ASYMMETRY IN REGARD TO THE EFFECTS OF MAGNOCELLULAR AND PARVOCELLULAR LESIONS

The placing of lesions in the magno- and parvocellular layers of the Lateral Geniculate Nucleus (LGN) of the visual stream has been used in attempts to assess the contributions of the two systems to various visual tasks. However, because there are about ten times as many parvocellular cells as magnocellular cells a lesion blocking the parvocellular input would be expected to have a larger deleterious impact than one blocking the magnocellular input. Thus, a visual task that depends upon all inputs, i.e. which is not linked specifically to either the magno- or parvocellular systems, would be expected to be more severely affected by a lesion in the parvocellular system than by one in the magnocellular system simply on the basis of the number of cells involved. A larger impact of a parvocellular lesion can, therefore, not be taken to mean that the task in question is specifically, or predominantly, linked to this system. Effects following magnocellular lesions (and not observed following parvocellular lesions), on the other hand, cannot be accounted for on the basis of cell number. There is, therefore, an asymmetry, in regard to the significance of the effects of lesions placed in the magno- and parvocellular layers of the LGN.

Magnocellular Bias in Exogenous Attention to Biologically Salient Stimuli as Revealed by Manipulating Their Luminosity and Color

Exogenous attention is a set of mechanisms that allow us to detect and reorient toward salient events—such as appetitive or aversive—that appear out of the current focus of attention. The nature of these mechanisms, particularly the involvement of the parvocellular and magnocellular visual processing systems, was explored. Thirty-four participants performed a demanding digit categorization task while salient (spiders or S) and neutral (wheels or W) stimuli were presented as distractors under two figure–ground formats: heterochromatic/isoluminant (exclusively processed by the parvocellular system, Par trials) and isochromatic/heteroluminant (preferentially processed by the magnocellular system, Mag trials). This resulted in four conditions: SPar, SMag, WPar, and WMag. Behavioral (RTs and error rates in the task) and electrophysiological (ERPs) indices of exogenous attention were analyzed. Behavior showed greater attentional capture by SMag than by SPar distractors and enhanced modulation of SMag capture as fear of spiders reported by participants increased. ERPs reflected a sequence from magnocellular dominant (P1p, ≃120 msec) to both magnocellular and parvocellular processing (N2p and P2a, ≃200 msec). Importantly, amplitudes in one N2p subcomponent were greater to SMag than to SPar and WMag distractors, indicating greater magnocellular sensitivity to saliency. Taking together, results support a magnocellular bias in exogenous attention toward distractors of any nature during initial processing, a bias that remains in later stages when biologically salient distractors are present.

Geometric-Optical Illusions Under Isoluminance?

This chapter briefly introduces nine classical geometric-optical illusions. These include the Delboeuf illusion, the Ebbinghaus illusion, the Judd illusion, the Müller-Lyer illusion, the Ponzo illusion, the vertical illusion, the Hering illusion, the Poggendorff illusion, and the Zoellner illusion. It then demonstrates that they persist under different luminance conditions and under isoluminance. The empirical findings show that our conscious percept is similarly affected by luminance conditions and isoluminance, suggesting that joint contour processing (chromatic and luminance) may extend well beyond early visual areas. The chapter further discusses these concepts in terms of the magnocellular system, the parvocellular system, and the koniocellular system.

Beyond Gaming

The interest around the utilization of video games as a component of rehabilitative therapy has dramatically increased over the past decade. Research efforts have confirmed the positive effects of repetitive gaming in improving visual outcomes; however, there is limited knowledge on the mechanism of action delivered by repetitive gaming. Utilizing knowledge of the visual system, including targeting specific cells in the retina with visual stimuli, the authors captured the training effects of gaming to augment pre-selected skills. Specifically, the authors embedded a homerun derby style baseball game with a contrast threshold test, to stimulate parvocellular retinal ganglion cells. Parvocellular cells are the first line of the ventral, or “what” pathway of visual processing. Repetitive stimulation of the parvocellular system shows promising preliminary results in improving batting performance.

Achromatic luminance contrast sensitivity in X-linked color-deficient observers: An addition to the debate

It is a matter of debate whether X-linked dichromacy is accompanied by enhanced achromatic processing. In the present study, we used sinusoidally modulated achromatic gratings under photopic conditions to compare the contrast sensitivity (CS) of protanopes, deuteranopes, and normal trichromats. 36 male volunteers were examined. CS was tested in static and dynamic conditions at nine different spatial frequencies. The results support the assumption that X-linked color-defective observers are at an advantage in terms of achromatic processing. Both protanopes and deuteranopes had significantly better CS than controls in both the static and the dynamic conditions. In the static condition, the advantage was observed especially at higher spatial frequencies, whereas in the dynamic condition, it was seen also at lower frequencies. The results are interpreted in terms of decreased chromatic modulation of the luminance channel and the early plasticity of the parvocellular system.

Magnocellular Channel Subserves the Human Contrast-Sensitivity Function

There is evidence that the human contrast-sensitivity function (CSF) is mediated by the spatiotemporal characteristics of magno and parvo neurons early in the visual pathway. In this study we use a measure of contrast gain derived from simple reaction times, to investigate the neural substrates of suprathreshold performance. The results reveal the activity of two mechanisms having distinctly different contrast-gain characteristics. Comparing these to neurophysiological data, we find that the magnocellular system dominates close-to-threshold detection and probably forms the basis of the achromatic CSF, whereas the parvocellular system dominates detection at higher contrasts, when the magnocellular system saturates.

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.