In age-related macular degeneration (AMD), the processing of fine details in a visual scene, based on a high spatial frequency processing, is impaired, while the processing of global shapes, based on a low spatial frequency processing, is relatively well preserved. The present fMRI study aimed to investigate the residual abilities and functional brain changes of spatial frequency processing in visual scenes in AMD patients. AMD patients and normally sighted elderly participants performed a categorization task using large black and white photographs of scenes (indoors vs. outdoors) filtered in low and high spatial frequencies, and nonfiltered. The study also explored the effect of luminance contrast on the processing of high spatial frequencies. The contrast across scenes was either unmodified or equalized using a root-mean-square contrast normalization in order to increase contrast in high-pass filtered scenes. Performance was lower for high-pass filtered scenes than for low-pass and nonfiltered scenes, for both AMD patients and controls. The deficit for processing high spatial frequencies was more pronounced in AMD patients than in controls and was associated with lower activity for patients than controls not only in the occipital areas dedicated to central and peripheral visual fields but also in a distant cerebral region specialized for scene perception, the parahippocampal place area. Increasing the contrast improved the processing of high spatial frequency content and spurred activation of the occipital cortex for AMD patients. These findings may lead to new perspectives for rehabilitation procedures for AMD patients.

Although blind people heavily depend on working memory to manage daily life without visual information, it is not clear yet whether their working memory processing involves functional reorganization of the memory-related cortical network. To explore functional reorganization of the cortical network that supports various types of working memory processes in the early blind, we investigated activation differences between 2-back tasks and 0-back tasks using fMRI in 10 congenitally blind subjects and 10 sighted subjects. We used three types of stimulus sequences: words for a verbal task, pitches for a non-verbal task, and sound locations for a spatial task. When compared to the sighted, the blind showed additional activations in the occipital lobe for all types of stimulus sequences for working memory and more significant deactivation in the posterior cingulate cortex of the default mode network. The blind had increased effective connectivity from the default mode network to the left parieto-frontal network and from the occipital cortex to the right parieto-frontal network during the 2-back tasks than the 0-back tasks. These findings suggest not only cortical plasticity of the occipital cortex but also reorganization of the cortical network for the executive control of working memory. (JINS, 2011, 17, 407–422)

We have studied the presence and distribution of α-amino-3–hydroxy-5–methyl-4–isoxazolepropionate (AMPA)-selective glutamate receptor subunits (GluR 1, 2, 3, and 4) in the adult cat visual cortical areas 17, 18, 19, and the lateral suprasylvian areas (LSA). Reverse transcription-polymerase chain reaction (RT-PCR) amplification indicated that the genes encoding GluR 1, 2, 3, and 4 are expressed in these areas and Western blot analysis revealed that the size of the corresponding peptides is similar to those described in the rat brain. In situ hybridization (ISH) using digoxigenin-labeled riboprobes showed that mRNAs coding for GluR1 and GluR3 were located in cells in all layers of the areas examined and also in the underlying white matter. GluR1 mRNA was relatively abundant throughout layers II–VI while GluR3 mRNA revealed a more laminated pattern of expression, preferentially labeling cells in layers II, III, V, and VI. The distribution of AMPA-selective receptor subunit peptides was studied by immunohistochemistry using subunit specific antibodies and found to be consistent with ISH results. In addition, we observed that most of the cells strongly labeled by the anti-GluR1 antibody were non-pyramidal neurons and that intense GluR2/3 immunoreactivity was seen preferentially in pyramidal neurons. Interestingly, double-labeling experiments indicated that neurons expressing γ-aminobutyric acid (GABA) as well as the GluR1 subunit were particularly abundant in deeper layers. The GluR4 peptide was predominantly found in a relatively low number of layer III and layer V neurons with either pyramidal or non-pyramidal morphology. Finally, the distribution of neurons expressing the various receptor subunits was similar in all the visual cortical areas studied. These findings indicate a high expression of GluR1–3 subunits in the cat visual cortex and that GluR1 and GluR2/3 subunits are particularly abundant in non-pyramidal and pyramidal neurons, respectively. In addition, the results described here provide a reference for future studies dealing with the effect of visual deprivation on the expression of this receptor type.

Here we examine the patterns of connections between the zona incerta (ZI) of the thalamus and the major visual centers of the rat brain, namely the retina, dorsal lateral geniculate nucleus (LGd), superficial layers of the superior colliculus (SCs), and occipital cortex (Oc1). Injections of the tracers biotinylated dextran or cholera toxin subunit b were made into each of these centers, as well as ZI itself, by using stereotaxic coordinates. Rat brains were then aldehyde-fixed and processed using standard methods. We show that the retina, LGd, SCs, and Oc1 all have connections with ZI; moreover, that each of these connections make a very distinct territory or subsector within the most lateral ZI regions. This subsector of connectivity with the visual centers does not respect the well-defined cytoarchitectonic sectors of ZI, being made up of small zones in the dorsal, ventral, and caudal sectors. Often, a distinctive “horse-shoe” pattern is evident, particularly after retinal and Oc1 injections. Tracer injections into topographically distinct regions of the LGd, SCs, or Oc1 results in no shift in the spatial location of labelling within ZI; after each injection, labelling is always seen within the lateral edge of the nucleus. Labelled terminals and cells are seen after LGd and SCs injections, while only labelled terminals are seen after retinal and Oc1 injections. Although the precise function of this novel visual subsector is not known, these early findings suggest that ZI may be in a position to integrate visual information together with the other somatosensory, motor, and visceral information that it receives.

Contralateral-control methods can be applied to psychophysiology and in particular to the study of visual memory. Visual memory possesses some degree of hemispheric organization, so that visual memory traces for laterally presented stimuli are stronger or more durable in the hemisphere contralateral to the hemifield where the stimuli were first presented. I first introduce the concept of hemispheric organization of function. Then I discuss how hemispheric organization can be exploited for obtaining information about the time course and brain localization of psychological processes, using a contralateral-control method. Behavioral and event-related brain potential data support the hemispheric organization view of visual memory, and the contralateral-control method, in conjunction with the recording of the event-related optical signal, can be used to reveal the existence of memory-driven processes in early stations of the visual system.