A two-dimensional computer-controlled visual stimulator

A computer-controlled instrument that creates complex two-dimensional patterns on a CRT monitor is described. These patterns are used to elicit visual evoked responses. Patterns are produced on a raster that is rotatable about its center. It is possible to assign to arbitrary regions in the raster any of four independent one-dimensional spatial-temporal functions. For each spatial-temporal function, the experimenter can select an arbitrary spatial profile, the spatial frequency of the profile, the starting phase of the profile, the temporal function, and the depth of modulation.     

Tachistoscopic software for Macintosh computers

The requirements for accurate presentation of short-duration stimuli on, raster display monitors are summarized, and existing packages for instantiating such displays are evaluated. A new soft-ware package that provides tachistoscopic functions for Macintosh computers is described. This software, called MacTach, performs video-buffer switching, allowing tachistoscopic presentation of large or multibit depth images that is not available with alternative methods. Compatibility routines and verification procedures allow compatibility with all Macintosh computers. Applications for animation, motion, and dichoptic presentations are discussed.     

A microcomputer-controlled complex reaction time instrument

An inexpensive microcomputer system that measures complex reaction time to a visual stimulus is described. This system can determine the rate of gain of information by utilizing a two-, four-, or eight-choice reaction time task encompassed within the same unit. The system turns on randomly chosen lights and monitors the time required for the subject to depress a corresponding switch. Information describing the stimulus and the subject’s response are printed on a Teletype after each response.     

Making quantized images appear smooth: Tricks of the trade in vision research

Digital image-processing systems combined with color raster television technology are opening new worlds for the visual psychophysicist. Some of the inherent limitations and practical hazards of these technologies are explored. Techniques for circumventing some of the current limitations are presented, especially with regard to the issue of real-time variation of visual images.     

Electromagnetic tracking of motion in the proximity of computer generated graphical stimuli: A tutorial

Electromagnetic motion-tracking systems have the advantage of capturing the tempo-spatial kinematics of movements independently of the visibility of the sensors. However, they are limited in that they cannot be used in the proximity of electromagnetic field sources, such as computer monitors. This prevents exploiting the tracking potential of the sensor system together with that of computer-generated visual stimulation. Here we present a solution for presenting computer-generated visual stimulation that does not distort the electromagnetic field required for precise motion tracking, by means of a back projection medium. In one experiment, we verify that cathode ray tube monitors, as well as thin-film-transistor monitors, distort electro-magnetic sensor signals even at a distance of 18 cm. Our back projection medium, by contrast, leads to no distortion of the motion-tracking signals even when the sensor is touching the medium. This novel solution permits combining the advantages of electromagnetic motion tracking with computer-generated visual stimulation.     

Usability of liquid crystal displays for research in the temporal characteristics of perception and attention

Recently, the use of liquid crystal displays (LCDs) in computer monitors has increased in popularity. Can LCDs produce results similar to those obtained in cathode-ray tube (CRT) displays in studies of temporal attention and perception tasks? Performance in two tasks (metacontrast masking and attentional blink) was examined using an LCD, a CRT oscilloscope, and a raster scan CRT display. Experiment 1 focused on metacontrast masking where a typical metacontrast function emerged irrespective of monitor type. Experiments 2 and 3 examined whether differences in monitors influence the attentional blink. Again, all displays elicited similar performance profiles for both the attentional blink and the trade-off between identification accuracy of the two targets. Although our results may not generalize to all LCD applications and all experimental paradigms, they indicate that LCDs can reproduce results similar to those found in metacontrast masking and attentional blink studies that were originally identified with CRT displays.     

Theoretical and methodological approaches to apparent movement of short-range process and pattern perception

A variable raster and vector display processor interfaced with a PDP-11/23 computer is briefly described. Several experiments using random-dot cinematograms (RDCs) (raster display) and line segments (vector display) are discussed. The experiments on cooperative processes of apparent movement of RDCs and spatial-frequency-filtered RDCs showed that the average cooperative neighborhood of neural interaction is about 15 min arc of visual angle. In pattern perception, based on Julesz’s texton theory (1981) and Shaw’s attention model (1980), this study showed that the aperture of attention in serial and parallel processing is a function of feature gradient, set size, and eccentricity.     

Frames of reference for the light-from-above prior in visual search and shape judgements

Faced with highly complex and ambiguous visual input, human observers must rely on prior knowledge and assumptions to efficiently determine the structure of their surroundings. One of these assumptions is the 'light-from-above' prior. In the absence of explicit light-source information, the visual system assumes that the light-source is roughly overhead. A simple, low-cost strategy would place this 'light-from-above' prior in a retinal frame of reference. A more complex, but optimal strategy would be to assume that the light-source is gravitationally up, and compensate for observer orientation. Evidence to support one or other strategy from psychophysics and neurophysiology has been mixed. This paper pits the gravitational and retinal frames against each other in two different visual tasks that relate to the light-from-above prior. In the first task, observers had to report the presence or absence of a target where distractors and target were defined purely by shading. In the second task, observers made explicit shape judgements of similar stimuli. The orientation of the stimuli varied across trials and the observer's head was fixed at 0, +/-45 or +/-60 degrees . In both tasks the retinal frame of reference dominated. Visual search behaviour with shape-from-shading stimuli (SFS) was modulated purely by stimulus orientation relative to the retina. However, the gravitational frame of reference had a significant effect on shape judgements, with a 30% correction for observer orientation. In other words, shading information is processed quite differently depending on the demands of the current task. When a 'quick and dirty' representation is required to drive fast, efficient search, that is what the visual system provides. In contrast, when the task is to explicitly estimate shape, extra processing to compensate for head orientation precedes the perceptual judgment. These results are consistent with current neurophysiological data on SFS if we re-frame compensation for observer orientation as a cue-combination problem.     

The importance of visual angle in word recognition: A “shrinking screen” modification for visual displays

The visual angle subtended by alphabetic stimuli seems to be given little regard in a large proportion of reading research. Existing empirical evidence suggests that this disregard may be unwise. We describe a modification to video monitors and oscilloscopes that allows the screen size of stimuli to be varied and permits words to be displayed at appropriate visual angles with accuracy and comparative ease. Other advantages of these modifications are also discussed.     

Visual display in the multipurpose laboratory: The benefits of video scan-conversion

A visual display system, based upon the PEP-400 video scan-converter, is described. For a cost of about $5,600, including nine video monitors, a laboratory can provide flexible, computer-generated, alpha-meric, and graphic displays simultaneously to eight subjects. Some performance results are given, as is discussion of interfacing and software required for a DEC lab system.     

Effects of a visual frame and of low radial accelerations on the visually perceived eye level

The purpose of this study was to determine how the combined effects of a reference frame and of very low gravito-inertial forces produced by centrifugation affect the visually perceived eye level (VPEL). Twenty subjects were instructed to set a luminous target to the VPEL under various experimental conditions involving two main factors: (1) visual context (frameless, frame centered, frame moved down 50 mm, and frame moved up 50 mm) and (2) gravito-inertial context (motionless, Gi₁=9.81001 m/sec² and Gi₂ = 9.95 m/sec²). The visual context significantly reduced the lowering of VPEL in darkness as caused by radial acceleration; this confirms the prevailing role of vision versus propriosomesthesis. However, under condition Gi₂, there was a significant effect on the VPEL in spite of the presence of the luminous frame; this demonstrates that VPEL processing involves both visual and propriosomesthesic information. Furthermore, the VPEL varied linearly with the vertical shift of the luminous frame for any of the gravito-inertial conditions used in this study, but, under condition Gi₂, the VPEL was shifted downward.     

Using hardware interrupts for timing visual displays and reaction-time key interfacing on the Commodore 64

The use of hardware interrupts for presenting and timing visual displays and for controlling reaction timing on the Commodore 64 is described. The three sources of interrupts discussed are reaction-time keys interfaced through the user port to on-board hardware timers, the alarm of the 60-Hz real-time clocks, and the video raster. In a demonstration program, these interrupts are used to measure display durations, to change screen displays, and to coordinate the onset of reaction timing with the onset of screen changes. In addition, an externally generated interrupt caused by a keypress is used to control reaction timing independently of CPU operations.     

Visual and auditory facilitation in auditory spatial localization

It has previously been shown that adults localize unseen auditory targets more accurately with their eyes open than closed. The interpretation usually proposed to explain this phenomenon is that auditory spatial information is referred or translated to a visual frame of reference. The present experiments show that the presence of an auditory reference point facilitates auditory localization judgements in the same manner as a visual reference point does. Although our results do not support the visual frame of reference hypothesis, they suggest that the auditory and the visual modalities are strongly linked in their localizing processes.     

Orientation contrast effects in the rod-and-frame test

Previous research indicates that a tilted visual display is capable of inducing eye torsion and an illusion of self-tilt in objectively upright observers. These effects may contribute to performance errors on the rod-and-frame test by rotating the perceived axes of visual space toward the tilted frame. The kinesthetic-matching method was used in the present study to see whether an effect of the visual orientation contrast between red and frame sides might also contribute to rod-and-frame test performance. Observers aligned invisible hand-held rods with the visual rod at various tilts under a control condition when the frame was absent, and under experimental conditions with the frame upright or set at 45°. The frame induced matching errors in the direction away from the frame sides which were most nearly parallel to the rod. Since no rotation of apparent visual axes should occur under these conditions, the data suggest that an orientation contrast effect is involved in the rod-and-frame test.     

High regularities in eye-movement patterns reveal the dynamics of the visual working memory allocation mechanism

With only two to five slots of visual working memory (VWM), humans are able to quickly solve complex visual problems to near optimal solutions. To explain the paradox between tightly constrained VWM and impressively complex human visual problem-solving ability, we propose several principles for dynamic VWM allocation. In particular, we propose that complex visual information is represented in a temporal manner using only a few slots of VWM that include global and local visual chunks. We built a model of human traveling salesman problem solving based on these principles of VWM allocation and tested the model with eye-movement data. Exactly as the model predicted, human eye movements during traveling salesman problem solving have precise quantitative regularities with regard to both the general statistical pattern of attentional fixations and how they vary across individuals with different VWM capacities. Even though VWM capacity is very limited, eye movements dynamically allocate VWM resources to both local and global information, enabling attention to fine details without loss of the big picture.     

An improved video multiplexer for behavioral research

Wellens and Revert (1979) described a solid-state circuit that allowed two sources of video information to be multiplexed and recorded using one videotape machine. On playback, the demultiplexed video signal allowed the original video channels to be viewed independently on separate television monitors. Recent changes in videocassette recorder technology have improved the playback quality of recorded images but disrupt the demultiplexing process when freeze frame or search options are selected. Improved multiplexing techniques that overcome these limitations are described.     

A consumer-grade LCD monitor for precise visual stimulation

Because they were used for decades to present visual stimuli in psychophysical and psychophysiological studies, cathode ray tubes (CRTs) used to be the gold standard for stimulus presentation in vision research. Recently, as CRTs have become increasingly rare in the market, researchers have started using various types of liquid-crystal display (LCD) monitors as a replacement for CRTs. However, LCDs are typically not cost-effective when used in vision research and often cannot reach the full capacity of a high refresh rate. In this study we measured the temporal and spatial characteristics of a consumer-grade LCD, and the results suggested that a consumer-grade LCD can successfully meet all the technical demands in vision research. The tested LCD, working in a flash style like that of CRTs, demonstrated perfect consistency for initial latencies across locations, yet showed poor spatial uniformity and sluggishness in reaching the requested luminance within the first frame. After these drawbacks were addressed through software corrections, the candidate monitor showed performance comparable or superior to that of CRTs in terms of both spatial and temporal homogeneity. The proposed solution can be used as a replacement for CRTs in vision research.     

Argumentation, the Visual, and the Possibility of Refutation: An Exploration

Taking the possibility of visual argumentation seriously, this essay explores how refutation might proceed. We posit three ways in which images can refute and be refuted in a mixed-media environment: (1) dissection, in which an image is broken down discursively; (2) substitution, in which one image is replaced within a larger visual frame by a different image; and (3) transformation, in which an image is recontextualized in a new visual frame. These strategies are illustrated in an analysis of three American documentary films on abortion.     

Adaptation to statistical properties of visual scenes biases rapid categorization

The initial categorization of complex visual scenes is a very rapid process. Here we find no differences in performance for upright and inverted images arguing for a neural mechanism that can function without involving high-level image orientation dependent identification processes. Using an adaptation paradigm we are able to demonstrate that artificial images composed to mimic the orientation distribution of either natural or man-made scenes systematically shift the judgement of human observers. This suggests a highly efficient feedforward system that makes use of “low-level” image features yet supports the rapid extraction of essential information for the categorization of complex visual scenes.     

The role of frame size on vertical and horizontal observers in the rod-and-frame illusion.

The rod-and-frame illusion was used to examine a proposed distinction between the mechanism responsible for frame effects on rod-adjustment errors with large displays and the mechanism responsible for errors with small displays. It was suggested that visual-vestibular mechanisms are involved only when the rod is surrounded by a large tilted frame. Errors in the perceived vertical with small frame would instead be due to purely visual mechanisms. To examine this dual process model, we compared errors at small and large frame when the body was vertical or horizontal. There is evidence to suggest that tilting the body affects visual-vestibular interactions, but there is no reason to expect that body tilt would affect intravisual interactions. Hence, we hypothesized that body tilt would increase errors for large frame, but not for small frame. Eight subjects were tested in four different conditions, corresponding to the combination of two body orientations (vertical versus horizontal) and two frame sizes (47.5 versus 10.5 deg of visual angle). Fourier analysis of data was performed. Repeated measures ANOVA tested the hypothesis about frame size and body orientation. The hypothesis was not confirmed. More specifically, we found that tilting the body increased errors for the small frame as well as for the large frame. The interaction between frame size and body orientation was not significant. Results are discussed in relation to the proposed dual-process model.