Interactions of signal and background variables in visual processing

Three variables which determine the opportunities for signal-noise confusions, display size (D), number of redundant signals per display (N), and number of alternative signals (A) were studied in relation to nature of the noise elements, confusable or nonconfusable with signals. Data were obtained in a forced-choice visual detection situation, the displays being linear arrays of letters on a CRT screen. For all three performance measures used, frequency of correct detections and correct and error latencies, strong interactions were obtained between all of the other variables and signal-noise confusability. The functions obtained, together with other data bearing on the role of confusions and on spatial relations among characters within the display, suggest a model whose initial phase is a parallel feature extraction process involving inhibitory relations among input channels.     

Effects of noise letters upon the identification of a target letter in a nonsearch task

During a 1-sec tachistoscopic exposure, Ss responded with a right or left leverpress to a single target letter from the sets H and K or S and C. The target always appeared directly above the fixation cross. Experimentally varied were the types of noise letters (response compatible or incompatible) flanking the target and the spacing between the letters in the display. In all noise conditions, reaction time (RT) decreased as between-letter spacing increased. However, noise letters of the opposite response set were found to impair RT significantly more than same response set noise, while mixed noise letters belonging to neither set but having set-related features produced intermediate impairment. Differences between two target-alone control conditions, one presented intermixed with noise-condition trials and one presented separately in blocks, gave evidence of a preparatory set on the part of Ss to inhibit responses to the noise letters. It was concluded that S cannot prevent processing of noise letters occurring within about 1 deg of the target due to the nature of processing channel capacity and must inhibit his response until he is able to discriminate exactly which letter is in the target position. This discrimination is more difficult and time consuming at closer spacings, and inhibition is more difficult when noise letters indicate the opposite response from the targe     

Form of redundancy as a determinant of tachistoscopic word recognition

Single letters and redundant CVCs and CCCs were presented at threshold durations and followed by a pattern mask. Performance on three-letter stimuli was greater than on single letters. The increased performance was consistent with a state model of perceptual independence. Conditional redundancy increased the frequency with which all three letters were accurately reported as compared to correlational redundancy. Meaning and pronounceability had little effect on performance.     

Effects of noise letters on decisions: Discrete or continuous flow of information?

Four forced-choice letter-detection experiments examined the effect on detection latency of noise letters that were visually similar to target letters. A single target letter was present in each display. Noise letters were similar to the target letter present in the display (the signal), to a different target letter assigned to the same response as the signal, or to a target letter assigned to the opposite response from the signal. Noise letters were present in either relevant or irrelevant display positions, and two quite different stimulus sets were used. The experiments were designed to test a prediction of models in which information about noise letters is transmitted continuously from the recognition to the decision process. These models predict that responses should be faster when noise letters are visually similar to a target assigned to the same response as the signal than when noise letters are similar to a target assigned to the opposite response. Statistically reliable effects of the type predicted by continuous models were obtained when noise letters appeared in relevant display positions, but not when they appeared in irrelevant positions.     

Further evidence concerning scanning and sampling assumptions of visual detection models

Following extended training in a visual detection task, functions were determined for individual Ss relating latency of detection responses to number of redundant signal elements embedded in tachistoscopic displays of letters and to distance between signal elements. Latency proved invariant with respect to number of redundant signals and varied non-monotonically with distance. Of the several types of models considered in relation to previous studies, substantial support was forthcoming only for an independent stimulus sampling model. It is suggested that the detection method effects a relatively clear separation of the perceptual from the mnemonic aspects of the standard visual apprehension experiment, and that the sampling process may constitute only the first phase of a more general model which includes both parallel and serial information processing.     

Response latency in visual search with redundancy in the visual display

Two experiments were run to investigate the effects of redundant display items upon response latency in a visual search task. In the first study, Ss searched five-letter displays for a predesignated critical letter. Both critical and noncritical letters could be repeated in the displays. Mean response latency decreased markedly with increasing redundancy in the critical letter and was affected to a lesser extent by redundancy in the noncritical letters. In the second study, Ss were required to detect the presence of redundant letters in displays of from two to five letters, first with no information as to what letter might be repeated, then with knowledge of which letter would be repeated if the display contained a redundant letter. Response latencies in the former case were much slower than in the latter. The implications of these findings for current views of visual information processing were discussed.     

Pattern redundancy and detection in very short-term memory

A tachistoscopically presented target letter is detected better in a background of redundant items than in nonredundant material. The present experiment extended this finding by showing that detection rate increased systematically across three levels of background letter redundancy. A conclusion based on the earlier findings was that a serial process of letters in memory could not account for the effects of background redundancy on detection accuracy. Contrary to this conclusion, the present analysis showed that detection for semiredundant displays was successfully predicted by two variants of a serial process model. One model assumed that the scan of a letter is faster when the letter is the same as any prior letter scanned in the matrix. The other model assumes that nonredundant letters are processed serially and that redundant letters are processed all at once, i.e., in parallel.     

Multisensory integration of vision and touch in nonspatial feature discrimination tasks

Multisensory integration of nonspatial features between vision and touch was investigated by examining the effects of redundant signals of visual and tactile inputs. In the present experiments, visual letter stimuli and/or tactile letter stimuli were presented, which participants were asked to identify as quickly as possible. The results of Experiment 1 demonstrated faster reaction times for bimodal stimuli than for unimodal stimuli (the redundant signals effect (RSE)). The RSE was due to coactivation of figural representations from the visual and tactile modalities. This coactivation did not occur for a simple stimulus detection task (Experiment 2) or for bimodal stimuli with the same semantic information but different physical stimulus features (Experiment 3). The findings suggest that the integration process might occur at a relatively early stage of object-identification prior to the decision level.     

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.     

Does backward masking by visual noise stop stimulus processing?

The identification of one, two, and four random letters was studied under three procedures: (1) backward masking by a visual noise; (2) concurrent masking by a visual noise; and (3) no masking. With backward masking the number of letters correctly identified was independent of the number presented. Direct judgments of the duration, brightness, contrast, sharpness, and texture of the letters were also made. Under backward masking the letters appeared to be on for a very brief duration, but with high apparent contrast. The results indicate that backward masking impairs identification by interrupting the stimulus processing, not by degrading the stimulus input.     

Positive effect of heterogeneity of difference on thesame-different disparity in letter matching

Subjects judged whether two adjacent letters were identical or different.Different pairs were similar, for example HM, DO, VY, or dissimilar, for example DY, HV,MO. According to the noisy-operator theory, increasing the heterogeneity of difference (external noise) by intermixing similar and dissimilardifferent pairs ought to produce faster but less accurate responses ondifferent trials. As predicted, the tendency to make more false-different responses (i.e., errors on same pairs) decreased when the similar and dissimilardifferent pairs were intermixed rather than presented in separate blocks. The fast-same effect did not change, however, seemingly due to criterion misadjustment. Proctor and Rao apparently found no effect of heterogeneity of difference on RT and errors because they used a less sensitive procedure (between-subjects design; different set of letters for more and less heterogeneous conditions). Consistent with the internal-noise principle, but not with the response-competition model of Eriksen, O’Hara, and Eriksen, the enhanced fast-same effect on blocks containing only similar (vs. dissimilar)different pairs was accompanied by an increased preponderance of false-different errors.     

The effects of location cuing on redundant-target processing

The present study is concerned with the redundany gain: the observation that subjects respond faster to simultaneously presented redundant targets than to single targets. This finding is usually interpreted as evidence for parallel, self-terminating, unlimited-capacity processing. Alternatively, it has been claimed that the reaction-time advantage with redundant targets is simply due to spatial uncertainty under single-target conditions. The present study tested this hypothesis. In Experiment 1, subjects responded when one, two, or three letters E were presented, and refrained from responding when one, two, or three letters F were presented. In half of the trials, location uncertainty was eliminated by presentation of a line segment at one of the locations of the subsequently appearing target letters. The results reject the alternative spatial-uncertainty explanation: even when the location of the impending target is cued in advance, there is no attenuation of the redundancy gain. Experiment 2 served as a control experiment and showed a clear redundancy gain, even in conditions in which it was ensured that, before display onset, attention was directed to a location of one of the impending targets.     

Preconscious analysis of global structure: Evidence from masked priming

According to the global precedence hypothesis, the perceptual processing of complex objects proceeds from global structure to the analysis of local elements. In the present study, we used a masked priming paradigm to explore whether the global or the local level of hierarchical letters is analysed at preconscious processing stages. Experiment 1 found masked priming only after global prime letters in focused‐attention conditions. Experiment 2 used a divided‐attention task in which attention was not focused specifically on either level of hierarchy and did not find any priming. Experiment 3 used otherwise the same task as Experiment 2 but biased attention either to the global or the local level by manipulating the probability that targets appeared at one level. Priming was found after global prime letters in the global‐bias condition but not in the local‐bias condition. Experiments 4a and 4b suggest that the size of the local letters was not responsible for the lack of priming after local primes. The results suggest a priority for global processing already at a preconscious level and that attentional factors may modulate processes at this level.     

Coactivation in the perception of redundant targets

Reaction time (RT) to redundant stimuli was investigated while controlling for distraction effects and response competition. In Experiment 1, a redundancy gain was found for 2 target letters with identical features (redundant) compared to trials in which 2 different targets shared the same response assignment (compatible) indicating coactivation of stimulus inputs. No difference in RTs was found between compatible displays and displays containing 2 targets with different responses (incompatible), suggesting (with other evidence) that letters were serially processed. In Experiment 2, a redundancy gain was again found. Unlike in Experiment 1, incompatible displays produced response competition, indicating a redundancy gain with parallel processing. Three forms of redundancy gains operating under specific conditions are discussed.     

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.     

Locus of the redundant-signals effect in bimodal divided attention: a neurophysiological analysis.

We reanalyzed the data from the study of Lamarre, Busby, and Spidalieri (1983). In that study, the activity of single neurons in area 4 of the motor cortex was recorded during a bimodal detection task in which a monkey (Macaca mulatta) had to respond as quickly as possible to a visual or an auditory signal or to both (redundant trials). Manual responses on redundant trials were speeded by the presence of both signals, as is typically found. The times between signal onsets and the first changes in neuronal activity were also speeded by redundant signals, but there was no difference between redundant-signals and single-signal trials in the time between the change in neuronal activity and movement onset. These results suggest that late motor processes are not speeded by redundant signals in bimodal detection tasks.     

Evidence in support of word unitization

In each of five experiments, the subjects viewed tachistoscopically presented pairs of letters and made speeded comparison judgments on the basis of name identity. On most trials, a noise letter string (word or anagram) was placed directly between the target letters. The results indicated that correct “same” RTs were a function of noise item type and its relation to target letters. Anagrams increased RTs more than their counterpart words, except when the noise word was either unmeaningflul or response incompatible with respect to the target letters (e.g., B Tea b). The interference effects were also found to be independent of sequence length. It was posited that the subjects were unable to completely ignore the irrelevant attributes of the displays and that under certain conditions, the subjects were able to identify the noise items in a holistic fashion. The data were interpreted in terms of a unitization hypothesis of word recognition, response competition, and a continuous-flow conception of information processing.     

Effects of noise similarity and redundancy on the information processed from brief visual displays

When a target letter form is presented tachistoscopicalfy and embedded in a matrix of irrelevant letter forms (noise), correct detections of the target at a given matrix size can be used to estimate indirectly the amount of information processed from the display, according to a model developed by Estes. In three experiments, the physical similarity between noise and target letters and noise-letter redundancy were varied to determine if the detection mechanism was responsive to these variables. It was found that increases in the physical differences between noise and target letters and increases in redundancy facilitated detection. It is argued that these data support a detection model that assumes parallel rather than serial information processing.     

Time-course of Spatial-attentional Focusing in the Case of High Processing Demand on the Peripheral Precue

Subjects had to identify pairs of spatially overlapping or neighbouring letters exposed successively at various locations around the central fixation. The first letter (S1) in a pair acted both as a physical precue to indicate the S2 location and as a stimulus letter to be identified. The second letter (S2) in a pair acted only as a letter to be identified. The SOA was varied between 0 and 320 ms in different experiments. Comparison of the time-course functions of S2 identification in the conditions of (1) single pair presentation and (2) presentation of the letter pairs in the context of distractor letters that accompanied S2 in time, but occupied alternative spatial positions, revealed that S2 processing from the covert spatial-attentional focus was open to non-decreasing interference from the distractor letters up to 240 ms SOAs. An unexpected result consisted in equal level of S2 identification regardless of whether S1 had to be identified or not. This shows that if the peripheral precue has to be processed with higher processing demands at the identification level beyond simple location processing (which by itself is sufficient to fulfil the spatial orienting function), then the narrowing down or zooming-in of the attentional area in response to a physical precue can take place as effectively as in the case of a lesser processing demand on that precue. By comparing the relative time-course functions of S1 and S2 identification, it was found that S2 identification gained significant advantage over S1 identification at intermediate SOAs even if the intensity of S1 exceeded that of S2. The results of this investigation refer to the spontaneous, automatic nature of spatial attentional focusing that is evoked by spatially localised transient signals, but also to the possibility of interference between the targets and distractors in the working memory representation.     

Locus of the redundant-signals effect in bimodal divided attention: A neurophysiological analysis

We reanalyzed the data from the study of Lamarre, Busby, and Spidalieri (1983). In that study, the activity of single neurons in area 4 of the motor cortex was recorded during a bimodal detection task in which a monkey (Macaca mulatta)had to respond as quickly as possible to a visual or an auditory signal or to both (redundant trials). Manual responses on redundant trials were speeded by the presence of both signals, as is typically found. The times between signal onsets and the first changes in neuronal activity were also speeded by redundant signals, but there was no difference between redundant-signals and single-signal trials in the time between the change in neuronal activity and movement onset. These results suggest that late motor processes are not speeded by redundant signals in bimodal detection tasks.