Wayne A. Hershberger

An approach through the looking-glass

Forty food-deprived cockerel chicks were tested individually in a straight runway containing a familiar food cup that moved when the chicks moved. The food cup always moved in the same direction as the chick: For 20 experimental chicks it moved twice as far as the chick; for 20 control chicks it moved half as far. In Lewis Carroll’s (1898/1926) picturesque terminology, the experimental chicks were tested in Alice’s “room through the looking-glass,” in which, in order to approach the food cup, they had to “walk the other way.” Although the control chicks performed well, the experimental chicks evinced the runway behavior that characterizes positive feedback: They persistently chased the food cup away. This means that the spatial polarity of visual feedback is critical and implies that an ordinary approach response is but an automatic (closed-loop) realization of an intended visual perception.

A conditioned weight illusion: Reafference learning without a correlation store

Each of 20 female college students repeatedly judged the weight of a heavy metal cylinder dropped repeatedly (180 times) into her waiting hand. The cylinder came to appear lighter to her when the release of the cylinder was accompanied by the onset of an indicator lamp than when it was not, but only providing the onset of the lamp preceded the release of the cylinder by a half second rather than being simultaneous with it. This conditional illusion has implications for von Holst and Mittelstaedt’s well-known thesis that every neural efference leaves an efference copy or corollary discharge of itself in the nervous system to be compared with concurrent neural reafference. Apparently some types of conditioned-efference leave no copies, and objects lifted in part by such unregistered efforts appear lightened accordingly. The illusion illustrates a form of reafference learning that need involve no correlation store (Held, 1961).

Saccadic eye movements and the perception of visual direction

The extant research literature concerning intrasaccadic stimuli implies that if a spot of light is flashed in the dark during a saccadic eye movement, one should subsequently look for the light in one direction while professing to see it lying in another. This paper accounts for this paradox in terms of two hypothesized varieties of sensed eye orientation, one estimating actual eye orientation (efference copy) and the other corresponding to intended eye orientation (afference copy).

Timing the shift in retinal local signs that accompanies a saccadic eye movement

Thephantom array was used to probe the time course of the shift in retinal local signs that accompanies a saccadic eye movement. The phantom array materializes when one saccades in the dark across a point light source blinking 120 times per second. One sees a stationary array of flashes—the first materializes discretely near the intended endpoint of the saccade, and subsequent flashes materialize progressively closer to the actual position of the blinking light. Four trained observers indicated the perceived location, relative to the phantom array, of a 1-msec marker flash (M) produced by two LEDs (light-emitting diodes) that vertically bracketed the blinking light. The marker was seen as spatially coincident with the first flash when it flashed 80 to 0 msec before the saccade, and was seen as spatially coincident with either the first flash or the actual position of the blinking light when it flashed mare than 80 msec before the saccade, indicating, respectively, that the shaft is presaccadic and rather abrupt.

TOUCH DOMINATES HAPTIC ESTIMATES OF DISCORDANT VISUAL-HAPTIC SIZE

Observers (72 college students) estimated the size of plastic squares that they held in their fingers and simultaneously viewed through a reducing lens that halved the squares’ visual size. The squares were grasped from below through a cloth that prevented direct sight of the hand. Each estimate was a match selected later, either haptically or visually, from a set of comparison squares. Vision dominated the visual estimates and touch dominated the haptic estimates, whether or not the observers knew in advance which type of estimate they would be asked to make. Neither modality inherently dominates perceived size.

Visual Direction Constancy: Perceiving the Visual Direction of Perisaccadic Flashes

Publisher Summary This chapter is concerned with the perceptual phenomenon known as visual direction constancy, particularly as it relates to saccadic movements of the eyes. It begins with the analysis of the visual perception of direction, conducted with a view to identifying the role that the oculomotor system must play in the constancy of visual direction. The chapter attempts to show how all three of these paradoxes may be resolved parsimoniously on the basis of a single coherent theoretical account. This theoretical account involves an elaboration of a pair of hypotheses advanced earlier as adjuncts to a theoretical model of the saccadic oculomotor system. The chapter illustrates that Robinsons closed-loop model controls eye orientation and utilizes two separate indices of the variable being controlled: a reference signal and a feedback signal. The chapter also explores that saccadic eye movements depend upon both neural copies whereas psychophysical judgments of visual direction depend only upon the afference copy. The hypothesis is called the sum-of- errors hypothesis, for reasons addressed in the chapter.

Visualizing the perisaccadic shift of spatiotopic coordinates

A point light source flickering on and off during a horizontal saccade projects a horizontal array onto the retina. The apparent visual direction of the tail end of the perceived (phantom) array reflects the amount of perisaccadic shift of spatiotopic coordinates that has been completed by the end of the saccade. Four men, saccading 8° to the right across a flashing light, judged the horizontal visual direction of the left (tail) end of the phantom array relative to the left end of a standard 8° array that had projected an image onto the retina before the saccade began. On average, the left ends appeared to be aligned when the last flash in the phantom array was imaged on the retina 7.4° to the right of the image of the left end of the standard array. This result implies that the shift of spatiotopic coordinates is virtually complete by the end of the saccade.

Haptic estimates of discordant visual—haptic size vary developmentally

In two size-conflict experiments, children viewed various squares through a reducing (1/2) lens while manually grasping them through a hand-concealing cloth. Then, using either vision or touch, they selected a match from a set of comparison squares. Forty 6-, 9-, and 12-year-olds participated in Experiment 1. Vision dominated the visual estimates of all three age groups; however, for the haptic estimates, the dominant modality varied developmentally: Vision dominated the 6-year-olds’ haptic estimates, whereas neither modality dominated the 9-year-olds’ haptic estimates, and touch dominated the 12-ear-olds’ haptic estimates. In Experiment 2, 24 six-year-olds were tested, as in Experiment 1; however, half of them were shown the size-distorting effects of the lens just prior to testing. Although this reduced the visual dominance of their haptic estimates, the effect was weak and short-lived. The haptic dominance seen in the data of the 12-year-olds was conspicuously absent.

The phantom array

The array seen when saccading across a point light source blinking in the dark is displaced in the direction of the saccade. This displacement reflects an abrupt shift of spatiotopic coordinates that precedes the actual eye movement. The extraretinal signal mediating this discrete shift appears to be an oculomotor reference signal, specifying intended eye orientation, that changes discretely before saccades.

Development of visual attention: A stereoscopic view

Five-year-olds, 8-year-olds, and adults (20 each) tachistoscopically viewed random-dot stereograms containing either uniform depth or an elevated disk whose perimeter fell 1.5°, 2.5°, or 4° beyond fixation. When subjects were forewarned as to the size of the disk to look for (precued trials), signal detection improved (d’ increased) for all three groups, and 5-year-olds improved the most. Without forewarning (self-cued trials), the 5-year-olds performed more poorly than the other two groups, which performed alike. Although young children are able to allocate their visual attention selectively, they do not appear to deploy it as quickly or as effectively as their elders when the focus of attention is theirs to choose.