Discrimination learning as a function of stimulus location along an auditory intensity continuum

Eight groups of rats were trained on an auditory intensity discrimination in which the discriminative stimuli were separated by 10 decibels (db). Four pairs of stimuli were selected from different regions along a 60–100 db (SPL) intensity continuum. Counterpart groups were trained on each stimulus pair, with the relative intensity positions of the reinforced stimulus (S^D) and the non-reinforced stimulus (S^Δ) reversed for the two groups. Discrimination acquisition curves were compared to determine whether stimuli separated by equal logarithmic units were of comparable “difficulty”, and to determine the relative effectiveness of an S^D serving as the more versus less intense member of a stimulus pair. It was concluded that: (1) When S^D is the more intense, auditory intensities of constant logarithmic separation are graded in “difficulty” along the intensity continuum; high intensity discriminative stimuli are most readily discriminated. When S^Δ is the more intense, this graded effect is not evident. (2) For a given continuum location, discrimination is inferior when S^Δ is the more intense. This effect is most pronounced at the high intensity end of the continuum and is chiefly attributable to differences in the rate of S^Δ responding.     

Detectability as a function of spatial location: effects of selective attention

In a series of previous reports we have described differences in detectability of a target in a background of nontarget patterns as a function of its spatial location. These differences, referred to as a "detectability gradient," have been attributed to target detection accomplished by a serial processing mechanism--a scan. The mathematical model of such a mechanism, developed in the previous report, is equally applicable to a series of attentional shifts or to a perceptual, i.e., a preattentive, mechanism. The present experiments were designed to test the hypothesis that this scan is attentional in nature. The results provide additional evidence for the scanning hypothesis but do not support the view that this scan represents a series of attentional shifts.     

Detectability as a function of target location: effects of spatial configuration

Marked differences in detectability as a function of spatial location, a "detectability gradient," are observed when subjects are required to detect a briefly exposed target pattern of uncertain location in the presence of a number of nontarget patterns. Target detectability also is inversely related to the number of nontarget patterns which are present in this search paradigm. These previous findings provide strong evidence for a serial process in which increasing probability of error occurs during a scan of a rapidly degrading neural representation of the visual image following a brief exposure to the stimuli. It is not yet established whether this scan is attentional or perceptual in nature. The present experiments test the hypothesis of an attentional scan by presenting the target and nontarget patterns in spatially segregated groups. If the scan is attentional, then target detectability under these circumstances would be expected to exhibit the characteristic phenomenon of "group processing"--a close clustering of detection performance for targets located within a group and large differences in detectability across groups. As no evidence for group processing was observed, the results fail to support the view that the scan is attentional in nature but are fully consistent with a nonattentional scan.     

Hemispheric asymmetries for global and local processing as a function of stimulus exposure duration

The experiments assess the relationship between stimulus exposure duration and hemispheric asymmetries for global and local processing. Six durations (27, 40, 53, 67, 80, and 147 ms) were tested in a selective attention task in which hierarchical letterforms were presented unilaterally to the left or right visual field. The results indicated that left hemisphere-local asymmetries were demonstrated more often than right hemisphere-global asymmetries and that asymmetries were most commonly found in the middle range of durations tested. The pattern of results emphasizes the importance of further study into those conditions under which the predicted asymmetries can be reliably demonstrated in healthy individuals.     

Peripheral apparent motion as a function of location, contrast, and direction of stimulus motion

This study investigated the perception of the direction of peripheral apparent motion as a function of stimulus location in the peripheral visual field, stimulus contrast, and the direction of the apparent motion. Results indicated that each of these independent variables was significant as a main effect while the interactions were not.     

Wholistic and analytic stimulus processing: The development of selective perceptual strategies

This experiment reports an investigation of the development of selective processing strategies as subjects become increasingly more practised at a serial self-paced RT task. It demonstrates the effect a preceding stimulus can have on the analysis of a current signal and the development of active analytic strategies in favour of passive wholistic processing, with practice. The stimuli used were letters with irrelevant visual noise dot patterns superimposed on them. The letter, or the dot pattern, or both, could be repeated on successive trials. Early in practice repetition of both stimulus components simultaneously produced short reponse latencies relative to repetition of the letter alone. The number of noise dots markedly affected RT. Late in practice, however, letter repetition RTs were small, irrespective of whether or not the noise dot pattern was repeated. Furthermore, the number of noise dots no longer had an effect on the RT to these stimulus transitions. The results suggest that subjects appear to be able to select relevant information to process as they become progressively more practised, even though there is evidence that they compare the current stimulus with an iconic representation of the immediately preceding one.     

Memory for spatial location: Cue effects as a function of field rotation

We developed theoretical extensions of Huttenlocher, Hedges, and Duncan's (1991) category-adjustment model of human spatial memory to incorporate the use of fuzzy boundariesand cue-determined prototypes. In two experiments, people reproduced locations of dots in a circle, while the number of external reference cues varied. In Experiment 1, the task field was stable and results were consistent with the use of fixed categories unaffected by number of cues. In Experiment 2, the task field was made dynamic by rotation on most trials, with results evaluated for nonrotation trials. The large cue effects observed for angular bias were consistent with the proposed cue-based fuzzy-boundary model. Large cue effects were also observed for absolute error, consistent with a model in which proximity to cues predicts stability of memory. Results point to the key role of orientation to the task environment in determining whether categorical encoding is based on cues.     

Familiarity changes as a function of perceptual shifts

This experiment compares the yes-no and forced recognition tests as methods of measuring familiarity. Participants faced a phase of 3 study-test recognition trials in which they studied words using all the letters of the alphabet (overlapping condition, O), and an additional phase in which targets and lures did not share any letters (non-overlapping condition, NO). Finally, subjects performed a forced-choice task in which they had to choose one of two new words, each from one of the subsets (Parkin et al., 2001). Results in the NO condition were better than in the O condition in the yes-no recognition test, while the forced-choice rate was significantly higher than .50, showing their sensitivity to familiarity. When the letter set of the words for study in the third list of the NO condition was switched, the difference between NO and O conditions disappeared in yes-no test, while the force-choice rate was not higher than .50. We conclude that both the yes-no test and the forced-choice test are valid and equivalent measures of familiarity under the right conditions.     

Different trends in perceptual pattern microgenesis as a function of the spatial range of local brightness averaging

Summary Stimulus patterns, quantized into small squares with the brightness level averaged within each square, were presented for recognition with various exposure durations. Qualitatively different recognition functions were obtained with varying spatial levels of quantization. The method used, if further elaborated, seems appropriate for the empirical differentiation between global and local levels of form and between integration and differentiation stages of a perceptual/attentional process.     

Hue discrimination as a function of stimulus luminance

The relation between hue discrimination and stimulus luminance was investigated. It was found that discrimination was best at the highest luminance and deteriorated at the lowest, except in the yellow region.