Spafiaf ProMssing Cfiorocterisftcs in ffce PerMpfJon of Brief Vfsuaf Arrays

A central issue in perceptual research concerns the spatial processing characteristics of mechanisms that extract information from briefly presented alpha-numeric arrays. Recent work on this issue by Estes and Taylor (1964, 1966) incorporated a methodology that avoided the short-term memory confoundings of prior designs. In the Estes and Taylor experiments, each trial consisted of the brief presentation of an array containing random "noise" letters plus one of two critical letters, the S_attempting to determine which critical letter appeared. As the number of noise letters was increased, the proportion of trials on which Ss selected the correct letter was found to decrease. This result was interpreted by Estes and Taylor, and by Rumelhart (1970) as demonstrating some limitation of perceptual capacity either a serial scan from a fading trace, or a parallel attentional nechanism of limited capacity. However, these experiments involved potentially critical methodological confoundings: stimulus arrays containing more letters w»re either larger in ilsa (visual angle) or were more "crowded" with adjacent letters closer together; both of these factors have been shown to decrease letter perceptibility independent of the factors manipulated In the Estes and Taylor studies. Experiment I in the present study was patterned after the Estes and Taylor paradigms, but controlled both angular size and crowding factors by means of a stimulus array Incorporating the lack of interaction found for Itemr. separated by 1° or more of visual angle (cf., Eriksen, Munsinger, & Grenspon, 1966). The results indicated that, notwithstanding these controls, Ss' performance decreased with increases from 4 to 16 in the r imber of letters in the array. Experiment II was similar to Experiment I, except that stimulus arrays were sub-span, containing from 1 to 4 letters; the results bncved the same performance decline as in Experiment I. The data from Experiments J and II supported models involving a limitation of perceptual capacity. However, there was evidence that Ss in detection experiments often confused noise letters with the critical alternatives; a mathematical model incorporating such confusions was developed and was found to predict the obtained decline in performance with increasing number of letters due to the decislonal structure of the detection paradigm, even though the perceptual stage embodied no limitation of capacity, i.e., the model conceptualized an independent, parallel perceptual channel for each stimulus letter. Experiment III attempted a critical test between previous limited-capacity models and the unlimitedcapacity "confusions" (UCC) model; it was similar to an experiment by Eriksen and Lappin (1967), and employed 1-4 letter arrays and a specially designed whole-report procedure. The results failed to duplicate the invariance of per-item identification accuracy found by Eriksen and Lappin. Such a performance invariance, along with the decrease in detection accuracy found in Experiment II for 1-4 letter arrays, would hav i been required to support the UCC model. Experiment IV attempted to resolve the discrepancy between the results of Eriksen and Lappin (1967) and Experiment III by means of an exact replication of the Eriksen and Lappin paradigm; the replication, however, failed to yield the invariance of per-item Identification accuracy required by the UCC model. vii It was concluded that, notwithstanding the methodological and theoretical considerations of Experiments I IV, limited-capacity conceptions such as Rumelhart's remain viable models for alpha-numeric character recognition under tachistoscopic conditions. Further considerations suggest, however, that a truly decisive rejection of unlimited-capacity conceptions may not be possible within current methodologies.