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Biomedical subjects

F M Miezin

Publications and source records attributed to F M Miezin.

At least 19 recordsLinked to original sources
PubMed2003

Automated method for extracting response latencies of subject vocalizations in event-related fMRI experiments.

For functional magnetic resonance imaging studies of the neural substrates of language, the ability to have subjects performing overt verbal responses while in the scanner environment is important for several reasons. Most directly, overt responses allow the investigator to measure the accuracy and reaction time of the behavior. One problem, however, is that magnetic resonance gradient noise obscures the audio recordings made of voice responses, making it difficult to discern subject responses and to calculate reaction times. ASSERT (Adaptive Spectral Subtraction for Extracting Response Times), an algorithm for removing MR gradient noise from audio recordings of subject responses, is described here. The signal processing improves intelligibility of the responses and also allows automated extraction of reaction times. The ASSERT-derived response times were comparable to manually measured times with a mean difference of -8.75 ms (standard deviation of difference = 26.2 ms). These results support the use of ASSERT for the purpose of extracting response latencies and scoring overt verbal responses.

Algorithms↗
PubMed2001

Direct comparison of prefrontal cortex regions engaged by working and long-term memory tasks.

Neuroimaging studies have suggested the involvement of ventrolateral, dorsolateral, and frontopolar prefrontal cortex (PFC) regions in both working (WM) and long-term memory (LTM). The current study used functional magnetic resonance imaging (fMRI) to directly compare whether these PFC regions show selective activation associated with one memory domain. In a within-subjects design, subjects performed the n-back WM task (two-back condition) as well as LTM encoding (intentional memorization) and retrieval (yes-no recognition) tasks. Additionally, each task was performed with two different types of stimulus materials (familiar words, unfamiliar faces) in order to determine the influence of material-type vs task-type. A bilateral region of dorsolateral PFC (DL-PFC; BA 46/9) was found to be selectively activated during the two-back condition, consistent with a hypothesized role for this region in active maintenance and/or manipulation of information in WM. Left frontopolar PFC (FP-PFC) was also found to be selectively engaged during the two-back. Although FP-PFC activity has been previously associated with retrieval from LTM, no frontopolar regions were found to be selectively engaged by retrieval. Finally, lateralized ventrolateral PFC (VL-PFC) regions were found to be selectively engaged by material-type, but uninfluenced by task-type. These results highlight the importance of examining PFC activity across multiple memory domains, both for functionally differentiating PFC regions (e.g., task-selectivity vs material-selectivity in DL-PFC and VL-PFC) and for testing the applicability of memory domain-specific theories (e.g., FP-PFC in LTM retrieval).

Adult↗
PubMed2000

Characterizing the hemodynamic response: effects of presentation rate, sampling procedure, and the possibility of ordering brain activity based on relative timing.

Rapid-presentation event-related functional MRI (ER-fMRI) allows neuroimaging methods based on hemodynamics to employ behavioral task paradigms typical of cognitive settings. However, the sluggishness of the hemodynamic response and its variance provide constraints on how ER-fMRI can be applied. In a series of two studies, estimates of the hemodynamic response in or near the primary visual and motor cortices were compared across various paradigms and sampling procedures to determine the limits of ER-fMRI procedures and, more generally, to describe the behavior of the hemodynamic response. The temporal profile of the hemodynamic response was estimated across overlapping events by solving a set of linear equations within the general linear model. No assumptions about the shape were made in solving the equations. Following estimation of the temporal profile, the amplitude and timing were modeled using a gamma function. Results indicated that (1) within a region, for a given subject, estimation of the hemodynamic response is extremely stable for both amplitude (r(2) = 0.98) and time to peak (r(2) = 0.95), from one series of measurements to the next, and slightly less stable for estimation of time to onset (r(2) = 0.60). (2) As the trial presentation rate changed (from those spaced 20 s apart to temporally overlapping trials), the hemodynamic response amplitude showed a small, but significant, decrease. Trial onsets spaced (on average) 5 s apart showed a 17-25% reduction in amplitude compared to those spaced 20 s apart. Power analysis indicated that the increased number of trials at fast rates outweighs this decrease in amplitude if statistically reliable response detection is the goal. (3) Knowledge of the amplitude and timing of the hemodynamic response in one region failed to predict those properties in another region, even for within-subject comparisons. (4) Across subjects, the amplitude of the response showed no significant correlation with timing of the response, for either time-to-onset or time-to-peak estimates. (5) The within-region stability of the response was sufficient to allow offsets in the timing of the response to be detected that were under a second, placing event-related fMRI methods in a position to answer questions about the change in relative timing between regions.

Adolescent↗
PubMed1998

Right anterior prefrontal cortex activation during semantic monitoring and working memory.

Areas of the adult human brain used for semantic monitoring were identified using positron emission tomography. For a series of tasks, subjects viewed a list of familiar English nouns and monitored the words for names of dangerous animals. The monitoring task used here also contained an instruction to keep track of the number or percentage of targets for report after the scan. Surface characteristics of the tasks such as stimulus rate, number of targets, and whether subjects were asked to count or estimate the number of targets were varied across multiple conditions within and between subjects. A passive word viewing condition was used as the control in all subjects. Reliable activations were identified in anterior and dorsal right prefrontal cortex [Brodmann areas (BA) 9 and 10] and left extrastriate cortex. The right prefrontal cortical locations are similar to areas that have been activated during many episodic memory tasks. This surprising finding led to a thorough review of the literature for examples of other activations within 16-mm vector distance of this right prefrontal area. Activations in the vicinity of right BA10 due to episodic memory retrieval, to various forms of working memory, and to miscellaneous tasks were found. The right prefrontal activations in the current experiment and the additional working memory and miscellaneous tasks demonstrate that, although right BA10 is routinely activated by episodic retrieval tasks, it is not uniquely activated by episodic retrieval tasks.

Adult↗
PubMed1998

Hemispheric specialization in human dorsal frontal cortex and medial temporal lobe for verbal and nonverbal memory encoding.

The involvement of dorsal frontal and medial temporal regions during the encoding of words, namable line-drawn objects, and unfamiliar faces was examined using functional magnetic resonance imaging (fMRI). Robust dorsal frontal activations were observed in each instance, but lateralization was strongly dependent on the materials being encoded. Encoding of words produced left-lateralized dorsal frontal activation, whereas encoding of unfamiliar faces produced homologous right-lateralized activation. Encoding of namable objects, which are amenable to both verbal and nonverbal encoding, yielded bilateral dorsal frontal activation. A similar pattern of results was observed in the medial temporal lobe. These results indicate that regions in both hemispheres underlie human long-term memory encoding, and these regions can be engaged differentially according to the nature of the material being encoded.

Adolescent↗
PubMed1998

Effect of motion contrast on human cortical responses to moving stimuli.

The cortical areas activated by motion-defined contours were studied in humans using positron emission tomography (PET). Subjects observed four types of random dot fields, displayed through a 21 degrees diam aperture: unidirectional motion of a translating dot field, motion in opposing directions of two superimposed translating fields, motion in opposing directions of dots in contiguous spatial regions (motion contrast), producing a square wave grating defined by motion, and luminance variation of stationary dots in contiguous spatial regions, producing a square wave grating defined by luminance. Relative to a static dot field, the unidirectional motion condition activated areas previously described, including areas 17/18, lateral temporal-occipital-parietal cortex (MT/MST), and the superior temporal sulcus. Motion-defined gratings increased the activation of areas 17/18 and MT/MST, but not the superior temporal sulcus, and added more dorsal areas in the cuneus, roughly corresponding to V3/V3a, and ventral areas in the lingual gyrus/collateral sulcus, roughly corresponding to V2/VP. Luminance defined gratings, relative to a static dot field, activated areas 17/18, regions in the dorsal cuneus similar to those activated by motion defined gratings, and a region near the left collateral sulcus, slightly lateral to the motion grating activation. They also activated a region in the right fusiform gyrus that was more weakly activated by the motion grating. These results indicate that adding motion contrast to large moving fields increases activity in areas 17/18 and MT/MST and adds both dorsal and ventral regions that are similar for motion and luminance defined contours.

Adult↗
PubMed1996

Functional anatomic studies of memory retrieval for auditory words and visual pictures.

Functional neuroimaging with positron emission tomography was used to study brain areas activated during memory retrieval. Subjects (n = 15) recalled items from a recent study episode (episodic memory) during two paired-associate recall tasks. The tasks differed in that PICTURE RECALL required pictorial retrieval, whereas AUDITORY WORD RECALL required word retrieval. Word REPETITION and REST served as two reference tasks. Comparing recall with repetition revealed the following observations. (1) Right anterior prefrontal activation (similar to that seen in several previous experiments), in addition to bilateral frontal-opercular and anterior cingulate activations. (2) An anterior subdivision of medial frontal cortex [pre-supplementary motor area (SMA)] was activated, which could be dissociated from a more posterior area (SMA proper). (3) Parietal areas were activated, including a posterior medial area near precuneus, that could be dissociated from an anterior parietal area that was deactivated. (4) Multiple medial and lateral cerebellar areas were activated. Comparing recall with rest revealed similar activations, except right prefrontal activation was minimal and activations related to motor and auditory demands became apparent (e.g., bilateral motor and temporal cortex). Directly comparing picture recall with auditory word recall revealed few notable activations. Taken together, these findings suggest a pathway that is commonly used during the episodic retrieval of picture and word stimuli under these conditions. Many areas in this pathway overlap with areas previously activated by a different set of retrieval tasks using stem-cued recall, demonstrating their generality. Examination of activations within individual subjects in relation to structural magnetic resonance images provided an-atomic information about the location of these activations. Such data, when combined with the dissociations between functional areas, provide an increasingly detailed picture of the brain pathways involved in episodic retrieval tasks.

Adolescent↗
PubMed1995

Superior parietal cortex activation during spatial attention shifts and visual feature conjunction.

Positron emission tomography was used to measure changes in the regional cerebral blood flow of normal people while they searched visual displays for targets defined by color, by motion, or by a conjunction of color and motion. A region in the superior parietal cortex was activated only during the conjunction task, at a location that had previously been shown to be engaged by successive shifts of spatial attention. Correspondingly, the time needed to detect a conjunction target increased with the number of items in the display, which is consistent with the use of a mechanism that successively analyzes each item in the visual field.

Attention↗
PubMed1995

Functional anatomical studies of explicit and implicit memory retrieval tasks.

Across three experiments, PET scans were obtained while subjects performed different word-stem completion and FIXATION control tasks designed to study the functional anatomy of memory retrieval. During each of three different word-stem completion scans, word-stem cues were visually presented in uppercase letters. The RECALL task required explicit retrieval of study words presented prior to the PET scan. The PRIMING task addressed the implicit effects of the prior study words without requiring intentional recall. The BASELINE task encouraged retrieval of information from a general knowledge store. Across experiments, the similarity between study words and word stems was manipulated by presenting prescan study words in either uppercase letters identical to the stems, in lowercase letters, or auditorily. The PRIMING task was not studied with auditory presentation. Many activations were consistent across experiments. The BASELINE task activated several regions in response to the reading and verbal-response demands of the task (visual, motor, and premotor cortices, cerebellum), as well as a left prefrontal region. The RECALL task additionally activated regions in anterior right prefrontal cortex. Bilateral occipitotemporal regions showed blood flow reductions during the PRIMING task as compared to the BASELINE task. Activation in the right hippocampal/parahippocampal region was observed only in one experiment, and no experiment showed activation in the left medial temporal lobe. These experiments suggest that areas of frontal cortex play a role in explicit recall and that an effect of priming may be to require less activation of perceptual regions for the processing of recently presented information.

Adult↗
PubMed1994

PET studies of parietal involvement in spatial attention: comparison of different task types.

Five experiments are described that concern the mechanisms that direct attention to spatial and non-spatial features of a stimulus and the effects that attention has on the visual system's analysis of that stimulus. Shifts of attention from one spatial location to another activated the superior parietal lobe and this activation was fairly independent of the task performed on the attended object, the response made to the attended object, and whether the shift of attention was controlled endogenously or exogenously. Maintaining attention tonically on a location or a particular visual feature such as shape, colour or motion did not produce a superior parietal response. Tonic attention to a feature (colour, shape, motion) or location, however, did produce enhancements in the response of various regions that are probably specialized for processing the attended visual feature. The activation of superior parietal cortex during shifts of spatial attention as well as the activation of parietal-occipital cortex when attention is tonically maintained on a location suggest that the parietal cortex plays an important role in spatial computations.

Animals↗
PubMed1993

A PET study of visuospatial attention.

Positron emission tomography (PET) was used to identify the neural systems involved in shifting spatial attention to visual stimuli in the left or right visual field along foveofugal or foveocentric directions. Psychophysical evidence indicated that stimuli at validly cued locations were responded to faster than stimuli at invalidly cued locations. Reaction times to invalid probes were faster when they were presented in the same than in the opposite direction of an ongoing attention movement. PET evidence indicated that superior parietal and superior frontal cortex were more active when attention was shifted to peripheral locations than when maintained at the center of gaze. Both regions encoded the visual field and not the direction of an attention shift. In the right superior parietal lobe, two distinct responses were localized for attention to left and right visual field. Finally, the superior parietal region was active when peripheral locations were selected on the basis of cognitive or sensory cues independent of the execution of an overt response. The frontal region was active only when responses were made to stimuli at selected peripheral locations. These findings indicate that parietal and frontal regions control different aspects of spatial selection. The functional asymmetry in superior parietal cortex may be relevant for the pathophysiology of unilateral neglect.

Adult↗
PubMed1992

Activation of the hippocampus in normal humans: a functional anatomical study of memory.

We studied regional cerebral blood flow using the H2(15)O method while normal subjects performed four similar tasks involving three-letter word beginnings (stems). Prior to each task, subjects studied a list of words. Local blood flow was then monitored during a 40-sec period while subjects (i) silently viewed word stems, (ii) completed stems to form the first words to come to mind, but the stems were not the beginnings of any study words (baseline), (iii) completed stems and half of them could form study words (priming), or (iv) tried to recall study words, and half of the stems could form these words (memory). There were three major findings. (i) The memory task engaged the right hippocampal region when the memory task was compared to either the baseline or the priming condition. The right hemispheric locus suggests that performance is driven by the visual characteristics of the words rather than by semantic or phonetic analysis. (ii) In the priming-minus-baseline comparison, there was reduction in blood flow in the right posterior cortex. (iii) Right prefrontal cortex was activated in the memory-minus-baseline condition. The results provide evidence for selective activation of the human hippocampal region in association with memory function. The results also lead to a suggestion about the neural basis of repetition priming: following presentation of a stimulus, less neural activity is required to process the same stimulus.

Adult↗
PubMed1991

Selective attention modulates extrastriate visual regions in humans during visual feature discrimination and recognition.

Positron emission tomography (PET) was used to identify regions of the human visual system which were selectively modulated by attention during feature discrimination and recognition tasks. In a first experiment, subjects were cued to the shape, colour or speed of visual stimulus arrays during a same-different match-to-sample paradigm. The psychophysical sensitivity for discriminating subtle attribute variations was enhanced by selective attention. Correspondingly, the neural activity (as measured by blood flow changes) in different visual associative regions was enhanced when subjects attended to different attributes of the same stimulus (intraparietal sulcus for speed; collateral sulcus and dorsolateral occipital cortex for colour; collateral sulcus, fusiform and parahippocampal gyri, superior temporal sulcus for shape). These regions appeared to be specialized for processing the selected attribute. Attention to a visual feature, therefore, enhances the psychophysical sensitivity as well as the neural activity of specialized processing regions of the human visual system. In a second experiment the effect of target probability (which biases attentional selection) was studied during visual search tasks involving the recognition of a single-feature (i.e. colour) or a feature-conjunction (i.e. colour x orientation) target. Target probability positively modulated neural activity of extrastriate visual regions, which were related to the single-feature or feature-conjunction processing level. These results suggest that selective attention can influence different processing levels in the visual system, possibly reflecting a facilitatory effect on different visual computations or task components.

Attention↗
PubMed1991

Selective and divided attention during visual discriminations of shape, color, and speed: functional anatomy by positron emission tomography.

Positron emission tomography (PET) was used to identify the neural systems involved in discriminating the shape, color, and speed of a visual stimulus under conditions of selective and divided attention. Psychophysical evidence indicated that the sensitivity for discriminating subtle stimulus changes in a same-different matching task was higher when subjects selectively attended to one attribute than when they divided attention among the attributes. PET measurements of brain activity indicated that modulations of extrastriate visual activity were primarily produced by task conditions of selective attention. Attention to speed activated a region in the left inferior parietal lobule. Attention to color activated a region in the collateral sulcus and dorsolateral occipital cortex, while attention to shape activated collateral sulcus (similarly to color), fusiform and parahippocampal gyri, and temporal cortex along the superior temporal sulcus. Outside the visual system, selective and divided attention activated nonoverlapping sets of brain regions. Selective conditions activated globus pallidus, caudate nucleus, lateral orbitofrontal cortex, posterior thalamus/colliculus, and insular-premotor regions, while the divided condition activated the anterior cingulate and dorsolateral prefrontal cortex. The results in the visual system demonstrate that selective attention to different features modulates activity in distinct regions of extrastriate cortex that appear to be specialized for processing the selected feature. The disjoint pattern of activations in extravisual brain regions during selective- and divided-attention conditions also suggests that preceptual judgements involve different neural systems, depending on attentional strategies.

Adult↗
PubMed1990

Attentional modulation of neural processing of shape, color, and velocity in humans.

Positron emission tomography (PET) was used to measure changes in regional cerebral blood flow of normal subjects, while they were discriminating different attributes (shape, color, and velocity) of the same set of visual stimuli. Psychophysical evidence indicated that the sensitivity for discriminating subtle stimulus changes was higher when subjects focused attention on one attribute than when they divided attention among several attributes. Correspondingly, attention enhanced the activity of different regions of extrastriate visual cortex that appear to be specialized for processing information related to the selected attribute.

Adult↗
PubMed1988

Transient and sustained responses in four extrastriate visual areas of the owl monkey.

Single neuron responses to stationary flashed bars were recorded from four extrastriate visual areas in the owl monkey: the middle temporal area (MT), the dorsal lateral area (DL), the dorsal medial area (DM), and the medial area (M). Data were collected at the optimum bar size and orientation for each cell. Each post-stimulus histogram was normalized to its maximum bin height. A cumulative histogram was produced for each area by adding together all the corresponding cell histograms. The cumulative histograms reveal a short latency, transient component and a longer latency, sustained component to the response for each of the areas. In all four areas there was a strong response, but the sustained component was much larger in DL and DM than in MT or M. The transient response in DL had a much longer latency than in the other areas. The dichotomy between areas which are slow-sustained responding and areas which are fast-transient responding is similar to the differences found between the magnocellular and parvocellular pathways.

Action Potentials↗
PubMed1987

Retinotopic organization of human visual cortex mapped with positron-emission tomography.

The retinotopic organization of primary visual cortex was mapped in normal human volunteers. Positron-emission tomographic measurements of regional cerebral blood flow were employed to detect focal functional brain activation. Oxygen-15-labeled water, delivered by intravenous bolus, was used as the blood flow tracer to allow multiple stimulated-state (n = 5) and control-state (n = 3) measurements to be acquired for each of 7 subjects. Responses were identified by applying a maximum-detection algorithm to subtraction-format images of the stimulus-induced change in cerebral blood flow. Response locales were described using a standardized system of stereotactic coordinates. Changes in stimulus location (macular, perimacular, peripheral, upper-field, lower-field) caused systematic, highly significant changes in response locale within visual cortex. Discrete extrastriate visual responses were also observed.

Adult↗