On the nature of perceptual limits in vision

Summary Studies of attention have generally assumed that the principal limit to visual perception involves the capacity to identify a stimulus. Analyses of responses in visual tasks, however, suggest that errors can arise from a loss of position information, as well as a loss of identity information, and that these two types of errors may be affected by different variables. Based on this, we have proposed that the difficult aspect of perception is not that of recognizing the form of the stimulus but rather determining its position relative to other items in the field. To demonstrate this, we conducted a series of experiments on lateral masking that showed that the failure to perceive a parafoveal stimulus surrounded by other items is due to two quite separate components: a loss of identity information induced by the immediately adjacent items and a loss of position information induced by both adjacent and non-adjacent items.This research was supported by grant A-9581 from the Natural Sciences and Engineering Research Council of Canada to the senior author. The first two experiments were reported in a paper to the annual meeting of the Psychonomics Society, San Antonio, 1984     

Attention shifts and the Simon effect: a failure to replicate Stoffer (1991)

The Simon effect refers to the intrusion of an irrelevant spatial dimension in the selection of a response to a stimulus. Recently, Stoffer (1991) proposed a specific role for attention orienting in the Simon effect. According to Stoffer, attentional shifts from an initial fixation point to a stimulus location are associated with the generation of a spatial code which specifies the position of the stimulus in relation to the last attended location. This spatial code forms the basis for the Simon effect because it is used to specify the selection of the right or left response. Three experiments were performed to examine further the attention-shift hypothesis forwarded by Stoffer. Experiment 1 was an attempt to replicate Stoffer (1991), but failed to do so. Experiments 2 and 3 were designed to optimize conditions for replicating crucial aspects of Stoffer's findings, but also failed to yield support for the attention-shift account.     

Reading skill and the use of orthographic knowledge by Mature Readers

Summary Butler and Hains (1979) found individual differences in the effect of word length on RT in word naming and lexical-decision tasks; subjects scoring high on a vocabulary test were much less affected by word length than subjects with low scores. The present study attempted to determine whether such differences could be due to differences in the use of intraword structure, since word recognition has been linked, both empirically (Mason, 1978; Scheerer-Neumann, 1981) and theoretically (Mewhort & Campbell, 1981) to the use of orthographic redundancy. In Experiment 1, subjects were shown tachistoscopically presented letter strings varying in approximation to English. High vocabulary subjects were more capable of capitalizing on the redundancy with the higher-order approximations. Experiment 2 then established that performance on the vocabulary test was correlated with reading skill, measured by the Nelson-Denny Reading Test, and that skilled readers were more sensitive to the syllable structure in words. In the experiment, subjects were shown eight-letter words presented as three separate word units which either matched or violated syllable boundaries in the word. Skilled readers were more sensitive to the difference between syllable and nonsyllable units than were less skilled readers. The results obtained by Butler and Hains appear to reflect differences in reading skill and the efficiency of letter sequence parsing prior to word recognition.     

Analog SKED

Analog data can now be handled by SKED. Through the use of F3s, input, output, decisions, and analyses can be performed on analog data. A new command has been added to SKED. The trade off between number of boxes and resolution is discussed.     

SKED systems: Paper tape,Tennecomp, and OS/8

The SKED system has been developed at different levels of complexity and power for differing hardware configurations. The simplest of these, and the least expensive, is the paper-tape SKED system. Some of the drawbacks associated with the use of paper tape are absent in the Tennecomp magnetic tape cartridge system. Users with mass storage capabilities can make use of the OS/8 operating system (copyright DEC) to speed up all aspects of the SKED system (especially editing, compilation, and loading of programs), and take advantage of a file-oriented system structure for data storage and analysis.     

Spatiotopic and retinotopic components of iconic memory

Summary Recently, there has been considerable interest in whether information in iconic memory is stored in retinotopic or spatiotopic coordinates. The present experiment examined the issue using a masking paradigm. In one set of conditions, subjects maintained fixation while a row of four letters appeared for 19 ms, centered about three degrees to the right of fixation. After a 153 ms ISI, the letters were followed by a blank field (no mask), a mask in the same position as the letters, or a mask displaced three degrees to the right of the letters. In a second set of conditions, the stimuli were the same but subjects were asked to shift fixation from the fixation point to the middle of the letter row during the interval between the letters and the mask. Subjects' eye movements were monitored in all conditions. Accuracy of report for the letters was lowered only with the mask over the letters with the no-eye-movement conditions and in both masking conditions with the eye movements. The results suggest that the icon includes two components, one that is retinotopic and one that is spatiotopic.