Journal of neurophysiology
-
Saccadic eye movements are thought to be influenced by blinking through premotor interactions, but it is still unclear how. The present paper describes the properties of blink-associated eye movements and quantifies the effect of reflex blinks on the latencies, metrics, and kinematics of saccades in the monkey. In particular, it is examined to what extent the saccadic system accounts for blink-related perturbations of the saccade trajectory. ⋯ These data strongly support the idea that blinks interfere with the saccade premotor circuit, presumably upstream from the neural eye-position integrator. They also indicated that a neural mechanism, rather than passive elastic restoring forces within the oculomotor plant, underlies the compensatory behavior. The tight latency coupling between saccades and blinks is consistent with an inhibition of omnipause neurons by the blink system, suggesting that the observed changes in saccade kinematics arise elsewhere in the saccadic premotor system.
-
The excitatory amino acid receptor (EAAR) types involved in the generation of light-evoked excitatory postsynaptic currents (EPSCs) were examined in X-type retinal ganglion cells. Using isolated and sliced preparations of cat and ferret retina, the light-evoked EPSCs of X cells were isolated by adding picrotoxin and strychnine to the bath to remove synaptic inhibition. N-methyl-D-aspartate (NMDA) receptors contribute significantly to the light-evoked EPSCs of ON- and OFF-X cells at many different holding potentials. ⋯ Experiments using selective EAAR modulators showed the AMPA receptor-selective modulator cyclothiazide potentiated glutamate-evoked currents on X cells, while the kainate receptor-selective modulator concanavalin A (ConA) had no effect on kainate-evoked currents. Whereas the present study confirms the general notion that AMPA EAAR-mediated currents are transient and NMDA receptor-mediated currents are sustained, current-voltage relations of the light-evoked EPSC at different time points showed the contributions of these two receptor types significantly overlap. Both NMDA and AMPA EAARs can transmit transient and sustained visual signals in X ganglion cells, suggesting that much signal shaping occurs presynaptically in bipolar cells.
-
Comparative Study Clinical Trial
Cortical responses to thermal pain depend on stimulus size: a functional MRI study.
Cortical activity patterns to thermal painful stimuli of two different sizes were examined in normal volunteers using functional magnetic resonance imaging (fMRI). Seven right-handed subjects were studied when the painful stimulus applied to the right hand fingers covered either 1,074-mm(2)-area large stimulator or 21-mm(2)-area small stimulator. Stimulus temperatures were adjusted to give rise to equivalent moderately painful ratings. fMRI signal increases and decreases were determined for the contralateral parietal and motor areas. ⋯ Painful stimuli were delivered to the fingers and vibrotactile thresholds were measured on the arm just distal to the elbow. Consistent with the fMRI results in the primary somatosensory cortex, painful thermal stimuli using the small stimulator increased vibrotactile thresholds on the forearm, whereas similarly painful stimuli using the large stimulator had no effect on forearm vibrotactile thresholds. These results are discussed in relation to the cortical dynamics for pain perception and in relation to the center-surround organization of cortical neurons.
-
In addition to maintaining the extracellular glutamate concentration at low ambient levels, high-affinity glutamate transporters play a direct role in synaptic transmission by speeding the clearance of glutamate from the synaptic cleft and limiting the extent to which transmitter spills over between synapses. Transporters are expressed in both neurons and glia, but glial transporters are likely to play the major role in removing synaptically released glutamate from the extracellular space. The role of transporters in synaptic transmission has been studied directly by measuring synaptically activated, transporter-mediated currents (STCs) in neurons and astrocytes. ⋯ We show that, at near-physiological temperatures (34 degrees C), high-frequency stimulation (HFS) does not affect the rate at which transporters clear glutamate from the extrasynaptic space. Thus, although spillover between synapses during "normal" stimulation may compromise the absolute synapse specificity of fast excitatory synaptic transmission, spillover is not exacerbated during HFS. Transporter capacity is diminished somewhat at room temperature (24 degrees C), although transmitter released during brief, "theta burst" stimulation is still cleared as quickly as following a single stimulus, even when transport capacity is partially diminished by pharmacological means.
-
Comparative Study
Electrical cochlear stimulation in the deaf cat: comparisons between psychophysical and central auditory neuronal thresholds.
Cochlear prostheses for electrical stimulation of the auditory nerve ("electrical hearing") can provide auditory capacity for profoundly deaf adults and children, including in many cases a restored ability to perceive speech without visual cues. A fundamental challenge in auditory neuroscience is to understand the neural and perceptual mechanisms that make rehabilitation of hearing possible in these deaf humans. We have developed a feline behavioral model that allows us to study behavioral and physiological variables in the same deaf animals. ⋯ The results of the present study include: 1) in the cats implanted with a scala tympani electrode array, the lowest ICC and A1 neural thresholds were virtually identical to the behavioral thresholds for intracochlear bipolar stimulation; 2) behavioral thresholds were lower than ICC and A1 neural thresholds in each of the cats implanted with a monopolar round window electrode; 3) EABR thresholds were higher than behavioral thresholds in all of the cats (mean difference = 6.5 dB); and 4) the cumulative number of action potentials for a sample of ICC neurons increased monotonically as a function of the amplitude and the number of stimulating biphasic pulses. This physiological result suggests that the output from the ICC may be integrated spatially across neurons and temporally integrated across pulses when the auditory nerve array is stimulated with a train of biphasic current pulses. Because behavioral thresholds were lower and reaction times were faster at a pulse rate of 30 pps compared with a pulse rate of 2 pps, spatial-temporal integration in the central auditory system was presumably reflected in psychophysical performance.