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.

Neuropsychological Perspective on Dyslexia

The aim of this chapter is to offer a neuropsychological approach to dyslexia. Firstly, the definition of dyslexia is addressed, as a specific learning disability that is neuropsychological in origin. Secondly, the clinical manifestations of dyslexia are discussed: academic, cognitive-linguistic, and socio-emotional. Thirdly, the main clinical explanations are explored, based on genetic theories (familial and twin heritability) and neurological theories, mainly neuroanatomical (brain asymmetry, corpus callosum morphology, cerebellar morphology, and variations in grey/white matter) and neurophysiological hypotheses (magnocellular system, connectivity between brain areas, and functional activity of brain areas). Finally, the main bases of an adequate neuropsychological intervention are detailed, such as training in visual perception, auditory perception, phonological processing, and orthographic processing.

Specificity of the visual perception and the thinking process impairments in their relationship with the functional state of the magno- and parvocellular neuronal systems in schizophrenia

The article examines the relationship between cognitive processes of perception and thinking in schizophrenia. An analysis of the functional state of the magnocellular and parvocellular visual systems, the specificity of their interaction was carried out. New data were obtained on the relationship between the mechanisms of global and local analysis of visual information and cognitive impairments in schizophrenia. The contribution of magnocellular and parvocellular systems (mechanisms of global and local analysis) in the implementation of the thinking processes has been discovered. It has been established that the activity of the magnocellular system is associated with the processes of abstraction and generalization. However, hyperactivation of the magnocellular system leads to a violation of the selectivity and goal setting in the thinking process. A decrease in the activity of the parvocellular system in patients with schizophrenia leads to distortions of the generalization process, an increase in the number of “latent” signs in the thinking process. The study obtained results that allow us to state that early sensory deficits are closely interrelated with the level and nature of the functioning of more highly organized cognitive processes. Sensory deficits identified in schizophrenia lead to higher-level cognitive deficits and are a key link in the structure of neurocognitive deficits in schizophrenia.

Exploring the Moderation Effect of Educational Stage on Visual Magnocellular Functioning Linked to Reading: A Study in French Primary School Children

Many studies have investigated the visual magnocellular system functioning in dyslexia. However, very little is known on the relationship between the visual magnocellular system functioning and reading abilities in typical developing readers. In this study, we aimed at studying this relationship and more specifically the moderation effect of educational stage on this link. We thus tested 82 French typical developing readers (40 beginning readers—Grade 1 and 42 advanced readers—Grade 5) with reading tests and a coherent dot motion task measuring the visual magnocellular functioning. Results indicate positive correlations between visual magnocellular functioning and reading for beginning readers but not for advanced readers. Moreover, moderation analyses confirm that reading proficiency moderates the relationship between magnocellular system functioning and reading outcomes. We concluded that the relationship between visual magnocellular pathway functioning and reading abilities in typical developing readers could depend on reading proficiency.

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.