On using Vernier acuity to assess magnocellular sensitivity

A recent study [Keri, S., & Benedek, G. (2009). Visual pathway deficit in female fragile × premutation carriers: A potential endophenotype. Brain and Cognition, 69, 291–295] has found Vernier acuity deficiencies together with contrast sensitivity defects consistent with a magnocellular deficit in female fragile × premutation carriers. This may appear to support the notion that Vernier acuity may serve as a test of magnocellular sensitivity. However, Vernier acuity deficiencies have been reported in other conditions (e.g., schizophrenia, amblyopia and cortical visual impairment) where there is little evidence for magnocellular deficits. The observation that Vernier acuity deficiencies can occur without magnocellular deficits indicates that Vernier acuity is not a reliable test of magnocellular sensitivity.     

Vernier acuity as affected by target length and separation

Vernier acuity was measured for vertical lines of different lengths; the threshold (about 4 sec of arc) was almost as good for the shortest stimuli (1 min 20 sec squares) as for the longest (21 min 20 sec × 1 min 20 sec rectangles) and did not change when two round dots were shown in positions corresponding to the squares. The threshold for the two dots measured in terms of minimum detectable lateral offset increased when the vertical separation between the dots increased, but, when replotted in terms of the angle of tilt between them with respect to vertical, the threshold improved with dot separation; moreover, at asymptote, the threshold was comparable to that obtained for detecting that an actual line was tilted out of vertical. Our data suggest that, in performing the vernier task, Ss do not extrapolate the edges of the vernier elements; instead, they judge the deviation of the inner ends of the stimuli from verticality. This hypothesis explains the effect of increasing separation between vernier elements and also accounts for other types of acuity, such as the detection of curvature.     

Vernier acuity and the magnocellular system revisited: Response to Skottun and Skoyles

Skottun and Skoyles (2009) recently presented a comment on Vernier acuity and magnocellular dysfunctions in fragile X premutation carriers (Kéri & Benedek, 2009). The authors concluded that our finding that the magnocellular deficit, as revealed by luminance-contrast sensitivity measurements, is associated with impaired Vernier acuity for achromatic gratings does not mean that this procedure is suitable for the investigation of the magnocellular pathways, given that deficits outside the magnocellular system also lead to impaired Vernier acuity. However, Skottun and Skoyles (2009) did not consider the dissociation between achromatic and isoluminant color Vernier and the convergence of Vernier and luminance-contrast sensitivity measurements.     

The relationship between visual acuity and the spatial duty cycle of periodic stimuli

Human visual acuity was measured with rectangular-wave gratings that had duty cycles (the proportion of each grating period that is light) that varied from 0.020 to 0.975. When adaptation level is held constant, the results are consistent with the hypothesis that only the amplitude of the fundamental Fourier component is detected at threshold.     

Visual acuity improvement following fading and feedback training—III. Effects on acuity for stimuli in the natural environment

The effectiveness of Fading and Feedback training for improving visual performance of myopic volunteers was evaluated using a randomized control design. Forty-eight myopic volunteers were randomly assigned to one of four treatments: Fading and Feedback Training, Fading and Feedback Training plus Incentives, No Training, or No Training plus Incentives. Subjects were pre- and post-tested on three measures of visual function: Performance—subjects played a video “pong” game for a brief period of time; Facial Expression Identification—subjects were asked to identify four different facial expressions; and Visual Search—subjects were asked to locate four household items placed on tables in a room. Subjects in the training condition significantly improved in their ability to recognize facial expressions and identify household objects. This occurred with or without incentives for training subjects. However, for both the facial expression and visual search tasks, incentives alone resulted in improved visual acuity in the absence of training. These data are discussed in terms of generalizability of Fading and Feedback training to stimuli in the natural environment.     

Vernier offset produced by rotary target motion

Two separated colinear lines appear displaced from colinearity when either the target or the subject’s head is rotated in a frontal plane. The direction of perceived offset is reversed for opposite directions of rotation. The present experiments prove that the effect depends on some property of the visual system that is responsive to stimulus motion per se and is not manifested in the response to stationary targets. Two mechanisms which may be responsible for the rotation-contingent effect are considered: (1) An induction mechanism based on the dynamics of induced tilt or of figural aftereffect displacement. (2) A mechanism based on variation of visual latency with stimulus energy/time.     

Relational judgments with remembered stimuli

Two experiments investigating the effect of the direction of a relational judgment on the speed of the judgment are reported. In both experiments, college students required more time to select the smaller of a pair of large animals than to select the larger. Conversely, the smaller of a pair of small animals was selected more quickly than was the larger. The magnitude of this “cross-over effect” was fully graded, increasing regularly with extremity, but the variability of the response times in each direction was unrelated to extremity. Individual animals were classified as “small” or “large” with almost perfect consistency. This pattern of results is used to evaluate several models of relational judgment; of these, the congruency model is shown to be inconsistent with these data.

     

Pavlovian conditioning with proximal stimuli

This experiment was conducted with the objective of demonstrating that the effective stimuli in Pavlovian Conditioning are not environmental stimuli but internal physiological processes elicited by environmental input (proximal stimuli). In order to achieve the objective, afterimages in color vision were used: looking at a diffuse lightened circle after seeing a red circle yields an image of a green circle. A differential conditioning paradigm with two sequential compounds was run. In one group (G⁺B^?: n₁=10), a red circle followed by a green circle was paired with shock, whereas a red circle followed by a blue circle remained unpaired. A second group (G^?B⁺: n₂=10) received red-blue paired trials and unpaired red-green trials. Immediately after that training, subjects were tested with a new, never trained sequential compound: a red circle followed by a diffuse lightened circle. Furthermore, they were tested with the already trained compounds. Taking the environmental point of view, the never trained stimulus should elicit an orienting response lying inbetween the excitatory reaction to the paired stimulus and the inhibitory reaction to the unpaired stimulus. From the proximal point of view, the diffuse light should elicit an excitatory reaction in group G⁺B^? and an inhibitory reaction in group G^?B⁺. Electrodermal conditioned anticipatory and omission responses were measured. The results supported the proximal hypothesis. Hence, defining input in environmental terms may be the wrong way. Instead, in conceptualizing the stimulus in conditioning, the following should be considered: the processing organism itself is creating the effective stimuli.     

Duplex perception with musical stimuli

Duplex perception, a phenomenon previously demonstrated for speech stimuli, is demonstrated here for musical stimuli. In the first experiment, major and minor chords are produced by dichotic fusion of two simultaneous piano notes presented to one ear (perfect fifth) with a “natural” or “flat” single note presented to the opposite ear. Musically trained subjects perceive simultaneously both the single tone and a fused (major or minor) chord. The chords are labeled more consistently than the single notes, even though the fused chords differ solely in terms of the contralateral notes. In a second experiment, using pure tones in place of piano notes, other musically trained subjects individually exhibited categorical perception for either the fused chord or the single tones, but never for both types of stimuli. The duplex phenomenon is discussed in terms of its implications for its specific component modes of perception.     

Picturing peripheral acuity

The grain of the retina becomes progressively coarser from the fovea to the periphery. This is caused by the decreasing number of retinal receptive fields and decreasing amount of cortex devoted to each degree of visual field (= cortical magnification factor) as one goes into the periphery. We simulate this with a picture that is progressively blurred towards its edges; when strictly fixated at its centre looks equally sharp all over.     

Categorization response time with multidimensional stimuli

Categorization response time (RT) was examined in three separate experiments, in each of which exemplars varied on two physical dimensions. Three different types of stimuli were used: (1) horizontal and vertical line segments of varying length that were joined at an upper left corner, (2) rectangles of varying width and height, and (3) circles or semicircles of varying size with a radial arm of varying orientation. No evidence was found that stimulus familiarity or the category prototypes played any special role in determining categorization RT. Instead, RT decreased with distance from the stimulus to the categorization decision bound.     

Unsupervised category learning with integral-dimension stimuli

Despite the recent surge in research on unsupervised category learning, the majority of studies have focused on unconstrained tasks in which no instructions are provided about the underlying category structure. Relatively little research has focused on constrained tasks in which the goal is to learn predefined stimulus clusters in the absence of feedback. The few studies that have addressed this issue have focused almost exclusively on stimuli for which it is relatively easy to attend selectively to the component dimensions (i.e., separable dimensions). In the present study, we investigated the ability of participants to learn categories constructed from stimuli for which it is difficult, if not impossible, to attend selectively to the component dimensions (i.e., integral dimensions). The experiments demonstrate that individuals are capable of learning categories constructed from the integral dimensions of brightness and saturation, but this ability is generally limited to category structures requiring selective attention to brightness. As might be expected with integral dimensions, participants were often able to integrate brightness and saturation information in the absence of feedback--an ability not observed in previous studies with separable dimensions. Even so, there was a bias to weight brightness more heavily than saturation in the categorization process, suggesting a weak form of selective attention to brightness. These data present an important challenge for the development of models of unsupervised category learning.     

Auditory–Visual stimuli: Effects on derived relations with compound stimuli

This research explored the effect of teaching conditional discriminations with three procedures on the derivation of 36 stimuli relations (derived relations). The stimuli used consisted of three characteristics musical instruments, along with the corresponding picture. In the first experiment six university students were trained with simple stimuli and tested with compound auditory–visual samples; therefore, a one‐to‐many structure was used. In the second experiment, auditory stimuli were replaced by visual stimuli, for the samples used, for new students. A third experiment was implemented with an extra phase of training with compound stimuli for six new students. The structure of the experiments was: pretests (Xbcd–A; Xacd–B; Xabd–C; Xabc–D), training (A–B; A–C; A–D), and posttests (same as pretests). The difference between these conditions was the kind of stimuli used and a new phase of teaching used in condition 3: (Xbcd–A). The results indicate that training with simple stimuli on discriminations that include stimuli that are easy to discriminate from each other (words and sounds) is a sufficient condition for good posttest performance. However, when comparisons are made difficult (words only), participants show better performance on new tests if they have a learning history with compound stimuli.     

Conditional discrimination with ambiguous stimuli.

Five pigeons learned a two-key conditional discrimination. When background color on both keys was red, pecks on the key with a horizontal line produced food. When the color was green, pecks on the key with a vertical line produced food. During part of the experiment, color was presented on only one of the keys. It was found that accuracy was higher when color was combined with the line stimulus correlated with nonreinforcement. In another part of the experiment, color was presented on both keys but a line was present only on one. Accuracy was higher when the line accompanied the nonreinforced option than when the line accompanied the reinforced option. Superior performance when the combined stimuli were displayed on the nonfood key may be explained by the association of different components of the compound stimuli with reinforcement or as the result of rules pigeons follow in solving conditional discriminations.     

Mental curve tracing with elementary stimuli

It has been proposed that certain spatial relations are determined by an operation, or "visual routine," that can trace along a boundary (Ullman, 1984). This proposal was supported by Jolicoeur, Ullman, and Mackay's (1986) finding that the time required to determine if two Xs are on the same curve increased monotonically with the separation of the Xs along that curve. In the present study the generality of the curve tracing hypothesis was explored across four experiments by using elementary stimuli that eliminated interweaving curves, displaced the fixation point away from the curves and target Xs, and provided a simple alternative to curve tracing--namely, determining whether or not the Xs fell on the same side of the figure. Stimuli consisted of two curves (150 degrees arcs) and two Xs, and each stimulus was presented for 150 ms. In Experiments 1 and 2, subjects were instructed to decide as quickly as possible if the two Xs fell on the same curve or on different curves. Even for these elementary stimuli, mean reaction time (RT) for same trials increased monotonically with the distance separating the Xs. Mean RT for different trials, however, decreased with the distance separating the Xs. In Experiments 3 and 4 alternatives to curve tracing were tested. For same trials the evidence strongly favored curve tracing. However, different trials were apparently solved on the basis of judgmental processes presumably operating in parallel with curve tracing. Curve tracing rates fluctuated across experiments and seemed to be partially governed by the width of the "pathway" provided for the trace.