Effects of stimulus spacing on steady state gradients of inhibitory stimulus control

Range effects in transient and maintained generalization gradients

In three experiments pigeons were trained to discriminate visual flicker rate stimuli. The stimulus set was varied so that the effects of overall stimulus range and border separation between positive and negative stimuli could be assessed. Experiments 1 and 2 showed that transient generalization gradients were lower in height and flatter with increases in either border separation or overall stimulus range. Border separation and overall range interacted in that the effect of range was greatest with relatively greater border stimulus separation. Experiment 3 showed that increased overall stimulus range reduced the magnitude of positive dimensional contrast in maintained gradients. Flattening of the maintained gradient with increases in stimulus range was similar to flattening of post-discrimination gradients found in Experiments 1 and 2. The results from all experiments indicate that increases in overall stimulus range do not decrease discriminability of unchanged stimulus values. Instead, increases in stimulus range may increase the variability of response or criterial factors in discrimination.

Massed and spaced stimulus sequences in dimensional discrimination

In two experiments, pigeons' responding to a visual flicker-rate continuum was established by a maintained generalization procedure. For both experiments, variable-interval reinforcement was available for responses during stimuli from one half of the stimulus continuum while responses during other stimuli were extinguished. The first experiment compared gradients of dimensional stimulus control produced by randomly presented positive and negative stimuli with gradients produced by massed positive and negative stimuli. The second experiment alternated the order of massed stimulus sequences. In both cases, massing of stimulus sequences diminished positive dimensional contrast effects, without seriously disturbing discrimination between positive and negative stimuli. The results indicate that stimulus sequences can have an important role in the production of dimensional contrast effects.

Local contrast and maintained generalization

Pigeons received variable-interval reinforcement for key pecking during presentations of horizontal and vertical line-orientation stimuli, while pecks during five intermediate orientations were extinguished. Lowest peck rates were observed during presentations of negative stimuli adjacent to the positive orientations while peck rate during 45 degrees (the intermediate negative orientation) was relatively high, i.e., there were negative contrast shoulders. When peck rates were manipulated in the positive orientations, peck rate in neithboring orientations changed in the opposite direction. Contrast shoulders faded after prolonged training. A second type of contrast, local contrast, was correlated with similarity of preceding stimulus and different average peck rates during different stages of the discrimination process. The data suggest that sequential local contrast accompanying the formation of a discrimination contributes to the form of generalization gradients. Blough's model of stimulus control predicts the changes in gradient form described here, but may not accurately depict the underlying process responsible for gradient form.

Behavioral and dimensional contrast in rats

Rats pressed a nose key for brain stimulation reinforcement presented on a fixed-interval schedule. Stimuli were drawn at random from a continuum of 12 white noise intensities in the range 62-95 decibels, spaced in 3 decibel steps. Experiment 1 varied the number of stimuli and the reinforcement contingencies associated with them. In Condition I (baseline) all stimuli signaled reinforcement; in Conditions II and III stimuli from one half of the continuum signaled reinforcement and those from the other half, extinction. However, in Condition II the 6 stimuli from the middle of the continuum were omitted. Experiment 2 held constant the number of stimuli and varied their spacing. In Condition I, each of 6 sounds signaled reinforcement. In Conditions II and II, three stimuli from one half of the continuum signaled reinforcement and three from the other half, extinction. However, in Condition II the stimuli were near the extremes of the continuum (Stimuli 1, 3, 4, 9, 10, 12). Condition III replaced Stimulus 3 with Stimulus 6 and Stimulus 10 with Stimulus 7. Behavioral contrast was seen in an increase over baseline in response rate to the stimuli associated with the constant schedule component when the variable component was changed to extinction. Dimensional contrast was seen in a further elevation of rate to intermediate positive stimulus values when stimuli were added to the border region between positive and negative values.