Biomedical subjects

M P Viggiano

Publications and source records attributed to M P Viggiano.

13 recordsLinked to original sources

PubMed2003

Hippocampal function and visual object processing in temporal lobe epilepsy.

Limbic event-related potentials related to verbal but not to visual recognition memory have been found to be attenuated within the epileptic hippocampus of patients with temporal lobe epilepsy (TLE). To identify hippocampal contributions to visual processing and memory we recorded intracranial ERPs directly from within the epileptic and the non-epileptic hippocampus in 12 patients with unilateral TLE during a visual object decision and naming task. While the non-epileptic hippocampus differentiated reliably between real and nonsense objects, this effect was completely eliminated within the epileptic mesial temporal lobe. This finding suggests that the hippocampus proper contributes to the semantic processing of visual objects and may help to explain visual memory deficits in TLE.

Adult↗

PubMed2001

Identification of spatially filtered stimuli as function of the semantic category.

The different weight of spatial frequency content in the identification of visual objects was investigated. Subjects were required to identify spatially filtered pictures of animals, vegetables and nonliving objects, displayed at 9 resolution levels of filtering following a coarse-to-fine order. Results showed that spatial frequency content differentially affected the three categories of stimuli. Data suggested a different involvement of low and high spatial frequency channels in visual processing of objects in relation to the semantic category.

Adult↗

PubMed2000

Identification of Poppelreuter-like pictures as indexed by blinking.

Blinking was recorded in 28 adult participants during the identification of superimposed pictures (similar to Poppelreuter figures), some of which had been presented individually in an earlier study phase. Participants were required to name the pictures at the end of the identification phase. The percentage of correct identifications was greater for combinations formed by old than new pictures, and decreased as the number of pictures in the combination increased. Attentional demands associated with mental load (number of pictures) affected both the rate of blinks produced during the identification process and the latency of the first blink produced after the stimulus onset. The first blink latency increased as the number of pictures increased, and also depended on material to be identified, with longer latencies associated with novel combinations. We suggest that blinking may be used as an index of memory processes involved in visual identification, even in absence of overt (verbal) responses.

Adult↗

PubMed2000

Overt and covert identification of fragmented objects inferred from performance and electrophysiological measures.

The authors investigated visual processing leading to object identification by manipulating the number of fragments and nature of the study. During the study, participants either named or drew objects in Experiment 1 and drew them all in Experiment 2. During the test, participants made an identification judgment at each of 6 different fragmentation levels for studied and new objects. Fewer fragments were needed to identify studied than unstudied objects. Reaction times were faster for studied than unstudied objects both at identification and at the preceding level. Event-related brain potentials (ERPs) to unidentified objects were characterized by a late negativity in contrast to a positivity to identified objects. ERPs to studied but not to new objects contained a smaller and later version of the identification positivity at level just prior to identification, which was not due to differential response confidence. Much covert visual analysis and even object identification may precede overt identification, depending on the nature of prior experience.

Adult↗

PubMed2000

Category effects on the processing of plane-rotated objects.

We examined the effects of plane rotations on the identification of exemplars of three semantic categories. In the first two experiments line drawings belonging to three categories (animals, inanimate objects, and vegetables) were presented at four orientations (0 degree, 60 degrees, 120 degrees, and 180 degrees of clockwise rotation). The response time was found to depend on stimulus category. In particular, whereas rotation effects were shown for animals, no effect at all was found for vegetables and only partial effects were found for inanimate objects. The unclear pattern found for inanimate objects was further examined in experiment 3 where the orientation effects on the identification of two subsets of the inanimate category were studied. The hypothesis of view-observation frequency was confirmed. In experiment 4, line drawings of objects at different orientations were presented in physically degraded versions. The minimum amount of visual information necessary to identify rotated stimuli was found to vary as a function of stimulus category as well. Results are discussed, combining current research on both viewpoint-dependence/independence and neural systems involved in category processing.

Adult↗

PubMed1998

The covert interplay between perception and memory: event-related potential evidence.

Neurocognitive models of visual object identification have focussed on processes at the moment of identification, when perceivers can actually name what they see. Less well known is the timecourse of processes preceding and leading to actual identification. To track neuromental processes involved in visual identification, behavioral measures and event-related potentials (ERPs) were recorded in two experiments prior to, during and after the identification of fragmented objects, half of which had been shown in their complete versions in a previous study phase. Each object was revealed in a sequence of frames wherein the object was represented by an increasingly less and less fragmented image up to the complete version. A shift in ERPs, around 300 ms and beyond, from negativity to positivity, marked the transition from non-identification to identification. However, while for new stimuli such a shift appeared abruptly from non-identification to identification, for recently-studied objects a late positive wave emerged in response to unidentified fragments at a level just prior to overt identification. Thus, ERPs reflected covert processes associated with a successful match between the current visual information and episodic recently-stored memory traces, which predicted overt identification.

Adult↗

PubMed1998

Identification of fragmented pictures in patients with brain damage.

Identification of fragmented pictures was investigated using line drawings (Snodgrass, Smith, Feenan, & Corwin, 1987) in 236 healthy participants and 43 patients with unilateral lesions of the right or left hemisphere. Perceptual closure (the integration of fragments into a meaningful perceptual whole) depended on the amount of physical visual information, the age of the patient, and the location of the lesion. Patients with right-side lesions showed more impairment than those with left-side lesions, confirming the importance of the right hemisphere in perceptual closure tasks. Right-hemisphere functions involved in perceptual closure, however, were not completely abolished, as performance improved with increasing visual information. The advantages of Snodgrass fragmented pictures as a neuropsychological test are discussed.

Journal Article↗

PubMed1996

Event-related potentials in brain-injured patients with neuropsychological disorders: a review.

Investigations using event-related potentials (ERPs) in brain-injured patients affected by neuropsychological disorders of perception, attention, memory, and language, and other special syndromes such as blind-sight, neglect, prosopagnosia, and apraxia are reviewed. These electrophysiological techniques can be used to assess the integrity of specific brain processes during the performance of cognitive tasks in which behavioral impairments are observed. A special feature of ERP technique is that it reveals ongoing and covert processing which may not be fully assessed by measuring only overt behavioral performance.

Brain Injuries↗

PubMed1995

Pattern reversal visual evoked potentials in patients with hemineglect syndrome.

To investigate basic visual information processing in patients with hemineglect syndrome, pattern reversal visual evoked potentials (VEPs) were recorded in 21 brain-injured patients (10 with neglect symptoms) and 6 healthy subjects. The stimulus was a checkerboard which varied in check size or temporal frequency, presented to the left or right visual field. VEPs recorded in neglect patients to stimuli presented in the subjectively neglected left visual field were comparable in amplitude to those recorded to stimuli presented in the normal right visual field. For stimuli presented centrally, there was no difference in VEPs between neglect patients and brain-damaged patients without neglect. These results support neuropsychological theories that state that the neglect syndrome is more closely related to deficits at post-sensory levels than to impairments in basic visual processing. Some evidence, however, suggests that the nature of the interaction between the two visual hemi-fields may also be altered in neglect patients.

Aged↗

PubMed1993

Hemispheric asymmetry in transient visual evoked potentials induced by the spatial factor of the stimulation.

In order to enhance the effect of spatial frequency on the hemispheric asymmetry of visual evoked potentials (VEP), the response amplitudes to ON-OFF modulated gratings were compared with the responses to pattern reversal stimulation. Sinusoidal gratings of different spatial frequencies were presented to six righthanders. VEPs were recorded from temporal leads on each hemisphere. In the left hemisphere, the amplitude was constant for the two modes of presentation and independent of spatial frequency. In the right hemisphere, the response amplitude was larger to the ONSET stage of ON-OFF stimulation than to reversal and presented the characteristic spatial frequency tuning curve. This asymmetry is assumed to reflect a difference in sensitivity of the two hemispheres to the spatiotemporal characteristics of the stimulus. The relevance of these findings is discussed in relation to the other hemispheric specialization models.

Evoked Potentials, Visual↗

PubMed1991

Pattern reversal visual evoked potentials in fencers.

Visual evoked potentials were recorded from occipital and temporal leads in the two cerebral hemispheres of eight fencers and eight control subjects. The stimulus was a checkerboard subtending a small (1 degree) or large (30 degrees) visual field. Significant differences in P60, N75 and P100 latency and amplitude were found between the two subject groups, especially during the processing of the large visual field. In fencers and left-handers shorter latencies were found for the large visual field condition, whereas right-handers showed an opposite trend. The results give further evidence of special patterns of visual processing in athletes, like fencers, in agreement with the literature.

Adolescent↗

PubMed1990

The effects of visual field size on hemispheric asymmetry of pattern reversal visual evoked potentials.

Visual evoked potentials (VEPs) were recorded in 17 females (10 right-handers and 7 left-handers) and 17 males (10 right-handers and 7 left-handers). The stimulus was a checkerboard pattern phase-reversed at a rate of 1 Hz, binocularly viewed by the subject. Two experimental conditions were tested: field size (the stimulus was enlarged from 1 to 30 degrees of visual angle) and occlusion (the central part of the stimulus was occluded by experimental scotomata of various size--from 1 to 16 degrees). Responses recorded at occipital and temporal leads of the two hemispheres were compared. The P100-N145 amplitude was larger at occipital than at temporal leads, it augmented increasing the size of the stimulus and diminished occluding progressively larger portions of the central field. Hemispheric asymmetries emerged with large field sizes and were not abolished by the presence of experimental scotomata. The hemispheric asymmetry was related to handedness (left-handers had larger amplitudes in the right hemisphere, right-handers in the left hemisphere), but not to sex. Hemispheric asymmetries in VEPs latency were not present. In comparison to males, females showed larger amplitudes and shorter latencies. These results point out that hemispheric asymmetries are found also for the processing of elementary visual stimuli like checkerboards, and they depend both on stimulus parameters (field size) and subject variables (handedness).

Adult↗

PubMedDate not supplied

Rhythmic variations of P100 component of pattern-reversal visual evoked potentials.

Pattern-reversal visual evoked potentials (EPs) were recorded in seven adult subjects to assess the existence of rhythmic variations in processing visual information at a primary stage. During a session of two hours, sixty EPs were recorded on the left and right cerebral hemispheres. The amplitude of N75-P100 component was measured. Remarkable variations were found with a periodicity from 15 to 60 min in six out of seven subjects. No hemispheric differences were found in the rhythmic variations. The results are in agreement with the behavioral data which show ultradian variations in visual performance depending on a general activation oscillator.

Activity Cycles↗