Interference in lateral masking stimuli - The effects of relative phase, position, orientation, and spatial frequency.

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].

A few remarks on spatial interference in visual stimuli.

Many vision experiments, e.g., tests of masking and visual crowding, involve the effect of adding a second stimulus to an initial one.

On separating the magnocellular from the parvocellular: Responses to two statements by Goodhew et al.

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 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.

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.

The need to differentiate the magnocellular system from the dorsal stream in connection with dyslexia.

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.

A few observations on linking VEP responses to the magno- and parvocellular systems by way of contrast-response functions.

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.

A few observations on metacontrast stimuli.

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.

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.

Subjective criteria and illusions in visual testing: some methodological limitations.

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.

Amplitude spectra of line-motion stimuli.

The line-motion illusion has been regarded as the result of attention. An alternative interpretation is that the illusion is related to apparent motion which would predict the stimuli to contain motion energy associated with the direction of the illusory motion. In order to examine this possibility Fourier transforms of x-t plots of line-motion stimuli were generated under a variety of conditions.

The time course of visual backward masking deficits in schizophrenia.

Schizophrenia, it has been hypothesized, is linked to a deficiency in the magnocellular portion of the visual system. Abnormal backward masking has been invoked as support for this hypothesis. The rationale for linking backward masking to the magnocellular system is the hypothesis that fast responses in the magnocellular systems catches up with, and then inhibits slower responses in the parvocellular system.

On the use of visual motion perception to assess magnocellular integrity.

It has been proposed, particularly in connection with dyslexia and schizophrenia, that motion perception can be used to assess magnocellular integrity. This suggestion is examined in this paper. The following observations are made: (1) motion information, i.

On identifying magnocellular and parvocellular responses on the basis of contrast-response functions.

It has been proposed that magnocellular and parvocellular sensitivity in schizophrenic individuals can be assessed using steady-state visually evoked potentials (VEPs) to either low-contrast stimuli or stimuli whose contrast is modulated around a high contrast "pedestal" (Green MF, Butler PD, Chen Y, et al. Schizophr Bull. 2009;35:163-181).

Temporal order judgment in dyslexia--task difficulty or temporal processing deficiency?

Dyslexia has been widely held to be associated with deficient temporal processing. It is, however, not established that the slower visual processing of dyslexic readers is not a secondary effect of task difficulty. To illustrate this we re-analyze data from Liddle et al.

L- and M-cone ratios and magnocellular sensitivity in reading.

It has been proposed that magnocellular deficits cause the reading problems in dyslexia. However, how magnocellular deficiencies are supposed to cause these problems is unclear. Recently it has been proposed that reading performance is limited by the L-/M-cone inputs to the magnocellular system.