Biomedical subjects

Brett A Clementz

Publications and source records attributed to Brett A Clementz.

8 recordsLinked to original sources

PubMed2005

Electroencephalography/magnetoencephalography study of cortical activities preceding prosaccades and antisaccades.

The temporal and spatial characteristics of brain activity preceding prosaccades and antisaccades were investigated using source reconstructions of 64-channel electroencephalography and 148-channel magnetoencephalography data. Stimulus-locked data showed early cuneus activity was stronger during antisaccades, and later occipital gyrus activity was stronger preceding prosaccades, which suggests a top-down influence on early visual processing. Response-locked data showed that supplementary eye field, prefrontal cortex, and medial frontal eye field activity was greater for antisaccades than for prosaccades prior to saccade generation. Lateral frontal eye field activity appeared to be inhibited prior to antisaccade response generation. The spatial and temporal resolution of combined electroencephalography/magnetoencephalography data allows the evaluation of specific cortical activities preceding saccades and for demonstration of how activities differ as a function of response contingencies.

Adult↗

PubMed2005

Stimulus sequence affects schizophrenia-normal differences in event processing during an auditory oddball task.

Schizophrenia patients have difficulty distinguishing relevant from irrelevant auditory information. Auditory oddball paradigms are commonly used to investigate the processing of stimulus relevance. The present study used dense-array EEG and distributed source reconstructions to examine schizophrenia-normal differences in the processing of targets and standards as a function of the temporal sequence of stimuli. Brain responses were evaluated separately for early and late standards (standards 1-3 and 4-6 following a target, respectively) and early and late targets (those following 2-3 standards and 4-6 standards, respectively). The latencies of peaks (N1, P2, P3) in the event-related potential (ERP) waveforms did not differ between schizophrenia and normal subjects. However, schizophrenia-normal differences in neural activity, derived from minimum norm estimation, occurred at specific times during stimulus processing as a function of stimulus sequence. Schizophrenia patients displayed smaller activity than normals in early ERPs (left hemispheric N1, right frontal P2) to late targets, and they produced P3-like responses to late standards. Furthermore, during the P2/N2 time interval, opposite patterns of brain activity were elicited in schizophrenia and normal subjects in response to standards, indicating different neural responses to the same stimulus events. These results suggest attention allocation to task-irrelevant stimuli in schizophrenia, consequent upon insufficient representation of stimulus significance and context. Thus, schizophrenia compromises the ability to properly use context to solve even simple cognitive problems.

Acoustic Stimulation↗

PubMed2004

Rate of stimulation affects schizophrenia-normal differences on the N1 auditory-evoked potential.

The present study examined how increasing the rate of steady-state stimulation affects schizophrenia-normal differences on the N1 auditory-evoked potential, an index of auditory integration. Dense-array EEG was recorded while schizophrenia and normal subjects heard 1 kHz tones amplitude modulated at 10, 20, 40, or 80 Hz. Spectral power across frequency and time was calculated. The typically lower N1 amplitude in schizophrenia, observed at the 10 Hz burst rate, increased to nearly equal that of normal individuals at 20 Hz. Unlike normal subjects, schizophrenia subjects' power at N1 failed to increase at the 40 and 80 Hz burst rates. These results suggest steady-state stimuli, up to a point, provide the extra information needed for schizophrenia patients to more efficiently integrate auditory information.

Acoustic Stimulation↗

PubMed2004

Inhibitory deficits in ocular motor behavior in adults with attention-deficit/hyperactivity disorder.

BACKGROUND: Many of the symptoms of attention-deficit/hyperactivity disorder (ADHD) have been attributed to deficits in behavioral inhibition mediated by the frontostriatal system. The ability to suppress unwanted saccadic eye movements is mediated by prefrontal cortex-basal ganglia circuitry and thus constitutes a useful measure of inhibitory ability. METHODS: To evaluate the functional integrity of this circuitry in ADHD, adult ADHD subjects unmedicated for at least 48 hours and normal comparison adults were studied by means of a comprehensive battery of ocular motor paradigms. RESULTS: On a prosaccade task, in which subjects were required to generate saccades toward a peripheral visual target after a short stimulus-free interval, ADHD subjects generated significantly more of anticipatory (premature) saccades (reaction time <90 msec) and of saccades toward the target on catch trials, in which they were supposed to inhibit eye movements. On the antisaccade task, in which they were required to inhibit gazing toward the target while moving their eyes in the opposite direction, ADHD subjects made significantly more directional errors than normal adults. The performance of ADHD adults was consistent with deficits in saccadic inhibition. CONCLUSIONS: Given the recent evidence for the interdependence between the brain systems mediating visual attention and ocular motor behavior, these findings support the notion that deficits in inhibitory mechanisms might underlie the inattention characteristic of ADHD. These results also implicate abnormalities in prefrontal cortex-basal ganglia circuitry in ADHD.

Adult↗

PubMed2004

Aberrant brain dynamics in schizophrenia: delayed buildup and prolonged decay of the visual steady-state response.

In schizophrenia, aberrant brain activity has been reported both during stimulus processing and at rest. Evoked response amplitude is a function of both the number and synchronization of neurons firing in relation to a stimulus. It is at present unclear whether schizophrenia patients have normal synchronization of neural activity in relation to stimulus processing, and whether the amount and time course of synchronization is related to their evoked response amplitudes. EEG brain dynamics in response to visual steady-state stimulation were assessed in 12 schizophrenia and 12 healthy subjects at three stimulation durations (2, 4, and 6 s). Group differences in the visual evoked potential, the visual steady-state response, and the local coherence of the visual steady-state response were evaluated over time. Schizophrenia patients had smaller and delayed event-related potentials. Moreover, they had a slower buildup of steady-state amplitude following stimulation onset and a prolonged decrease after stimulation offset. Groups did not differ during mid-segments of steady-state stimulation. Increase in coherence to stimulation onset did not differ between-groups, but coherence decay of the visual steady-state response following stimulus offset was delayed in schizophrenia patients. The initial response to visual stimulation among schizophrenia subjects, therefore, may be reduced in amplitude due to weak signal strength, not poor coordination between distant cortical regions. The prolonged recovery function of schizophrenia patients' visual system may indicate abnormal nonlinearity in neural response. These findings have implications understanding the nature of evoked response differences between schizophrenia and normal groups especially in repetitive stimulus paradigms.

Adolescent↗

PubMed2003

Visual stimuli activate auditory cortex in deaf subjects: evidence from MEG.

Studies using fMRI have demonstrated that visual stimuli activate auditory cortex in deaf subjects. Given the low temporal resolution of fMRI, it is uncertain whether this activation is associated with initial stimulus processing. Here, we used MEG in deaf and hearing subjects to evaluate whether auditory cortex, devoid of its normal input, comes to serve the visual modality early in the course of stimulus processing. In line with previous findings, visual activity was observed in the auditory cortex of deaf, but not hearing, subjects. This activity occurred within 100-400 ms of stimulus presentation and was primarily over the right hemisphere. These results add to the mounting evidence that removal of one sensory modality in humans leads to neural reorganization of the remaining modalities.

Adult↗

PubMed2003

Ear of stimulation determines schizophrenia-normal brain activity differences in an auditory paired-stimuli paradigm.

Schizophrenia patients have abnormalities of auditory information processing, theoretically associated with dysfunction of neuronal excitation. Auditory paired-stimuli (S1-S2) paradigms are used to evaluate the nature of these abnormalities. It is unknown whether patients' abnormalities during S1-S2 paradigms are attributable to specific hemispheric differences in cortical processing. The present studies used whole head magnetoencephalography and monaural or binaural versions of the paired-stimuli paradigm to evaluate auditory processing among 38 schizophrenia and 38 normal subjects. The strengths of auditory-evoked brain responses over time were quantified using distributed source reconstructions with L2 minimum norm constraint and realistic head models. For left ear stimuli, schizophrenia and normal groups did not differ on either left or right hemisphere activity over auditory cortex. For right ear and binaural stimuli, schizophrenia patients had less activity over left auditory cortex from 80 to 120 ms post-stimulus but did not differ from normal on activity over right auditory cortex. Additionally, in response to monaural stimulation, schizophrenia patients had significantly less activity than normal over right temporal parietal junction from 60 to 120 ms post-stimulus. These data are consistent with four propositions about schizophrenia: (i). right auditory cortex is functioning normally; (ii). processing of simple auditory stimuli is abnormal in left auditory cortex, probably specifically in supra-granular layers; (iii). auditory localization abilities are deficient; and (iv). auditory cortex abnormalities are not a function of deficient hemispheric communication because they are evident early in processing as long as stimuli are delivered directly to left hemisphere.

Acoustic Stimulation↗

PubMedDate not supplied

Knowledge of stimulus repetition affects the magnitude and spatial distribution of low-frequency event-related brain potentials.

Rate effects are defined as a reduction in amplitude of an evoked brain response with increasing stimulus frequency. In auditory paired-stimulus paradigms, a smaller amplitude evoked response to the second stimulus at a latency of 50 ms has been proposed to index a preattentive sensory gating mechanism. The present study investigated the possibility that expectancy and/or attentional biases could alter evoked potentials associated with rate effects. EEG data were recorded from 30 channels while subjects received 240 trials of 1, 2 or 3 click stimuli (with successive stimuli being separated by 500-ms intervals). Half of the subjects knew (blocked condition) and half of the subjects did not know (mixed condition) how many stimuli they would receive on a given trial. Subjects in the blocked condition had a significantly larger rate effect than subjects in the mixed condition. This effect was present only for low-frequency components of the event-related brain potential (ERP; below 10 Hz) and occurred from 30 to 60, 90 to 160 and 190 to 260 ms after stimulus presentation (P(1)-N(1)-P(2) complex). Mixed condition subjects also had larger contributions to their ERPs from temporal channels. These results suggest that the rate effect can be significantly altered by expectancy, and they are inconsistent with the thesis that ERPs near 50 ms in a paired-stimulus paradigm solely index a preattentive sensory gating mechanism.

Adolescent↗