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Haptic information is critically important in complex sensory-motor tasks such as manipulating objects. Its comparable importance in spatial orientation is only beginning to be recognized. We have shown that postural sway in humans is significantly reduced by lightly touching a stable surface with a fingertip at contact force levels far below those physically necessary to stabilize the body. To investigate further the functional relationship between contact forces at the hand and postural equilibrium, we had subjects stand in the tandem Romberg stance while being allowed physically supportive (force contact) and non-physically supportive (touch contact) amounts of index fingertip force on surfaces with different frictional characteristics. Mean sway amplitude (MSA) was reduced by over 50% with both touch and force contact of the fingertip, compared to standing without fingertip contact. No differences in MSA were observed when touching rough or slippery surfaces. The amplitude of EMG activity in the peroneal muscles and the timing relationships between fingertip forces, body sway and EMG activity suggested that with touch contact of the finger or with force contact on a slippery surface, long-loop “reflexes” involving postural muscles were stabilizing sway. With force contact of the fingertip on a rough surface, MSA reduction was achieved primarily through physical support of the body. This pattern of results indicates that light touch contact cues from the fingertip in conjunction with proprioceptive signals about arm configuration are providing information about body sway that can be used to reduce MSA through postural muscle activation.

The tonotopicity of the cat's primary auditory cortex (AI) is thought to provide the framework for frequency-specific processing in that field. This study was designed to assess this postulate by examining the spatial distribution of neurons within AI that are activated by a single tonal frequency delivered to the contralateral ear. Distributions obtained at each of several stimulus levels were then compared to assess the influence of stimulus amplitude on the spatial representation of a given stimulus frequency in AI. Data were obtained from 308 single units in AI of four adult, barbiturate-anesthetized cats, using extracellular recording methods. Stimuli were 40-ms tone pulses presented through calibrated, sealed stimulating systems. In each animal, the CF (stimulus frequency to which the unit is most sensitive), threshold at CF, response/level function at CF, and binaural interactions were determined for isolated neurons (usually one per track) in 60–90 electrode tracks. For each unit, regardless of its CF, responses to 40 repetitions of contralateral tones of a single frequency, presented at each of four or five sound pressure levels (SPLs) in the range from 10 to 80 dB were obtained. Different test frequencies were used in each of four cats (1.6, 8.0, 11.0, and 16.0 kHz). For tones of each SPL, we generated maps of the response rates across the cortical surface. These maps were then superimposed on the more traditional maps of threshold CF. All units whose CF was equal to the test frequency could be driven at some SPL, given an appropriate monaural or binaural configuration of the stimulus. There was a clear spatial segregation of neurons according to the shapes of their CF tone response/level functions. Patches of cortex, often occupying more than 2 mm2, seemed to contain only monotonic or only nonmonotonic units. In three cortices, a patch of nonmonotonic cells was bounded ventrally by a patch of monotonie cells, and in one of these cases, a second patch of monotonic cells was found dorsal to the nonmonotonic patch. Contralateral tones of any given SPL evoked excitatory responses in discontinuous cortical territories. At low SPLs (10, 20 dB), small foci of activity occurred along the isofrequency line representing the test frequency. Many of these cells had nonmonotonic response/level functions. At mid- and high SPLs, the CFs of neurons activated by a pure tone varied across 3 octaves. At the highest SPL used (80 dB), most of the neurons with nonmonotonic response/level functions were inactive, or responded poorly; the active neurons were widely spread across the cortex, and the distribution of activity had a pattern bearing little relationship to the threshold CF contour map. These data indicate that only isolated patches of units within the relevant isofrequency contour are activated by a given suprathreshold contralateral tone. At suprathreshold stimulus levels, the region of cortex containing active patches extends widely beyond the threshold isofrequency contour region corresponding to the test stimulus frequency. The spatial representation of a stimulus delivered to the contralateral ear appears, therefore, to be highly level dependent and discontinuous. These observations suggest that in the cat's AI, tonotopicity and isofrequency contours are abstractions which bear little resemblance to the spatial representation of tonal signals.

The role of binocular vision in grasping has frequently been assessed by measuring the effects on grasp kinematics of covering one eye. These studies have typically used three or fewer objects presented at three or fewer distances, raising the possibility that participants learn the properties of the stimulus set. If so, even relatively poor visual information may be sufficient to identify which object/distance configuration is presented on a given trial, in effect providing an additional source of depth information. Here we show that the availability of this uncontrolled cue leads to an underestimate of the effects of removing binocular information, and therefore to an overestimate of the effectiveness of the remaining cues. We measured the effects of removing binocular cues on visually open-loop grasps using (1) a conventional small stimulus-set, and (2) a large, pseudo-randomised stimulus set, which could not be learned. Removing binocular cues resulted in a significant change in grip aperture scaling in both conditions: peak grip apertures were larger (when reaching to small objects), and scaled less with increases in object size. However, this effect was significantly larger with the randomised stimulus set. These results confirm that binocular information makes a significant contribution to grasp planning. Moreover, they suggest that learned stimulus information can contribute to grasping in typical experiments, and so the contribution of information from binocular vision (and from other depth cues) may not have been measured accurately.

Maturation and differentiation of electrical properties of neurons and synaptic transmission are modulated by neuronal interaction. In vitro experiments have shown that these processes also seem to be regulated by signals from non-neuronal elements such as glial cells. It is not known, however, whether glial alterations in intact neural networks may also affect the maturation of electrical properties and synaptic transmission during development. We used the taiep rat, a neurological mutant with a progressive demyelination and astrogliosis, as an experimental model to study the postnatal development of motoneurons in an altered glial environment. Using the patch-clamp technique, we made intracellular recording from motoneurons of Rexed’s lamina IX in spinal cord slices of neonatal rats (postnatal day P4–P10). The electrical properties of normal motoneurons changed significantly with age, showing decreasing input resistance (Rin) and increasing membrane capacity (Cm). The rheobase increased with age, accompanied by an increase of the amplitude and a decrease of the duration of action potentials (APs). In contrast, mutant neurons showed no age-dependent changes of Rin, Cm, or AP characteristics. After blocking inhibitory transmission, intralaminar bipolar stimulation elicited, in both control and taiep motoneurons, fast glutamatergic excitatory postsynaptic potentials (EPSPs). Two types of taiep motoneurons were identified according to the temporal patterns of synaptic responses; (1) taiep SYN neurons, which showed no significant differences to control motoneurons, and (2) taiep ASYN neurons, in which the initial EPSP was followed by a variable number of delayed, asynchronous EPSP responses (for up to 300 ms). All these electrophysiological findings suggest that the mutation in taiep rats interfered with the development of the electrical properties of neurons and with the maturation of synaptic transmission, probably due to alterations in the neuron-glia interactions.

Muscle tendon vibration was applied during voluntary step-tracking arm target-movements performed by normal human subjects. Vibration (freq. = 120 Hz) was applied over either the biceps or triceps tendons. During non-visually guided (eyes closed) trials, vibration of the muscle antagonistic to the movement being performed resulted in an undershoot of the required target. Thus, biceps vibration produced an undershoot of the extension target and triceps vibration an undershoot of the flexion target. The same effect occurred if the vibration was applied continuously over several movements or only during the course of individual movements. In contrast, vibration of the muscle acting as the prime mover had no effect on the correct attainment of the required target. It is suggested that the central nervous system may monitor muscle afferent activity of the lengthening (antagonist) muscle during simple, step movements.

Reflexive attentional shift in response to another individual’s gaze direction has been reported, but it remains unknown whether this process can occur subliminally. We investigated this issue using facial stimuli consisting of drawings (Experiment 1) and photographs (Experiment 2). The gaze direction was expressed by the eye gaze direction (Experiment 1), and the eye gaze and head direction (Experiment 2). The gaze cue was presented either supraliminally or subliminally in the center of the visual field, before target presentation in the periphery. The task for participants was to localize the target as soon as possible. The reaction time needed to localize the target was consistently shorter for valid than invalid gaze cues for both types of gaze cues in both subliminal and supraliminal conditions. These findings indicate that attentional shift can be triggered even without awareness in response to another individual’s eye gaze or head direction.

The activity of 249 neurons in the dorsomedial frontal cortex was studied in two macaque monkeys. The animals were trained to release a bar when a visual stimulus changed color in order to receive reward. An acoustic cue signaled the start of a series of trials to the animal, which was then free to begin each trial at will. The monkeys tended to fixate the visual stimuli and to make saccades when the stimuli moved. The monkeys were neither rewarded for making proper eye movements nor punished for making extraneous ones. We found neurons whose discharge was related to various movements including those of the eye, neck, and arm. In this report, we describe the properties of neurons that showed activity related to visual fixation and saccadic eye movement. Fixation neurons discharged during active fixation with the eye in a given position in the orbit, but did not discharge when the eye occupied the same orbital positions during nonactive fixation. These neurons showed neither a classic nor a complex visual receptive field, nor a foveal receptive visual field. Electrical stimulation at the site of the fixation neurons often drove the eye to the orbital position associated with maximal activity of the cell. Several different kinds of neurons were found to discharge before saccades: 1) checking-saccade neurons, which discharged when the monkeys made self-generated saccades to extinguish LED's; 2) novelty-detection saccade neurons, which discharged before the first saccade made to a new visual target but whose activity waned with successive presentations of the same target. These results suggest that the dorsomedial frontal cortex is involved in attentive fixation. We hypothesize that the fixation neurons may be involved in codifying the saccade toward a target. We propose that their involvement in arm-eye-head motor-planning rests primarily in targeting the goal of the movement. The fact that saccaderelated neurons discharge when the saccades are self initiated, implies that this area of the cortex may share the control of voluntary saccades with the frontal eye fields and that the activation is involved in intentional motor processes.

 Developmental changes in the effects of quadriceps (Q) nerve stimulation on the locomotor rhythm induced by a mixture of N-methyl-d-aspartic acid and 5-hydroxytryptamine were examined using in vitro preparations from neonatal rats at postnatal days (P) 1–6. The effects of such stimulation on the rhythm were dependent both on stimulus strength and on the age of the animal. Low-intensity stimulation (≤3.0×T, where T=threshold for the monosynaptic reflex) during the flexor phase reset the rhythm via a prolongation of the flexor burst in most rats at P1–3, but via flexor burst truncation at P4–6. At any age, low-intensity stimulation during the extensor phase had no consistent effect on the ongoing rhythm. Activation of muscle afferents evoked via isometric contraction of the Q muscle caused effects similar to those obtained on low-intensity electrical stimulation in all age groups. In all age groups, high-intensity stimulation (≥5.0×T) caused resetting when delivered during the flexor phase via a prolongation of the flexor burst and during the extensor phase via a truncation of the extensor burst. These results suggest that the type of resetting evoked from low-threshold muscle afferents changes drastically during postnatal week1, while effects evoked from high-threshold afferents remain unchanged.