Asher Cohen

Attention and structure in sequence learning

In this study we investigated the role of attention, sequence structure, and effector specificity in learning a structured sequence of actions. Experiment 1 demonstrated that simple structured sequences can be learned in the presence of attentional distraction. The learning is unaffected by variation in distractor task difficulty, and subjects appear unaware of the structure. The structured sequence knowledge transfers from finger production to arm production {Experiment 2), suggesting that sequence specification resides in an effector-independent system. Experiments 3 and 4 demonstrated that only structures with at least some unique associations (e.g., any association in Structure 15243... or 4 to 3 in Structure 143132...) can be learned under attentional distraction. Structures with all items repeated in different orders in different parts of the structure (e.g., Sequence 132312...) require attention for learning. Such structures may require hierarchic representation, the construction of which takes attention.

Isolating attentional systems: A cognitive-anatomical analysis

Recently our knowledge of the mechanisms of visual-spatial attention has improved because of studies employing single cell recording with alert monkeys and others using performance analysis of neurological patients. These studies suggest that a complex neural network that includes parts of the posterior parietal lobe and midbrain is involved in covert shifts of visual attention. Is this system an isolated visual attentional module or is it part of a more general attentional system? Our studies employ the dual-task technique to determine whether covert visual orienting can take place while a person’s attention is engaged in a language processing task. We find clear evidence of interference between the two tasks, which suggests some common operations. However, the results also indicate that whatever is common to the two tasks does not have the same anatomical location as that of visual-spatial attention.

On the modularity of sequence representation.

Abstract A modular theory of motor control posits that the representation of an action sequence is independent of the effector (motor) system that implements the sequence. Three experiments tested this theory. Each used a variant of a method developed by Nissen and Bullemer (1987) in which subjects responded to visual signals occupying different spatial positions by pressing a key corresponding to each signal position. Sequence learning is indicated when reaction times to signals that follow a sequence become faster with practice than reaction times to random signals. The first experiment showed transfer of sequential learning of key pressing from the fingers to the arms, or vice versa. Similar transfer was found when a distraction task was added that likely blocked an attentional form of learning (cf. Curran & Keele, 1993). In a third experiment, much but not all of the sequential learning transferred from a situation in which the response was a key press to one with a vocal response, suggesting that at ...

Illusory conjunctions inside and outside the focus of attention.

This article addresses 2 questions that arise from the finding that visual scenes are first parsed into visual features: (a) the accumulation of location information about objects during their recognition and (b) the mechanism for the binding of the visual features. The first 2 experiments demonstrated that when 2 colored letters were presented outside the initial focus of attention, illusory conjunctions between the color of one letter and the shape of the other were formed only if the letters were less than 1 degree apart. Separation greater than 2 degrees resulted in fewer conjunction errors than expected by chance. Experiments 3 and 4 showed that inside the spread of attention, illusory conjunctions between the 2 letters can occur regardless of the distance between them. In addition, these experiments demonstrated that the span of attention can expand or shrink like a spotlight. The results suggest that features inside the focus of attention are integrated by an expandable focal attention mechanism that conjoins all features that appear inside its focus. Visual features outside the focus of attention may be registered with coarse location information prior to their integration. Alternatively, a quick and imprecise shift of attention to the periphery may lead to illusory conjunctions among adjacent stimuli.

Density effects in conjunction search : evidence for a coarse location mechanism of feature integration

Four experiments used the visual search paradigm to examine feature integration mechanisms. Reaction time to determine the presence or absence of a conjunctive target is relatively fast and exhaustive for low-density displays. Search rate is slow and self-terminating for high-density displays. Density effects do not arise when the target is defined by a unique feature. Two mechanisms are proposed for feature integration. A fast mechanism integrates features on the basis of coarse location information coded with the initial registration of the features. This coarse location mechanism requires that display items be spaced apart. A second, slower mechanism is used when objects are clumped together. The 2-mechanism hypothesis provides a resolution to conflicting findings in the visual search and illusory-conjunction literature. A possible interpretation of the findings with a single guided search mechanism for feature integration is also discussed.

Perceptual dimensional constraints in response selection processes.

Most processing stages theorists assume that response selection processes are largely autonomous from earlier perceptual analysis of information. We report findings that challenge this assumption and suggest that a much more complex interaction exists between perception and action. Specifically our study demonstrates that the initial analysis of visual objects into features from different dimensions strongly constrains postperceptual processes of response selection. When a target that is associated with a response on the basis of one dimension is presented simultaneously with flankers that are associated with a response on the basis of another dimension, the response to the target is not affected by the identity of the flankers. These findings hold under a variety of conditions and appear to be specific to perceptual dimensions. When both target and flankers are associated with a response on the basis of the same dimension, the response to the target is affected by the identity of the flankers even when the target and flankers are perceptually dissimilar or belong to different semantic categories (digits and letters). We propose a model of response selection that can account for these findings. The model assumes that initial response selection processes for simple features are performed within dimensional modules and suggests a specific mediating role for spatial attention. We argue that this model is compatible with several other lines of research.

Tests of a temporal theory of attentional binding

Different features of stimuli present in the field of view appear to be registered in different cortical maps. How, then, are the features that come from the same object bound together rather than mistakenly assembled with features coming from other simultaneously present objects? One theory supposes that an attentional mechanism intercepts input coming from particular retinal locations at a way station prior to parsing of the features from the same object. Any enhancement (or facilitation) at that stage will cause all the features from that object to be modified simultaneously in the downstream registers. The imposed temporal synchronicity serves as the essential binding cue. Five experiments provided no support for the theory. There is no tendency for synchronicity of features to cause binding unless the features come from the same location. Location, rather than temporal synchronicity, appears to be the essential cue for binding.

Attention and Feature Integration: Illusory Conjunctions in a Patient With a Parietal Lobe Lesion

This article explores the relation between spatial attention and feature integration in a parietal patient. When the patient had to focus her attention in the middle of the visual field by performing a digit task, she committed a much larger number of illusory conjunctions on the side contralateral to her lesion than on the side ipsilateral to her lesion. When the digit task was eliminated, thereby relaxing the requirement of focused attention, the number of illusory conjunctions on the contralateral side was dramatically reduced. These results indicate that the spatial attention mechanism plays a central role in feature integration.

Intra- and cross-dimensional visual search for single-feature targets.

Cross-dimensional visual search for single-feature targets that differed from the distractors along two dimensions (color and orientation) was compared with intradimensional search for targets that differed from the distractors along a single dimension (either orientation or color). The design of the first three experiments differed from those of previous studies in that participants were required to respond differently to each of the targets. Experiments 1–3 were similar except that in Experiment 1, the distractors were homogeneous; in Experiment 2, two types of distractors were used in equal proportions; and in Experiment 3, two types of distractors were used but one of the distractors was a singleton. The findings, contrary to those of previous studies, revealed that cross-dimensional search is at least as efficient and for some targets even more efficient than intradimensional search. These results suggest that the details of stimulus-to-response mapping are essential in comparing intra- and cross-dimensional tasks. Experiment 4 used a priming design and did not support an explanation based on grouping processes. We outline an explanation for all the findings based on a recent cross-dimensional response selection model by Cohen and Shoup (1997).

Visual Attention and Coactivation of Response Decisions for Features from Different Dimensions

The role of visual attention in task performance has been extensively debated. On the basis of the dimensional-action model, we hypothesized that a major role of attention is to transfer response decisions from targets on which it is focused to high-level centers dealing with response execution. This hypothesis predicts that response decisions for two targets will interact only when attention is focused on both targets, and only when the response to the targets is defined by different dimensions. Three experiments, using the redundancy-gain paradigm, tested and confirmed this prediction. Experiment 1 showed that coactivation of two cross-dimensional targets occurred only when the targets were positioned in the same location, not when they were in separate locations. Experiment 2 manipulated the focus of attention and showed that coactivation can occur even for targets positioned in different locations if they are both within the attentional focus. Experiment 3 showed that this attention-induced coactivation does not occur for targets from the same dimensional module. These results suggest that a major role of attention is postperceptual and involves gating of selected responses to executive functions.