Temporal Frequency and the Magnocellular and Parvocellular Systems

CFS-crafter: An open-source tool for creating and analyzing images for continuous flash suppression experiments

Continuous flash suppression (CFS) is a popular masking technique used to manipulate visual awareness. By presenting a rapidly changing stimulus to one eye (the 'mask'), a static image viewed by the other (the 'target') may remain invisible for many seconds. This effectiveness affords a means to assess unconscious visual processing, leading to the widespread use of CFS in several basic and clinical sciences. However, the lack of principled stimulus selection has impeded generalization of conclusions across studies, as the strength of interocular suppression is dependent on the spatiotemporal properties of the CFS mask and target. To address this, we created CFS-crafter, a point-and-click, open-source tool for creating carefully controlled CFS stimuli. The CFS-crafter provides a streamlined workflow to create, modify, and analyze mask and target stimuli, requiring only a rudimentary understanding of image processing that is well supported by help files in the application. Users can create CFS masks ranging from classic Mondrian patterns to those comprising objects or faces, or they can create, upload, and analyze their own images. Mask and target images can be custom-designed using image-processing operations performed in the frequency domain, including phase-scrambling and spatial/temporal/orientation filtering. By providing the means for the customization and analysis of CFS stimuli, the CFS-crafter offers controlled creation, analysis, and cross-study comparison. Thus, the CFS-crafter-with its easy-to-use image processing functionality-should facilitate the creation of visual conditions that allow a principled assessment of hypotheses about visual processing outside of awareness.

Age- and frequency-dependent changes in dynamic contrast perception in visual snow syndrome

Objective

Patients with visual snow syndrome (VSS) suffer from a debilitating continuous (“TV noise-like”) visual disturbance. They report problems with vision at night and palinopsia despite normal visual acuity. The underlying pathophysiology of VSS is largely unknown. Currently, it is a clinical diagnosis based on the patient’s history, an objective test is not available. Here, we tested the hypothesis that patients with VSS have an increased threshold for detecting visual contrasts at particular temporal frequencies by measuring dynamic contrast detection-thresholds.

Methods

Twenty patients with VSS were compared to age-, gender-, migraine- and aura-matched controls in this case-control study. Subjects were shown bars randomly tilted to the left or right, flickering at six different frequencies (15 Hz, 20 Hz, 25 Hz, 30 Hz, 35 Hz, 40 Hz). The contrast threshold (CT) for detection of left or right tilt was measured in a two-alternative adaptive forced-choice procedure (QUEST). The threshold was defined as the Michelson contrast necessary to achieve the correct response in 75% of the cases.

Results

The CT increased for higher flicker frequencies (ANOVA: main effect frequency: F (5,180) = 942; p < 0.001), with an additional significant frequency*diagnosis interaction (ANOVA: F (5,180) = 5.00; p < 0.001). This interaction effect was due to an increased CT at a flicker frequency of 15 Hz in the VSS cohort (VSS: MC = 1.17%; controls: MC = 0.77%). At the other frequencies, group comparisons revealed no differences. Furthermore, in the VSS cohort we observed an increase of CT with higher age (r = 0.69; p < 0.001), which was not seen in controls (r = 0.30; p = 0.20).

Conclusions

This study demonstrates a lower visual contrast sensitivity exclusively at 15 Hz in VSS patients and demonstrates frequency-dependent differences in dynamic contrast vision. The peak sensitivities of both parvo- and magnocellular visual pathways are close to a frequency of about 10 Hz. Therefore, this frequency seems to be of crucial importance in everyday life. Thus, it seems plausible that the impairment of contrast sensitivity at 15 Hz might be an important pathophysiological correlate of VSS. Furthermore, the overall age-related decrease in contrast sensitivity only in VSS patients underscores the vulnerability of dynamic contrast detection in VSS patients. Dynamic CT detection seems to be a promising neurophysiological test that may contribute to the diagnosis of VSS.

Dyslexia and the magnocellular-parvocellular coactivaton hypothesis

Previous studies showed that the lateral masking of a fast-moving low spatial frequency (SF) target was strong when exerted by static flankers of lower or equal to the target SF and absent when flankers' SF was higher than the target's one. These masking and unmasking effects have been interpreted as due to Magnocellular-Magnocellular (M-M) inhibition and Parvocellular-on-Magnocellular (P-M) disinhibitory coactivation, respectively. Based on the hypothesis that the balance between the two systems is perturbed in Developmental Dyslexia (DD), we asked whether dyslexic children (DDs) behaved differently than Typically Developing children (TDs) in conditions of lateral masking. DDs and TDs performed a motion discrimination task, of a .5c/deg Gabor target moving at 16 deg/sec, either isolated or flanked by static Gabors with a SF of .125, .5 or 2 c/deg (Experiment 1). As a control, they also performed a contrast detection task of a static target, either isolated or flanked (Experiment 2). DDs did not perform any different from TDs with either a static target or an isolated moving target of low spatial frequency, thus suggesting efficient feedforward Magnocellular (M) and Parvocellular (P) processing. Also, DDs showed similar contrast thresholds to TDs in the M-M inhibition condition. Conversely, DDs did not recover from lateral masking in the M-P coactivation condition. In addition, their performance in this condition negatively correlated with non-words accuracy, supporting the suggestion that an inefficient Magno-Parvo coactivation may possibly be associated to both higher visual suppression and reduced perceptual stability during reading.

A few words on differentiating magno- and parvocellular contributions to vision on the basis of temporal frequency

A number of authors have proposed that changes in temporal frequency within the range of 0-30Hz may be used to differentiate contributions from the magno- and parvocellular systems. The present analyses estimate the percentage of active magnocellular cells as a function of frequency based on published cut-off values for magno- and parvocellular cells. These analyses indicate that varying the temporal frequency over the range of 0-30Hz has little effect upon the percentage of active magnocellular cells. The analyses were also carried out for a series of hypothetical cut-off frequencies and standard deviations of these frequencies for magnocellular cells. The results of these simulations indicate that even large alterations in these values do not alter the above conclusion to a noteworthy extent.

A few remarks on the utility of visual motion perception to assess the integrity of the magnocellular system or the dorsal stream

It has been proposed that visual motion perception may be used to assess magnocellular or dorsal stream integrity. It is here pointed out, based on recently published data from dyslexic readers, that it is possible for deficient motion perception to exist without there being deficiencies in neither the magnocellular system nor in the dorsal stream. This makes it difficult to rely upon tests of motion perception to assess the integrity of these structures.

On the use of spatial frequency to isolate contributions from the magnocellular and parvocellular systems and the dorsal and ventral cortical streams

Many authors have claimed that suprathreshold achromatic stimuli of low and high spatial frequency can be used to separate responses from different entities in the visual system. Most prominently, it has been proposed that such stimuli can differentiate responses from the magnocellular and parvocellular systems. As is reviewed here, investigators who have examined stimulus specificity of neurons in these systems have found little difference between magno- and parvocellular cells. It has also been proposed that spatial frequency can be used to selectively activate the "magnocellular-dorsal stream". The present review indicates that cells in Area MT of the dorsal stream do prefer very low spatial frequencies. However, the review also shows that cells in Area V4 of the ventral stream respond, not only to relatively high spatial frequencies, but also to low frequency stimuli. Thus, low spatial frequencies cannot be relied upon to selectively activate the dorsal stream.

On using very high temporal frequencies to isolate magnocellular contributions to psychophysical tasks

In connection with dyslexia several authors have sought to employ stimuli of very high temporal frequency to isolate magnocellular contributions to visual tasks. It is here pointed out that considerable evidence indicate that the ability to see the very highest temporal frequencies is limited by cortical mechanisms. This suggests that variations and abnormalities in this ability may reflect cortical factors rather than magnocellular ones. It is therefore difficult to rely upon very high temporal frequency stimuli to isolate contributions from the magnocellular system.

Near peripheral motion detection threshold correlates with self-reported failures of attention in younger and older drivers

Motion contrast thresholds for 0.4cycle/degree drifting Gabor stimuli were assessed at 15 degrees eccentricity in the right and left visual fields for 16 younger drivers (ages 24-42), and 15 older drivers (ages 65-84), using a temporal two-alternative forced choice staircase procedure. Two self-report questionnaires that assess failures of attention while driving-the Driver Perception Questionnaire (DPQ5), and an abridged Aging Driver Questionnaire (ADQ15)-were administered. The three UFOV((R)) subtests of attention and processing speed were also administered. Mean peripheral motion contrast threshold (PMCT) of older drivers was significantly higher than that of younger drivers. When controlling for age, PMCT thresholds correlated significantly with both DPQ5 and ADQ15 while the UFOV((R)) subtests were found not to correlate with PMCT results. The potential value of the PMCT as an assessment of drivers' hazard detection capacity is discussed.

On using Vernier acuity to assess magnocellular sensitivity

A recent study [Keri, S., & Benedek, G. (2009). Visual pathway deficit in female fragile x 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 x 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.