Phosphene Threshold
Phosphene Threshold - Related Videos
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State-Dependency Effects on TMS: A Look at Motive Phosphene Behavior
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In this article, we examine the effects of visually relevant state dependency on TMS induced motive phosphenic presentations.
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Phosphenes are transient percepts of light that can be induced by applying Transcranial Magnetic Stimulation (TMS) to visually sensitive regions of cortex. We demonstrate a standard protocol for determining the phosphene threshold value and introduce a novel method for quantifying and analyzing perceived phosphenes.
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Source: Laboratory of Jonathan Flombaum—Johns Hopkins University Psychophysics is the name for a set of methods in perceptual psychology designed in order to relate the actual intensity of stimuli to their perceptual intensity. One important aspect of psychophysics involves the measurement of perceptual thresholds: How bright does a light need to be for a person to be able to detect it? How little pressure applied to the skin is detectable? How soft can a sound be and still be heard? Put...
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FAQs
A phosphene threshold reflects the excitability of the stimulated visual pathway, because stimulation must raise activity in retinal or visual cortical neurons to a level associated with conscious perception. Lower or higher values therefore provide a functional measure of how readily that pathway responds. This makes threshold measurement useful for comparing neural responsiveness across experimental conditions.
The measurement can probe different parts of the visual system depending on whether stimulation engages retinal neurons or visual cortical neurons. Electrical, magnetic, or mechanical stimulation may therefore provide information about distinct neural locations rather than a single uniform response. Comparing these conditions helps researchers examine where visual pathway excitability changes occur.
Stimulus duration and waveform shape the neural response produced by the same nominal intensity. Consequently, a threshold measured with one timing or waveform condition may not be directly interchangeable with a value obtained under another. Researchers must keep these parameters consistent when comparing participants or sessions so that differences more clearly reflect neural responsiveness rather than altered stimulation conditions.
Researchers generally begin with a low stimulation intensity and increase it gradually while monitoring whether the participant perceives a flash without external illumination. The threshold is assigned when the stimulation produces the relevant visual percept. Recording the stimulus location, duration, waveform, and individual response helps place the result in its experimental context and supports meaningful comparisons.
In transcranial magnetic stimulation research, phosphene thresholds provide a noninvasive index of visual cortical excitability. Changes in the intensity needed to produce the percept can indicate altered responsiveness of the stimulated visual system. This makes the measure useful for studying sensory processing and for evaluating how experimental conditions influence activity in visual cortical neurons.
Visual neuroprosthesis studies can use threshold measurements to examine how much stimulation is needed to evoke a visual percept, while disease or treatment studies can track changes in neural responsiveness. Because values vary across individuals and stimulation conditions, repeated or carefully matched measurements help researchers interpret whether an observed shift reflects altered visual pathway or cortical function.