Perceptual latencies to sinusoidal gratings

Simple visual reaction time (RT) to suprathreshold sinusoidal gratings was determined as a measure of perceptual latency. As parameters, spatial frequency, exposure duration and modulation were varied. With three subjects, RT was found to increase with increasing spatial frequency. High contrast gives shorter RTs than low contrast. In the range investigated the effect of contrast is small compared with that of spatial frequency. At constant spatial frequency exposure durations of 30–60 msec produce shortest RTs. Underlying physiological mechanisms are considered. Some evidence comes from different temporal behaviour of X- and Y-system but central mechanisms are also supposed to be involved.     

Evoked potential estimates of the time course of adaptation and recovery to counterphase gratings

Scalp-recorded evoked potentials (VEP) were sequentially sampled in humans during adaptation to and recovery from prolonged viewing of counterphase sinusoidal grating targets. The sum of the power at the first and second harmonics of the Fourier-transformed VEP components was found to decrease during adaptation and increase during recovery. Time constants (T) for the adaptation and recovery processes as estimated from exponential functions ranged from 2.9 to 19 sec, varying non-monotonically with the spatial frequency and contrast of the stimulus. The observed T values are shorter than those reported in psychophysical studies of adaptation but overlap estimates derived from single cell studies. An unexpected finding was the occurrence of a 3-6 sec delay in the appearance of the maximum VEP response after the onset of the adaptation stimulus. The delay occurred in all subjects and at all spatial frequencies when moderate to high adapting contrasts (e.g. greater than 0.2) were used. The data support a feature-selective, multi-channel lateral inhibitory model of spatial vision and suggest the presence of tonic inhibition between the channels.     

Perceptual learning of oriented gratings as revealed by classification images

Classification image analysis is a psychophysical technique in which noise components of stimuli are analyzed to produce an image that reveals critical features of a task. Here we use classification images to gain greater understanding of perceptual learning. To achieve reasonable classification images within a single session, we developed an efficient classification image procedure that employed designer noise and a low-dimensional stimulus space. Subjects were trained across ten sessions to detect the orientation of a grating masked in noise, with an eleventh, test, session conducted using a stimulus orthogonal to the trained stimulus. As with standard perceptual learning studies, subjects showed improvements in performance metrics of accuracy, threshold, and reaction times. The clarity of the classification images and their correlation to an ideal target also improved across training sessions in an orientation-specific manner. Furthermore, image-based analyses revealed aspects of performance that could not be observed with standard performance metrics. Subjects with threshold improvements learned to use pixels across a wider area of the image, and, apposed to subjects without threshold improvements, showed improvements in both the bright and dark parts of the image. We conclude that classification image analysis is an important complement to traditional metrics of perceptual learning.     

Accommodation to size and blur changing in counterphase

We explored the influence of changing size and blur on accommodation by presenting the two stimuli sinusoidally in counterphase. The frequency response of the accommodative system (0.05 to 1 Hz) was determined using a high-speed infrared optometer while the subject viewed the target in a Badal optometer. Blur was provided by moving the target dioptrically toward and away from the subject, and size of the target was altered by a variable aperture. Both stimuli were varied sinusoidally at the same frequency, but in counterphase. We find that both size and blur can have an influence on accommodation: blur is particularly powerful at low temporal frequencies, whereas size becomes effective at moderate and high temporal frequencies.     

Perceptual learning with spatial uncertainties

In perceptual learning, stimuli are usually assumed to be presented to a constant retinal location during training. However, due to tremor, drift, and microsaccades of the eyes, the same stimulus covers different retinal positions on sequential trials. Because of these variations the mathematical decision problem changes from linear to non-linear (). This non-linearity implies three predictions. First, varying the spatial position of a stimulus within a moderate range does not deteriorate perceptual learning. Second, improvement for one stimulus variant can yield negative transfer to other variants. Third, interleaved training with two stimulus variants yields no or strongly diminished learning. Using a bisection task, we found psychophysical evidence for the first and last prediction. However, no negative transfer was found as opposed to the second prediction.     

Perceptual learning in patients with Stargardt disease

ObjectiveTo evaluate the efficacy of Perceptual Learning in improving the peripheral reading performance of patients with Stargardt disease (STGD).DesignProspective observational randomized study.ParticipantsFourteen consecutive patients (7 females, 7 males; median age of 50.4 ± 12.8 years) with STGD were analyzed and divided into two groups: Group A received “Win-flash” as Perceptual Learning training and Group B was used as control.MethodsSubjects underwent an ophthalmic evaluation at baseline, after perceptual learning training and at 6 months of follow-up. Outcomes measured included reading speed, contrast sensitivity and fixation stability.ResultsReading speed improved of 51,7% after training in group A. Visual acuity, contrast sensitivity and fixation stability enhanced in group A after training from 0.89 (±0.09) LogMAR to 0.75 (±0.2) LogMAR (t(6)= 3.6, p= 0.001), from 0.8 (±0.3) LogC (0.6 ? 0.9) to 1.3 (±0.3) LogC (t(13)=3.17, p= 0.003) and from 59.3 % (± 24.3) to 71.5 % (± 20.4) (t(13)=1.8 p= 0.04), respectively. No changes were found in group B. At 6-monts of follow-up, visual acuity and contrast sensitivity decreased in group A.ConclusionsSTGD patients receiving “Win-flash training”, as PL technique, showed an improvement of reading performance on a real-world task. Early follow-up for perceptual learning re-intervention should be considered.     

Masking by fast gratings

Perception of an oriented pattern is impaired in the presence of a superimposed orthogonal mask. This masking effect most likely arises in visual cortex, where neuronal responses are suppressed by masks having a broad range of orientations. Response suppression is commonly ascribed to lateral inhibition between cortical neurons. Recent physiological results, however, have cast doubt on this view: powerful suppression has been observed with masks drifting too rapidly to elicit much of a response in cortex. We show here that the same is true for perceptual masking. From contrast discrimination thresholds, we estimated the cortical response to drifting patterns of various frequencies, and found it greatly reduced above 15-20 Hz. In the same subjects, we measured the strength of masking by the same patterns and found it equally strong for masks drifting slowly (2.7 Hz) as for masks drifting rapidly (27-38 Hz). Fast gratings thus cause strong masking while eliciting weak cortical responses. Our results might be explained by inhibition from cortical neurons that respond to unusually high frequencies, and yet do not make their signals fully available for perceptual judgments. A more parsimonious explanation, however, is that masking does not involve lateral inhibition from cortex. Masking might operate in retina or thalamus, which respond to much higher frequencies than cortex. Masking might also be due to thalamic signals to cortex, perhaps through depression at thalamocortical synapses.     

Perceptual benefits of objecthood

Object-based attention facilitates the processing of features that form the object. Two hypotheses are conceivable for how object-based attention is deployed to an object's features: first, the object is attended by selecting its features; alternatively, a configuration of features as such is attended by selecting the object representation they form. Only for the latter alternative, the perception of a feature configuration as entity ("objecthood") is a necessary condition for object-based attention. Disentangling the two alternatives requires the comparison of identical feature configurations that induce the perception of an object in one condition ("bound") and do not do so in another condition ("unbound"). We used an ambiguous stimulus, whose percept spontaneously switches between bound and unbound, while the stimulus itself remains unchanged. We tested discrimination on the boundary of the diamond as well as detection of probes inside and outside the diamond. We found discrimination performance to be increased if features were perceptually bound into an object. Furthermore, detection performance was higher within and lower outside the bound object as compared to the unbound configuration. Consequently, the facilitation of processing by object-based attention requires objecthood, that is, a unified internal representation of an "object"-not a mere collection of features.     

Perceptual changes during metacontrast

Phenomenal reports of perceptual changes during metacontrast are abundant in the literature. In the present study four such variables (brightness of the center, edge, and whole test stimulus, and apparent duration of the test stimulus) were quantified using a magnitude estimation procedure. While all four are described by U-shaped functions, apparent duration was at a minimum at a significantly longer SOA than brightness. Implications for processing mechanisms are discussed.     

Masking by spatially-modulated gratings

Contrast- or quasi-frequency-modulated masker gratings consisting of three high frequency components (8.8, 11 and 13.2 c/deg) affect the detectability of a 2.2 c/deg signal grating, to an extent that is strongly dependent upon the relative phase between signal and masker. Unmodulated high frequency maskers have no such phase-dependent effects. This paper explores the possibility that the visual system's nonlinear response to luminance is responsible for these phenomena. A specific hypothesis is proposed according to which the effects of the spatially modulated maskers are due entirely to a distortion product at 2.2 c/deg caused by the visual nonlinearity. Although some of the predictions of this hypothesis are borne out by the experimental findings, others are contradicted.     

Perceptual learning without signal

Perceptual learning is characterized by an improvement in a perceptual task following practice. Several studies have demonstrated that top-down processes, such as attention and task-related expectations, can be necessary components of perceptual learning [Ahissar & Hochstein, 1993, 2000, 2002; Fahle & Morgan, 1996; Seitz, Lefebvre, Watanabe, & Jolicoeur, 2005; Seitz, Nanez, Holloway, Koyama, & Watanabe, 2005; Seitz & Watanabe, 2003; Shiu & Pashler, 1992]. Here, we report an experiment that isolated top-down processes in perceptual learning, using a variant of the Gosselin and Schyns (1992) no-signal procedure. Results indicate that top-down processes can be sufficient to produce substantial, possibly long-lasting and rotation-invariant perceptual learning.     

Perceptual objects capture attention

A recent study has demonstrated that the mere organization of some elements in the visual field into an object attracts attention automatically [Kimchi, R., Yeshurun, Y., & Cohen-Savransky, A. (2007). Automatic, stimulus-driven attentional capture by objecthood. Psychonomic Bulletin & Review, 14(1), 166-172]. We tested whether similar results will emerge when the target is not a part of the object and with simplified task demands. A matrix of 16 black L elements in various orientations preceded the presentation of a Vernier target. The target was either added to the matrix (Experiment 1), or appeared after its offset (Experiment 2). On some trials four elements formed a square-like object, and on some of these trials the target appeared in the center of the object. No featural uniqueness or abrupt onset was associated with the object and it did not predict the target location or the direction of the target’s horizontal offset. Performance was better when the target appeared in the center of the object than in a different location than the object, even when the target appeared after the matrix offset. These findings support the hypothesis that a perceptual object captures attention (Kimchi et al., 2007), and demonstrate that this automatic deployment of attention to the object is robust and involves a spatial component.     

Perceptual compensation for eye torsion

To correctly perceive visual directions relative to the head, one needs to compensate for the eye's orientation in the head. In this study we focus on compensation for the eye's torsion regarding objects that contain the line of sight and objects that do not pass through the fixation point. Subjects judged the location of flashed probe points relative to their binocular plane of regard, the mid-sagittal or the transverse plane of the head, while fixating straight ahead, right upward, or right downward at 30 cm distance, to evoke eye torsion according to Listing's law. In addition, we investigated the effects of head-tilt and monocular versus binocular viewing. Flashed probe points were correctly localized in the plane of regard irrespective of eccentric viewing, head-tilt, and monocular or binocular vision in nearly all subjects and conditions. Thus, eye torsion that varied by +/-9 degrees across these different conditions was in general compensated for. However, the position of probes relative to the midsagittal or the transverse plane, both true head-fixed planes, was misjudged. We conclude that judgment of the orientation of the plane of regard, a plane that contains the line of sight, is veridical, indicating accurate compensation for actual eye torsion. However, when judgment has to be made of a head-fixed plane that is offset with respect to the line of sight, eye torsion that accompanies that eye orientation appears not to be taken into account correctly.     

Perceptual classification of chromatic modulation

We measured the regions of the equiluminant plane that are exploited by observers during a Yes/No detection task. The signal was a 640-ms Gaussian modulation (sigma(t) = 160 ms) of a Gaussian spatial patch (sigma(s) = 2.4 deg) presented in chromatically bivariate uniform noise. One component of the noise was along the direction axial with the signal in color space, the other perpendicular. Four signal directions were tested: along cardinal LM and S axes and two intermediate directions to which the cardinal axes were equally sensitive. The distribution of noise chromaticities from each trial was correlated with the observers' responses and the presence and absence of the signal to build a classification image of the distribution of chromaticities on which the decision of the observer was based. The images show a narrowly selective peak in the signal direction flanked by regions with a broader selectivity. These results raise the possibility that detection judgments are mediated by both linear and nonlinear mechanisms with peak sensitivities between the cardinal directions.     

Perceptual learning of temporal structure

We investigated the extent to which the ability to perceive spatial form from temporal structure (TS) improves with practice. Observers trained monocularly for a number of consecutive days on a shape discrimination task, with one group of observers judging shape defined by luminance contrast between target and background elements and another group judging shape defined by correlated TS (synchronized changes in motion direction between target and background elements). Substantial learning was found for both shape tasks, with complete interocular transfer of training. Observers trained on TS showed no transfer of learning to the luminance condition, but observers trained using the luminance display with incidental synchronized changes did show transfer to the TS task. Possible underlying neural changes are discussed.     

Perceptual consequences of feature-based attention

Attention modulates visual processing along at least two dimensions: a spatial dimension, which enhances the representation of stimuli within the focus of attention, and a feature dimension, which is thought to enhance attended visual features (e.g., upward motion) throughout the visual field. We investigate the consequences of feature-based attention onto visual perception, using dual-task human psychophysics and two distant drifting Gabor stimuli to systematically explore 64 combinations of visual features (orientations and drift speeds) and tasks (discriminating orientation or drift speed). The resulting single, consistent data set suggests a functional model, which predicts a maximum rule by which only the dominant product of feature enhancement and feature benefit by feature relevance may benefit perception.