Contrast is the differential luminance between one object and another. Contrast sensitivity (CS) quantifies the ability to detect this difference: estimating contrast threshold provides information about the quality of vision and helps diagnose and monitor eye diseases. High contrast visual acuity assessment is traditionally performed in the eye care practice, whereas the estimate of the discrimination of low contrast targets, an important complementary task for the perception of details, is far less employed. An example is driving when the contrast between vehicles, obstacles, pedestrians, and the background is reduced by fog. Many conditions can selectively degrade CS, while visual acuity remains intact. In addition to spatial CS, “temporal” CS is defined as the ability to discriminate luminance differences in the temporal domain, i.e., to discriminate information that reaches the visual cortex as a function of time. Likewise, temporal sensitivity of the visual system can be investigated in terms of critical fusion frequency (CFF), an indicator of the integrity of the magnocellular system that is responsible for the perception of transient stimulations. As a matter of fact, temporal resolution can be abnormal in neuro-ophthalmological clinical conditions. This paper aims at considering CS and its application to the clinical practice.
Background: To settle the fundamentals of a numerical procedure that relates retinal ganglion-cell density and threshold sensitivity in the visual field. The sensitivity of a generated retina and visual pathways to virtual stimuli are simulated, and the conditions required to reproduce glaucoma-type defects both in the optic-nerve head (ONH) and visual fields are explored.
Methods: A definition of selected structural elements of the optic pathways is a requisite to a translation of clinical knowledge to computer programs for visual field exploration. The program is able to generate a database of normalized visual fields. The relationship between the number of extant receptive fields and threshold sensitivity is plotted for background sensitivity and corresponding automated perimetry. A solution in two planes to the 3D distribution of axons in the ONH is proposed. Visual fields with induced damage in the optic disc are comparable in pattern and quantity to glaucomatous records.
Results: The two-level simulation of the ONH facilitates the analysis of optic-cup/retinal defects. We can generate the virtual optic pathways tailored to the age and morphology of the patient’s eye, and it is possible to reproduce glaucomatous damage by “reverse engineering” engineering. The virtual cortical model renders a quantitative relationship between visual defect and neural damage.
Conclusions: A two-level computing of the retina/optic nerve facilitates the analysis of neuroretinal defects and can be incorporated to automatic perimeters to facilitate visual field analysis.
Background: Research suggests that the analysis of facial expressions by a healthy brain would take place approximately 170 ms after the presentation of a facial expression in the superior temporal sulcus and the fusiform gyrus, mostly in the right hemisphere. Some researchers argue that a fast pathway through the amygdala would allow automatic and early emotional treatment around 90 ms after stimulation. This treatment would be done subconsciously, even before this stimulus is perceived and could be approximated by presenting the stimuli quickly on the periphery of the fovea. The present study aimed to identify the neural correlates of a peripheral and simultaneous presentation of emotional expressions through a frequency tagging paradigm.
Methods: The presentation of emotional facial expressions at a specific frequency induces in the visual cortex a stable and precise response to the presentation frequency [i.e., a steady-state visual evoked potential (ssVEP)] that can be used as a frequency tag (i.e., a frequency-tag to follow the cortical treatment of this stimulus. Here, the use of different specific stimulation frequencies allowed us to label the different facial expressions presented simultaneously and to obtain a reliable cortical response being associated with (I) each of the emotions and (II) the different times of presentations repeated (1/0.170 ms =~5.8 Hz, 1/0.090 ms =~10.8 Hz). To identify the regions involved in emotional discrimination, we subtracted the brain activity induced by the rapid presentation of six emotional expressions of the activity induced by the presentation of the same emotion (reduced by neural adaptation). The results were compared to the hemisphere in which attention was sought, emotion and frequency of stimulation.
Results: The signal-to-noise ratio of the cerebral oscillations referring to the treatment of the expression of fear was stronger in the regions specific to the emotional treatment when they were presented in the subjects peripheral vision, unbeknownst to them. In addition, the peripheral emotional treatment of fear at 10.8 Hz was associated with greater activation within the Gamma 1 and 2 frequency bands in the expected regions (frontotemporal and T6), as well as desynchronization in the Alpha frequency bands for the temporal regions. This modulation of the spectral power is independent of the attentional request.
Conclusions: These results suggest that the emotional stimulation of fear presented in the peripheral vision and outside the attentional framework elicit an increase in brain activity, especially in the temporal lobe. The localization of this activity as well as the optimal stimulation frequency found for this facial expression suggests that it is treated by the fast pathway of the magnocellular layers.
Abstract: Contrast is the differential luminance between one object and another. Contrast sensitivity (CS) quantifies the ability to detect this difference: estimating contrast threshold provides information about the quality of vision and helps diagnose and monitor eye diseases. High contrast visual acuity assessment is traditionally performed in the eye care practice, whereas the estimate of the discrimination of low contrast targets, an important complementary task for the perception of details, is far less employed. An example is driving when the contrast between vehicles, obstacles, pedestrians, and the background is reduced by fog. Many conditions can selectively degrade CS, while visual acuity remains intact. In addition to spatial CS, “temporal” CS is defined as the ability to discriminate luminance differences in the temporal domain, i.e., to discriminate information that reaches the visual cortex as a function of time. Likewise, temporal sensitivity of the visual system can be investigated in terms of critical fusion frequency (CFF), an indicator of the integrity of the magnocellular system that is responsible for the perception of transient stimulations. As a matter of fact, temporal resolution can be abnormal in neuro-ophthalmological clinical conditions. This paper aims at considering CS and its application to the clinical practice.
Abstract: Focal intraretinal alterations have been studied to advance our understanding of the pathology of neurodegenerative diseases. The current literature involving focal alterations in the intraretinal layers was reviewed through PubMed using the search terms “focal alteration”, “region of interest”, “optical coherence tomography”, “glaucoma”, “multiple sclerosis”, “Alzheimer’s disease”, “Parkinson disease”, “neurodegenerative diseases” and other related items. It was found that focal alterations of intraretinal layers were different in various neurodegenerative diseases. The typical focal thinning might help differentiate various ocular and cerebral diseases, track disease progression, and evaluate the outcome of clinical trials. Advanced exploration of focal intraretinal alterations will help to further validate their clinical and research utility.
