Biomedical subjects

Merav Sabri

Publications and source records attributed to Merav Sabri.

8 recordsLinked to original sources

PubMed2007

Involvement of the left anterior insula and frontopolar gyrus in odor discrimination.

Discriminating between successively presented odors requires brief storage of the first odor's perceptual trace, which then needs to be subsequently compared to the second odor in the pair. This study explores the cortical areas involved in odor discrimination and compares them with findings from studies of working-memory, traditionally investigated with n-back paradigms. Sixteen right-handed subjects underwent H(2) (15)O positron emission tomography during counterbalanced conditions of odorless sniffing, repeated single odor detection, multiple odor detection, and conscious successive discrimination between odor pairs. Eight odorants were delivered using a computer-controlled olfactometer through a birhinal nasal cannula. Conscious successive odor discrimination evoked significantly greater activity in the left anterior insula and frontopolar gyrus when compared to reported sensory detection of the identical odors. Additional activation was found in the left lateral orbital/inferior frontal and middle frontal gyri when discrimination was compared to the odorless condition. The left anterior insula is likely involved in the evaluation of odor properties. Consistent with other studies, frontopolar and middle frontal gyrus activation is more likely related to working memory during odor discrimination.

Adult↗

PubMed2005

Neural correlates of olfactory change detection.

Detecting changes in a stream of sensory information is vital to animals and humans. While there have been several studies of automatic change detection in various sensory modalities, olfactory change detection is largely unstudied. We investigated brain regions responsive to both passive and active detection of olfactory change using fMRI. Nine right-handed healthy, normosmic subjects (five men) were scanned in two conditions while breathing in synchrony with a metronome. In one condition, subjects mentally counted infrequent odors (Attend condition), whereas in the other condition, subjects' attention was directed elsewhere as they counted auditory tones (Ignore condition). Odors were delivered via a nasal cannula using a computer-controlled air-dilution olfactometer. Infrequently occurring olfactory stimuli evoked significant (P < .05, corrected) activity in the subgenual cingulate and in central posterior orbitofrontal cortex, but only in the Ignore condition, as confirmed by direct comparison of the Ignore session with the Attend session (P < .05, corrected). Subgenual cingulate and posterior orbital cortex may therefore play a role in detecting discrepant olfactory events while attention is otherwise engaged in another sensory modality.

Adult↗

PubMed2005

Is the failure to detect stimulus deviance during sleep due to a rapid fading of sensory memory or a degradation of stimulus encoding?

The mismatch negativity (MMN) is thought to reflect the outcome of a system responsible for the detection of change in an otherwise repetitive, homogenous acoustic environment. This process depends on the storage and maintenance of a sensory representation of the frequently presented stimulus to which the deviant stimulus is compared. Few studies have been able to record the MMN in non-rapid eye movement (NREM) sleep. This pattern of results might be explained by either a rapid fading of sensory memory or an inhibition of stimulus input prior to entry into the cortical MMN generator site. The present study used a very rapid rate of presentation in an attempt to capture mismatch-related negativity prior to the fading of sensory memory. Auditory event-related potentials were recorded from 12 subjects during a single sleep period. A 1000 Hz standard stimulus was presented every 150 ms. At random, on 6.6% of the trials, the standard was changed to either a large 2000 Hz or a small 1100 Hz deviant. In wakefulness, the large deviant elicited an extended negativity that was reduced in amplitude following the presentation of the small deviant. This negativity was also apparent during REM sleep following the presentation of the large deviant. These deviant-related negativities (DRNs) were probably a composite of N1 and MMN activity. During NREM sleep (stage 2 and slow-wave sleep), only the large deviant continued to elicit a DRN. However this DRN might be overlapped by the initial activity of a component that is unique to sleep, the N350. There was little evidence of the DRN or the MMN during sleep following the presentation of the small deviant. A rapid rate of presentation, therefore, does not preserve the MMN following small deviance within sleep. It is possible that inhibition of sensory input occurs before entry into the MMN generating system in the temporal cortex.

Adolescent↗

PubMed2004

Neural correlates of auditory sensory memory and automatic change detection.

An auditory event-related potential component, the mismatch negativity (MMN), reflects automatic change detection and its prerequisite, sensory memory. This study examined the neural correlates of automatic change detection using BOLD fMRI and two rates of presentation previously shown to induce either a large or no MMN. A boxcar block design was employed in two functional scans, each performed twice. A block consisting of 1000-Hz standards (S) alternated with one consisting of 1000-Hz standards and 2000-Hz infrequent deviants (S + D). Presentation rate was either 150 or 2400 ms. Fourteen participants were instructed to ignore all auditory stimulation and concentrate on a film (no audio) by reading subtitles. Data analysis used SPM99 and random effects approach. Cluster statistics (P < 0.05, corrected) were employed at a height threshold of P < 0.001. At the short ISI, there was a significant BOLD response in the right superior temporal gyrus (STG), the left insula, and the left STG (including parts of primary auditory cortex). There were no suprathreshold clusters at the long rate, with S + D blocks inducing no greater activity than S blocks. These results support the hypothesis that the automatic detection of auditory change occurs in the STG bilaterally and relies on the maintenance of sensory memory traces.

Adult↗

PubMed2004

Olfactory system activation from sniffing: effects in piriform and orbitofrontal cortex.

Neuroimaging studies suggest that piriform cortex is activated at least in part by sniffing. We used H(2)(15)O positron emission tomography (PET) to study 15 healthy volunteers while they participated in four conditions, two of which were sniffing odorants and odorless air. The remaining two conditions involved a constant, very low flow of either odorized or odorless air during velopharyngeal closure (VPC), a technique that prevents subject-induced airflow through the nasal passages. Contrary to expectation, sniffing under odorless conditions did not induce significant piriform and surrounding cortical (PC+) activity when compared to odorless VPC, even at a liberal statistical threshold. However, a small correlation emerged in PC+ between the difference signal of [odorless sniffing - odorless VPC] and peak rate of nasal pressure change. PC+ activity was, however, strongly evoked by odorant exposure during sniffing and VPC, with neither technique showing greater activation. Activity in orbitofrontal (olfactory association) cortex was absent during odorant stimulation (OS) with VPC, but present during odorant sniffing. Sniffing may therefore play an important role in facilitating the higher-order analysis of odors. A right orbitofrontal region was also activated with odorless sniffing, which suggests a possible orbitofrontal role in guided olfactory exploration.

Adult↗

PubMed2004

Alcohol-related olfactory cues activate the nucleus accumbens and ventral tegmental area in high-risk drinkers: preliminary findings.

BACKGROUND: The mesocorticolimbic dopamine system is implicated in motivation and reward and may be involved in the development of alcoholism. METHODS: We used functional magnetic resonance imaging to study the blood oxygen level-dependent (BOLD) response to alcohol-related olfactory stimuli (AROS; odors of beer and whiskey) and non-alcohol-related olfactory stimuli (NAROS; odors of grass and leather) in 10 high-risk (HR) drinkers (average drinks per week, 19.99; SD, 6.99; all with > or = 2 first- or second-degree alcoholic relatives) and 5 low-risk (LR) social drinking controls (drinks per week, 2.82; SD, 2.87; 1 subject had 1 second-degree alcoholic relative). Data were analyzed with SPM99 and random effects analysis by using regions of interest and corrected cluster statistics (p < 0.05) to focus on the nucleus accumbens (NAc) and ventral tegmental area (VTA). RESULTS: In HR subjects, there was a greater BOLD signal increase in the NAc during AROS than during clean air. BOLD signal increases during AROS were also greater in the NAc than the signal increases induced by NAROS. The AROS signal was significantly greater than the NAROS signal in a small number of voxels in the VTA. Finally, the AROS/NAROS difference signal was larger in HR drinkers in both the NAc and VTA. CONCLUSIONS: Alcoholic olfactory cues may invoke the dopaminergic mesocorticolimbic system to a greater degree than nonalcoholic odors and could be effective tools in exploring the role of the dopamine system in susceptibility to alcoholism.

Adult↗

PubMed2003

Effects of sleep onset on the mismatch negativity (MMN) to frequency deviants using a rapid rate of presentation.

This study examined the effects of sleep onset-the transition from a waking, conscious state to one of sleep and unconsciousness-on the mismatch negativity (MMN) following frequency deviants when a rapid rate of stimulus presentation is employed. The MMN is thought to reflect a brief-lasting sensory memory. Rapid rates of stimulus presentation should guard the sensory memory from fading. A 1,000 Hz standard stimulus was presented every 150 ms. At random, on 6.6% of the trials, the standard was changed to either a large 2,000 or a small 1,100 Hz deviant. During alert wakefulness (when subject ignored the stimuli and read a book), the large deviant elicited a larger deviant related negativity (DRN) than did the small deviant. This negativity may be a composite of both N1 and MMN activity while that following the small deviant is probably a 'true' MMN. The large deviant continued to elicit a DRN in relaxed wakefulness (eyes closed) and Stages 1 and 2 of sleep, although it was much reduced in amplitude. A significant MMN was recorded for the small deviant only in alert wakefulness. The failure to observe an MMN to small deviance and the attenuation of the DRN to large deviance at sleep onset therefore is probably not due to a decay of sensory memory. It is more likely that cortical encoding of both the standard and deviant is weakened during sleep onset because of prior thalamic inhibition of sensory input.

Acoustic Stimulation↗

PubMed2002

The effects of digital filtering on mismatch negativity in wakefulness and slow-wave sleep.

The mismatch negativity (MMN) is a response to a deviant auditory stimulus that occurs infrequently in a sequence of otherwise repetitive, homogeneous standard auditory stimuli. The MMN is presumed automatic and independent of conscious awareness. Recording of the MMN during unconscious states may be problematic. The frequency content of the long-lasting MMN may overlap and summate with other event-related slow potentials and low-frequency background electroencephalogram (EEG) activity. The purpose of this study is to determine the optimal filter settings for recording the MMN during unconscious states. Auditory event-related potentials (ERPs) were recorded from eight subjects in an oddball paradigm during wakefulness and Stages 3 and 4 of sleep [slow-wave sleep (SWS)] using a 0.16-35 Hz analogue bandpass. Deviant probability was 0.033. Stimulus-onset asynchrony was 150 ms. The EEG data were subsequently digitally filtered in the frequency domain. The low-pass filter was set at either 24, 12 or 6 Hz, and the high-pass filter at either 1, 2, 3 or 4 Hz. Applying a low-pass filter down to 12 Hz had a minimal impact on the waking or sleeping MMN amplitude. On the other hand, increasing the high-pass setting from 2 to 3 Hz permitted the visualization of the MMN recorded during sleep. The 4 Hz filter showed a similar trend but also markedly attenuated the amplitude of the waking MMN. A high-pass setting of 3 Hz provides a reasonable compromise. It has only a slight effect on the MMN when the subject is conscious, but still attenuates most of the unwanted slow potential activity when the subject enters SWS.

Acoustic Stimulation↗