Bernt C. Skottun

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 We review published and unpublished evidence which indicates that this simple and objective criterion reliability divides neurons of the striate cortex in both cats and monkeys into two groups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 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.

Contrast sensitivity and magnocellular functioning in schizophrenia

It has been suggested that schizophrenia is associated with a magnocellular deficit. This would predict a loss of contrast sensitivity at low spatial and/or at high temporal frequencies. We here review research that tested contrast sensitivity in individuals with schizophrenia. We find that the results of this research tend to show uniform reductions in contrast sensitivity that are generally not consistent with a magnocellular deficit. While much of this data may be consistent with an attentional deficiency on the part of the schizophrenic individuals, it is difficult to link such an attentional deficiency specifically to the magnocellular system. The conclusion of the present review is that contrast sensitivity data do not indicate the existence of an association between magnocellular deficits and schizophrenia.

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 Wilsons [(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.

On the directional selectivity of cells in the visual cortex to drifting dot patterns.

It is well established that cortical neurons frequently show different preferred drift directions for random dots and gratings. Dot stimuli often produce two preferred directions which are arranged symmetrically on either side of the preferred directions for gratings. Based on their filter properties in three-dimensional (3-D) Fourier space and on the 3-D power spectra of drifting dot patterns, we estimated the optimal direction to drifting dots for ten neurons in the striate cortex of five adult cats. These estimates frequently gave two optimal directions, one on either side of the optimal direction to gratings. The angle between the two estimated peaks increases with drift speed. Predicted and actual angles were in reasonably good agreement. We conclude, therefore, that the directional selectivity of cortical neurons to drifting random dot patterns can be understood from linear filtering properties. For this reason, the directional tuning to drifting dot patterns seems to reflect the same mechanisms that mediate the responses to sinusoidal gratings and do not require a separate directional mechanism.

On the use of metacontrast to assess magnocellular function in dyslexic readers

It has been proposed that dyslexia is the result of a deficit in the magnocellular system. Reduced metacontrast masking in dyslexic readers has been taken as support for this view. In metacontrast, a masking stimulus reduces the visibility of a spatially adjacent target stimulus when the target stimulus precedes the masking stimulus by about 30–100 msec. Recent evidence indicates that the latency difference between the magnocellular and parvocellular subcortical pathways is at most 20 msec and may be as small as only 5 msec, or even less. This makes it difficult to attribute the latency in metacontrast to the latency differences between the magnocellular and parvocellular systems. It is therefore problematic to attribute reduced metacontrast masking to a deficit in the magnocellular system.

On the use of red stimuli to isolate magnocellular responses in psychophysical experiments: a perspective.

Neurophysiological recordings have shown that activity of magnocellular neurons may be reduced by red backgrounds. This has led some researchers to use red light, or red filters, in attempts to determine the magnocellular contribution to psychophysical tasks. This requires that red light not affect parvocellular neurons, or at least that it is possible to control for the effect on the parvocellular system by using other colors. The present report investigates these assumptions by calculating the effect of red, green, and blue filters on the three cone pigments and on the four parvocellular color-opponent cell mechanisms. It is found that a red filter has a large effect on the long- and middle-wavelength cone pigments and on the red-green color-opponent mechanisms. A green filter, on the other hand, has little effect. A blue filter has a fairly pronounced effect but this effect is distinctly different from that of the red filter. These results indicate that one ought not rely upon red light to isolate magnocellular activity in psychological experiments. The results also indicate that it is difficult to use colors other than red to control for the effect of this color on the parvocellular system.

Neurophysiological evaluation of the differential response model for orientation and spatial-frequency discrimination

Recent models have attempted to reconcile low psychophysical orientation and spatial-frequency discrimination thresholds with relatively broad orientation and spatial-frequency tuning of cortical neurons. These models have relied on the ability of the neurons to convert small stimulus changes into reliable response changes. We have examined this ability in a sample of neurons from the cats striate cortex. We present here data from two cells that reliably signaled the smallest orientation and spatial-frequency differences. Using receiver operating characteristic analysis, we find that these cells could reliably signal orientation differences of 1.84 deg and spatial-frequency differences of 0.073 octave. We compare these single-cell results to cat and human behavioral discrimination thresholds.

Temporal Frequency and the Magnocellular and Parvocellular Systems

Can the magnocellular system be stimulated exclusively, or predominantly, by using a particular temporal frequency? There are two problems. (1) Many researchers have confused contrast reversal rates with luminance modulation frequencies, even though it is the latter (which is half the reversal rate) that may be compared to neuronal responses. (2) Many of the frequencies used are too low to selectively activate magnocellular neurons. While a pure magnocellular response may be obtained by employing particular combinations of temporal frequency, spatial frequency and contrast, however, this possibility is limited to stimuli close to detection threshold, and such stimuli give only weak responses.