Lateral masking has been defined as the perception of a visual target stimulus being impaired when other stimuli are present in its adjacent surroundings. In such cases it has generally been assumed that the target stimulus presented along with a masking stimulus has the same stimulus power as when presented alone and that the reduced visibility reflects interactions in the visual system. It has, however, become clear that there may be interference between such stimuli [5]. Such interference, which takes place in the stimuli and is independent of the visual system, has the potential to reduce the stimulus power of target stimuli. The present report asks, employing 2-Dimensional Gabor functions as stimuli, how interference effects may depend on (1) relative spatial phase, (2) separation between target and masking stimuli, (3) difference in orientation, and (4) difference in spatial frequency between masking and target stimuli. Interference was estimated numerically based on the sums of the amplitudes in the Fourier spectra and the norms of these spectra. Clear evidence for interference was demonstrated with both measures. All the four parameters have the ability to influence the amount of interference. These findings, therefore, emphasize that one cannot count on a target stimulus presented along with masking stimuli to have the same stimulus power as when it is presented by itself.

Many vision experiments, e.g., tests of masking and visual crowding, involve the effect of adding a second stimulus to an initial one. The effects of such additions are generally considered in terms of physiological mechanisms and the possibility of interference in the stimuli is generally not considered. In the present study, interference between two stimuli was assessed by comparing the sum of amplitudes in the combined stimulus to the sums of the amplitudes in the two stimuli determined separately. With this approach, evidence for interference was found. It was also found that adding a second stimulus may alter the phase angles. These observations mean that the same stimulus presented together with other stimuli may have less stimulus power than when presented by itself. Thus, it is necessary to take account of the possibility of interference when interpreting results from experiments in which the effect of one stimulus element upon another is explored.

Interference may occur between visual stimuli or between separate elements in visual stimuli. Such interference takes place in the stimuli as these exist independently of vision and visual processing. Interference manifests itself in reduced ”stimulus power” which makes this a central concept in this context. The present report examines three potential measures of stimulus power: Amplitude sum, norm, and sum of the absolutes of wavelet coefficients. These are examined with very simple 1-Dimensional stimuli as examples. It is shown how there may be interference with each of these measures. Thus, interference is not only linked to the Fourier Transform. Due to interference a given stimulus may have less stimulus power when presented along with a second stimulus than when presented alone. This has consequences for the concept of stimulus sinceit makes it possible for the same physical stimulus to have different stimulus power in different contexts.

Enns and Di Lollo (1997, Psychological Science, 8, 135-139) described a kind of visual masking termed”masking by substitution” in which a diamond-shaped target stimulus was masked by four small squares.The present study addresses the question of if , or to what extent, interference in the stimuli could reducetheir power. Interference was assesses based on (1) the sums of the amplitudes in the Fourier spectra,and (2) the norms of the Fourier spectra. The two methods gave interference effects of 17 % and 28%, respectively. It is concluded that in the case of the stimuli used by Enns and Di Lollo it cannot beassumed that the target stimulus has the same stimulus power when presented together with the maskingstimulus as when presented alone. The interference takes place in the stimuli and does not depend uponthe visual system.

Goodhew et al. (Attention Perception & Psychophysics, 79, 1147-1164, 2017) claim we (Skottun & Skoyles) hold: (1) that it is not possible to separate contributions from the magno- and parvocellular systems to psychophysical tasks, and (2) that there are no differences between magno- and parvocellular cells. Neither of these claims is correct.

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.

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.

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.

A number of authors have postulated a "magnocellular-dorsal stream" deficit in dyslexia. Combining the magnocellular system and the dorsal stream into a single entity in this context faces the problem that contrast sensitivity data do not point to a magnocellular deficiency linked to dyslexia, while, on the other hand, motion perception data are largely consistent with a dorsal stream dysfunction. Thus, there are data both for and against a "magnocellular-dorsal stream" deficit in connection with dyslexia. It is here pointed out that this inconsistency is abolished once it is recognized that the magnocellular system and the dorsal stream are separate entities.

•Do magnocellular and parvocellular cells differ with regard to spatial frequency?•It is here found that there is little difference between the systems.•Do the dorsal and ventral streams differ with regard to spatial frequency?•This question is examined.•It is difficult to differentiate the systems based on low spatial frequencies.

GENERAL COMMENTARY article Front. Hum. Neurosci., 05 December 2014Sec. Speech and Language Volume 8 - 2014 | https://doi.org/10.3389/fnhum.2014.00983

It has been proposed that magno- and parvocellular contributions to Visually Evoked Potentials (VEPs) can be isolated, or differentiated, by noting the contrast-response relationships of the responses. This suggestion is examined quantitatively by determining the similarity between various sets of VEP data that have been attributed to the magno- and parvocellular systems and previously reported contrast-response functions for different kinds of neurons (magno- and parvocellular neurons and V1, V4, and MT cells) and combinations of the contrast-response functions for these neurons. It is found that other neurons, or combinations of other neurons, typically give better fits to the data than do magno- and parvocellular cells. Thus, to attribute VEP responses to the magno- or parvocellular systems based on contrast-responses properties faces difficulties.

GENERAL COMMENTARY article Front. Hum. Neurosci., 06 October 2014Sec. Cognitive Neuroscience Volume 8 - 2014 | https://doi.org/10.3389/fnhum.2014.00786

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.

It is argued that illusions cannot generally be investigated with criterion-independent methods. This limits the value of the data obtained from them. This is particularly important when the results are compared between groups of subjects, for example, between dyslexic readers and controls, since it is possible that the differences between the groups reflect differences with regard to criteria rather than real perceptual differences.

Isoluminant (or equiluminant) color stimuli (i.e., those that contain variations only in chromaticity) have been employed in attempts to separate magno- and parvocellular responses in psychophysical and noninvasive electrophysiological experiments. The justification for this has been the assumption that magnocellular cells, unlike parvocellular neurons, do not respond to stimuli varying only in hue. However, several problems are associated with this notion: (1) under many conditions, magnocellular neurons are not fully silenced at isoluminance, and (2) in many circumstances, parvocellular responses are substantially reduced at isoluminance. To rely upon isoluminant stimuli to “bias” stimuli toward the parvocellular system also faces obstacles. Therefore, caution is required when attempting to use isoluminant color to separate magno- and parvocellular responses.

Metacontrast is a form of visual masking in which the target and mask are non-overlapping. In metacontrast, the masking effect is typically largest when the mask is presented some time after the target. This is known as Type-B masking. The present report examines to what extent Type-B metacontrast masking can be explained based on the stimuli involved. The assumption is made that the visibility of the target is, at least in part, determined by the correlation between the amplitude spectrum of the target-and-mask combination and that of the target alone. It is found that the correlation is higher when the stimuli are presented at the same time relative to when they are presented at different times. This relationship follows from the stimuli alone. Thus, one would expect the masking to be weakest when the two stimuli are simultaneous. Type-B correlation functions, in which the largest reductions occur only when the mask is presented after the target, can be obtained by further assuming a temporal integration window with a rapid onset and a shallow decline. In agreement with psychophysical masking studies; the analyses yield functions that are most similar to Type-B masking for moderate mask intensities and become less Type-B like at higher mask intensities. The effects of dark adaptation and spatial separation of target and mask are also modeled.

GENERAL COMMENTARY article Front. Psychol., 27 December 2013Sec. Psychopathology Volume 4 - 2013 | https://doi.org/10.3389/fpsyg.2013.00999

Zorzi et al. (1) reported in PNAS that enlarging interletter spacing increases reading speed and accuracy in dyslexic readers. They attributed this to an abnormally strong crowding effect. An important question here is whether or not these effects are, in fact, specific to dyslexia. Zorzi et al. (1) sought to examine this by including a control group. Based on the results from this group, they concluded “... that increased crowding is most likely a fundamental deficit in dyslexia....” We make two observations.

Background: It has been claimed that schizophrenia can be linked to the magnocellular system by way of N-methyl-d-aspartate (NMDA) receptors. The present report examines this claim.Methods: A review is made of relevant research literature.Results: The NMDA studies that have been referenced to connect visual deficits in schizophrenia to the magnocellular system are based on the cat, a species whose visual system is fundamentally different from that of primates. The cat visual system cannot easily be divided into magno- and a parvocellular portions.Conclusions: Owing to the substantial differences between the visual systems of cats and primates, it is difficult to link sensory abnormalities in schizophrenia specifically to the magnocellular system based on data from the cat.