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Related Concept Videos

Color Vision01:24

Color Vision

Color perception begins in the retina, the light-sensitive layer at the back of the eye. Two main theories explain how colors are seen: the trichromatic theory and the opponent-process theory. The trichromatic theory, proposed by Thomas Young in 1802 and extended by Hermann von Helmholtz in 1852, suggests that color vision is based on three types of cone receptors in the retina. These cones are sensitive to different but overlapping ranges of wavelengths corresponding to red, blue, and green.
Vision01:24

Vision

Vision is the result of light being detected and transduced into neural signals by the retina of the eye. This information is then further analyzed and interpreted by the brain. First, light enters the front of the eye and is focused by the cornea and lens onto the retina—a thin sheet of neural tissue lining the back of the eye. Because of refraction through the convex lens of the eye, images are projected onto the retina upside-down and reversed.

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Related Experiment Video

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Stimulus-specific Cortical Visual Evoked Potential Morphological Patterns
09:42

Stimulus-specific Cortical Visual Evoked Potential Morphological Patterns

Published on: May 12, 2019

Transient and sustained BOLD responses to sustained visual stimulation.

Kâmil Uludağ1

  • 1Max-Planck-Institute for Biological Cybernetics, High-Field Magnetic Resonance Center, Tübingen, Germany. kamil.uludag@tuebingen.mpg.de

Magnetic Resonance Imaging
|May 16, 2008
PubMed
Summary

Functional magnetic resonance imaging (fMRI) reveals transient blood-oxygen-level-dependent (BOLD) signal responses during visual stimulation. These phasic responses, alongside sustained tonic activity, can complicate data interpretation in rapid experimental designs.

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Area of Science:

  • Neuroimaging
  • Brain Physiology
  • Functional Magnetic Resonance Imaging (fMRI)

Background:

  • The blood-oxygen-level-dependent (BOLD) signal in fMRI is used to study brain physiology.
  • BOLD signal analysis typically focuses on sustained (tonic) responses.
  • Transient (phasic) BOLD signal changes, such as overshoots, are occasionally observed but less understood.

Purpose of the Study:

  • To investigate the nature and implications of transient (phasic) BOLD signal responses during visual stimulation.
  • To differentiate between neuronal activity and vascular uncoupling as causes of phasic BOLD signals.
  • To assess the impact of phasic and tonic responses on fMRI data interpretation, particularly in rapid event-related designs.

Main Methods:

  • Utilized hemifield visual stimulation in human subjects.
  • Employed functional magnetic resonance imaging (fMRI) to measure BOLD signal changes.
  • Applied a biomechanical 'balloon model' of the BOLD signal to interpret findings.

Main Results:

  • Observed sustained, contralateral activation in the visual cortex and LGN, as expected.
  • Detected bilateral phasic BOLD responses that only partially overlapped with the sustained network.
  • The 'balloon model' suggested that both phasic neuronal activity and vascular uncoupling can generate phasic BOLD signals.

Conclusions:

  • Sustained visual stimulation elicits both bilateral phasic and contralateral sustained neuronal responses.
  • Phasic and tonic BOLD networks may be difficult to separate in rapid event-related fMRI designs, potentially confounding results.
  • Combined phasic and tonic responses in a region of interest could mimic BOLD responses seen in adaptation studies.