A temporally dynamic context effect that disrupts voice onset time discrimination of rapidly successive stimuli

Across three experiments, voice onset time discrimination along a /ba/-/pa/ continuum was found to be influenced by the order of presentation of rapidly successive stimuli. Specifically, discrimination was disrupted when a relatively unambiguous /pa/ syllable was presented before, rather than after, a more ambiguous /pa/ or /ba/ syllable. In Experiments 1 and 2, for between-category discrimination, this order effect was significant at interstimulus intervals (ISIs) below 250 msec, but not at 250 or 1,000 msec. In Experiments 2 and 3, the order effect was also significant for within-category discrimination at ISIs below 250 msec. In addition, in Experiment 3 this order effect was not diminished by provision of performance feedback across eight testing sessions. These findings reveal a particular vulnerability of phonological processing in response to rapidly successive stimuli and may have implications for mathematical and neural models of speech processing of normal and impaired populations.     

Minimally-effective interpolated stimuli in weight discrimination

The question of minimally effective weights was investigated by using an interpolated anchor paradigm. The weight series of 100 to 300 g,at 50-g-step intervals, was shown to be significantly affected by an interpolated anchor of 0.5 g, thus demonstrating a reduction in the region of ineffectiveness. The results were discussed in terms of procedural artifacts in the weight-judging methods reported in previous literature.     

Emergent simple discrimination in children: Preference for non-preferred stimuli

Previous research (Smeets, 1991) suggested that when given a new discrimination, children respond on the basis of physical similarity with previously discriminated stimuli. They respond to a stimulus similar to another preferred stimulus (S+ transfer) and respond away from a stimulus similar to another nonpreferred stimulus (S- transfer). When both types of transfer apply to the same stimulus, S+ transfer prevails, S+ Priority Transfer (S+PT). The present study demonstrated that S+PT also occurs when the criterion task consists of two nonpreferred stimuli. When given a choice between two previously nonpreferred stimuli, one similar and one dissimilar to other preferred stimuli, children select the first one. They do not so, however, when a nonpreferred stimulus resembling another preferred stimulus is presented with a new nonpreferred stimulus. These findings suggest that the children's preferences were not based on the physical resemblance with other (non)preferred stimuli but on the functions (S+, S-, S0) of individual stimulus components. A theoretical model is presented that accounts for all experimental data reported in the previous and present study. The model implies that discriminative responding not only results from but also determines the functional properties of individual stimulus elements.     

Numerosity discrimination of tactile stimuli.

The ability of 40 college students to discriminate the number of tactile stimuli presented simultaneously was measured in two experiments. Simulation was provided by 12 solenoids fixed to points on the arms and legs. Exp. I showed a mean correct discrimination level of between 1.55 and 2.10 solenoids. Exp II compared discrimination of numerosity by a group who received immediate feedback with a no-feedback control group. Feedback produced a small increase in this ability. However, the most stimulation points correctly discriminated did not exceed three. The data are discussed in terms of the possible role of cutaneous masking.     

Conditional discrimination with ambiguous stimuli.

Five pigeons learned a two-key conditional discrimination. When background color on both keys was red, pecks on the key with a horizontal line produced food. When the color was green, pecks on the key with a vertical line produced food. During part of the experiment, color was presented on only one of the keys. It was found that accuracy was higher when color was combined with the line stimulus correlated with nonreinforcement. In another part of the experiment, color was presented on both keys but a line was present only on one. Accuracy was higher when the line accompanied the nonreinforced option than when the line accompanied the reinforced option. Superior performance when the combined stimuli were displayed on the nonfood key may be explained by the association of different components of the compound stimuli with reinforcement or as the result of rules pigeons follow in solving conditional discriminations.     

Newborns' discrimination of chromatic from achromatic stimuli

Two experiments assessed newborns' ability to discriminate chromatic from achromatic stimuli. In Experiment 1, newborns differentiated gray from green, from yellow, and from red: For each of these hues they preferred chromatic-and-gray checkerboards over gray squares matched in mean luminance, even though the luminance of the gray checks was varied systematically over a wide range so as to minimize nonchromatic cues. However, newborns showed no evidence of differentiating gray from blue: At some luminances they showed no preference for a blue-and-gray checkerboard over a gray square. In Experiment 2, newborns differentiated red from gray but appeared not to differentiate blue from gray: Following habituation to a series of gray squares of varying luminance, they looked longer at a red square than at a gray square of novel luminance but showed no such pattern of recovery to a blue square. The results imply that newborns have some, albeit limited, ability to discriminate chromatic from achromatic stimuli and hence, that they are at least dichromats.     

Associative learning of discrimination with masked stimuli

Great controversy exists on whether associative learning occurs without awareness. In Experiment 1, 31 participants received discrimination training by repeated presentations of two stimulus sequences (S1_A → S2_A, and S1_B → S2_B), S1 being a masked stimulus. S2 were imperative stimuli for a reaction time (RT) task. After the acquisition phase, all participants were tested with 20 presentations of congruent (S1_A → S2_A and S1_B → S2_B) and incongruent (S1_A → S2_B and S1_B → S2_A) stimulus sequences. The RT in the testing phase was faster in congruent than in incongruent stimulus sequences. These results are considered strong evidence of associative learning without awareness of the contingency between the stimuli. A second experiment was designed with SOA varied between three groups (23, 58, and 117 ms). The results showed that the participants responded more quickly to congruent stimulus sequences and that the SOA did not affect RT. The SOA did not modify the effect of congruence either, although the interaction was near significance.     

Newborn's discrimination among mid- and long-wavelength stimuli

Recent anatomical and behavioral evidence suggests that important visual functions like color vision may be severely limited at birth. Therefore, we examined human newborns' ability to discriminate among stimuli of different wavelength by habituating them to a 16 degrees chromatic square of widely varying luminance (as wide as 1.1 log cd/m2) and then testing for recovery of habituation to a different chromatic square of the same size. In the first experiment, newborns showed evidence of recovery to a 650-nm (peak wavelength) red square after habituating to 545-nm green squares, but they did not recover to the red or the green after habituating to 585-nm yellow squares. In a second experiment using the same procedure, newborns did not show evidence of discriminating the 650-nm red from the 545-nm green when the stimulus size was reduced to 8 degrees. These data suggest that human newborns are capable of making a chromatic discrimination within the spectral region above 540 nm (the Rayleigh region) but their ability is limited to chromatic stimuli of very wide spectral separation and of very large size. Possible neurological bases underlying this immaturity are discussed.     

Transfer across unconditioned stimuli in serial feature discrimination training

The transfer of conditioned modulation across conditioned stimuli (CS) and unconditioned stimuli (US) was examined in 3 experiments that used Pavlovian appetitive training procedures with rats. In Experiment 1, after training in a positive patterning discrimination (X-->A+/X-/A-), X increased conditioned responding elicited by another trained-then-extinguished CS as long as that CS had been trained with the same US as was used in discrimination training. In Experiment 2, after training with a feature-negative discrimination (X-->A-/A+), X inhibited conditioned responding elicited by another trained-then-extinguished CS as long as that CS had been trained with the same US. Experiments 1 and 2 used a between-groups design, whereas Experiment 3 used a more powerful within-groups design. In Experiment 3, rats were trained in a feature-positive discrimination (X-->A+/A-). In transfer tests, X increased conditioned responding elicited by another CS trained then extinguished with the same US from training. This increase was greater than the X increased conditioned responding elicited by another CS trained then extinguished with a different US from training. The results supported the suggestion that features trained in serial discrimination tasks influence behavior indirectly by transiently raising or lowering the threshold for activation of the US representations by its target stimuli and by any other stimuli that may be associated with that US. Other interpretations of the findings were also considered.     

Spatial coherence does not affect contrast discrimination for multiple Gabor stimuli

Gestalt grouping rules imply a process or mechanism for grouping together local features of an object into a perceptual whole. Several psychophysical experiments have been interpreted as evidence for constrained interactions between nearby spatial filter elements and this has led to the hypothesis that element linking might be mediated by these interactions. A common tacit assumption is that these interactions result in response modulation which disturbs a local contrast code. We addressed this possibility by performing contrast discrimination experiments using two-dimensional arrays of multiple Gabor patches arranged either (i) vertically, (ii) in circles (coherent conditions), or (iii) randomly (incoherent condition), as well as for a single Gabor patch. In each condition, contrast increments were applied to either the entire test stimulus (experiment 1) or a single patch whose position was cued (experiment 2). In experiment 3, the texture stimuli were reduced to a single contour by displaying only the central vertical strip. Performance was better for the multiple-patch conditions than for the single-patch condition, but whether the multiple-patch stimulus was coherent or not had no systematic effect on the results in any of the experiments. We conclude that constrained local interactions do not interfere with a local contrast code for our suprathreshold stimuli, suggesting that, in general, this is not the way in which element linking is achieved. The possibility that interactions are involved in enhancing the detectability of contour elements at threshold remains unchallenged by our experiments.     

Vibrotactile frequency discrimination at short durations

This experiment investigated how frequency discrimination of a sinusoidal, mechanical vibration applied to the tip of the right index finger is affected by shortening the duration of the stimuli from 200 ms to 30 ms. Using a standard stimulus of 100 Hz at 30 dB above threshold, seven comparison frequencies (at intervals of 10 Hz) were judged as higher or lower in frequency according to the method of constant differences. Vibrotactile stimuli were matched for subjective intensity across both frequency and duration. Difference limens for vibrotactile frequency were found to decline slightly from 200 ms to 50 ms (attributable to practice) and to increase noticeably at 30 ms. This result is discussed in relation to the seemingly contradictory results for auditory pitch discrimination.     

Temporal-order judgment and reaction time for short and long stimuli

Summary The effect of intensity on visual latency was investigated. Visual latency was measured by two methods: simple motor reaction time (RT) and temporalorder judgment (TOJ). It was found, in accordance with previous studies, that the changes in RT were larger than TOJ latency when measured in the same range of intensity. Moreover, the relationship between TOJ latency and intensity was compared in two conditions: under so-called with-offset, the information about offset order was available for the subject, while under without-offset conditions it was not. It was hypothesized on the ground of Jakowski's 1991 study that the TOJ latency-intensity relation should differ for these two conditions because of the effect of intensity on visual persistence. We were unable to find any such effect.     

Effects of response-produced stimuli upon conditional discrimination performance

In zero-delay matching procedures the performance of three groups of pigeons was examined when exteroceptive stimuli, response-produced stimuli associated with the completion of either of two fixed ratios, or a compound of exteroceptive and response-produced stimuli were available as samples. Exteroceptive samples were found to control a higher level of matching accuracy than response-produced samples, while compound samples controlled a higher level of accuracy than did exteroceptive samples alone. When all subjects were placed on a transfer procedure, during which the previously used red and green samples were replaced by horizontal and vertical lines, the availability of sample-specific fixed-ratios facilitated acquisition of the task.     

Superior discrimination between similar stimuli after simultaneous exposure

Human participants received unsupervised exposure to difficult-to-discriminate chequerboard stimuli (e.g., AX and BX), before learning a discrimination between them. Experiment 1 demonstrated that prior exposure enhanced later discrimination and that intermixed exposure (AX, BX, AX, BX …) resulted in better subsequent discrimination than did blocked exposure (CY, CY … DY, DY …). Experiment 2 showed that simultaneous exposure to two similar stimuli (AX–BX, BX–AX …) facilitated the later acquisition of a successive discrimination, more than successive exposure (AX–AX, BX–BX …). These results parallel those observed by Mundy, Honey, and Dwyer (2007) who used pictures of human faces as stimuli and establish the generality of the fact that simultaneous exposure produces a particularly marked perceptual learning effect.     

Same/different discrimination learning with trial-unique stimuli

A long-standing issue in same/different discrimination learning concerns the possible role of individual stimulus memory through repeated presentation. The aim of eliminating any effect of repetition prompted us to devise a new method for generating trial-unique stimuli. These stimuli were arrays of 16 mosaics, each containing 16 cells, which could be filled with 16 possible luminance levels. In Experiment 1, we successfully trained 4 pigeons with these trial-unique stimuli in a two-alternative forced choice same/different discrimination task to 80% correct-choice performance. We later conducted two tests that explored the nature of this discrimination and suggested that pigeons compared the mosaics in the arrays on the basis of their spatial configurations, not on the basis of lower level perceptual properties. In Experiment 2, college students responded similarly to the same sequence of training and testing. Our results suggest that pigeons and people may use similar mechanisms in relational discrimination learning.