Double flashes from single pulses of light

Under some conditions, a single pulse of light appears as two. The following parameters of the double-flash phenomenon were studied in two experiments: room illumination, stimulus size, peripheral angle of viewing, latency between the two flashes, and flash wavelength. It was found that the phenomenon only appeared consistently under mesopic levels of white or green illumination and from 13 to 55 deg peripherally. The mean latency between the flashes was estimated at 104 msec. The data are considered to support an explanation in terms of differential latency between the rod and cone systems.     

Building a programming language for a small computer: Reinventing the wheel

The language SP-12 for a PDP-12 computer is described. SP-12 is an example of how a special-purpose language can be constructed from a set of machine-language subroutines that are called by a simple interpreter. Symbolic source programs make it easy to learn the language. Preliminary translation of the source program into binary code, using an assembler, greatly increases the speed of the interpreter.     

On the relation between identifying and locating masked targets in visual search

Three experiments were designed to determine whether naming is contingent on locating in a visual search task. Subjects were required to identify a masked target whose location was known (I|L) or unknown (I) and to locate a masked target whose identity was known (L|I) or unknown (L). The location-contingent hypothesis predicts a relationship among the tasks such that P(L) P(I|L) = P(I), since P(I) and P(L) P(I|L) both estimate the joint probability of identifying and locating the target (i.e. P(IλL)). This relationship held in Experiment I where targets were presented alone, and in Experiment II where targets were presented with dots as noise elements, but not in Experiment III where Xs were noise elements. The results are discussed in terms of the generality of the location-contingent hypothesis.     

The apparent length of rotating arcs under conditions of dark adaptation

The apparent contraction of a rotating light arc occurred during the first 20 rain, but not after 25 min, of dark adaptation. Estimates of length obtained after 25 min were affected by the level of luminance of the arc but not by its speed of rotation, by dark gaps in the arc, or by instructions to estimate its length in terms of a brighter region. There was no tendency for a rotating dark arc to appear shorter at any stage of adaptation.     

Visual adaptation to tilt and displacement: Sameor different processes?

Visual adaptation to tilt and displacement were compared to test whether they were dependent on the same or different processes. Although interocular transfer was essentially complete for both transforms, marked differences occurred between the two kinds of optical transforms in terms of rate of adaptation as a function of exposure time and transform magnitude, level of compensation, and rate of decay. Tilt and displacement appear to be quantitatively different, consistent with the idea of a different locus for each adaptation effect. This conclusion was supported by the absence of a correlation between individual performance under the two transforms. The possibility is discussed that displacement and tilt adaptation involve independent visual systems for the perception of location and form.     

Storage and later recognition of exemplars of concepts

This paper concerns the problem of abstraction: whether when we encounter several exemplars of a concept, we retain only the abstracted concept, only the exemplars, or both. Although many studies concur that both are stored, a recent article argued strongly that only the abstracted concept is stored. The present study, aimed at replication of this recent finding, follows the earlier procedural details but adds appropriate controls and uses simpler material. A set of 24 exemplars of four concepts, in the form of four-tuples of letters and numbers, was presented to Ss who, after presentation, rated a larger set of exemplars for recognition. One group of Ss experienced the conceptual exemplars; control group Ss experienced items that were similar in composition but not exemplars of a concept. Two major results appeared: Unlike the study on which this was based, all Ss were able to distinguish those items that were originally experienced from those that were not. And, the more completely an exemplar fit the concept (the longer the item), the more confident the S was that it had been presented. In contrast, in the control condition, the longer the item, the more confident the S was that it had not been presented earlier. Two models are described to account for these results. One is based on the S's initial storage of the exemplars in a concept-plus-correction format; the other is based on a procedure whereby the S can make recognition judgments without having previously abstracted and retained the concepts.     

Stimulus suffix effects with speech and nonspeech sounds

The stimulus suffix effect (SSE) was examined with short sequences of words and meaningful nonspeech sounds. In agreement with previous findings, the SSE for word sequences was obtained with a speech, but not a nonspeech, suffix. The reverse was true for sounds. The results contribute further evidence for a functional distinction between speech and nonspeech processing mechanisms in auditory memory.