The visual discrimination of shape by Octopus: The effects of stimulus size

Octopuses were trained in a successive situation to discriminate between vertical and horizontal rectangles: Group S was trained with rectangles of side length 5 × 1 cm., Group M with 10 × 2 cm. rectangles, and Group L with 20 × 4 cm. rectangles. The larger the shapes used, the more readily were they discriminated, both in terms of speed of learning and of the asymptote of performance. After training, each group was given transfer tests with the two pairs of rectangles not used in training and with two further pairs of 2.5 × 0.5 cm. and 40 × 8 cm. The results can be summarized in three generalizations: (1) When the size of a shape is changed, performance is worse than on the original training shape. (2) The bigger the change in proportionate size, the less transfer is shown. (3) For corresponding changes of proportionate size, there is better transfer to larger shapes than to smaller. These generalizations are supported by data from earlier experiments on the question of transfer to different sized shapes: some of these data were reworked and are presented in detail here. The theoretical implications of the results are discussed.     

A test of two theories of shape discrimination

An experiment is reported which confirms a prediction from the writer's theory of shape recognition. The same experiment is held to refute an alternative theory, but this refutation is not conclusive, since the alternative theory has not been clearly stated and can be interpreted in several conflicting ways. It is shown that these possible interpretations of the alternative theory all yield predictions which are not confirmed by the present findings.     

Disruptive effects of stimulus intensity on two variations of a temporal discrimination procedure

Previous reports using stimulus intensity changes to disrupt temporal discrimination have shown shifts in the psychophysical curve for time, while studies using other disruptors have shown a flattening of the curve. The current study investigated the impact of increases and decreases in stimulus intensity on temporal discrimination in pigeons, to determine if a flattening of the curve could be extended to this disruptor. The brightness of the sample to be timed was manipulated under two procedural variations, in which the response alternatives were differentiated by color or location. Results showed that all subjects in the color procedure, and one in the location procedure, showed a flattening of the psychophysical curve when they experienced increased stimulus intensity in descending order. No subjects exposed to an ascending order of stimulus intensities, and none of the other subjects in the location procedure, showed any impact of changed stimulus intensity. Minimal disruption was found when test sessions presented decreased stimulus intensity levels in a second series. These results, together with those using other types of disruptors, add to the evidence of a flattening of the psychophysical curve when temporal discrimination is disrupted.     

Response bias and the discrimination of stimulus duration

Pigeons discriminated stimulus duration in a psychophysical choice situation. Following presentation of any duration from a set of short duration (11 to 15 sec), responses on a red key were reinforced intermittently. Following presentation of any duration from a set of long durations (16 to 22 sec), responses on a green key were reinforced intermittently. Relative reinforcement rates were manipulated for choice responses across conditions. As relative reinforcement rates were varied, psychometric functions showed shifts in green-key responses at all durations. A signal-detection analysis showed that sensitivity remained roughly constant across conditions while response bias changed as a function of changes in relative reinforcement rate. Relative error rates tended to match relative reinforcement rates.     

The pigeon's discrimination of shape and location information

Two groups of pigeons (n = 4) were trained with 16 line drawings portraying a fixed shape and a variable shape. The four variable shapes (a wedge, a cone, a cylinder, and a handle) appeared to the left of, to the right of, above, or below the fixed shape (a cube). Group Shape (S) was required to discriminate the identity of the variable shape that was mated with the cube, whereas Group Location (L) was required to discriminate where the variable shape appeared relative to the cube. Three of the four pigeons in each group mastered their respective tasks. Later tests revealed that the two groups of pigeons had attended to different aspects of the shape pairs, even though the visual stimuli and general procedures they had been given were the same. Group S had attended to the identity of the variable shape and had ignored the identity and location of the cube, whereas Group L had attended to the configuration of the two shapes. The methods and stimuli could be useful in delineating the biological bases of shape and location perception.     

The failure of stimulus control after presence-absence discrimination of click-rate

Pigeons were trained to discriminate a slow click-rate from its absence, or to discriminate it from a faster click-rate. Subsequent click-rate generalization tests produced the usual steepened gradients after the intradimensional discrimination but produced flat gradients after presence/absence discrimination. The occurrence of stimulus control only after intradimensional discrimination, combined with previous results showing stimulus control sometimes after nondifferential reinforcement and sometimes after presence/absence discrimination, argues for a reformulation of the problem of stimulus control. A theoretical framework, relying upon blocking effects inherent in the different discrimination procedures, was presented to account for the diversity of results.     

A role for stimulus generalization in conditional discrimination learning

In each of three experiments on discrimination learning by rats, whether or not a 10-sec target stimulus was followed by food was determined by the nature of a 2-min background stimulus that accompanied it. A conditional discrimination was employed in Experiment 1 such that background A indicated food would follow one target but not the other, whereas this relationship between the targets and food was reversed in the presence of background B. Experiment 2 employed two feature-positive discriminations. Subsequent test trials revealed that the background for one discrimination was able to enhance responding during the target for the other discrimination. Experiment 3 employed a feature-positive and a feature-negative discrimination prior to test trials in which each target was presented separately during a compound of both background stimuli. The compound enhanced responding to the target from the feature-positive discrimination and reduced it to the target from the feature-negative discrimination. We suggest that to accommodate all these findings, the best explanation is provided by a configural model of Pavlovian conditioning.     

A component model for stimulus variables in discrimination learning

A general function is derived describing the conditioning of a single stimulus component in a discriminative situation. This function, together with the combinatorial rules of statistical learning theory [5, 12], generates empirically testable formulas for learning of classical two-alternative discriminations, probabilistic discriminations, and discriminations based on the outcomes of preceding trials in partial reinforcement experiments.The researches on which this paper is based were facilitated by a grant from the National Science Foundation.     

The discrimination of two racial samples

Nineteen different anthropometric measures were obtained on all members of each of two racial groups. A procedure was developed and applied to the data to give maximum differentiation between the groups. The method is applicable wherever we have a large number of independent variables and a dependent variable. In such cases, the conventional methods for determining multiple regression constants are very laborious. An iteration method is presented which is more rapid than any with which the writers are familiar. The method selects in sequence those variables which together yield the largest multiple correlation with the criterion. At each step in the procedure, rapid estimates of the regression weights and the multiple correlation at that point are available.     

The strength and generality of stimulus over-selectivity in simultaneous discrimination procedures

Stimulus over-selectivity refers to behavior being controlled by one element of the environment at the expense of other equally salient aspects of the environment. This is a common problem for many individuals, including those with autism spectrum disorders, and learning difficulties, and presents a considerable problem for information processing in many important situations involving complex cues and environments. Three experiments explored the strength and generality of the over-selectivity effect in non-clinical adults undertaking a cognitively demanding task, by training and testing participants on a two-component trial-and-error discrimination learning task. The over-selectivity effect was found in a variety of test conditions, including when the comparison test stimulus was neutral (Experiment 1), novel (Experiment 2), neutral with no conditioning history (Experiment 3), or when punished during training (Experiments 1, 2 and 3). Such results provide ubiquity to the phenomenon, making it important to investigate further.