Attentional demands following perceptual skill training.

Practicing simple visual tasks induces substantial improvement. We investigated whether increased efficiency is accompanied by automaticity and immunity to across-task interference. We found that although practice speeds orientation feature detection, it does not abolish susceptibility to interference from introduction of concurrent central-letter identification, which takes priority. Yet following training with each task observers successfully managed to perform the tasks concurrently. The effectiveness of separate training implies that the role of improved intertask coordination in achieving concurrent performance was minor. Indeed, even when initial training was concurrent, improvement on the two tasks was sequential, and the higher-priority (central) task was learned first. However, automatic processing was not accomplished either, because increasing the difficulty of the higher-priority task interfered with performance of both tasks. What appears to be orchestrated posttraining performance is actually mainly an emergent property of speeded initial processes rather than either eliminated bottlenecks or improved central executive management.

Learning pop-out detection: building representations for conflicting target-distractor relationships.

Studies of perceptual learning consistently found that improvement is stimulus specific. These findings were interpreted as indicating an early cortical learning site. In line with this interpretation, we consider two alternative hypotheses: the 'earliest modification' and the 'output-level modification' assumptions, which respectively assume that learning occurs within the earliest representation which is selective for the trained stimuli, or at cortical levels receiving its output. We studied performance in a pop-out task using light bar distractor elements of one orientation, and a target element rotated by 30 degrees (or 90 degrees). We tested the alternative hypotheses by examining pop-out learning through an initial training phase, a subsequent learning stage with swapped target and distracted orientations, and a final re-test with the originally trained stimuli. We found learning does not transfer across orientation swapping. However, following training with swapped orientations, a similar performance level is reached as with original orientations. That is, learning neither facilitates nor interferes to a substantial degree with subsequent performance with altered stimuli. Furthermore, this re-training does not hamper performance with the original trained stimuli. If training changed the earliest orientation selective representation (specializing it for performance of the particular performed task) it would necessarily affect performance with swapped orientations, as well. The co-existence of similar asymptotes for apparently conflicting stimulus sets refutes the 'earliest modification' hypothesis, supporting the alternative 'output level modification' hypothesis. We conclude that secondary cortical processing levels use outputs from the earliest orientation representation to compute higher order structures, promoting and improving successful task performance.

Synapse formation between dissociated basal forebrain neurones and hippocampal cells in culture.

Dissociated neurones from rat basal forebrain and hippocampus were co-cultured in vitro for 8-15 days. Patch-clamp recordings from individual presumed hippocampal pyramidal cells revealed synaptic currents following focal extracellular stimulation of single presumed basal forebrain neurones. Of 18 neurone pairs, 13 showed inward (excitatory) synaptic currents, 4 showed outward (inhibitory) synaptic currents, and 1 showed a mixed current. Latencies varied from 4 to 15 ms, suggesting both mono- and polysynaptic currents. These experiments indicate that synaptic connections can be established between basal forebrain and hippocampal cells in dissociated cell culture.

Electrophysiology of degenerating neurones in the vagal motor nucleus of the guinea-pig following axotomy.

1. The electrophysiological properties of motoneurones in the dorsal motor nucleus of the vagus in the guinea-pig were studied at different times following cervical vagotomy. The results were compared both to normal neurones and to results obtained at the same time from intact neurones located in the contralateral nucleus. 2. The input resistances of axotomized neurones are significantly higher than those of normal neurones (66 +/- 29 compared to 45 +/- 17 M omega). This difference was seen during the first month following axotomy without any sign of a time-dependent process. On the other hand, no change in resting potential was observed. 3. Significant reduction in action potential amplitude was observed 1 month after axotomy (from 97.8 +/- 8 to 87 +/- 7 mV) and was followed by slow recovery lasting more than 1 year. Neither the Na+ conductance nor the voltage-dependent K+ conductance responsible for the fast rise and fall of the action potential, respectively, were affected by axotomy. 4. One month after axotomy the action potential duration in axotomized neurones was found to be shorter than that of normal neurones (0.9 +/- 0.1 ms compared to 1.1 +/- 0.04 ms). We show that this decrease in duration reflects a reduction in the depolarizing hump on the falling phase of the action potential, which is known to express the Ca2+ conductance activated during the action potential. A slow recovery of the spike duration was observed, although an age-dependent reduction in duration was also observed in neurones in the contralateral nucleus. 5. Two K+ conductances, the Ca2+-dependent and the A type, decrease 1 month after axotomy and follow a similar time course of recovery to that of the reduction in action potential duration and amplitude. 6. The firing pattern of axotomized neurones undergoes profound alteration, manifested as an increase in firing duration as a response to a rectangular current pulse. Examination of these alterations reveals that the reduction in both K+ conductances is responsible for the observed changes. 7. The results are discussed within the framework of the degenerative response known to take place in the nucleus following axotomy. We hypothesize that the observed phenomena reflect an increase in intracellular Ca2+ concentration which, in turn, inactivates the Ca2+ and K+ conductances. Furthermore this rise in intracellular Ca2+ may eventually be responsible for cell death.

Time course and distribution of motoneuronal loss in the dorsal motor vagal nucleus of guinea pig after cervical vagotomy.

Cells in the dorsal motor vagal nucleus (DMVN) of the adult guinea pig were counted at different times after unilateral cervical section of the vagus nerve. The counts were made from serial 30 microns coronal sections throughout the DMVN in normal and operated animals. There are three types of cells in the DMVN of guinea pig: medium-sized motoneurons that are retrogradely filled by HRP from the site of the vagotomy, small neurons, and glial cells. An interesting observation was a change in distribution of cells in the DMVN with age in unoperated guinea pigs. Following vagotomy degeneration was seen only in the motoneurons. Disappearance of motoneurons was slow and only 27% were present after 1 year. During that time the decrease in the total number of motoneurons was exponential with a time constant of 8.6 months, but degeneration in different parts of the nucleus was not uniform. Thirty-four percent of motoneurons in the caudal area of DMVN disappeared in the first month after vagotomy, while the rostral area was almost unchanged. The rostral area, however, showed rapid degeneration between 3 and 6 months after vagotomy. The central part of the nucleus degenerated at a constant rate between those of the rostral and caudal regions. At the end of 1 year, cell loss in all parts of the nucleus was approximately equal. Surviving motoneurons showed morphological changes: rounding of the soma, continuous reduction of the cell volume, and shrinkage of the nucleus. Occasional abnormal forms showing vacuolization or invaginated nuclei were seen. Calculations show that the process of degeneration lasts 25 days on the average. The marked degeneration found in dorsal vagal motoneurons, in contrast to recovery from axotomy in somatic motoneurons, is similar to that found in intrinsic neurons of the central nervous system. The slow and continuous time course of disappearance of motoneurons after vagotomy, however, is exceptional. It is reasonable to postulate that the increased vulnerability of these motoneurons may be sufficient to result in degeneration in response to what are normally nonpathological metabolic demands.

Properties of the on-transient of the intracellular response in the barnacle photoreceptor.

We have studied the on-transient of the receptor potential of the barnacle photoreceptor. Its amplitude has previously been shown to depend on light intensity and state of light-dark adaptation. We have examined its dependence on 1) the presence of a prolonged depolarizing afterpotential (PDA), 2) a background light, 3) added alcohol, or 4) decreased K+ concentration in the bath. We find that the relative on-transient amplitude tends to increase initially with increasing depolarization arising from 1)-4) and then to decrease again at higher depolarization. This behavior is qualitatively explainable by the cell's current-voltage characteristics and by the adapting effect of the stimulus on the conductances arising from the PDA, the background light and the alcohol.