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Biomedical subjects

T A Hackett

Publications and source records attributed to T A Hackett.

12 recordsLinked to original sources
PubMed2001

Architectonic identification of the core region in auditory cortex of macaques, chimpanzees, and humans.

The goal of the present study was to determine whether the architectonic criteria used to identify the core region in macaque monkeys (Macaca mulatta, M. nemestrina) could be used to identify a homologous region in chimpanzees (Pan troglodytes) and humans (Homo sapiens). Current models of auditory cortical organization in primates describe a centrally located core region containing two or three subdivisions including the primary auditory area (AI), a surrounding belt of cortex with perhaps seven divisions, and a lateral parabelt region comprised of at least two fields. In monkeys the core region can be identified on the basis of specific anatomical and physiological features. In this study, the core was identified from serial sets of adjacent sections processed for cytoarchitecture, myeloarchitecture, acetylcholinesterase, and cytochrome oxidase. Qualitative and quantitative criteria were used to identify the borders of the core region in individual sections. Serial reconstructions of each brain were made showing the location of the core with respect to gross anatomical landmarks. The position of the core with respect to major sulci and gyri in the superior temporal region varied most in the chimpanzee and human specimens. Although the architectonic appearance of the core areas did vary in certain respects across taxonomic groups, the numerous similarities made it possible to identify unambiguously a homologous cortical region in macaques, chimpanzees, and humans.

Acetylcholinesterase↗
PubMed2001

Sensory enrichment after peripheral nerve injury restores cortical, not thalamic, receptive field organization.

Sensory perception can be severely degraded after peripheral injuries that disrupt the functional organization of the sensory maps in somatosensory cortex, even after nerve regeneration has occurred. Rehabilitation involving sensory retraining can improve perceptual function, presumably through plasticity mechanisms in the somatosensory processing network. However, virtually nothing is known about the effects of rehabilitation strategies on brain organization, or where the effects are mediated. In this study, five macaque monkeys received months of enriched sensory experience after median nerve cut and repair early in life. Subsequently, the sensory representation of the hand in primary somatosensory cortex was mapped using multiunit microelectrodes. Additionally, the primary somatosensory relay in the thalamus, the ventroposterior nucleus, was studied to determine whether the effects of the enrichment were initiated subcortically or cortically. Age-matched controls included six monkeys with no sensory manipulation after median nerve cut and regeneration, and one monkey that had restricted sensory experience after the injury. The most substantial effect of the sensory environment was on receptive field sizes in cortical area 3b. Significantly greater proportions of cortical receptive fields in the enriched monkeys were small and well localized compared to the controls, which showed higher proportions of abnormally large or disorganized fields. The refinements in receptive field size and extent in somatosensory cortex likely provide better resolution in the sensory map and may explain the improved functional outcomes after rehabilitation in humans.

Animals↗
PubMed2000

Subdivisions of auditory cortex and processing streams in primates.

The auditory system of monkeys includes a large number of interconnected subcortical nuclei and cortical areas. At subcortical levels, the structural components of the auditory system of monkeys resemble those of nonprimates, but the organization at cortical levels is different. In monkeys, the ventral nucleus of the medial geniculate complex projects in parallel to a core of three primary-like auditory areas, AI, R, and RT, constituting the first stage of cortical processing. These areas interconnect and project to the homotopic and other locations in the opposite cerebral hemisphere and to a surrounding array of eight proposed belt areas as a second stage of cortical processing. The belt areas in turn project in overlapping patterns to a lateral parabelt region with at least rostral and caudal subdivisions as a third stage of cortical processing. The divisions of the parabelt distribute to adjoining auditory and multimodal regions of the temporal lobe and to four functionally distinct regions of the frontal lobe. Histochemically, chimpanzees and humans have an auditory core that closely resembles that of monkeys. The challenge for future researchers is to understand how this complex system in monkeys analyzes and utilizes auditory information.

Animals↗
PubMed2000

Thalamic and cortical contributions to neural plasticity after limb amputation.

Little is known about the substrates for the large-scale shifts in the cortical representation produced by limb amputation. Subcortical changes likely contribute to the cortical remodeling, yet there is little data regarding the extent and pattern of reorganization in thalamus after such a massive deafferentation. Moreover, the relationship between changes in thalamus and in cortex after injuries of this nature is virtually unexplored. Multiunit microelectrode maps were made in the somatosensory thalamus and cortex of two monkeys that had long-standing, accidental forelimb amputations. In the deprived portion of the ventroposterior nucleus of the thalamus (VP), where stimulation to the hand would normally activate neurons, new receptive fields had emerged. At some recording sites within the deprived zone of VP, neurons responded to stimulation of the remaining stump of the arm and at other sites neurons responded to stimulation of both the stump and the face. This same overall pattern of reorganization was present in the deprived hand representation of cortical area 3b. Thus thalamic changes produced by limb amputation appear to be an important substrate of cortical reorganization. However, a decrease in the frequency of abnormal stump/face fields in area 3b compared with VP and a reduction in the size of the fields suggests that cortical mechanisms of plasticity may refine the information relayed from thalamus.

Amputation, Surgical↗
PubMed1999

Prefrontal connections of the parabelt auditory cortex in macaque monkeys.

In the present study, we determined connections of three newly defined regions of auditory cortex with regions of the frontal lobe, and how two of these regions in the frontal lobe interconnect and connect to other portions of frontal cortex and the temporal lobe in macaque monkeys. We conceptualize auditory cortex as including a core of primary areas, a surrounding belt of auditory areas, a lateral parabelt of two divisions, and adjoining regions of temporal cortex with parabelt connections. Injections of several different fluorescent tracers and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were placed in caudal (CPB) and rostral (RPB) divisions of the parabelt, and in cortex of the superior temporal gyrus rostral to the parabelt with parabelt connections (STGr). Injections were also placed in two regions of the frontal lobe that were labeled by a parabelt injection in the same case. The results lead to several major conclusions. First, CPB injections label many neurons in dorsal prearcuate cortex in the region of the frontal eye field and neurons in dorsal prefrontal cortex of the principal sulcus, but few or no neurons in orbitofrontal cortex. Fine-grain label in these same regions as a result of a WGA-HRP injection suggests that the connections are reciprocal. Second, RPB injections label overlapping prearcuate and principal sulcus locations, as well as more rostral cortex of the principal sulcus, and several locations in orbitofrontal cortex. Third, STGr injections label locations in orbitofrontal cortex, some of which overlap those of RPB injections, but not prearcuate or principal sulcus locations. Fourth, injections in prearcuate and principal sulcus locations labeled by a CPB injection labeled neurons in CPB and RPB, with little involvement of the auditory belt and no involvement of the core. In addition, the results indicated that the two frontal lobe regions are densely interconnected. They also connect with largely separate regions of the frontal pole and more medial premotor and dorsal prefrontal cortex, but not with the extensive orbitofrontal region which has RPB and STGr connections. The results suggest that both RPB and CPB provide the major auditory connections with the region related to directing eye movements towards stimuli of interest, and the dorsal prefrontal cortex for working memory. Other auditory connections to these regions of the frontal lobe appear to be minor. RPB has connections with orbitofrontal cortex, important in psychosocial and emotional functions, while STGr primarily connects with orbital and polar prefrontal cortex.

Animals↗
PubMed1999

Auditory processing in primate cerebral cortex.

Auditory information is relayed from the ventral nucleus of the medial geniculate complex to a core of three primary or primary-like areas of auditory cortex that are cochleotopically organized and highly responsive to pure tones. Auditory information is then distributed from the core areas to a surrounding belt of about seven areas that are less precisely cochleotopic and generally more responsive to complex stimuli than tones. Recent studies indicate that the belt areas relay to the rostral and caudal divisions of a parabelt region at a third level of processing in the cortex lateral to the belt. The parabelt and belt regions have additional inputs from dorsal and magnocellular divisions of the medial geniculate complex and other parts of the thalamus. The belt and parabelt regions appear to be concerned with integrative and associative functions involved in pattern perception and object recognition. The parabelt fields connect with regions of temporal, parietal, and frontal cortex that mediate additional auditory functions, including space perception and auditory memory.

Acoustic Stimulation↗
PubMed1999

Callosal connections of the parabelt auditory cortex in macaque monkeys.

Auditory cortex of macaque monkeys is located on the lower bank of the lateral sulcus and the adjoining superior temporal gyrus. This region of cortex contains a core of primary-like areas surrounded by a narrow belt of associated fields. Adjacent to the lateral belt on the superior temporal gyrus is a parabelt region which contains at least two subdivisions (rostral and caudal). In previous studies we defined the parabelt region as cortex with topographic cortical connections with the belt areas surrounding the core, and connections with the dorsal and magnocellular divisions of the medial geniculate complex, but minimal connections with the core region and ventral division of the medial geniculate complex. The callosal connections of the parabelt auditory cortex were determined by placing injections, of up to six distinguishable tracers, into different locations of the parabelt region in each of four macaque monkeys. The results indicated that the strongest callosal projections arise from homotopic areas in parabelt cortex, and they roughly matched the rostrocaudal levels of the medial and lateral belt cortex. Weaker callosal inputs to the parabelt originate from the corresponding levels of the superior temporal gyrus and superior temporal sulcus. The core region does not contribute significant callosal projections to the parabelt region. The results provide further support for the conclusion that the parabelt region represents a third level of auditory cortical processing beyond direct activation by primary subcortical and cortical auditory structures.

Animals↗
PubMed1998

Thalamocortical connections of the parabelt auditory cortex in macaque monkeys.

The auditory cortex of macaque monkeys contains a core of primary-like areas surrounded by a narrow belt of associated fields that encompass much of the superior temporal plane in these animals. Adjacent to the lateral belt on the superior temporal gyrus is a parabelt region that contains at least two subdivisions (rostral and caudal). In a previous study (Hackett et al. [1998] J. Comp. Neurol. 394:475-495), we determined that the parabelt has topographic connections with the belt areas surrounding the core, but minimal connections with the core itself. In this study, we describe the thalamocortical connections of the parabelt auditory cortex based on multiple injections of neuronal tracers into this region in each of five macaque monkeys. Injections confined to the parabelt labeled large numbers of neurons in the dorsal (MGd) and magnocellular (MGm) divisions of the medial geniculate complex (MGC), suprageniculate (Sg), limitans (Lim), and medial pulvinar (PM) nuclei. Only when injections encroached on the lateral belt cortex were substantial numbers of labeled neurons found in the ventral (MGv) division of the MGC, consistent with the absence of significant connections between the parabelt and core fields. The rostrocaudal topography of the parabelt region was maintained in the thalamocortical connections, supporting the parcellation of this region of cortex. The results suggest that the parabelt region represents a third level of auditory cortical processing, which is not influenced by direct inputs from primary cortical or subcortical auditory structures.

Animals↗
PubMed1998

Subdivisions of auditory cortex and ipsilateral cortical connections of the parabelt auditory cortex in macaque monkeys.

Auditory cortex of macaque monkeys can be divided into a core of primary or primary-like areas located on the lower bank of the lateral sulcus, a surrounding narrow belt of associated fields, and a parabelt region just lateral to the belt on the superior temporal gyrus. We determined patterns of ipsilateral cortical connections of the parabelt region by placing injections of four to seven distinguishable tracers in each of five monkeys. Results were related to architectonic subdivisions of auditory cortex in brain sections cut parallel to the surface of artificially flattened cortex (four cases) or cut in the coronal plane (one case). An auditory core was clearly apparent in these sections as a 16- to 20-mm rostrocaudally elongated oval, several millimeters from the lip of the sulcus, that stained darkly for parvalbumin, myelin, and acetylcholinesterase. These features were most pronounced caudally in the cortex assigned to auditory area I, only slightly reduced in the rostral area, and most reduced in the narrower rostral extension we define as the rostrotemporal area. A narrow band of cortex surrounding the core stained more moderately for parvalbumin, acetylcholinesterase, and myelin. Two regions of the caudal belt, the caudomedial area, and the mediolateral area, stained more darkly, especially for parvalbumin. Rostromedial and medial rostrotemporal, regions of the medial belt stained more lightly for parvalbumin than the caudomedial area or the lateral belt. The parabelt region stained less darkly than the core and belt fields. Injections confined to the parabelt region labeled few neurons in the core, but large numbers in parts of the belt, the parabelt, and adjacent portions of the temporal lobe. Injections that encroached on the belt labeled large numbers of neurons in the core and helped define the width of the belt. Caudal injections in the parabelt labeled caudal portions of the belt, rostral injections labeled rostral portions, and both caudal and rostral injections labeled neurons in the rostromedial area of the medial belt. These observations support the concept of dividing the auditory cortex into core, belt, and parabelt; provide evidence for including the rostral area in the core; suggest the existence of as many as seven or eight belt fields; provide evidence for at least two subdivisions of the parabelt; and identify regions of the temporal lobe involved in auditory processing.

Animals↗
PubMedDate not supplied

Subdivisions of auditory cortex and levels of processing in primates.

In a series of experiments on New World and Old World monkeys, architectonic features of auditory cortex were related to tone frequency maps and patterns of connections to generate and evaluate theories of cortical organization. The results suggest that cortical processing of auditory information involves a number of functionally distinct fields that can be broadly grouped into four or more levels of processing. At the first level, there are three primary-like areas, each with a discrete pattern of tonotopic organization, koniocortical histological features, and direct inputs from the ventral division of the medial geniculate complex. These three core areas are interconnected and project to a narrow surrounding belt of perhaps seven areas which receive thalamic input from the major divisions of the medial geniculate complex, the suprageniculate/limitans complex, and the medial pulvinar. The belt areas connect with a lateral parabelt region of two or more fields that are almost devoid of direct connections with the core and the ventral division of the medial geniculate complex. The parabelt fields connect with more distant cortex in the superior temporal gyrus, superior temporal sulcus, and prefrontal cortex. The results indicate that auditory processing involves 15 or more cortical areas, each of which is interconnected with a number of other fields, especially adjoining fields of the same level.

Animals↗