 min, both in the presence and absence of distracters. For shooters, time did not have an effect on fixation stability, and they had more stable fixation than controls in the distracters condition. Results indicate a difference between groups on both the temporal span of attention and selective attention. In the saccadic task, subjects were asked to saccade, as fast as possible, towards a peripherally displayed target. Two conditions were used: simple reaction to target onset and discrimination between targets and distracters. Shooters had faster saccadic latency to targets than controls in both conditions. Finally, to evaluate the effect of exercise on saccadic latency, we trained one control subject to saccade to a target displayed at a constant spatial position. At the end of the training, saccadic latency reached a value comparable to that recorded in shooters. Learning was largely retinotopic, not showing transfer to untrained spatial positions.

Benefits of Physical Exercise on the Aging Brain: The Role of the Prefrontal Cortex

Motor planning in older adults likely relies on the overengagement of the prefrontal cortex (PFC) and is associated with slowness of movement and responses. Does a physically active lifestyle counteract the overrecruitment of the PFC during action preparation? This study used high-resolution electroencephalography to measure the effect of physical exercise on the executive functions of the PFC preceding a visuomotor discriminative task. A total of 130 participants aged 15–86 were divided into two groups based on physical exercise participation. The response times and accuracy and the premotor activity of the PFC were separately correlated with age for the two groups. The data were first fit with a linear function and then a higher order polynomial function. We observed that after 35–40 years of age, physically active individuals have faster response times than their less active peers and showed no signs of PFC hyperactivity during motor planning. The present findings show that physical exercise could speed up the response of older people and reveal that also in middle-aged people, moderate-to-high levels of physical exercise benefits the planning/execution of a response and the executive functions mediated by the PFC, counteracting the neural overactivity often observed in the elderly adults.

Influence of the radial and vertical dimensions on lateral neglect

Abstract. The influence of radial (near-far) and vertical (upper-lower) dimensions on lateral visuo-spatial neglect was studied using two horizontal line-bisection tasks (one motor and one perceptual). A group of 15 patients with neglect and a group of 14 right-brain damaged patients without neglect were examined. This latter group was used to define the range of variability in line-bisection performance that was independent of neglect. For the radial dimension, some neglect patients showed greater errors in far space than in near space (for both stimuli presented in the upper and lower space). Fewer patients showed the opposite pattern (i.e., greater errors for near-space stimuli). These near-far asymmetries were present for both the motor and perceptual conditions and showed a good degree of intra-individual consistency. This finding contradicts the hypothesis that the motor component is critical for yielding such asymmetry. For the vertical dimension, the results indicated that neglect patients make more bisection errors for lower-space stimuli than for upper-space stimuli. This vertical asymmetry was nearly always confined to stimuli in near space. Asymmetries along the vertical dimension were present for both perceptual and motor conditions, although intra-individual consistency was low. When perceptual and motor conditions were directly compared, several neglect patients showed greater errors in the perceptual than in the motor task.

The neurophysiology of central and peripheral fatigue during sub-maximal lower limb isometric contractions

Fatigue has been defined as an exercise-induced decline in force generation capacity because of changes at both the peripheral and central levels. Movement is preceded and accompanied by brain activities related to the preparation and execution of movement (movement related cortical potentials, MRCP), which have been correlated with the perception of effort (RPE). We combined force measurements, surface electromyography (sEMG), peripheral electrical stimulation (maximal twitch, MT) and MRCP analysis to further our understanding of the neural correlates of peripheral and central changes during a fatiguing task involving the lower limbs. Eighteen healthy volunteers performed 4 blocks of isometric knee extensions at 40% of the maximal voluntary contraction (MVC) for a total of 240 2-s contractions. At the baseline and after each block, we measured RPE, MT and MVC. We simultaneously recorded the force of the knee extensor muscles, root mean square (RMS) of the sEMG of the vastus lateralis muscle, and electroencephalography (EEG) from 64 channels. The MRCPs were extracted from the EEG recordings and averaged in the early (Block 1–2) and late (Block 3–4) blocks. Two cohorts were obtained by cluster analysis based on the RPE (i.e., perception of effort) and MT (i.e., peripheral fatigue). We observed a significant decline in both the MVC (−13%) and RMS (−25%) of the sEMG signal over the course of the task; thus, muscle fatigue had occurred in all of the participants regardless of the cohort. The MRCP amplitude was larger in the fatigued than the non-fatigued MT cohort in the supplementary and premotor areas, whereas the MRCP amplitude was larger in the fatigued than the non-fatigued RPE cohort in the aforementioned areas, and also in the primary motor and prefrontal cortices (PFC). The increase in the positive activity of the PFC, along with the perception of effort, represents a novel result, suggesting that it is modulated more by the perception of effort than peripheral fatigue.