Biomedical subjects

Franco Lepore

Publications and source records attributed to Franco Lepore.

At least 19 recordsLinked to original sources

PubMed2007

Developmental delay and magnocellular visual pathway function in very-low-birthweight preterm infants.

This study investigated the effect of very preterm birth (gestation < or =30wks) and very low birth weight (< or =1500g) on the development of magnocellular and parvocellular visual processing streams. Participants were preterm infants (n=55: 31 females, 24 males) born between 24 and 30 weeks'gestation (mean 27.4wks [SD 1.3]), weighing between 720 and 1470g (mean 1015g [SD 215]) and term infants (n=52: 27 females, 25 males) born between 38 and 42 weeks'gestation (mean 39.4wks [SD 0.9]), weighing between 2670 and 4405g (mean 3549g [SD 440]). Visual-evoked potentials to phase-reversing sine-wave gratings, varying in spatial frequency and contrast, were used to elicit magnocellular and parvocellular specific responses. Previous studies found that the N1 component reflects the parvocellular response, while P1 reflects the magnocellular response in adults and infants. Findings from the current study indicate significantly lower P1 amplitudes in preterm compared with term infants under most conditions. No difference was found for the amplitude of the N1 waveform. Results indicate that, for the age-range tested, preterm birth has little effect on the development of parvocellular function, while it appears to disrupt the development of magnocelluar function.

Contrast Sensitivity↗

PubMed2006

A role for the inferior colliculus in multisensory speech integration.

Multisensory integration can occur at relatively low levels within the central nervous system. Recent evidence suggests that multisensory audio-visual integration for speech may have a subcortical component, as acoustic processing in the human brainstem is influenced by lipreading during speech perception. Here, stimuli depicting the McGurk illusion (a demonstration of auditory-visual integration using speech stimuli) were presented to a 12-year-old child (FX) with a circumscribed unilateral lesion of the right inferior colliculus. When McGurk-type stimuli were presented in the contralesional hemifield, illusory perception reflecting bimodal integration was significantly reduced compared with the ipsilesional hemifield and a group of age-matched controls. These data suggest a functional role for the inferior colliculus in the audio-visual integration of speech stimuli.

Acoustic Stimulation↗

PubMed2006

Functional cerebral reorganization for auditory spatial processing and auditory substitution of vision in early blind subjects.

Early blind (EB) individuals can recognize bidimensional shapes using a prosthesis substituting vision with audition (PSVA) and activate right dorsal extrastriate visual cortex during the execution of this task. The present study used repetitive transcranial magnetic stimulation (rTMS) to further examine the functional role of this structure in the successful use of the PSVA. Moreover, we investigated which auditory parameter used in the prosthesis (pitch, intensity, or spatial location) might contribute to this occipital activation. Results revealed that rTMS applied to right dorsal extrastriate cortex in EB subjects interferes with both the PSVA use and the auditory spatial location task but not with pitch and intensity discriminations. By contrast, rTMS targeting the same cortical areas in sighted subjects did not affect performance on any auditory tasks. Early visual deprivation thus leads to functional cerebral cross-modal reorganization in the processing of auditory information and auditory-to-visual sensory substitution. The findings also point to the specific involvement of the dorsal visual stream for auditory spatial processing in blind subjects. Moreover, this suggests that sensory substitution prostheses can be developed using these additional neural resources to perform tasks that partially compensate for the loss of vision.

Adult↗

PubMed2006

Impact of vision on the development of topographical orientation abilities.

The current experiment examined the importance of visual input on the construction of inner spatial representations. Early and late-onset blind and paired control participants performed a tactile spatial orientation task. No significant group differences were observed, indicating that the blind can represent space. More errors, however, were committed by the early blind than by the late blind and sighted individuals in portions of the task that involved mental rotation skills, suggesting a potential facilitating role for vision in the proper development of spatial constructs.

Adult↗

PubMed2006

A positron emission tomography study during auditory localization by late-onset blind individuals.

Individuals deprived of vision early in life often demonstrate exceptional abilities in their remaining sensory modalities in order to compensate for their handicap. Recent studies have shown that some of these abilities also extend to those who have lost their sight later in life. It is not clear, however, what mechanisms underlie these abilities. Here, we examined cortical activation using positron emission tomography in late-onset blind participants during a free-field auditory localization task. Even though no behavioral enhancements were observed in this testing condition relative to sighted controls, the results revealed that the occipital cortex was nonetheless activated during task execution. We conclude that late-onset blind individuals do manifest cerebral reorganization, although its functional relevance to the task is less clear.

Acoustic Stimulation↗

PubMed2006

Effect of connectivity and bistability on the visual potentials evoked by illusory figures.

The present study aimed at testing functional hypotheses regarding two brain potentials elicited by illusory figures. Accordingly, the N1 potential indexes mechanisms connecting the separate parts of the illusory form, whereas a subsequent negative potential indexes compensatory processes triggered by perceptual difficulty. Here, perceptual difficulty was induced by bistability; that is, by equating the probability of perceiving the illusory form to that of perceiving the independent separate parts. We compared the brain potentials evoked by a strongly connected illusory square, with almost no bistability, with those evoked by a weakly connected illusory square presenting strong bistability. Consistent with our hypotheses, the latter figure evoked the smallest N1 and a larger negative component peaking at 360 ms (N360). These results strengthen the link between N1 and connection and between negativity to perceptual difficulty and perceptual difficulty.

Adult↗

PubMed2006

Neuropsychology: traditional and new methods of investigation.

The neuropsychological assessment is an integral part of the clinical investigation of patients suffering from epilepsy. The aim of the evaluation is to determine disease-related and treatment-related effects on cognition and behavior in order to orient therapeutic interventions, by taking into account the compensatory mechanisms that are available to the patient. Examples of the tests best illustrating the classical neuropsychological protocol are presented. Neuropsychology also plays an important role in the assessment of language lateralization in patients slated for epilepsy surgery. Traditionally, this has been achieved by means of the rather invasive Wada procedure. However, with the advent of new neuroimaging techniques, this procedure is gradually being replaced by minimally invasive or noninvasive methods, such as functional magnetic resonance imaging, positron emission tomography, and optical imaging. In the present paper, we discuss some of the newer techniques that are available to the neuropsychologist for the study of the impact of epilepsy on cerebral functioning.

Amobarbital↗

PubMed2005

The effect of interpolation and perceptual difficulty on the visual potentials evoked by illusory figures.

Completion is the process by which the brain unifies and segregates the parts of an incomplete form. It is qualified as amodal when the form is placed behind an obstacle and modal when the form is at the foreground and closed by illusory contours. The N1, and sometimes the N2, deflections of the visual evoked potentials are known to be larger for modal figures, such as the Kanizsa triangle, than for control figures. This result is generally linked to completion or illusory contours, but it could also be related to a third process: the interpolation of the form by connecting its separate parts. To test the influence of interpolation, a modal triangle, an amodal triangle, a figure with outlined inducers, and a no-triangle figure were randomly presented to 26 subjects. The N1 evoked by the three triangle figures were all larger than the N1 to the no-triangle figure. These results suggest that the N1 amplitude is largely determined by the possibility of interpolating a form in the figure. The greatest N1 to the modal figure further suggests that interpolation may be increased by modal completion and decreased by the features that diminish the saliency of triangle in the amodal figure and the figure with outlined inducers. On the other hand, the largest N2 was evoked by the amodal figure. This effect may index processes activated in response to the great difficulty in perceiving the triangle in the amodal figure, a difficulty that is initially caused by a conflict of perceptions characterizing this figure.

Adult↗

PubMed2005

Development of visual-evoked potentials to radially modulated concentric patterns.

The visual processing of radially modulated concentric patterns was studied in human participants, aged 3-22 years, by recording event-related potentials. These stimuli are known to activate the fusiform face area as well as area V4 in normal adults. The electrophysiological data showed a P1 latency that reached a maturation asymptote before 3 years of age, whereas that of N1 and P2 became adultlike by 13 years of age. In addition, the distribution of the P2 component over the scalp was focalized in the primary visual cortex before adolescence and became distributed over the entire brain after adolescence. Radially modulated concentric stimuli thus induce brain activation that is not mature until 13 years of age.

Adolescent↗

PubMed2005

A functional neuroimaging study of sound localization: visual cortex activity predicts performance in early-blind individuals.

Blind individuals often demonstrate enhanced nonvisual perceptual abilities. However, the neural substrate that underlies this improved performance remains to be fully understood. An earlier behavioral study demonstrated that some early-blind people localize sounds more accurately than sighted controls using monaural cues. In order to investigate the neural basis of these behavioral differences in humans, we carried out functional imaging studies using positron emission tomography and a speaker array that permitted pseudo-free-field presentations within the scanner. During binaural sound localization, a sighted control group showed decreased cerebral blood flow in the occipital lobe, which was not seen in early-blind individuals. During monaural sound localization (one ear plugged), the subgroup of early-blind subjects who were behaviorally superior at sound localization displayed two activation foci in the occipital cortex. This effect was not seen in blind persons who did not have superior monaural sound localization abilities, nor in sighted individuals. The degree of activation of one of these foci was strongly correlated with sound localization accuracy across the entire group of blind subjects. The results show that those blind persons who perform better than sighted persons recruit occipital areas to carry out auditory localization under monaural conditions. We therefore conclude that computations carried out in the occipital cortex specifically underlie the enhanced capacity to use monaural cues. Our findings shed light not only on intermodal compensatory mechanisms, but also on individual differences in these mechanisms and on inhibitory patterns that differ between sighted individuals and those deprived of vision early in life.

Auditory Perception↗

PubMed2005

Role of primary visual cortex in the binocular integration of plaid motion perception.

This study assessed the early mechanisms underlying perception of plaid motion. Thus, two superimposed gratings drifting in a rightward direction composed plaid stimuli whose global motion direction was perceived as the vector sum of the two components. The first experiment was aimed at comparing the perception of plaid motion when both components were presented to both eyes (dioptic) or separately to each eye (dichoptic). When components of the patterns had identical spatial frequencies, coherent motion was correctly perceived under dioptic and dichoptic viewing condition. However, the perceived direction deviated from the predicted direction when spatial frequency differences were introduced between components in both conditions. The results suggest that motion integration follows similar rules for dioptic and dichoptic plaids even though performance under dichoptic viewing did not reach dioptic levels. In the second experiment, the role of early cortical areas in the processing of both plaids was examined. As convergence of monocular inputs is needed for dichoptic perception, we tested the hypothesis that primary visual cortex (V1) is required for dichoptic plaid processing by delivering repetitive transcranial magnetic stimulation to this area. Ten minutes of magnetic stimulation disrupted subsequent dichoptic perception for approximately 15 min, whereas no significant changes were observed for dioptic plaid perception. Taken together, these findings suggest that V1 is not crucial for the processing of dioptic plaids but it is necessary for the binocular integration underlying dichoptic plaid motion perception.

Adult↗

PubMed2004

EEG coherence in early-blind humans during sound localization.

Human blind individuals have demonstrated cross-modal plasticity in research over the past decade. In one such study, we showed that early-blind subjects were able to localize sound sources accurately despite the lack of visual input for the calibration of their auditory space. A further ERP study with these subjects also revealed N1 and P3 components during a sound localization task to be more posteriorly distributed than for sighted controls, indicating an involvement of posterior regions in sound localization for blind subjects not present for sighted subjects. In the current study, we analyzed these data for EEG power and coherence in theta, alpha, beta, and gamma frequency bands to see whether blind individuals would show increased coherence reflecting increased connectivity between the central and posterior cortical regions. Blind and sighted subjects did not differ with respect to overall EEG power in any frequency range. However, EEG coherence was significantly increased in blind subjects compared to sighted in the theta, alpha, and beta frequency bands. These results have implications for cortical plasticity affected by sensory deprivation in humans.

Acoustic Stimulation↗

PubMed2004

Early- and late-onset blind individuals show supra-normal auditory abilities in far-space.

Blind individuals manifest remarkable abilities in navigating through space despite their lack of vision. They have previously been shown to perform normally or even supra-normally in tasks involving spatial hearing in near space, a region that, however, can be calibrated with sensory-motor feedback. Here we show that blind individuals not only properly map auditory space beyond their peri-personal environment but also demonstrate supra-normal performance when subtle acoustic cues for target location and distance must be used to carry out the task. Moreover, it is generally postulated that such abilities rest in part on cross-modal cortical reorganizations, particularly in the immature brain, where important synaptogenesis is still possible. Nonetheless, we show for the first time that even late-onset blind subjects develop above-normal spatial abilities, suggesting that significant compensation can occur in the adult.

Acoustic Stimulation↗

PubMed2004

Neuropsychology: pitch discrimination in the early blind.

Do blind people develop superior abilities in auditory perception to compensate for their lack of vision? They are known to be better than sighted people at orientating themselves by sound, but it is not clear whether this enhanced awareness extends to other auditory domains, such as listening to music or to voices. Here we show that blind people are better than sighted controls at judging the direction of pitch change between sounds, even when the speed of change is ten times faster than that perceived by the controls--but only if they became blind at an early age. The younger the onset of blindness, the better is the performance, which is in line with cerebral plasticity being optimal during the early years.

Acoustic Stimulation↗

PubMed2004

Disparity sensitivity in the superior colliculus of the cat.

The present study aims at evaluating the spatial disparity response profiles of binocular cells in the superficial layers of the superior colliculus of the cat using drifting light bars and phase-shifted spatial frequency gratings. Results show that a total of 64% of the cells were sensitive to phase disparities and had large tuning profiles. Similarly, a large proportion (75%) of those tested with position offsets showed one of the four classic disparity profiles, those of the tuned cells being rather coarse. When tested with both position and phase disparities, 54% of the cells showed sensitivity profiles to the two types of stimuli. The overall results suggest that the superior colliculus is involved in the analysis of coarse stereopsis and/or the planning and initiation of saccades during vergence eye movements and/or the control of fine adjustments to maintain fixation as the stimulus moves in depth.

Action Potentials↗

PubMed2004

Effective binocular integration at the midline requires the corpus callosum.

To study the role of the corpus callosum (CC) in midline binocular integration, the effects of late callosotomy and congenital CC agenesis on the ability to perceive dichoptic plaid motion was assessed. Coherent motion was well perceived at all locations in the visual field under dioptic viewing but not along the vertical meridian (VM) when the components were dichoptically presented. This deficit was totally abolished in the agenesis subject and reduced in the callosotomized individual when stimulus size was increased beyond the VM. Electrophysiological correlates were also examined by recording visual evoked potentials and these showed that the P1/N2 components were abnormal for small dichoptic stimuli presented on the midline. These findings attest to the importance of the contribution of CC to midline binocular integration and the effects of cerebral plasticity.

Adult↗

PubMed2003

Comparison of sensitivity to first- and second-order local motion in 5-year-olds and adults.

We compared sensitivity to first- versus second-order motion in 5-year-olds and adults tested with stimuli moving at slower (1.5 degrees s(-1)) and faster (6 degrees s(-1)) velocities. Amplitude modulation thresholds were measured for the discrimination of the direction of motion (up vs. down) for luminance-modulated (first-order) and contrast-modulated (second-order) horizontal sine-wave gratings. At the slower velocity (1.5 degrees s(-1)), the differences in threshold between 5-year-olds and adults were small but significant for both first- and second-order stimuli (0.02 and 0.05 log units worse than adults' thresholds, respectively). However, at the faster velocity (6 degrees s(-1)), the differences in threshold between the children and adults were 8 times greater for second-order motion than for first-order motion. Specifically, children's thresholds were 0.16 log units worse than those of adults for second-order motion compared to only 0.02 log units worse for first-order motion. The different pattern of results for first-order and second-order motion at the faster velocity (6 degrees s(-1)) is consistent with models positing different mechanisms for the two types of motion and suggests that those mechanisms mature at different rates.

Adult↗

PubMed2002

Phase-disparity coding in extrastriate area 19 of the cat.

Binocular interactions were investigated in area 19 of the anaesthetized cat using dichoptically presented phase-shifted static spatial frequency gratings that flickered at a fixed temporal rate. More than two-thirds of the binocular cells showed phase specificity to static phase disparities leading to either summation or facilitation interactions. This proportion of spatial disparity selectivity was higher than that shown for the same area (one-third of the units) when drifting light bars or drifting spatial frequencies were used to create disparities. The range of phase disparities encoded by binocular cells in area 19 is inversely related to the optimal spatial frequency of the dominant eye. Thus, cells in this area are tuned to coarse spatial disparities which, as supported by behavioural studies, could reflect its involvement in the analysis of stereoscopic pattern having gross disparities but devoid of motion cues. Because of the nature of its interconnections with numerous visual cortical areas, area 19 could serve as a way station where stereoscopic information could be first analysed and sent to other higher order areas for a complete representation of three-dimensional objects.

Animals↗