Okihide Hikosaka

Rostrocaudal gradients in the neuronal receptive field complexity in the finger region of the alert monkey's postcentral gyrus.

SummaryIn the primate postcentral gyrus, the cytoarchitectonic characteristics gradually shift from those of koniocortex to more homotypical parakoniocortex along its rostrocaudal axis. To find the physiological correlates of these changes we examined a large body of data accumulated during a series of our experiments with alert monkeys. Along the rostrocaudal axis of the postcentral gyrus, we found a gradual and continuous increase in the number of neurons with converging receptive fields and those in which receptive field positions or submodalities were not determined. Deep or skin submodality neurons were dominant in area 3a or 3b respectively. The proportion of skin submodality neurons decreased gradually from area 3b to the more caudal part of the gyrus. The proportion of deep submodality neurons was almost constant from area 3b to area 2 inclusive; they were not the majority in area 2. The data are consistent with the hierarchical scheme, i.e., within the postcentral gyrus sensory information is processed from the primary sensory receiving stage to the more associative, integrative stages.

Deficits in manipulative behaviors induced by local injections of muscimol in the first somatosensory cortex of the conscious monkey.

Injection of muscimol (GABA agonist) in the finger region of area 2 of the alert monkey led to the striking yet reversible loss of finger coordination, thus disabling the monkey from picking up a small piece of food from a small hole or a funnel. Symptoms varied depending on sites of injection within the finger region. Neither weakness of hand or finger movements nor deficit in hand reach was observed.

Functional subdivisions representing different finger regions in area 3 of the first somatosensory cortex of the conscious monkey

SummaryThe representation of fingers in the first somatosensory cortex was studied in conscious monkeys by recording single neuronal activity, and the following results were obtained:(1)In area 3a, most neurons responded to joint manipulation or other types of deep stimuli. The representation of five fingers was somatotopically arranged.(2)In area 3b, 77.7% and 20.9% of identified neurons responded to cutaneous and deep stimuli respectively.(3)Neurons responding to light mechanical stimuli and with receptive fields on the distal finger segment were found in the most anterior part of area 3b while those responding better to specific mechanical stimuli, such as rubbing, scraping, pinching, tapping, etc. of finger glabrous skin, were found in the more posterior part. The representation of the five fingers was somatotopically arranged.(4)Neurons responding to light or specific mechanical stimulation of the dorsal hairy skin of fingers were found in the posterior part of area 3b. The independent somatotopic representation of four fingers was recognized within this region.(5)Neurons responding to mechanical stimulation of the palmar skin were found in two separate regions, the medial one for the ulnar half and the lateral one for the radial half of the palm.(6)These results indicate that the representation of fingers in areas 3a and 3b of the conscious monkey is divided into multiple somatotopic subdivisions each representing a functional region of the hand and fingers.(7)Neurons with multi-finger receptive fields were occasionally found in area 3b, mostly in layer VI. Some of them had inhibitory receptive fields. Multifinger type receptive fields were more commonly found in area 1.

Converging patterns of finger representation and complex response properties of neurons in area 1 of the first somatosensory cortex of the conscious monkey

SummaryThe representation of the hand and fingers in area 1 of the first somatosensory cortex was studied in conscious monkeys by recording single neuronal activity. The results are as follows.(1)We found multi-finger type receptive fields which cover more than one finger discontinuously or wide-field type ones which cover both finger and palmar skin or two halves of the palmar skin together. Multi-finger type receptive fields were also found in some joint manipulation neurons. Multifinger or wide-field type receptive fields were found in nearly 40% of area 1 neurons. The rate was even higher, up to 70%, in the medial part of the cortical finger region. Consequently, the finger representation in area 1 was less discretely somatotopic than that in area 3b.(2)The submodality content of area 1 was almost identical to that of area 3b: 74.5% and 20.9% of identified neurons were, respectively, cutaneous and deep. The distribution of neurons with different submodalities overlapped in area 1.(3)Among area 1 neurons with multi-finger type receptive fields, response characteristics of those with inhibitory receptive fields, those with directional selectivity to moving stimuli, and those with converging afferent inputs, were studied in detail. Evidence is presented to suggest that information from different parts of the body, or from the same body parts but different afferent sources, is integrated in area 1.(4)It is proposed that, within the SI, area 1 is the initial stage of integration of sensory information coming from the thalamus and from area 3a or 3b via cortico-cortical connections.

Overlapping representation of fingers in the somatosensory cortex (area 2) of the conscious monkey

Finger representation on the first somatosensory cortex was mapped in conscious monkeys by single-unit recording. While representation was somatotopic in area 3, this was not the case in area 2, where the representation of different fingers overlapped. Our results indicate that area 2 is not a pure replica of the receptor sheet of the body surface.

Cell activity in monkey caudate nucleus preceding saccadic eye movements

SummarySingle cell activity was recorded in the monkey caudate nucleus. The monkey fixated a visual target, and, if the target jumped, followed it by making a saccade. A group of cells showed spike discharges before a contralateral saccade to the target. This activity was related to the saccade rather than the onset of the target, but was conditional in that it was unrelated to spontaneous saccades which were made without a particular target. Some caudate cells showed activity only when the saccade was made to a remembered position of a target. It is suggested that caudate cells contribute to the initiation of saccades in a selective manner by removing nigrocollicular tonic inhibition.

Vertical neuronal arrays in the postcentral gyrus signaling active touch: a receptive field study in the conscious monkey

SummarySingle neuronal activity was recorded in the crown of the postcentral gyrus (areas 1 and 2) in 5 conscious monkeys. A total of 88 penetrations entered perpendicularly to the cortical surface. Among them, 6 examples are chosen and the receptive field characteristics of constituent neurons were described in some detail. Receptive fields of neurons recorded along a particular penetration were variable in their positions, but the largest receptive field usually covered the others. Neurons with the largest receptive fields were found most frequently in the infragranular layers. Often they included inhibitory receptive fields. The inhibitory receptive fields were arranged side-by-side to the excitatory ones, rather than in a center-surround fashion. The key stimulus common to neurons in a vertical penetration was the contact of an object to the receptive field achieved during animals active behavior to manipulate the object. We thus designated the largest receptive field as a functional surface. Our results demonstrate that a vertical array of neurons in this cortical region can be regarded as a functional assemblage which deals with a set of information concerning one of various aspects of active touch.

Diversity in receptive field properties of vertical neuronal arrays in the crown of the postcentral gyrus of the conscious monkey.

SummarySingle neuronal activity was recorded in the crown of the postcentral gyrus (areas 1 and 2) in 5 conscious monkeys. A total of 93 penetrations were made in the hand and finger region of 9 hemispheres and 827 neurons were isolated. The receptive field characteristics of neurons recorded along each of 88 penetrations which entered perpendicularly to the cortical surface were compared. The majority of neurons in this region were responsive to skin stimulation. In 54 penetrations, neurons related to different sensory submodalities were mixed. In 30, skin neurons predominated, and in 8, ‘deep’ neurons, while in the remaining 16 penetrations neurons related to different submodalities were equally mixed. In 16 penetrations, neurons responded exclusively to stimulation of skin, hair or nails. In 9 penetrations, neurons were exclusively related to joint manipulation or other types of ‘deep’ submodality. In 9 penetrations, unidentified neurons were in the majority. In each penetration, the receptive field positions varied considerably on the same finger or encompassed more than one finger. Although neurons of the same submodality, either skin or ‘deep’ tended to be set in an array, the most adequate stimulus could vary among neurons of a given array. The variability in the receptive field positions or the most adequate stimuli remained constant irrespective of the angle of the electrode penetration in the cortex. The results are compatible with the idea that vertically arranged neuronal array receive inputs of multiple sources, both thalamacortical and corticocortical, so that interactions between different inputs can readilly occur.

Eye movements induced by microinjection of GABA agonist in the rat substantia nigra pars reticulata

Injection of muscimol (GABA agonist) in the substantia nigra pars reticulata (SNr) of the alert rat induced a continual repetition of fast eye movements to the contralateral side, each of which was followed by a slow returning movement. The fast eye movements were similar to spontaneous saccades. Larger saccades were accompanied by contralateral neck muscle activity. We suggest that the SNr plays an important role in control of eye movements in the rat.

Role of basal ganglia in initiation of voluntary movements

A motor system called the basal ganglia facilitates movement initiation by removing its powerful inhibition on other motor areas. It may also facilitate activity in the cerebral cortex with disinhibition and ensure sequential processing of motor signals.