Selective response activation can begin before stimulus recognition is complete: a psychophysiological and error analysis of continuous flow

In discussions of process models of human information processing, the continuous flow conception (Eriksen and Schultz 1979) plays a prominent role. A central prediction of this conception is that any information in a display associated with a response activates that response as soon as it becomes available in the perceptual system. If it concerns the correct response channel, then response facilitation occurs. If it concerns the incorrect response channel, then response competition occurs. To assess these mechanisms more directly, we used psychophysiological measures as well as reaction time (RT). We used the latency of the P3 component of the event related brain potential (ERP) as an index of stimulus evaluation duration, the onset of lateralized motor activity derived from the ERP as an index of selective central motor activation, and the onset of electromyographic activity as an index of the start of peripheral motor activation. Subjects were required to respond to target letters that were either flanked by letters that signalled the opposite response (incompatible arrays), by the target itself (compatible arrays), by letters not associated with a response (neutral arrays), or by no other letters (targets alone). Our results replicated the basic findings obtained in this paradigm. RTs to targets alone did not differ from RTs to compatible arrays. The latter were faster than RTs to neutral arrays, which were faster than RTs to incompatible arrays. P3 latencies were longer on incompatible than on neutral trials, and longer on compatible than on target alone trials. Incorrect central response activation on incompatible trials and correct central response activation on compatible trials, both began earlier than on target alone trials. Peripheral responding on both trial types, however, began later than on target alone trials. More incompatible but less compatible trials than neutral ones exhibited incorrect peripheral response activation. Peripheral response execution was faster and more accurate on compatible than on target alone trials, while it was slower and less accurate on incompatible than on neutral trials. These results indicate, that the flankers activated their associated response channel while display evaluation was still going on, and that response facilitation and competition occurred. After applying criteria proposed by Miller (1988), it was concluded that the set of stimulus recognition processes and the set of response activation processes cannot be regarded as independent stages of processing.     

Capacity, duration, and position effects in visual memory following a successive field procedure

This study considered the efficacy of a Successive Field procedure in revealing properties of iconic memory. 8 subjects were required to identify differences between sequentially presented stimulus and target arrays of letters. While results were broadly in line with previous estimates of the capacity and duration of iconic storage, there was also evidence that identification of target letters depended on their position in the array.     

Recognition time for letters and nonletters: effects of serial position, array size, and processing order

Three experiments address the question of whether end-effects (the advantage for the first and last letters in multi-letter arrays) are due to sensory or cognitive factors. The basic strategy used was to compare letters with symbol stimuli that do not lend themselves to top-down processing. Serial position functions using central fixation were obtained in the first experiment for letters, digits, and symbols. End-effects were present with both letters and digits, indicating that processes limited to word recognition cannot be responsible for end-effects. Five-symbol arrays, however, yielded U-shaped reaction time functions, with poorest performance at the initial and terminal positions. The effects of array size and retinal placement were investigated with letters and symbols in a second experiment. Multi-letter and symbol arrays differed primarily in that letters showed end-effects that were independent of retinal placement, whereas symbols did not. The conclusion that multi-letter and symbol arrays are not processed in the same way was tested in a third experiment. Using a paradigm that eliminated processing order as a variable, the experiment obtained identical effects of ordinal position within an array for letters, digits, and symbols. It is concluded that sensory factors cannot provide a viable account of letter end-effects and that letter and nonletter comparisons can improve our understanding of the sensory and cognitive factors involved in letter perception.     

Letter string processing and visual short-term memory

The present study investigated whether expertise with letter string processing influences visual short-term memory capacity. Specifically, we examined whether performance in a change-detection task would vary as a function of stimulus type (letters vs. symbols) and type of display (horizontal, vertical, and circular). Participants were asked to detect a one-character change in a briefly presented character array following a delay of 900?ms. Concurrent articulation was used to limit effects of rehearsal. Type of display significantly affected performance with letters, but not with symbols, with a selective increase in change-detection accuracy for horizontally presented letter arrays compared with vertical and circular arrays. These findings confirm the standard limits of storage in visual short-term memory, but critically reveal a selective advantage for letter arrays over symbol arrays when presented horizontally. Such an advantage is probably due to the utilization of a specialized encoding mechanism built up over years of reading experience.     

Effects of array organization on same-different judgments

Two experiments investigated the role of array organization in the simultaneous matching task. Multielement arrays of letters were arranged in either “good” or “poor” configurations and subjects were asked to determine whether all the letters were the same or if at least one was different. Correct “different” responses were slower when the letters were presented in good configurations than in poor configurations. Correct “same” responses, however, were not affected by the organizational quality (goodness) of the array. This effect of pattern goodness on matching performance was not affected by the confusability of the letters being compared, the relative eccentricity (spread) of the configuration, or the ratio of different letters present on different trials. The results suggest an encoding stage locus for the effect of pattern goodness and provide evidence supporting a two-process model of same-different judgments.     

A diffusion model of early visual search: Theoretical analysis and experimental results

Summary Meinecke (1989, Exp. 1, cond. HO) showed that the detectability of a visual target embedded in a linear noise array decreases with increasing retinal eccentricity, while the reaction time (RT) of the hits increases. One of the most interesting features of her results was that the RT of the correct rejections is consistently larger than the RT for signals presented near the fovea. This finding suggests that initially visual attention is concentrated near the fixation point and then diffuses across the stimulus array to perform a serial, exhaustive search. We present a diffusion model of early visual-search processes that quantitatively describes this evolution of attention in time and space; in contrast to most previous conceptions, it is based on a genuine relation between the spatial and temporal dimensions of the search processes performed. The model predicts quantitatively both detection performance and RT. We conducted an experiment similar to that of Meinecke (1989), but with an additional variation of the presentation time. All the main features of the 189 predictions could be explained by the model. The interpretation of the four model's parameters is discussed in some detail and compared with previous estimates of the microscopic search speed derived from alternative models. Finally, we consider some possible modifications related to results of Kehrer (1987, 1989), and some generalizations to multi target detection and two-dimensional stimulus arrays.     

Scanning,holistic encoding,and the word-superiority effect

The role of holistic encoding induced by scanning in the superior identifiability target letters in orthographic strings (words) enjoy over those in strings of unrelated letters was examined in two experiments. Both the set of potential targets and the critical position in the letter arrays were predesignated. One group in each experiment was induced to scan each array from left to right in a dot-counting task. A dot could precede the array to its left and/or appear with the array to its right. Only this group displayed the word superiority. For the second group in the first experiment, any dot(s) present always appeared with the array. The second group in the second experiment was shown no dots but, rather, performed the letter-identification task only. The absence of a word-superiority effect in other studies is related to holistic encoding stemruing from scanning requirements, and the implications for positional uncertainty explanations of the phenomenon are discussed.     

Evidence for Mixed Processes in Normal/Mirror Discrimination of Rotated Letters: A Bayesian Model Comparison Between Single- and Mixed-Distribution Models1

When determining whether a rotated letter is normal or mirrored, an observer mentally rotates the letter to its canonical orientation. To account for patterns of response times (RTs) for the normal/mirror discrimination of rotated letters, previous research formulated a model that postulated a mixture of trials with and without mental rotation. While this model could explain the curvilinear relationship that has been found between averaged RT and letter orientations, the curved RT function is still open to alternative explanations without assuming mixed processes. To address this issue and test the mixed-process hypothesis more directly, we analyzed trial-by-trial RT data instead of averaged RTs by employing a Bayesian model comparison technique. If rotation and non-rotation trials are mixed, trial-by-trial RTs for letters in a particular orientation should not follow a single distribution but a mixed one formed from the superposition of two separate distributions, one for rotation and one for non-rotation trials. In the present study, we compared single- and mixed-distribution models. Bayes-factor analysis showed decisive support for the mixed-distribution model over the single-distribution model. In addition, using the widely applicable information criterion (WAIC), the predictive accuracy of the mixed-distribution model was found to be as high as that of the single-distribution model. These results indicated the involvement of mixed processes in normal/mirror discrimination of rotated letters. The usefulness of statistical modeling in psychological study and necessary precautions to take in the interpretation of the parameters of unconfirmed models are also discussed.     

Temporal separation of two part-letter arrays and size changes in a nonmasking work-superiority effect

In two experiments subjects were asked to report the identity of a position-cued critical letter in an array of four letters. Four types of arrays were used: (i) unpronounceable nonwords; (ii) pronounceable nonwords ('pseudowords'); (iii) words in which the critical letter was minimally constrained by the context letters; and (iv) words in which the critical letter was maximally constrained by the context letters. All four-letter stimuli were presented in two parts. A leading array in which the information from two quadrants of a vertical by horizontal division of each letter was presented, and, after intervals of 0, 20, 40, 80, 100, 120, 160, 320, and 480 ms and infinity (ie, no trailing array), a trailing array of the complementary letter parts. In experiment 1 a single group of eight subjects responded to the one hundred and sixty combinations of the four types of letter strings, the four serial positions, and the ten stimulus onset asynchrony values. In experiment 2 the stimulus onset asynchrony values were varied among subjects, with twelve subjects responding at each value. The results from these two studies were generally similar. Performance in the word conditions was consistently superior to performance in the nonword conditions, and the magnitude of this difference (ie, the word-superiority effect) increased with increasing stimulus onset asynchrony up to 120 ms, and then gradually declined. The fact that the magnitude of the word-superiority effect initially increased with the separation of leading and trailing arrays was interpreted as support for Johnston's suggestion that letters in words are represented during visual encoding both in the form of individual letter percepts and in a decay-resistant word percept, as opposed to letters in nonwords, which are represented only as decay-susceptible letter percepts. The experimental findings are discussed in relation to the 'interactive activation' model of word perception.     

Bilateral saccades increase intrahemispheric processing but not interhemispheric interaction: Implications for saccade-induced retrieval enhancement

Retrieval of memories is enhanced when bilateral saccades are made immediately before attempting retrieval. One hypothesis is that saccades enhance retrieval by increasing interaction of the brain hemispheres. To test this, subjects viewed arrays of lateralized letters and indicated whether target letters matched either of two probe letters. Matching targets and probes were presented to either the same hemisphere (within-hemisphere trials) or separate hemispheres (across-hemisphere trials). Match detection requires interhemispheric interaction on across-hemisphere trials but primarily intrahemispheric processing on within-hemisphere trials. Subjects performed letter matching following saccades and a fixation control condition. Saccades increased match-detection accuracy on within-hemisphere trials only, suggesting that, counter to the hypothesis, saccades enhance intrahemispheric processing but not interhemispheric interaction. Across-hemisphere accuracy was higher, however, for subjects who were not strongly right-handed, versus those who were, and the absence of strong right-handedness may reflect greater interhemispheric interaction. We discuss implications for accounts of saccade-induced retrieval enhancement.     

Repetition blindness for locations: evidence for automatic spatial coding in an RSVP task

The authors report a new phenomenon called repetition blindness (RB) for locations: When 3 or 4 letters are presented rapidly and sequentially at random locations within a spatial array, experimental participants have difficulty reporting pairs of letters appearing in the same location within 250 ms of each other. This deficit occurs both during report of letter identities and during report of the locations in which the letters appear; it can also be found using a partial report task. During letter report, the deficit is found for 4-location arrays but not for 8-location arrays. In contrast, letter RB is not found during location report even when the letters are always chosen from a set of 4. These results indicate that a small number of locations--but not letters--can be encoded automatically even when they are not explicitly reported. The authors argue that RB for locations results from a difficulty individuating 2 tokens at the same spatial location.     

Separating effects of orthographic similarity and contour summation in the identification of masked letter strings

We separated effects of contour summation and orthographic similarity under masking conditions, by comparing identification with simultaneous and sequentially presented letter strings, which either did or did not overlap spatially. With overlapping simultaneous stimuli, performance was better for strings with similar contours than for strings with the same letters (the orthographic similarity condition). This suggests that contour summation effects were strongest in the condition where stimuli had similar contours. With sequential presentations, performance in the similar contour and the orthographically similar conditions was equated when the stimuli were overlapping. However, effects of contour summation decreased when prime and target letters were spatially displaced, whereas performance in the orthographically similar condition was maintained. We conclude that effects of orthographic similarity can be distinguished from effects of contour summation, under masking conditions.     

Selective attention and target search with brief visual displays

In a series of four experiments, subjects were presented with eight item arrays tachistoscopically. In two tasks, the subject was required to recall a digit presented among seven letters or a letter among seven digits. The remaining tasks required the subject to detect the presence of a specific letter either in the context of seven random letters or seven digits. An examination of the effects of a masking stimulus for various target positions suggested that subjects could attend selectively when the target was a different category than its context but had to process the entire array when the target was the same category as the context. The effects of selective attention and masking are explained in terms of parallel and sequential identification processes.     

Rapid proportion comparison with spatial arrays of frequently used meaningful visual symbols

It has been shown that when two arrays of Arabic numerals were briefly presented, observers could accurately indicate which array contained the larger number of a target numeral. This study investigated whether this rapid proportion comparison can be extended to other meaningful symbols that share some of notable properties of Arabic numerals. We tested mainly several Japanese Kanji letters, each of which represents a meaning and can work as a word. Using physically identical stimulus sets that could be interpreted as different types of letters, Experiment 1 first confirmed the rapid proportion comparison with Arabic numerals for Japanese participants. Experiment 2 showed that the rapid proportion comparison can be extended to Kanji numerals. Experiment 3 successfully demonstrated that rapid proportion judgments can be found with non-quantitative Kanji letters that are used frequently. Experiment 4 further demonstrated the rapid proportion comparison with frequently used meaningful non-letter symbols (gender icons). The rapid processing cannot be attributed to fluent processing of familiar items, because it was not found with familiar phonograms (Japanese Kana letters). These findings suggest that the rapid proportion comparison can be commonly found with frequently used meaningful symbols, even though their meaning is not relevant to the task.     

Interference in letter identification: A test of feature-specific inhibition

The role of perceptual interference in letter identification was investigated in three experiments designed to test the feature-specific inhibition model proposed by Bjork and Murray (1977). According to their extension of Estes’ (1972, 1974) interactive channels model, input channels leading to the same feature detector inhibit one another more than do channels leading to different detectors. The model therefore predicts perceptual interference between two letters to be a function of the degree of their feature overlap. Experiment 1 confirmed the feature-specific inhibition model and Bjork and Murray’s finding that the accuracy of report is lower when a briefly presented target letter is flanked by an identical letter than when flanked by another target letter or by a nontarget letter. Results from Experiment 2 indicated that single-target performance is a function of the degree of feature similarity between the target letter and background characters in a stimulus display. Experiment 3 ruled out a spatial-uncertainty explanation of feature-specific inhibition in a new paradigm that does not require subjects to process a poststimulus cue. The results of these experiments are discussed in relation to recent studies exhibiting strong effects of noise letters at the response stage of processing. It is suggested that discrepancies between feature-specific interference and response-interference studies may be a function of the particular mode of stimulus presentation and of the dependent measures that are used.     

Intrusion and location errors in the naming of letters in words and non-words

Summary In two experiments subjects were asked to report the identity of a position-cued critical letter in a linear array of letters. The identification errors made were classified either as intrusion (i.e., a report of a letter not in the array), or as location (i.e., a report of a noncritical or context letter from the array), and the arrays used were either unpronounceable non-words or words. In the first experiment all stimuli were four letters long and were presented in two parts: a leading array in which the information from two quadrants of a vertical by horizontal division of each letter was presented, and, after intervals of 40 and 160 ms, a trailing array of the complementary letter parts. The cue for the critical letter appeared either 80 ms before the initiation of the leading array or 320 ms after. The results showed a word advantage in all conditions, and an analysis of mislocations as proportions of total errors did not show that they differed significantly for non-words and for words. In the second experiment the stimuli were eight letters long and they were presented intact, followed after 140 ms by a patterned mask. The critical letter was again pre- and post-cued, and arrays subtended either 1.6° or 5° of visual angle. The results again showed a general word advantage and no difference in the proportions of mislocations for non-words and words. The results of both studies failed to provide support for the notion that the non-words probably required more attention than the words, and since this differential-attention hypothesis questioned the equality of encoding assumption of the Interactive Activation model of word perception, its rejection supported the latter proposition. It was concluded, therefore, that the encoding of the strings of letters in these studies was independent of their linguistic properties.     

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.     

Modulations of the processing of line discontinuities under selective attention conditions?

We examined whether the processing of discontinuities involved in figure-ground segmentation, like line ends, can be modulated under selective attention conditions. Subjects decided whether a gap in collinear or parallel lines was located to the right or left. Two stimuli were displayed in immediate succession. When the gaps were on the same side, reaction times (RTs) for the second stimulus increased when collinear lines followed parallel lines, or the reverse, but only when the two stimuli shared the same orientation and location. The effect did not depend on the global form of the stimuli or on the relative orientation of the gaps. A frame drawn around collinear elements affected the results, suggesting a crucial role of the "amodal" orthogonal lines produced when line ends are aligned. Including several gaps in the first stimulus also eliminated RT variations. By contrast, RT variations remained stable across several experimental blocks and were significant for interstimulus intervals from 50 to 600 msec between the two stimuli. These results are interpreted in terms of a modulation of the processing of line ends or the production of amodal lines, arising when attention is selectively drawn to a gap.     

Evaluating the unequal-variance and dual-process explanations of zROC slopes with response time data and the diffusion model

We tested two explanations for why the slope of the z-transformed receiver operating characteristic (zROC) is less than 1 in recognition memory: the unequal-variance account (target evidence is more variable than lure evidence) and the dual-process account (responding reflects both a continuous familiarity process and a threshold recollection process). These accounts are typically implemented in signal detection models that do not make predictions for response time (RT) data. We tested them using RT data and the diffusion model. Participants completed multiple study/test blocks of an "old"/"new" recognition task with the proportion of targets and the test varying from block to block (.21, .32, .50, .68, or .79 targets). The same participants completed sessions with both speed-emphasis and accuracy-emphasis instructions. zROC slopes were below one for both speed and accuracy sessions, and they were slightly lower for speed. The extremely fast pace of the speed sessions (mean RT=526) should have severely limited the role of the slower recollection process relative to the fast familiarity process. Thus, the slope results are not consistent with the idea that recollection is responsible for slopes below 1. The diffusion model was able to match the empirical zROC slopes and RT distributions when between-trial variability in memory evidence was greater for targets than for lures, but missed the zROC slopes when target and lure variability were constrained to be equal. Therefore, unequal variability in continuous evidence is supported by RT modeling in addition to signal detection modeling. Finally, we found that a two-choice version of the RTCON model could not accommodate the RT distributions as successfully as the diffusion model.     

Too much to count on: impaired very small numbers in corticobasal degeneration

Patients with corticobasal degeneration (CBD) have calculation impairments. This study examined whether impaired number knowledge depends on verbal mediation. We focused particularly on knowledge of very small numbers, where there is a precise relationship between a cardinality and its number concept, but little hypothesized role for verbal mediation. We evaluated accuracy and reaction time (RT) for matching dot arrays and Arabic numerals involving smaller (2-4) and larger (5-9) cardinalities in non-aphasic patients with CBD (n=16), frontotemporal dementia (FTD; n=23), and healthy controls (n=15). CBD were less accurate and slowed at judging smaller Arabic numeral-dot array stimuli compared to FTD patients and controls. Moreover, only CBD showed longer RTs judging successively larger number-dot array pairs among the smaller cardinalities. Difficulty judging very small numbers is impaired in CBD, suggesting degraded representation of precise number knowledge that does not depend on language functioning.