Microelectrode recordings were used to investigate the tonotopic organization of auditory cortex of macaque monkeys and guide the placement of injections of wheat germ agglutinin-horse radish peroxidase (WGA-HRP) and fluorescent dyes. Anatomical and physiological results were later related to histological distinctions in the same brains after sections were processed for cytoarchitecture, myeloarchitecture, acetylcholinesterase (AchE), or cytochrome oxidase (CO). The experiments produced several major findings. (1) Neurons throughout a broad expanse of cortex were highly responsive to pure tones, and best frequencies could be determined for neurons in arrays of recording sites. (2) The microelectrode recordings revealed two systematic representations of tone frequencies, the primary area (AI) and a primary-like rostral field (R) as previously described. The representation of high to low frequency tones in A1 was largely caudorostral along the plane of the sulcus. A reversal of the order of representation of frequencies occurred in R. (3) AI and R together were coextensive with a koniocellular, densely myelinated zone that expressed high levels of AchE and CO. These architectonic features were somewhat less pronounced in R than AI, but a clear border between the two areas was not apparent. (4) Cortex bordering AI and R was less responsive to tones, but when best frequencies for neurons could be determined, they matched those for adjoining parts of AI and R. (5) Architectonically distinct regions were apparent within some of the cortex bordering AI and R. (6) The major ipsilateral cortical connections of AI were with R and cortex immediately lateral and medial to AI. (7) Callosal connections of AI were predominantly with matched locations in the opposite AI, but they also included adjoining fields. (8) Neurons in the ventral (MGV), medial (MGM), and dorsal (MGD) nuclei of the medial geniculate complex projected to AI and cortex lateral to AI. (9) Injections in cortex responsive to high frequency tones labeled more dorsal parts of MGV than injections in cortex responsive to low frequency tones.

After limited sensory deafferentations in adult primates, somatosensory cortical maps reorganize over a distance of 1 to 2 millimeters mediolaterally, that is, in the dimension along which different body parts are represented. This amount of reorganization was considered to be an upper limit imposed by the size of the projection zones of individual thalamocortical axons, which typically also extend a mediolateral distance of 1 to 2 millimeters. However, after extensive long-term deafferentations in adult primates, changes in cortical maps were found to be an order of magnitude greater than those previously described. These results show the need for a reevaluation of both the upper limit of cortical reorganization in adult primates and the mechanisms responsible for it.

The prevailing hierarchical model of cortical sensory processing holds that early processing is specific to individual modalities and that combination of information from different modalities is deferred until higher-order stages of processing. In this paper, we present physiological evidence of multisensory convergence at an early stage of cortical auditory processing. We used multi-neuron cluster recordings, along with a limited sample of single-unit recordings, to determine whether neurons in the macaque auditory cortex respond to cutaneous stimulation. We found coextensive cutaneous and auditory responses in caudomedial auditory cortex, an area lying adjacent to A1, and at the second stage of the auditory cortical hierarchy. Somatosensory-auditory convergence in auditory cortex may underlie effects observed in human studies. Convergence of inputs from different sensory modalities at very early stages of cortical sensory processing has important implications for both our developing understanding of multisensory processing and established views of unisensory processing.

Retinal cells have been induced to project into the medial geniculate nucleus, the principal auditory thalamic nucleus, in newborn ferrets by reduction of targets of retinal axons in one hemisphere and creation of alternative terminal space for these fibers in the auditory thalamus. Many cells in the medial geniculate nucleus are then visually driven, have large receptive fields, and receive input from retinal ganglion cells with small somata and slow conduction velocities. Visual cells with long conduction latencies and large contralateral receptive fields can also be recorded in primary auditory cortex. Some visual cells in auditory cortex are direction selective or have oriented receptive fields that resemble those of complex cells in primary visual cortex. Thus, functional visual projections can be routed into nonvisual structures in higher mammals, suggesting that the modality of a sensory thalamic nucleus or cortical area may be specified by its inputs during development.

Area 2 is a traditional architectonic subdivision of anterior parietal cortex in macaque monkeys, but its overall somatotopic organization and responsiveness to different types of somatic stimuli are poorly understood, and there are uncertainties concerning its rostral and caudal extent. The goals of the present study were to define the rostral and caudal borders of area 2 better, and to describe its overall organization and responsiveness. Somatic receptive fields were defined for hundreds of closely spaced microelectrode recording sites in postcentral parietal cortex of individual macaque monkeys anesthetized with ketamine. Electrophysiological and architectonic evidence suggested that a 3-4 mm-wide strip of cortex along the caudal border of area 1 includes all or most of area 2. The most lateral explored portion of area 2 adjoined the representation of the face in area 1. Much of this sector of area 2 was activated by cutaneous stimulation of the face, especially the chin, but more caudal parts of the head also were represented there. Medially, an adjacent sector of area 2 represented the hand. Rostrally, in the cortex within 1.5 mm from the area 1 border, the glabrous surfaces of digits 5 through 1 were represented in a mediolateral cortical sequence, and from tip to base in a rostrocaudal sequence, mirroring the organization in the adjacent portion of area 1. More caudally at this mediolateral level of area 2, digit tips and other phalanges were represented for a second time. The pads of the palm and the dorsal surfaces of the hand were represented laterally and medially within the portion of area 2 devoted to the hand. More medially, the wrist, forearm, and arm were represented in a lateromedial cortical sequence in area 2, roughly matching the mediolateral organization within the bordering area 1. However, immediately caudal to the representation of the occiput, neck, and shoulder in area 1, a rostrocaudal strip of cortex extending across area 2 represented the arm and forearm for a second time in area 2. This cutaneously activated strip of cortex extended into area 5, where the proximal portion of the hand was represented. More medially, next to the trunk representation in area 1, area 2 was devoted to the trunk and limbs. Next to the representations of the ankle, leg, and thigh in area 1, area 2 was activated from similar locations on the hindlimb.(ABSTRACT TRUNCATED AT 400 WORDS)