Mental curve tracing with elementary stimuli

It has been proposed that certain spatial relations are determined by an operation, or "visual routine," that can trace along a boundary (Ullman, 1984). This proposal was supported by Jolicoeur, Ullman, and Mackay's (1986) finding that the time required to determine if two Xs are on the same curve increased monotonically with the separation of the Xs along that curve. In the present study the generality of the curve tracing hypothesis was explored across four experiments by using elementary stimuli that eliminated interweaving curves, displaced the fixation point away from the curves and target Xs, and provided a simple alternative to curve tracing--namely, determining whether or not the Xs fell on the same side of the figure. Stimuli consisted of two curves (150 degrees arcs) and two Xs, and each stimulus was presented for 150 ms. In Experiments 1 and 2, subjects were instructed to decide as quickly as possible if the two Xs fell on the same curve or on different curves. Even for these elementary stimuli, mean reaction time (RT) for same trials increased monotonically with the distance separating the Xs. Mean RT for different trials, however, decreased with the distance separating the Xs. In Experiments 3 and 4 alternatives to curve tracing were tested. For same trials the evidence strongly favored curve tracing. However, different trials were apparently solved on the basis of judgmental processes presumably operating in parallel with curve tracing. Curve tracing rates fluctuated across experiments and seemed to be partially governed by the width of the "pathway" provided for the trace.     

When are nonwords easy to see?

The influences of presentation mode (mixed vs. blocked trials) and target variability on the detection of targets in words and in random letter strings were examined. The results indicated that there was a substantial word superiority effect in mixed lists of words and nonwords, but that this effect was eliminated when pure lists of words and nonwords were compared. Also, target variability affected the shape of the serial position curve. When subjects searched repeatedly for a single target, the serial position curve had only a significant linear component. However, when the identity of the target varied from trial to trial, the serial position curve had a significant quartic component (i.e.. it was M-shaped). These results were interpreted in terms of strategies and feature learning.     

Expectancy and stimulus frequency: A comparative analysis in rats and humans

We examined whether expectancy, one of several factors influencing attention, is similarly affected in rats and humans by manipulation of relative stimulus frequency, A two-choice reaction time (RT) task was developed for rats, and an analogous task was used for humans. Errors, RTs, discriminability, and response bias were measured. Both rats and humans shifted their response bias to the more frequent stimulus, with no change in overall discriminability. As stimulus probability or stimulus repetition increased, RTs and errors decreased. These results illustrate the similarity of expectancy in rats and humans. This two-choice RT task for rats can be used in future studies to examine the neuronal mechanisms of expectancy and attention.     

Further evidence for a time-independent shift of the focus of attention

The separation between stimuli was manipulated in a same-different matching task. In Experiment 1, stimuli were upright Ts and/or Ls, whereas in Experiment 2, they were rotated Ts and/or Ls. In both experiments, mean reaction time (RT) for the same-different judgment did not increase as a function of interletter separation, suggesting either that the time needed to relocate attention was independent of distance, or that the stimuli were processed in parallel. These alternatives were tested in a third experiment, with a diagnostic for parallel processing proposed by Egeth and Dagenbach (1991). The diagnostic indicated that the rotated Ts and Ls in Experiment 2 were processed serially. If serial processing implies the utilization of attention, then the results of Experiment 2 suggest that relocation of attention is time-invariant with respect to distance.     

Orientation perception in Williams Syndrome: Discrimination and integration

Williams Syndrome (WS) is a rare neurodevelopmental disorder, which stems from a genetic deletion on chromosome 7 and causes a profound weakness in visuospatial cognition. Our current study explores how orientation perception may contribute to the visuospatial deficits in WS. In Experiment 1, we found that WS individuals and normal 3–4 year olds had similar orientation discrimination thresholds and had similar prevalence of mirror-reversal errors for diagonal targets (±45 deg). In Experiment 2, we asked whether this immaturity in orientation discrimination would also be reflected in a task requiring integration of oriented elements. We found that sensitivities of WS individuals for detecting orientation-defined contours were higher than sensitivities of normal 3–4 year olds, and were not significantly different from sensitivities of normal adults. Together, these results suggest that orientation discrimination and orientation integration have different maturational trajectories in normal development and different susceptibilities to damage in WS. These may reflect largely separate visuospatial mechanisms.     

Does a salient distractor capture attention early in processing?

Kim and Cave (1999) used spatial probes in order to measure the effects of bottom-up and top-down factors on the allocation of spatial attention over time. Subjects searched for a target with a unique shape, with a uniquely colored distractor present on each trial. The singleton distractor captured attention early in processing, whereas attention homed in on the target’s location later on. Kim and Cave (1999) concluded that top–down factors cannot prevent the presence of a salient distractor from delaying target selection. The present study tested the idea that such results were obtained only because subjects adopted the strategy of searching for the most salient item. Kim and Cave’s (1999) finding was replicated in Experiment 1. In Experiment 2, instead of a feature search, subjects performed a conjunction search—that is, a task that could not be performed using a salience–based strategy. Probe response times were longest at the salient distractor’s location at both the short and the long stimulus onset asynchronies. These results suggest that, early in processing, top–down factors can exert their influence and prevent the capture of attention by a salient distractor.     

Effects of top-down guidance and singleton priming on visual search

Recent literature suggests that observers can use advance knowledge of the target feature to guide their search but fail to do so whenever the target is reliably a singleton. Instead, they engage in singletondetection mode—that is, they search for the most salient object. In the present study, we aimed to test the notion of a default salience-based search mode. Using several measures, we compared search for a known target when it is always a singleton (fixed-singleton search) relative to when it is incidentally a singleton (multiple-target search). We examined the relative contributions of strategic factors (knowledge that the target is a singleton) and intertrial repetition effects (singleton priming, or the advantage of responding to a singleton target if the target on the previous trial had also been a singleton). In two experiments, singleton priming eliminated all the differences in performance between fixed-singleton and multiple-target search, suggesting that search for a known singleton may be feature based rather than salience based.     

Object-based selection: the role of attentional shifts

The objective of this paper was to investigate under what conditions object-based effects are observed. Recently, Watson and Kramer (1999) used a divided-attention task and showed that unless top-down factors induce a bias toward selection at a higher level, object-based effects are obtained when same-object targets belong to the same uniformly connected (single-UC) region, but not when they belong to different single-UC regions grouped into a higher order object (grouped-UC regions). We refine this claim by proposing that a critical factor in determining whether or not object-based effects with grouped-UC regions are observed is the need to shift attention. The results of four experiments support this hypothesis. Stimuli and displays were similar to those used by Egly, Driver, and Rafal (1994). Subjects had to make size judgments. Using different paradigms, we obtained object-based effects when the task required shifts of attention (spatial cuing, same vs. different judgment with asynchronous target onsets), but not when attention remained either broadly distributed (same vs. different judgment with simultaneous targets) or tightly focused (response competition paradigm).     

Made you blink! Contingent attentional capture produces a spatial blink

Previous studies have shown that the capture of attention by an irrelevant stimulus can be eliminated by fore knowledge of the spatial location of the relevant target stimulus. To explore whether spatial certainty is sufficient to eliminate capture, four experiments are reported in which the spatial location of the target is certain but the temporal position is uncertain. Subjects viewed a central rapid serial visual presentation stream in which a target letter was defined by a particular color (e.g., red). On critical trials, irrelevant color singletons appeared in the periphery. In Experiments 1 and 2, peripheral singletons produced a decrement in central target identification that was contingent on the match between the singleton color and the target color. Experiments 3 and 4 provided evidence that this decrement reflected a shift of spatial attention to the location of the distractor. The results suggest that spatial certainty is not sufficient to eliminate attentional capture and that attentional capture can result in a spatial "blink" that is conditional on top-down attentional control settings.