Inhibition between channels selective to contour orientation and wavelength in the human visual system

Recent studies have demonstrated inhibition between channels selective to contour orientation in the human visual system. On the basis of adaptation studies, it has also been suggested that the human visual system contains channels jointly responsive to both contour orientation and wavelength. The present paper investigates inhibition between such channels. Two experiments demonstrated that, with simultaneous presentation of a center vertical target grating and a concentric surround grating, the threshold and the apparent orientation of the center grating depended on the relative orientations of the two gratings and also on whether they were viewed in the same or in different colored light. Color selectivity in both experiments was found across a wider range of angular separations than has generally been reported for successive presentation of the two stimuli. These results suggest inhibition between channels selective to combinations of contour orientation and wavelength in the human visual system.     

Inhibitory interaction between channels sensitive to direction and velocity in the human visual system

Two paradigms for the study of velocity contrast have been employed. Paradigm one has focused on perceived velocity changes across a field. Paradigm two has focused on perceived velocity changes at the juncture of disparate velocities. The current experiments employ both paradigms to look at oppositely directed motion. Using paradigm two, it was determined that, with oppositely directed motion, the effect on perceived velocity could not be attributed to lateral inhibitory interaction--in contrast to earlier reports of lateral interaction when motion was in the same direction. Using paradigm one, it was determined that the effect upon perceived velocity with oppositely directed motion appeared to involve inhibition across entire fields of motion.     

Orientational anisotropy in the human visual system

When a target stimulus in a predesignated location is identified by a keypress response, responses are slightly faster if noise stimuli in adjacent locations are identical to the target than if they are a different stimulus assigned to the same response (a repeated-stimulus superiority effect). An exception to this result has been found in experiments that used randomly intermixed letter and digit stimuli. These experiments showed slower responding for identical noise than for nonidentical, response-compatible noise (a repeated-stimulus inferiority effect). The present study investigated these phenomena in three experiments. Experiments 1 and 2 established that both the superiority and inferiority effects can be obtained in the same experiment. They also provided evidence that the repeated-stimulus inferiority effect is a function of the intermixing of letters and digits and not of the larger target-set size that has been used for mixed lists. Experiment 3 demonstrated that, with unmixed presentation, the repeated-stimulus superiority effect is enhanced by an increase in the number of stimuli assigned to each response. The experiments are consistent with accounts that attribute the repeated-stimulus superiority effect to competition that occurs when different internal recognition responses are activated. Moreover, the experiments suggest that the repeated-stimulus inferiority effect has its basis in processes that occur subsequent to feature extraction.     

Task-irrelevant visual motion and flicker attenuate the attentional blink

Our reduced ability to correctly report two sequentially presented targets is seen in the robust effect known as the attentional blink (AB; Raymond, Shapiro, & Arnell, 1992). One recent report (Olivers & Nieuwenhuis, 2005) strikingly reveals the AB to be virtually abolished when non-task-demanding music occurs in the background. The authors suggest that a diffuse attentional state is the mediating factor. Here, we seek to broaden the finding’s generality by determining if task-irrelevant visual motion and flicker also attenuate the AB. In our experiments, the AB task was presented together with a background field of moving dots that could moveaway from ortoward the central AB task, or flicker. In the control condition, the dots remained static. The AB was attenuated—though to different degrees—in all experimental conditions, but not in the static condition. Our findings add to the generality of the previous conclusions, and we emphasize an account based on the overallocation of attention.     

Effects of fasting on visual flicker fusion

Two experiments are reported which investigated the effects of fasting on visual flicker fusion (VFF). In Exp. 1 the VFF of 40 undergraduate students, 20 men and 20 women, whose mean age was 22.5 yr., was measured in nonfasting conditions 1 to 2 weeks before and in fasting conditions during the Muslim fasting month of Ramadan. In Exp. 2 the VFF of a comparable group of 15 men and 15 women, whose mean age was 23.2 yr., was measured in a different month in both fasting and nonfasting conditions. Subjects were assigned randomly to these two conditions. Analysis shows that fasting reduced VFF significantly in both experiments. This finding indicates that fasting is likely to reduce perceptual sensitivity. The results were explained in terms of fatigue and physical exhaustion effects produced by fasting.     

Changes in critical flicker frequency during prolonged visual deprivation

Subjects were tested on the critical flicker frequency during a 1-week period of visual deprivation (either homogeneous illumination or darkness). Deprived subjects showed no significant differences from a confined control group. Results were discussed in relation to previous research, Schultz’s theory of “sensoristasis” and Sharpless’ revision of the “law of denervation.”     

Spatiotemporal isosensitivity fields in the human visual system

The human visual system is capable of detecting correlations, manifested perceptually as global pattern, in mathematically constrained dynamic textures. This ability has given rise to speculation that correlative mechanisms in the human visual system exist and that they have a neural basis similar to the orientationally selective structures discovered in area 17 of the mammalian visual cortex. The limits to the detection of correlation were mapped, spatially and temporally, by means of a psychophysical technique. Evidence is presented that, at least in the spatial domain, the correlation mechanism may be served by a population of such neural units.     

Frequency specific chromatic adaptation in the human visual system

Subjects were exposed to a vertical chromatic grating alternating with horizontal chromatic grating of the identical frequency. They were then tested with a series of test gratings of varied spatial frequencies to examine whether the responses were effected by the spatial frequency of the adaptation pattern in relation to the test pattern. It was found that maximum response occurred when adaptation and test gratings had the same spatial frequency, the effect was asymmetric and finally that enhancements were found at octaves. Thus the experiment further demonstrated that neural elements specific to spatial frequency exist in the human visual system.     

Representational content and computation in the human visual system

Summary Information-processing systems can be characterized by their ability to transform systematically certain internal representational states (symbols) into one another. The presence of such an information-processing capacity calls for an explanation. How could such an explanation in principle be formulated? How is it possible to specify internal representational states and to ascribe to them certain representational contents? What has to be demonstrated by such explanations is how an information-processing capacity is actually instantiated in a system. In this paper, the outlines of an explanation by instantiation are sketched for a specific human visual capacity. In addition, some fundamental problems facing the development of this explanation will be discussed.     

Differential luminance sensitivity of the human visual system

In this paper the differential sensitivity of the visual system is investigated by means of two simultaneously presented stimuli in a yes-no procedure. The sensitivity measure σ_I appears to be proportional to stimulus intensity (i.e., Weber’s law). The curve displaying Weber’s law is little affected by variation of the background intensity or of the adaptation level. An increment threshold experiment using only one stimulus yields a proportionality of σ_I with the square root of the background intensity. An additional experiment shows that the sensitivity measure σ_I for two flashes decreases first, from dark up to a particular background intensity, and increases when the background tends to mask the flashes. So, in general, two background levels exist with the same differential sensitivity. The results cannot easily be explained by the simple quantum fluctuation concept. A model based partially on electrophysiological data from the literature is proposed which encounters a particular adaptation mechanism, a transducer with a limited dynamical range, and a range setting mechanism.     

The shape of inhibitory fields in the human visual system

Five point sources of light were displayed sequentially in a horizontal line, with .45 in. of separation between each point, employing a computer-based cathode-ray tube display system. If a particular display order and appropriate display rate is employed, then the first two points being displayed will not be perceived. By systematically varying the spatial location of these two blanked points relative to the three unblanked points, a start is made towards mapping the shape of visual inhibitory fields in man, and this general technique is related to the work of Hartline and of Ratliff on lateral inhibition and to that of Hubel and Wiesel with receptive fields in the cat and monkey striate cortex.