Visual attention and inferotemporal cortex in rhesus monkeys

Four rhesus monkeys with inferotemporal lesions were inferior to their normal controls in relearning a visual discrimination problem with variable, irrelevant cues, and in shifting responses to the previously irrelevant cues. Reducing the variability and/or amount of information improved the performance of the inferotemporal group. Results are interpreted in terms of a “trade-off” model of visual attention.     

Central locomotor programming in the rabbit

In decorticate, unanaesthetized and curarized rabbit preparations, with both hindlimbs deafferented, locomotor-like discharges were recorded from nerves to flexors and extensors and their time patterns were compared. The bursts of rhythmic activities recorded from nerves to several flexor muscles acting at either joint were shown to be synchronous, with no differences in their time of onset. The same was true for the extensors. Nerves to bifunctional muscles (biceps posterior, semi-tendinosus and tenuissimus) acting on two consecutive joints (knee flexors, hip extensors) could display two consecutive bursts in each locomotor cycle, one being pure flexor and the other an extensor discharge. It is shown here that this functional bivalence is programmed centrally and can be modulated, i.e. the relative importance of the flexor and the extensor bursts can be changed in a predictable way through afferent (proprioceptive or cutaneous) influences, or through activation of the descending monoaminergic pathways. In extreme cases, complete functional reversal was observed in these bifunctional muscle nerves.     

Operant conditioning of vertical eye movements without visual feedback in the midpontine pretrigeminal cat

An operant conditioning of vertical eye movements was achieved in the midpontine pretrigeminal cat in total darkness by contingent reinforcement of spontaneous eye movements with lateral hypothalamic (LHT) reward stimulation, when each movement (upward direction was chosen in this experiment) exceeded a preset amplitude. However, the response rates in the dark were lower than those in the light and the time to reach the peak response rate was much longer. Recording of evoked potentials to optic chiasma (OC) stimulation revealed enhancement of late components of the visual cortex (VC) and superior colliculus (SC) responses in relation to eye movements. Sequential records of the averaged evoked responses associated with eye movements indicated that the amplitudes of the late components of the VC and SC waves gradually increased in the course of establishment of the operant conditioning, and decreased gradually during extinction. In a yoked control test, increase in amplitudes of the late components was much less significant during non-contingent reinforcement given independently of the eye movements. These results suggest that 'corollary discharge' may play a critical role as a cue in acquisition of the operant conditioning of vertical eye movements when visual feedback is absent in total darkness.     

Effects of posterior neocortical lesions on wavelength,light/dark and stripe orientation discrimination in ground squirrels

Thirteen-lined ground squirrels (Citellus tridecemlineatus) were trained on three two-choice visual discrimination problems: light/dark, color and stripe orientation. After posterior neocortical lesions in one or two stages, they were tested on all three discriminations. The results demonstrate that animals with large posterior neocortical lesions which produced retrograde changes throughout the dorsal lateral geniculate (LGNd) were capable of light/dark and wavelength discrimination. These animals were not able to discriminate stripe orientation. It is proposed that wavelength discrimination depends on extrageniculostriate mechanisms in posterior neodecorticates of this species.     

Suppression of sexual receptivity in the female hamster: Neuroanatomical projections from preoptic and anterior hypothalamic electrode sites

Electrical stimulation (ES) through fine-wire electrodes chronically implanted in the preoptic area and anterior hypothalamus suppressed the lordosis response in freely moving hormone-primed female hamsters. ES at these sites never induced or facilitated lordosis. When behavioral testing was completed small lesions were made at the tips of the electrodes, and the Fink-Heimer method was used to trace degenerating axons away from the sites of ES. This was done in an effort to determine the neural pathways important for the modulation of this behavior. The hypothalamic distribution of the stria terminalis was also charted for the hamster and was found to be similar to that in the rat, including projections to both the shell and core of the ventromedial nucleus (VMN). No single pattern of degeneration was common to all females showing ES-produced lordosis suppression. These data demonstrate that stimulation of several different pathways can produce behavioral states that are incompatible with the lordosis response. We suggest that ES in the medial preoptic-medial anterior hypothalamic continuum suppresses sexual receptivity by influencing neurons in the VMN region, while ES through more laterally placed electrodes suppresses receptivity by producing an incompatible state of behavioral activation, without directly influencing VMN region neural activity.     

Laterality and reading proficiency

Lateral asymmetry in tachistoscopic perception of words was investigated in good and poor readers between 7:6 and 8:7 years old. Five-letter words were presented unilaterally to left or right of a fixation point. Good readers showed greater right over left field superiority than poor readers. Several possible causal factors are discussed. It is concluded that good and poor readers of the same age differ in the degree of lateralization in terms of cerebral hemispheric specialization of language functions. In addition, poor readers were superior to good readers on left hemifield presentations. This possibly suggests superior linguistic function in the right cerebral hemisphere of less lateralized individuals. Boys showed greater asymmetry than girls.     

Serotonin synthesis and release in brain slices: Independence of tryptophan

The purpose of the present study was to examine the role of substrate availability in the regulation of the release and synthesis of serotonin by brain slices. Electrical field depolarization of the brain slices stimulated the synthesis and release of serotonin in the absence of changes in intracellular tryptophan concentration, in the absence of tryptophan in the incubation bath, and in the absence of changes in total tryptophan uptake. Furthermore, electrical stimulation decreased the apparent Km for tryptophan required for synthesis of serotonin by the slices. Several conclusions were drawn. (1) Rates of serotonin release and synthesis in brain slices may increase independently of the tissue tryptophan concentration or tryptophan uptake. (2) There is little difference in the synthetic rate of serotonin in tissues exposed to pargyline or not at all. (3) Blockade of monoamine oxidase by pargyline appears to give estimates of the synthetic rates of serotonin which are comparable to estimates derived from [3H]-tryptophan incorporation. (4) Newly synthesized serotinin is preferentially released. (5) Only in tissue pretreated with pargyline do increasing concentrations of tryptophan increase the releasable pool of 5-HT. (6) Electrical depolarization may allosterically activate tryptophan hydroxylase; an effect which may be mimicked by homogenization of the tissue.     

Synthesis and release of serotonin by brain slices: Effect of ionic manipulations and cationic ionophores

The effect of various ionic manipulations and cationic ionophores on the rates of release and synthesis of serotonin were investigated in brain slices prepared from adult male rats. Two depolarizing, univalent cationic ionophores, gramacidin (10 μg/ml) and valinomycin (10 μ/ml), and the non-depolarizing, calcium-specific ionophore, A23187 (190 μM), stimulated both the release and synthesis of serotonin in this tissue preparation. Electrical field depolarization of the brain slices also stimulated both the release and synthesis of serotonin, effects which were completely blocked by a calcium-free media and 1 mM EGTA or by 10 mM Mg²⁺. Lithium partially blocked the release of serotonin by the stimulated brain slices, but markedly augmented the rates of serotonin biosynthesis. Although electrical stimulation significantly increased the rate of [³H]tryptophan uptake by the slices, the increased rates of release and synthesis of serotonin following treatment of the tissues with the ionophores and lithium were not associated with increased rates of tryptophan uptake. Furthermore, despite the inhibition of the release and synthesis of serotonin by stimulated slices incubated in the presence of magnesium or in the absence of calcium, rates of tryptophan uptake by the stimulated brain slice remained increased. These results taken together with the results of other studies suggest that release and synthesis of serotonin by the serotonergic neuron is tightly coupled to the ionic events attending depolarization and further argue than transcellular calcium fluxes may be of special importance in this regulation. These results do not support the postulate that regulation of serotonin release and synthesis is dependent upon substrate availability as a major variable.     

The spinal course and medullary termination of myelinated renal afferents in the rat

Myelinated afferent fibers, recorded in the left renal nerve of rats, were antidromically activated by discrete electrical stimulation of the cervical spinal cord and the caudal medulla. The lowest thresholds for activation of these fibers were found in the most medial portion of the ipsilateral fasciculus gracilis. This region of minimum threshold continued rostrad through the nucleus commissuralis. Based on threshold vs depth contours, fibers appeared to terminate in the ipsilateral nucleus gracilis and nucleus solitarius. Myelinated fibers could be activated by punctate pressure on the renal hilus. Action potentials generated by hilar pressure collided with antidromically-conducted action potentials elicited by electrical stimulation at cervical levels. We conclude that myelinated renal afferents carry information from intrarenal receptors, via the dorsal column system, to both visceral afferent and dorsal column nuclei.     

Opioid peptides excite pyramidal neurons and evoke epil-eptiform activity in hippocampal transplants in oculo

The effect of opiate peptide administration on the electrical activity of intraocular hippocampal transplants was studied. Similar to observations in situ, the administration of beta-endorphin or methionine enkephalin produces a concentration-dependent increase in the firing rate of identified pyramidal neurons within hippocampal formation transplants. In addition, these peptides elicit a profound increase in 'EEG' amplitude, which ultimately develops into epileptiform activity. The ability of naloxone to either reverse or prevent the peptide-induced changes in both single unit and EEG activity supports the hypothesis that the excitatory response of the hippocampus to opioid peptides is mediated via an opiate receptor. The results of this study also suggest that the excitatory response to the opiate peptides in hippocampus is the result of alterations in intrinsic neuronal circuitry and is not dependent upon extra-hippocampal afferents.     

The role of GABA in the regulation of the dopamine/tyrosine hydroxylase-containing neurons of the rat retina

Dopamine (DA) and gamma-aminobutyric acid (GABA) are putative neurotransmitters in two separate populations of amacrine neurons in the mammalian retina. Pharmacological studies have been conducted to determine if GABA neurons regulate the neuronal activity of the neurons that secrete DA. Tyrosine hydroxylase (TH) activity, a biochemical indicator of changes in activity of DA/TH-containing neurons, was low in dark-adapted retinas and high in light-exposed retinas. Muscimol (a GABA receptor agonist) produced a dose-related, biphasic effect on the light-evoked activation of TH, when the drug was injected into the vitreous (intravitreal injection) of dark-adapted rats. At low doses, (35 and 60 pmol) muscimol enhanced the light-evoked activation of TH, but at higher doses (greater than or equal to 120 pmol) it inhibited the light-evoked increase in enzyme activity. Muscimol had no significant effect on the TH activity of dark-adapted retinas. GABA antagonists, bicuculline and picrotoxin, produced effects on TH activity that were dependent on both dose and light-exposure. At low doses (0.4-0.5 nmol), bicuculline and picrotoxin both inhibited the light-evoked activation of TH, but had no effect on TH activity of the dark-adapted retinas. At a higher dose (2.0 nmol), both antagonists increased TH activity in the dark-adapted retina and attenuated the further activation of the enzyme by light. Rat retinas were dissociated into suspensions of viable cells in order to investigate the direct effects of muscimol and picrotoxin on the DA/TH-containing cells. The process of dissociating dark-adapted retinas resulted in an apparent activation of TH. Incubation of the cells with muscimol resulted in a decrease of TH activity in a concentration-dependent manner. Picrotoxin antagonized the inhibitory effect of muscimol, but had no effect when incubated alone. The biphasic effects of GABA agonists and antagonists in vivo suggest that a certain subpopulation of GABA neurons are involved in the activation of the DA/TH-containing neurons by photic stimulation, while another subpopulation of GABA neurons produce a tonic inhibition of the DA/TH-containing neurons in darkness. The experiments with retinal cell suspensions indicate that the tonic inhibition is probably mediated by synapses of GABA neurons directly onto the DA/TH-containing cells.     

“Late” recovery of the right ear dichotic score following cerebrovascular accident: A case report

Late recovery of the right ear score on a digit dichotic listening test was observed in a 45-yr-old female patient. The right ear score was at chance levels of accuracy through the ninth month post-onset of a cerebrovascular accident and then improved dramatically to 67% correct by month 12. Possible bases for this pattern of performance are discussed     

The influence of naloxone on the C fiber response of dorsal horn neurons and their inhibitory control by raphe magnus stimulation

In intact rats anesthetized with chloralose, the effects of naloxone were studied on the responses of spinal cord dorsal horn neurons to C fiber stimulation and upon the inhibition induced on these responses by stimulation of the nucleus raphé magnus (NRM). (1) A mean 44% facilitatory effect on responses to C fibers was observed for 12/19 units. (2) A mean 30% reduction of the inhibitory effects of NRM was found for 14/29 units. (3) However there is no clear relationship between these facilitatory effects and the diminution of the efficiency of NRM stimulation. These results demonstrate a facilitatory effect of naloxone upon the transmission of noxious messages at the spinal level and confirm that opiate endogenous substances are implicated in the inhibitory mechanisms activated by stimulation of NRM.     

The relationship between GABA concentrations in brain and cerebrospinal fluid

GABA concentrations in cerebrospinal fluid (CSF) and brain of rats and cats were determined before and after intraperitoneal injection of three drugs that increase brain GABA levels. GABA exists in the CSF in two forms: free and conjugated GABA. In the CSF of untreated animals, there is very little free GABA (65 ± 12pmol/ml) but considerable amounts of conjugated GABA (2885 ± 100pmol/ml). After IP administration of γ-vinyl GABA to rats, CSF concentrations of both free and conjugated GABA rise in a dose-dependent manner. There is an exponential correlation (r = 0.92, P < 0.001) between rat whole brain GABA concentrations and free GABA in the CSF. Concentrations of brain GABA and conjugated CSF GABA are linearly correlated (r = 0.84, P < 0.001). γ-Acetylenic GABA has qualitatively similar effects to γ-vinyl GABA. Treatment with ethanolamine-O-sulfate i.p. at a dose not affecting brain GABA concentrations markedly increases serum GABA, leaves conjugated CSF GABA unchanged and significantly elevates free GABA in the CSF.These findings suggest that total CSF GABA concentrations are related primarily to brain GABA levels and are minimally affected by the changes in the peripheral GABA concentrations. Determination of the levels of free and conjugated GABA in the CSF may be useful for the estimation of brain GABA concentration in patients on therapy intended to alter brain GABA levels.