On using isoluminant stimuli to separate magno- and parvocellular responses in psychophysical experiments-a few words of caution.

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.

Photopic and scotopic flicker sensitivity of a rod monochromat.

The sensitivity to sine-wave flicker of a rod monochromat was compared with that of a normal subject at photopic and scotopic levels of luminance. The sensitivity of the rod monochromat in the low-frequency region (below 3 Hz at scotopic levels and below 12 to 14 Hz at photopic levels) was found to be superior to that of the control. This superiority was most pronounced at photopic levels, where the rod monochromat frequently showed a two-peak sensitivity curve.

Orientation discrimination in amblyopia.

Using extended sinusoidal gratings to avoid potential problems of eccentric fixation, the authors have studied orientation discrimination in amblyopia. For all subjects, elevated orientation discrimination thresholds at high spatial frequencies were found. However, raised thresholds decrease with decreasing spatial frequency, and can be normal at low frequencies. Orientation discrimination thresholds for both amblyopic and non-amblyopic eyes are independent of contrast over most of the visible range. Therefore, amblyopic orientation discrimination thresholds cannot be mimicked in non-amblyopic eyes by reducing contrast. Control experiments show that the orientation discrimination deficits are not restricted to vertical stimuli and that they are not a result of exaggerated cyclotorsional eye movements.

Rod monochromat sensitivity to sine wave flicker at luminances saturating the rods.

The flicker sensitivity (temporal modulation transfer function) of a rod monochromat was measured at luminances in the range of 8 to 782 scotopic td. The results could be fitted by a single curve shifted vertically, suggesting that rod saturation is independent of the temporal properties of the stimulus. This is consistent with the hypothesis that rod saturation is a pure receptor phenomenon.

Neuronal responses to plaids.

The majority of neurons in the visual cortex are orientation selective. When presented with a plaid, i.e. a stimulus generated by adding two gratings of different orientations, these neurons respond to the individual gratings making up the plaid. However, there are some pattern selective neurons in Area MT of the monkey visual cortex which respond in accordance with the combined plaid. The present study used computer simulation to investigate the response properties of simulated MT neurons to orthogonal plaids. The MT neurons were simulated by first multiplying the outputs of conventional orientation selective V1 neurons and then normalizing the product. It was discovered that pattern selective responses may emerge when the outputs from two orientation selective neurons, which differ in optimal orientation by more than about 50 degrees, are combined in this manner. This demonstrates that pattern selectivity may be the result of a very simple although nonlinear mechanism.

The magnocellular deficit theory of dyslexia: the evidence from contrast sensitivity.

A number of authors have made the claim that dyslexia is the result of a deficit in the magnocellular part of the visual system. Most of the evidence cited in support of this claim is from contrast sensitivity studies. The present review surveys this evidence. The result of this survey shows that the support for the magnocellular deficit theory is equivocal. In the case of spatial contrast sensitivity there clearly are results that are consistent with the magnocellular deficit theory; however, these results are outnumbered both by studies that have found no loss of sensitivity and by studies that have found contrast sensitivity reductions that are inconsistent with a magnocellular deficit. Many of the studies of temporal contrast sensitivity are also difficult to reconcile with a magnocellular deficit. The evidence from studies of contrast sensitivity is therefore highly conflicting with regard to the magnocellular system deficit theory of dyslexia.

A model for end-stopping in the visual cortex.

A model for end-stopping that requires only excitatory inputs is presented. This model is based on multiplication of the outputs from two orientation tuned and spatial-frequency selective neurons. Computer simulations show that, provided the optimal orientations of the two neurons are sufficiently different, the resulting product will display orientation-independent end-stopping. Neurons simulated in this manner display the main characteristics of actual hypercomplex cells.

Hyperacuity and the estimated positional accuracy of a theoretical simple cell.

Receiver operating characteristic (ROC) analysis was used to estimate positional discrimination thresholds for a theoretical simple cell by assuming Poisson distribution of the response, average response strength, and an optimal spatial frequency of 16.0 cyc/deg. These thresholds were compared to the positional difference required to generate a response change of one action potential. This comparison indicated that the inability to alter the response by less than one spike may be limiting positional accuracy. Taking account of this limitation, displacement thresholds were, depending on parameters, estimated to be as small as, or smaller than, the lowest psychophysical thresholds of about 2 sec of arc. This suggests that it may be possible to account for even the lowest human hyperacuity thresholds in terms of single cortical neurons.

Effects of contrast and spatial frequency on vernier acuity.

We have examined vernier acuity using sinusoidal luminance gratings. Vernier thresholds were affected by both grating contrast and spatial frequency. With fixed (50%) contrast gratings, vernier thresholds reached minimum values of approximately 10 sec of arc at spatial frequencies between 6 and 16 c/deg. Vernier thresholds for all spatial frequencies are related to contrast by a power law with exponents of approximately -0.8. Thresholds approach half a grating period (180 deg phase shift) as grating contrast approaches detection thresholds. We discuss our results in relation to three models for vernier detection. Most of our data are consistent with the predictions of Wilson's [(1986) Vision Res. 26, 453-469] model. Detection of vernier off-sets at low spatial frequencies may depend on detection of the horizontal border formed between the two halves of the grating.

The effects of large orientation and spatial frequency differences on spatial discriminations.

We have examined two questions: (1) can the finest orientation discrimination be achieved only between stimuli with similar spatial frequency content? and likewise, (2) can the lowest spatial frequency discrimination thresholds be achieved only with parallel gratings? In 2 AFC tests we found that neither type of discrimination was affected by stimulus differences along the other dimension. However, some small decreases in method of adjustment matching accuracy were associated with large differences along the secondary dimensions. Considering the neurophysiological implications, these data suggest that fine orientation and spatial frequency discrimination can occur even though separate populations of neurones in the primary visual cortex may be activated by the two stimuli to be discriminated.

Classifying simple and complex cells on the basis of response modulation.

Hubel and Wiesel (1962; Journal of Physiology, London, 160, 106-154) introduced the classification of cortical neurons as simple and complex on the basis of four tests of their receptive field structure. These tests are partly subjective and no one of them unequivocally places neurons into distinct classes. A simple, objective classification criterion based on the form of the response to drifting sinusoidal gratings has been used by several laboratories, although it has been criticized by others. We review published and unpublished evidence which indicates that this simple and objective criterion reliability divides neurons of the striate cortex in both cats and monkeys into two groups that correspond closely to the classically-described simple and complex classes.

The possible relationship between visual deficits and dyslexia: examination of a critical assumption.

Numerous studies have found that visual deficits are associated with dyslexia. The prevailing theory regarding this association is that dyslexia is the result of a deficit in the magnocellular system (earlier called the transient system) in the visual pathway. An essential assumption of this theory is that the parvocellular system (formerly called the sustained system) is suppressed by the magnocellular system at the time of saccadic eye movements. This assumption is examined on the basis of published studies of saccadic suppression. The evidence from six studies indicates quite unequivocally that the magnocellular system, not the parvocellular system, is suppressed during saccadic eye movements. It seems, therefore, that an essential premise of the magnocellular deficit theory of dyslexia is incorrect.

A note on the possibility of explaining why a color cannot be both red and green.

To provide a neurophysiological basis for the opponent nature of color vision it has been previously argued that a color cannot be both red and green because color-opponent neurons cannot respond to both red and green at the same time. The present analysis shows that such arguments hinge on the possibility of excluding statements of the kind "a color can be both red and green." For an empirical fact to exclude such statements, these statements would have to be meaningful. However, statements like "a color is both red and green" are not meaningful and are not allowed in our language. Thus, the properties of neurons are not in a position to exclude the possibility of "a color that is both red and green." This means that this attempt to establish a neurophysiological basis for opponent colors is flawed.

Amplitude and phase in the Müller-Lyer illusion.

It has previously been claimed that the Müller-Lyer illusion is the result of low-pass spatial filtering. One way to understand this would be that the distribution of amplitudes is what generates this illusion. This possibility was investigated by computing the 2-D Fourier transforms of the two Müller-Lyer stimuli and extracting their phase and amplitude spectra. These spectra were combined to create hybrid spectra having the phase of one Müller-Lyer figure and the amplitudes of the other. Images were then created by computing the inverse Fourier transform of the hybrid spectra. Except in cases where the analysis was performed patchwise on very small patches, the figures generated with the phase spectrum of the stimuli having outward-pointing fins appear the longer. This was also the case when stimuli were generated with flat amplitude spectra. Because they show that the Müller-Lyer illusion does not depend on any particular distribution of amplitudes, these demonstrations do not support the theory that the Müller-Lyer illusion is the result of low-frequency filtering.