Noise Letters Research Articles

Attentional distribution in the visual field during same-different judgments as assessed by response competition.

Judgments of same and different on a comparison task have been found to be subject to response competition if an irrelevant stimulus is presented in the display along with the target stimuli. For example, the reaction time for judging two letters the same is markedly increased if a different but irrelevant letter is also present in the display (C. W. Eriksen, O'Hara, & B. [A.] Eriksen, 1982). We have made use of this competition effect to map the visual attentional field in two dimensions. In two experiments, we varied the size of the attended area by varying the separation of the comparison stimuli. The boundaries of the attended area were mapped by varying the location of a response-competitive irrelevant noise letter. On this task, the attended area was found to be elliptical in shape, with the location of the target stimuli defining the major axis. The minor axis of the ellipse increased in direct proportion to increases in the major axis. Rather than interpret these field effects in terms of areas of enhanced processing, we propose that instead they represent the limits or failures in inhibition of competing stimulation.

Age-related differences in the timing of stimulus and response processes during visual selective attention: performance and psychophysiological analyses.

In this study, age-related differences in the selection of visual information were investigated. Two groups of younger and older subjects performed focused- and divided-attention (i.e., visual search) tasks. In the focused-attention task, centrally presented target letters could be flanked by compatible or incompatible noise letters. In the visual-search task, targets could be cued or uncued, and target locations could be spatially compatible or incompatible with the responding hand. P3 latency, lateralized readiness potentials, the electromyogram, and reaction times were used to detect possible age-related differences in the timing of stimulus- and response-related processes during selective processing of information. In the focused-attention task, performance of older subjects showed greater interference by incompatible flankers than did that of younger subjects because of stronger response competition caused by partial activation of the incorrect response channel by the incompatible flankers. No evidence was found of specific age-related differences in the efficiency of visual search in a divided-attention task. Furthermore, in both tasks, younger subjects showed an earlier start of response execution (in the electromyogram) relative to the onset of response preparation (lateralized readiness potential) and a higher percentage of incorrect electromyographic activity than did older subjects.

Time courses in the negative and positive repetition effects.

Time courses of the negative repetition effect (NRE), a poorer detection of the target in noise-same-as-target than in noise-alternative-target displays, and its opposite, the positive repetition effect (PRE), were examined. Experiment 1 showed that displays in which a low-contrast target was present with a high-contrast noise produced a larger NRE than did displays in which a contrast relationship between items was reversed. A negative contrast repetition effect (NCRE) was also found that was comparable to the NRE. Experiment 2 showed that dimensional Korean letters (e.g., [symbol: see text]) whose configural differences were apparent in orientation produced the largest PRE at a 0-ms stimulus onset asynchrony, whereas featural Korean letters (e.g., [symbol: see text]) that differed in the number of elements yielded the largest NRE when a noise letter preceded a target letter by 50 ms. Experiments 3A and 3B indicated that the NCRE may arise from spatial attention.

What makes targets redundant?

Two letter-classification experiments that investigated target-redundancy effects on reaction time (RT) were conducted. Both experiments were replicated with choice reaction time (CRT) and go/no-go (GNG) procedures. In each experiment, there were two single-target conditions, one with a noise letter and one without. In one experiment, the letter classes were two letters that could be of either case. In the second experiment, each class consisted of two different capital letters. In both experiments, there were two redundant-targets conditions, one with identical targets and one with the different members of a class. In both of the GNG experiments, redundancy gains were obtained comparing the different-targets condition with the no-noise, single-target condition. Redundant stimuli are ones that lead to the same response. Visually different stimuli may be processed in parallel and jointly activate a response. GNG procedures are more sensitive than CRT in the investigation of redundancy effects.

Probability effects on stimulus evaluation and response processes.

This study investigated the effects of probability information on response preparation and stimulus evaluation. Eight subjects responded with one hand to the target letter H and with the other to the target letter S. The target letter was surrounded by noise letters that were either the same as or different from the target letter. In 2 conditions, the targets were preceded by a warning stimulus unrelated to the target letter. In 2 other conditions, a warning letter predicted that the same letter or the opposite letter would appear as the imperative stimulus with .80 probability. Correct reaction times were faster and error rates were lower when imperative stimuli confirmed the predictions of the warning stimulus. Probability information affected (a) the preparation of motor responses during the foreperiod, (b) the development of expectancies for a particular target letter, and (c) a process sensitive to the identities of letter stimuli but not to their locations.

Reducing the effects of adjacent distractors by narrowing attention.

Three experiments explored the gradual narrowing of visual attention to a letter target when other letters were positioned close by. The method by which attention was narrowed involved presenting a digit target immediately prior to the latter target and in the same location for progressively shorter durations and requiring the subject to identify both the digit target and the letter target before responding. The response time data from the first 2 experiments indicated that shorter durations of the digit reduced the amount of information processed from noise letters positioned on either side of the letter target. In the third experiment, in which separation of letters was increased slightly, the response times indicated that the information from flanking noise letters may have been virtually eliminated.

How much processing do nonattended stimuli receive? Apparently very little, but...

The early versus late selection issue in attention models was examined by means of a new methodology. Through cues or precues, attention was directed to one location of a multistimulus visual display and, while attention was so engaged, the identity of a stimulus located at a different position in the display was changed. By varying the time after display onset before the stimulus was changed, we controlled the preview time that the original stimulus was represented on the retina. Then, using a marker cue, we directed the subject's attention to the location of the changed stimulus. The subject's response was a timed discrimination between two possible target letters. The data of main interest was the effect of preview time upon the subject's latency in identifying the new target that appeared in the changed location. We found that the preview time of the original stimulus, before RT was affected to the new target, depended upon whether the original stimulus was a neutral (noise) letter or whether it was the alternative target. When the original stimulus was a noise letter, RTs to the new target were just as fast as those obtained in the control condition in which the target was present throughout the preview interval and did not change its identity. Significant effects upon RT were obtained at preview times of 83 msec when the original stimulus was one of the targets that changed to the alternative target. Preview times also varied as a function of precuing. Preview times were correspondingly shortened when the first cue occurred 50 msec before display onset, thus providing an extra 50 msec for attention to be directed to the first display location. The results were interpreted in terms of two separate information-processing systems in the human: an automatic system and an attentional system. Even though a stimulus may have been automatically processed, when the attention system is directed to that stimulus, processing starts at the beginning again.

Redundancy phenomena are affected by response requirements

Results are reported for two go/no-go reaction time (RT) experiments, in which the redundant targets advantage was investigated. These experiments were replications of two earlier choice reaction time (CRT) experiments, in which letter stimuli were used. Important differences between the go/no-go RT experiments and the CRT experiments were obtained. Equal and significant redundancy advantages were obtained whether redundant targets were compared with a single target presented with a noise letter or without noise. In the CRT experiments, the advantage was not obtained in the comparison with a single target presented alone. Noise letters did not slow the RTs to single targets with which they were presented as was the case with CRT. Since the differing results of the two procedures depend on the response requirements, explanation of differing CRT data in terms of perceptual or attentional concepts is probably inappropriate. The presence and absence of response competition in the two situations may be the best interpretation. The results tend to support a conclusion of the parallel processing of two letter stimuli separated spatially by as much as 3 degrees.

Schizophrenic deficit in span of apprehension.

We investigated two hypotheses about the span of apprehension task in schizophrenia: (a) schizophrenics show performance deficit on the forced-choice (FC) version but not on the full-report (FR) version, and (b) schizophrenic impairment on the FC version is greater when the display subtends a wide visual angle than when it subtends a narrow one. Schizophrenic (n = 21) and normal (n = 22) groups were tested on 3 versions of the task. A narrow-angle FC version was matched psychometrically with a wide-angle one by use of a greater number of noise letters in the narrow version. Schizophrenics reported fewer correct letters than normals on the FR version but did not differ on either the wide or the narrow FC versions. The results imply that schizophrenic deficit is not specific to the FC version and that on the FC version, visual angle is not more important than number of noise letters for demonstrating schizophrenic deficit.

Processing redundant signals: coactivation, divided attention, or what?

The evidence for and against a redundancy gain in reaction time (RT) when the target is repeated in the visual display is reviewed. We consider the relevance of redundancy gains under these circumstances to the question of whether attention can be simultaneously directed to separate locations in the visual field. In the present experiments, two capital letters were the target stimuli in a two-alternative forced-choice RT paradigm. In addition to the usual conditions of single-target trials, trials on which the target is repeated in the display, and trials on which the target occurs with a noise letter, we introduced the innovation of a condition in which both targets occur in the display. In our two experiments, RT was fastest with single-target displays and slowest with displays containing a target and a noise letter. There was no significant difference in RT to displays in which the target was repeated and displays in which both targets were presented. Both conditions showed a redundancy gain when compared with displays containing a target and a noise letter. The lack of response competition in the both-targets condition and the overall pattern of the results were well explained by a unitary attentional focus that serially processed the letters in the display. Analyses of minima and maxima RTs were consistent with this interpretation.

Effects of size of letters on selective identification of letters

An experiment was performed to examine the effects of the size of target and noise letters on selective identification of a target letter. Twelve undergraduate students were required to identify a target letter on a visual display which presents a target letter alone or a target letter flanked by two noise letters. The reaction time of pressing buttons to the target letter was measured under combined conditions of size of target and noise letters. The large target letter was identified faster than the small one whether it was accompanied by noise letters or not. The large noise letters exerted stronger effects on the identification of a target letter than the small one. The results agreed with the prediction based on the assertion by Eriksen and Schultz (1979). Further, it was found that the effects of size of letters varied according to the relation between target letters and noise letters and to the similarity of a target letter to noise letters.

A response-selection basis for the mixed-category, repeated-stimulus inferiority effect.

When subjects identify a target stimulus with an assigned keypress response, flanking noise stimuli produce interference if they signal an alternative response and slight facilitation if they are identical to the target. However, when the possible stimuli come from two distinct categories Getters and digits), interference also occurs if the noise letters are identical to the target. Four experiments were conducted to determine whether this mixed-category, repeated-stimulus inferiority effect is due to stimulus-identification or response-selection processes. The inferiority effect was (1) absent when letters were assigned to one response and digits to another; (2) absent when the target stimulus was named, rather than identified by a keypress response; and (3) eliminated when subjects practiced with mixed assignments of letters and digits. These findings converge on a response-selection basis for the inferiority effect.

Temporal changes in the distribution of attention in the visual field in response to precues.

This experiment measured the changes in the distribution of attentional processing in the visual field in response to precues. The hypothesis of an analogue movement of the attentional “spotlight” across the visual field was also tested. Choice reaction time to stimuli was determined in a response competition paradigm. Subjects viewed a central fixation point and the stimuli (the letters A and U) were presented at retinal eccentricities of 1°, 2°, and 3° along the horizontal meridian to the left and right of fixation. Two letters were simultaneously presented in all experimental conditions. For compatible noise, the second letter was identical to the target, and for incompatible noise, the second letter was the other letter of the target set. The target letter in a trial was designated by an underline that either occurred simultaneously with the onset of the two-letter display or preceded the display by 25, 50, 75, 100, or 175 msec. The target letter occurred equally often in each of the six display positions and the noise letter equally often in each of the remaining positions. The response competition effect of an incompatible noise letter was found to be a joint function of target and noise-letter location, an effect that changed over time from onset of the underline precue to the onset of the display. With simultaneous onset, interference was largely independent of target and noise location, but with increasing asynchrony of onset, noise interference became increasingly confined to locations adjacent to the target. No evidence was found for an analogue movement of an attentional “spotlight” across the visual field.

Dependence of target redundancy effects on noise conditions and number of targets.

In a letter identification task using a four-position array, reaction time (RT) decreased as a negatively accelerated function of the number of targets, both with and without noise letters in nontarget positions. The effect was much greater in the noise condition, in which each redundant target replaced a noise letter. In such situations, the redundant-targets effect should probably be attributed to noise reduction rather than to redundancy. Scaling analysis of the RT distributions indicated that neither noise nor redundancy affects the rate of growth of excitatory strength, but that both affect the time at which growth begins. Excitatory strength grew as a function of the number of redundant targets according to a principle of decreasing gain, with much greater growth in the noise conditions. Further experiments indicated that whether noise is manipulated between or within subjects is important. Unless all stimuli present are reliabl targets, redundancy effects are eliminated or reduced in no-noise conditions.

Visual attention within and around the field of focal attention: a zoom lens model.

The operation of attention in the visual field has often been compared to a spotlight. We propose that a more apt analogy is that of a zoom or variable-power lens. Two experiments focused upon the following questions: (1) Can the spatial extent of the attentional focus be made to vary in response to precues? (2) As the area of the attentional focus increases, is there a decrease in processing efficiency for stimuli within the focus? (3) Is the boundary of the focus sharply demarked from the residual field, or does it show a gradual dropoff in processing resources? Subjects were required to search eight-letter circular displays for one of two target letters and reaction times were recorded. One to four adjacent display positions were precued by underlines at various stimulus onset asynchronies before display presentation. A response competition paradigm was used, in which the “other target” was used as a noise letter in noncued as well as cued locations. The results were in good agreement with the zoom lens model.

Recognition memory and attentional selection: serial scanning is not enough.

In two experiments, using memory sets of up to 10 letters, the response competition paradigm was employed to investigate the extent to which extraneous visual stimuli interfere with or affect the process of memory search. It was assumed that if selective attention could exclude the effect of noise letters from a Sternberg-type memory comparison process, then there would be an increase in intercept for the reaction time-set size functions but no increase in slope. This result was obtained. However, a large difference in response times to both positive and negative set targets was found when the accompanying noise letters indicated a competing response, as opposed to when they indicated the same response as the target. This implies rapid identification of the nature of both target and noise, independent of a serial comparison process. A modification of a dual process model (Juola, Fischler, Wood, & Atkinson, 1971) in which stimuli activate a familiarity value independent of memory search was suggested to account for these results.

Temporal characteristics of noise conditions producing facilitation and interference.

In reaction time (RT) experiments, a target letter was flanked by noise letters with differing relations to the target. When noise letters that are identical to the target, or response-compatible but not identical, precede the target by 100 msec, RT is facilitated. Facilitation by identical noise is greater. Scaling analysis of the RT distributions indicated that facilitation by compatible noise is a very short-latency, transient effect. Facilitation by identical noise is more persistent. It is suggested that there are two facilitative processes-direct response priming by compatibility, and facilitation of stimulus coding by stimulus identity. Facilitation is not obtained when these noise letters are presented simultaneously with the target. Associative interference is produced by response-incompatible noise letters, both with simultaneous presentation and with noise preceding the target. Maximum interference is earlier when noise precedes the target. This confirms previous findings and a prediction of continuous growth theory.

An electromyographic examination of response competition

Subjects were required to discriminate the letters S and H in a choice reaction time task. They responded with a thumbpress of one hand to one of the letters and with the thumb of the other hand to the other letter. In the incompatible noise condition, the target letter was flanked by the opposite response letter, and in the compatible noise condition, the flanking noise letters were identical to the target. In addition to overt reaction time, associated electromyographic activity was recorded in both arms on each trial. In keeping with previous findings, incompatible noise significantly and appreciably increased reaction time for the overt response. More importantly, in the incompatible condition, the occurrence of a correct thumb response was frequently accompanied by an EMG in the arm appropriate to the noise letters. This result was interpreted as showing that the noise letters were processed to the point of incipient response activation simultaneously with the target letter. The results were predicted by the continuous flow model of C. W. Eriksen and Schultz (1979) and the variable criterion theory of Grice, Nullmeyer, and Spiker (1982).

Effects of noise letters upon selective identification of letters

Eighteen undergraduate students participated in two experiments which were designed to investigate the effects of noise letters upon selective identification of target letters. The reaction time of pressing buttons to the target letter was measured under various combinations of accompanying noise letters located 1 degree 48' apart from a target in visual angle, and added noise letters inserted between the target and accompanying noise letters. The results provided evidence in favor of the hypothesis which attributes the interfering effects of noise letters not to the processing level, but to the response level. Moreover, the results suggested that the noise letter adjacent to a target plays an important part in this interference, though the noise letters can be processed even in the position as much as 1 degree 48' apart from a target, and that it is especially important what relations the adjacent noise letter has with the response of the target.

A psychophysiological investigation of the continuous flow model of human information processing.

Twelve subjects responded to target letters "H" or "S" by squeezing dynamometers with the left or right hand. Targets could be surrounded by compatible (e.g., HHHHH) or incompatible noise (SSHSS) letters. Measures of the P300 component of the event-related brain potential and of correct and incorrect electromyographic and squeeze activity were used to study stimulus evaluation and response-related processes. When incorrect squeeze activity was present, execution of the correct response was prolonged, indicating a process of response competition. This process occurred more often under incompatible noise conditions, which were also associated with a delayed P300. Thus, the noise/compatibility manipulation influenced both stimulus evaluation and response competition processes. In contrast, a warning tone that preceded array presentation on half the trials, increased response speed without influencing evaluation time. The data suggest that the latency and accuracy of overt behavioral responses are a function of (a) a response activation process controlled by an evaluation process that accumulates evidence gradually, (b) a response priming process that is independent of stimulus evaluation, and (c) a response competition process.