Biology of elderly suicide.

Most studies of suicide in younger patients have demonstrated significant alterations in the serotonin system. Although a high percentage of completed suicides occur in late-life, to date very few studies of the biology of suicide have focused on this age group. This chapter describes age-related changes in the central nervous system pertinent to the biology of suicide, then reviews post-mortem biological studies of the brains of suicides and suicide attempters. As suicide attempts in the elderly are characterized by the use of violent means, biologic studies of impulsive violence are discussed. Finally we describe data on the effect of degenerative diseases on the serotonin system and the possible link to increased suicidal behavior in affected patients. This review underscores the need for further study of the biology of suicide in the geriatric age group.     

The recovery of structure from motion: no evidence for a special link with the convergent disparity mechanism

Convergent and divergent stereo mechanisms were compared in their ability to recover structure from motion. Contrary to a recent result reported by Richards and Lieberman, no difference in their performance was found; both mechanisms appeared equally capable of supporting the perception of good structure from motion. Possible reasons for the disparate results are discussed.     

Selective attention and stimulus-reinforcer interactions in the pigeon

The study supplies further evidence that non-associative effects and temporal-spatial similarities between certain combinations of cue and consequence cannot explain all instances of stimulus-reinforcer interactions. Pigeons were trained to press a treadle in the presence of a discriminative compound stimulus either to avoid shock or to obtain a food reinforcer. The compound stimulus was composed of diffuse tone and light cues which had identical temporal patterns of onset, duration and offset. With the avoidance schedule the auditory cue acquired more control than the visual cue; however, when food was the reinforcer, the visual cue exerted more control. This pattern of stimulus control on the appetitive schedule did not change if random shocks were also added, even though these shocks were equal in density to the food presentations and equal in magnitude to those used for the avoidance schedule. Other changes in the appetitive procedure, such as making the tone spatially contiguous with food and removing the light in the food hopper, also failed to alter the relative control by the different cues. Prior training with a food reinforcer did not produce any change in the relative control by the two cues when the birds were retrained on the shock-avoidance schedule. The results suggest that some frequently stated alternatives to selective associability are not adequate explanations of this instance of a stimulus-reinforcer interaction.     

Successive negative contrast in one-way avoidance learning in rats

In three experiments, successive negative contrast was examined in one-way avoidance learning. Reward magnitude in first (pre-shift) and second (post-shift) phases was manipulated by time spent in the safe compartment. Experiment 1 demonstrated that when time in the danger compartment was held constant, a group shifted from a large reward--30 sec spent in the safe compartment--to a small reward--1 sec--showed poor performance and longer response latency than a group conditioned with the small reward in both phases. Experiment 2 replicated this effect with a less intense shock and also demonstrated that a group shifted from large to small reward performed more poorly than a group exposed to large reward--30 sec--in both phases. Finally, Experiment 3 showed that changes in intertrial interval, defined as total time spent in the safe compartment and the danger compartment before the onset of the warning signal, were not responsible for this contrast effect. These results suggest that time spent in a safe place can act as appetitive incentive during one-way avoidance learning.