No abstract

The waveform of the cortical event-related potential is extremely sensitive to variations in the sequence of stimuli preceding the eliciting event. The waveform changes were manifested primarily in the amplitudes of the negative component of the potential that peaked at 200 milliseconds, the positive component that peaked at 300 milliseconds, and the slow-wave components. A quantitative model was developed relating the waveform changes to changes in event expectancy. Expectancy is assumed to depend on a decaying memory for events within the prior sequence, the specific structure of the sequence, and the global probability of event occurrence. For stimuli relevant to the task, the less expected the stimulus the larger the amplitudes of late components of the event-related potentials.

Infrequent, attended, auditory and visual stimuli evoke large potentials in the human limbic system in tasks that usually evoke endogenous potentials at the scalp. The limbic potentials reverse polarity over small distances and correlate with unit discharges recorded by the same electrodes, indicating that they are locally generated.

Pigeons' responses in the presence of two concurrently available (initial-link) stimuli produced one of two different (terminal-link) stimuli. Entrance into the mutually exclusive terminal links was arranged by different and independent variable-interval schedules for each key, while responses during the mutually exclusive terminal-link stimuli produced a single food reinforcement according to identical and independent variable-interval schedules. The pigeons emitted more initial-link responses on the key with the shorter average interreinforcement interval in the initial link. This difference in initial-link response rates varied directly with the difference between the average inter-reinforcement intervals of the initial-link schedules and decreased when the initial-link schedule with the longer average interreinforcement interval was followed by several consecutive food reinforcements on the variable-interval schedule in the terminal link on that key. These results are incompatible with previous formulations of choice behavior with the concurrent-chains procedure. A modified formulation with a multiplier for the overall rate of primary reinforcement obtained on each key provides a better description of choice. In addition, the new formulation applies to behavior in simple (concurrent) choice situations, an advantage not achieved by previous formulations.Since the concurrent-chains procedure was introduced by Autor (1960), the effects of several variables on choice behavior have been studied. In this procedure the organism responds on two concurrently available keys, each of which is illuminated by the stimulus associated with the initial link of one of the chains. Responses on each key occasionally produce the stimulus for the terminal link of the chain on that key. Responses in the presence of either of the mutually exclusive terminal-link stimuli are reinforced with food. The dependent variable has generally been the distribution of choice responses in the initial links as a function of some difference between the events occurring during the two terminal links.Both Autor (1960), using variable-interval (VI), variable-ratio (VR) and response-independent schedules in the terminal links, and Herrnstein (1964a) using VI and VR schedules, found that the relative rates of choice responding (the number of choice responses on one

The Trail Making Test (TMT) is primarily a test of motor speed and visual attention. In Trail Making, Part A, the subject's task is to quickly draw lines on a page connecting 25 consecutive numbers. In Part B, the subject must draw the lines alternating between numbers and letters. To determine what makes Part B harder than Part A, variations of the standard Trail Making Test were assessed. Forty college students (20 male, 20 female) were given four forms of the Trail Making Test. The results show that Trail Making, Part B with just numbers took longer to complete than the standard Part A with numbers. Part B is 56 cm longer and has more visually interfering stimuli than Part A. These results indicate that Part B is more difficult than Part A not only because it is a more difficult cognitive task, but also because of its increased demands in motor speed and visual search.